Structural controls on hydrothermal alteration and gold-antimony mineralisation in the Hillgrove area, NSW, Australia

Abstract

The Hillgrove gold-antimony deposit is hosted in late Palaezoic, biotite-grade metasedimentary rocks and Permo-Carboniferous granitoid intrusions of the New England Orogen. Mineralisation occurred at a range of structural levels during rapid uplift in the orogen at 255-245 Ma. Hydrothermal fluids were controlled by extensional faults in a regional-scale sinitral strike-slip fault system. Principal faults in this system were developed in, and possibly evolved from, mylonite zones which were active during Late Permian tectonics. Earliest mineralisation formed scheelite-bearing quartz veins, and these were followed by auriferous arsenopyrite-pyrite-quartz-carbonate veins with minor base metal sulphides. This latter type was accompanied by sericitisation and carbonation of the host rock, with addition of sulphur, arsenic and gold, in zones up to 20 m from veins. Quartz-stibnite veins with electrum, gold, aurostibnite, and arsenopyrite form a prominent and economically important hydrothermal type, with little wall-rock alteration but extensive hydrothermal breccia formation and local open-space filling textures. Below a mining depth of 300-500m, this type passes over a short distance downwards into stibnite-poor gold-bearing veins. Late-stage carbonate-stibnite veins with gold and silver sulphosalts cut all earlier veins, and have open-space filling textures. Aspects of the Hillgrove deposit have similarities to many other orogenic gold deposits in the SW Pacific which have been formed at different structural levels. Hillgrove is distinctive in having evidence for mineralisation at this wide range of structural levels in the one deposit, formed progressively during syn-orogenic uplift.