Mesoproterozoic Gold in the Central Gawler Craton, South Australia: Geology, Alteration, Fluids, and Timing

Abstract

Similarities in mineralization and alteration style, host rocks, fluid compositions, and alteration-related 40 Ar/ 39 Ar ages from several recently discovered gold prospects support the existence of a significant Mesoproterozoic gold province spanning an arcuate region at least 300 km in length in the central Gawler craton, South Australia. At the recently discovered prospects of Tunkillia, Nuckulla Hill, Barns, and Weednanna, as well as at the historically mined Au deposit at Tarcoola, gold occurs as disseminated and veinlet-hosted mineralization focused in brittle to brittle-ductile faults and shear zones. Host rocks are principally ~1715 to ~1680 Ma granites but also include Hutchison Group (2000-1860 Ma) and Tarcoola Formation (~1650 Ma) metasedimentary rocks. Hydrothermal alteration is characteristically zoned around gold mineralization, with intense sericitepyrite alteration and quartz veining proximal to gold mineralization and chlorite ± epidote ± hematite alteration distal from mineralization. Alteration was either synchronous with or, in some cases, continued after deformation. Gold is associated with pyrite and minor to trace galena, sphalerite, and chalcopyrite. Iron oxides are low in abundance in mineralized zones, which correspond to demagnetized zones. Fluid inclusion analyses suggest that multiple fluids were present in some systems, but one key fluid type occurs in each of the gold prospects: a low-to moderate-salinity (up to 10 wt % NaCl equiv) fluid with homogenization temperatures mostly in the range of ~150°to 300°C and commonly containing CO2 or associated with CO2-rich inclusions. Several features of the prospects are similar to those of orogenic-and intrusion-related gold deposits, but neither the prospect-scale geology nor the regional geologic setting in the central Gawler craton is sufficiently well understood to establish a genetic model or to confidently classify the mineralizing system. Hydrothermal white micas from several of the central Gawler craton gold prospects yield reproducible 40 Ar/ 39 Ar plateau ages of between 1567 and 1583 Ma. These ages, together with U-Pb zircon ages of granitic host rocks, constrain the timing of mineralization to the interval ~1690 to 1570 Ma, although we interpret the 40 Ar/ 39 Ar results to indicate a narrower mineralizing interval at ~1580 ± 10 Ma. This age range overlaps with, and is indistinguishable from, the range of U-Pb zircon ages reported from regional Hiltaba Suite granites and Gawler Range Volcanics, although igneous rocks of this age have not been identified locally at each of the prospects.

Proxies for Basement Structure and Its Implications for Mesoproterozoic Metallogenic Provinces in the Gawler Craton

“…The electrical conductors observed across the craton (e.g., Figures , , and ) are interpreted as zones of fluid flow along shear zones probably during the Paleoproterozoic to Mesoproterozoic, which decreased the resistivity through aqua‐carbonic fluid entrapment (Heinson et al, ; Thiel & Heinson, ) or sulfide precipitation (Heinson et al, ). The significant domain boundaries within Gawler Craton (Figures b and ) appear to have acted as agents for energy and fluid flux between different geochemical reservoirs across the craton basement along secondary structures (Bastrakov et al, ; Direen & Lyons, ; Fraser et al, ; Haynes et al, ; Kontonikas‐Charos et al, , ; Uvarova et al, ) including shallow levels in the overlying sedimentary basins and basement (Cherry et al, ; Reid & Fabris, ). This is evident from the regional alteration and chemical footprint observed in both northern and southern sectors of the IOCG province (Kontonikas‐Charos et al, , ) and similarities in timing and composition of the hydrothermal alterations between the IOCG and CGG provinces (Bastrakov et al, ; Fraser et al, ).…”

“…The significant domain boundaries within Gawler Craton (Figures b and ) appear to have acted as agents for energy and fluid flux between different geochemical reservoirs across the craton basement along secondary structures (Bastrakov et al, ; Direen & Lyons, ; Fraser et al, ; Haynes et al, ; Kontonikas‐Charos et al, , ; Uvarova et al, ) including shallow levels in the overlying sedimentary basins and basement (Cherry et al, ; Reid & Fabris, ). This is evident from the regional alteration and chemical footprint observed in both northern and southern sectors of the IOCG province (Kontonikas‐Charos et al, , ) and similarities in timing and composition of the hydrothermal alterations between the IOCG and CGG provinces (Bastrakov et al, ; Fraser et al, ). This fundamental role of the major structures as pathways persists through time due to reactivation associated with tectonic switches, as previously envisaged around the Olympic Dam deposit (Direen & Lyons, ; Heinson et al, , ; Korsch & Doublier, ; Stewart & Betts, , ).…”

“…We have developed a study case on the application of satellite gravity data over the Gawler Craton in South Australia (Figure 1), which hosts the Olympic iron oxide-copper-gold (IOCG; Bastrakov et al, 2007;Skirrow et al, 2007) and the Central Gawler Gold (CGG; Ferris & Schwarz, 2003;Fraser et al, 2007) metallogenic provinces. Both provinces formed during the earliest Mesoproterozoic (approximately 1,600 to 1,570 Ma; Skirrow et al, 2007;Fraser et al, 2007) over an Archean-Palaeoproterozoic cratonic shield (Hand et al, 2007).…”

“…We have developed a study case on the application of satellite gravity data over the Gawler Craton in South Australia (Figure 1), which hosts the Olympic iron oxide-copper-gold (IOCG; Bastrakov et al, 2007;Skirrow et al, 2007) and the Central Gawler Gold (CGG; Ferris & Schwarz, 2003;Fraser et al, 2007) metallogenic provinces. Both provinces formed during the earliest Mesoproterozoic (approximately 1,600 to 1,570 Ma; Skirrow et al, 2007;Fraser et al, 2007) over an Archean-Palaeoproterozoic cratonic shield (Hand et al, 2007). Models for the genesis and geodynamic setting of these provinces are varied, including plumemodified orogenesis and magmatism (Betts et al, 2007(Betts et al, , 2009, the development of extensional sedimentary basins (Cherry et al, 2017;McPhie, Kamenetsky, Chambefort, et al, 2011;McPhie et al, 2016), a convergent plate-boundary settings (Direen & Lyons, 2007;Fraser et al, 2007), and postsubduction settings (Skirrow et al, 2018).…”

Tectonic Framework and Evolution of the Gawler Craton, Southern Australia

“…The Olympic Cu-Au province is host to IOCG systems, including Olympic Dam as well as Prominent Hill and Moonta and the Carrapateena prospect, and numerous other prospects (Skirrow et al, 2002. In contrast, the central Gawler Au province contains shear-hosted Au systems that contain little Cu and are largely devoid of the Fe-bearing alteration that characterizes the IOCG systems (Ferris and Schwarz, 2003;Budd and Fraser, 2004;Fraser et al, 2007).…”