Silver–antimony deposits of Central Asia: physico-chemical model of formation and sources of mineralisation

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Questions on the origin of Ag–Sb deposits are considered for several large economic ore districts in Central Asia: southeast Pamir (Tajikistan), Talas in northern Tien Shan (Kyrgyzstan), southeast Altai and northwest Mongolia, and Verkhoyansk province (Yakutia). The Ag–Sb mineralisation of Central Asia is spatially and temporally associated with alkaline mafic magmatism, and occasionally with coeval granitoid magmatism. Ag–Sb deposits have close spatial and temporal relationships with granitoid and alkaline mafic magmatism. The Ag–Sb deposits were formed in the temperature range 50–280°C, from Ag-specific reduced two-phase hydrothermal fluids containing a highly concentrated chloride solution (NaCl > FeCl2 > CaCl2 > KCl > MnCl2) and a high-density CO2+ CH4 ± N2 gaseous phase. Fluid salinities range widely from 40 to 0.1 wt% NaCl equiv. Geochemical specialisation of Ag–Sb ore is caused by chemical features of chloride brines enriched in Ag, Sb, Pb and Cu, with a relatively low Au content (≤10−6 mol/kg) within a pH of 3.5–4. The highest temperatures, concentrations of solutions and ore elements in the ore fluids were found in early quartz of siderite–tetrahedrite veins from below-ore level of the ore-forming system. Vertical zonation of the Ag–Sb mineralisation is caused by precipitation of different mineral associations from the same solution with temperatures decreasing towards the upper parts of the system. Sufficient vertical range of mineralisation (600–800 m) and relatively low Ag contents in the ore (300–600 ppm) result from the temperature decrease and absence of geochemical barriers. Higher temperature gradient and dilution of magmatogenic fluid with meteoric water was a geochemical barrier for the Ag-rich ore formation in a narrow (about 400 m) main ore deposition zone. Lateral zonation of Ag–Sb mineralisation is related to a staged development of the ore-forming process and deposition of different stage mineral associations at the beginning and end of the hydrothermal process. The Ag–Sb deposits are polygenic in terms of sources of ore elements and ore-forming fluids. Ore elements may have been inherited from magmatic and terrigenous host-rocks of different ages. Temporal correlation of the Ag–Sb mineralisation with alkaline mafic magmatism, He isotope composition and Hg contents in the ores confirm participation of a mantle source in the formation of the Ag–Sb deposits.