A New Type of Ore Mineralization in Precambrian Conglomerates on the Western Slope of the Southern Urals

摘要

Precambrian conglomerates are known in the Middle Riphean Mashak Formation (R_2) on the western slope of the southern Urals. The most complete Mashak section is observed in the Shatak Graben area within the Middle Riphean paleoriftogenic structure (Fig. 1). The section is composed of terrigenous, magmatic, and vol-canosedimentary rocks. Based on lithology and bedding succession, the Mashak Formation is divided into eight subformations. Volcanosedimentary rocks dominate in subformations 1 and 6; volcanic rocks, in sub-formations 2 and 4; and terrigenous rocks, in subformations 3, 5, 7, and 8. Terrigenous rocks (～75 vol %) are mainly composed of the coarse-grained varieties (sandstones and conglomerates of the quartz-quartzite composition) with a subordinate amount of siltstones and carbonaceous shales. Conglomerates make up units up to 20-25 m thick and 10-12 km long in the Kuz"elga (R_2ms_1), Kuyantav (R_2ms_5), and Karan (R_2ms_6) subformations. They are observed to be pebbles and rubbles that include all transitional varieties. The clastic material consists of well-rounded pebbles and rubbles of quartzitic sandstones, quartzites, and less common quartz. The matrix is composed of the quartz-sericite, quartz-sericite-chlorite, and less common epidote-chlo-rite aggregates with a variable content of magnetite, hematite, and sulfldes.