Abstract

Lithogeochemical research at the Groundrush gold deposit, Tanami region, Northern Territory, has identified new lithologies and provides a new understanding of the hydrothermal alteration, metamorphism and weathering processes that have affected the suite of metamorphosed igneous host rocks. Metamorphosed hornblende–chlorite–plagioclase-bearing mafic–intermediate intrusive rocks were classified, using conserved constituents, as dolerite and previously unrecognized basaltic andesite and diorite, the last of which hosts Au mineralization. The diorite forms a steeply dipping dyke, whereas the dolerites are sills intruding clastic sedimentary rocks.

Major element geochemical variation in dolerite is consistent with the fractionation of plagioclase and pyroxene, whereas diorite is more chemically homogenous and does not exhibit evidence of fractional crystallization. Hydrothermal alteration, principally developed within the diorite, was accompanied by addition of S and carbonate, and the loss of Ca and Na, giving rise to a propylitic hydrothermal alteration assemblage of chlorite, albite, carbonate and pyrite.

A regolith profile of c. 40 m thickness overlies these rocks. If originally present, Fe-rich upper units have been removed. The major weathering processes observed in the regolith profile are: (1) sulphur oxidation and (in diorite), calcite dissolution at 40 m depth; (2) ferrolysis (Fe oxidation) and the production of smectite, goethite and quartz from chlorite and hornblende at 20 m depth; (3) breakdown of albite to kaolinite, breakdown of apatite, and deposition of secondary carbonate in the top 5 to 15 m.

No evidence for significant near-surface gold depletion is evident, suggesting that regolith processes remobilizing gold were generally not operating to any significant extent. However, gold was leached from a metre-scale quartz vein, possibly as thiosulphate complexes formed from pyrrhotite oxidation in a relatively low-oxygen environment.

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