Fluid-rock interactions recorded by bulk-rock and mica compositions in the Xiuwacu granite-related quartz-vein W–Mo deposit, western Yunnan Province, China

Abstract

The Xiuwacu deposit is a typical granite-related quartz-scheelite-molybdenite vein type deposit hosted within sandstones and sandy slates in the southern Yidun Terrane, western Yunnan Province, China. The Xiuwacu complex comprises the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry (MGP) and aplite granite. Extensive and intensive alteration including potassic, propylitic, phyllic, silicic and argillic alteration developed within the Late Cretaceous monzogranite porphyry. Greisenization and pegmatization occurred in the roof of the Xiuwacu complex, suggesting large-scale fluid-rock interactions. However, the effects of fluid-rock interactions on the elemental exchange and ore-forming processes at Xiuwacu still remained unclear. This study firstly reported whole-rock major and trace elements of representative altered rocks and in situ major and trace elements of micas from fresh and altered rocks, and quantitively constrained the mobilization of elements by normalized isocon diagrams. The altered MGP are characterized by higher loss on ignition (LOI) values (>2), lower light rare earth elements (LREEs) and variable large ion lithophile elements (LILEs) contents, with higher Cs, K, Rb and Ba contents and lower Sr content than fresh MGP. The classification diagram based on mica composition shows that magmatic biotites from the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry belong to Fe biotites. Hydrothermal micas from pegmatoid and greisen belong to Li-phengites and zinnwaldites, and hydrothermal micas from quartz-molybdenite vein are classified as zinnwaldites. Li-phengites and zinnwaldites have relatively lower F contents, higher W, Sn, Li, Cs contents than biotites, intergrown with calcite and ankerite, suggesting CO2-rich, F-poor and rare metal-rich nature of hydrothermal fluids. Based on a compilation of fluid inclusions, H–O–S-Sr isotopes, scheelite geochemistry from previously published literatures and compositions of altered rocks and micas from this study, the tungsten ore-forming processes at Xiuwacu could be concluded as follows: (1) the initial moderately-oxidized, F-rich fluids exsolved from parental magmas of MGP form fluorite-quartz assemblages (pegmatoid; 400–300 °C); (2) the F-poor, W-rich, moderately acid fluids cause the breakdown of Ca-rich plagioclases, which consumes H+ and provides Ca2+ for the precipitation of early-stage scheelites (greisen; approximately 300 °C); the evolved nearly neutral and medium- to low-temperature fluids (300–150 °C) drive the fluid-rock interactions to supply Ca2+ for the deposition of main-stage scheelites. The deposition of Mo was not determined by the fluid-rock interactions, but by the ore-forming fluids transformed from oxidized to reduced nature.