GEOLOGY,GEOCHEMISTRY AND GENESIS OF THE HANSHAN DUCTILE BRITTLE SHEAR ZONE GOLD DEPOSIT IN GANSU PROVINCE
Mao Jingwen 1,Yang Jianmin 1,Wang Zhiling 1,Zhang Zuoheng 1,(1 Institute of Mineral Deposits,Chinese Academy of Geological Sciences,Beijing,100037)
The Hanshan deposit is a recently discovered large size gold deposit in the western part of the North Qilianshan Caledonian orogenic belt.It is controlled by a NWW trending ductile brittle shear zone more than 10 km long and 200￣600 m wide.The shear zone experienced three stages of deformation,viz.,ductile deformation,brittle deformation during the gold mineralization,and ductile deformation caused by gravity after the gold mineralization. The host rock of the gold mineralization is mainly Ordovician basaltic andesite,which has been altered into mylonite and mylonitic rocks of low greenschist facies.A total of 69 tecto alteration zones have so far been found in the shear zone by geological survey and mapped at 1∶100000 scale.These tecto alteration zones are exclusively gold mineralized zones with grades in excess of 0 5 g/t.The mineralized zones are from 50 meters to 1000 meters long and one to ten meters wide,trending N10°W or N10°E and dipping 50° to 70°.The average grades of various ore zones vary from 0 5 g/t to 49 g/t.Quartz sericitization is strongly developed in the whole shear zone,but is much stronger in and surrounding the mineralized zones.Carbonatization is less strongly developed,but can be frequently found. Mineralization in the Hanshan gold deposit displays two episodes,i.e.,endogenetic and exogenetic episodes.The endogenetic episode comprises three stages of mineralization.Stage Ⅰ mineralization comprises auriferous sericite (quartz)sulfide stockworks developed in the greenschist within the shear zones.Stage Ⅱ mineralization,consisting of auriferous muscovite quartz(carbonate)veins and stockworks,is frequently superimposed upon Stage Ⅰ mineralization and responsible for gold mineralization in the deposit.Stage Ⅲ mineralization composed of auriferous sericite quartz veins and quartz calcite veins is superimposed on the previous mineralization alteration zones.Geochemical study on the ores and altered rocks reveals that large quantities of silicon,alkaline components,CO 2,LREE were brought into the metallogenic system during mineralization and alteration.The primary mineralization is dated to be (312 95±3 10)×10 6 a through measurement of sericite quartz veins by K Ar method,which is coincident with the regional geological evolutionary history of the North Qilianshan orogenic belt. The Hanshan deposit is also the first yellow gossan type(or yellow weathering type) gold deposit found in China.The oxidized ore zone occurs in the upper part from the surface to 20￣90 m downward.Qilian mountain,the northeastern margin of the Tibet plateau,has been a region of arid climate since Tertiary.Gold originally contained in sulfides was leached out and accumulated in the gossan,especially in the secondary enrichment zone.