Features of orogenic gold-quartz mineralization of the Fedorovsky ore-placer field (Gornaya Shoriya) on the example of Lazaretny and Fedorovsky ore occurrences

New mineralogical-geochemical and thermobarogeochemical data on the nature and conditions of the formation of orogenic gold-quartz mineralization of the Lazaretny and Fedorovsky ore occurrences were derived. They are localized in the identical geological structure, however differ in their content composition and genesis. Both objects were an important source of native gold from the Fedorovka River placer, the largest placer in Gornaya Shoria, and deserve to be studied. The aim of the study is to identify the genetic features of gold mineralization that caused such significant mineralogical and geochemical differences. The age of mineralization (³⁹Ar/⁴⁰Ar by the sericite method) and the age of granodiorites of the Fedorovskaya intrusion (U-Pb by the zircon method) were determined to clarify the role of the intrusion in the formation of gold mineralization. The gold mineralization is concluded in the long-lived tectonic zone at the northern face of the Mrassky anticlinorium. It is represented by vein-metasomatic linear zones among essentially black-shale deposits and volcanogenic rocks V-Ci metamorphosed in the green-shale facies. The gold mineralization is located near dikes and sills of gabbro-dolerites of C and Di.₂ ages. It was formed as a result of tectonic processes associated with the closure of the Paleoasiatic Ocean at the end of C1. Mineral associations and geochemical profiles of mineralization indicate the metamorphogenic-hydrothermal (Au-As) character of the Lazaretny ore occurrence and the magmatogenic-metamorphogenic (Au, Ag, Te, Ba, Se, As, Sb, Pb, W, Bi, Cr, Ni) Fedorovsky. The first has a simple mineral composition and is formed in 2 stages: pyrite-arsenopyrite-pyrrhotite and chalcopyrite-sphalerite-tetrahedrite. Native gold is small and fine, has relatively high fineness, it is found both in quartz veins and in metasomatites . The Fedorovsky ore occurrence has an early pyrite-pyrrhotite and a late, sulfide-sulfosol-sulfotelluride productive stage of mineralization. Native gold is larger, lower fineness, associated with low-temperature minerals (hessite, argentite, freibergite, pyrargyrite, polybasite, etc.). Instead of arsenopyrite, gersdorfite is found in the ores. The main amount of gold is found in quartz veins. The study of gas-liquid inclusions in quartz by cryothermometry and Raman spectroscopy showed that the gold mineralization was formed at a temperature of 190-275 °C by weakly concentrated NaCl solutions (3.3-6.6 wt.% NaCl-eq.). The gas phase of the inclusions consists of CO2 and N2, with a small amount of CH4. CO2 predominates over N2. If carbonaceous shales predominate among the rocks containing mineralization, then there is a weak direct correlation between the increased N2 content in the inclusions and gold in the sample. The age of the gold mineralization is very different from the age of the granodiorites : 322±4-291±1.3 million years (C2-P1) and 410±1.5 million years (D1), but the gold-bearing devonian dolerite dikes, probably paragenetically related to granodiorites, played an important ore-forming role in the Fedorovsky ore occurrence. The dikes were an important source of gold during the metamorphogenic-hydrothermal ore formation in the period from C2 to P1, which caused the magmatogenic-metamorphogenic character of gold mineralization.

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