Origin and significance of pre-mineralization pyrite in the Jinya gold deposit, northwestern Guangxi Province, China: in situ trace element and sulfur isotope constraints

The Jinya Au deposit is a typical Carlin-type deposit in the Golden Triangle of Yunnan, Guizhou, and Guangxi provinces, China. The ore bodies occur in the Middle Triassic Baifeng Formation, which are turbidites deposited on the edge of a carbonate platform. The main Au-bearing minerals are arsenopyrite and pyrite, and previous studies have found that pyrite can be divided into three types: pre-, syn-, and post-mineralization. In this study, electron microscopy, laser ablation inductively coupled plasma mass spectrometry, and secondary ion mass spectrometry were used to determine the in situ elemental and S isotopic composition of pre-mineralization pyrite in the Jinya Au deposit. The pre-mineralization pyrite can be divided into syn-sedimentary (Py I) and diagenetic pyrite (Py II). Pyrite Co/Ni ratios are < 1, with an obvious sedimentary origin. Pyrite Co/Ni and Se/Tl ratios indicate that the formation temperature of Py II was higher than that of Py I. δ 34 S values of Py I range from –8.61 to –2.63‰, with an average of –3.68‰, which are similar to the δ 34 S values of sulfides formed in Permian–Triassic oceans. δ 34 S values of Py II range from + 10.13 to + 13.30‰, with an average of + 12.13‰, and are typical of thermochemical sulfate reduction formation of pyrite. This study shows that the syn-sedimentary pyrite (Py I) precipitated from an undersaturated solution of Fe and S at low temperatures and that Cu, Pb, Zn, As, Se, Ag, Sb, and Au in the detrital material resulted in strong cataclastic dissolution and regrowth of syn-sedimentary pyrite during diagenesis. In the later stages of diagenesis, strong thermochemical sulfate reduction in the sediments reduced the range of δ 34 S values, and Py II began to precipitate, resulting in pre-enrichment of Au and other elements.