Constraints on depositional conditions and ore-fluid source for orogenic gold districts in the West Qinling Orogen, China: Implications from sulfide assemblages and their trace-element geochemistry

摘要

Yangshan, Mian-Lue-Ning and Manaoke represent three large orogenic gold districts in the Triassic West Qinling Orogen, the third largest gold province in China (similar to 1100 t gold resource). Although their component gold deposits have similar geological characteristics, representative deposits from these three gold districts have contrasting ore-mineral assemblages and gold speciation suggestive of diversity in gold source and/or ore-forming depositional conditions or processes. Decrease in P-T conditions of ore formation and metamorphic grades of host rocks in deposits from the Mian-Lue-Ning through Yangshan to Manaoke districts is matched by variation in their mineral paragenesis from pyrite-chalcopyrite-sphalerite-galena-native gold through pyrite-arsenopyrite-stibnite to scheelite-pyrite-arsenopyrite-stibnite-realgar. Invisible gold in the deposits of the Yangshan and Manaoke gold districts, contrasts with native gold in the Mian-Lue-Ning gold district. Concentrations of Ag-As-Au-Bi-Cu-Pb-Sb-Tl in hydrothermal pyrite show a gradual systematic increase from Mian-Lue-Ning through Yangshan to Manaoke. Pyrite in the Huachanggou gold deposit from the Mian-Lue-Ning district has high Co and low As and Au concentrations, in contrast with pyrite which is zoned with respect to As and Au in gold deposits from the other two districts. These variations between gold deposits in the different districts could be due to variable local ore-fluid sources derived from the different host-rock sequences during metamorphic devolatilization, as also allowed by mass-balance calculations. However, previously unmetamorphosed voluminous source rocks that have experienced amphibolite facies metamorphism, with liberation of gold and other metals during conversion of gold-bearing pyrite into pyrrhotite, are limited. It appears more likely that an external deeply-sourced fluid, most logically from devolatilization of the pyrite-bearing sediment wedge above the Triassic subduction zone associated with the formation of the West Qinling Orogen, was responsible for gold mineralization. In this model, variation between districts was due to fluid interactions with contrasting host-rock sequences under contrasting P-T conditions.