Geology, fluid inclusion, bitumen and isotope geochemistry of the organic-matter-rich Nanmushu lead–zinc deposit, Mayuan, the northern margin of the Yangtze platform, China

The Nanmushu lead–zinc deposit is located in the northern margin of Yangtze plate, midwest China. Ore bodies are hosted in Sinian carbonate rocks, which is controlled by the Sinian Dengying Formation. Metal mineral includes sphalerite, galena, pyrite, gangue minerals including dolomite, quartz and small amount of barite and calcite. The ores have breccia, veins, disseminated and geode structure. Three stages of hydrothermal mineralization are: an early stage of bitumen (B 1 ) (Stage I), a middle stage of sphalerite–galena (Stage II) and a late stage of bitumen (B 2 ) (Stage III) based on the ore structure, textures, crosscutting relationship and mineral assemblages. The reflectivity and total organic carbon content (TOC) values of natural bitumen, combining crosscutting relationships between the bitumen and various minerals, clarify that the B 1 (early-stage bitumen) is closely related to mineralization; the Cambrian Guojiaba Formation provides abundant organic matter for natural bitumen in the Dengying Formation. Four types of FIs were identified: liquid-rich (L 0 -type), vapor-rich (V-type), single-phase, aqueous (ML-type) and single-phase, pure-gas FIs (MV-type). The ore-forming fluids were characterized by medium-to-low temperature (100–280 °C), medium salinity (2–16 wt% NaCl eq.), a medium density (0.91–1.07 g/cm 3 ), contained CH 4 and small amounts of bitumen, H 2 S. The petrography and microthermometric data indicate that the small scale of fluid boiling is inferred to take place in sphalerite–galena stage. The δD (-113.0% to -41.6%) and δ 18 O H2O (2.69% to 12.73%) data from stage I to stage III indicate that the ore-forming fluids were derived from basin fluids and in the late stage of hydrothermal mineralization mixed with organic water or meteoric water. The δ 13 C PDB and δ 18 O SMOW values of hydrothermal calcite range from –3.1 to –1.6 ‰ and from 15.6 to 18.3 ‰, respectively, and are plotted near the dissolution line of carbonate rock and marine carbonate rock, indicating that the carbon in the ore-forming fluids may have been derived from the dissolution of marine carbonates in the sedimentary rocks. The δ 34 S values (δ 34 S Py , δ 34 S Sp and δ 34 S Ga ) have a narrow range, inferring a single sulfur source. The δ 34 S CDT values of sphalerite, galena, and pyrite from hydrothermal ore-forming stage 2 range from 13.7 to 19.4 ‰, average 17.09 ‰, 12 to 16.8 ‰, average 14.61 ‰, 18.6 to 18.8 ‰, average 18.7 ‰, respectively. The barite has obviously different δ 34 S CDT values, ranging from 31.3 to 33‰, average 32.2 ‰. It is concluded that thermochemical sulfate reduction (TSR) is the main formation mechanism of reduced sulfur in ore-forming fluids by comprehensive analysis.