LA-ICP-MS trace element analysis of pyrite from the Chang'an gold deposit, Sanjiang region, China: Implication for ore-forming process

The Chang'an gold deposit, Yunnan province is one of five large gold deposits in the Ailaoshan gold belt that is the most important ore belt of the Sanjiang Tethyan metallogenic domain. Geochemical study of pyrite in the deposit was conducted using laser ablation inductively coupled plasma mass spectroscopy. Three types of hydrothermal pyrite were identified in the ores and wall rocks, i.e., the coarse euhedral crystals in syenite (Py1), the coarse grains disseminated in altered sandstones or sandstone ores (Py2), and the fine-grained euhedral pyrite in sandstone ores (Py3). Their trace elements exhibit different concentrations, associations and rim–core zoning, implying different geneses and crystallization processes. The cores of Py1 were formed by magmatic fluids and have the lowest concentrations of Au, As, Cu and Zn. The cores of Py2 were formed by metamorphic fluids and have relatively high Au, Ag, Ni, Pb and Cu concentrations. The Py3 grains and the growth rims of Py1 and Py2 show consistently high contents of Au, As, Pb and Co, suggesting that all of them were rapidly deposited from a mixing fluid systemthatwas probably composed of fluids ofmetamorphic andmagmatic origins. This interpretation is supported by the observation that the high-grade ores generally contain lots of Py2 and Py3. Hence we consider that the deposit was formed during India–Asia collision, slightly postdate the 33–35 Ma tectono-magmatism.