Petrographic and mineralogical composition of rocks and ores of the Urinsky ore field (Berelekh ore district, Northeast Russia)

The article presents the results of studing the mineral composition of ores and host rocks of three manifestations of gold ore mineralization in the Uryinsky ore field, which is part of the Chay-Yurinsky ore-placer unit of the Berelekh mineralogical region of the Yano-Kolyma metallogenic belt. Mineral and petrographic studies were carried out according to the traditional method with an emphasis on the degree of metasomatic changes and deformations of the constituent minerals. It was established that gold ore mineralization is paragenetically related to hydrothermal-altered dikes of the Nera-Bohapchinsky complex (J 3 nb), which in composition correspond to quartz diorite-porphyrites and diorite-porphyrites. Metasomatic changes are represented by two types of transformations: propylitization (chlorite, sericite, epidote, albite) and widely developed beresitization (quartz, sericite, chorite, carbonate, pyrite). Veins of quartz, chlorite-carbonate-quartz, carbonate-quartz, albite-carbonate-quartz composition with ore mineralization are superimposed on metasomatically transformed rocks. Three stages of ore formation were identified – metasomatic, hydrothermal and hypergenic. During the metasomatic stage, leucoxene and xenomorphic pyrite I, (less commonly) pyrrhotite, chalcopyrite and sphalerite (with a micro-mixture of Cd) are deposited in the rock. The hydrothermal stage is divided into four stages (plutonogenic – I, II) and volcanic-plutonogenic (III, IV); three are productive. The first stage is associated with the development of veins of drusoid quartz I, to the cracks of which clay minerals and limonite are confined; in the second stage, bismuth-containing galena and high-grade gold (929 ‰) are deposited; the third stage is characterized by the formation of quartz (II) feldspar veins with carbonate nests and the inclusion of short-prismatic arsenopyrite and pyrite II. Polysulfide minerals and native gold (692 ‰) were found along the cracks of the cataclase and in the caverns of these minerals. In the fourth stage, veins of chalcedony-like quartz are formed, containing rare inclusions of native gold, and framboidal pyrite, marcasite and long-prismatic arsenopyrite are deposited in the rock. The described manifestations of the Uryinsky ore field in terms of mineral composition and host medium can be attributed to the gold-quartz ore formation of the dyke type