Australian Journal of Earth Sciences (2004) 51, 1–14
Geochronological constraints on the polymetamorphic evolution of the granulite-hosted Challenger gold deposit: implications for assembly of the northwest Gawler Craton* A. G. TOMKINS,1† W. J. DUNLAP2 AND J. A. MAVROGENES1,2 1 2
Department of Geology, Australian National University, ACT 0200, Australia. Research School of Earth Sciences, Australian National University, ACT 0200, Australia. A temperature–time history for the granulite-hosted Challenger gold deposit in the Christie Domain of the Gawler Craton, South Australia, has been derived using a range of isotopic decay systems including U–Pb, Sm–Nd, Rb–Sr and 40Ar/39Ar. Nd model ages and detrital zircon ages suggest a protolith age of ca 2900 Ma for the Challenger Gneiss. Gold mineralisation was probably introduced under greenschist/amphibolite-facies conditions towards the end of the Archaean, between 2800 and 2550 Ma. However, evidence for the exact age and P–T conditions of this event was almost completely removed by granulite-facies metamorphism during the Sleafordian Orogeny, which peaked around ca 2447 Ma. Cooling to 350C occurred before 2060 Ma. It is possible that the Christie Domain was then subject to further sedimentation and volcanism in the period ca 2000–1800 Ma before reburial and a second period of orogeny around ca 1710–1615 Ma. During this second orogeny, the eastern Christie Domain experienced heterogeneous fluid-induced retrograde metamorphism at lower greenschist- to amphibolite-facies conditions, with metamorphic grade varying between structural blocks. At this time, the Challenger deposit was subject to greenschist-facies conditions (not significantly hotter than 350C), while at Mt Christie (50 km to the south) lower amphibolite-facies conditions prevailed and to the west the Ifould Block experienced extensive plutonism. A third very low-temperature thermal pulse around ca 1531 Ma, which reached ~150–200C, is recorded at the Challenger deposit. It is likely that the global Grenvillian Orogeny (1300–1000 Ma) was a major period of domain exhumation and juxtaposition. KEY WORDS: Archaean, Challenger, Gawler Craton, geochronology, gold, tectonics.
INTRODUCTION Due to the paucity of outcrop, lack of commercial interest and complex geology, the northwest Gawler Craton in South Australia is probably the least understood semiexposed continental basement region in Australia. Previous workers (initially Thomson 1980) divided the craton into a series of domains of similar magnetic character that are separated by laterally extensive shear zones (Figure 1). The history of each of these domains varies considerably, and comparisons between these domains have been used to derive the current evolutionary model put forward by Teasdale (1997) and Daly et al. (1998). However, these studies rely on correlation across large distances, and they cannot offer a complete understanding of a terrane characterised by fault-bound crustal blocks with heterogeneous metamorphic and structural histories. For example, the current understanding of the Christie Domain is primarily based on analysis of gneisses and intrusions at Lake Ifould and at Mt Christie, two areas that are ~200 km apart and separated by several crustal-scale shear zones (Figure 2). To go beyond the current understanding of regional tectonics in this area, an analysis of the separate structural blocks within each larger domain is required. The majority of geochronological studies undertaken in the northwest Gawler Craton have utilised only zirconbased techniques, a method that records only high-temper-
ature zircon-forming events. Although a small amount of dating of other minerals has taken place at some localities, no study has yet employed a broad range of isotopic systems to evaluate a more complete metamorphic history. This is despite the commonly held perception that two high-temperature metamorphic cycles may have influenced the region (Fanning 2002). In the present study we use a range of geochronometers to evaluate the timing of metamorphic and thermal events that affected the Challenger gold deposit. The Challenger Gneiss (the granulite facies gneisses that host the deposit) is located in the Christie Domain (Figures 1, 3). We use 40 Ar/39Ar dating of biotite, retrograde muscovite and K-feldspar and Rb–Sr dating of biotite to investigate the cooling history of these rocks. The results are tied to the existing whole-rock, garnet, zircon and monazite geochronological data to develop a temperature–time path for
†
Corresponding author and present address: Department of Geology & Geophysics, University of Calgary, Calgary, AB Canada T2N1N4 (
[email protected]). *Tables 4–14 [indicated by an asterisk (*) in the text and listed at the end of the paper] are Supplementary Papers; copies may be obtained from the Geological Society of Australia’s website (http://www.gsa.org.au) or from the National Library of Australia’s Pandora archive (http://nla.gov.au/nla.arc-25194).
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the evolution of the Challenger Gneiss. This will not only enable a better understanding of the Challenger Gneiss, but it will also provide some new constraints on the tectonic evolution of the northwest Gawler Craton.
REGIONAL GEOLOGY OF THE WESTERN GAWLER CRATON The Gawler Craton consists of a diverse association of geological units dating from the Late Archaean to Mesoproterozoic (Drexel et al. 1993). The basement rocks are divided into two broad groupings, the Archaean Sleaford and Mulgathing Complexes (Figure 3), which outcrop sporadically in the western and northwestern portions of the craton (Daly & Fanning 1993; Drexel et al. 1993; Teasdale 1997; Daly et al. 1998). They include paragneisses and orthogneisses with mafic to ultramafic intrusive and extrusive rocks. The protolith age of the Sleaford and Mulgathing Complexes has been variously estimated at between 3150 and 2950 Ma, based on detrital zircon ages and Nd whole-rock ages (Daly & Fanning 1993; Daly et al. 1998). Studies conducted on the Sleaford Complex in the Eyre Peninsula (Figures 1, 3), summarised by Daly and Fanning (1993), indicate peak metamorphism of ~860C and 700– 900 MPa at 2450 Ma (Fanning et al. 1986; Webb et al. 1986) during the Sleafordian Orogeny of Thomson (1980). The
Sleafordian Orogeny (2637–2300 Ma) is inferred to have affected the entire basement of the Gawler Craton (Daly & Fanning 1993). The Archaean Christie Gneiss, which is the basal unit of the Mulgathing Complex (Daly & Fanning 1993), is considered to be correlative with the Challenger Gneiss (Daly et al. 1998). At Mt Christie, the type locality for the Christie Gneiss (Figure 3), Daly et al. (1998) interpreted peak metamorphism to have occurred during the Sleafordian Orogeny. At this same locality, Teasdale (1997) estimated these peak conditions at 750–800C and 450– 550 MPa, and confirmed the timing of peak metamorphism by dating zircons to between 2450 and 2420 Ma. Initially recognised on the Eyre Peninsula, the Kimban Orogeny (1845–1700 Ma), involved peak metamorphic conditions of 800–850C and 700–900 MPa in the southern Eyre Peninsula, and 600–675C and 500–700 MPa in the central Eyre Peninsula (Parker 1993). Retrograde metamorphism has been inferred at Mt Christie during the Kimban Orogeny, although the P–T conditions for this event were not firmly established (Daly & Fanning 1993). Further deformation and metamorphism is interpreted by Daly et al. (1998) to represent the Kararan Orogeny, associated with collision between the Mawson Continent of Fanning et al. (1996) and the proto-Yilgarn Craton. Localised ultrahigh temperature metamorphism, assigned to the Kararan Orogeny and associated with the Karari Fault Zone (near the western edge of the craton), has been dated at
Figure 1 Structural domains as defined by Thomson (1980) and refined by Teasdale (1997) and Daly et al. (1998). Domains studied by Teasdale (1997) are named in bold and contain information on grade and age of peak metamorphism. Challenger is situated in the Christie Domain. Rectangles show the location of Figures 2 and 3.
Thermochronometry, Challenger gold deposit ca 1650 Ma. Extensive granitic plutonism occurred over the period 1680–1670 Ma near Lake Ifould in the western Christie Domain (Teasdale 1997). Subsequent granulitefacies metamorphism (ca 1565–1540 Ma) assigned to the Kararan Orogeny has been noted on the Mabel Creek Ridge and in the Coober Pedy Domain (Daly et al. 1998). In a comparative study of the evolution of Gawler Craton structural domains (Figure 1), Teasdale (1997) was able to define significant differences between each domain
Figure 2 (a) Imaged aeromagnetic data from the northwest Gawler Craton (total magnetic intensity, first vertical derivative). (b) Interpreted shear structures from (a). Domain boundaries in thick black lines are largely as interpreted and partially ground-truthed by Teasdale (1997). Smaller structural block boundaries are in grey. Thin black lines indicate subsidiary faults and general structural grain. Potential shear zones indicated by grey and thin black lines are purely interpretive and have not been ground-truthed. See Figure 1 for location.
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(Figure 4). Although granulite-facies metamorphism has affected many domains, the ages of peak metamorphism were commonly significantly different. In contrast, the Wilgena and Nuyts Domains were found to have experienced much lower grades of metamorphism than the adjacent Fowler and Christie Domains. Four subsidiary structural blocks also occur in the Fowler Domain (Teasdale 1997). Each of these blocks experienced different metamorphic histories, ranging from amphibolite facies
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and 500 MPa in the west to granulite facies and 1000 MPa in the east, representing ~20 km of variation in crustal depth in the Fowler Domain alone. Moreover, the timing of peak metamorphism in these blocks is variable: the western blocks experienced peak metamorphism at ca 1710 Ma and the eastern blocks, from 1560 to 1490 Ma. Three generations of regional shear zones were also delineated by Teasdale (1997). Although cross-cutting relationships suggest that some were active after ca 1425 Ma, circumstantial evidence from one shear zone (the Tallacootra Shear Zone) suggests that it was still active after ca 1175 Ma. A period of differential exhumation and lateral accretion along the crustal-scale shear zones (Figure 2) around ca 1150–1100 Ma was proposed. It has been suggested that a series of ca 1300–1000 Ma orogenic belts was instrumental in the assembly of the supercontinent Rodinia (Unrug 1997), and it may be that the western Gawler Craton was part of this globally interconnected orogenic system (the Grenvillian Orogen) (Davidson 1995; Teasdale 1997).
Regional structure and metamorphism of the Christie Domain A total magnetic intensity map of the northwest Gawler Craton, centred on the Christie Domain, is shown in Figure 2a. Using this image, and correlating with regional basement mapping, it is possible to define a series of northnortheast-trending, regionally extensive shear zones (Figure 2b). The oldest visible major structures in the Christie Domain trend east–west, and are locally rotated into parallelism with the north-northeast-trending structures. Note that many of these structures cannot be ground-truthed due to lack of exposure, although some have been partially ground-truthed by Teasdale (1997). The Christie Domain is dissected by several of these structures, for example, a north-northeast-trending lineament separates the Mt Christie and Challenger areas. Thus, there are similarities with the Fowler Domain, where Teasdale (1997) noted considerable variation in geological history across shear zones with a similar orien-
Figure 3 Regional geology of the Gawler Craton (simplified from Daly et al. 1998). Because much of the area shown is under cover, the distribution of rocks is based largely on interpretation of aeromagnetic data as well as sparse rock outcrops and a small number of drillholes. Note that Daly et al. (1998) do not distinguish between the Sleaford and Mulgathing Complexes. The Sleaford Complex has been identified and studied on the Eyre Peninsula, whereas the Mulgathing Complex has been identified and studied in the northwest Gawler Craton. See Figure 1 for location.
Thermochronometry, Challenger gold deposit tation. Consequently, similar variation in metamorphic grade and geochronology might be anticipated in the Christie Domain. There are subtle variations in metamorphic grade between the Challenger and Mt Christie areas. Thermobarometry for peak metamorphism suggests conditions of 800–850C and ~750 ± 150 MPa at the Challenger deposit (Tomkins & Mavrogenes 2002; Tomkins 2002), and 750–800C and 450–550 MPa for the Christie Gneiss (Teasdale 1997; Tomkins 2002). In addition, the retrograde metamorphism at the Challenger deposit is greenschist facies (see below), whereas at Mt Christie it may be lower amphibolite facies (based on green hornblende rims on orthopyroxene). Rb–Sr dating of retrograde biotite from Mt Christie gave an age of ca 1650 Ma (Teasdale 1997), an age that approximately corresponds to the late fluid-induced retrograde metamorphism event proposed for the Challenger area (below).
CHALLENGER GEOLOGY The economic geology and evolution of the Challenger gold deposit has been discussed in detail by Tomkins and Mavrogenes (2002). In addition, that study described the local geology and metamorphic petrology of the deposit,
Figure 4 Variation in geological history of several adjacent structural domains of the western Gawler Craton (simplified from Teasdale 1997). The results of the current study are not incorporated. The orogenic episodes outlined by Daly et al. (1998) and corresponding metamorphic/ plutonic episodes are italicised. The Grenvillian event was not discussed by Daly et al. (1998)
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and included information on thermobarometry. These details are briefly summarised here. The dominant lithology at the Challenger deposit is a garnet–biotite gneiss with localised cordierite and orthopyroxene (Figure 5a). Other lithologies include a volumetrically minor two-pyroxene granulite and a series of late mafic–ultramafic dykes and sills. Thermobarometry conducted on the gneisses (see Tomkins & Mavrogenes 2002 for a detailed discussion) suggests that the Challenger Gneiss has been subjected to P–T conditions of 750 ± 150 MPa and 800–850C, and the occurrence of orthopyroxene-bearing pelitic assemblages supports this result. Evidence of partial melting is recognised by the presence of stromatic migmatite. This migmatite is interpreted to be the consequence of a series of vapour-absent melting reactions resulting from the breakdown of muscovite and biotite (Tomkins & Mavrogenes 2002; Tomkins 2002). Although the mafic–ultramafic dykes and sills are unaffected by granulite-facies metamorphism, they are affected by greenschist-facies metamorphism. This is recognised by the presence of chlorite–biotite–actinolite assemblages, which are typical of the unit. In addition, the Challenger Gneiss is variably overprinted by greenschistfacies retrograde metamorphism, particularly in the vicinity of the dykes and sills, where very fine-grained
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(99% pure) of perthitic K-feldspar and biotite were obtained from samples AGT089, AGT090 and AGT091, and retrograde muscovite was obtained from samples AGT083 and AGT087. 40 Ar/39Ar step-heating results are presented as age spectra. Ages quoted for each sample are derived from the plateau-like segment of each age spectrum; they are apparent ages and their interpretation as related to actual events will be discussed below. Ages are step-size weighted means, based on percentage of 39Ar in the contributing steps. Correction factors to account for K- and Ca-derived Ar isotopes are (36Ar/37Ar)Ca = 3.5 10–4, (39Ar/37Ar)Ca = 7.86 10–4, and (40Ar/39Ar)K = 2.7 10–2. Data tables for stepheating experiments are given in Tables 4*–14*.
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The three samples of biotite from pristine Challenger Gneiss (unaffected by greenschist-facies retrograde metamorphism) give plateau-like ages over a considerable range. The youngest age was given by sample AGT090 at 1860 ± 8.8 Ma, and the oldest by sample AGT089 at 2060 ± 10.3 Ma. Sample AGT091 gave a plateau-like age of 1894 ± 6.1 Ma. None of these ages are within error of each other. The age spectra for these samples are presented in Figure 7a–c. Samples of muscovite from retrograde metamorphosed Challenger Gneiss give plateau-like ages of 1614 ± 6 Ma (sample AGT087) and 1615 ± 10 Ma (sample AGT083). The average age is 1614.5 Ma. Age spectra for these samples are shown in Figure 7d, e. Because K-feldspar is not a hydrous mineral, it is possible to extract argon at temperatures up to ~1100C
Table 2 Descriptions of the samples dated by the 40 Ar/39Ar technique. Sample
Lithology
Mineralogy
Description
AGT083, AGT087
Pelitic schist
Quartz, chlorite, muscovite (
