Genesis of the Mianhuakeng granite-related uranium deposit, South China: Insights from cathodoluminescence imaging, fluid inclusions, and trace elements composition of hydrothermal quartz

Abstract

The Mianhuakeng deposit is the largest granite-related U deposit in China, with multiple hydrothermal quartz veins. However, its genesis and ore-forming processes are still under discussion. In the present study, we have attempted to unveil the characteristics of ore-forming fluids and the metallogenesis of Mianhuakeng deposit with the help of cathodoluminescence (CL) imaging, fluid inclusions (FIs), trace elements, as well as hydrogen-oxygen (H-O) isotope compositions of hydrothermal quartz associated with U mineralization. Based on the crystallography and CL textures, four generations of hydrothermal quartz have been identified: early stage QtzⅠ quartz, main stage QtzⅡa and QtzⅡb quartzes, and late stage QtzIII quartz. QtzⅠ was formed by a continuous fluid flow in an open fluid system. The formation of QtzIIa was associated with a brief period of fluid flow in a compressive fluid system, whereas QtzIIb crystallized in an open system with a stable fluid flow. QtzIII was produced by fluid chemical fluctuation in an open system. These quartzes show high Al, variable Li, Na, K, and Ca, moderate Rb and Ba, and low Ti, Fe, Ge, and U contents. Al3+ and Ge4+ were incorporated into the quartz crystal lattice by substituting Si4+, helped by the change compensators composed of Li, Na, K, Rb, Ca, and Ba. The FIs microthermometry results suggested that ore-forming fluids were NaCl–H2O ± CO2 system with moderate to low temperatures and salinities. A gradually decreasing trend has been recorded for temperatures from 225 to 353 °C, through 153–302 °C, and toward 105–161 °C, respectively, with salinities changing from 6.0 to 10.1 wt% NaCl equiv, through 0.6–10.1 wt% NaCl equiv, to 0.5–5.7 wt% NaCl equiv from early to late stages. The H-O isotopic results indicated that the ore-forming fluids were originated from a mixture of deep magmatic and meteoric waters, with an increase in the amount of meteoric water from early to late stages. Fluid decompression and immiscibility were triggered by episodic fracturing of NS-trending fractures, which resulted in a fluid flow transition from extrusion to opening and the breakdown of uranyl complexes. The Fe2+ from altered granites reduced free U6+ to U4+, forming pitchblende (UO2) in hydrothermal quartz veins. Therefore, the Mianhuakeng deposit is thought to be of polygenic in origin and is controlled by several geological factors such as fracturing, fluid immiscibility, and redox reaction.