Metallogenic Model And Exploration Potential Of The Gangcha Gold Deposit In Gansu

The western Qinling is an important part of"Shanxi-Gansu-Sichuan”golden triangle in China and hosts several major gold deposits.The Xiehe-Hezuo zone is newly found Au metallogenic belt,where several large gold deposits are developed such as the Zaozigou,Jiagantan,Laodou,Zaorendao,Laibuzaikaand,etc.The Gangcha gold deposit,located in the northern part of Xiahe-Hezuo fault and to the east of Dewulu granodiorite intrusion,is a newly discovered medium-scale epithermal deposit and has significant ore-prospecting potential.The deposit is controlled by faults and developed in?carbonaceous?slate,limestone,andesite,andesitic volcanic breccia of the Longwuhe Group.The major metallic minerals in the ore are natural gold,pyrite,arsenopyrite,sphalerite,galena,chalcopyrite,and stibnite,and the gangue minerals are sericite-illite and montmorillonite iron,quartz,calcite,dolomite etc.The gold in oxidized ore occur in the dissolved pores of hematite and limonite in the form of microscopic visible gold grains,and in primary ores pyrite,arsenopyrite and sphalerite in the form of invisible gold.In this study,we obtained zircon LA-ICP-MS U-Pb age of andesite and diorite which show 243.0±2.6 Ma and 239.1±2.0 Ma,respectively.The rocks belongs to alkali and calc-alkali series and are characterized by high Mg#,relative enrichment of Th,U and Pb and Rb,Ba,and relative depletion of Ba,Nb,Ta,P,Ti and LREE,consistent with the geochemical features of rocks formed in active continental margin.The magmas that generated andesite,dioritic extrusion-facies rocks and diorite were from derived the partial melting of mantle wedge induced by subducted slab-derived fluids.The magma might have undergone limited crustal contamination during ascent.Fluid inclusion study shows that the ore-forming fluid is characterized by low temperature?190 to 250°C?,low salinity?3.17 to4.91%?and low density?0.80 to 0.87 g/cm3?,and is rich in CO₂.We estimate the metallogenic depth as 1.56 to 2.13 km.The S,Pb,Sr isotope characteristics of the ore show that the ore-forming fluid and ore-forming materials were derived from deep sources and involved crust-mantle interaction.The Rb-Sr isochron age of pyrite is 225.3±3.4 to 229.9±4.7Ma,which is considered to represent the age of the mineralization,suggesting that the deposit formed in an early-middle Triassic active continental margin associated with the subduction of the oceanic slab of Animaqing.A summary of the available data on the relationship between regional strata,structure,magmatic rocks and gold mineralization shows that there no preferential affinity to any strata for the Au mineralization and the different stratigraphic units in different areas host the Au mineralization.It is proposed that structure played a decisive role for Au mineralization and the Au deposits are general developed within the range of 0 to 10 km of NW-trending deep faults.The Au deposits occur in general within the range of 5 km of the magmatic intrusions of the corresponding period.Based on geochemical analyses for prospecting procedures,a total gold resources of up to 850 tons is estimated in this area suggesting prospecting potential for super large or large deposit.It is also suggested that the key prospecting approach should focus on the deep structures towards the southern part of the mining area.