Geology, age, and fluid inclusions of the Tanjianshan gold deposit, western China: Two orogenies and two gold mineralizing events

The Tanjianshan gold deposit, located at the tectonic boundary between the Oulongbuluk block (OB) and North Qaidam orogenic belt (NQO) on the northern margin of the Qaidam basin (NMQB), western China, underwent Early Palaeozoic and Late Palaeozoic to Early Mesozoic multiple accretions and collisional orogenies. It is a typical example of a deposit resulting from two gold mineralizing events related to two collisional orogenies. Two phases of ore-controlling NW-trending shear zones have been identified in the deposit: dextral brittle–ductile shear deformation during the late Early Palaeozoic collisional orogeny and sinistral strike-slip shear deformation during the Late Palaeozoic–Early Mesozoic orogeny. N–S-trending folds developed in the NW-trending shear zone and resulting from sinistral strike-slip motion of the shear zone are nose-shaped in plan view and control most gold orebodies in the deposit. The major gold orebodies trend NNE and are concentrated in the hinge zones of these N–S-trending folds, occurring along the fault zones parallel to both limbs of these folds. These gold orebodies, composed of altered mylonite schist type ore and altered dyke type gold ore, occur as saddle-shaped bodies, beds and lenses mainly within altered carbonaceous quartz–sericite mylonite schist of a highly strained zone and locally in altered igneous dykes of the highly strained zone. Alteration related to gold mineralization includes silicification, pyritization and sericitization. A progressive increase of SiO2, S, Au, Ag, As and Sb from unaltered or weakly altered rocks through altered rocks to gold ore is consistent with progressively enhanced silicification and pyritization, suggesting the flow of mineralizing fluids along the NW-trending shear zone and N–S-trending folds. Native gold and electrum are present as very fine grains, mainly enclosed in disseminated pyrite or occurring in fractures within the pyrite. Three generations of pyrite and associated hydrothermal quartz are observed.The fabrics of gold ore, generations of auriferous pyrite, field relationship between multiple ore-controlling structures and gold mineralization, and the bimodal distribution of ore elements in the gold district suggest the occurrence of two main hydrothermal and mineralizing events. The first event, marked by a mineral assemblage of first-generation quartz (qz 1) and first-generation pyrite (py 1), was restricted to the carbonaceous mylonite schist within the NW-trending shear zone. Sericite from the ore-controlling NW-trending shear zone gave a 40Ar–39Ar age of 409.4 Ma, which is consistent with the age of the regional late Early Palaeozoic collisional orogeny in the region. The second event, marked by a mineral assemblage of second-generation quartz (qz 2) and second-generation pyrite (py 2), occurred within the N–S-trending folds and was related to late Palaeozoic orogeny in the region. We have obtained a K–Ar age of 268.9 ± 4.3 Ma and an Rb–Sr isochron age of 288 ± 9.7 Ma for hydrothermal minerals from gold ores. The latter age is closer to the age of this event of gold mineralization. Mineralization of this event was superimposed onto gold mineralization of the early phase and formed most of the gold orebodies in the deposit.The distribution and microthermometric data of fluid inclusions indicate that late Early Palaeozoic ore fluids belong to the H2O–CO2–CH4–NaCl fluid system formed at 186 to 250 °C, whereas Late Palaeozoic fluids comprise two kinds of immiscible fluid: a H2O–NaCl fluid with salinity of 1.6 to 10.8 wt.% NaCl equiv., which probably formed at temperatures of 381 to 449 °C and, and H2O–CO2–NaCl fluids with salinities of 1.8 to 7.9 wt.% NaCl equiv., which are genetically the product of evolution of former immiscible magmatic fluids, and which formed at temperatures of 274 to 410 °C.