New constraints unifying Au mineralised systems in the central Gawler Craton: alteration, mineralisation and fluids

New reconnaissance studies of Au mineralisation, alteration, geochronology, and palaeo-fluids in the central Gawler Craton support the existence of a major Au metallogenic province. The results show that Au mineralising systems were active along a &gt;300 km belt at similar times (~1570 Ma to ~1590 Ma; Fraser et al., 2004, this volume) during the early Mesoproterozoic. The scale of this metallogenic belt is a positive indicator for Au prospectivity. Recent company results suggest that the potential for economic resources is high.
The main similarities in the Tarcoola, Tunkillia, Nuckulla Hill, and Barns Au systems are:
Gold occurs in both disseminated and veinlet-hosted styles. 
The main host rocks are ~1680-1720 Ma granitoids and, at Tarcoola, metasedimentary rocks (partly carbonaceous).
Hydrothermal alteration is characteristically zoned around the Au mineralisation, with intense sericite-pyrite alteration and quartz veining proximal to Au mineralisation, and chlorite, epidote, hematite alteration distal (metres to 100s of metres) to mineralisation.
Deformation was synchronous with the proximal sericitic hydrothermal alteration and mineralisation and was generally of brittle to brittle-ductile style including shearing.
Au is associated with disseminated and vein-hosted pyrite and minor to trace galena, sphalerite, and chalcopyrite.
Iron oxides have very low abundance in mineralised zones, where they are present mainly as haematite, manifest as de-magnetised zones in magnetic images.
Hydrothermal white micas at Tarcoola, Tunkillia, Barns, and Weednanna all yield similar Ar-Ar ages of ~1570-1600 Ma. Sericitic alteration and Au mineralisation are interpreted to have formed within this period.
Multiple fluids were present in some systems, but one fluid-type occurs in each of the Au prospects: a low-moderate salinity (up to 10 eq. wt.% NaCl; mostly 3-6 %) fluid with homogenisation temperatures mostly in the range of ~200 C to 300?C and trapping temperatures of ~300-400 C. This fluid is commonly associated with CO2-rich fluid inclusions.
Galena yields broadly similar Pb isotope compositions at Tarcoola and Tunkillia. 
Differences between the Au mineralised systems at Tarcoola, Tunkillia, Nuckulla Hill, and Barns, such as the intensity of deformation, relative abundances of base metal sulfides, and sulfur isotope values, reflect differing local structural and lithostratigraphic settings.
The Weednanna Au and nearby Mawson Au-Cu prospects bear some similarities to the other Au prospects of the central Gawler gold province, but we consider them to be possible hybrids of the early high-temperature alteration style of iron-oxide Cu-Au systems overprinted by Au hydrothermal systems.
Late-stage epithermal-style quartz-carbonate-adularia veins in the Nuckulla Hill and Weednanna areas indicate potential for epithermal Au mineralisation in the region.
Preliminary depth constraints suggest high-level crustal environments of mineralisation, possibly around 3-6 km deep, at temperatures up to 300-400 C. Spatial and genetic relationships with Hiltaba Suite magmatism remain cryptic, despite the broadly coeval timing of magmatism and Au mineralisation. The Au mineralisation style does not neatly fit deposit classes such as 'lode-gold', 'orogenic/mesothermal gold', 'intrusion-related gold', 'epithermal gold', or 'porphyry Au'.

Tags: AU-SA, Earth Sciences, PowerPoint, Published_External, geochemistry, geochronology, mineral exploration
Organization: Geoscience Australia Data