Geological setting, nature of ore fluids and sulphur isotope geochemistry of the Fu Ning Carlin-type gold deposits, Yunnan Province, China

Carlin-type gold deposits in southern China are present in Palaeozoic to Mesozoic siliciclastic and carbonate rocks. The border region of Yunnan, Guizhou and Guangxi Provinces contains gold deposits on the south-western margin of the Pre-Cambrian South China Craton in south-eastern Yunnan Province. The Fu Ning gold deposits host epigenetic, micron-sized disseminated gold in: (i) Middle Devonian (D1p) black carbonaceous mudstone at the Kuzhubao gold deposit and (ii) fault breccia zones at the contact between Triassic gabbro (βμ51) and the Devonian mudstone (D1p) at the Bashishan gold deposit. The deposits are associated with zones of intense deformation with enhanced permeability and porosity that focused hydrothermal fluid flow, especially where low-angle N-S striking thrust faults are cut by NW striking strike-slip and/or NE striking normal faults. Major sulphide ore minerals in the Fu Ning gold deposits are pyrite, arsenopyrite, arsenic-rich pyrite, stibnite and minor iron-poor sphalerite. Gangue minerals are quartz, sericite, calcite, ankerite and chlorite. Hypogene ore grades range from 1 to 7 gt-1 Au and up to 18 gt-1 Au at the Kuzhubao gold deposit and are generally less than 3 gt-1 Au at the Bashishan gold deposit. Sub-microscopic gold mineralization is associated with finely disseminated arsenic-rich pyrite in the Stage III mineral assemblage. Two types of primary fluid inclusions have been recorded: Type I liquid-vapour inclusions with moderate-to-high liquid/vapour ratios, and Type II inclusions containing moderate liquid/vapour ratios with CO2 as determined from laser Raman analysis. Temperature of homogenization (Th) data collected from these primary fluid inclusions in gold-ore Stage III quartz ranged from 180 to 275°C at the Kuzhubao gold deposit and 210 to 330°C at the Bashishan gold deposit. Salinity results indicate that there were possibly two fluids present during gold deposition, including: (i) an early fluid with 0.8-6.5 wt.% NaCl equivalent, similar to salinity in shear-zone-hosted gold deposits with metamorphic derived fluids; and (ii) a late fluid with 11.8-13.4 wt.% NaCl equivalent, indicating possible derivation from connate waters and/or brine sources. CO2 and trace CH4 were only detected by laser Raman spectrometry in gold-ore-stage primary fluid inclusions. Results of sulphur isotope studies showed that δ34S values for pyrite and arsenopyrite associated with gold-ore mineralization during Stage III at the Kuzhubao and Bashishan gold deposits are isotopically similar and moderately heavy with a range from +9 to +15 per mil, and also fall into the range of δ34S values reported for Carlin-type gold deposits. Sulphur isotopes suggest that the Fu Ning gold deposits were formed from connate waters and/or basinal brines. Fluid geochemistry data from the Fu Ning gold deposits suggest a Carlin-type genetic model, involving fluid mixing between: (i) deep CO2-rich metamorphic fluids, (ii) moderately saline, reduced connate waters and/or basinal brines; and (iii) evolved meteoric waters.