Geochemical variation of copper sulphides within the Carrapateena deposit, South Australia

Abstract

Carrapateena is an Iron Oxide Copper-Gold (IOCG) deposit which is located on the eastern margin of the Gawler Craton, 100km southeast of Olympic Dam. Mineralisation at Carrapateena is restricted to a steeply plunging, pipe-like body of hematite and hematite-granite breccias termed the Carrapateena Breccia Complex (CBC). Primary Cu mineralisation within the CBC occurs in hematite-sericite-chlorite dominated breccias as chalcopyrite (CuFeS2) and bornite (Cu5FeS4) with some trace chalcocite (Cu2S). Secondary covellite (CuS) and digenite (Cu9S5) is also observed in association with primary chalcopyrite and bornite. A geochemical Cu/S zonation pattern extends laterally outward with a transition from bornite → bornite/chalcopyrite → chalcopyrite → pyrite. The precipitation of main stage Cu mineralisation chalcopyrite is believed to have been through interface-coupled dissolution-reprecipitation reaction (ICDR) and overgrowth of pre-existing pyrite and hematite. Bornite then undergoes similar reaction replacing the previous generation of chalcopyrite. Utilising Scanning Electron Microscopy (SEM) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) 8 polished rock pucks sampled from drillhole EG20CAR2038 were analysed. Evidence of back reaction ICDR replacement through exsolution and annealing textures of chalcopyrite, covellite and digenite from bornite grains was found. In instances of bornite-digenite overgrowth, significant increases in abundance of Ag and Cu were discovered at depths of 1150m. Exsolution of chalcopyrite from bornite at shallower depths of 878m, 923m, 966m and 1099m resulted in no increase of Ag, Ni, Co and an overall decrease in Cu from the original bornite grains. This study shows that during the cooling stages (150-250⁰C) of Carrapateena’s development significant movement of trace elements and major elements amongst Cu sulphides occurred via the interaction of low temperature, basic pH fluids resulting in enrichment at depth within the deposit.