Alkalic Epithermal Or Porphyry? Hydrothermal Alteration And Vein Paragenesis At The E41 Gold Deposit Cowal District, New South Wales, Australia

Epithermal and porphyry styles of mineralization and alteration occur within the Endeavour 41 (E41) gold prospect of the Ordovician Cowal district, New South Wales, Australia. The deposit is located in an Ordovician subaqueous volcano-sedimentary succession that has been intruded by multiple sills and dykes. Gold is associated with quartz-pyrite and quartz-sphalerite-carbonate veins. Alteration facies include potassic, calcic-potassic, phyllic and propylitic mineral assemblages. The nature and distribution of these assemblages has been controlled by rock types, structure and geometry of the mineralised zones. The paragenetic sequence is complex, with certain minerals (e.g. epidote, pyrite, sericite, quartz and carbonate) forming at several stages in the deposit evolution. Early hydrothermal alteration produced actinolite-magnetite-albite-chlorite and garnet-epidote-carbonate assemblages. Syn-mineralization alteration facies are characterised by K-feldspar, sericite (muscovite/illite), chlorite, epidote and arsenopyrite, whereas late alteration comprises epidote-carbonate-prehnite. The hydrothermal system evolved from early high-temperature actinolite-magnetite-albite-chlorite and garnet alteration and vein facies, characteristic of an alkalic porphyry environment, to assemblages more typical of an epithermal style gold deposit (e.g. quartz, carbonate, chalcedony, adularia, gold, sphalerite, galena and illite). The paragenetic history of the E41 gold prospect appears to record the transition from deep to shallow-level magmatic-hydrothermal activity, and implies un-roofing of the system synchronous with mineralization.