GEOCHEMISTRY, MINERALOGY, AND SOURCES OF SUBSTANCE OF QUARTZ VEINS AND QUARTZITES IN THE SOUTH OF THE BORSHCHOVOCHNYI RIDGE (<i>Transbaikalia</i>)

Quartz veins and quartzites of the Unda quartz vein zone (southern part of the Borshchovochnyi Ridge) of the Argun block of the Mongol–Okhotsk Orogenic Belt have been studied using modern high-precision methods. The mineralogical, petrographic, and geochemical features of quartz of the Unda zone have been first described in detail. The vein quartz and quartzites sampled at the tectonic contact between the Middle–Late Jurassic igneous granitoids of the Borshchovochnyi complex and the overlying volcanosedimentary rocks of a Middle Jurassic accretionary complex (Talangui Formation) are intensely deformed and mylonitized. The quartz veins that formed in metamorphic rocks located at a distance from the tectonized zone (Dzhida site) are weakly granulated. The parameters of fluid inclusions in vein quartz at the Borshchovochnyi and Talangui sites were determined. The quartz veins and quartzites are genetically related to the host rocks and inherit their geochemical and isotopic features. Deformation processes led to a decrease in the content of impurity elements and a change in their proportion in the quartz veins and quartzites.