Anthony Reid1, 2
1 Geological Survey of South Australia, Department of the Premier and Cabinet
2 Department of Earth Sciences, University of Adelaide
Download this article as a PDF (0.7 MB); cite as MESA Journal 84, pages 23–27
Introduction
Hydrothermal processes that precipitate ore minerals are very sensitive to the thermal state of the crust into which those hydrothermal fluids move. This is because changes in fluid chemistry caused by drops in temperature and pressure are important factors in formation of hydrothermal ore deposits (Lester, Ord and Hobbs 2012). The most extreme changes in temperature and pressure occur above the brittle–ductile transition, and typically within the upper 10 km of the crust, and it is the upper portions of a mineral system that are typically the most prospective. Being able to demonstrate that a particular rock mass was at low temperature and therefore shallow crustal levels during a known mineralisation event has implications for understanding the mineral prospectivity of a region.
For example, in hematite-rich iron oxide – copper–gold (IOCG) systems, ore-bearing fluids are typically in the order of 200 to 300 °C and result from the mixing of multiple fluid sources, often with an input of magmatic fluid with starting temperatures >600 °C (Williams et al. 2005). Therefore, one of the indicators of prospectivity for IOCG systems includes the preservation of shallow crustal levels from the time of the mineralisation event.
This article summarises recently published 40Ar/39Ar thermochronology that investigated the region of crust between the Olympic Dam and Prominent Hill mines within the Olympic Cu–Au Province of the eastern Gawler Craton (Reid, Jourdan and Jagodzinski 2017; Fig. 1). Both of these deposits and the region between them are covered by up to 500 m of Neoproterozoic to Cenozoic sediment, making inferences about prospectivity reliant on geophysical interpretation and sparse drillhole information. The study aimed to determine the thermal history of the Archean rocks to see if there was any evidence for thermal or alteration effects associated with the c. 1.59 Ga event that formed the majority of the IOCG mineralisation in the Olympic Cu–Au Province (Skirrow et al. 2007).
Figure 1 Location of drillholes sampled in the Reid, Jourdan and Jagodzinski (2017) study. (a) Surface geology at 1:100,000 scale. (b) Bouguer gravity image. Inset shows location of the study area with respect to the Gawler Craton and the Olympic Cu–Au Province. Geological and geophysical data available via South Australian Resources Information Gateway (SARIG). Reprinted from Reid, Jourdan and Jagodzinski (2017; figs 2a and 2b) with the permission of the Geological Society of Australia.
Geological setting
The Gawler Craton is a Mesoarchean to Mesoproterozoic basement province in South Australia that contains evidence for multiple stages of crustal reworking and rejuvenation (Hand, Reid and Jagodzinski 2007; Reid and Hand 2012). The Olympic Cu–Au Province is a region of IOCG ± U mineralisation and related alteration that occurs along the eastern margin of the Gawler Craton (Skirrow et al. 2002; Skirrow et al. 2007). The Gawler Craton basement within the Olympic Cu–Au Province comprises Neoarchean granite and volcanics (Devils Playground Volcanics) that have been intercepted in mineral exploration drillholes between Olympic Dam and Prominent Hill (Cowley and Fanning 1991; Reid, Fricke and Cowley 2009; Reid, Jourdan and Jagodzinski 2017). This basement is intruded and overlain by Paleoproterozoic metasedimentary and igneous units including granites of the c. 1.85 Ga Donington Suite and the c. 1.75 Ga Wallaroo Group (Jagodzinski 2005). A major igneous event occurred at c. 1.59 Ga with the intrusion of granites of the Hiltaba Suite and extrusion of bimodal volcanics of the Gawler Range Volcanics (Johnson and Cross 1995; Allen et al. 2008; McPhie et al. 2011). Alteration and mineralisation related to this major thermal event occurred throughout the Olympic Cu–Au Province (Skirrow et al. 2007; Ciobanu et al. 2013; Reid et al. 2013).
The Olympic Cu–Au Province is buried beneath a veneer of Neoproterozoic to Cenozoic sedimentary successions including those associated with the Adelaide Rift Complex, along with Mesozoic rocks of the Eromanga Basin.
Sampling strategy
Reid, Jourdan and Jagodzinski (2017) attempted to constrain the thermal history and the timing of fluid flow by applying the 40Ar/39Ar method to magmatic mica and K-feldspar and hydrothermal sericite in rocks from mineral exploration drillhole SH 7 located in the region between Olympic Dam and Prominent Hill (Fig. 1). SH 7 (SA Geodata drillhole number 227935) intersected a weakly oxidised and hematite-altered tonalite dated at 2.529 ± 0.005 Ga (Reid, Jourdan and Jagodzinski 2017) beneath 1,427 m of sedimentary cover sequences that include >1,000 m of Mesoproterozoic Pandurra Formation (Sawyer et al. 2009). The tonalite is variably hematite-altered, in places pervasively so, where iron oxide (hematite) is associated with sericite, chlorite and clays (Fig. 2). Biotite, muscovite, K-feldspar and sericite were dated from two samples of the tonalite via the 40Ar/39Ar method.
Analytical methods
The K-bearing mineral phases were analysed by the 40Ar/39Ar method (McDougall and Harrison 1999) at Curtin University within the Western Australia Argon Isotope Facility using the methods of Jourdan et al. (2010). The minerals were extracted via standard crushing and density separation, and hand-picked to concentrate high-purity separates. The samples were then irradiated in the US Geological Survey nuclear reactor (Denver, Colorado) to convert 39K into 39Ar, so that the parent and a proxy for the daughter isotope could be measured simultaneously. The samples were degassed in a high-vacuum mass spectrometer via a continuous wave Nd-YAG (IR; 1064 nm) laser rastered during 1 min over individual grains or multi-grain aliquots. This heating causes the crystal structure of the mineral to degrade and release the trapped Ar. More details of the analytical methods and the full analytical data tables are found in the original paper (Reid, Jourdan and Jagodzinski 2017).
Figure 2 Representative images of tonalite from drillhole SH 7. Abbreviations: bt, biotite; chl, chlorite; Ksp, K-feldspar; mu, muscovite; plag, plagioclase; qtz, quartz; ser, sericite; TiO, titanium oxide (leucoxene). Reprinted from Reid, Jourdan and Jagodzinski (2017; figs 4a, 4c, 4e and 4f) with the permission of the Geological Society of Australia.
Figure 2a Moderately hematite-altered tonalite, 1,465 m. Drill core is 3.5 cm wide. (Photo 415922)
Figure 2b Photomicrograph of sample 1965413, 1,465.8–1,467.0 m, cross-polarised light showing K-feldspar and biotite within primary igneous texture of the tonalite. (Photos 415923)
Figure 2c Photomicrograph of sample 1965411, 1451.5–1451.6 m, cross-polarised light. Muscovite occurs as both coarse (>250 µm) and smaller (<50 µm) flakes, the latter of which commonly occur as inclusions within plagioclase. (Photo 415924)
Figure 2d Scanning electron microscope image of sample 1965411, showing a zone of sericite–chlorite alteration, which overprints the primary igneous texture of the rock. (Photo 415925)
Results
Figure 3 Summary of 40Ar/39Ar age spectra obtained by Reid, Jourdan and Jagodzinski (2017). (a) Composite age spectra showing results from sample 1965411, muscovite; and sample 1965413, biotite and K-feldspar. (b) Age spectra obtained from two step heating experiments from two aliquots of sericite from sample 1965411.
Muscovite from SA Geodata rock sample 1965411 yielded an age spectrum that starts at a minimum of c. 1.45 Ga in the initial heating step and then progresses to older ages. Although no plateau age can be calculated, most of the heating step ages are around c. 2.0 Ga (Fig. 3). Analysis of biotite from sample 1965413 yielded a simple step heating age spectrum with the majority of steps yielding ages also around c. 2.0 Ga (Fig. 3). K-feldspar, also from sample 1965413, yielded an age spectrum that varies from c. 1.56 Ga to a maximum of c. 1.86 Ga. As noted by Reid, Jourdan and Jagodzinski (2017), this type of age spectrum is difficult to interpret; however, it most likely indicates a reheating or recrystallisation event that has modified the age distribution within the K-feldspar sometime around or possibly younger than c. 1.6 Ga.
Sericite occurs within a sample of the tonalite as small flakes that form mats of sericite after plagioclase (Fig. 3), and dating of sericite can potentially constrain the timing of hydrothermal alteration (Verati and Jourdan 2014). Two step heating experiments were performed on sericite aliquots from sample 1965411. The age spectra are very similar from both experiments and show a relatively constant age profile, with ages clustering around 1.6 Ga (Fig. 3). Although no statistical plateau age can be calculated, c. 1.6 Ga most likely approximates the age of the alteration event.
Discussion and conclusion
Figure 4 Summary of the thermal history of tonalite from drillhole SH 7 based on the 40Ar/39Ar data obtained by Reid, Jourdan and Jagodzinski (2017).
Argon isotopic data from drillhole SH 7 gives muscovite and biotite ages close to c. 2.0 Ga. The near coincidence of the ages from the two mica species could be a function of a distinct thermal event that affected the tonalite of SH 7 at around this time. Nominal closure temperatures of around 400 °C for muscovite and 350 °C for biotite (Hodges 1991; McDougall and Harrison 1999), suggest that the tonalite cooled below 350 °C by around 2.0 Ga (Fig. 4).
Argon isotopic data from magmatic K-feldspar within the tonalite of SH 7 is consistent with cooling below 300 °C prior to c. 1.86 Ga, the oldest age from the step heating age spectra. The lower end of the age spectra at c. 1.6 Ga is consistent with a thermal event at this time or possibly younger, that has partly reset the K-feldspar. Assuming a typical K-feldspar closure temperature window of between ~150 °C and ~300 °C (McDougall and Harrison 1999), the data suggests cooling to below 150 °C occurred by c. 1.6 Ga.
The sericite argon isotopic data yields consistent ages of c. 1.6 Ga, which was interpreted by Reid, Jourdan and Jagodzinski (2017) as evidence for an alteration event at this time. This suggests that the low-temperature fluid-related alteration of the tonalite in drillhole SH 7 occurred at broadly the same time as the major magmatic, hydrothermal and mineralisation event that affected other portions of the Olympic Cu–Au Province (Skirrow et al. 2007; Reid et al. 2013) and indeed large regions elsewhere in the Gawler Craton (Daly, Fanning and Fairclough 1998; Fraser, Skirrow and Holm 2007; Hand, Reid and Jagodzinski 2007).
K-feldspar and sericite within low-temperature hematite–sericite–chlorite alteration of the tonalite from drillhole SH 7 appear to record the influence of a hydrothermal event at c. 1.6 Ga. This age is identical to the timing of the major mineralisation event that formed the Olympic Cu–Au Province and should encourage future mineral exploration in the vicinity of this drillhole.
Acknowledgements
This study was undertaken as part of the PACE Geochronology program in collaboration with Steve Hogan and Monax Mining Ltd. Miles Davies, former PACE General Manager, is thanked for his support of the PACE Geochronology program. Liz Jagodzinski (GSSA) and Fred Jourdan (Curtin University, WA) are gratefully acknowledged for their work in the original study.
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