Study Of The Geological Characteristics And Genesis Of Dachang Gold Deposit, Qinghai Province

The Dachang gold deposit is located in the North Bayan Har orogenic belt within theSongpan-Ganzi Fold Belt, a late Triassic orogen in the northeast part of the TibetanPlateau, NW China. Gold ore bodies mainly occur along secondary faults or interstratifiedfracture zones in the Bayan Har group composed of metaturbidites. Auriferous veinsconsist of silicificated cataclasites which often contain <15%sulfides, and sulfideminerals are mainly composed of arsenopyrite, pyrite and stibnite. Gold are commonlyenclosed within arsenopyrite and pyrite. The host rocks mainly experienced silicification,sericitization and weak carbonatization.Based on microthermometic analyses of fluid inclusions, it was considered thatmetallogenic stages could be divided into barren mineralized quartz, quartz sulfide,quartz stibnite, visible-gold-bearing quartz and quartz calcite ones early and late. Barrenmineralized quartz veins （metallogenic temperatures are about350℃, and homogenoustemperatures are280³60℃）, native gold-quartz-sulfide cataclastic ores （metallogenictemperatures are about301℃, and homogenous temperatures are220²80℃）,gold-quartz-stibnite ores （homogenous temperatures are160²20℃）, and nativegold-quartz veins （homogenous temperatures are160²20℃）, formed respectively in theformer four stages, and little silicification and carbonification （homogenous temperatureis low than160℃） could happen within theabove four types of rocks or ores during the latest quartz calcite stage. Accompany with Laser Raman spectrum analyses, it wasconcluded that the metallogenic fluids underwent low salinity H₂O-CO₂-NaCl fluidsystem in lithostatic pressure system （metallogenic pressures is215MPa, metallogenicdepth is8.1km） in the early stage, low salinity H₂O-NaCl fluid system in lithostatic tohydrostatic pressure system （metallogenic pressures are49¹08MPa, metallogenicdepths are5.5⁸.6km） in the middle stage and low salinity H₂O-NaCl fluid system underhydrostatic pressure system （metallogenic pressures may be low than40MPa） in the lateststage, respectively.By isotopic studying, gold-bearing quartz grains have δ¹⁸O values ranging from16.9to21.2‰and the δ¹⁸O values of fluid that was equilibrium with the quartz range from6.2to9.6‰. δD values for fluid inclusions in quartz range from-90to-72‰. δ34S valuesof gold-bearing sulfides mainly range from-5.9‰to-2.8‰. Pyrite and arsenopyrite ofthe ores have²⁰⁶Pb/²⁰⁴Pb ratios ranging from18.2888to18.4702,²⁰⁷Pb/²⁰⁴Pb ratios from15.5763to15.6712, and²⁰⁸Pb/²⁰⁴Pb ratios from38.2298to38.8212. These isotopiccompositions indicate that the ore-forming fluids are of a metamorphic origin, and thesources of S and Pb may have derived from the host metaturbidites of the Bayan HarGroup. The Dachang deposit has geological and geochemical features similar to typicalorogenic gold deposits, suggesting that the Dachang gold deposit belongs to orogenicgold deposit and formed when the Songpan-Ganzi Fold Belt was intensely deformed intoan orogenic belt by folding and thrusting in the late Triassic. Deep dehydratedmetamorphic water migrated along regional thrust faults and leached mineralizedmaterial from metaturbidite of Bayan Har group, then the ore-forming fluid migrateupward to the secondary faults or interstratified fracture zones and finally auriferousveins formed.