ORE-FORMING CONDITIONS AND GOLD PROSPECTNG CRITERIA IN CHANGMA REGION ,WEST GANSU PROVINCE

The region of interest is located in the intersection site of northern Qilian orogen and Arjin fault.Ten gold deposits have been found in the region.The bigger ones of them are Hanshan,Yingzuishan,Chelugou and Laochelugou.Based on regional geology and metallogenic geologic background.we have analysed their ore-controlling factors and have put forward their ore guide.The authors think that the gold deposits belong to the altered rock type and can be divided into three types:the first one transformed by brittle-ductile shear belt(GTS),the second one controlled by brittle-ductile shear belt(GCS) and the third one controlled by the fracture zone of brittle fault(GCB).The authors think that the gold deposits occurring in Yinzuishan region were strongly transformed by ductile shear action.and formed in the subduction-collision environment between Huabei-Tarim plate and middle Qilian plate.Their metallogenic epoch belongs to Caledonian and their orebodies directly contact with adjacent rock.Their mineralization temperatures were 320-200 ℃.The study also reveals that gold deposits occurring in Hanshan formed in ductile shear zone and with metallogenic epoch of 303±10 Ma.Their orebodies gradually chang into wall rock.The ore-forming temperatures were 160-163℃ and the deposits occurred where the ductile shear zone was superimposed by EW-striking fault in Ordovician volcanics.The gold deposits of Chelugou region belong to GCB and their metallogenic epoch is late Variscan-Indosinian epoch.The Au contents of both Ordovician and Cambian, the source bed of gold deposits in the study area,are 0.1×10-9-0.3×10-9 and 0.1×10-9-0.4×10-9 respectively. NWW-striking ductile shear zone and NNW-striking fracture zone provided the place where ore -forming fluid can travel, accumulate,leach their mother rock and precipitate.The authors conclude that in the ductile shear zone of Yingzuishan-Huwanzi region,the more Caledonian ultrabasic rock and Cambrian volcanics, the more bucking ore and the bigger their scale are. That the age of Qingshan granodiorite (310 Ma) is close to the age of Hanshan deposit(303±10Ma) suggests that heat source of the metallogenesis in Hanshan may came form Middle Variscian intrusion.Late Variscian granodiorite is related to the gold deposits in Chelugou, the nearer to it, the bigger orebody is,and higher its ore grade is.The ductile shear zone locating in Yingzuishan-Huwanzi where the volcanics and ultrabasic rocks occur is promising for prospecting GTS.Hanshan ductile shear zone superimposed by EW-striking fault is promising for prospecting GCS.The brittle fracture zone of NNW-striking located in the outer contact zone of Late Variscan granodiorite is prospective for exploring GCB.