Abstract
The Uyanga ore knot district of the Khangay metallogenic zone are hosted by the lower-middle Devonian volcanogenic-sedimentary Erdenetsogt formation. About 40 samples were collected from the host rocks, veins and quartz veins in the Uyanga ore knot district in 2016. The new Burgetei, Ult and Senjit gold occurrences were studied. The quartz-sulfide, gold-arsenic and gold-antimony-mercury mineralization are determined in the berecitization, silicification, limonitization and glauconization altered metasomatic zones within the Uyanga ore knot districts. The rocks of the Erdenetsogt formation have an irregular gold content: 0.96 g/t Au is determined in quartz vein taken from trench of the Burgetei occurrence (BG-7/16), Au content is highest up to 3.5 g/t in the quartzite-jasper (Ult-7/16 and Ult-9/16) cut by quartz veins in the Ult occurrence. The Senjit occurrence represents Au-Hg-Sb epizonal level of orogenic gold deposits structure with highest Hg content up to 851 ppm. This year no sulfide minerals were found in the siltstone of this occurrence. The Au content of arsenical pyrite of the Burgetei and Ult is below the detection limit by electron microprobe analysis. The Au content of arsenopyrite of the Ult occurrence is highest (up to 238 ppm). Therefore, the ore-mineral assemblages in the gold occurrences reflect the differences between the three explored sites, formed in the course of fluid evolution during the fluid-rock interaction. Variable concentrations of indicative elements (As, Te, Sb, Hg) and their ratios confirm this fact. The geodynamic position, the type of the hydrothermal alteration of both igneous and sedimentary rocks, textures and mineral assemblages, the mineralization sequences are consistent with orogenic classification for the Burgetei, Ult and Senjit gold occurrences.
Highlights
The classical theory implies that orogenic Au deposits form during the late stage of orogeny and are typically hosted by rocks of greenschist to amphibolite metamorphic grade that may be spatially related to granitoid intrusions (Groves et al, 1998; Goldfarb et al, 2001)
Arsenopyrite and pyrite represent the most abundant sulphide minerals that occur in association with the Au mineralization, which formed from low salinity, CO2-bearing, reduced and near-neutral pH fluids that show a wide range in fluid temperatures from about 200 to 500°C (Goldfarb et al, 2005; Keith et al, 2018)
The siltstone of volcanogenic-terrigenic Erdenetsogt formation cut veins and structures of metasomatic alteration; ore controlling transition zone with the brittle-ductile deformation is clearly seen in the second trench of the Burgetei occurrence (Fig. 2a)
Summary
The classical theory implies that orogenic Au deposits form during the late stage of orogeny and are typically hosted by rocks of greenschist to amphibolite metamorphic grade that may be spatially related to granitoid intrusions (Groves et al, 1998; Goldfarb et al, 2001). The primary ore occurrences of the Uyanga gold ore knot district of the Khangay gold zone are hosted in volcanogenic-sedimentary rocks of the lowermiddle Devonian Erdenetsogt formation (Fig. 1). Orolmaa and other (2008) investigated the Permian-Triassic granitoid magmatism and metallogeny of the Khangay upland (Fedorova,1975) They have indicated that gold mineralization in the area that is confined within the Khangay gold belt appears to be independent of the granitoid intrusions. In terms of the geological conditions, mineral assemblages, chemical composition of ore minerals, ratios of indicator elements and isotopic investigation an assumption is made to clarify the genetic processes of the ore occurrences formation, because they are perhaps the typical orogenic gold occurrences. Mineralization age of gold deposits depends on geodynamic condition of each specified area, most significant genetic type among them is orogenic gold deposits (Delgertsogt, 2017)
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