Genesis of the world-class Dachang gold deposit, northern Qinghai-Tibet Plateau: A multiproxy approach

Dachang is a world-class gold deposit that occurs in the Bayan Har Terrane, northern Qinghai-Tibet Plateau, with Au resources of 121 t at an average grade of 3.0 g/t. Although this deposit was investigated in some previous studies, its genesis remains debated. The deposit is controlled by the Gande-Maduo anticlinorium and thrust fault zone, and the orebodies occur in subsidiary interlayer faults or fractures in thin-bedded, stratatoid, lenticular and vein forms. The host strata belong to the Lower Triassic Bayan Har Shan Group consisting of metasandstone, metasiltstone, slate, and carbonaceous slate interlayered with phyllite. The mineralization of Dachang can be divided into the following four stages. Stage 1 gold-sulfide-quartz veins, stage 2 gold-sulfide-carbonate-quartz veins, stage 3 stibnite-quartz veins and stage 4 quartz-carbonate veins. There are two types of ores: altered rocks and quartz veins. The main ore minerals are pyrite, arsenopyrite, stibnite and native gold, with trace amounts of tetrahedrite, chalcopyrite, galena and sphalerite. The wallrock alteration comprises mainly sulfida-tion, silicification, carbonatization, and sericitization. The element association is As, Sb, Au, Te, Bi, Ag, W and Pb in order of enrichment factors, whereas Cu, Mo, and Sn are depleted relative to their crustal abundances.In this study, we identified three types of primary fluid inclusions in quartz from veins of the four minerali-zation stages as follows: type 1 liquid-dominated two-phase aqueous inclusions; type 2 aqueous-carbonic in-clusions, including aqueous-dominated (type 2a) and carbonic-dominated inclusions (type 2b); and type 3 two-phase hydrocarbon inclusions. Microthermometric measurements of fluid inclusion assemblages (FIAs) and bulk fluid inclusion analyses show that the ore-forming fluids had similar low-salinity compositions (1.7 to 6.9 wt% NaCl equiv.) in the H2O-CO2-N2 system with trace amounts of hydrocarbons. The mineralization temperatures range from 150 to 200 degrees C, and estimated depths are approximately 5 to 6.5 km. Fluid/wallrock reactions and immiscibility or boiling during mineralization in response to pressure fluctuations in a brittle faults were the main triggers for gold precipitation at Dachang.The hydrogen, oxygen, carbon and He-Ar isotopic compositions indicate that the ore-forming fluids originated from metamorphic devolatilization of immediate-and deep-level country rocks and no meteoric water or mantle volatiles were involved. The organic carbon proportions account for 42 to 53% of the total carbon in ore-forming fluid, and carbon fractions from sedimentary carbonates range from 47 to 58%. The gold mineralization followed immediately after the regional metamorphism (240-225 Ma) and occurred when the host rocks had exhumated into a brittle environment. The Dachang deposit is an epizonal orogenic gold deposit.