Using combined texture-element-isotope indicators of sulfides to trace fluid mixing and evolution in Paleozoic IOCG system

Abstract

Geochemistry of sulfides is widely used to constrain sources and ore-forming processes of various mineral deposits. However, its application on Fe–oxide Cu–Au (IOCG) deposits is not well constrained due to multiphase pyrite and Cu minerals and their complex inheritance relationships. The Shuanglong deposit is an IOCG-like deposit in the Eastern Tianshan characterized by abundant Py1 and Py2 in the Fe mineralization stage (II) and Py3 and Py4 in the Cu mineralization stage (III). Chalcopyrite is divided into three types where Ccp1 is formed by replacing Py1 and Py2, Ccp2 coexists with Py4, and higher grade Ccp3 is in quartz–hematite–chalcopyrite veins without pyrite. Py1 and Py2 are associated with multiphase magnetite in stage II Fe mineralization, whereas Py3 coexists with the early epidote replaced by the late calcite–hematite–Py4–Ccp2 assemblage in stage III Cu mineralization.

The increasing Co contents and Co/Ni ratios and decreasing δ34S values (∼8‰ to 4 ‰) from core to rim in Py1 and Py2 indicate temperature and oxygen fugacity increases during the input of magmatic-hydrothermal fluids. Decreasing Co/Ni, increasing Au, and δ³⁴S_fluid (from ∼ 6 ‰ to 30 ‰) from Py3 to Py4 show fluid mixing between the magmatic-hydrothermal fluid and oxidized non-magmatic sulfur such as seawater or basinal brine sulfate during the stage III Cu mineralization. Ccp1 inherited its sulfur and certain trace elements such as Pb and Zn from pyrite, whereas the addition of external sulfur contributed to local high-grade Cu mineralization. Such external sulfur may be from the seawater or basinal brine sulfate, with an increasing contribution during precipitation of Ccp2 and Ccp3 shown by fluid δ³⁴S values (from 26 ‰ to 36 ‰). Combined with the decreasing Cd/Zn ratios from Ccp1 to Ccp3 caused by increasing total sulfur in fluids, this suggests that the sulfate sulfur contribution may have played a significant role in the high-grade Cu mineralization in IOCG deposits. This study also highlights the importance of the detailed texture of pyrite using acid etching, coupled with in-situ sulfur isotope and trace elements of pyrite and chalcopyrite for IOCG deposit research.