| Laozuoshan gold deposit， which is located in the central-north part of Jiamusimassif，the east section of xing’anling-mongolian orogenic belt，is divided into threeore belts:east belt, middle belt and west belt.outcrop geological bodies in mining areamainly is Mashan Group, Hercynian period granite and Yanshanian intermediate aciddike, such as diorite porphyrite, granite porphyry，plagioclase granite and so on. faultstructure are composed entirely of NW、NWW and NE, while NW and NWW faultare ore-controlling fracture.Study on geological characteristics of the three ore belts can further determinethe genetic type of the east and middle ore belts to be skarn type gold (iron) deposit,the orebody is mainly hosted in NW-NWW tensional fracture, or in the structuralcontact zone of granite and calcareous marble or gneiss, mineralization is closelyassociated with skarn; while the type of the west ore belt is hydrothermal vein typegold-copper deposit, the orebody is also hosted in NW-NWW tension-torsionalfracture, gold mineralization is concentrated in the east and middle ore belts. Themain metallic mineral are pyrrhotite, pyrite, arsenopyrite, chalcopyrite，sphalerite，galena etc. and gangue mineral are garnet, diopside, epidote, chlorite, quartz, calciteetc. wall rock alteration includes silicification, potassic alteration, sericitization，skarnization, actinolitization, epidotization, chloritization and carbonation etc.Study on fluid inclusion and H-O isotopes show that the fluid inclusion fromLaozuoshan gold deposit from with a homogenization temperature520℃to57℃,salinity w (NaCl eq) from1.73%to39.68%. Among them, early skarn stage has aore-forming temperature of520℃to470℃, salinityfrom1.73%to8.9%, late skarnstage has a ore-forming temperature of470℃to410℃, salinity from4.6%to34.8%, early stage of quartz-sulfide with ore-forming temperature of410℃to300℃,salinity from3.37%to38.64%,late quartz-sulfide stage with ore-forming temperatureof300℃to150℃, salinity from1.73%to39.68%, stage of quartz-calcite with a ore-forming temperature of150℃to57℃, salinity from3.03%to8.67%. Theore-forming fluid in the mass is a CO2－CH4－H2O system, with characteristics ofmoderate-high salinity and moderate-low density, and early ore-forming fluids aremainly mantle fluids, late are mainly atmospheric water.Zircon LA-ICPMS U-Pb dating of ore-hosting granite and dioritic porphyritedike that cut gold lodes are262.6±3.9Ma and105Ma separately. The plateau age of40Ar/39Ar of alteration phlogopite from west ore belt is108.76±0.70Ma, togetherwith the Rb-Sr isochron dating results of pyrite from the middle belt，which is239+34Ma, we holds that the deposit has two endogenetic mineralization periods, the skarnmineralization stage occured in239±34Ma, while hydrothermal superimpositionstage occured in102Ma， the early mineralization occurred during subduction ofPaleo-Asian Ocean to Jiamusi massif, and consistent with Hercynian magmatismactivities in late middle Permian, while the late mineralization environment iscontinental marginis, caused by Pacific Plate subducted in Late Mesozoic, and isrelated with intermediate-acid magmatism activitiesc.Based on the study mentioned above, we can come to the conclusion that goldmineralization happens on the base of dry skarn and wet skarn metasomatism ofPaleozoic granites and calcareous rock series and tectonic magmatism. |
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