Pyrite geochemistry and its implications for ore formation in reduced copper skarn systems: A case study of the Saibo deposit, Northwest China

Copper skarns are the main source of Cu worldwide, and they are classified into two types according to their redox state: oxidized and reduced. Numerous studies have investigated the mineralization processes of oxidized Cu skarn deposits in terms of pyrite trace element compositions. However, it remains unclear how the compositional and micro-textural features of pyrite are linked to specific physicochemical processes in reduced Cu skarn systems. The Saibo deposit in the northern part of the western Tianshan orogenic belt is a typical reduced Cu skarn deposit, characterized by abundant Ti-rich garnet, arsenopyrite, pyrrhotite, and CH₄-rich inclusions. In this study, we examined pyrite geochemical and micro-textural features and related these features to Cu-forming processes at Saibo. Copper skarn mineralization was recognized to occur in the contact zone between the granodiorite porphyry and the Kusongmuqieke Group limestone as veins, bands, massive, and disseminations. Five stages were identified: (I) prograde skarn, (II) retrograde skarn, (III) quartz–pyrite–arsenopyrite (Py1), (IV) quartz–polymetallic sulfides (Py2), and (V) quart–calcite–polymetallic sulfides (Py3).

Pb, Sb, Ag, Cu, Zn, and Bi were typically present in the host pyrite as micro- and nanoscale mineral inclusions, whereas Co, Ni, Se, Te, and As were mostly present as lattice substitutions for S and Fe. In situ δ³⁴S values of Py1 to Py3b for the Cu skarn ore at Saibo defined a narrow range from 4.86 ‰ to 7.67 ‰, which was well consistent with the published S values of sulfide separates (0.2 to 9.1 ‰). These values are similar to those of Middle–Late Devonian granitoids, such as the Lamasu plagiogranite, and different from those of sediments. This indicates that the dominant source of sulfides in the Saibo deposit is the Middle–Late Devonian granodiorite porphyry. The overall increasing concentrations of As, Sb, Te, Pb, Au, and Bi, and decreasing concentrations of Co, Ni, and Se from Py1 to Py3b indicate that prior to Cu precipitation, ore-forming fluids were in low oxygen fugacity conditions that were associated with fluid-rock interactions and fluid boiling. Furthermore, the main controlling factors during Cu precipitation were sulfur loss, the increase in pH, and the decrease in temperatures of ore-forming fluids, which could be attributed to fluid-rock interactions, later-stage fluid boiling, and mixing with meteoric water, respectively. The PLS-DA results show that pyrite composition varies following skarn redox state. Pyrite from reduced skarns have in common high concentrations of Au, Bi, and Ni, while those from oxidized have in common high As, Sb, and Pb. Pyrite trace element composition is effective in discriminating different redox states of skarns.