Sulfur isotope distribution in Late Silurian volcanic-hosted massive ...

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Volcanic-hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South .... Trough, the principal stratotectonic units hosting VHMS-.
Australian Journal of Earth Sciences (2004) 51, 123–139

Sulfur isotope distribution in Late Silurian volcanic-hosted massive sulfide deposits of the Hill End Trough, eastern Lachlan Fold Belt, New South Wales P. M. DOWNES1* AND P. K. SECCOMBE2 1 2

Geological Survey of New South Wales, PO Box 536, St Leonards, NSW 2065, Australia. Tectonics & Earth Resources Research Group, University of Newcastle, NSW 2308, Australia. Volcanic-hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat–Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured 34S values range from –7.4‰ to 38.3‰, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from –1.7‰ to 5.9‰. Ore-forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average 34S values ranging from 5.4‰ to 8.1‰. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The 34S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5‰, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6‰ to 38.3‰. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6‰, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by 34S-enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature. KEY WORDS: Hill End Trough, Lachlan Fold Belt, New South Wales, Silurian, sulfur isotopes, volcanichosted massive sulfides.

INTRODUCTION The eastern Lachlan Fold Belt (Figure 1) contains a number of small to large volcanic-hosted massive sulfide (VHMS) deposits hosted by Late Silurian felsic (meta-) volcanic and associated siliciclastic (meta-) sedimentary rocks. Despite the metallogenic importance of this terrane, few data have been published on the distribution of sulfur isotopes in VHMS deposits of the region, with the exception of studies at Captains Flat (Stanton & Rafter 1966) and Woodlawn (Ayres et al. 1979; Glen et al. 1995), both in the Captains Flat – Goulburn Trough, and Sunny Corner (Seccombe et al. 1984) in the Hill End Trough. Additional

limited data on the sulfur isotope distribution are available from Burns and Smith (1976) for the Currawang deposit (Captains Flat – Goulburn Trough) and the Mt Bulga and Colo Creek (Kempfield) deposits in the Hill End Trough. However, Burns and Smith (1976) did not attempt a geological interpretation for the isotopic results for their study nor were the detailed data included. The aim of the present study was to bring together a number of previously unpublished studies and new reconnaissance data to provide an overview of the distribution of sulfur

*Corresponding author: [email protected]

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isotope data for principal examples of VHMS mineralisation in the Hill End Trough, eastern Lachlan Fold Belt, New South Wales. In the present paper, the term 'VHMS deposit' follows the usage of Large (1992) and is restricted to base-metal sulfide mineralisation that occurs in a sequence dominated by submarine volcanic rocks. These deposits generally contain stratiform or stratabound massive sulfide lenses and related stringer zones or massive replacement pipes, all of which are considered to be approximately the same age as the host volcanic sequences. The basin-wide approach adopted in the present study provides a test for competing sulfur sources available in a submarine volcanic environment. Four principal sulfur reservoirs may make a contribution to the sulfur budget of VHMS systems associated with the Hill End Trough: (i) sulfur derived from seawater; (ii) biogenic sulfur; (iii) sulfide–sulfur leached from the host rocks; and (iv) sulfur contributed directly from synmineralisation magmatic sources (Rye & Ohmoto 1974; Ohmoto & Rye 1979). The present study attempts to assess the role of each sulfur reservoir by the following approach. First, a number of barite-bearing VHMS deposits (e.g. Commonwealth, Stringers and Kempfield) can provide a reference for the composition of contemporaneous seawater sulfate during VHMS formation. In addition, sulfur isotope data for barite and coexisting sulfide offer a test of the degree of anoxia of the depositional environment (Goodfellow & Jonasson 1984) and, by use of sulfate–sulfide isotope geothermometry, of constraints on depositional temperatures for individual deposits. Second, a biogenic sulfur signature may be obtained from the analysis of texturally early pyrite commonly encountered in black shale units. Some of these black shale

Figure 1 Location of the Hill End Trough and Captains Flat – Goulburn Trough in the eastern Lachlan Fold Belt of New South Wales.

units are lateral equivalents of massive sulfide lenses. This approach was taken for samples from the Peelwood area (Cordillera, John Fardy and Peelwood mine), where contrasting sulfur isotope signatures were obtained among different stratigraphic units at both the district and deposit scales, and where 32S depletion noted in sulfide from black shales implies input of biogenic sulfur. Last, a basin-wide approach is important in assessing mineralising events in space and time. Recent remapping of key 1:250 000 map sheet areas [Dubbo, (Morgan et al. 1999a) and Bathurst (Raymond et al. 1998)] and current remapping of the Goulburn map sheet have been undertaken by the New South Wales Department of Mineral Resources and Geoscience Australia. This has provided high-resolution biostratigraphic and radiometric age determinations for the host formations to VHMS mineralisation in the Hill End Trough (Pogson & Watkins 1998; Meakin & Morgan 1999). Also, the volcanic facies of host formations can be used to constrain the probable location of some volcanic centres in the trough. Proximity to a volcanic centre and water depth may influence a number of environmental factors in VHMS ore formation, such as the amount of magmatic sulfur contributed to the system; the size of the hydrothermal system; and the relation between water depth and its effect on the style of mineralisation. Although the present study cannot be considered comprehensive, it addresses all of the above issues for those VHMS systems in the Hill End Trough where geological and isotopic data permit.

GEOLOGICAL SETTING The Lachlan Fold Belt of eastern Australia represents part of a >1000 km-wide orogenic system that developed along the Pacific margin of the Australian craton during Palaeozoic time (Foster et al. 1999). The focus of the present paper is the eastern Lachlan Fold Belt, an outboard, convergentmargin terrane, dominated by mafic to felsic volcanic rocks and thick turbidite successions of Ordovician to Devonian age (Glen 1998; Foster et al. 1999). Critical to the development of VHMS mineralisation, the eastern Lachlan Fold Belt was subjected to 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). During mid-Silurian to Middle Devonian time, a number of north-trending, backarc rift basins formed in the eastern Lachlan Fold Belt, as a result of crustal extension (Glen 2002). These include the Hill End Trough and the Captains Flat – Goulburn Trough. Depositional sequences in these back-arc basins are characterised by quartz-rich turbidites, felsic and mafic volcanic rocks, volcaniclastic rocks and black shales. The age of inversion and deformation of these back-arc basin sequences remains poorly constrained in the eastern Lachlan Fold Belt. A Middle to Late Devonian age (380– 370 Ma) for greenschist-facies regional metamorphism and cleavage formation is favoured for the Hill End Trough from radiometric dating of metamorphic mica (Cas et al. 1976; Lu et al. 1996; Packham 1999) and field relations linking trough-fill and cover sequences. The contribution

Sulfur isotopes, Hill End Trough, NSW

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Figure 2 Location of volcanic-hosted massive sulfide (VHMS) systems and distribution of Late Silurian felsic volcanic and related sedimentary units within and adjacent to the Hill End Trough, eastern Lachlan Fold Belt. Geology adapted from the Bathurst (Raymond et al. 1998) Dubbo (Morgan et al. 1999a) 1:250 000 and Crookwell (Johnson et al. 2000) 1:100 000 map sheets.

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to regional cleavage formation of an overprinting Carboniferous deformation remains unresolved (Glen & Watkins 1999). The eastern Lachlan Fold Belt contains VHMS mineralisation in Late Silurian felsic volcanic and associated siliciclastic sedimentary rocks of the Hill End Trough, one of the longer lived backarc basins at the eastern margin of the orogen. Within the Hill End Trough, the principal stratotectonic units hosting VHMSrelated mineralisation include the Campbells Group, Mumbil Group, Chesleigh Group and the Tannabutta Group (Figure 2). These volcaniclastic sequences, which are now recognised to be more extensive than previously mapped, were discussed in detail by Meakin and Morgan (1999), and Pogson and Watkins (1998). The Mumbil Group is interpreted as a shelf and slope sequence that outcrops extensively in two north-trending belts on either side of the Hill End Trough (Figure 2). On the western side of the Trough, the Mumbil Group includes the Anson Formation, Mullions Range Volcanics, Barnby Hills Shale and Gleneski Formation. The Anson Formation hosts VHMS mineralisation at the major Lewis Ponds deposit and the adjacent Mt Bulga deposit, as well as a number of small VHMS occurrences. Scott and Meakin (1998) described the Anson Formation in the vicinity of the Lewis Ponds deposit to include pyritic and calcareous siltstone, limestone (submarine mass flow: Agnew 2002, 2003) and rhyolitic volcanics towards the top of the sequence. The overlying Mullions Range Volcanics, consisting of volcaniclastic sandstone with minor trachytic and rhyolitic lavas, tuffaceous siltstone and volcanic breccia (Scott et al. 1998), hosts the nearby Calula deposit. Further north, siltstone and quartz crystal tuff of the Gleneski Formation host VHMS deposits in the Commonwealth area (Commonwealth mine and Stringers deposit). On the eastern side of the Hill End Trough, the Bells Creek Volcanics (Mumbil Group) host a number of small VHMS and copper–zinc occurrences. Undifferentiated sedimentary rocks of the mid- to Upper Silurian Campbells Group dominate the southern part of the Hill End Trough. The Campbells Group (formerly Campbells Formation) has been redefined and now includes the Kangaloolah Volcanics (M. Scott pers. comm. 2002). The Kangaloolah Volcanics, which consist of rhyolitic and dacitic volcanic rocks, and feldspathic sandstone and siltstone (Wyborn et al. 1998), host barite–basemetal mineralisation at Kempfield. Further south, in the Peelwood area, a number of base-metal deposits (John Fardy, Peelwood and Cordillera) are located adjacent to the contact between undifferentiated feldspathic sandstone, siltstone and slate of the Campbells Group, and felsic volcaniclastic rocks of the Kangaloolah Volcanics. West of Peelwood, undifferentiated sediments of the Campbells Group host the Elsinora mineralisation. The Tannabutta Group, to the southeast of Mudgee, contains several base-metal occurrences including the Accost prospect (Figure 2). The mineralisation is associated with the Dungeree Volcanics, which comprise rhyolite to dacite lava, fine- to coarse-grained volcaniclastic rocks, conglomerate and limestone (Colquhoun et al. 1999a). The Dungeree Volcanics are interpreted to represent a shallow marine to emergent volcanic pile, fringed by an adjacent

slope facies of clastic sedimentary and carbonate rocks (Colquhoun et al. 1997). The Chesleigh Formation has been upgraded to group status, with the felsic volcaniclastic rocks of the upper Chesleigh Formation now included in the Piambong Formation (Colquhoun et al. 1999b). The Piambong Formation hosts significant massive sulfide mineralisation at Sunny Corner towards the eastern margin of the Hill End Trough, and the Belara deposit in the north of the region.

SAMPLE PREPARATION For the new isotopic analyses reported in the present study, sulfur-bearing samples were collected mainly from diamond drillcore, most of which is stored at the New South Wales Department of Mineral Resources core facility at Londonderry, New South Wales. Some of the new data are from samples collected from mine dumps (Belara, Calula, Commonwealth, Peelwood and Stringers) or mineralisation exposed in mine openings (Kempfield). Care was taken to ensure that samples collected from mine dumps were representative of that deposit. Samples were slabbed prior to mineral separation to provide additional detail on host-rock lithology, textures, and alteration assemblages and mineral paragenesis. Sulfides and barite were extracted using a microdrill and a binocular microscope. Contamination of mineral separates by other sulfide phases was minimised by selecting coarse-grained material where possible, but was inevitable for samples containing fine-grained, mixed sulfides. In the latter cases, sample purity was estimated from the slabbed and polished surfaces. Samples analysed in earlier studies of the Commonwealth and Stringers deposits (James 1984) and the Calula area, were concentrated from drillcore or dump samples, after crushing, sizing and heavy-media separation. Polished mounts of the mineral separates were checked for impurities. Prior to isotopic analysis, purified sulfides from all studies reported here were converted to sulfur dioxide gas in a vacuum line at 950C, using cuprous oxide as an oxidant, after the procedure of Robinson and Kusakabe (1975). Barite samples were initially reduced with carbon at 1150C, to form barium sulfide (Bailey & Smith 1972), which was converted to silver sulfide before combustion to sulfur dioxide. Vacuum lines at the University of Newcastle and the Centre for Isotope Studies at CSIRO, North Ryde, were used for the preparation of both sulfide and barite samples. Isotopic analysis was undertaken on a range of gassource mass spectrometers, at the University of Tasmania (Calula, Commonwealth and Stringers) or at the Centre for Isotope Studies for other deposits. Data are reported to an accuracy of ±0.2‰, relative to Cañon Diablo Troilite (CDT) sulfide and a variety of secondary standards.

MINERALISATION AND RESULTS The VHMS mineralisation occurs in a number of areas in the eastern Lachlan Fold Belt. The results of this and past investigations into the sulfur isotope distribution for

Sulfur isotopes, Hill End Trough, NSW VHMS-related mineralisation confined to the Hill End Trough are summarised in Table 1 and Figure 3, and are discussed below. All unpublished 34S data are listed in Appendix 1. Geological descriptions of individual deposits have been summarised in regional reviews by Davis (1990), Downes (1998, 1999, 2003), Gilligan (1975), Huston et al. (1997) and Stevens (1975). The study area has been divided into the western Hill End Trough, Capertee Zone and southern Hill End Trough for the purpose of providing a geographical grouping for individual systems.

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Western Hill End Trough A number of VHMS deposits is located on the western side of the Hill End Trough in a discontinuous belt of Upper Silurian felsic volcanic and volcaniclastic units. The zone includes the Lewis Ponds and nearby Mt Bulga deposits, as well as the Calula, Commonwealth, Stringers and Belara systems further north (Figure 2). The Lewis Ponds VHMS system, 14 km east of Orange, consists of a stacked sequence of concordant gold–silver– copper–lead–zinc-rich massive sulfide lenses, which extend

Table 1 Features of volcanic-hosted massive sulfide deposits in the Hill End Trough, eastern Lachlan Fold Belt. Deposit

Commodities major (minor)

Mineralogy major (minor)

Host unit

34S(‰)

Accost

Zn, Pb, (Cu)

py, sph, (cpy, ga)

Dungeree Volcanics

Belara

Zn, Pb, Cu, (Ag, Au)

py, sph, ga, cpy, (au, as, po, tet)

Piambong Formation

Calula

Zn, Cu, (Pb)

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

Commonwealth

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

py, sph, ga, cpy, ba (tet, bo, cov)

Mullions Range Volcanics Gleneski Formation

py = 1.9–3.4a ga = 1.4, 1.9a py = 1.2–4.3a cpy = 2.3, 2.8a sph = 2.2–3.6a py = 4.0–7.6b

Cordillera

Zn, Pb, Cu, Ag, (Au)

py, sph, cpy, ga, as, tet, (po, mar, au, cc, cov, sch, bou, jam)

Campbells Group

Elsinora

Zn, Pb, Cu, (Ag, Au)

py, sph, ga, (cpy, as, po, bis, au)

Campbells Group

John Fardy

Zn, Cu, Pb, Ag, (Au)

py, sph, cpy, ga, (ten-tet, as, cov, cc, au)

Kangaloolah Volcanics

Kempfield

Ba, Ag, (Pb, Zn)

ba, py, sph, ga, cpy, (tet)

Kangaloolah Volcanics

Lewis Ponds

Au, Pb, Zn, Ag, Cu

py, sph, ga, cpy, po, au

Anson Formation

Mount Bulga

Cu, Pb, Zn, (Ag, Au)

py, po, sph, cpy, ga, (cub, mar, as, tet, au, ag)

Anson Formation

Peelwood

Zn, Pb, Cu, Ag, (Au)

py, sph, cpy, ga, (tet, as, mar, cc, bo, cc, au)

Kangaloolah Volcanics

Sunny Corner

Au, Cu, Ag

py, sph, cpy, ga, (bo, as, po, au)

Chesleigh Group (Piambong Formation)

Stringers

Pb, Cu, (Zn)

py, cpy, sph, ga, ba, (cov, au)

Gleneski Formation

py = 3.3–10.1c py–sph (mixed) = 7.4–9.8c sph = 8.2c ba = 23.4–34.6a py = –7.4–4.8a,d cpy = 3.9–4.8a,d ga = 2.6, 4.4a sph = 4.7–7.5a py = 7.2–13.3, 18.5, 22.5a ga = 9.6a sph = 7.0, 9.9a py = –2.2, –1.9, 1.1, 4.5–7.5, 10.2–15.4d cpy = 9.2, 11.4d sph = 13.0, 14.1d py = 2.0, 9.9–17.4a as = 5.8 a ga = 9.0a ba = 12.6, 25.1–29.0a py = 3.8–5.9a cpy = 4.3, 5.1a ga = 1.7a sph = 3.3, 3.5a py = 2.7–3.7e ga = –1.7–0.3e sph = 1.5–3.0e po = 1.5–2.9e py = –0.7, 0.1, 5.1–8.7a, d cpy = 6.4–9.9a, d sph = 6.6–9.1a, d sph–ga (mixed) = 8.5–9.8a py = 1.7–14.2f ga = 7.3–8.8f sph = 3.4–7.9f py = 10.5, 15.1c ba = 26.9–30.6, 38.3a

ag, native silver; as, arsenopyrite; au, gold; ba, barite; bis, bismuthinite; bo, bornite; bou, bournonite; cc, chalcocite; cov, covellite; cpy, chalcopyrite; cub, cubanite; ga, galena; jam, jamesonite; mar, marcasite; py, pyrite; po, pyrrhotite; sch, scheelite; sph, sphalerite; ten, tennantite; tet, tetrahedrite. 34S data: apresent study; b P. K. Seccombe & R. G. Skirrow unpubl. data, c James (1984); d Brown (1999); e Chisholm (1976); f Seccombe et al. (1984). Commodity and mineralogical data: present study; NSW Department of Minerals Resources Metallic Mineral Occurrence (METMIN) Database.

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Figure 3 Distribution of sulfur isotope values (34S) for volcanic-hosted massive sulfide (VHMS) deposits in the Hill End Trough. Deposits in the Peelwood area (Cordillera, John Fardy, Peelwood mine) and barite-bearing VHMS deposits (Commonwealth/Stringers and Kempfield) are grouped. Data sources: Accost, present study; Belara, present study; Calula, P. K. Seccombe & R. G. Skirrow (unpubl. data); Commonwealth and Stringers, James (1984) and present study; Cordillera, Brown (1999) and present study; Elsinora, present study; John Fardy, modified from Brown (1999); Kempfield, present study; Lewis Ponds, present study; Mt Bulga, Chisholm (1976); Peelwood, Brown (1999) and present study; Sunny Corner, Seccombe et al. (1984).

for 1.6 km along a north-northwest-trending zone. The mineralisation is hosted by the Upper Silurian Anson Formation. Agnew (2002, 2003) noted that subvolcanic rhyolite bodies of the Mullions Range Volcanics have intruded the host sequence. The sulfur isotope values for the massive sulfide mineralisation at the Lewis Ponds deposit vary from 1.7‰ to 5.9‰. In the Main zone, the 34S composition of pyrite ranges from 4.8‰ to 4.9‰ (three analyses). A single sphalerite gave a 34S value of 3.3‰ and a single galena sample yielded a value of 1.7‰. Sulfur isotopic composition of pyrite from the smaller Toms zone in the southern part of the area ranges from 3.8‰ to 5.0‰ (five analyses), and

34S values of 4.3‰ and 3.5‰ were obtained from single samples of chalcopyrite and sphalerite, respectively. Pyrite from the discordant footwall stockwork zone, located in the underlying felsic volcanic pile, gave 34S values of 5.7 and 5.9‰ (duplicate analysis). A single analysis of chalcopyrite from the latter zone yielded a 34S value of 5.1‰. The Mt Bulga VHMS system, 9 km east of Orange, is also hosted in siltstones of the Upper Silurian Anson Formation. The deposit may have formed at a similar stratigraphic level to that of the Lewis Ponds deposit (M. Agnew pers. comm. 2002). Chisholm (1976) found that the sulfur isotope values for the Mt Bulga mineralisation occupy a

Sulfur isotopes, Hill End Trough, NSW

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Figure 3 Contd

narrow range from –1.7‰ to 3.7‰. In addition he observed a well-developed isotopic zonation defined by a decrease in the 34S values for sulfides stratigraphically upwards through the deposit. The 34S zonation is coincident with sulfide zoning noted at Mt Bulga, which grades from a basal chalcopyrite–pyrite zone, through a central pyrite interval, to an upper galena–sphalerite zone. This trend in 34S values may reflect a declining temperature profile upwards through the ore-forming environment at the deposit. The data are also consistent with the order of isotopic fractionation among equilibrated sulfides, whereby 34S values for pyrite > sphalerite > galena (Ohmoto & Goldhaber 1997). In the Calula area, 24 km north-northwest of Orange, a number of base-metal occurrences are identified, including the Calula (Pyrite) mine (Figure 2) (Skirrow 1983). The massive and disseminated sulfide mineralisation at the Calula mine is hosted by the Mullions Range Volcanics. In the mine area the Mullions Range Volcanics consists of a

sequence of epiclastic volcanic sedimentary rocks with possible rare pyroclastic units present in the footwall (Skirrow 1983). The available sulfur isotope data for pyrite from the Calula mine range from 4.0‰ to 7.6‰. Further north, gold-rich VHMS mineralisation is present in a region 10 km east-southeast of Wellington. These deposits include the Commonwealth mine and adjacent Stringers mine (Figure 2). The mineralisation is located within a structurally complex fault-bounded block of felsic to intermediate volcanic and volcaniclastic rocks of the Upper Silurian to Lower Devonian Gleneski Formation. James (1984) found that massive sulfide mineralisation at the Commonwealth mine had 34S values ranging from 3.3‰ to 10.1‰ (mean 7.9‰) and proposed that the sulfur was derived from both seawater and magmatic sources as part of a submarine exhalative process. Sulfur isotopic compositions of 10.5 and 15.1‰ for pyrite from the nearby Stringers mine, located 1.8 km east-southeast of the

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Commonwealth mine, suggest that the primary source of sulfur was seawater. To better constrain the conditions of mineralisation and the likely source of sulfur, samples of abundant barite coexisting with sulfides were analysed from both locations. Barite from the Commonwealth mine yielded 34S values ranging from 23.4‰ to 34.6‰ (mean 28.5‰). This is similar to that for barite from the Stringers mine (34S range from 26.9‰ to 30.6‰; mean 28.6‰, plus one outlying analysis of 38.3‰). Figure 3 shows the combined sulfur isotope distribution for samples from both the Commonwealth and Stringers deposits. The Belara VHMS deposit is located 34 km northeast of Wellington (Figure 2). Mineralisation is hosted by a sequence of interbedded felsic volcaniclastic and epiclastic, fine- to medium-grained and locally coarse-grained sedimentary rocks of the Upper Silurian Piambong Formation (Downes 1999, 2003). The sulfur isotope values for the Belara mineralisation occupy a narrow range from 1.2‰ to 4.3‰ (mean 3.0‰: 13 analyses). The Belara samples generally show isotopic fractionation consistent with equilibration among coexisting sulfides (34S values for pyrite > sphalerite). However, the samples were not suitable for geothermometry: the samples rarely represent texturally coexisting sulfides, being commonly extracted from separate layers in massive banded ore; and finegrained impurities, including galena and/or chalcopyrite, were invariably noted in the analysed pyrite or sphalerite samples.

Capertee Zone The Capertee Zone lies adjacent to the eastern edge of the Hill End Trough. The zone hosts several VHMS-related base-metal and barite prospects southeast of Mudgee including the Accost prospect, and the historic Sunny Corner massive sulfide deposits, east of the Bathurst Batholith (Figure 2), which were mined extensively between 1875 and 1896 (Seccombe et al. 1984). The Accost prospect is located 18 km east-southeast of Mudgee. Disseminated and vein-style base metal sulfide mineralisation is hosted by quartz–sericite altered Upper Silurian Dungeree Volcanics. Facies analysis of the Dungeree Volcanics in this area suggests a depositional

Figure 4 Distribution of sulfur isotope values according to ore type for the John Fardy deposit (including the Central Hill zone). Data reinterpreted from Brown (1999).

setting involving a shallow marine to emergent dacitic volcanic edifice, fringed by clastic and carbonate units developed on adjacent slopes (Colquhoun et al. 1997). The origin of the mineralisation has yet to be resolved. Downes (1999, 2003) suggested that, although this occurrence has similarities to other VHMS systems, the shallow-marine setting may indicate that massive sulfide mineralisation was unable to form, perhaps due to boiling of hydrothermal fluids as a result of insufficient water pressure. The sulfur isotope signature for the Accost zone varies from 1.4‰ to 3.4‰, with values for pyrite ranging from 1.9‰ to 3.4‰ (mean 2.7‰: eight analyses) and those for galena from 1.4‰ and 1.9‰ (two analyses). At Sunny Corner, massive sulfide mineralisation occurs on the moderately dipping, western limb of a complexly faulted anticlinal (or domal) structure (Lau 1979; Seccombe et al. 1984). The host rocks are siltstone and fine-grained siliceous rhyolitic tuffs in the upper part of the Silurian Chesleigh Group (formerly Chesleigh Formation). These units are herein interpreted to be part of the Piambong Formation. The sulfur isotope values for the Sunny Corner mineralisation range from 1.7‰ to 14.2‰, with an average of 7.4‰ (Seccombe et al. 1984).

Southern Hill End Trough The southern part of the Hill End Trough, south of the Bathurst Batholith, is different in character from the areas in the north. Trough-fill sequences south of the Bathurst Batholith are dominated by felsic volcanic-derived turbiditic sedimentary rocks, which form the bulk of the mid- to Upper Silurian Campbells Group. Included in the Campbells Group are the Upper Silurian Kangaloolah Volcanics, which host a group of VHMS-related districts, including Kempfield, Peelwood and Elsinora (Figure 2). The Kempfield area, 50 km southwest of Bathurst, contains a number of VHMS zones including the Kempfield, Sugarloaf, Inco and Shell zones. Siltstone, felsic volcanic and volcaniclastic rocks, and minor limestone and chert of the Kangaloolah Volcanics host the stratiform barite–basemetal mineralisation. Burns and Smith (1976) reported 34S ranges of 3–6‰ for galena, 4–8‰ for sphalerite, 8–10‰ for pyrite and a ?single 34S determination for barite of 29‰ (note: Colo Creek is a former name for Kempfield). However, individual analyses were not reported by Burns and Smith (1976), thus we were unable to include their data in Table 1 and Figure 3. In the present study it was found that the 34S values range from 12.6‰ to 29.0‰ for barite, and 2.0–17.4‰ for pyrite. Single analyses of arsenopyrite and galena gave 34S compositions of 5.8‰ and 9.0‰, respectively. Although there is general agreement among the results for both barite and pyrite from the two studies, sampling is insufficient to clearly define the sulfur isotope signature of the mineralisation in the Kempfield area. A number of VHMS deposits occurs in the Peelwood district 80 km south-southwest of Bathurst. These include the Peelwood, John Fardy and Cordillera deposits (Figure 2). In general, the massive sulfide lenses are located at, or close to, the generally strongly sheared boundary between undifferentiated sedimentary rocks of the Campbells Group (including the informal 'Peelwood Shale') and units of the Kangaloolah Volcanics.

Sulfur isotopes, Hill End Trough, NSW The principal massive sulfide zones at the Peelwood mine are the conformable Magazine lode (Markham 1961) that is closely associated with a pyritic black shale unit, and the discordant Cornish lode (O'Brien 1973). Other mineralised zones include the Paddys lode. The sulfur isotopic composition of the Peelwood mineralisation ranges from 6.6‰ to 9.1‰ for sphalerite, 6.4–9.9‰ for chalcopyrite, 5.1–8.7‰ for pyrite and 8.5–9.8‰ for mixed galena–sphalerite. In addition, 34S values for early pyrite, associated with black shales, are –0.7‰ and 0.1‰ (two analyses). Brown (1999) investigated the sulfur isotope distribution of the John Fardy deposit, 1.1 km north-northwest of the Peelwood mine. Based on new information regarding drillhole collar locations, Brown’s data (Brown 1999) have been reinterpreted because some samples assigned by him to the John Fardy deposit are now known to have come from the nearby Peelwood and Cordillera deposits. Figure 4 summarises the distribution of sulfur isotope values for the John Fardy deposit (including Central Hill) according to ore type. Pyrite from black shales has low 34S values (–2.2‰ and –1.9‰). Chalcopyrite and pyrite from a cherty exhalite, conformable with adjacent layered massive sulfide, have sulfur isotope values ranging from 4.5‰ to 11.4‰ (mean 8.3‰). The massive sulfides have higher 34S values (11.9–14.5‰; mean 13.2‰) than those from the cherty exhalite. A pyritic silica-rich zone at the Central Hill (adit) zone, 300 m southeast of the main mineralised zone, yielded similar values to the main zone of mineralisation (34S range 13.1–15.4‰; mean 13.8‰: four analyses). Finally, disseminated pyrite in vein quartz formed during the D2 deformation defined by Brown (1999) at Peelwood, have 34S values ranging from 1.1‰ to 1.9‰ (John Fardy zone 1.9‰, Central Hill zone 1.1‰ and 1.5‰). At the Cordillera deposit, 3.8 km north–northwest of the Peelwood mine (Figure 2), massive sulfide mineralisation is contained in four overlapping, conformable pyrite– galena–chalcopyrite lenses hosted by undifferentiated units of the Campbells Group. The lenses are adjacent to the steeply dipping contact with coarse-grained felsic volcanics of the Kangaloolah Volcanics. 34S values for pyrite range from –7.4‰ to 1.3‰ (mean –2.4‰: eight analyses), excluding a single 34S analysis of 4.8‰. For sphalerite, the 34S values range from 4.7‰ to 7.5‰ (mean 6.2‰: nine analyses), for chalcopyrite the values range from 3.9‰ to 4.8‰, whereas two 34S analyses for galena returned 2.6‰ and 4.4‰. A number of sulfur reservoirs are identified from the data. These include early biogenic sulfur from black shale host rocks (including 34S analyses in the range from –7.4‰ to –5.8‰: Appendix 1), as well as

sulfur derived from probable magmatic (rock sulfide) and seawater sources. At Elsinora (or Elsienora), 87 km south-southwest of Bathurst and 10 km southwest of the Peelwood mine (Figure 2), disseminated and thin massive bands of pyrite, with some fine-grained galena and sphalerite in the more massive pyritic zones, are hosted by felsic volcanic and metasedimentary rocks of the Campbells Group. The mineralisation is interpreted to be VHMS-related (Stevens 1975). The sulfur isotope values for pyrite form two groupings, with the majority of sulfur isotope data for pyrite data ranging from 7.2‰ to 13.3‰ (mean 10.0‰: seven analyses). Two outlying pyrite analyses gave 34S values of 18.5‰ and 22.5‰. 34S values of 7.0‰ and 9.9‰ were obtained from sphalerite (two samples), and a single galena sample measured 9.6 34S‰.

DISCUSSION Based on the range and distribution of sulfur isotope values, the majority of the deposits were classified into two groups. Group 1 deposits have 34S values close to 0‰ and show in general a narrow compositional range, whereas group 2 deposits have higher 34S values and in general a broader range of compositions. The 34S signature for the Lewis Ponds, Mt Bulga (Chisholm 1976), Belara and Accost mineralisation are all very similar and range from –1.7‰ to 5.9‰ (group 1). Since the Lewis Ponds, Mt Bulga and Belara deposits contain primary pyrrhotite in their sulfide assemblages, in addition to pyrite and base-metal sulfides, the ore-forming fluids for these deposits are likely to have been more reducing than for other deposits where pyrrhotite is absent. Thus the isotopic composition of these group 1 sulfides would correspond closely to the isotopic composition of total dissolved sulfur in the mineralising fluids (34S sulfide  34S S fluid: Ohmoto & Goldhaber 1997), and the sulfur isotope composition of the sulfides will reflect the sulfur isotope composition of the sulfur source. 34S values in the range –1.7‰ to 5.9‰ for this group of Lachlan Fold Belt ores overlap the range of 34S values for normal igneous sulfur (0 ± 2‰: Ohmoto & Rye 1979). This suggests that sulfur in these deposits was derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or, indirectly, through dissolution and recycling of rock sulfide in the host volcanic sequences. Skew of the 34S data for this group towards 34S-enrichment, with several 34S values near 6‰, may indicate that a minor component of sulfur

Table 2 Summary sulfur isotope data for ore sulfides from group 2 deposits in the Hill End Trough. Deposit

34S range o/oo

34S mean o/oo

Reference

Calula Commonwealth Cordilleraa Kempfield Peelwood minea Sunny Corner

4.0–7.6 3.3–10.1 2.6–7.5 3.0–11.7 5.1–9.9 1.7–10.7

6.1 7.9 5.4 Insufficient data 8.1 7.8

P. K. Seccombe & R. G. Skirrow unpubl. data James 1984 Brown 1999; present study Burns & Smith 1976; present study Brown 1999; present study Seccombe et al. 1984

a

131

Excluding data for probable early biogenic sulfides, or sulfides from syntectonic veins.

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was derived from reduced seawater sulfate. The data also conform closely to sulfur isotope distributions for pyrite and base-metal sulfides associated with modern sea-floor hydrothermal vents, in which magmatic and rock sulfide make a major contribution to the sulfur budget [e.g. East Pacific Rise: 34S range 1.4–4.5‰ (Arnold & Sheppard 1981; Zierenberg et al. 1984)]. The 34S values for the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine, and Sunny Corner (group 2) are similar. The average 34S values for these deposits range from 5.4‰ to 8.1‰ (Table 2). Pyrrhotite is generally absent, or exists only as a minor or trace mineral, in this group of deposits. Moreover, some of the deposits contain zones of massive barite, especially Kempfield, Commonwealth and Stringers (Table 1). Unlike the group 1 deposits (pyrrhotite-bearing), this second group of VHMS ores appears to have formed from ore fluids that were more oxidising, with a relatively high content of aqueous sulfate, in addition to sulfide in the hydrothermal fluids (i.e. SO42–/H2S  1). 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, 34S values higher than normal igneous sulfur values are anticipated in the sulfides (Ohmoto et al. 1983). High 34S values are typical for sulfides of VHMS origin throughout the Phanerozoic, particularly in host-rock sequences dominated by felsic volcanics (Hutchinson 1973; Ohmoto & Rye 1979). The source of sulfate in the Lachlan Fold Belt deposits, attributed to Silurian seawater, is discussed below. The remaining three deposits of the present study (Stringers, Elsinora and John Fardy) show higher 34S sulfide values than other group 2 deposits. In the case of the Stringers deposit, whereas barite 34S values are consistent with other group 2 barite 34S data, only two sulfide analyses are available (Table 1). At the Elsinora deposit, pyrite 34S data include outlying values as high as 22.5‰, but the 34S values for sphalerite and galena (34S range 7.0–9.9‰) are similar to other group 2 deposits. Although sampling is limited at Elsinora, the results suggest that sulfur in this system was derived dominantly from a seawater source. Brown (1999) inferred multiple sources of sulfur for the John Fardy deposit at Peelwood. A clear stratigraphic control on 34S composition of sulfides is evident: 34S values for pyrite from black shales (–2.1‰ and –1.9‰) are lower than those for chalcopyrite and pyrite from a cherty exhalite horizon adjacent to the ore horizon (range 4.5– 11.4‰; mean 8.3‰). The massive sulfide zones have higher 34S values (11.9–14.5‰; mean 13.2‰), similar to a pyrite– silica zone (13.1–15.4‰; mean 13.8‰) at the Central Hill adit. The 34S values for the ore zone (massive sulfide) at John Fardy are the highest in our dataset, suggesting a greater sulfur contribution derived from the reduction of seawater sulfate, compared to other VHMS deposits in the Hill End Trough. The near-zero 34S values (–2.1‰ and –1.9‰) for pyrite in black shales at John Fardy (Figure 4) are likely to represent a rock sulfide signature, with no significant contribution from fluids involved in the formation of the massive sulfides at John Fardy. Pyrite 34S values as low as –7.4‰ in

black shales at the nearby Cordillera mine probably reflect a component of biogenic sulfur, incorporated in the synsedimentary environment. By contrast, the 34S composition of sulfides associated with later syntectonic vein quartz from the district have a range of 1.1–1.9‰. Vein sulfides have compositions that are intermediate among the 34S range for the various sulfide-bearing lithologies in the Peelwood district and may represent remobilised sulfides. Variations in the sulfur isotope signature at a depositand district-scale are influenced by a number of factors including 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; Huston 1999). The preservation of distinct sulfur isotope signatures in three very similar massive sulfide deposits in the Peelwood district (Peelwood, John Fardy, Cordillera: Figure 2) along a >3 km strike-length of the host stratigraphy suggests that the hydrothermal systems responsible for these deposits were small. This is in contrast to the district-scale consistency in the sulfur isotope data for large economic VHMS deposits in the Lachlan Fold Belt, such as Woodlawn (Ayres et al. 1979) and Captains Flat (Stanton & Rafter 1966). It is important to note that, despite variations in the sulfur isotope data, the lead isotope signatures for all of the deposits in the Peelwood area are similar (Seccombe et al. 2000). This suggests that lead was derived from a similar crustal source in all cases, and similar Silurian model ages are inferred for all VHMS deposits in the Peelwood district. The sulfur isotopic composition of barite in VHMS ores of the Hill End Trough and elsewhere in the eastern Lachlan Fold Belt is invariably enriched in 34S. Most 34S data for barite in our study occupy the 23.4–38.3‰ range, with more than 75% of the samples having a sulfur isotope composition between 23.4‰ and 30.6‰. Host rocks of the barite-bearing VHMS deposits in the Hill End Trough are of Late Silurian to earliest Devonian age. The age of the Gleneski Formation, host to the Commonwealth and Stringers deposits, is Late Silurian (mid-Ludlow) to ?earliest Devonian (Morgan et al. 1999b). A Late Silurian age is accepted for the Kangaloolah Volcanics, which include the Kempfield barite deposit. Further south in the Captains Flat–Goulburn Trough barite 34S data (Table 3) are available for the Gurrundah barite deposit (Maier 2002) and the Woodlawn polymetallic VHMS deposit (Ayres et al. 1979; Glen et al. 1995). Stratiform lenses of massive barite–pyrite mineralisation at Gurrundah (Gilligan 1975; Maier 2002) are confined to the Wet Lagoon Volcanics of the Campbells Group of probable mid-Silurian age (Wenlock to early Ludlow: I. Percival pers. comm. 2002). A Late Silurian age is assigned also to the felsic volcanic rocks of the Woodlawn Volcanics that host the Woodlawn massive sulfide deposit (Felton & Huleatt 1977; Abell 1991). The sulfur isotopic composition of seawater sulfate during Late Silurian to earliest Devonian time was between 24‰ and 25‰ (Claypool et al. 1980). Such values correspond generally to the lower end of the range of 34S data for barite from the eastern Lachlan Fold Belt. Hence it

Sulfur isotopes, Hill End Trough, NSW is inferred that barite with a 34S signature close to 25‰ formed from barium of hydrothermal origin in combination with coeval seawater sulfate. Consistency among these barite 34S data provides supporting evidence that the Commonwealth, Stringers, Kempfield, Gurrundah and Woodlawn systems formed concurrently with the Late Silurian volcanic event in the eastern Lachlan Fold Belt. Barite with 34S values significantly higher than 25‰ among the Lachlan Fold Belt barite samples can be attributed to closed-system isotopic fractionation during partial reduction of seawater sulfate (Maynard & Okita 1991). Rye et al. (1978) also noted that the sulfur isotopic composition of barite responds to the timing of mineral growth from syngenetic to diagenetic stages. During diagenesis, progressive enrichment of 34S is evident in barite due to preferential removal of 32S by pyrite formation. Thus, diagenetic barite may record significant 34S enrichment relative to contemporaneous seawater sulfate (Maynard & Okita 1991). 34 S depletion noted in one barite sample from the Kempfield deposit (34S = 12.6‰: Table 1) may represent a hybrid signature, involving seawater sulfate mixed with sulfate of hydrothermal origin produced from the oxidation of vented H2S in the sea-floor environment. A similar explanation for 34 S depletion in barite was proposed by Seccombe et al. (1990, 1991) to account for barite with anomalously 'light' 34S signatures in the range 5.1–12.2‰, encountered in the Upper Devonian Buttle Lake VHMS deposits in British Columbia, Canada. In that district the mixing proportions for sulfate of hydrothermal and seawater origins determined the resulting 34S value of the 'light' barite. Temperatures of barite and sulfide formation for baritebearing deposits can be estimated using sulfur isotope geothermometry (Ohmoto & Rye 1979; Ohmoto & Goldhaber 1997). For those deposits where sufficient 34S data are available for coexisting sulfate and sulfide minerals (Table 3; Appendix 1), mean calculated temperatures range from 338C (Commonwealth) to 379C at Kempfield. Using the data of Maier (2002), a calculated mean sulfur isotope temperature of 352C was obtained from coexisting barite and pyrite at the Gurrundah barite deposit in the Captains Flat – Goulburn Trough. These estimates are based on the isotopic fractionation factors of Ohmoto and Lasaga (1982) and involve errors of ±30C. Despite reasonable agreement

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between these calculated temperatures, those recorded for modern high-temperature seafloor vents (Seyfried et al. 1999), and those inferred for examples of VHMS mineralisation in the geological record (Large 1992), the results should be treated with caution. The principal problem involves a demonstrated lack of isotopic equilibrium among aqueous sulfate and sulfide in vent fluids associated with VHMS ores (Ohmoto et al. 1983), which may lead to an overestimation of true depositional temperatures. Isotopic disequilibrium is evident among different sulfide phases for many coexisting sulfide pairs in the overall dataset for the present study. Although different sulfide minerals extracted from the same drillcore interval or deposit may show the correct trend of isotopic fractionation for sulfide mineral pairs formed in isotopic equilibrium (34S value for pyrite > sphalerite > chalcopyrite > galena: Ohmoto & Rye 1979), there are many exceptions, and isotopic reversals are common. Replacement of early formed sulfides by later formed sulfides during deposit formation is commonly observed in VHMS systems (Eldridge et al. 1983). Additionally, the recrystallisation of ductile sulfides as a result of metamorphic overprinting at greenschist grade is commonly encountered in Lachlan Fold Belt VHMS systems. Therefore, isotopic disequilibrium is to be expected (Eldridge et al. 1983). Thus the application of sulfur isotope geothermometry to coexisting sulfide minerals in many VHMS systems in the Lachlan Fold Belt is unlikely to produce meaningful data relating to sulfide deposition. The sulfur isotope distribution appears to be independent of the host-rock units, and their geographical distribution in the Hill End Trough. However, there appears to be a relationship linking the sulfur isotopic composition of different deposits to defined centres of felsic volcanism and intrusion. Agnew (2002, 2003) noted the presence of subvolcanic bodies of rhyolite emplaced in an inferred rhyolite cryptodome in the footwall to the Lewis Ponds deposit. Cross-sections constructed through the Mt Bulga deposit indicate bodies of massive rhyolite and dacite adjacent to the massive sulfide lens (Campbell & Kirk 1974; Chisholm 1976). The Accost occurrence is hosted by the Dungeree Volcanics, which are dominated by coherent rhyolite and dacite. These group 1 deposits have sulfur isotope signat-

Table 3 Comparative sulfur isotope data for volcanic-hosted massive sulfide-style mineralisation in the Captains Flat–Goulburn Trough, eastern Lachlan Fold Belt. Deposit

Commodities major (minor)

Mineralogy major (minor)

Host unit

34S (‰)

Captains Flat Gurrundah

Zn, Pb, Cu, Au, Ag, (S) Ba, (Zn, Pb, Cu, Ag)

py, sph, ga, (as, bou, cpy, au, po, ten) ba, py (cpy, ga, sph, as)

Kohinoor Volcanics Wet Lagoon Volcanics

Woodlawn

Zn, Pb, Cu, Ag, Au

py, sph, cpy, ga, tet, (cc, cov, ba)

Woodlawn Volcanics

mixed sulfides = 6.0–10.2a py = –13.7, 6.0–11.6b ba = 27.8–30.9b py = 6.7–9.2c cpy = 6.4–7.0c ga = 2.8–5.5c sph = 5.2–8.6c ba = 26.9–30.7, 35.4c,d

as, arsenopyrite; au, gold; ba, barite; bo, bornite; bou, bournonite; cc, chalcocite; cov, covellite; cpy, chalcopyrite; ga, galena; py, pyrite; po, pyrrhotite; sph, sphalerite; ten, tennantite; tet, tetrahedrite. 34S data: aStanton & Rafter (1966); bMaier (2002); cAyres et al. (1979), ore horizon only; dGlen et al. (1995). Commodity and mineralogical data: NSW Department of Minerals Resources Metallic Mineral Occurrence (METMIN) Database.

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ures (34S range –1.7 to 5.9‰) that reflect a greater contribution of magmatic sulfur. By contrast, the John Fardy, Cordillera, Sunny Corner, Peelwood mine, Calula, Kempfield and Commonwealth–Stringers areas are confined to clastic units and reworked volcanogenic sediments, with lesser amounts of coherent volcanics in the local stratigraphy. This latter group of deposits is characterised by 34 S enrichment. They have signatures indicating a lesser contribution of magmatic sulfur that can be explained by the distal location of mineralisation relative to a volcanic centre. An exception to this is the Belara deposit, which is hosted by reworked felsic volcanic rocks of the Piambong Formation and which has a pronounced magmatic sulfur isotope signature.

CONCLUSIONS Sulfur isotope distribution in VHMS mineralisation from the Late Silurian Hill End Trough of the eastern Lachlan Fold Belt is reported from a sample base comprising 105 new analyses, and a compilation of earlier, largely unpublished data. Results of the sulfur isotope studies show that sulfur was derived from a range of reservoirs, including contributions from: magmatic (rock sulfide); reduced seawater sulfate; and biogenic sulfur. Two principal groupings of 34S distribution are apparent from the data. Group 1 deposits (Lewis Ponds, Mt Bulga, Belara and Accost) are located generally near inferred felsic volcanic centres, have 34S values in the range from –2–6‰, and are influenced strongly by sulfur of magmatic or rock sulfide. Group 2 deposits (Calula, Commonwealth, Cordillera, Kempfield and Sunny Corner deposits, and Peelwood mine) are located in more distal volcano-sedimentary environments, have 34S signatures that range from 5.4‰ to 8.1‰, and indicate a mixed sulfur contribution from seawater and magmatic sources. In addition, sulfur of metamorphic origin has been identified in syntectonic quartz–sulfide veins that overprint VHMS mineralisation in the Peelwood district. The 34S analysis of barite from the VHMS deposits (e.g. Commonwealth, Stringers and Kempfield) indicates a clear contribution from sulfate of seawater origin and Late Silurian age, with a 34S composition commonly in the range 23–31‰. Sulfate–sulfide isotope geothermometry indicates depositional temperatures near 350C for some barite-bearing deposits, although these results need to be treated with caution. The basin-wide approach adopted for the present study has been important in providing constraints on the relative contribution of the various sulfur reservoirs to individual systems. This in turn suggests that, although sulfur isotope studies may not provide vectors to mineralisation for mineral exploration, such studies are useful in discriminating between multiple small systems in (relatively) close spatial proximity and larger systems that may have greater economic significance. The present study highlights the need to undertake well-constrained and systematic sulfur isotope studies to provide complementary information about the source of sulfur in individual deposits, to contribute to a better understanding of the variation in the depositional environ-

ments for VHMS systems, and to provide information useful to mineral explorers.

ACKNOWLEDGEMENTS We wish to thank Remy Dehaan and Craig Parry for providing samples from the Belara and Kempfield areas. Richard Bale kindly assisted with sample preparation at the University of Newcastle. We are grateful to Anita Andrew and Brad McDonald, CSIRO Division of Exploration and Mining, North Ryde, for undertaking the isotopic analyses through the Centre for Isotope Studies, a collaborative facility involving New South Wales universities and the CSIRO. Phillip Carter and Olivier Rey Lescure drafted the figures. We are grateful to Vlad David, Jyrki Pienmunne, Richard Facer, Ken McQueen and Michael Agnew for reviewing drafts of this paper. The present study has been supported, in part, by funding from the University of Newcastle. Publication by Peter Downes is with the permission of the Director-General, New South Wales Department of Mineral Resources.

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Received 27 June 2003; accepted 12 December 2003

APPENDIX 1: NEW SULFUR ISOTOPE DATA FOR VOLCANIC-HOSTED MASSIVE SULFIDE-TYPE MINERALISATION FROM THE HILL END TROUGH, EASTERN LACHLAN FOLD BELT Prospect

Sample no.

Location

Mineral

34S (‰)

Reference

Accost

ACC1/2

Accost

ACC1/3

Galena Pyrite Pyrite

1.4 3.4 3.0

Present study Present study Present study

Accost

ACC1/4

Pyrite

2.7

Present study

Accost

ACC1/5

Pyrite

3.0

Present study

Accost

ACC1/6

Accost

ACC1/7

Pyrite Pyrite (repeat) Pyrite

2.5 2.2 1.9

Present study Present study Present study

Accost

ACC1/8

Belara

BE 1

Australian Anglo-American DDH1 (4862/3075): 12.10–12.15 m Australian Anglo-American DDH1 (4862/3075): 11.50–11.55 m Australian Anglo-American DDH1 (4862/3075): 91.85–91.90 m Australian Anglo-American DDH1 (4862/3075): 106.10–106.1 m Australian Anglo-American DDH1 (4862/3075): 131.55–131.60 m Australian Anglo-American DDH1 (4862/3075): 92.30–92.35 m Australian Anglo-American DDH1 (4862/3075): 14.12–14.17 m Mine dump

Belara

BE 2

Mine dump

Belara Belara Belara Belara Belara Belara Belara Calula

BE 3 BE 4 BE 4/1 BE 4/2 BE 5 BE 5/1 BE 5/2 SK247

Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump DDH2: 73.9

Galena Pyrite Chalcopyrite Pyrite Chalcopyrite Pyrite Sphalerite Sphalerite (repeat) Pyrite Sphalerite Pyrite Pyrite Pyrite Sphalerite Sphalerite Pyrite

1.9 3.2 2.8 2.8 2.3 3.8 3.6 3.6 3.4 2.2 1.2 3.7 4.3 2.8 3.0 7.6

Calula

SK248

DDH2: 73.9–89.3 m

Pyrite

6.3

Calula

SK256

DDH2: 119–154.5 m

Pyrite

6.3

Calula

SK268

DDH4: 26.5 m

Pyrite

6.2

Calula

SK270

DDH4: 28.7–31.4 m

Pyrite

6.1

Calula

Sk272

DDH4: 31.4 m

Pyrite

6.8

Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data)

Sulfur isotopes, Hill End Trough, NSW 34S (‰)

Prospect

Sample no.

Location

Mineral

Calula

SK273

DDH4: 31.4 m

Pyrite

7.4

Calula

SK296

Mine dump

Pyrite

4.0

Calula

SK297

Mine dump

Pyrite

4.3

Calula

SK298

Mine dump

Pyrite

7.0

Calula

SK299

DDH2: 77.7 m

Pyrite

5.4

Calula

SK300

Mine dump

Pyrite

4.9

Calula

SK301

Mine dump

Pyrite

5.8

Calula

SK302

Mine dump

Pyrite

6.4

Calula

SK303

Mine dump

Pyrite

5.6

Calula

SK304

Mine dump

Pyrite

7.1

Calula

SK305

Mine dump

Pyrite

6.9

Calula

SK306

Mine dump

Pyrite

6.0

Calula

SK307

Mine dump

Pyrite

5.7

Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth

DDHCM4-104.0 DDHCM4-106.5 DDHCM4-107.9 DDHCM4-109.6 DDHCM4-119.2 DDHCM4-127.3 DDHCM4-132.4

DDHCM4: 104.0 m DDHCM4: 106.5 m DDHCM4: 107.9 m DDHCM4: 109.6 m DDHCM4: 119.2 m DDHCM4: 127.3 m DDHCM4: 132.4 m

Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Commonwealth Cordillera

DDHCM4-133.8 13 21 9 20 14 C1 C2 C3 C4 CORDDH1/1

DDHCM4: 133.8 m Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Peelwood DDH1: 112.9 m

Cordillera Cordillera Cordillera

CORDDH1/10 CORDDH1/2 CORDDH1/3

Peelwood DDH1: 131.5 m Peelwood DDH1: 116.1 m Peelwood DDH1: 120.5 m

Cordillera

CORDDH1/4

Peelwood DDH1: 121.5 m

Cordillera

CORDDH1/5

Peelwood DDH1: 121.9 m

Cordillera

CORDDH1/6

Peelwood DDH1: 122.4 m

Cordillera

CORDDH1/7

Peelwood DDH1: 123.6 m

Cordillera

CORDDH1/8

Peelwood DDH1: 124.0 m

Cordillera

CORDDH1/9

Peelwood DDH1: 125.1 m

Cordillera

SB08/2

Peelwood DDH1 (depth not recorded)

Pyrite Pyrite Pyrite Pyrite Pyrite/Sphalerite Sphalerite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite/Sphalerite Pyrite/Sphalerite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite Sphalerite Pyrite Sphalerite Pyrite Sphalerite Galena Sphalerite Chalcopyrite Sphalerite Pyrite Sphalerite Galena Pyrite Sphalerite Pyrite Sphalerite Pyrite Sphalerite Pyrite

3.3 7.2 5.3 9.5 7.4 8.2 8.8 10.1 10.0 7.7 7.0 9.8 7.4 8.2 23.4 34.6 27.5 28.5 –1.9 7.5 1.3 7.5 –3.3 5.4 4.4 5.7 3.9 5.5 –7.4 6.0 2.6 –5.8 7.2 –0.5 6.2 4.8 4.7 1.0

137

Reference

Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) Seccombe and Skirrow (unpubl. data) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) James (1984) Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Brown (1999)

138

P. M. Downes and P. K. Seccombe

Prospect

Sample no.

Location

Mineral

34S (‰)

Reference

Cordillera Cordillera Cordillera Elsinora

SB09 SB10 SB10/2 ELSDDH1/1

Peelwood DDH1 (depth not recorded) Peelwood DDH1 (depth not recorded) Peelwood DDH1 (depth not recorded) Elsinora DDH1: 79.0 m

Elsinora

ELSDDH1/2

Elsinora DDH1: 83.4 m

Elsinora Elsinora Elsinora Elsinora Elsinora Elsinora Elsinora John Fardy

ELSDDH1/3 ELSDDH1/4 ELSDDH1/6 ELSDDH3B/1 ELSDDH3B/2 ELSDDH3B/3 ELSDDH3B/4 SB12

4.1 4.8 –2.2 9.6 9.6 9.9 8.3 7.0 7.2 10.9 12.6 22.5 18.5 8.4 13.3 1.1

Brown (1999) Brown (1999) Brown (1999) Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Brown (1999)

John Fardy

SB12/3

Pyrite

1.5

Brown (1999)

John Fardy

SB13a

Pyrite

13.3

Brown (1999)

John Fardy

SB13b/2 SB14 SB14/2 SB14/3 SB15

John Fardy John Fardy John Fardy John Fardy John Fardy John Fardy

SB16 SB17 SB18 SB19a SB19b SB20a

JF34: 88.1 m JF2: 37.5 m JF3: 58.5 m JF2: 58.5 m JF2: 58.5 m JF3: 65.2 m

John Fardy John Fardy John Fardy John Fardy John Fardy John Fardy John Fardy John Fardy Kempfield Kempfield Kempfield Kempfield

SB20b SB21 SB22 SB23a SB23a/2 SB23b SB23b/2 SB24 HS1 HS2 KS1 KS2

JF3: 65.2 m JF2: 50.0 m JF4: 34.1 m JF4: 35.0 m JF4: 35.0 m JF4: 35.0 m JF4: 35.0 m JF6: 33.5 m Outcrop Henry’s Adit/Patience Outcrop Henry’s Adit/Patience Outcrop GKF030: 90–92 m

Kempfield Kempfield

KS3 KS4

GKF059: 42–44 m GKF037: 58–60 m

Kempfield

KS6

GKF074: 97–98 m

Kempfield Kempfield Kempfield Lewis Ponds Lewis Ponds Lewis Ponds Lewis Ponds

KS7 KS8 KS9 TLPD 18/1 TLPD 18/2 TLPD 18/3 TLPD 18/4

GKF074: 101–103 m Outcrop in quarry GKF074: 101–103 m Main zone TLPD18: 334 m Main zone TLPD18: 374.2 m Main zone TLPD18: 386 m Main zone TLPD18: 372.5 m

Lewis Ponds

TLPD 53/1

Toms zone TLPD53: 275.6 m

Lewis Ponds

TLPD 53/2

Toms zone TLPD53: 294.7 m

Pyrite Pyrite (repeat) Pyrite Pyrite Pyrite Pyrite Pyrite (repeat) Pyrite Pyrite Pyrite Pyrite Chalcopyrite Pyrite Pyrite (repeat) Chalcopyrite Pyrite Pyrite Pyrite Pyrite Sphalerite Sphalerite Pyrite Pyrite Pyrite Arsenopyrite Galena Pyrite Barite Barite Pyrite Barite Pyrite Barite Barite Pyrite Pyrite Pyrite Pyrite Galena Sphalerite Chalcopyrite Pyrite Pyrite

12.8 11.7 15.4 13.5 13.1 –2.2 –1.9 1.9 7.5 13.5 10.7 11.4 4.6 4.5 9.2 10.2 12.4 14.5 12.8 14.1 13.0 11.9 10.7 11.7 5.8 9.0 9.9 29.0 27.0 2.0 25.1 15.2 12.6 27.2 17.4 4.9 4.8 4.9 1.7 3.3 4.3 5.0 4.5

Brown (1999)

John Fardy John Fardy John Fardy John Fardy

Elsinora DDH1: 93.9 m Elsinora DDH1: 114.9 m Elsinora DDH1: 130.7 m Elsinora DDH3B: 295.2 m Elsinora DDH3B: 302.7 m Elsinora DDH3B: 320.3 m Elsinora DDH3B: 327.2 m Central Hill PWD80 (depth not recorded) Central Hill PWD80 (depth not recorded) Central Hill PWD80 (depth not recorded) Central Hill PWD80 (depth not recorded) Central Hill adit dump Central Hill adit dump Central Hill adit dump JF3: 77.0–m

Pyrite Chalcopyrite Pyrite Galena Pyrite Sphalerite Pyrite Sphalerite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite

Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study

Sulfur isotopes, Hill End Trough, NSW Prospect

Sample no.

Location

Mineral

34S

Reference

(‰) Lewis Ponds Lewis Ponds Lewis Ponds

TLPD 53/3 TLPD 53/4 TLPD 66/1

Toms zone TLPD53: 302.8 m Toms zone TLPD53: 312 m Footwall stockwork zone TLPD66: 367.4 m

Lewis Ponds

TLPD 66/2

Mt Bulga

7/102.46

Toms zone (equivalent) TLPD66: 217.3 m DDHMB7: 102.46

Mt Bulga

7/105.28

DDHMB7: 105.28

Mt Bulga

8/93.65

DDHMB7: 93.65

Mt Bulga

9/80.77

DDHMB7: 80.77

Mt Bulga

26/69.70

DDHMB7: 69.70

Mt Bulga

34/203.22

DDHMB7: 203.22

Peelwood mine Peelwood mine Peelwood mine

PM01 PM02 PM03

Mine dump Mine dump Mine dump

Peelwood mine

PM04

Mine dump

Peelwood mine

PM05

Mine dump

Peelwood mine

PM06

Mine dump

Peelwood mine

PM07

Mine dump

Peelwood mine

PM08

Mine dump

Peelwood mine

PM09

Mine dump

Peelwood mine Peelwood mine Peelwood mine Peelwood mine Peelwood mine Peelwood mine Peelwood mine Stringers Stringers Stringers Stringers Stringers Stringers Stringers Stringers

SB01 SB02 SB03 SB04 SB05 SB06 SB07 10 11 S1 S2 S3 S4 S5 S6

PD2: 32.3 m APR31: 100.6 m APR32: 91.5 m APR32: 162.0 m APR31: 76.6 m PD2: 41.8 m Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump Mine dump

All 34S values have been rounded to one decimal place. The Calula mine is also known as the Pyrite mine.

Pyrite Pyrite Chalcopyrite Pyrite Pyrite (duplicate) Pyrite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Galena Pyrite Pyrrhotite Sphalerite Pyrite Sphalerite Chalcopyrite Sphalerite/galena Chalcopyrite Sphalerite Sphalerite Sphalerite/galena Chalcopyrite Sphalerite Chalcopyrite Sphalerite/galena Chalcopyrite Sphalerite/galena Chalcopyrite Sphalerite Pyrite Chalcopyrite Pyrite Pyrite Pyrite Sphalerite Pyrite Pyrite Pyrite Barite Barite Barite Barite Barite Barite

3.8 4.2 5.1 5.9 5.7 4.7 3.5 0.0 3.1 2.1 2.1 –1.7 2.7 1.5 1.5 –1.6 3.1 1.5 1.5 0.1 3.0 2.6 2.5 –1.0 3.7 2.9 3.0 0.3 3.0 1.5 1.5 8.0 8.8 9.9 9.8 9.6 9.1 7.6 8.6 7.6 8.4 8.6 8.7 8.8 8.5 6.7 6.6 7.2 6.4 –0.7 5.1 0.1 6.7 8.7 10.5 15.1 30.0 30.6 26.9 38.3 28.2 27.4

Present study Present study Present study Present study Present study Present study Present study Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Chisholm (1976) Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) Brown (1999) James (1984) James (1984) Present study Present study Present study Present study Present study Present study

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