Greg Corbett, Consultant Geologist
AIG Presidential Lecture 2001-2002
Epithermal gold (± Cu & Ag) deposits form at shallower crustal levels than porphyry Cu-Au systems, and are primarily distinguished as low and high sulphidation using criteria of varying gangue and ore mineralogy, deposited by the interaction of different ore fluids with host rocks and groundwaters. Low sulphidation deposits are in turn further divided according to mineralogy related to the depth and environment of formation, while high sulphidation systems vary with depth and permeability control, and are distinguished from several styles of barren acid alteration.
Low sulphidation epithermal Au + Cu + Ag deposits develop from dilute near neutral pH fluids and are divided into two groups: those which display mineralogies derived dominantly from magmatic source rocks (arc low sulphidation), and others with mineralogies dominated from circulating geothermal fluid sources (rift low sulphidation). The former are classed with decreasing crustal level as: quartz-sulphide gold + copper, passing to polymetallic gold-silver veins, carbonate-base metal gold and shallowest epithermal quartz gold-silver. These ore types are zoned in time and space with shallower styles overprinting the deeper, and metal contents which vary as high Cu at depth, to Ag and Au dominant in elevated crustal settings. Low sulphidation adularia-sericite epithermal gold-silver systems comprise the rift low sulphidation style. These are dominated by gangue mineralogies deposited from meteoric water rich circulating geothermal fluids, typically formed in rift settings. Sediment hosted replacement gold deposits are interpreted to develop from low sulphidation fluids in reactive carbonate bearing rocks.
High sulphidation Au + Cu ore systems develop from the reaction with host rocks of hot acidic magmatic fluids to produce characteristic zoned alteration and later sulphide and Au + Cu + Ag deposition. Ore systems display permeability controls governed by lithology, structure and breccias and changes in wall rock alteration and ore mineralogy with depth of formation. One of the challenges is to distinguish the mineralised systems from a group of generally non-economic acidic alteration styles including lithocaps or barren shoulders, steam heated, magmatic solfatara and acid sulphate alteration.