In situ trace element and sulfur isotopic analysis of multi-stage pyrite in Mingsai gold deposit, southern Tibet: Insights into mineralization and gold remobilization

Abstract

Pyrite is the main gold-bearing mineral and can provide information about the formation of gold deposits. This study focuses on the Mingsai gold deposit, a newly discovered deposit in the east of the Tethyan Himalayan Sequence. We analyzed the texture, composition, and formation process of pyrite in the deposit to understand how gold was formed. We identified five types of pyrite: diagenetic pyrite (Py0), pre-ore stage pyrite (Py1), gold-rich pyrite (Py2a and Py2b), and post-stage pyrite (Py3). All of these types contain varying amounts of “invisible” gold. Py0 is framboidal pyrite enriched in certain elements but depleted in gold. It has high δ34S values (mean 10.75 ‰). Py1 has a porous structure and contains silicate inclusions. It has low gold content and high Co/Ni ratios. The δ34S values of Py1 cluster around ∼ 0 ‰. Py2a is subhedral-euhedral pyrite found in the middle of disseminated pyrite and quartz-carbonate-sulfide veins. It has abundant gold (0.16 – 103 ppm) and relatively low δ34S (1.17 ‰ – 3.50 ‰). Py2b occurs as the rim of disseminated pyrite and sulfide inclusions at the interface with Py2a. It has lower gold content (0.90 – 9.85 ppm) and distinctly high δ34S (6.58 ‰ – 8.87 ‰) compared to Py2a. Py3 is vein-like pyrite in metamorphic tuff, which does not contain significant amounts of gold (Au < 1 ppm) and has higher δ34S values (9.86 ‰ – 11.69 ‰) than Py2b. The concentrations of metal elements, including gold, in Py2b are intermediate between those of Py2a and Py3. The researchers propose that Py2a is the main source of gold, and Py2b is a mixture of Py2a and Py3. During the mixing process, gold is released from Py2a and re-precipitates in Py2b through dissolution-reprecipitation reactions. The sulfur isotope composition suggests that the materials that formed Py2a (∼2‰) may have originated from magmatic-hydrothermal sources and undergone transformation. The detailed geochemical evidence indicates a close connection between the Mingsai gold deposit and other gold-antimony deposits in the Zhaxikang ore district, which are likely related to early Miocene magmatism in the Cuonadong dome.