矿床地球化学国家重点实验室知识库

The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO₂-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C₂H₆, vapor CH₄, CH₄–H₂O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and ⁴⁰Ar/³⁹Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.