In situ pyrite sulfur isotope and trace element analyses of the world-class Dachang gold deposit, northern Qinghai-Tibetan Plateau: Implications for metallogenesis

Abstract

Dachang is a world-class gold deposit with gold reserves of 121 t and an average grade of 3.02 ppm. However, the genesis, and in particular the origin of the Au, S and associated trace elements of the deposit, remain under debate. The deposit occurs in black turbidites of the Lower Triassic Bayan Har Shan Group, northern Qinghai-Tibetan Plateau. The orebodies are composed of gold-sulfide-quartz veins and gold-sulfide-altered rocks. The ore minerals are pyrite, arsenopyrite, native gold, and stibnite, with trace amounts of tetrahedrite, chalcopyrite, galena and sphalerite. The gangue minerals are quartz, carbonate minerals, mica, feldspar, and supergene kaolinite.Five types of pyrite (Py1 to Py5) have been identified in the host strata and ore, according to their occurrences and textures. In situ SIMS sulfur isotope and LA-ICP-MS trace element analyses indicate that the pre-ore pyrites (Py1 to Py3) have a large range of δ34SVCDT values from −8.5 to 38.3‰; the δ34S values of the ore pyrites (Py4 and Py5) range from −9.4 to 1.0‰, and the ore sulfides also have relatively large δ34S values ranging from −10.8 to 6.5‰, which is approximately in the range of the δ34S values of the pre-ore pyrite; and the pre-ore pyrites have trace element associations of Au, As, Pd, Sb, Ag, Cu, W, Bi and Pt consistent with those of the ore-stage pyrites. These results suggest that the S, As, Au and other metals in the ore were mainly sourced from the host rocks, and pyrite was the main source. The ultimate source was Early Triassic seawater, which had high concentrations of Au and other metals, attributed to high levels of chemical weathering and erosion globally. The recrystallization and metamorphic reaction of pre-ore pyrites during late diagenesis and/or regional metamorphism and deformation released S, As, Au and other metals, which produced metamorphic gold-bearing ore fluids and led to the formation of the orogenic Dachang gold deposit. Au occurs as a solid solution in the pre-ore and ore pyrite structures, and pyrite is the main Au-bearing mineral in the Dachang ore.