Genesis of the Bangbu gold deposit in the southern Tibet: Evidenced from in-situ sulfur isotopes and trace element compositions of pyrite

The Bangbu orogenic gold deposit in the North Himalaya of the southern Tibet contains more than 40 t Au at an average grade of 7.0 g/t. In this deposits, gold-bearing quartz veins were controlled by nearly E-trending Qusong-Cuogu-Zhemulang shear zone and occurred within the secondary faults which crosscut Late Triassic greenschistfacies rocks. To further understand the sulfur source and ore-forming process, we have conducted a compressive study of in-situ SIMS sulfur isotopes and LA-ICP-MS trace element compositions of two stages of pyrite at Bangbu. Early-stage pyrite (Py1) is coarse-grained (mostly 0.2-2 mm) and euhedral, and has gold concentrations of less than 0.3-54 ppm (mean of 20 ppm) and delta S-34 values of 1.6-5.1 parts per thousand (mean of 3.2 parts per thousand). Latestage pyrite (Py2) is generally fine-grained (mostly < 50 mu m to 1 mm) and subhedral to anhedral, and has gold concentrations of 3.6-115 ppm (mean of 30 ppm) and delta S-34 values of 0.9-5.2 parts per thousand (mean of 2.7 parts per thousand). Gold occurs mainly as invisible refractory within Py1 and Py2, and to a lesser extent as native gold within quartz, pyrite, arsenopyrite and sphalerite. Sulfur for Bangbu gold mineralization was probably sourced from the Greater Himalayan crystalline complex. Release of ore-forming fluids was likely related to amphibolite-facies metamorphism during similar to 50-45 Ma. Ore fluids deposited Au-rich pyrite during early and late mineralization stage and precipitation of native gold was probably related to fluid boiling and/or remobilization of invisible gold within pyrite.