Study On The Geological Characteristics And The Genesis Of Kushuiquan Gold Deposit In The Northern Margin Of Qaidam Basin

The Kushuiquan gold deposit lies in the middle segment of the northern margin of the Qaidam basin.It is a gold deposit discovered only recently.The strata in the mining area are relatively simple,and mainly developed the paleoproterozoic dakendaban group biotite plagioclase gneiss..Intrusive rocks in the area typically include stock-like quartz diorite and fine-grained diorite dikes.The former is emplaced in the Dakendaban group while the latter is emplaced in the quartz diorite.Structures in the area typically include NE and NW-trending faulted structures.Gold mineralization there is mainly constrained by a group of NE-trending and a group of NW-trending faulted structures.The NW-trending faults are a little younger in age than the NE-trending faults.The mineralization is of typical vein type.The deposit is seated in the quartz diorite with a close spatial connection with the fine-grained diorite dikes.The fine-grained diorite has been dated to have an average weighted zircon U-Pb age of 428.0±1.9 Ma.Geochemical characterization has indicated that the rocks are a high-K calc alkaline to calc alkaline and paraluminous series,with Rb/Sr ratios of 0.142⁰.246,ε_Hf（t）values of-22.14 to-25.96.The original magma should have originated from partial melting of basic lower crust material.The rocks are abundant in LILEs Ba,K,and Rb and active incompatible elements like Th and U;but are significantly deficient in HFSEs Nb and Ta,deficient in Sr,and low in Yb,showing the characteristics of arc island or continental margin arc magma.The fine-grained diorite has a close spatial connection with the gold vein.It must have provided thermodynamic,hydrothermal,and metallogenic material for the mineralization.Hence it is a mineralization-related intrusive rock.All evidence shows that a hydrothermal gold mineralization event occurred in a subduction tectonic setting in 428 Ma±of the Silurian.Two stages of mineralization have been detected in the area.The earlier stage was constrained by an NE-trending compressional-torsional fault in the early stage and an NW-trending compressional-torsional fault in the late stage.The NW-trending fault has evidently dislocated the NE trending,although the offset if quite modest.The mineralization stage constrained by the NE-trending fault characterizes dextral compressional-torsional movement.Its along-strike attitude is easterly oriented,while the relaxed part of its along-dip attitude falls at the point where the fault is locally flared,providing a favorable destination for mineralization.The along-strike mineralization shows evident rhythm of stepwise enrichment.In the meantime,the dextral compressional-torsional movement of the NE-trending fault has caused the orebody to pitch northeastward.The pitching of the orebody is perpendicular to the movement direction of the fault at the time of mineralization.The NW-trending fault occurs quite steeply,dipping northward with local inversions.Overall it is of a compressional nature.The orebody is not obviously pitched.Fluid inclusion test shows that the Kushuiquan gold deposit is host to mainly gas-liquid two-phase and CO₂-containing three-phase inclusions.The fluids in stage I are mainly gas-liquid two-phase inclusions with homogenization temperature of319.7°C³34.5°C,salinity of 6.29¹1.84 wt%NaCl and density of 0.74⁰.79 g/cm³.They are high-temperature,low-salinity,low-density inclusions.The mineralization depth is estimated at 2.84³.67 km.The fluids in stage II include both gas-liquid two-phase and CO₂-containing three-phase inclusions-immiscibility has taken place in the fluids-with homogenization temperature of 202.4°C²71.2°C,salinity of 9.73¹8.27 wt%NaCl and density is 0.86⁰.99 g/cm³ for the gas-liquid two-phase inclusions,and homogenization temperature of 202.1°C²85.6°C,salinity of 5.05⁶.81 wt%NaCl and density of0.80⁰.89 g/cm³ for the CO₂-containing three-phase inclusions.The mineralization depth is estimated at 2.14².85 km.The salinity of the gas-liquid two-phase fluids is higher while that of the CO₂-containing three-phase inclusions is lower.The fluids in stage III are gas-liquid two-phase inclusions with homogenization temperature of 156.0°C¹89.8°C,salinity of 4.63¹9.22 wt%NaCl and density of 0.92¹.04 g/cm³.The salinity is highly variable but the density is greater,mainly attributable to the runoff of gas and CO₂.The mineralization depth is estimated at 1.34².18 km.The fluids in stage IV are gas-liquid two-phase inclusions with homogenization temperature of 117.0°C¹41.2°C,salinity of9.60²1.80 wt%NaCl and density of 1.00¹.09 g/cm³.Increased salinity and density are possibly attributable to the absorption of the salt content in the shallow crust formations.The mineralization depth is estimated at 1.30¹.94 km.Type of deposit:（1）The deposit originates from the high-angle deep subduction of the South Qilian Oceanic Crust Plate in 428±1 Ma.The mineralization-related diorites formed as a result of partial melting of lower crust basic rocks caused by the underplating of mantle-derived magma;（2）Mineralization was strictly constrained by the NE and NW-trending faulted structures and fine-grained diorite dikes.The ore-bearing hydrothermal fluids enriched and deposited through filling and replacement processes;（3）The ore minerals are simply composed.Metal minerals primarily include pyrite.The ore-forming fluids are mesothermal,low-salinity fluids with presence of CO₂-containing three-phase fluids.This is almost identical to the typical mesothermal hydrothermal gold deposites found in China’s Jiaodong,Qinling,and Jiapigou;（4）It can be decided that the deposit is genetically a mesothermal hydrothermal vein type deposit.Judged from the tectonic setting of mineralization,it should be a shallow orogenic gold deposit.Recommendations on further ore prospecting and exploration:The well-mineralized part in the area revealed so far is the NE-trending vein III.In addition to main orebody M3inside it,trenching projects may be conducted along the NE trending,at the point where the attitude of the fault zone turns easterly and relaxes,namely near the AS⁸ Au anomaly concentration center,to trace stepwise enrichment sections.Inside the area,parallel veins to vein III should be traced,including fault F10 already outcropping to the southeast of vein III and possibly hidden alteration zones,with more attention paid to NE-trending parallel ore veins in Au anomaly regions along the NW direction of the major axis.According to the transverse corresponding law of mineralization,trenching projects should first be carried out at the point where fault F10transversely corresponds to the ore-rich portion of orebody M3 along vein III（the part of F10 corresponding to the Au-richest trench 15QTC11 on orebody M3）.Mineralization in the area is highly selective of the surrounding rock—mainly limited to the location of quartz diorite.The fine-grained diorite dikes have the closest spatial connection with the mineralization.The locations of the Dakendaban group metamorphic rocks and Maoniugou formation clastic rocks are not favorable ore-prospecting destinations.