Genetic Mineralogy And Prospectsof The Gangcha-Kemo Gold Deposit,Hezuo City,Southern Gansu Province,China

The Gangcha-Kemo gold deposit is a mid-sized deposit which is located inthe Xiahe-Hezuo region,northern margin of the West Qinling Orogen.The deposit consist of two parts.The north part is Gangcha gold deposit,which is a fault-controlledvein gold deposit,hosted in the Triassic volcanic rocks and Permian sedimentary rocks.The adjacent south part is Kemo ore block of Diruigang gold deposit,in which fault-controlled copper-lead-zinc polymetallic mineralization can been found.The Gangcha-Kemo gold deposit is mainly hosted in the Gangcha volcanic-subvolcanic-volcaniclastic rocks,consisting of andesites,dacites,and quartz diorite porphyry,with minor amounts of volcanic breccia,volcanic agglomerate and tuff.Bulk-rock zircon saturation temperature range from 768? to 818?.Together with geochemical characteristic,it is suggested that the partial melting of ancient lower crustal material and the magma-mixing of mantle with crustal-derived melts may have played an important role in the generation of the Gangcha volcanic-subvolcanic-volcaniclastic rocks.According to the detailed mineral paragenetic studies,five types of pyrite have been identified in the ore.Time-resolved depth profiles demonstrate that As,Co and Ni occur as solid solutions in pyrite,whereas Cu,Pb,Zn and Ag occur mainly as inclusions.The trace element characteristics of different pyrite typesindicate the chemical evolution of the ore-forming fluids.Combining with the geologic characteristics,we infer that the ore-forming fluids were closely linked to magmatic activity.Abundant ellipsoidal and sub-spheroidal polymetallic globules occur in the Gangcha volcanic-subvolcanic-volcaniclastic rocks.Detailed mineralogical study reveal that these globules are composed of quartz,chlorite,epidote,muscovite,K-feldspar,calcite,together with droplets of sulfides(sulfides are mainly pyrite ±arsenopyrite ±pyrrhotite ±sphalerite ±chalcopyrite ±galena).Based on the mineral assemblage,two types of globules are recognized.The geochemical comparison between the globules and the host rocksuggests that they are cogenetic.The chlorite and two-feldspar geothermometerssuggest that the globules formed at temperature in the 297~435?.Overall,we infer that the globules were formed as deep fluid-rich vesicles moved into the shallow domains and were then trapped by cooled magma,inhibiting the escape of fluids,and ultimately leading to the formation of the globules.The close relationship between globules and gold mineralization suggests that globules might be important markers for deep-seated ore mineralization.Soil geochemical medium–low temperature element anomalies(Cu,Pb,and Zn)are nearly consistent withthe globules occurrence area.Further exploration should be carried on in this area.