External fluid incursion during Cu-mineralization stage of Mina Justa iron oxide copper-gold (IOCG) deposit: Evidence from triple sulfur isotope geochemistry of chalcopyrite

Abstract

IOCG deposits are major host for Cu, rare earth elements (REEs), U and other metals, and whose source(s) of ore-forming fluids are highly debated. Particularly, whether or not external fluid was involved during Cu-mineralization stage is of great interest and intense debate. In this contribution, we investigate this topic using triple sulfur isotope geochemistry of Cu-mineralization stage sulfides in the Mina Justa deposit. The results show that the chalcopyrite that intensely replaced early-stage pyrite has similar triple sulfur isotopic composition to early-stage pyrite, which largely in the range of magmatic sulfur (Δ33S = 0 ± 0.05 ‰, Δ33S = δ33S − 1000 × [(1 + δ34S/1000)0.515 – 1]). However, chalcopyrite that occurs alone or with only minimal replacement of coexisting pyrite shows triple isotope (Δ33S = 0.05 to 0.20 ‰) that deviates from magmatic sulfur. This indicates that the sulfur of chalcopyrite with intense replacement of early-stage pyrite mainly inherits sulfur from early-stage pyrite, while the sulfur of chalcopyrite that exist alone or with minimal replacement of pyrite obtained their sulfur from a non-magmatic source. Collectively, the triple sulfur isotope data indicates that external fluid is present in the Cu-mineralization stage.