Abstract
In the 1980s, Ag-Sb deposits were discovered in a new ore cluster of the Delyun-Yustyd back-arc rift basin in southeastern Altai and northwestern Mongolia. The Delyun-Yustyd basin is filled with up to 9 km thick Devonian volcanosedimentary and terrigenous rocks lying over Vendian-Cambrian carbonate-terrigenous strata. Volcanosedimentary rocks are intruded by D 3-C 1 granites of the Yustyd complex, as well as by pre-granite mafic rocks, post-granite gabbro-diabase, diabase porphyry dikes, and younger lamprophyre dikes of the Chuya complex (245–236 Ma Ar-Ar biotite age). Ag-Sb ores in the Yustyd cluster formed at the Early Mesozoic stage at 240±1.6 Ma (Early-Middle Triassic boundary) and were thus nearly coeval with the Chuya lamprophyre intrusion. The age of Cu-Ag-Sb-Hg mineralization is about 234.4±1.0 Ma (Ar-Ar, sericite). Sb-Hg mineralization is the latest in the province (231.5±1.0 Ma, Ar-Ar, sericite). Gradual change of Ag-Sb (siderite-tetrahedrite) ores to complex Cu-Ag-Sb-Hg (Hg-Ag-tetrahedrite) and Sb-Hg (cinnabar with Sb sulfosalts and stibnite) mineralization along the Kurai-Kobdo and Terekta-Tolbonur large faults is interpreted as development of stage metallogenic zoning along major ore-controlling faults.
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