Pyrite textures and trace element compositions from the Balong gold deposit in the Eastern Kunlun Orogenic Belt, Northern Tibetan Plateau: Implications for gold mineralization processes

The formation of gold deposits may involve multiple stages of gold enrichment, which makes it difficult to differentiate the potential multistage processes of ore material enrichment. Determining whether these events represent the remobilization of gold from pre-existing deposits or the introduction of new gold during a distinct epigenetic event is often challenging. The Balong gold deposit is a representative lode gold deposit in the East Kunlun metallogenic belt in China. Gold mineralization is hosted in Triassic granitoids and is characterized by multi-stage quartz-sulfide veins. Pyrite is the most abundant sulfide in the ore and is also the most important host for gold. Three types of pyrite have been identified. The porous Py1 exhibits low trace element content, with an absence of gold. Subhedral Py2-1 contains various Cu-Pb-Zn-Ag mineral inclusions. Py2-2 shows a significant increase in As (median 17, 073 ppm) and Au (median 3.79 ppm), exhibiting obvious distinctions between Py2-1 and Py2-2.

Gold in the Balong deposit consists of both visible and invisible gold. Visible gold is found within the micro-fractures of pyrite and arsenopyrite, appearing as irregular inclusions or infillings. In addition to visible gold grains, the majority of the invisible gold in Py2-2 exists as solid solutions (Au). Backscattered Electron imaging and trace-element analyses show that invisible gold occurs only in the As-rich bands. Pyrite records a narrow range of δ⁺³⁴S values from -1.6 to 5.4‰, reflecting sulfur from a deep magmatic source. In conjunction with fluid inclusion studies and the estimated age of the related magmatic activity, our results point to magmatic-hydrothermal fluids as the main contributors of ore materials. Coupled dissolution-reprecipitation reactions of early pyrite are a key factor for visible gold precipitation and later invisible gold enrichment. Our pyrite data constrain the evolution of ore-forming processes and offer new perspectives on zonal pyrite formation.