Formation Conditions and ore potential of black shales of the Udokan Group, East Siberia

Abstract

Peculiarities of formation and ore potential of black shales of the Paleoproterozoic Udokan Group are considered. They are compared to the stratotype (Khokholma Formation, Sukhoi Log gold deposit). The black shales are confined to lower parts of the Paleoproterozoic sequence of the Udokan Group. They are observed in sections of the Chitkanda, Inyr, Ayan, and Ikab’ya formations. The black shale complex of the Udokan Group is divided into three levels of Au concentration. The first level with background Au contents is typical of barren shales; the second level is confined to sulfidized rocks; and the third level is related to sulfide-quartz veins in the black shales. The average Au content is 0.12 g/t in the sulfidized shales and 0.28 g/t in the sulfide-quartz veins. Black shales of the Udokan Group and Khokholma Formation were formed in a relatively deep shelf zone, while those of the Shaman Formation precipitated in a shallow zone. Protoliths of black shales of the Udokan Group contained graywackes, diorites, syenites, basic rocks, and tuffites. Protoliths of black shales of the Khokholma Formation were produced by the intense weathering of basic and ultrabasic rocks.