Isotope signatures of selected Silurian to Devonian

Quarterly Notes Geological Survey of New South Wales

Isotope signatures of selected Silurian to Devonian mineral systems in the Nymagee area, central Lachlan Orogen, New South Wales June 2018

Authors Peter M. Downes1 and Simon R. Poulson2 Geological Survey of New South Wales, 516 High Street, Maitland NSW 2320 2 Dept. Geological Sciences & Engineering, University of Nevada, Reno, USA 1

No.

151

Keywords: sulfur isotopes, lead isotopes, Nymagee, Ordovician basement, Cobar Basin, Mount Hope Trough, Winduck Shelf, Canbelego–Mineral Hill Volcanic Belt, Lachlan Orogen, mineralisation, ore deposit geology, oreforming fluids

ISSN 0155-3410 (print) ISSN 2204-4329 (online)

Contents Abstract Introduction Geological setting Sample preparation and techniques Mineralisation and results Ordovician basement units Mount Hope Trough Winduck Shelf Southern Cobar Basin Kopyje Shelf (Canbelego–Mineral Hill Volcanic Belt) Discussion Sources of sulfur Sources of lead Implications for mineralisation Conclusions Acknowledgements References

1 1 3 5 9 9 11 14 17 21 24 24 28 30 31 31 32

Quarterly Notes is published to give wide circulation to results of studies in the Geological Survey of New South Wales. Papers are also welcome that arise from team studies with external researchers. Contact: [email protected] Production co-ordination and general editing Geneve Cox and Simone Meakin Design Carson Cox Layout Kate Holdsworth Geospatial information Cassie Yarnold Technical editing Lyn Day © State of New South Wales through Department of Planning and Environment 2018. You may copy, distribute and otherwise freely deal with this publication for any purpose, provided that you attribute the Department of Planning and Environment as the owner. The information (and links) contained in this publication is based on knowledge and understanding at the time of writing (October 2017). However, because of advances in knowledge, users are reminded of the need to ensure that the information upon which they rely is up to date and to check the currency of the information with the appropriate officer of the Department of Planning and Environment or the user’s independent adviser. The product trade names in this publication are supplied on the understanding that no preference between equivalent products is intended and that the inclusion of a product name does not imply endorsement by the department over any equivalent product from another manufacturer.

Abstract

Introduction

The Nymagee area is a key mineralised part of the central Lachlan Orogen in New South Wales. Despite the metallogenic importance of the area, few sulfurand lead-isotope analyses have been published. This study presents 313 new sulfur- and 44 new lead-isotope analyses, together with additional 118 sulfur- and 105 lead-isotope analyses compiled from unpublished studies for 22 mineralised zones. Measured δ34S values range from -20.9‰ to 31‰, with the data suggesting that there are significant variations between unit/setting and deposit type. Three systems included in the present study (Condobolin, Melrose, Tallebung) have average sulfurisotope values close to zero (Group 1) suggesting that sulfur for these deposits came from reservoirs containing magmatic sulfur. A second group (Group 2) generally have average sulfur-isotope values of 4–9‰ (Anomaly 3, Hera, Mineral Hill, Mount Hope, Mount Solar, Nymagee and Pipeline Ridge) indicating that sulfur in these deposits was sourced from reservoirs containing both magmatic and reduced seawater sulfate and/or that mixing occurred between these two end-member reservoirs. Group 3 deposits (Blind Calf, BMW, Great Central, R7, Sandy Creek, Shuttleton and Wagga Tank) have average sulfurisotope values of 9–12‰, suggesting that these deposits included sulfur largely derived from reduced seawater sulfate reservoirs. Several Group 4 deposits (Gundaroo, Mallee Bull, MD2–Siegals, Mount Allen, Manuka and Yellow Mountain) have average δ34S values greater than 12‰, with some zones having a trend towards heavier values. This suggests that some sulfur in these deposits came from low-temperature, less fractionated/ unfractionated seawater sulfur/sulfate reservoirs.

The Nymagee mineral systems project area is in central western New South Wales (Figure 1). It covers a complex geological environment with different mineralisation styles and multiple mineralising events. The project area lies within the central part of the Central ‘Subprovince’ (Glen 2005, 2013) of the Lachlan Orogen and covers the southern continuation of the Cobar polymetallic mining district. This study is part of an integrated project, being undertaken by the Geological Survey of New South Wales, to better constrain the timing of mineralisation and magmatism, sources of metals/fluids and mineral deposit types in this key part of the Lachlan Orogen (see Downes et al. 2013; Downes, Blevin et al. 2016; Downes, Tilley et al. 2016; Fitzherbert et al. 2016; Fitzherbert, Mawson et al. 2017). Some of these studies are ongoing and will be reported separately.

Lead isotope data for most deposits in the study area closely conform to the LFB crustal growth curve model for the Lachlan Orogen and, with the possible exception of Condobolin, there is little evidence for the incorporation of lead from more primitive mantle-derived sources. However, lead from older less-evolved crustal sources such as Ordovician basement was included into younger systems particularly at Mallee Bull, which is hosted by late Silurian–earliest Devonian units. The lead isotope data for Mineral Hill and Pipeline Ridge support the interpretation that these deposits were formed coeval with magmatism in the latest Silurian. Furthermore, the lead-isotope data for several intrusion-related systems including Condobolin, MD2–Siegals, Mount Allen, Sandy Creek and Tallebung suggest that these deposits formed in the late Early to Middle Devonian. Similarly, the data for Hera, Nymagee, and several structurally controlled high-sulfide zones including Blind Calf, BMW, Shuttleton, Wagga Tank and Yellow Mountain, together with Manuka (carbonate Pb–Zn) and Gundaroo (sandstone Pb–Zn), all have leadisotope signatures consistent with these deposits having formed during late Early to Middle Devonian time. Cover: Pyrrhotite–chalcopyrite filled fractures hosted by a silicified alteration zone downdip of the Engine lode in diamond drillhole KDD004 (495072E 6393089N, Zone 55) at the Blind Calf prospect. The field of view is about 5 cm across.

The Nymagee area contains small to large gold-only, gold–base metal, base metal-rich and tin–tungsten systems hosted either by the late Silurian–Early Devonian Cobar Basin and related units of the ‘Cobar Superbasin’ i.e. Mount Hope Trough, Winduck Shelf, Mouramba Shelf and Kopyje Shelf (see David & Glen 2004; David 2006, 2010), or by the Ordovician units adjacent to, and forming probable basement to, the Cobar Basin. Despite the metallogenic importance of this area only limited sulfur- and lead-isotope data are available. Published data for the area include three sulfur-isotope analyses from Rayner (1969) and 10 lead-isotope analyses from Huston et al. (2016). In addition, there are unpublished analyses by Bush (1980), Northcott (1986), Ryan (1987), Spandler (1998), David (2005), Mernagh (2008), the CSIRO Pbisotope database (see Forster et al. 2010) and Page (2011). However, there is no regional synthesis of the sulfur- and lead-isotope data available. This contribution aims to develop an isotopic framework for mineralisation in the southern Cobar Basin and adjacent areas to better constrain the sources of metals and fluids and to provide a better understanding of the metallogenesis of the area. This study presents 313 new sulfur-isotope analyses for sulfide-rich samples from 22 deposits/zones, together with 44 new lead-isotope analyses for lead-rich samples from 12 deposits/zones. In addition, 118 unpublished sulfur-isotope analyses and 115 (including 105 unpublished) lead-isotope analyses from earlier studies have been collated. Regional sulfur and lead-isotope studies combined with insights from the deposits and geological setting can provide constraints as to the sources of metals, fluids and ore-forming process at both deposit and district scales. These inputs are important for understanding how mineral systems form, and controls to the ore-forming processes. This approach provides a test for competing sulfur and lead sources available for inclusion in individual mineralised zones/systems.

Quarterly Notes 151

1

Four principal sulfur reservoirs may contribute to the sulfur budget of a system. These are: (i) sulfur derived from sea water

In addition, Forster et al. (2015) summarised the history and use of lead-isotope data for understanding metallogenic events and terrains. Importantly, leadisotope samples are described as high- or low-lead. High-lead samples include galena, lead-rich minerals and samples with >700 ppm Pb (see Gulson 1986). For these samples, there is generally insufficient 238U, 235U and/or 232Th present to significantly change their original lead-isotope ratio through radioactive decay, and these samples would normally be considered to preserve an ‘initial’ ratio (Gulson 1986). In contrast, for samples with low levels of lead, the presence of even small amounts of 238U, 235U and/or 232Th can significantly change the initial lead ratio by adding radiogenic lead after mineral deposition (increasing the 208Pb/204Pb, 207Pb/204Pb and/ or 206Pb/204Pb ratio) so that the interpretation of these data needs to be treated with caution (see Downes 2009; Forster et al. 2015). The identification of high- and lowlead samples is critical for understanding and interpreting lead-isotope data. Given that we also include data from >30 years of analyses by the CSIRO Pb-isotope laboratory (North Ryde) and that the context for many of the historic

(ii) biogenic sulfur (iii) sulfide–sulfur leached from the host rocks (iv) sulfur contributed directly from syn-mineralisation magmatic sources (Rye & Ohmoto 1974; Ohmoto & Rye 1979). For lead, the principal isotopic reservoirs likely to be present include: (i) lead leached from Ordovician crustal basement reservoirs (ii) lead leached from late Silurian–Early Devonian intrusions and the derived quartzofeldspathic volcano-sedimentary rocks forming the fill to the basin (iii) radiogenic lead added to basin/basement reservoirs through radiogenic decay of uranium and thorium (iv) primitive lithospheric mantle-derived lead, recycled from or derived from units such as the Macquarie Arc.

QUEEN SLAN D

Bourke BOURKE

LOUTH

SOU T H AU ST R ALIA

Wilcannia

BARNATO

WALGETT

COBAR

NYNGAN Nyngan

COBAR

Dubbo

Nymagee

Broken Hill IVANHOE

NYMAGEE Mount Hope

NARROMINE

Newcastle

BOOLIGAL

Lake Cargelligo CARGELLIGO

FORBES

SYDNEY SOUTH PACIFIC OCEAN Wollongong

Hay Wagga Wagga

REFERENCE

Canberra

Nymagee project area

A.C.T

Central Lachlan Orogen Cobar Basin Kopyie Shelf Mount Hope Trough

VICTORIA

0

100

200 km

Mouramba Shelf Rast Trough Eastern Lachlan Orogen 1:250 000 map sheet area 2017_160

Figure 1. Project location and geological summary map, showing the distribution of major geological elements of the Lachlan Orogen discussed in the text.

2

June 2018

analyses was not recorded, this study has focused on only those samples from the CSIRO Pb-isotope database that contain high lead values, in order to reduce the risk of misinterpretation of that dataset. This study seeks a better understanding of the sulfur- and lead-isotope reservoirs supplying sulfur and lead (metals) to mineral systems in the area. Although this study is not comprehensive, it addresses all the above issues for those systems where geological and isotopic data permit.

Geological setting The Lachlan Orogen is part of the >1500 km-wide Tasmanides that developed along the Pacific margin of the Australian craton during Palaeozoic time (Foster et al. 1999; Glen 2005). The Tasmanides developed in a series of tectonic cycles defined by periods of prolonged crustal extension, in response to slab rollback at the leading edge of the palaeo-Pacific oceanic plate and a retreating subduction boundary (Collins 2002; Collins & Richards 2008), followed by regional deformation events. The Lachlan Orogen includes three major cycles: the Benambran Cycle (490–433 Ma) which was terminated by the earliest Silurian Benambran Orogeny; the Tabberabberan Cycle (433–380 Ma) which was terminated by the Middle Devonian Tabberabberan Orogeny and the Kanimblan Cycle (380–~340 Ma) which was terminated by the Early Carboniferous Kanimblan Orogeny (Glen 2005; Collins & Richards 2008; Glen 2013). The focus of the present study is Nymagee 1:250 000 map sheet area and adjacent areas in the Central ‘Subprovince’ (Glen 2005, 2013) of the Lachlan Orogen (Figure 1). Basement to that area includes Ordovician turbiditic sedimentary rocks of the Girilambone and Wagga groups (Figure 2). The Wagga Group is Lancefieldian to early Gisbornian in age (Colquhoun, Hendrickx & Meakin 2005). However, the age of Girilambone Group units in the study area is poorly constrained, with no fossils identified. Late Ordovician conodonts are present 20 km north of Condobolin (Percival 1999) in the east of the study area, whilst Early to Middle Ordovician conodonts are present to the north of the study area, north and northeast of Cobar (see Percival 2006, 2007; Burton et al. 2012). Importantly, minor mafic units, which may be sources of copper, gold and magmatic sulfur, are also within the Girilambone Group. These Ordovician sequences were deformed and metamorphosed during the Benambran Orogeny in the early Silurian (443–433 Ma; Glen 2005). This is supported by 40Ar/39Ar dates on deformationrelated mica of 440.2 ± 2.6 Ma, 435.2 ± 2.6 Ma and 434.3 ± 3.5 Ma (Fergusson et al. 2005). Following the Benambran Orogen, the area underwent extension that resulted in extensive granitic plutonism and the development of north-trending shallow- to deep-water basins/rifts during the late Silurian to Early Devonian. These palaeogeographic units include the Mount Hope Trough in the western part of the study area, and the Cobar Basin and poorly exposed Rast Trough in the central part of the area (Figure 2). Flanking these are

the Mouramba Shelf and Kopyje Shelf (which hosts the Canbelego–Mineral Hill Volcanic Belt) to the east of the Cobar Basin, and the Winduck Shelf that formed to the west of the Cobar Basin (Glen et al. 1985), see Figure 2. Together, these units form the ‘Cobar Superbasin’ (see David & Glen 2004; David 2006, 2010). The Mount Hope Trough hosts mineralisation on the western side of the study area. These zones include the Great Central mine area (Great Central mine, Anomaly A, Comet mine), BMW prospect, MD2–Siegals, Mount Allen (Au) and Wagga Tank. The sequences in the Mount Hope Trough are dominated by felsic volcanic and derived sedimentary rocks of the late Silurian to earliest Devonian (Pridolian to early Lochovian; Sherwin 2013; Downes, Blevin et al. 2016) Mount Hope and Broken Range groups (Figure 2). The Mount Hope Group includes the Mount Halfway, Mount Kennan and Nombiginni volcanics (not shown; see Notes 1 and 2) — and similar aged fine- to coarse-grained clastic rocks of the Broken Range Group (see MacRae 1987a; Scheibner 1987) that David (2005) suggests formed the sag-phase of the Mount Hope Trough. Note 1: Simpson (2014) (see also Downes, Blevin et al. 2016) proposed that the stratigraphy of the Mount Hope Group be simplified, with the Ambone Volcanics, Regina Volcanics, Goona Volcanics and part of Coando Volcanics being grouped into the Mount Halfway Volcanics. In addition, lavas of the Double Peak Volcanics are very similar to those in the Mount Kennan and/or Nombiginni volcanics. This simplification of the stratigraphy of the Mount Hope Group is shown on the Cobar Special 1:500 000 Metallogenic Map (Fitzherbert et al. 2016). Note 2: Individual formations/members are not shown in figures unless otherwise stated, due the scale and available space. East of the Mount Hope Trough is the deepwater late Silurian to earliest Devonian (Sherwin 2013; Downes, Blevin et al. 2016) Cobar Basin. The Mallee Bull, R7, Sandy Creek and Shuttleton zones are located in the central part of the basin, whereas Hera and Nymagee, as well as the Cobar gold–base metal district to the north of the study area, are located adjacent to the eastern edge of the basin. The basin is dominated by fine- to medium-grained turbidites (MacRae 1987a, b) of the late Silurian to earliest Devonian (Pridolian to early Lochovian; Downes, Blevin et al. 2016; Fitzherbert et al. 2016) Amphitheatre Group (Figure 2) which is subdivided into the lower Amphitheatre Group, the Shume Formation and the upper Amphitheatre Group. Both the lower and upper Amphitheatre Group consist of a sequence of quartz-rich to quartz-lithic sandstones and siltstones, whilst the Shume Formation also includes minor felsic volcanic and volcaniclastic rocks (MacRae 1987b).

Quarterly Notes 151

3

145°30′E 31°26′S

147°12′E 31°27′S

COBAR Hermidale

Canbelego

Pipeline Ridge

Gundaroo (De Nardi, Ridge)

Nymagee

4

1

2

The Canbelego–Mineral Volcanic Belt, also known as the Mineral Hill Rift (see Scheibner & Basden 1998), is separated from the Cobar Basin area by outcropping Ordovician basement (Girilambone Group, Wagga Group). This late Silurian to earliest Devonian (Sherwin 2013; Downes, Blevin et al. 2016) volcanic belt is part of the Kopyje Shelf (see Suppel & Gilligan 1993) and hosts mineralisation at Mineral Hill, Pipeline Ridge and Yellow Mountain. In the study area (Figure 2), the Canbelego– Mineral Volcanic Belt consists of the Kopyje Group (MacRae 1987b; Pogson 1991) which is three felsicdominated volcanic units – Badinda Volcanics, Majuba Volcanics, Mineral Hill Volcanics – and the fine-grained terrigenous to shallow-marine sedimentary rocks of the Baledmund Formation (Pogson 1991). Individual units are not shown on the figure.

Nymagee

Crowl Creek South Shuttleton R7 prospect

Hera Sandy Creek

Manuka

6

Mallee Bull

3

MD2–Siegals

Yellow Mountain

5 Fountaindale

Mineral Hill

Blind Calf Wagga Tank

BMW Melrose Mount Allen

Tallebung

Mount Hope Great Central mine area Mount Solar

Condobolin Condobolin Euabalong 33°09′S 145°28′E

33°10′S 147°12′E

REFERENCE Ordovician

Kopyje Shelf

Faults

Girilambone Group

Kopyje Group

1

Jackermaroo Fault

Mine or prospect

Wagga and Bendoc groups

Ootha Group

2

Buckwaroon Fault

Locality

3

Blue Mountain Fault

Highway

Mouramba Group

4

Crowl Creek Fault

Major road, sealed

Walters Range Group

5

Scotts Craig Fault

Major road, unsealed

6

Rookery Fault

Unit boundary

Fifield Suite Silurian to Early Devonian (Cobar Superbasin) Cobar Basin Amphitheatre Group Mount Hope Trough Mount Hope Group Broken Range Group Bootheragandra Group Rast Trough

Mouramba Shelf

Winduck Shelf Mulga Downs Group

Fault

Winduck Group

Project area

Yarra Yarra Creek Group Silurian to Early Devonian granites S-, I- and A-type granites (undifferentiated)

Rast Group

0

15

30 km 2017_161

Figure 2. Generalised geology for the Nymagee project area. The map shows the location of major mineralised zones and geological units at group level. Individual formations referred to in the text are not shown.

4

June 2018

To the east of the Cobar Basin is the shallow to deepwater Mouramba Shelf (Figure 2). This shelf includes the late Silurian to earliest Devonian (Sherwin 2013; Downes, Blevin et al. 2016) Mouramba Group (Figure 2) which is dominated by the Burthong Formation (not shown). MacRae (1987b) and Pogson (1991) described the Burthong Formation as a sequence of very fine- to medium-grained interbedded sandstones and siltstones with minor basal conglomeratic units and minor localised felsic volcanic/volcaniclastic horizons.

North of the Mount Hope Trough and west of the Cobar Basin is the Winduck Shelf, which is represented by the Early Devonian Winduck Group (Figure 2). Within this area carbonate-hosted base metal–silver mineralisation (Manuka, formerly Wonawinta) and sandstone-hosted base metal–silver mineralisation (Gundaroo) are hosted by the Pragian to early Emsian (Sherwin 2013) Gundaroo Sandstone and the Lochkovian to early Emsian Buckambool Sandstone respectively (see Sherwin 2013). Scheibner (1987) and MacRae (1987a) noted that the Gundaroo Sandstone includes thinly to thickly bedded sandstones and siltstones, with MacRae (1987a) also noting the presence of minor limestone units including the Booth Limestone Member, which is exposed in the wall of the Manuka silver–lead mine. Extensive felsic magmatism occurred within the study area during the Tabberabberan Cycle. Granites intruded both Ordovician basement units and the late Silurian to earliest Devonian volcano-sedimentary sequences overlying the basement (Figure 2). Many of the major granites were emplaced in the late Silurian (e.g. Derrida, Erimeran, Thule and Gilgunnia granites) at ~427–419 Ma; however, other intrusions (i.e. Boolabone Granite) are younger at ~415– 412 Ma. The new dating is summarised in Downes, Blevin et al. (2016). The timing of basin inversion and deformation of the late Silurian to earliest Devonian sequences, which host the majority of the mineralisation, is poorly constrained. Glen et al. (1992) proposed that the initial deformation of the Cobar Basin occurred 395–400 Ma (their ‘Cobar Deformation’), while Sun et al. (2000) proposed a later timing at ~385–389.2 Ma which essentially correlates this event with the Tabberabberan Orogeny. More recently,

Downes, Blevin et al. (2016) reviewed the available data and supported the interpretation that basin inversion and deformation occurred during the Tabberabberan Orogeny. Importantly, with the exception of the late diageneticzone assemblage associated with the Winduck Shelf, this event resulted in mineral assemblages across the Mount Hope Trough, Cobar Trough and Kopyje Shelf (including the Canbelego–Mineral Hill Volcanic Belt) consistent with sub- to lowest-greenschist facies regional metamorphic conditions (Downes, Tilley et al. 2016; Fitzherbert, Mawson et al. 2017). In addition, Fitzherbert, Mawson et al. (2017) noted the presence of zones of elevated metamorphic grade within the study area. They include zones of contact metamorphism surrounding intrusions in the Mount Hope Trough; as well as a narrow zone of hydrothermal biotite-zone greenschist facies metamorphism extending to the north of Mallee Bull; and importantly, a zone of hydrothermal biotite-zone greenschist to amphibolite facies metamorphism in the Hera–Nymagee area. Mineralisation styles within the project area include gold–base metal epithermal systems such as Mineral Hill and Pipeline Ridge; carbonate-hosted silver–lead–zinc ‘Mississippi Valley’-type mineralisation (e.g. Manuka); structurally controlled low-sulfide gold (orogenic gold — Mount Solar); and high-sulfide structurally controlled gold–base metal ‘Cobar-type’ systems including Mallee Bull, South Shuttleton and Wagga Tank. In addition, there are a number of intrusion-related systems such as Melrose (Cu), Mount Allen (Au), R7 (Cu–Zn), Sandy Creek (Pb–Zn) and Tallebung (Sn–W) whilst the Hera (Au–Pb–Zn–Ag) and Nymagee (Zn–Pb–Cu) deposits are now recognised as being intrusion-related skarns (see Fitzherbert, McKinnon & Blevin 2017; Fitzherbert, Blevin & McKinnon 2017; McKinnon & Fitzherbert 2017).

Sample preparation and techniques In the present study, samples for isotopic analyses were collected mainly from diamond drillcores, many of which are stored at the WB Clarke Geoscience Centre at Londonderry. Some of the new data are from samples collected from mineralised zones exposed in mine openings at Mineral Hill and Hera, or in the case of background low-lead data from unweathered outcrop/ unaltered drillcore. Sulfide-rich powders for sulfurisotope analysis were extracted using a microdrill and a binocular microscope. Samples for lead-isotope analysis were generally small pieces of core with visible galena. Contamination of mineral separates by other sulfide phases was minimised by selecting coarse-grained material where possible, but it was inevitable for samples containing fine-grained, mixed sulfides. In the latter cases, sample purity was estimated. In addition, samples were submitted for background sulfur- and lead-isotope studies (not reported here). Samples for background sulfurisotope analysis were fine-crushed powders of whole-rock geochemical samples, while the samples for low-lead lead-isotope analyses were feldspar separates from the coarse crush fraction of whole-rock geochemical samples.

Quarterly Notes 151

5

Sulfur-isotope analyses were undertaken at the Nevada Stable Isotope Laboratory, University of Nevada, Reno, USA, using the procedure as outlined by Giesemann et al. (1994) and Grassineau et al. (2001). Individual analyses used 40–50 μg of sulfur (i.e. the equivalent of 80–100 μg of pure pyrite). Actual samples supplied for analyses were two to ten times the minimum required for the analysis in order to allow for repeated analyses or possible analytical problems. The analytical procedure included the combustion of the sample in an elemental analyser and separation of SO2 from other gases by gas chromatography. Sulfur dioxide gas then entered the ion source of the mass spectrometer through a split interface. Reference gas for isotopic calibration was supplied from a SO2 bottle by injection into the carrier gas (helium) between the elemental analyser and the split interface. Within the Micromass Isoprime stable-isotope ratio mass spectrometer, the ion currents for masses 64 and 66 were measured and integrated. A comparison to the corresponding masses of a standard gas was made to determine the isotopic ratio, with correction for oxygen isotope contributions. Data are reported in the usual δ notation to an accuracy of ± 0.2‰ relative to ViennaCañon Diablo Troilite (VCDT), with calibration by means of sulfur-isotope standards obtained from the International Atomic Energy Agency (Vienna, Austria). New sulfurisotope data are summarised in Table 1 with individual analyses and locations included in Appendix 1 in Downes and Poulson 2017 and discussed below. Lead isotope analyses were undertaken by Mark Schmitz and the group at the Isotope Geology Laboratory, Boise

State University, Boise, USA (Appendix 2 in Downes & Poulson 2017). The majority of samples analysed for their lead-isotope value contained galena or were from zones that contained high (%) levels of lead. Samples with low amounts of lead were lightly crushed and leached using hot aqua regia. Samples were analysed using an IsotopX Phoenix X62 multicollector thermal-ionization mass spectrometer with lead-isotope ratios measured on single Re filaments with a silica gel–phosphoric acid emitter solution in static mode and maintaining a 3V 208Pb beam for 200 cycles. Tabulated lead-isotope ratios presented in this study include an external fractionation correction of 0.12 ± 0.02% (1-sigma) per a.m.u., based upon over 100 NBS-981 lead-standard measurements, by the Isotope Geology Laboratory at Boise State University, using similar-sized ion beams at the same run temperatures undertaken. That laboratory also noted that fractionation uncertainty imposes the following minimum absolute uncertainties (1-sigma): 208Pb/204Pb = 0.019; 207Pb/204Pb = 0.007; 206 Pb/204Pb = 0.008; 208Pb/206Pb = 0.0009; 207Pb/206Pb = 0.0004. The new lead-isotope results are presented in Appendix 2 of Downes and Poulson 2017 and discussed below. Errors, expressed as 2-sigma analytical precision, are shown in the upper left-hand corner of lead-isotope diagrams. This error is very similar to, and largely overlaps with, the reported error for analyses reported by Mernagh (2008) and Huston et al. (2016) that were undertaken at the Isotope Geochemistry laboratory at the School of Earth Sciences, University of Melbourne (see Huston et al. 2017).

Table 1. Summary of sulfur isotope data and mineralogy for mineralisation in the Nymagee project area, central Lachlan Orogen Deposit (deposit type)

Commodities major (minor)

Primary ore mineralogy major (minor)

Host unit

Sulfur isotope average (per mil)

Sulfur isotope range by mineral (per mil)(source in superscript)

Deposit (deposit type)

Commodities major (minor)

Primary ore mineralogy major (minor)

Host unit

Sulfur isotope average (per mil)

Sulfur isotope range by mineral (per mil)(source in superscript)

Great Central (high sulfide structurally controlled)

Cu

cpy, po, (py, sph, ga, Mount Halfway asp) Volcanics

11.8

py = 8.9–13.21 cpy = 11.0–13.01 po = 11.4–13.01 sph = 11.5–13.21

Gundaroo (inc. Ridge, De Nardi) (sandstone-hosted Pb)

Ag, Zn, Pb (Cu)

sph, ga, py (cpy, tet) Buckambool Sandstone

12.2 (excludes outliers)

py = 11.7–14.51 cpy = 10.5, 13.41 ga = 9.3–10.91 sph = 12.5–14.1, 19.6, 29.81 tet = 13.81

Hera (skarn)

Au, Pb, Zn, Ag

po, ga, sph, py, cpy, lower Amphitheatre 5.1 (asp, au, bi, bis, Group, Burthong Fm bo, cc, cub, mar, scheelite, stb, ten, tet) (see McKinnon & Fitzherbert 2017 for full list)

py = 4.9, 5.63,4 asp = 4.7–5.01 cpy = 3.2–7.43,4 ga = 2.9–6.21,3,4 ga–sph = 5.0, 5.11 po = 4.9–7.41,3,4 sph = 4.0–6.61,3,4 sph–asp = 5.01

Mallee Bull (high sulfide structurally controlled)

Cu, Au (Pb, Zn, Ag)

po, py, cpy, ga, sph (asp, mag, tet, cas, el, asb)

upper Amphitheatre two signatures Group, Shume Fm 12.9 & 19.8

py = 14.9–17.81 asp = 11.5–13.71 cpy = 10.6–15.5, 20.21 ga = 10.4–13.51 po = 14.6, 21.51 sph = 11.81

Manuka, formerly Wonawinta (carbonate-hosted Pb–Zn)

Pb, Zn, Ag

ga, py, sph

Gundaroo Sandstone

insufficient data

py = 6.2, 18.4, 20.21,4 ga = 10.8, 12.74 sph = 9.3, 11.24

MD 2–Siegals

Zn (Pb, Cu, Ag)

po, py, ga, sph (cpy) Mount Halfway Volcanics

12.6

cpy = 10.4–13.21 po = 10.4–13.31 sph = 13.31

Melrose (intrusion-related)

Cu

py, sph, cpy, mo

-2.1

py = -2.31 cpy = -3.6– -0.41

Mineral Hill (intermediate sulfidation epithermal)

Cu, Au (Ag, Pb, Zn, Bi)

py, cpy, ga, sph, tet, Mineral Hill (bo, mag, bi, bis, Volcanics asp, au, cc, cov, cup, dig, idaite, mol, el, stb)

8.8

py = 5.0–16.81, 3, 6 bo = 5.76 cpy = 5.1–11.71, 3, 5, 6 ga = 5.1–9.91, 3, 5, 6 sph =5.1–10.93, 5, 6

Mount Allen (intrusion-related)

Au, Fe

mag, py (cpy, wof, bis, po, au, secondary bismuth minerals)

Double Peak Volcanics

12.2

py = 11.2–13.41 bis = 11.41

Girilambone Group

Anomaly 3 (high sulfide structurally controlled)

Au, Ag (Pb)

po, sph, ga, (cpy, py) ?Mount Halfway Volcanics

7.9

py = 6.2–9.12 cpy = 6.1–8.82

Blind Calf (high sulfide structurally controlled)

Cu

cpy, py (po, sph, bo, cc, cov, cup, ga, mag, tet)

Girilambone Group

8.3

py = 8.4, 8.71 cpy = 7.6–8.6, 9.2,10.01 ga = 7.21 po = 7.2–8.21

BMW prospect (high sulfide structurally controlled)

Pb, Zn (Cu, Au)

po, py (cpy, ga, mag, sph)

Mount Halfway Volcanics

10.2

cpy = 12.51 ga = 5.5, 11.71 po = 8.7, 11.51 sph = 11.61

Mount Hope (high sulfide structurally controlled)

Cu (Ag, Au)

cup, cpy (mag, py, po, sph, cc)

Broken Range Group

7.8

py = 7.6–7.71 cpy = 8.01

Condobolin (inc. Mascotte, Phoenix, Potters) (intrusion related)

Au, Pb (Ag, Zn, Cu)

au, py, ga (sph, cpy, asp)

Girilambone Group

2.9

py = 1.6–3.7, 6.51 ga = 0.6, 2.01 sph = 1.7–7.21

Mount Solar (low sulfide structurally controlled)

Au

py (ga, sph cpy, po, au, cc)

Broken Range Group

5.5

py = 5.4–5.71

6

June 2018

Quarterly Notes 151

7

Deposit (deposit type)

Commodities major (minor)

Primary ore mineralogy major (minor)

Host unit

Sulfur isotope average (per mil)

po, cpy, sph ga (mag, asp, cub, vallerite, tet, mackinawite, bi argentite)

lower Amphitheatre 7.4 Group

Sulfur isotope range by mineral (per mil)(source in superscript)

Nymagee (high sulfide structurally controlled)

Zn, Pb, Cu

cpy = 5.4–10.21,7 ga = 8.51 ga–sph = 6.1–7.11 po = 6.6–7.87 sph = 5.0–10.21,7

Pipeline Ridge (intermediate sulfidation? epithermal)

Au, Ag, Cu (Zn, Pb)

py, cpy, sph (ga, au, el, mar, stromeyerite, tet)

Baledmund Fm

10.3

py = 4.8–14.01 cpy = 11.4, 11.71 ga = 7.4–10.21 sph = 12.41

R7 prospect (intrusion-related)

Cu, Zn

py, po, sph, cpy

Shume Fm, R7 granite

8.5 (excludes py = 31.0)

py = 8.9–9.8, 31.01 cpy = 8.21 po = 7.8–9.21

Sandy Creek( intrusion-related)

Zn, Pb (Cu, Ag)

asp, cpy, po, sph

Shume Fm

12.0

cpy = 12.9–13.01 ga = 10.2–11.51 po = 12.7–12.91 sph = 11.7–12.21

Shuttleton (South Shuttleton) (high sulfide structurally controlled)

Cu (Pb, Zn, Ag)

cc, cpy, py, po (ga, mag, sph, cub, cup)

Shume Fm

11.6

py = 10.3–13.11,3 cpy = 10.5, 12.71 ga–sph = 11.21 po = 10.9, 11.61,3

Tallebung (intrusion-related)

Sn, W

cas, wof (asp)

Clements Fm

-0.9 (excludes asp = asp = -0.1, 6.11 6.1) po = -2.3–1.11

Wagga Tank (high sulfide structurally controlled)

Zn, Pb, Cu (Au, Ag)

cc, cpy, dig, ga, py, sph (cub, po)

Mount Kennan Volcanics

11.1 (excludes outliers)py = -20.9, -11.1, -1.2– 2.2, 6.7, 10.1–13.51,8 cpy = 11.58 ga = 7.7–11.28 sph = 9.9–13.91,8

Yellow Mountain (high sulfide structurally controlled)

Zn, Pb, Ag, Cu (Au)

ga, py, sph (asp, cpy, cov, cup, mar, po, tet)

Majuba Volcanics

15.1

py = 13.8–16.91 cpy = 13.9–16.61 ga = 14.1, 15.11 ga–sph = 14.4–15.21 sph = 14.9–15.41

Mineral abbreviations: ag = native silver; asb = aurostibnite, asp = arsenopyrite, au = native gold, bi = native bismuth, bis = bismuthinite, bo = bornite, cas = cassiterite, cc = chalcocite, cov = covellite, cpy = chalcopyrite, cub = cubanite, cup= cuprite, dig = digenite, el = electrum, ga = galena, mar = marcasite, mag = magnetite, mol = molybdenite, po = pyrrhotite, py = pyrite, sph = sphalerite, stb = stibnite, ten = tennantite, tet = tetrahedrite, wof = wolframite. δ 34S data: 1present study; 2Northcott (1986), 3Mernagh (2008), 4David (2005), 5Bush (1980), 6Spandler (1998), 7Downes & Page (reported in Page 2011), 8Ryan (1987). per mil = ‰ Commodity and mineralogical data: present study; GSNSW Metallic Industrial and Exploration (MetIndEx) database. The deposit classifications are based on data from Fitzherbert et al. (2016); Fitzherbert, Blevin & McKinnon (2017); McKinnon & Fitzherbert (2017).

For samples analysed by the CSIRO Pb-isotope laboratory (now included in the CSIRO Pb-isotope database) precision estimates representing two standard deviations are 0.05% for the 207Pb/204Pb ratio and 0.1% for all other ratios (Gulson et al. 1984). Errors, expressed as analytical precision, are shown in the upper left-hand corner of lead-isotope diagrams as 95% confidence ellipses based on over 1400 analyses (Carr et al. 1995). Estimates of lead model ages – the age that dates the extraction of lead from the lead-reservoir – were made using

8

June 2018

unpublished calculators developed by David Champion and David Huston of Geoscience Australia (see Huston et al. 2017). These calculators use the Cumming and Richards (1975) global crustal growth curve and Carr et al. (1995) Lachlan Fold Belt (LFB) growth curve models. The Lachlan Fold Belt crustal growth curve is a modified last stage to the Cumming and Richards (1975) growth curve that passes through the average lead-isotope values for the Woodlawn volcanic-associated massive sulfide (VAMS) system at an assumed age of 420 Ma (Carr

et al. 1995). It is shown on lead-isotope figures included in this study. Derived lead model ages were checked against data plotted in PbGraph, which is a lead-isotope plotting program developed by the CSIRO which used the Cumming and Richards (1975) crustal growth curve model. Stated error estimates for lead model ages are around ± 15 Ma for data analysed by the former CSIRO Pbisotope laboratory (i.e. CSIRO Pb-isotope database; David 2005, 2008; Carr et al. 1995). They are estimated at 18.3, which are significantly more evolved than other analyses from Condobolin and reflect a younger, possibly Carboniferous, event. The lower-precision data for the Potters zone from the CSIRO database also have a wide range of values, with lead model ages ranging from the early Silurian to mid-Devonian, in a similar fashion to those for the higher-precision data. Overall, the lead-isotope data for Condobolin reflect mixing between a mid-Silurian dominantly crustal reservoir and an early to mid-Devonian crustal reservoir.

Melrose Minor copper mineralisation is present in drillhole AOG8A that tested the Melrose anomaly approximately 76 km southeast of Nymagee and 12 km west of Mineral Hill. The mineralisation is hosted by the Early Ordovician Narrama Formation (Girilambone Group) see Figure 2. However, Re–Os dating of molybdenite from AOG8A gave an age of 424.7 ± 1.5 Ma (Creaser pers. comm. 2003 quoted in Blevin 2003) indicating that the mineralisation is younger than the host sequence. Limited sulfur-isotope data are available for the mineralisation associated with the Melrose anomaly, with δ34S values occupying a narrow range between -3.6‰ and -0.4‰ (mean -2.1‰; 7 analyses) (Figure 3). Chalcopyrite has δ34S values between -3.6‰ and -0.4‰ (mean -2.1‰; 6 analyses), with a single δ34S analysis for pyrite of -2.3‰.

Tallebung Suppel and Gilligan (1993) have briefly described the large intrusion-related Tallebung tin–tungsten system,

80 km south of Nymagee. The mineralised veins are hosted by sedimentary units of the Clements Formation (Wagga Group; Figure 2). U–Pb dating of cassiterite from Tallebung suggests that the system formed at 418 ± 6 Ma (Downes, Blevin et al. 2016), which is consistent with widespread magmatism across the study area. Sulfur-isotope values for Tallebung (Figure 3) lie in a narrow range between 2.1‰ and 1.1‰ (mean -0.9‰; 10 analyses; one analysis for arsenopyrite of 6.1‰ excluded). Data for pyrrhotite range from -2.1‰ to 1.1‰ (mean -1.0‰; 9 analyses), whilst arsenopyrite has δ34S values of -0.1‰ and 6.1‰. One lead-isotope analysis for pyrrhotite–galena is available from Huston et al. (2016). The data plot adjacent to the crustal growth curve, within the Silurian VHMS field of Carr et al. (1995), and had an Early Devonian LFB lead model age of 408 ± 3 Ma (Figure 4). This analysis suggests that lead was sourced from an Early Devonian reservoir. The model age is within error of, and overlaps with, a U–Pb SHRIMP date for cassiterite of 413.2 ± 8.8 Ma from Barry (2015).

Mount Hope Trough BMW prospect Sulfide-rich stringer and disseminated mineralisation has been intersected at the BMW prospect 83 km southwest of Nymagee. Here pyrrhotite, pyrite, sphalerite, galena and chalcopyrite are hosted by chloritically altered units of the Mount Halfway Volcanics (Mount Hope Group), see Figure 2. Sulfides from mineralised zones at BMW have δ34S values of 5.5–12.7‰ (6 analyses; Figure 5). Galena has δ34S values of 5.5‰ and 11.7‰, pyrrhotite has values of 8.7‰ and 11.5‰, and the single δ34S values for chalcopyrite and sphalerite are 12.5‰ and 11.6‰ respectively (Figure 5).

Quarterly Notes 151

11

Twenty-eight sulfur-isotope analyses are available for the Great Central zone (Figure 5). Nineteen analyses are for the Great Central mine (present study) and a further 9 analyses, from Northcott (1986), are for the Anomaly 3 zone which is 900 m further south. δ34S values for the Great Central mine lie between 8.9‰ and 13.2‰ (mean 11.8‰; 19 analyses). Chalcopyrite has δ34S values between 11.0‰ and 13.0‰ (mean 12.0‰; 6 analyses), and those for pyrite lie between 8.9‰ and 13.2‰ (mean 11.0‰; 5 analyses). Those for pyrrhotite are between 11.4‰ and 13.0‰ (mean 12.0‰; 5 analyses), with values for sphalerite of 12.1‰ and 13.2‰. The sulfur-isotope values for Anomaly 3 are lower and range from 6.1‰ to 9.1‰ (mean 7.9‰; 9 analyses), with values for chalcopyrite between 6.1‰ and 8.8‰ (mean 7.8‰; 5 analyses) and for pyrite between 6.2‰ and 9.1‰ (mean 7.9‰; 4 analyses).

Two galena-rich samples were analysed for their leadisotope signature as part of the present study. The data plot adjacent to the crustal growth curve, within the Silurian VHMS field of Carr et al. (1995), see Figure 6, and have a latest Early Devonian signature (~403 Ma).

Great Central Discontinuous base-metal mineralisation occurs over a 2.7-km zone around the Great Central mine which is 100 km south-southwest of Nymagee and 5 km southsouthwest of Mount Hope. This mineralised structure hosts massive, stringer and/or disseminated sulfides at the Comet mine, Great Central mine–Hodge lode and at Anomaly 3 (individual deposits not shown). Sheared and Mg-chlorite–phengitic white mica altered felsic volcaniclastic rocks (see Downes, Tilley et al. 2016) of the late Silurian Mount Halfway Volcanics (Mount Hope Group; see Downes, Blevin et al. 2016, figure 2) host the zone. The mineralisation may be part of a deformed stringer (feeder) zone to a VAMS system (Suppel & Gilligan 1993) or part of a structurally controlled high-sulfide epigenetic base metal deposit.

Ordovician basement 15.70

Analytical precision CSIRO Pb isotope database this study

Condobolin n=24 8

6

6

13

15

17

19

207

-5

-3

-1

1

3

5

7

11

13

15

17

19

15.60

tle mi xin so

gi

Siluro-Devonian Cu–Au

18.10

18.20 206

2

ron ch

15.55 18.00

n ro ch iso

4

ng

Ordovician Besshitype VAMS

6

Frequency

an

Tallebung n=11

ixi

m

8 Melrose n=7

2

-m

tle

δ34S%

4

Frequency

Devonian VAMS

an

δ34S%

9

tal

11

rus

9

ac

7

l-m

5

ta

3

us

1

cr

-1

a

-3

0M

Devonian granite-related

M

-5

Silurian VAMS

Carboniferous granite-related

35

0

0

40

0

a

2

curve

M

2

crustal growth

a

4

0M

Pb/204Pb

35

4

15.65

0

Frequency

8

Huston et al. (2016)

40

Frequency

The MD2–Siegals base metal system is located 66 km southwest of Nymagee. In this system, disseminated and

Ten lead-isotope analyses are included in the CSIRO database for the Comet mine, which lies on the Great Central structure some 600 m to the north of the Great Central mine. The majority of the samples have 206Pb/204Pb values >18.11 and contain less than 700 ppm Pb (values of 400 ppm, 360 ppm, 275 ppm, 180 ppm, 135 ppm, 40 ppm and 5 ppm). They are not shown on Figure 6 as they

Blind Calf n=24

Sulfur-isotope data from the MD2–Siegals zone range between 10.4‰ and 13.4‰ (mean 12.6‰; 10 analyses) with values for chalcopyrite in the range of 10.4–13.2‰ (mean 12.5‰; 4 analyses), those for pyrrhotite are in the range of 10.4–13.3‰ (mean 12.7‰; 5 analyses), with a single value for sphalerite of 13.3‰ (Figure 5).

MD2–Siegals

10

10

vein-hosted sulfides (pyrrhotite–sphalerite–chalcopyrite– galena) are hosted by chlorite–white mica altered felsic volcanic rocks of the Mount Halfway Volcanics (Mount Hope Group; Figure 2). Some mineralised zones are associated with peperitic breccia units located between coherent rhyodacitic sills (Burrell et al. 2012); however, the timing and controls to mineralisation have yet to be resolved.

are not likely to represent initial ratio data. Three analyses contain >1000 ppm Pb, have 206Pb/204Pb values between 18.08 and 18.11, plot within error of the crustal growth curve and are likely to represent initial ratio data, but do not cluster (Figure 6). Two of these points plot within the Silurian VHMS field of Carr et al. (1995), whilst the third point has a 206Pb/204Pb value of 18.105 and plots within the Devonian granite field of Carr et al. (1995). However, all three analyses have a very similar Early Devonian LFB lead model age (~411 Ma), which suggests that mineralisation along the Great Central structure is younger than the host volcanic sequence.

18.30

Pb/204Pb

REFERENCE 0

0 -5

-3

-1

1

3

5

7

9

11

13

15

17

19

-5

-3

-1

1

3

δ34S%

5

7

9

11

13

15

17

19

δ34S%

Condobolin (Huston et al. 2016)

Condobolin (this study)

Condobolin (CSIRO database)

Tallebung (Huston et al. 2016)

Blind Calf (this study) arsenopyrite pyrite

chalcopyrite pyrrhotite

galena

2017_163

sphalerite 2017_162

Figure 3. Distribution of sulfur isotope data for mineralised zones (Blind Calf, Condobolin, Melrose and Tallebung) hosted by Ordovician basement units included in the present study. Data from the present study.

12

June 2018

Figure 4. Distribution of all 207Pb/204Pb vs 206Pb/204Pb data for the Blind Calf, Condobolin and Tallebung systems. Data shown are for the area where 206Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events and crustal growth curve from Carr et al. (1995). Data from the CSIRO database (shown as points), Huston et al. (2016) and the present study (both shown as 2-sigma error ellipses). The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database (yellow), the 2-sigma uncertainty for the present study (red), and Huston et al. (2016) (light green).

Quarterly Notes 151

13

Wagga Tank At Wagga Tank, 86 km southwest of Nymagee and 27 km northwest of Mount Hope, base-metal mineralisation is hosted by breccias at the contact between felsic volcaniclastic rocks (and related sedimentary rocks; Scott et al. 1991) and siltstone–slate units (Ryan 1987), both of which are part of the Mount Kennan Volcanics (Mount Hope Group; Figure 2). The mineralisation here varies from massive and colloform-banded sulfides to quartz– sulfide veins and breccias and disseminated sulfides (Scott et al. 1991). Ryan (1987) suggested that the mineralisation formed at temperatures of around 325°C, based on fluid inclusion studies and chlorite geothermometry. The origin of the mineralisation at Wagga Tank is unclear with Suppel and Gilligan (1993) suggesting that the mineralisation may be a deformed volcanogenic zone or a structurally controlled ‘Cobar-type’ deposit. Thirty-two sulfur-isotope analyses are available from Ryan (1987; 26 analyses) and the present study (6 analyses; Figure 5). The majority of δ34S values lie between 7.7‰ and 13.9‰ (mean 11.1‰; 26 analyses), with six outliers

14

June 2018

-3

-1

1

3

5

7 9 δ34S%

11

13

15

17

19

-5

10

-3

-1

1

3

5

7 9 δ34S%

11

13

15

17

19

10 Great Central n=19

6 4

MD2–Siegals n=10

8 Frequency

8

2

6 4 2

0

0 -5

-3

-1

1

3

5

7 9 δ34S%

11

13

4

15

17

19

-5

-3

-1

1

3

5

7 9 δ34S%

11

13

4

Mount Allen n=8

2

15

17

19

Mount Hope n=3

2

0

0 -5

Gundaroo

Twenty-one sulfur-isotope analyses are available for sulfides from Gundaroo (20 for Ridge, 1 for De Nadi; present study; Figure 7). Sulfur-isotope values lie between 9.3‰ and 14.1‰ (mean 12.2‰; 19 analyses) with outliers of 19.6‰ and 28.8‰ (for sphalerite from the Ridge prospect). Excluding outliers, δ34S values for sphalerite are between 12.5‰ and 14.1‰ (mean 13.2‰; 7 analyses), values for galena are between 9.3‰ and 10.9‰ (mean 9.8‰; 5 analyses), values for chalcopyrite are 10.5‰ and 13.4‰ (2 analyses) and a single δ34S analysis of 13.8‰ was obtained from tetrahedrite.

2

0 -5

Winduck Shelf Scattered lead–zinc–silver mineralisation is present at Gundaroo, 49 km west of Nymagee. Two major mineralised zones have been identified by drilling — the De Nadi deposit and the Ridge prospect. Sandstones of the Early Devonian Buckambool Sandstone (Winduck Group; Figure 2) unconformably overlie the late Silurian Thule Granite and host both mineralised zones.

Frequency

Frequency

0

Frequency

One lead-isotope analysis for sulfide is available from the present study for Mount Allen. The analysis plots adjacent to the crustal growth curve, within the Devonian granite field of Carr et al. (1995). It has a lead model age of 402 ± 3 Ma (Figure 6) which is significantly younger than the host sequence.

BMW prospect n=6

2

Other

For Mount Solar there are two lead-isotope analyses in the CSIRO database. One of the analyses (207Pb/204Pb vs 206 Pb/204Pb ratio = 18.095/15.637) has a poor data quality score (measure of analytical reliability by the CSIRO of 9), and is not shown on Figure 6. The second analysis has a high lead concentration (2410 ppm Pb) and a 207Pb/204Pb vs 206Pb/204Pb ratio of 18.171/15.726 which plots outside error and significantly above the crustal growth curve (Figure 6). This suggests that it includes excess 207Pb from the isotopic decay of 235U and is thus not diagnostic.

4

-3

-1

1

3

5

7 9 δ34S%

11

13

4

15

17

-5

19

-3

-1

1

3

5

7 9 δ34S%

10

Mount Solar n=3

2

11

13

15

17

19

Wagga Tank n=30 excluding pyrite= -20.9,-11.1

8

Frequency

The δ34S data for Mount Allen lie in a narrow range of 11.2– 13.4‰ (mean 12.2‰; 8 analyses; Figure 5). δ34S values for pyrite lie between 11.2‰ and 13.4‰ (mean 12.4‰; 7 analyses), with a single value for bismuthinite of 11.4‰.

Four lead-isotope analyses for galena are available from the CSIRO database for mineralisation at Wagga Tank. The analyses plot in a tight group centred at 207Pb/204Pb vs 206 Pb/204Pb ratio = 15.641/18.122 within error, along the same fractionation curve, adjacent to the crustal growth curve and within the Devonian granite field of Carr et al. (1995), see Figure 6. The data have a mid-Devonian lead model age (~383 Ma).

Limited isotopic data are available for the Mount Hope copper mine and Mount Solar gold mine near Mount Hope, 95 km south-southwest of Nymagee (Figure 2), both of which are hosted by units of the Broken Range Group. For Mount Hope, pyrite has δ34S values of 7.6‰ and 7.7‰, with a single value of 8.0‰ from chalcopyrite (Figure 5). Sulfur-isotope values for pyrite from Mount Solar range between 5.4‰ and 5.7‰ (average 5.5‰, 3 analyses; Figure 5).

4

Anomaly 3 n=9

6

Frequency

Fine-grained clastic and volcaniclastic rocks of the late Silurian to earliest Devonian Nombiginni Volcanics (Mount Hope Group, Figure 2) host gold and silver with minor bismuth at the Mount Allen gold mine, 16 km north of Mount Hope and 72 km southwest of Nymagee. The mineralisation styles include massive hematite–magnetite zones, hematite–magnetite-bearing siltstones and quartz– sulfide veined chloritic siltstones (Suppel 1984). Suppel (1984) suggested that the mineralisation is intrusion related whilst Suppel and Gilligan (1993) proposed that the mineralisation is similar to other structurally controlled ‘Cobar-type’ deposits.

8

Frequency

Mount Allen

between -20.9‰ and 6.7‰. Pyrite has δ34S values in the range of 10.1–13.5‰ (mean 11.0‰; 8 analyses) with outliers of -20.9‰, -11.1‰, -1.2‰, 2.2‰, 2.2‰ and 6.7‰. Values for galena lie between 7.7‰ and 11.2‰ (mean 9.4‰; 6 analyses), those for sphalerite lie between 9.9‰ and 13.8‰ (mean 11.4‰; 11 analyses) and a single δ34S analysis for chalcopyrite has a value of 11.5‰.

Frequency

David (2008) included six lead-isotope analyses for galena (Figure 6). All the samples were analysed at the CSIRO Pbisotope laboratory. Five of the analyses plot largely within error and adjacent to the crustal growth curve, whilst the remaining analysis has a 207Pb/204Pb vs 206Pb/204Pb value of 15.618/18.146, suggesting that some lead is from a more-evolved reservoir. LFB lead model ages for the data range between 414 ± 15 and 388 ± 15 Ma (average ~400 Ma). This suggests that the system is significantly younger than the host sequence and is younger than the Boolahbone Granite (415 ± 3.1 Ma U–Pb SHRIMP date on zircon in Downes, Blevin et al. 2016) which appears to be the youngest intrusion in the area.

6 4 2

0

0 -5

-3

-1

1

3

5

7 9 δ34S%

11

bismuthinite pyrite

13

15

17

19

chalcopyrite pyrrhotite

-5

-3

-1

galena

1

3

5

7 9 δ34S%

11

13

15

17

19

mixed sulfide

sphalerite 2017_164

Figure 5. Distribution of sulfur isotope data for mineralised zones (BMW, Great Central (Anomaly 3, Great Central), MD2–Siegals, Mount Allen, Mount Hope, Mount Solar and Wagga Tank zones) hosted by units of the Mount Hope Trough included in the present study. Data from Northcott (1986; Anomaly 3 (Great Central)), Ryan (1987; Wagga Tank) and the present study.

Quarterly Notes 151

15

Manuka

Five samples were analysed for their lead-isotope signature. Four of the data contain visible galena, plot in a tight cluster centred at 207Pb/204Pb vs 206Pb/204Pb ratio = 15.625/18.096 adjacent to the crustal growth curve (Figure 8) and are mostly within the Silurian VHMS field of Carr et al. (1995). The remaining analysis has a 207 Pb/204Pb vs 206Pb/204Pb ratio of 15.665/18.132, did not contain visible galena and has been discarded as it may have been modified by the addition of radiogenic lead. The data for the four galena-rich samples have an average lead model age of ~405 Ma (range 412 ± 15 to 398 ± 15 Ma), suggesting that the majority of lead was sourced from a younger (more-evolved) lead reservoir than the Lochkovian to early Pragian (Downes, Blevin et al. 2016) host sequence.

The Manuka (formerly Wonawinta) oxide silver mine (Figure 2) is located 57 km west-southwest of Nymagee. At this location, sulfide-rich mineralisation (below the base of weathering) is associated with the dolomitic Booth Limestone Member and overlying carbonaceous shale of the Pragian to earliest Emsian Gundaroo Sandstone (Winduck Group; Downes, Blevin et al. 2016). This package unconformably overlies the late Silurian Thule Granite. The mineralisation is part of a carbonate-hosted Pb–Zn–Ag system with similarities to Mississippi Valleytype systems (David 2005; Lenard 2010). This system formed at temperatures less than 150°C (Giles 1993, based on a reconnaissance fluid inclusion study). In addition, below the mineralised zones there are gypsumrich zones (see Downes, Tilley et al. 2016).

Mount Hope Trough 15.75

Analytical precision CSIRO Pb isotope database and David (2008) this study

Limited sulfur-isotope data are available for Manuka from David (2005) and the present study. The data have a wide range of δ34S values of 6.2–20.2‰ (7 analyses; Figure 7), with pyrite having values of 6.2‰, 18.4‰ and 20.2‰, while galena has values of 10.8‰ and 12.7‰, and sphalerite has δ34S values of 9.3‰ and 11.2‰.

± 15 Ma, suggesting that the majority of lead was sourced from a mid-Devonian reservoir.

Eleven lead-isotope analyses are available from David (2005) and the CSIRO database with an additional analysis from the present study. Although the majority of data are for analyses of galena and/or samples containing galena, they have a wide range of 207Pb/204Pb vs 206Pb/204Pb values (between 15.622/18.106 and 15.661/18.66). Five analyses have a data quality score of nine, indicating poor reliability, and are therefore excluded from this study and from Figure 8. The remaining data plot within individual error of the crustal growth curve and within or adjacent to the Devonian granite field of Carr et al. (1995), see Figure 8, but again the data do not cluster. The sample from the present study has a 207Pb/204Pb vs 206Pb/204Pb ratio of 15.6246/18.1120, with a LFB lead model age of 398 ± 3 Ma. The remaining (less precise) data from the CSIRO database and David (2005) have 207Pb/204Pb vs 206Pb/204Pb ratio ranges between 15.622/18.106 and 15.662/18.166, with LFB lead model ages between 403 ± 15 Ma and 352

At Hera, 5 km south of Nymagee, gold–base metal mineralisation is in a high-strain zone adjacent to the contact between the lower Amphitheatre Group and the Mouramba Group (Burthong Formation; David 2005). The host sequence includes quartz-lithic greywacke, siltstones, shales and minor felsic volcanic rocks (David 2005; Page 2011). The mineralisation is steeply dipping and extends for at least 750 m along strike. It consists of skarn-like garnet–quartz–tremolite/actinolite– zoisite±phlogopite–anorthite–wollastonite–titanite alteration and vein assemblages associated zones of intense Au–Ag–sulfide±W-rich breccias and veins (Fitzherbert, McKinnon & Blevin 2017; Fitzherbert, Blevin & McKinnon 2017). From north to south, the major zones are North pod > Far West Lower > Far West > Main North > Main South > 1530 > Main SE with the Hays North and South zones on a subsidiary structure to the southwest of the Main South–1530 zone (McKinnon & Fitzherbert 2017). The sulfide-rich mineralisation formed at 381.9 ± 2.2 Ma (Downes & Phillips 2018). Page (2011) suggested that mineralisation formed at temperatures of 270–365°C (an average of 319°C is based on chlorite geothermometry and limited fluid inclusion data). More recently, Fitzherbert, Mawson et al. (2017) noted an early hightemperature, pre-deformation assemblage that includes garnet–actinolite/tremolite–quartz ± chalcopyrite– galena–sphalerite as well as scheelite, and suggested that peak hydrothermal temperatures exceeded 450°C.

15.70

8

Gundaroo n=20 excluding sph=29.8

15.65 00

0

35

a

M

a

M

crustal growth

Frequency

207

Pb/204Pb

6

curve

2

4

Devonian granite-related

5

7

9 δ34S%

11

13

15

n

Siluro-Devonian Cu–Au

17

19

Manuka n=7

6

n ro

ro

ch

iso

iso

ng

18.30

Pb/204Pb

4

2

BMW (this study)

Mount Allen (this study)

Comet (CSIRO database)

Mount Solar (CSIRO database)

MD 2–Siegals (David 2008)

Wagga Tank (CSIRO database)

0 -5 2017_165

Figure 6. Distribution of selected 207Pb/204Pb vs 206Pb/204Pb data for the BMW prospect, Great Central (Comet mine), MD2–Siegals, Mount Allen (Au) mine, Mount Solar (Au) mine and Wagga Tank. Data shown are for the area where 206Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events and crustal growth curve from Carr et al. (1995). Data from the CSIRO database and David (2008) are shown as diamonds or squares. Data from the present study are shown as shown as coloured 2-sigma error ellipses. The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database and David (2008) (yellow) and the 2-sigma uncertainty for the present study (red).

June 2018

3

8

REFERENCE

16

1

ng

ixi

18.20 206

-1

ixi

m

-3

m

tle

tle

an

ch

18.10

-5

Frequency

l-m

an l-m

ta

ta

us

Ordovician Besshitype VAMS 15.55 18.00

0

us

cr

cr

a

a

Devonian VAMS

M

M

0

Carboniferous granite-related

0

15.60

35

40

Silurian VAMS

4

-3

-1

1

3

5

7

9 δ34S%

chalcopyrite

galena

pyrite

sphalerite

11

13

15

17

19

mixed sulfide

2017_166

Figure 7. Distribution of sulfur isotope data for mineralised zones (Gundaroo, Manuka) hosted by units of the Winduck Shelf. Data from David (2005; Manuka) and the present study.

Southern Cobar Basin Hera

Sixty-one sulfur-isotope analyses are available for Hera. They include data from David (2005; 12 analyses), Mernagh (2008; 22 analyses) and the present study (27 analyses). All data plot in a tight cluster between 2.9‰ and 7.4‰ (mean 5.0‰; Figure 9). δ34S ranges for arsenopyrite are 4.7–5.0‰ (mean 4.9‰; 3 analyses), for chalcopyrite are 3.2–7.4‰ (mean 5.4‰; 10 analyses), for galena are 2.9–6.2‰ (mean 4.4‰; 16 analyses), for pyrrhotite are 4.9–7.4‰ (mean 6.3‰; 6 analyses) and for sphalerite are 3.9–6.6‰ (mean 4.9‰; 21 analyses). In addition, mixed galena–sphalerite has δ34S values of 5.0‰ and 5.1‰, pyrite has values of 4.9‰ and 5.6‰ and sphalerite–arsenopyrite has a value of 5.0‰ (Table 1). Table 3 summarises the available data by mineralised zone for the Hera deposit. Where there is sufficient data (North Pod, Main North, Main South), there is a systematic change in the sulfur-isotope values from north to south along the main mineralised structure (i.e. average δ34S values for sulfides from the North pod > Main North > Main South). In addition, North pod has lower average δ34S values for ore sulfides (chalcopyrite, galena, sphalerite) compared to Main North. Similarly Main North has lower average δ34S for ore sulfides compared to Main South, although chalcopyrite has the reversed trend. There are insufficient data for the Far West Lower and Far West lodes; however, the data suggest that the lowest

Quarterly Notes 151

17

Table 3. Summary of available sulfur isotope δ34S data for ore zones at Hera. Data for mineralised zones are organised from north to south (i.e. North pod to Main SE) with the Hays North and South lying on a subsidiary structure. δ34S range ‰ by mineral

δ34S average ‰ by mineral

No. of analyses

(δ34S Average ‰)

North pod

3.3–5.0 (4.5)

asp = 4.7–5.0 ga = 3.3 sph = 3.9, 4.4 sph–asp = 5.0

asp = 4.9 ga = 3.3 sph = 4.1 sph–asp = 5.0

3 1 2 1

Far West Lower

5.1–5.3 (5.2)

ga = 5.1 sph = 5.3

ga = 5.1 sph = 5.3

1 1

Far West

3.5–4.0 (5.0)

ga = 3.5 po = 7.4 sph = 4.0

ga = 3.5 po = 7.4 sph = 4.0

1 1 1

Main North

2.9–7.4 (5.0)

cpy = 3.2–7.4 ga = 2.9–6.1 ga–sph = 5.0, 5.1 po = 6.2, 6.8 sph = 4.5–6.2

cpy = 5.4 ga = 4.4 ga–sph = 5.0 po = 6.5 sph = 5.0

7 10 2 2 10

3.8–6.6 (5.2)

cpy = 4.4, 6.1 ga = 3.8–6.2 po = 4.9 py = 4.9 sph = 4.6–6.6

cpy = 5.2 ga = 4.8 po = 4.9 py = 4.9 sph = 5.5

2 3 1 1 4

5.4 (5.4)

po = 5.4

po = 5.4

1

Hays North

4.0, 7.0 (5.5)

po = 7.0 sph = 4.0

po = 7.0 sph = 4.0

1 1

Hays South

5.6 (5.6)

cpy = 5.6 py = 5.6

cpy = 5.6 py = 5.6

1 1

Main South

Main SE

The historic Nymagee copper mine is immediately west of the village of Nymagee. Here units of the lower Amphitheatre Group adjacent to the boundary with the Mouramba Group host the mineralisation. The sequence is deformed and includes quartz-lithic greywacke, siltstones, shales and minor felsic volcanic units (David 2005). Mineralised zones are steeply dipping and consist of disseminated sulfides and sulfide veinlets with pyrrhotite–chalcopyrite-, pyrrhotite–galena–sphaleriteand pyrrhotite-rich lodes (Suppel & Gilligan 1993). Page (2011) suggested that mineralisation at Nymagee

Winduck Shelf 15.70

Analytical precision CSIRO Pb database and David (2005) this study

0M

35 a

a Carboniferous granite-related

tal -m

tle

xin mi

an

tle an

l-m ta us ng n ro ch

iso

18.10

Siluro-Devonian Cu–Au

ron

ch so gi

ixi m

15.55 18.00

rus

cr

Ordovician Besshi-type VAMS

ac

a

M

Devonian VAMS

0M

35

Devonian granite-related

0

15.60

Silurian VAMS

40

Brown et al. (2013) briefly discussed the geological setting and mineralisation at the Mallee Bull deposit 44 km southwest of Nymagee. They described the mineralisation as being adjacent to the contact between the Shume Formation and the upper Amphitheatre

curve

M

Mallee Bull

Sulfides from Mallee Bull have a wide range of δ34S values between 11.5‰ and 21.5‰ (mean 14.1‰; 18 analyses; Figure 9). Those for arsenopyrite have δ34S values between 11.5‰ and 13.7‰ (mean 12.7‰; 4 analyses), those for galena are similar and range between 10.4‰ and 13.5‰ (mean 11.9‰; 3 analyses), whilst single analyses of sphalerite and mixed galena–sphalerite have δ34S values of 11.8‰ and 10.6‰ respectively. By contrast, chalcopyrite, pyrite and pyrrhotite have a wider range of values. Chalcopyrite has δ34S values of 10.6‰, 15.5‰ and

crustal growth

0

High-precision lead-isotope data for galena are available from Mernagh (2008; 4 analyses), Huston et al. (2016; 1 analysis) and the present study (3 analyses), with an additional six analyses from David (2005) that were analysed at the CSIRO Pb-isotope laboratory. All but one analysis form a tight cluster in Figure 10, with 207Pb/204Pb/ vs 206Pb/204Pb ratios between 15.618/18.094 and 15.632/18.104 whilst the remaining analysis from David (2005) is also within error. The data lie adjacent to the crustal growth curve and have a late Early Devonian lead model age (~400 Ma; range 412 ± 3–397 ± 3 Ma; data from this study, Mernagh 2008 and Huston et al. 2016).

Group (both Amphitheatre Group; Figure 2). Locally, the host lithologies include fine-grained sandstones to mudstones, felsic volcanic-derived sandstones, pebbly sandstones and diamictite. The sulfides occur as massive to semi-massive, sulfide-rich lenses and as disseminated sulfides within a pyrrhotitic alteration envelope, with a chalcopyrite stringer zone as the lowermost mineralised zone (Chapman 2012; Brown et al. 2013). Chapman (2012) suggested that the ore-forming fluids were cooler at the top of the mineral system (288–320°C) than in the stringer zone (340–400°C).

40

δ34S values along the structure may occur in the Far West lode area.

June 2018

Nymagee

15.65

Data from David (2005), Mernagh (2008) and the present study. Mineral abbreviations: asp = arsenopyrite, cpy = chalcopyrite, ga = galena, po = pyrrhotite, py = pyrite, sph = sphalerite.

18

Eight lead-isotope analyses (including repeats) for galena were carried out as part of the present study, with one analysis from Huston et al. (2016). Six of the analyses centred at 207Pb/204Pb vs 206Pb/204Pb = 15.619/18.053 plot within error, adjacent to the crustal growth curve and are interpreted to be initial ratio data (Figure 10). These data lie largely within the Ordovician Besshi field of Carr et al. (1995). Four of the analyses (labelled MB1 on Figure 10) have LFB lead model ages between 436 ± 3 and 426 ± 3 (mean ~432 Ma), suggesting that much of the lead in these samples came from a Pb-reservoir less evolved (older) that the host sequence. A further two analyses from the same group and two more-evolved samples have LFB lead model ages (labelled MB2 on Figure 10) between 423 ± 3 Ma and 419 ± 3 Ma (mean ~422 Ma), suggesting

Pb/204Pb

(approx. north to south)

δ34S range ‰ ore sulfides (only)

that some lead was also sourced from the fill of the Cobar Basin. The final analysis (4MRCD006261.06m) plots outside error, significantly above the crustal growth curve and may not represent initial ratio data.

207

Lode/zone

20.2‰, those for pyrite are 14.9‰, 15.2‰ and 17.8‰, with δ34S values for pyrrhotite of 14.6‰ and 21.5‰.

18.20 206

18.30

Pb/204Pb

REFERENCE Gundaroo (this study)

Manuka (CSIRO database)

Manuka (this study)

Manuka (David 2005) 2017_167

Figure 8. Distribution of all 207Pb/204Pb vs 206Pb/204Pb data for the Gundaroo and Manuka deposits. Data shown are for the area where 206 Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events and crustal growth curve from Carr et al. (1995). Data from the CSIRO database and David (2005) are shown as diamonds or squares. Data from the present study are shown as coloured 2-sigma error ellipses. The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database and David (2005) (yellow) and the 2-sigma uncertainty for the present study (dark orange).

Quarterly Notes 151

19

formed at temperatures between 292–394°C (average 335°C),based on chlorite geothermometry and 240– 360°C, based on fluid inclusion studies. In addition, Fitzherbert, Mawson et al. (2017) noted a garnet– tremolite–actinolite–scheelite–phlogophite assemblage and suggested that this implied T >400°C in the hottest part of the system. Suppel and Gilligan (1993) suggested that the deposit formed pre-deformation and that it was similar to other ‘Cobar-type’ deposits. More recently, Fitzherbert, Blevin and McKinnon (2017) proposed that Nymagee was a calcic Zn–Cu to Fe–Cu skarn based

Thirty sulfur-isotope analyses are available for sulfides from Nymagee from Page (2011) and the present study. All δ34S data plot in a tight group between 5.0‰ and 10.2‰ (mean 7.4‰; Figure 9). Sulfur-isotope values for chalcopyrite range from 5.4‰ to 10.2‰ (mean 7.4‰; 15 analyses), pyrrhotite has values between 6.6‰ and 7.8‰ (mean 7.1‰; 8 analyses), sphalerite has values of 5.0‰, 8.5‰ and 10.2‰, mixed galena–sphalerite has 6

30

Hera n=61

25

Mallee Bull n=17 excluding pyrrhotite=21.5

20

4

Frequency

Frequency

on the presence of remnant high T garnet–anorthite– zoisite–titanite–tremolite and retrograde quartz–chlorite– muscovite–illite assemblages.

15

2

10 5

0

0 -5

-3

-1

1

3

5

7 9 δ34S%

11

13

15

17

-5

19

20

-3

-1

1

3

5

7 9 δ34S%

11

15

17

19

6 Nymagee n=30

R7 prospect n=9 excluding pyrite=31

15 4

Frequency

Frequency

13

10

2

5

0

0 -5

-3

-1

1

3

5

7 9 δ34S%

11

13

15

17

19

-5

-3

-1

1

3

5

7 9 δ34S%

11

13

15

16

6 Sandy Creek n=9

17

19

Shuttleton n=24

14

4

Frequency

Frequency

12

8 6 4 2 0

0 -5

-3

-1

1

3

5

7 9 δ34S%

11

13

15

17

-5

19

-3

-1

1

arsenopyrite

chalcopyrite

galena

mixed sulfide

pyrite

sphalerite

3

5

7 9 δ34S%

11

13

15

17

19

pyrrhotite

2017_168

Figure 9. Distribution of sulfur isotope data for mineralised zones (Hera, Mallee Bull, Nymagee, R7, Sandy Creek, Shuttleton) hosted by units of the southern Cobar Basin. Data from David (2005; Hera), Mernagh (2008; Hera, Shuttleton) and the present study.

20

June 2018

Four high-precision lead-isotope analyses are available from the present study, with an additional analysis from Huston et al. (2016). In addition, there are 15 analyses (including three for galena) in the CSIRO database from earlier studies. However, three of the samples in the CSIRO database contain less than 700 ppm Pb (value of 345 ppm) or the lead-content is unknown, and these are not shown on Figure 10 since they may not represent initial ratio data. The remaining CSIRO analyses cluster along the same fractionation curve and plot adjacent to the crustal growth curve, largely within error, with the majority of data plotting within the Silurian VHMS field of Carr et al. (1995). The data from Huston et al. (2016) and the present study overlap with data from the CSIRO database. They form a tight cluster on Figure 10, but have a wide range of LFB lead model ages with two groupings between 424 ± 3 to 415 ± 3 Ma and 407 ± 3 to 396 ± 3 Ma. This suggests that mixing occurred between two separate lead reservoirs: a late Silurian reservoir and a more-evolved mid-Devonian reservoir. The less precise CSIRO data have a similar range of lead model ages and groupings supporting the interpretation that mixing occurred between two separate lead-reservoirs.

R7 prospect The R7 intrusion-related base metal system, 33 km westsouthwest of Nymagee, is associated with a late Silurian granite (422.8 ± 4.9 Ma; Chisholm et al. 2014) hosted by sedimentary rocks of the Shume Formation (Amphitheatre Group; Figure 2). Ten sulfur-isotope analyses are available for sulfides from mineralised zones at R7. Sulfur-isotope values for pyrrhotite range between 7.8‰ and 9.2‰ (mean 8.4‰; 6 analyses), pyrite has values of 8.9‰, 9.8‰ and 31‰ and a single analysis for chalcopyrite has a value of 8.2‰ (Figure 9).

Sandy Creek The intrusion-related Sandy Creek Pb–Zn–(Cu, Ag) prospect is located 27 km southwest of Nymagee. Here sulfide-rich veins are hosted by chloritic-altered and silicified sandstones and siltstones of the Shume Formation (Amphitheatre Group; Figure 2), which has been intruded by a coarse-grained granite (Mackenzie & Pienmunne 2008).

10

2

values of 6.1‰, 7.1‰ and 7.1‰ and a single analysis for galena has a value of 8.5‰. The δ34S data suggest that sulfur came from a reservoir with both magmatic and reduced seawater sulfate or that mixing occurred between these two end-member reservoirs.

Sulfides from Sandy Creek gave δ34S values between 10.6‰ and 13.0‰ (mean 12.0‰; 9 analyses; Figure 9). Galena has δ34S values between 10.2‰ and 11.5‰ (mean 10.8‰; 3 analyses), chalcopyrite has values of 12.9‰ and 13.0‰, those for pyrrhotite are 12.7‰ and 12.9‰ and those for sphalerite are 11.7‰ and 12.2‰. Three galena-rich samples were analysed for their leadisotope signature as part of the present study, with an additional analysis for galena from Mernagh (2008), see Figure 10. These data plot in a tight group within the

Silurian VHMS field of Carr et al. (1995), plot along the same fractionation curve and have a crustal signature. The data have Early Devonian LFB lead model signatures of 417 ± 3 Ma (1 analysis) and 406 Ma (3 analyses), suggesting that mixing occurred between lead sourced from both the host sequence and a younger reservoir.

Shuttleton Polymetallic (Cu–Pb–Zn–Ag) mineralisation at Shuttleton (Crowl Creek and South Shuttleton deposits), 24 km west-southwest of Nymagee, is hosted by or adjacent to a rhyolitic body which locally underlies sandstones and siltstones of the Early Devonian Shume Formation (Amphitheatre Group; Figure 2; Suppel & Gilligan 1993). Twenty-one sulfur-isotope analyses for South Shuttleton were undertaken as part of the present study, with an additional three analyses from Mernagh (2008) (Figure 9). Sulfides have S-isotope values between 10.3‰ and 13.2‰ (mean δ34S 11.6‰). The values for pyrite range between 10.3‰ and 13.2‰ (mean δ34S 11.6‰; 19 analyses), while chalcopyrite has values of 10.5‰ and 12.7‰, pyrrhotite has values of 11.0‰ and 11.6‰ and a single value for mixed galena–sphalerite was 11.2‰ (Figure 9). The data support the interpretation that sulfur was sourced from a reservoir containing reduced seawater sulfate. Fourteen lead-isotope analyses are included in the CSIRO database for Shuttleton, with an additional analysis for galena from the present study. Twelve of the analyses in the CSIRO database have a wide range of 207Pb/204Pb/ vs 206Pb/204Pb ratios (between15.619/18.098 and 15.667/18.742), and contain less than 600 ppm Pb. These samples are excluded from the present study and are not shown on Figure 10 as they may not represent initial ratio data. The remaining analyses (two for sulfides from the CSIRO database with 700 ppm Pb; one for galena from the present study) plot adjacent to the crustal growth curve (within error) and are within the Devonian granite field of Carr et al. (1995). These analyses are interpreted to represent initial ratio data. Both the lower precision CSIRO data and the sample from this study have late Early Devonian LFB lead model ages at ~400 Ma, which is significantly younger than the host sequence.

Kopyje Shelf (Canbelego–Mineral Hill Volcanic Belt) Mineral Hill Approximately 85 km southeast of Nymagee is the Mineral Hill Ag–Pb–Zn and Cu–Au epithermal system. Many studies have described the geology and mineralisation at Mineral Hill. They include McClatchie (1971), Bush (1980), Spandler (1998), Morrison et al. (2004) and Jones and Mackenzie (2007). The deposit is hosted by felsic volcanic rocks, and limestones and siltstones of the late Silurian (Sherwin 2013; Downes, Blevin et al. 2016) Mineral Hill Volcanics (Kopyje Group) see Figure 2. The mineralisation is complex and includes massive stratabound sulfiderich zones as well as sulfide-rich veins and disseminated

Quarterly Notes 151

21

Three samples of galena were analysed for their leadisotope signature as part of the present study. In addition, there are three analyses from earlier studies in the CSIRO database, but these analyses are excluded from Figure 12 as one has poor data quality (Q = 9) whilst the others contain less than 700 ppm Pb. Data from the present study plot in a tight group adjacent to the crustal growth curve and within the Silurian VHMS field of Carr et al. (1995). The data have a late Silurian signature (~422 Ma) that overlaps

Sixteen sulfur-isotope analyses for sulfides from Pipeline Ridge gave δ34S values between 4.8‰ and 14.0‰ (mean 10.4‰; 16 analyses; Figure 11). The δ34S data for pyrite vary from 7.7‰ to 14.0‰ (mean 10.4‰ with one outlier of 4.8‰), those for galena are between 7.4‰ and 10.2‰

Southern Cobar Basin Analytical precision

15.70

CSIRO Pb isotope database this study Huston et al. (2016) Mernagh (2008)

15.65 0M

35

Pb/204Pb

a

tal -m an

5.4–10.3

cpy = 6.0–10.3 ga = 5.4–8.9 sph = 6.6, 8.5

cpy = 8.1 ga = 7.4 sph = 7.5

10 6 2

Red Terror

7.5–10.5

cpy = 7.6–10.5 ga = 7.5–8.1

cpy = 9.2 ga = 7.9

9 4

cpy = 8.6–11.7 ga = 8.9 sph = 10.9

cpy = 9.8 ga = 8.9 sph = 10.9

7 1 1

ga = 5.1, 9.9 sph = 5.1, 10.0

ga = 7.5 sph = 7.5

2 2

Southern Ore Zone

West Iodide

8.6–13.0

5.1–10.0

Data from Bush (1980), Spandler (1998), Mernagh (2008) and the present study. Mineral abbreviations: cpy = chalcopyrite, ga = galena, sph = sphalerite.

22

June 2018

Siluro-Devonian Cu–Au

n

Parkers Hill

ro

3 1

ch

cpy = 7.2 sph = 6.8

ron

iso

cpy = 5.1–9.6 sph = 6.8

ch

ng

5.1–9.6

so

ixi

Jacks Hut

gi

m

No. of analyses

xin

tle

an

Ordovician Besshi-type VAMS

mi

l-m

ta

tle

us

cr

δ34S mean ‰ by mineral

rus

a

δ34S range ‰ by mineral

Carboniferous granite-related

ac

M

Devonian VAMS

0M

35

Silurian VAMS

0

Forty kilometres north of Nymagee is the Pipeline Ridge base metal–gold epithermal system, which is briefly described by Gilligan et al. (1995). At Pipeline Ridge, mineralisation is associated with colloform- and

MB2

40

15.60

δ34S range ‰ ore sulfides (only)

curve

a

MB1

Devonian granite-related

Table 4. Summary of S-isotope data for chalcopyrite, galena and sphalerite (only) from the Jacks Hut, Parkers Hill, Red Terror, Southern Ore Zone and West Iodide zones at the Mineral Hill mine. Zone

crustal growth

M

Pipeline Ridge

(mean 9.0‰; 3 analyses), with analyses for chalcopyrite of 11.4‰ and 11.7‰ and a single δ34S analysis for sphalerite of 12.4‰.

0

For Mineral Hill, there are 14 lead-isotope analyses in the CSIRO database, with an additional two analyses for galena from Mernagh (2008). Five analyses in the CSIRO database are for galena-rich samples from Jacks Hut; however, two of these analyses are not shown in Figure 12 due to poor data quality (Q = 9). The remaining analyses in the CSIRO database are for non-located samples from the 1983 BHP study, with no details of the material sampled available. Most analyses from the CSIRO database, together with those from Mernagh (2008), plot within analytical error adjacent to the crustal growth curve and largely within the Silurian VHMS field of Carr et al. (1995) (Figure 12). The data can be grouped into two populations at ~412 ± 15 Ma (range 416–409 Ma) and a late Early Devonian signature at ~396 Ma (range 405–378 Ma) that includes the higher precision data from Mernagh (2008). The mineralisation has been dated at 420.5 ± 2.7 Ma (Downes & Phillips 2018), suggesting that more-evolved lead was added to the system by later events.

crustiform-banded quartz–sulfide–carbonate veins surrounded by zones of intense silica–white mica alteration. The mineralisation is hosted by felsic volcanic units and fine- to medium-grained clastic rocks of the late Silurian Baledmund Formation (Kopyje Group; Figure 2) which was dated by Bodorkos et al. (2015) at 419.3 ± 2.8 Ma (U–Pb SHRIMP dating of zircon).

40

Sixty-one sulfur-isotope analyses are available for Mineral Hill from studies by Bush (1980; 11 analyses), Spandler (1998; 9 analyses), Mernagh (2008; 20 analyses) and the present study (21 analyses), although the locations for some samples are poorly known. Sulfur-isotope data have a wide range of values between 5.0‰ and 13.4‰ (mean δ34S 8.8‰). The δ34S range for chalcopyrite is 5.1–11.8‰ (mean 8.8‰; 31 analyses), for galena is 5.1–9.9‰ (mean 7.7‰; 13 analyses), for pyrite is 5.0–11.1‰ (mean 10.2‰; 10 analyses), and for sphalerite is 5.1–10.9‰ (mean 8.0‰; 6 analyses), with single δ34S analyses for bornite of 5.7‰ (Figure 11).

There are systematic changes in the δ34S values for chalcopyrite and galena from Mineral Hill suggesting that Parkers Hill is the centre of a zoned system. Sulfur-isotope values for chalcopyrite from Parkers Hill average 8.1‰ (range 6.0–10.3‰), those for the adjacent Red Terror zone are higher and average 9.2‰ (range 7.6–10.5‰), whereas those from the Southern Ore Zone (500 m to the south) are higher again and average 9.8‰ (range 8.6–11.7‰). In addition, the limited δ34S data for galena from Parkers Hill (average 7.4‰) and Red Terror (average 7.9‰) also suggest a similar zonation, see Table 4. This zonation is further reviewed in the discussion section below.

207

sulfides. The system is geochemically and mineralogically zoned (see Bush 1980; Spandler 1998; Downes, Tilley et al. 2016). The zonation includes Parkers Hill which is copper-rich (copper–lead–zinc with gold associated with the adjacent Red Terror zone) and the Eastern Ore Zone (EOZ) which is gold-rich with variable but relatively low copper (O. Thomas pers. comm. October 2015 in Downes, Tilley et al. 2016). Southwest of these are the Jacks Hut and Iodide Lode areas. Jacks Hut is copper-rich with gold and low lead–zinc, whereas the Iodide Lode is polymetallic (Pb–Zn–Ag–Au ± Cu). By contrast, the Pearse deposit is an Au–Ag zone with both arsenopyrite and stibnite. The deposit formed at temperatures up to ~350°C (fluid inclusion data, Bush 1980) with Fitzherbert, Mawson et al. (2017) suggesting temperatures of ~150°C for iron-rich chlorite distal to mineralisation and ~300°C for chlorite proximal to mineralisation (based on chlorite analyses from Bush 1980). Downes, Tilley et al. (2016) suggested that Parkers Hill was proximal to the centre of the Mineral Hill hydrothermal system; that the Southern Ore Zone was slightly more distal; and that the Pearse deposit was more distal again and possibly represented a fluid discharge/outflow zone. That study was based on the interpreted alteration-related mineralogy interpreted from HyLoggerTM spectral data of scanned diamond drillcore.

15.55 18.00

18.10

18.20 206

18.30

Pb/ Pb 204

REFERENCE Mallee Bull (this study)

Shuttleton (this study)

Hera (Mernagh 2008)

Mallee Bull (Huston et al. 2016)

Shuttleton (CSIRO database)

Hera (David 2005)

Sandy Creek (this study)

Hera (this study)

Nymagee (this study)

Sandy Creek (Mernagh 2008)

Hera (Huston et al. 2016)

Nymagee (Huston et al. 2016) Nymagee (CSIRO database) 2017_169

Figure 10. Distribution of selected 207Pb/204Pb vs 206Pb/204Pb data for Hera, Mallee Bull, Nymagee, Sandy Creek, and Shuttleton systems. Data shown are for the area where 206Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events and crustal growth curve from Carr et al. (1995). Data from the CSIRO database and David (2005) are shown as circles, squares and triangles. Data from Mernagh (2008), Huston et al. (2016) and the present study are shown as coloured 2-sigma error ellipses. The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database and David (2005) (yellow) and the 2-sigma uncertainty for the present study (dark orange), and Mernagh (2008) and Huston et al. (2016) (both light green).

Quarterly Notes 151

23

with the age of the host which is a late Silurian–earliest Devonian sequence.

20

Yellow Mountain

16

Three sulfide-rich samples containing galena were analysed for their lead-isotope signature, with an additional two analyses for galena from the CSIRO Pb-isotope database. All samples plot adjacent to the crustal growth curve (Figure 12) and within the Silurian VHMS field of Carr et al. (1995). Sample YD16-621 has a 207 Pb/204Pb/ vs 206Pb/204Pb ratio of 15.619/18.066 and a LFB lead model age ~422 Ma, suggesting that it contains lead from the host sequence. All other analyses plot along the same fractionation curve, with the data from this study having an average 207Pb/204Pb/ vs 206Pb/204Pb ratio of 15.623/18.094 and a lead model age of 407 Ma, indicating that lead was sourced from a more-evolved Early Devonian reservoir or, more likely, this is a mixed signature that reflects lead sourced from a late Silurian lead-reservoir in addition to a more-evolved reservoir. It is likely that this second reservoir is of mid-Devonian age as a CSIRO galena analyses (YD13/5 from DDH YD13 296.67.80m) has a LFB lead model age of 390 ± 15 Ma.

Discussion Sources of sulfur Many of the mineralised zones included in the present study contain primary pyrrhotite and/or arsenopyrite in their sulfide assemblages, in addition to pyrite and basemetal sulfides (Table 1). They include Anomaly 3– Great Central, Blind Calf, BMW, Condobolin, Hera, Mallee Bull, MD2–Siegals, Mineral Hill, Mount Hope, Mount Solar, R7, Sandy Creek, Shuttleton, Tullabung, Wagga Tank and Yellow Mountain. The ore-forming fluids for these deposits are likely to have been more reducing than for other

24

June 2018

Frequency

14 12 10 8 6 4 2 0 -5

-3

-1

1

3

5

7

9

11

13

15

17

19

δ34S%

6 Pipeline Ridge n=16

4 Frequency

As part of the present study, 19 samples were analysed for their sulfur-isotope value from Yellow Mountain. All δ34S analyses form a tight group between 13.8‰ and 16.9‰ (mean 15.1‰; 19 analyses; Figure 11). Chalcopyrite has δ34S values ranging between 13.9‰ and 16.6‰ (mean 15.6‰; 4 analyses), those for pyrite range between 13.8‰ and 16.9‰ (mean 15.0‰; 7 analyses), values for sphalerite are between 14.9‰ and 15.4‰ (mean 15.2‰; 3 analyses), mixed galena–sphalerite has values between 14.4‰ and 15.2‰ (mean 14.8‰; 3 analyses), whilst galena has δ34S values of 14.1‰ and 15.1‰. The data indicate that sulfur was sourced from a reduced seawater sulfate reservoir.

18

2

0 -5

-3

-1

1

3

5

7

9

11

13

15

17

19

δ34S%

10 Yellow Mountain n=19

8

Frequency

Located approximately 65 km southeast of Nymagee is the Yellow Mountain base metal system (Figure 2). At Yellow Mountain, disseminated base metal sulfides are hosted by felsic volcanic and derived sedimentary rocks of the Majuba Volcanics (Kopyje Group; Figure 2) adjacent to the contact with the Yellow Mountain Granite and Ordovician basement units (Girilambone Group). Previous workers have suggested that this system may be a VAMS zone (see Cyprus Mines Corporation 1971; Suppel & Gilligan 1993). However, it is more likely that this is an epigenetic base-metal system.

Mineral Hill n=61

6

4

2

0 -5

-3

-1

1

3

5

7

9

11

13

15

17

19

δ34S% chalcopyrite

galena

mixed sulfide

pyrite

pyrrhotite

sphalerite 2017_170

Figure 11. Distribution of sulfur isotope data for mineralised zones (Mineral Hill, Pipeline Ridge, Yellow Mountain) hosted by units of the Kopyje Shelf (including the Canbelego–Mineral Hill Volcanic Belt). Data from Bush (1980), Spandler (1998) (all for Mineral Hill), Mernagh (2008; Mineral Hill) and the present study.

deposits where pyrrhotite and/or arsenopyrite are absent, or present only as a trace mineral. Thus, the isotopic composition of these sulfides should correspond closely to the isotopic composition of total dissolved sulfur in the mineralising fluids (δ34S sulfide~ δ34S ∑S fluid; Ohmoto & Goldhaber 1997), and the sulfur-isotope composition of sulfides will reflect the sulfur-isotope composition of the sulfur source. By contrast, pyrrhotite and arsenopyrite are generally absent, or are present only as a trace mineral

at Gundaroo, Manuka and Pipeline Ridge, with sulfatebearing minerals (gypsum) being noted by Downes, Tilley et al. 2016 at Manuka. These deposits appear to have formed from ore fluids with a higher redox state (more oxidised) than those where pyrrhotite and/or arsenopyrite are present, and are likely to have a higher concentration of aqueous sulfate, in addition to sulfide in the hydrothermal fluids. Under these circumstances, and depending on factors such as the oxidation state of the fluids, their temperature, and the proportion of igneous sulfide derived from the rock column relative to sulfate, δ34S values higher than normal igneous sulfur values are anticipated for these sulfides (Ohmoto et al. 1983). As discussed in detail by Marini et al. (2011), degassing and crystallisation processes can also lead to significant changes in the δ34S values of magmatic fluids, and are particularly dependent upon redox conditions, and whether the fluids evolve under closed vs open system conditions. In the absence of any quantitative indicators of the redox state necessary to try and account for possible δ34S changes, it has been assumed that changes in δ34S values due to degassing and/or crystallisation are relatively small. Potential sources of sulfur for mineralised zones in the study area include magmatic sulfur (δ34S = 0 ± 2; Ohmoto & Rye 1979), rock-sulfur leached from local country rocks, and reduced sulfur provided by either high-temperature (inorganic) or low-temperature (bacteriogenic) reduction of seawater sulfate. Reduced sulfur derived from modified sea water has probably contributed to the sulfur budget for many of the systems included in the present study. The Ordovician Girilambone Group was deposited under marine conditions and the isotopic composition of seawater sulfate at that time would be near 24–30‰ (Claypool et al. 1980; Kampschulte & Strauss 2004). Similarly, units forming the fill to the Cobar Basin, Kopyje Shelf, Mount Hope Trough and Mouramba Shelf are late Silurian–earliest Devonian age and the isotopic composition of seawater sulfate at that time was near 23–30‰ (Kampschulte & Strauss 2004). The Winduck Group is Lochlovian–earliest Emsian in age, with the mineralisation at Manuka hosted by Pragian-age limestones. The isotopic composition of seawater sulfate fell from 23–27‰ in the earliest Lochlovian to 18–20‰ in the Pragian (Kampschulte & Strauss 2004). High-temperature inorganic reduction of seawater sulfate by deep crustal circulation, coupled with some sulfate deposition in shallow intake zones, can produce sulfides depleted in 34S by as much as 15‰ with respect to their initial δ34S∑s (Ohmoto & Rye 1979; Styrt et al. 1981). Thus, it would be anticipated that reduced sulfur from modified Ordovician and/or late Silurian–earliest Devonian sea water would have an isotopic composition around 10– 15‰ (assuming T ≥ 300°C; i.e. Wagga Tank, Hera, Mallee Bull, Mineral Hill, Nymagee). However, the average δ34S value for all deposits hosted by Ordovician and/or by late Silurian–earliest Devonian units in the study area is 8.7‰ which indicates that other sources of sulfur need to be considered.

Variations in the sulfur-isotope signature at a depositand district-scale are influenced by several factors. They include variations in temperature and/or oxidation state of the mineralising fluid, the size and nature of the hydrothermal system (i.e. diffuse vs focused upflow), variations in the composition of the hydrothermal fluids, and variations in the relative contributions of different sulfur reservoirs (Rye & Ohmoto 1974; Ohmoto & Rye 1979). Based on the range, distribution and average sulfurisotope values, deposits forming the present study are grouped into four groups that largely reflect the relative contribution of different sulfur reservoirs. Group 1 deposits, including Condobolin, Melrose and Tallebung, have average sulfur-isotope values close to zero (Table 1). Sulfur-isotope values close to zero overlap with the range of δ34S values for normal igneous sulfur (0 ± 2‰; Ohmoto & Rye 1979) and those for mafic units from the study area (Break O’Day Amphibolite which is located at 32.404°S, 146.607°E; Hill 239 Gabbro -32.755°N, 146.903°E — individual units not shown on Figure 2) with δ34S values of -0.2‰ and 1.4‰ respectively — Appendix 3 in Downes & Poulson 2017). This suggests that sulfur in these deposits came from reservoirs dominated by magmatic sulfur. These deposits are interpreted to be intrusion-related, with a feature of the Condobolin system being its metal zonation (Au-dominant core > Au–Ag > Cu–Pb–Zn peripheral to the system). Furthermore, the data for Mascotte which lies within the central core at Condobolin averages 2.2‰ (range 0.6–3.5‰), whilst that for Phoenix which is to the west is higher and averages 3.6‰ (range 1.8–7.2‰), and that for Potters which is to the east of Mascotte averages 3.3‰ (values 2.0‰, 4.6‰). This suggests that this system is also isotopically zoned. In addition, the trend towards higher ppm values at Phoenix (e.g. sphalerite = 6.5‰, 6.8‰; pyrite = 7.2‰) indicates that some sulfur was sourced from a second, probably reduced, seawater sulfate reservoir. Although data are limited, similar values were noted at Tallebung which suggests that magmatic fluid–wall rock interactions were important for sulfide precipitation in these systems. A second group of deposits have average sulfur-isotope values between 4‰ and 9‰ (Group 2). They include Anomaly 3 (Great Central zone), Blind Calf, Hera, Mineral Hill, Mount Hope, Mount Solar, Nymagee and R7. Values between 4‰ and 9‰ suggest that these systems incorporate sulfur from a reservoir with a mixed magmatic–reduced seawater sulfate signature (e.g. δ34S values for Derrida Granite = 4.0‰, unaltered Mineral Hill Volcanics = 4.4‰, Mount Halfway Volcanics = 6.1‰ and Gilgunnia Granite = 6.2‰ — Appendix 3 in Downes & Poulson 2017) or that mixing of sulfur-bearing fluids occurred between these two end-member reservoirs. The range of values and average sulfur-isotope signatures for these deposits also overlap with the data for deposits in the central part of the Cobar mineral field (see Seccombe et al. 2017). In addition, Pipeline Ridge also has a wide range of δ34S values (4.8–12.4‰) and, although its average δ34S value is 10.3‰, values as low as 4.8‰ indicate that some sulfur was derived from a magmatic reservoir. The

Quarterly Notes 151

25

range of values for Pipeline Ridge overlaps with that for Mineral Hill (discussed below). Unlike other deposits in this group, Pipeline Ridge has little or no pyrrhotite/ arsenopyrite, suggesting that the ore-forming fluids were relatively oxidised compared to other deposits in this group. As noted earlier, under these circumstances higher δ34S values are anticipated for sulfides in this deposit.

a host that was largely sourced from dominantly hightemperature, reduced seawater sulfate reservoir(s) such as those derived from sea water associated with the host Ordovician and late Silurian to earliest Devonian sedimentary sequences. The McKinnons gold deposit which lies to the north of the study area on the western edge of the Cobar Basin has a similar range (8.1–12.1‰) and average (10.3‰) δ34S values to these deposits (see Forster & Seccombe 1999).

Group 3 deposits have average sulfur-isotope signatures that lie between 9 ppm and 12 ppm. They include BMW, Great Central, Sandy Creek, Shuttleton and Wagga Tank, which all contain variable pyrrhotite and/or arsenopyrite (Table 1) indicating relatively reduced ore-forming fluids in these deposits. These values overlap with the predicted reduced Silurian–earliest Devonian seawater sulfate signature and suggest that this sulfur came from

Six deposits have average δ34S values greater than 12 ppm (Group 4). They include Gundaroo, Mallee Bull (two signatures), Manuka, MD2–Siegals, Mount Allen and Yellow Mountain. The range of sulfur-isotope values for these deposits (Table 1), including the δ34S data that trend towards the heaviest δ34S values (>14‰) for Gundaroo,

Kopyje Shelf 15.70

Analytical precision CSIRO Pb isotope database this study Mernagh (2008)

curve

crustal growth

40

Pb/204Pb

0M

35

a

0M

a

15.65

tal

g

so

n ixi

gi

m

Ordovician Besshi-type VAMS

xin

tle

mi

an

tle

l-m

an

ta

us

-m

cr

Devonian VAMS

rus

a

M

Silurian VAMS

15.60

Carboniferous granite-related

ac

207

0M

35

0

40

Devonian granite-related

n ro ch

ron

ch

iso

Siluro-Devonian Cu–Au 15.55 18.00

18.10

18.20 206

18.30

Pb/204Pb

REFERENCE Mineral Hill (Mernagh 2008)

Yellow Mountain (this study)

Mineral Hill (CSIRO database)

Yellow Mountain (CSIRO database)

Pipeline Ridge (this study) 2017_171

Figure 12. Distribution of selected 207Pb/204Pb vs 206Pb/204Pb data for the Kopyje Shelf (Mineral Hill, Pipeline Ridge and Yellow Mountain). Data shown are for the area where 206Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events and crustal growth curve from Carr et al. (1995). Data from the CSIRO database are shown as diamonds or triangles. Data from Mernagh (2008) and the present study are shown as 2-sigma error ellipses. The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database (yellow), the 2-sigma uncertainty for the present study (dark orange), and Mernagh (2008) (light green).

26

June 2018

Mallee Bull, Manuka and Yellow Mountain, suggests that some of the sulfur in these systems originated from a different sulfur-isotope source than the ‘standard’ hightemperature, reduced seawater sulfate reservoir derived from modified Ordovician and/or late Silurian–earliest Devonian sea water. Downes, Tilley et al. (2016) noted that gypsum occurred adjacent to the primary (sulfide) mineralisation at Manuka. The sulfides are hosted by dolomitised zones, suggesting that the gypsum may be alteration-related. In addition, Giles (1993) suggested that the deposit formed at temperatures less than 150°C (based on reconnaissance fluid inclusion data), consistent with the late digenetic metamorphic conditions (~150°C) observed by Fitzherbert, Mawson et al. (2017; based on conodont alteration data from Talent et al. 2003). The low-temperature environment and heavy sulfur-isotope values suggest that neutral to slightly alkaline conditions prevailed in the ore-forming fluid and that isotopic equilibrium was not achieved in the Manuka system (see Ohmoto & Lasaga 1982). The mineralisation at Gundaroo probably evolved under similar conditions. At Mallee Bull, the heavy sulfur-isotope values and absence of a magmatic signature were unexpected, given the interpreted high temperature of ore formation and a range in δ34S values between 11.5‰ and 21.5‰ (mean 14.1‰). This contrasts with δ34S data for typical epigenetic ‘Cobar-style’ deposits hosted by the Cobar Basin (e.g. CSA, Great Cobar, Perseverence, The Peak etc.) that have average δ34S values between 7.0‰ and 10.0‰ (see Seccombe et al. 2017). The mineralisation at Mallee Bull is associated with stilpnomelane, biotite and magnetite (Chapman 2012), with sphalerite–galena–pyrite-dominant mineralisation forming at temperatures 150°C/km) existed across this region. In addition, the presence of magnetite indicates that some ore-forming fluids were near neutral to slightly alkaline. The available data support a model whereby fluid mixing occurred, out of isotopic equilibrium, between hightemperature, sulfur-poor, reduced metal-transporting fluids and low-temperature, near neutral to slightly alkaline connate fluids that transported sulfur from an 34 S-enriched sulfur reservoir (as suggested for Manuka above; see Ohmoto & Lasaga 1982). Hera and Nymagee are ~5 km apart, with sulfur-isotope data for mineralised zones at Hera lying in a narrow range between 2.9‰ and 7.4‰ (average 5.0‰) and supporting the interpretation of a magmatic sulfur source. Those for Nymagee have higher δ34S values, lie between 5.0‰ and 10.2‰ (average 7.4‰), and suggest that sulfur from both magmatic and reduced seawater sulfate fluid sources was incorporated into the system. Fitzherbert,

Mawson et al. (2017) noted that both deposits lie within a high-temperatures zone, with early (pre-deformation) assemblages at Hera indicating that peak temperatures here exceeded 450°C. Those authors also suggested that the mineralisation at Hera is an intrusion-related skarn (see Fitzherbert, McKinnon & Blevin 2017; Fitzherbert, Blevin & McKinnon 2017). The available sulfur-isotope data for Hera supports a magmatic origin for that deposit and is consistent with the δD and δ18O data reported by Fitzherbert, Blevin and McKinnon (2017). The data for Nymagee suggests that this system may be more distal from the mineralising pluton(s), allowing for mixing of fluids from different sources. In addition, whilst the data for Hera are limited, the lowest sulfur-isotope values occur in the northern part of the main mineralised structure, the North pod area, while the Main North–Main South area has generally higher δ34S values (Table 3; see discussion above). It is suggested that a major fluid up-flow zone may be present in the northern part of the main mineralised structure. The observed zonation is consistent with very limited mixing of sulfur from a second 34S-enriched reservoir (such as the host rock sequence), under opensystem conditions, during hydrothermal fluid transport southwards along the main structure. Potential temperature differences between the Hera and Nymagee mineralising systems appear unrelated to the δ34S distribution for the two deposits. Isotopic fractionation for the sphalerite–H2S pairs is approximately 0.1‰ for a temperature decrease from 400°C to 300°C, whilst that for galena–H2S pairs varies by -0.5‰ for the same temperature range (based on fraction values of Li & Liu 2006). These values are smaller than the δ34S values obtained for galena and sphalerite as summarised in Table 3. Furthermore, the data in Table 3 are not consistent with closed-system 34S–32S fluid–mineral fractionation occurring within the system. Closed-system fractionation would have resulted in depletion of 34S, resulting in a trend towards lower sulfur-isotope values. For example, in the Dargues Reef–Majors Creek system located in the Eastern Subprovince of the Lachlan Orogen, Dargues Reef which is the main deposit has an average sulfur-isotope value of -1.3‰, whereas the nearby Snobs system has an average of -4.4‰ and the lowest individual δ34S value of -6.7‰ (see Forster et al. 2014). There are also systematic changes in δ34S values for chalcopyrite and galena at Mineral Hill, as summarised by Table 3 and discussed above, which support the interpretation by Downes, Tilley et al. (2016) that Parkers Hill is the centre of a zoned system. It appears unlikely that this zonation is temperature-related, as the lower δ34S values are at Parkers Hill which is copper–gold-rich, whereas the Southern Ore Zone is gold-rich but copperpoor. Nor can it be due to closed-system fractionation, since isotopically lower values again should be peripheral to the system. It is proposed that this zonation reflects the mixing of fluids carrying isotopically light sulfur from a magmatic reservoir adjacent/beneath Parkers Hill with fluids carrying isotopically heavy sulfur from a reduced seawater sulfate source distal to Parkers Hill. Fluid mixing is an important part of the ore-forming process for other

Quarterly Notes 151

27

Pb/204Pb

350 Ma

450 Ma

400 Ma

15.65

e ntl ma al-

Devonian VAMS

tle

gi xin mi

an l-m

Ordovician Besshi-type VAMS

ixi m

ron

n

ro

ch

iso

18.10

Siluro-Devonian Cu–Au

ch so

ng

15.55 18.00

Carboniferous granite-related

t urs

Silurian VAMS

Devonian granite-related

ac 0M 35

15.60

curve crustal growth chards (1975) Ri & g Cummin curve crustal growth 5) 99 (1 . al Carr et

a st

New data from this study are consistent with data from David (2005), David (2008), Mernagh (2008), Huston et al. (2016) and the earlier lead-isotope data from the CSIRO database (see Forster et al. 2010). The major difference between recent analyses carried out at Boise State

this study

r cu

June 2018

Two crustal growth curves are generally used for the analysis of Lachlan Orogen lead-isotope data. These are the Cumming and Richards (1975) crustal growth curve and the Carr et al. (1995) Lachlan Fold Belt (LFB) model. Lead model ages calculated using these two growth curves will give different estimates of the lead model age, with many data varying by ~20 Ma. LFB lead model ages are generally younger than those of Cumming and Richards (1975). New high-precision lead-isotope data from this study lie adjacent to and largely within error of the crustal growth curve of Carr et al. (1995) and, in general, plot below the crustal growth curve of Cumming and Richards (1975), see Figure 13. This suggests that lead included in mineralised zones within the Nymagee study area is mainly derived from crustal reservoirs consistent with the LFB model of Carr et al. (1995). Thus we have used LFB lead model ages as the basis for this study (summarised in Table 2) and the following discussion. These data gave results consistent with the ages of geological units/mineralising events within the study area. Table 2 also includes the Cumming and Richards (1975) lead model ages calculated for the same analyses as used to calculate LFB lead model ages.

CSIRO Pb isotope database

a M

28

Sources of lead

Analytical precision

0

Only minor sulfur from biogenic sources is apparent in the sulfur-isotope data for the study area. δ34S values significantly below zero can reflect biogenic sources where significant closed-system fractionation can be excluded. Carbonaceous and/or black shale units are rare in the Mount Hope Trough, thus δ34S values of -20.9‰, -11.1‰ and -1.2‰ for Wagga Tank suggest that some sulfur was sourced from basement to the trough, rather than from within the trough fill. Both the Wagga and the Bendoc groups contain black shale/carboniferous units on the Cargelligo 1:250 000 map sheet area to the south of the study area (see Colquhoun et al. 2005) suggesting that these units may form part of the basement to the Mount Hope Trough.

Replacement of early formed sulfides by later formed sulfides during deposit formation has been observed in many systems elsewhere (e.g. VAMS deposits, Eldridge et al. 1983). Additionally, the recrystallisation of ductile sulfides due to later events can often occur. Therefore, isotopic disequilibrium is not unexpected (Eldridge et al. 1983) and hence the application of sulfurisotope geothermometry calculations to coexisting sulfide minerals in the study area is unlikely to produce meaningful data relating to sulfide deposition.

Present Study 15.70

40

For mineralisation along the Great Central structure near Mount Hope, there are systematic differences in δ34S values between Anomaly 3 and the Great Central mine (1 km to the north). δ34S values for Anomaly 3 are between 6.1‰ and 9.1‰ (Northcott 1986), whilst those for Great Central are between 8.9‰ and 13.2‰ (present study). This difference probably reflects systematic differences in sulfur sources. The δ34S data from Anomaly 3 and Great Central suggest sulfur contributions from both magmatic and reduced seawater sulfate reservoirs, but the Great Central δ34S data indicate a higher proportion of sulfur from a reduced seawater sulfate reservoir. Given that the sulfur-isotope data for Anomaly 3 and Great Central do not overlap, this suggests that separate hydrothermal cells were responsible for the development of each zone. Alternatively, a single hydrothermal cell, with fluid mixing or closed-system fractionation occurring along the Great Central structure, could form both systems. Alteration along the Great Central structure is remarkably uniform, with a change from an outer Fe-chlorite–muscovite assemblage (distal) to a more central Mg-chlorite–phengitic white mica assemblage associated with mineralisation along the entire zone (Downes, Tilley et al. 2016). The area has undergone high heat flow (Fitzherbert, Mawson et al. 2017) and metal zonation along the 2.7 km-long structure involves Zn–Pb in the south, through Au–Ag (Anomaly 3), to Cu (Great Central–Hodge lode) to Cu with minor Zn–Pb in the north of the field. The authors propose that the Great Central system is a zoned magmatic–hydrothermal system with an intrusion located near Anomaly 3. Fluid mixing along the structure would best account for the observed sulfur-isotope signatures, as the sulfur-isotope data and metal zonation are not compatible with closed-system fractionation (under this latter scenario, Great Central should have lower δ34S values than Anomaly 3 or be Pb–Zn rich). The timing of mineralisation is constrained by the available lead-isotope data and is discussed below.

Isotopic disequilibrium is evident among different sulfide phases for many of the coexisting sulfide pairs in the overall dataset of the present study. Although different sulfide minerals extracted from the same drillcore interval or deposit may show the expected trend of isotopic enrichment for sulfide mineral pairs formed in isotopic equilibrium (δ34S pyrite > sphalerite > chalcopyrite > galena; Ohmoto & Rye 1979; δ34S pyrrhotite > greenockite > sphalerite > chalcopyrite > cubanite > sylvanite > bornite > violarite > galena; Li & Liu 2006), there are many exceptions, and isotopic reversals are common. Examples of isotopic reversal include: • sample RT1165-07 (Red Terror zone at Mineral Hill); chalcopyrite = 7.6‰, galena = 8.1‰, sphalerite = 12.5‰ • sample CCR 30-160.2 from Gundaroo; chalcopyrite = 10.5‰, galena = 10.9‰, sphalerite = 12.5‰ • sample NYM–7–1 from Nymagee; chalcopyrite = 8.7‰, sphalerite = 8.5‰.

207

intermediate sulfidation epithermal systems. Hayba (1997) suggested that mixing between deeply circulating hydrothermal fluids and overlying dilute groundwater was the dominant ore-forming mechanism in the central and southern portions of the Creede intermediate-sulfidation epithermal district. Downes (2007) proposed that fluid mixing was the dominant ore-precipitation mechanism for the Yerranderie intermediate-sulfidation epithermal system west of Sydney.

18.20 206

18.30

Pb/204Pb 2017_172

Figure 13. Distribution of all 207Pb/204Pb vs 206Pb/204Pb data from the present study (only), shown as 2-sigma error ellipses, compared to the crustal growth curve of Cumming & Richards (1975) and that of Carr et al. (1995). Data shown are for the area where 206Pb/204Pb lies between 18.00 and 18.30 and are compared to the signature of Ordovician to Carboniferous metallogenic events from Carr et al. (1995). The ellipses in the upper left-hand corner of the ratio plot are the analytical error at 95% confidence for samples from the CSIRO database (yellow) and the 2-sigma uncertainty for the present study (dark orange).

University (present study) and the University of Melbourne (Mernagh 2008; Huston et al. 2016) compared with earlier lead-isotope analyses from the CSIRO Pb-isotope laboratory is the higher analytical precision and resulting tighter lead model ages for the more recent data. A feature of the eastern Lachlan Orogen is the presence of mantle-derived or unevolved lead-isotope signatures, primarily associated with units of the Macquarie Arc (see Carr et al. 1995; Forster et al. 2011; Forster et al. 2015), but also evident in the data for many younger systems (see Carr et al. 1995; Downes 2009). Except for Condobolin, there is little or no evidence for the input of mantle-derived lead for the Nymagee study area, suggesting that the substrate to basement in this area is very different to that associated with the eastern Lachlan Orogen. Also, the absence of values from the study area that plot in the Silurian Cu–Au and Ordovician Cu–Au fields of Carr et al. (1995) indicates that the proto-Australian plate on which the study area lies had little or no interaction with the proto-Pacific plate. Huston et al. (2017) suggested that the majority of lead-isotope signatures present in the Tasmanides can be accounted for by the interaction between the proto-Australian and protoPacific plates, with both systems contributing lead from crustal and/or mantle sources to individual mineral systems.

Blind Calf, Condobolin and Mallee Bull include lead with mid-Silurian LFB lead model signatures. Blind Calf and Condobolin are hosted by the Ordovician Girilambone Group, whereas Mallee Bull is hosted by late Silurian to earliest Devonian units of the Cobar Basin (Shume Formation). Both Blind Calf and Condobolin include moreevolved lead, suggesting that these systems may be as young as the late Early Devonian. This is supported by U–Pb SHRIMP dating of zircons from felsic dykes at Blind Calf that gave a date of 413.9 ± 3.2 Ma (see Downes, Blevin et al. 2016) which is within error of the 410 ± 3 Ma LFB lead model age for that deposit. Alternatively, later events may have added radiogenic lead into these structures. For Mallee Bull, a LFB lead model age of 432 ± 3 Ma indicates that some lead included in that deposit was sourced from a reservoir significantly older than the host sequence. This is inferred to be basement to the Cobar Basin. Late Silurian–earliest Devonian lead-isotope signatures are evident for several zones, including Mallee Bull, Mineral Hill, Nymagee, Pipeline Ridge, Sandy Creek, and Yellow Mountain. The data for Nymagee, Sandy Creek and Yellow Mountain probably reflect a contribution of unevolved lead from the host late Silurian to earliest

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Devonian sequences (i.e. lower Amphitheatre Group, Shume Formation and Majuba Volcanics respectively; Table 1). The data for Pipeline Ridge and Mineral Hill, both of which are interpreted to be intermediate sulfidation epithermal systems hosted by latest Silurian to earliest Devonian sequences, reflect the age of the mineralising event. A 422 ± 3 Ma LFB lead model age for Pipeline Ridge is supported by U–Pb SHRIMP dating for zircons of 419.3 ± 2.8 Ma (Bodorkos et al. 2015) for the host Baledmund Formation. Similarly, a 412 ± 15 Ma LFB lead model age for Mineral Hill overlaps with the Pridolian fossil assemblages and U–Pb SHRIMP dating for Mineral Hill Volcanics (see Downes, Blevin et al. 2016) and the timing of mineralisation (420.5 ± 2.7 Ma, Downes & Phillips 2018). Many of the zones included in the present study have late Early Devonian LFB lead model ages (411–390 Ma). These include Blind Calf, BMW prospect, Condobolin, Great Central (Comet zone), Gundaroo, Hera, Manuka, MD2– Siegals, Mount Allen, Nymagee, Sandy Creek, Shuttleton, Tallebung, Wagga Tank and Yellow Mountain (Table 2). Hera, MD2–Siegals, Mount Allen, Nymagee, Sandy Creek and Tallebung are now interpreted to be intrusionrelated (see Fitzherbert et al. 2016; Downes, Blevin et al. 2016; Fitzherbert, Mawson et al. 2017). For these systems, LFB lead model ages of 408–399 Ma suggest that there are buried late Early Devonian intrusions along the margins and within the Cobar Basin which have yet to be identified. For the Hera–Nymagee area, on the eastern side of the Cobar Basin, Fitzherbert, Mawson et al. (2017) outlined a zone of biotite-grade greenschist to amphibolite grade metamorphism. In addition, those authors noted the presence of an early garnet–tremolite– actinolite–scheelite—phlogophite assemblage at Hera and suggested that peak temperatures were >450°C. The timing of this thermal event has yet to be resolved. Given that the timing of galena–sphalerite mineralisation at Hera is 381.9 ± 2.2 Ma (Downes & Phillips 2018) and that the lead-isotope data for Hera and Nymagee have LFB lead model ages ~400 Ma, it is likely that this thermal event may be latest Early Devonian in age. This is younger than the Nymagee felsic dykes that were emplaced at 415 ± 2.7 Ma (U–Pb dating of zircon; Downes, Blevin et al. 2016). The Blind Calf, BMW, Shuttleton, Wagga Tank and Yellow Mountain zones are all interpreted to be structurally controlled high-sulfide epigenetic zones which are hosted by late Silurian–earliest Devonian units. However, the timing of mineralisation for these zones is poorly constrained. These systems all have lead-isotope data with LFB model ages between 410 and 393 Ma, with data from the present study for BMW and Shuttleton being around 400 ± 3 Ma. These lead signatures are only a little older than the Middle Devonian Tabberabberan Orogeny, which Glen (2005) suggested was Eifelian (393.3 ± 1.2 to 387.7 ± 0.8 Ma; Gradstein et al. 2012) in age. It is suggested that these zones formed during the Tabberabberan Orogeny and that the lead-isotope data reflects some mixing between lead sourced from the host rock sequences and more-evolved lead. Similarly, the late Early Devonian lead model ages for Gundaroo and Manuka are also consistent

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with lead being sourced from the Lochkovian to earliest Emsian Winduck Group. However, the trend towards younger values in both zones suggests that additional radiogenic lead has been added to these systems during later events. The timing of mineralisation for Great Central is still poorly constrained, with only data from the CSIRO database being available. A LFB lead model age of 411 ± 15 Ma is within error of the age of the host unit which is theMount Halfway Volcanics — U–Pb SHRIMP dates for zircons of 422.8 ± 2.6 (Chisholm et al. 2015) and 418.7 ± 3.1 Ma (Downes, Blevin et al. 2016). Note the Regina Volcanics is now included in the Mount Halfway Volcanics. However, based on the sulfur-isotope zonation and observed alteration and metal zonation (discussed above), it appears more likely that this system formed in the mid Early or possibly late Early Devonian.

Implications for mineralisation Mineralisation in the study area reflects contributions from multiple lead and sulfur reservoirs. The lead sources include Ordovician basement, late Silurian–Early Devonian sedimentary sequences, mid-Silurian to ?latest Early Devonian intrusions and younger tectonic events. By contrast, the sulfur-isotope reservoirs are dominated by magmatic and reduced seawater sulfate reservoirs, with little input from biogenic sources. A mid-Silurian LFB lead model signature for Mallee Bull is consistent with some lead being sourced from Ordovician-age basement reservoirs rather than from the late Silurian to earliest Devonian host sequence. Mixing of lead from older less-evolved reservoirs with lead sourced from younger more-evolved reservoirs such as intrusions/ and evolving late Silurian to mid-Devonian lead reservoirs could potentially skew lead model ages within mineral systems of the southern Cobar Basin, including the Mount Hope Trough and Kopyje Shelf, towards older values. Such lead could have been added to lead-reservoirs through the breakdown of biotite and other minerals where lead is incorporated into the crystal lattice but where uranium and/or thorium are excluded. This may account for a number of the potentially anomalous older LFB lead model ages derived for prospects including Blind Calf, Condobolin, Great Central, MD2–Siegals, Nymagee, Sandy Creek and Yellow Mountain. Many of the zones included in the present study have sulfur-isotope values that reflect a contribution of sulfur from magmatic sources. These include Condobolin, Hera, Melrose, Mineral Hill, Mount Solar, Nymagee, Pipeline Ridge and Tallebung. With the exception of Tallebung (Sn–W), Sandy Creek and Yellow Mountain (both Zn–Pb dominant), the remaining zones all include copper and/or gold as a major commodity. A copper–gold endowment, together with the presence of sulfur from a magmatic reservoir and/or lead from basement source(s), suggests that most of the copper and gold was sourced from units that formed the basement to the study area and/or from younger intrusions. It is unlikely that the late Silurian to Early Devonian quartzofeldspathic sequences forming

the fill to the Cobar Basin (e.g. the host to the Mallee Bull and Shuttleton deposits) were a significant source of copper and gold. Seccombe et al. (2017) suggested that the Girilambone Group in the Cobar area was the source of copper and gold in the Cobar mineral field. In that area, the Girilambone Group is interpreted to form the basement to the Cobar Basin (see Glen et al. 1994; Stegman & Pocock 1996) and it contains mafic volcanic units to the north and east of the Cobar Basin (see Felton 1981; Pogson 1991; Burton et al. 2012). However, MacRae (1987b) did not describe any mafic units in the Girilambone Group in the Nymagee area, immediately adjacent to the Cobar Basin. In addition, the basement to the southern Cobar Basin/Mount Hope Trough is interpreted to be Wagga Group (see van der Wielen & Korsch 2008; supported by figure 5 of Fitzherbert, Mawson et al. 2017) which lacks mafic units (see Colquhoun et al. 2005). Thus, other sources of copper and gold need to be considered. As suggested by Fitzherbert, Mawson et al. (2017), the available sulfur-isotope data indicates that much of the copper and gold came from intrusions either as a direct magmatic input or through recycling of metals and sulfur due to later events.

Acknowledgements Many people need to be thanked for helping with this study. We wish to acknowledge the ongoing support given by many individuals and companies. They include Paul Burrell, Ian Cooper, Vlad David, Stuart Jeffrey, Trangie Johnston, Adam McKinnon, Shane Melle, Geoff Oaks and Owen Thomas. In addition, Dan Page is thanked for providing samples from his honours project for sulfur-isotope analysis. Mark Schmitz and his group at the Isotope Geology Laboratory, Boise State University undertook the lead-isotope analyses, whilst David Huston and David Champion of Geoscience Australia provided the lead-isotope lead model age calculators used in this study. The outstanding contribution by David Suppel also needs to be acknowledged as he submitted many of the samples now included in the CSIRO Pb-isotope database. Phil Blevin provided ongoing support and guidance, David Forster reviewed an early draft of this paper and Joel Fitzherbert is thanked for the many thought-provoking discussions. Phil Seccombe kindly reviewed this paper for publication whilst Stewart Watson and Cassie Yarnold are thanked for preparing the figures.

Conclusions The Nymagee project area, which covers a key part of the central Lachlan Orogen is associated with a diverse range of mineralisation styles including structurally controlled low- and high-sulfide deposits, carbonate- and sandstonehosted epigenetic deposits and a range of intrusionrelated systems. The sulfur and lead-isotope data for the project area reflect the interaction between basement and overlying basinal units with fluids of both magmaticand basinal derivation and younger sources/events. Multiple sources of sulfur and lead have contributed to mineralisation in the study area and, by extension, to the central Lachlan Orogen. The terrain-wide approach adopted for the present study has been important in providing constraints on the relative contribution of the various sulfur and lead reservoirs to individual systems. This suggests that, although isotope studies may not provide regional vectors to mineralisation for mineral exploration efforts, such studies are useful in understanding the sources of metals and fluids and the zonation present within individual mineral systems. Understanding the zonation present within the individual system may help vector towards further mineralised zones. The present study highlights the need to undertake well-constrained and systematic sulfur- and lead-isotope studies using appropriate analytical techniques to provide complementary information about the source of metals and sulfur in individual deposits in order to contribute to a better understanding of the variation in the ore-forming system and the controls to mineralisation.

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Mackenzie I. & Pienmunne J. 2008. Sandy Creek project annual report of exploration activities on exploration licence 6817 for the period ending 27 June 2008. Geological Survey of New South Wales, File GS2008/0737. MacRae G.P. 1987a. Geology of the Lachlan Downs 1:100,000 Sheet 8033. New South Wales Geological Survey, Sydney. MacRae G.P. 1987b. Geology of the Nymagee 1:100,000 Sheet 8133. New South Wales Geological Survey, Sydney. Marini L., Moretti R. & Accornero M. 2011. Sulfur Isotopes in Magmatic-Hydrothermal Systems, Melts and Magmas. In: Behrens H. & Webster J.D. eds. Sulfur in Magmas and Melts: Its Importance for Natural and Technical Processes, Reviews in Mineralogy & Geochemistry 73, 423–492. McClatchie L. 1971. Base metal mineralisation at Mineral Hill central western New South Wales. Geological Survey of New South Wales Memoir 11. McKinnon A.R. & Fitzherbert J.A. 2017. New developments at the Hera Au-Pb-Zn-Ag mine, Nymagee, New South Wales. Mines and Wines - Discoveries in the Tasmanides 2017. Extended Abstracts, AIG Bulletin 67. Mernagh T.P. 2008. Question 3: What and where are the fluid reservoirs for a mineral system? In: van der Wielen S. & Korsch R. eds. 3D architecture and predictive mineral analysis of the Central Lachlan Subprovince and Cobar Basin, New South Wales, final report for the pmd*CRC T11 project, pp. 85–116. Geoscience Australia, Canberra. Morrison G., Blevin P.L., Miller C., Hill P. & Mackenzie I. 2004. Age and setting of the Mineral Hill Au–Base metal deposits. In: Bierlein F.P & Hough M.A. eds. Tectonics to mineral discovery — Deconstructing the Lachlan Orogen, pp. 83–93. Geological Society of Australia Abstracts 74. Northcott M.J. 1986. The geology and alteration associated with the Great Central copper mine and Anomaly 3 polymetallic gossan, Mount Hope, centralwestern New South Wales. BSc Hons thesis, University of Adelaide (unpubl.). Ohmoto H. & Goldhaber M.B. 1997. Sulfur and carbon isotopes. In: Barnes H. L. ed. Geochemistry of hydrothermal ore deposits, pp. 517–611. 3rd edition. John Wiley and Sons, New York. Ohmoto H. & Lasaga A. 1982. Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems. Geochimica et Cosmochimica Acta 46, 1727–1745.

Percival I.G. 1999. Microfossiliferous cherts from the Forbes 1:250 000 map sheet, New South Wales. Palaeontological report 99/02. Geological Survey of New South Wales, Report GS1999/513 . Percival I.G. 2006. Sussex 1:100 000 sheet: Palaeontological determinations. Palaeontological Report 2006/01. Geological Survey of New South Wales, File GS2006/845. Percival I.G. 2007. Byrock 1:100 000 sheet: Palaeontological determinations. Palaeontological Report 2007/01. Geological Survey of New South Wales, File GS2007/856. Pogson D.J. 1991. Geology of the Bobadah 1:100 000 sheet 8233. Geological Survey of New South Wales, Sydney. Rayner E.O. 1969. The copper ores of the Cobar region, New South Wales. Memoirs of the Geological Survey of New South Wales Geology 10. Ryan S.J. 1987. The geology and genesis of the polymetallic Wagga Tank prospect, Mount Hope, N.S.W. BSc Hons thesis, University of Adelaide (unpubl.). Rye R. O. & Ohmoto H. 1974. Sulfur and carbon isotopes and ore genesis: a review. Economic Geology 69, 826– 842. Scheibner E. 1987. Mount Allen 1:100 000 Geological Sheet 8032, Explanatory Notes. Geological Survey of New South Wales, Sydney. Scheibner E. & Basden H. 1998. Geology of New South Wales — Synthesis: Volume 2 Geological Evolution. Geological Survey of New South Wales, Memoir Geology 13 (2). Scott K.M., Rabone G. & Chaffee M.A. 1991. Weathering and its effect upon geochemical dispersion at the Polymetallic Wagga Tank deposit, NSW, Australia. Journal of Geochemical Exploration 40, 413–426.

Styrt M.M., Brackmann A.J., Holland H.D., Clark B.C., Pisutha-Arnond V., Eldridge C.S., & Ohmoto H. 1981. The mineralogy and the isotopic composition of sulphur in hydrothermal sulphide/sulphate deposits on the East Pacific Rise, 21°N latitude. Earth and Planetary Science Letters 82, 36–48. Suppel D.W. 1984. A study of mineral deposits in the Cobar Supergroup, Cobar region, New South Wales. MSc thesis, University of New South Wales, Sydney (unpubl.). Suppel D.W & Gilligan L.B. 1993. Metallogenic study and mineral deposit data sheets, Nymagee 1:250 000 metallogenic map. Geological Survey of New South Wales, Sydney. Sun Y., Jiang Z., Seccombe P.K. & Feng Y. 2000. New dating and a review of previous data for the development, inversion and mineralization in the Cobar Basin. In: McQueen K.G. & Stegman C.L. eds. Central West Symposium Cobar 2000. Geology, Landscapes and Mineral Exploration WA CSIRO Extended Abstracts 113–116. Talent J.A., Winchester-Seeto T. & Mawson R. 2003. Final report for Eastern Star Gas and NSW Department of Mineral Resources on information gleamed from Darling Basin cores. Macquarie University Palaeobiology. Geological Survey of New South Wales, Report GS2006/128. van der Wielen S. & Korsch R. (eds.) 2008. 3D architecture and predictive mineral analysis of the Central Lachlan Subprovince and Cobar Basin, New South Wales, final report for the pmd*CRC T11 project. Geoscience Australia, Canberra.

Seccombe P.K., Jiang Z. & Downes P.M. 2017. Sulfur isotope and fluid inclusion geochemistry of metamorphic Cu–Au deposits, central Cobar area, NSW, Australia. Australian Journal of Earth Sciences 64, 537–556. Sherwin L. 2013. Revised Nymagee 1:250 000 Siluro– Devonian time space plot. Geological Survey of New South Wales, Report GS2013/1885 (unpubl.).

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Now open: Port Macquarie Coastal Geotrail The Port Macquarie Coastal Geotrail is an easily accessible 4 km walk along the coast from Shelly Beach to Rocky Beach that tells the story of plate tectonics and how Earth’s crust was formed. The geotrail features rocks formed by volcanoes, microscopic marine animals, ocean currents, and extreme temperatures and pressures, 100 km below Earth’s surface.

New release: Braidwood 1:100 000 map and explanatory notes The Braidwood 1:100 000 geological map and explanatory notes are finally finished! This 160 page full colour book with DVD outlines the geological history, structure, metamorphism and mineral systems of the area, and contains detailed descriptions of all stratigraphic units. There is also a detailed palaeontological appendix.

$19.80 incl. GST

Download our free app! Explore the geotrail at your own pace, with a tour guide at your fingertips.

NSW GeoTours

$37.00 incl. GST

Grab a free brochure from Sea Acres Rainforest Centre. Visit pmhc.nsw.gov.au/coastalgeotrail for more information. The geotrail is a collaborative project led by the University of Newcastle with the Department of Planning and Environment’s Geological Survey of NSW (GSNSW) and the National Parks and Wildlife Service (NPWS).

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June 2018

Contact [email protected]

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New release: New England Orogen 1:750 000 metallogenic map First edition A new map is now available for the mineral-rich New England region. On the back of the map is a comprehensive poster with the mining history, a brief geological history, and images of some of the marvellous minerals found in the New England region.

$19.80 incl. GST

Please specify flat or folded when ordering.

Enquiries [email protected]

Geological Survey of New South Wales NSW Department of Planning and Environment 516 High Street, Maitland NSW 2320 PO Box 344 Hunter Region Mail Centre NSW 2310 Tel: 1300 736 122 (toll free) or +61 (0)2 4063 6500

resourcesandgeoscience.nsw.gov.au