Ag-Sb deposits of the Yustid depression, eastern Russia and northwest Mongolia

Ag-Sb deposits of a new economic district that occurs on the boundary of th e southeastern Altay Republic of eastern Russia and northeast Mongolia are herein described. Mineralization of this region is divided into two periods -(1) Late Hercynian (Mo-W deposits) and (2) Mesozoic (Sn-W, as well as Ni-C o-As, Ag-Sb, and Sb-Hg deposits). The deposits of main economic interest, s ilver-antimony, consist of siderite-sulfosalt veins and vein zones that occ ur in contact-metamorphosed coal-bearing terrigenous rocks of Devonian age. These veins occur to depths of up to 900 m, and occur along strike for 3 t o 3.5 km; their thicknesses range up to 12 m. The deposits are mineralogica lly and geochemically similar to silver deposits of the Yakutian, Pamir, Sl ovakian, Coeur D'Alene (USA), and Freiberg (Germany) ore districts. The mai n ore minerals are quartz, carbonate, and Ag tetrahedrite, with minor chalc ostibnite, chalcopyrite, bournonite, zinkenite, arsenopyrite, and pyrite. S ulfides are characterized by widespread isomorphism of bismuth and antimony . The deposits are characterized by a metal suite of Ag, Sb, Ri, Cu, Yb, Zn , As, and Hg, with the Ag content in the ores ranging from 250 to 600 g/t. Fluid inclusion studies demonstrate that the siderite-sulfosalt veins forme d over a temperature interval of 200 to 70 degrees C and pressures of 500 t o 120 bar from highly concentrated NaCl-CaCl2-FeCl2 solutions, with CO2, CN 4, and N-2 dominating the gas phase. A vertical gradient of 9 degrees C/100 m characterized the beginning of ore formation. Investigations of O, S, Pb , and Sr isotope compositions show that ore-bearing fluids were related to the final stages of crystallization of a deep magmatic center, with subsequ ent crystallization of Sn-W, Cu-Co-W, Ni-Co-As, and Ag-Sb ores.