首页|Gold deposits and occurrences of the Greater Caucasus, Georgia Republic: Their genesis and prospecting criteria
Gold deposits and occurrences of the Greater Caucasus, Georgia Republic: Their genesis and prospecting criteria
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The south-central part of the Greater Caucasus region, Georgia Republic, represents an extremely prospective region for significant orogenic gold deposits. Gold-bearing quartz veins are concentrated in two extensive WNW-trending belts, the Mestia-Racha and Svaneti districts, within the northern margin of the Southern Slope Zone of the Great Caucasus orogen. This metalliferous region is dominated by Early to Middle Jurassic slates, which are part of a terrane that likely accreted to the continental margin from late Paleozoic to Jurassic. The slates were subsequently intruded by both Middle to Late Jurassic and Neogene granitoids. Quartz veins in the more carbonaceous slate units are most consistently enriched in As, Au, Hg, Sb, and W, and show mineralization styles most consistent with typical orogenic gold deposits. Quartz veins in the Mestia-Racha district were mined in Soviet times for As, Sb, and W, but many of these are now being recognized as gold resource targets. The veins occur in the footwall of a thrust fault between the Southern Slope zone and an earlier accreted terrane, the Main Zone, to the north. Many veins in the district continue along strike for >1 km and some cut Neogene intrusions, constraining ore formation to the most recent 4 to 5 million years. Gold deposition thus correlates with final collision of the Arabian plate to the south and uplift of the ore-hosting Greater Caucasus. The Zopkhito deposit, previously mined for antimony, contains an estimated 55 t Au at a cutoff grade of 0.5 g/t. The veins are localized in an area where smaller-order structures show a major change in strike from N-S to more E-W trends. A pyrite-arsenopyrite ore stage includes gold concentrated in both sulfide phases; it is overprinted by a later stibnite-dominant stage. Fluid-inclusion studies of ore samples from the Zopkhito deposit indicate minimum trapping temperatures of 300 to 350 deg C and 200 to 300 deg C for the two stages, respectively, and minimum trapping pressures of 0.2 to 0.5 kbar. Ore-forming fluids, with approximately 5 to 20 mol percent non-aqueous gas, evolved from N_2-dominant to CO_2-dominant during evolution of the hydrothermal system. delta~(34)S values of +1 to +4 percent, for ore-related sulfides at Zopkhito are consistent with a sedimentary rock source for the sulfur, and delta~(34)O quartz measurements of 16 to 21 per thousand are consistent with either a magmatic or metamorphic fluid. More than 60 gold-bearing lodes and placers in the Svaneti district occur along the thrust between the Southern Slope and Main Zones. Lode gold potential was first recognized in the historic placer district in the 1980s, with many auriferous quartz veins cutting Middle Jurassic igneous rocks. Brecciated veins in the 18 t Au Lukhra deposit cut a small granodioritic to dioritic stock; the latter intrudes Devonian schist immediately north of the thrust. Presently, there are three recognized ore zones in the deposit, with the most significant occurring over an area 140 m in length and 12 m-wide, with typical grades of 7 to 9 g/t Au. Reconnaissance fluid-inclusion studies of ore samples from the Lukhra deposit indicate minimum trapping temperatures of 220 deg C. Measurements of delta~(18)O_(quartz) of about 10 per thousand suggest buffering of isotopic composition by the igneous host rocks.
Greater CaucasusGeorgia RepublicOrogenic goldMestia-Racha districtSvaneti district
Sergo A. Kekelia、Maren A. Kekelia、Simon I. Kuloshvili
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Geological Institute of Academy of Sciences of Georgia, M. Alexidze str.1/9, Tbilisi, 0193 Georgia
NSTL
