Geology and mineralogy of East Kazakhstan gold-bearing jasperoids (by the example of the Baybura ore field)

Gold-bearing ore occurrences and deposits in jasperoids are objects of a new unconventional type widespread in the West Kalba auriferous belt in eastern Kazakhstan. We present original data on the structural and geologic position and mineralogy of gold-bearing jasperoids of the Baybura ore field, which is a standard object of this type in eastern Kazakhstan. Gold-bearing jasperoids are localized here in shale–carbonate strata of island-arc type (C₁) that underwent metasomatism in superintrusive zones of the Kunush complex plagiogranites (C₃). Free finely dispersed gold shows typical signs of hypogenous changes. The content of Au varies from 0.1 to 33.5 ppm, and that of Ag, from 0.05 to 2.5 ppm. Based on the data obtained, a geologo-genetic model of the formation of the Baybura gold-bearing jasperoids is proposed. According to the model, the ore deposition is related to the impact of juvenile fluids of plagiogranitic magma on carbonate substrate. The main factors determining the evolution of gold-bearing metasomatites are their localization in zones of high tectonic activity and a drastic change in the acidity–alkalinity regime of solutions in the carbonate environment. Native gold grains are extremely fine and thin and contain a permanent impurity of Hg (0.2–0.7 wt.%). In mineralogical and geochemical features the gold mineralization of the Baybura ore field is similar to that of the Suzdal’ deposit and can be assigned to the Carlin type.