Mineralogical and Geochemical Features and Formation Conditions of the Tardan Gold–Sulfide–Quartz Deposit (Northeastern Tuva)

Abstract

—We studied the mineralogical and geochemical features and formation conditions of productive mineral assemblages of the Tardan gold–sulfide–quartz deposit located in the endo-/exocontact zone of the Kopto–Bai-Syut gabbro-diorite–plagiogranite pluton of the Early Ordovician Tannu-Ola complex (O1tn). Postskarn mineralization of vein–dissemination type in skarns, quartz diorites, and carbonate rocks is limited by tectonic crushing zones and conjugated with beresitization and listwanitization of the ore-bearing rocks. Mineralogical and geochemical research has shown the formation of ultrahigh-fineness (986–952‰) and high-fineness (947–918‰) gold at the first productive gold–quartz–calcite substage, of high-fineness gold (918–904‰) → medium-fineness gold (896–809‰) → low-fineness gold (798–756‰) ± hessite Ag2Te ± volynskite AgBiTe2 at the second productive gold–telluride–sulfide–quartz–carbonate substage, and of medium-fineness gold (897–802‰) → low-fineness gold (799–717‰) → electrum (691–612‰) → mercurian electrum (471–451‰) ± hessite Ag2Te ± acanthite Ag2S ± matildite AgBiS2 at the third productive gold–sulfosalt–sulfide–quartz substage. High- and medium-fineness gold prevails in the ores, ultrahigh- and low-fineness gold is subordinate, and electrum and mercurian electrum are scarce. The fineness of native gold in the ores varies from 451 to 986‰, averaging 858‰. The productive mineral assemblages of the Tardan deposit formed from aqueous fluids containing Mg, Na, and K chlorides (salinity of 6.1–12.9 wt.% NaCl eq.), with a decrease in the mineral formation temperature from 380 to 150 ºC and variations in fO2, fS2, fSe2, and fTe2.