Study On Metallogenesis And Prognosis Of Yajishan Mo(Cu) Deposit, Inner Mongolia

Yajishan molybdenum-copper deposit lies in the eastem Chifeng city with its exploitation in a certain scale.It is necessary to determine a reasonable scale in production and construction on prospecting resources clearly,and get the best profit from it.The studied area is located in the central of Inner Mongolia Fold Belt,Mongolia-Okhotsk orogenic belt,inner ondor Temple-Wengniuteqi Caledonian fold zone;on the north side of Chengde-Chifeng deep fault,southeast side of Zhaluteqi-Balihan deep fault,near the crossroads of two parts.The strata in the region are exposed from Paleozoie to the Cenozoic,mainly lower Permian, upper Jurassic and Quaternary.Lithological classifications are as the followings:mottled slate, sandstone and feldspar-quartz sandstone;mafic volcanic rocks interlayer volcanic sedimentary rocks;dust-color or sub-brown sandy soil,sub-clay,intercalated with silt and gravel layers.The main magma intrusion in the region is the Yanshanian granite,ore bodies mainly occur in the altered fracture zone of adamellite.EW faults(F₁) is the most important ore-conducting and hosting structure,which controls the migration of ore-bearing hydrothermal and provides favorable space for the ore forming.The sub-ordered NE tensional fracture zone is the main ore-controlling structure developed in the adamellite body within hanging wall of F₁.Ore bodies in 1,2 veins occur in the inner contact of adamellite and middle-fine -grained granite and the inner altered fracture zone of adamellite.Structure plays an important role in the ore-controlling and the fracture alteration zone has a tendency to undulating in both striking and dipping direction,leading to an alternative between rich and barren ore blocks.The intersection of EW and NE structure is considered as the main spot hosting ore bodies.Ore minerals in this deposit are pyrite,chalcopyrite,molybdenite,sphalerite,as well as a small amount of galena,chalcocite,bornite,tetrahedrite,and so on.Gangue minerals are quartz, feldspar,sericite,calcite,muscovite,biotite,and so on.Ore textures are mostly in the form of idiomorphic crystal,hypautomorphic crystal,relic and allotriomorphic granular texture.Ore structures are mainly disseminated structure,vein structure,network structure and mass structure. Wall-rock alteration in the area is often mylonite,potassium,sericite,silicification,followed by chlorite there,carbonate,and so on.Fluid inclusion characteristics showed that three-phase(containing sub-crystal) and two-phase(gas and liquid) fluid inclusions are predominant with less vapor-rich fluid inclusions.The three-phase type is in high temperature,high salinity saturation,mainly occurred in the first quartz stage,with homogenization temperature of 320～380℃,salinity of 39.38%～44.00%NaCleqv,mostly originated from the magmatic fluids.The two-phase type is in low temperature,low salinity,often occurred in quartz and calcite with the homogenization temperature 167～245℃and salinity 2.06%～6.72%NaCleqv,which mainly originated from meteoric.Three stage of fluid evolution are involved in the process ore forming:late in magma crystallization,saturated fluid bearing sub-crystal in sulfide phase was isolated from magma, molybdenum and other sulphophile elements combined into sulfide phase,ore-bearing magmatic fluid in high temperature migrated upward along the fluid channel,partly filled in the cracks along the wall rock,stage 1 ends.In stage 2,with the ore-bearing fluid rapidly moved upward along the crack or joint,fluid pressure and temperature rapidly reduced,meteoric water and hot ore-bearing fluid mixed,forming a cycled hydrothermal system.Such mixed fluid again reduced in salinity and density,and filling,precipition,metasomatic replacement with the wall rock and deposition;the high-temperature,high salinity and saturated fluid mixing with the low-temperature,low salinity meteoric water or interlaminar water,contributes to the enrichment and deposition of the molybdenite.Gradually moving into Phase 3,the fluid temperature and salinity get much lower.The intrusive rock closely related to mineralization is the high-K calc-alkaline granite,the intro-continental orogenic products.Mineralization of Molybdenum and other metal has an intimate relationship to Yanshanian(J_1-2) intro-continental orogeny on the northern margin of North China craton.Molybdenum mineralization is controlled by the sub-order EW fracture of EW cross-crust deep fault on the northern margin of the North China craton,ore bodies distributed in sub-EW fault and sub-order NE fault.The first and second stages count in molybdenum mineralization,it is concluded as a mesothermal vein type deposits,early with high temperature,high salinity and high density of the magma fluid and late with mesothermal,low salinity and low density of mixed fluid,the depth of the deposit is 5.21～8.59km.Based on the study of mineralization conditions,geological characteristics and metallogensis,combining with effective prospecting methods,it is prospected potential areas at the depth(surroundings) of the existing deposit.Copper and molybdenum ore bodies and ore-bearing altered fracture zone are both of "low-resistance low polarization"; while adamellite containing pyrite is "low-resistance and high polarization";adamellite and granite far away from the ore body which not bearing pyrite or just with very little amount of pyrite are obviously "high-resistance low-polarization".Mineralized block is recognized by the verification of drilling,indicating a good prospecting potential in the depth beneath the main ore body in the dipping direction,and a certain perspective at its surroundings.