China'S First Independent Cobalt Deposit And Its Metallogenic Mechanism: Evidence From Fluid Inclusions And Isotopic Geochemistry

The Tuolugou cobalt deposit is the first independent large-scale Co- and Au-bearing deposit discovered in northwestern China. It is located in the eastern Kunlun orogenic belt in Qinghai Province, and occurs conformably in low-grade metamorphic volcano-sedimentary rock series with well-developed Na-rich hydrothermal sedimentary rocks and typical hydrothermal sedimentary ore fabrics. Fluid inclusions and isotopic geochemistry studies suggest that cobalt mineralizing fluid is dominated by NaCl-H(2)O system, accompanied by NaCl-CO(2)-H(2)O-N(2) system responsible for gold mineralization. Massive, banded and disseminated pyrite ores have similar compositions of He and Ar isotopes from the mineralizing fluid, with (3)He/(4)He range between 0.10 to 0.31Ra (averaging 0.21Ra), and (40)Ar/(36)Ar between 302 and 569 (averaging 373), which reflects that Co mineralizing fluids derived dominantly from meteoric water deeply circulating. delta(34)S values of pyrite approaches to zero (delta(34)S ranging from -4.5 parts per thousand to +1.5 parts per thousand, centering around -1.8 parts per thousand to -0.2 parts per thousand), reflecting its deep source. Ore lead is characterized by distinctly high radiogenesis, with (206)Pb/(204)Pb>19.279, (207)Pb/(204)Pb>15.691 and (208)Pb/(204)Pb>39.627, and its values show an increase trend from country rocks, regional Paleozoic volcanic rocks to ores. This may have suggested that high radiogentic ore Pb derived mainly from country rocks by leaching meteoric water-dominated hydrothermal fluid during its circulation at depth. Cobalt occurs mainly in sulfide phase (such as pyrite), but cobalt enrichment, and presence and increasing contents of Co-bearing minerals have a positive correlation with metamorphic degree. The Tuolugou deposit and other typical strata-bound Co-Cu-Au deposits have striking similarities in the geological features and metallogenic pattern of primary cobalt All of them are syngenetic hydrothermal exhalative sedimentation in origin.