Ore fluid evolution in the giant Marcona Fe-(Cu) deposit, Perú: Evidence from in-situ sulfur isotope and trace element geochemistry of sulfides

Abstract

The Marcona magnetite deposit, located in the iron oxide copper-gold (IOCG) province of southern Perú, is an important Fe deposit with sub-economic Cu and Zn mineralization. Three generations of sulfides were identified at the Marcona alteration/mineralization Stages IV, V and VII. Magnetite, pyrrhotite, pyrite and minor chalcopyrite started to form at Stage IV. Sulfides in Stage V are dominated by chalcopyrite and pyrite, whereas only pyrite was present in Stage VII. In-situ SIMS sulfur isotope and LA-ICP-MS trace element analyses were conducted on the sulfides of these stages. Stage IV sulfides contain δ34S largely in the range of 0 to +4.0‰, indicating a dominantly magmatic origin. In particular, δ34S values of Stage IV pyrite increase from core to rim, and the corresponding trace elements concentrations vary along with δ34S. Positive δ34S vs. Se/S correlation of Stage IV pyrite indicates that the ore-forming system was open. The Stage V pyrite are rich in Mn, Cu, Ag, Sb, Te, Pb and Bi, and all Stage V sulfides have also elevated δ34S (up to +7.8‰), indicative of possible seawater incursion during its formation. The δ34S values of Stage VII pyrite are highly variable from −13.4 to 5.4‰. This, together with ubiquitous disseminated voids, low Co/Ni ratios and enrichments of Mn, Se, Sb, Te, Tl and Pb, suggests that Stage VII pyrite may have formed by sulfate reduction associated with organic materials. We propose that the ore-forming fluids of Stage IV are initially magmatic with involvement of external (e.g., seawater) at late, where the fluids of Stage V are likely mainly exotic. The external fluids, together with organic matter, may have contributed to the ore-forming fluids of Stage VII, indicating a complex fluid evolution history for this giant Fe (-Cu) deposit.