| Machangqing Copper-Molybdenum Polymetallic Deposit is one of theporphyry-type deposits related to alkali-rich porphyries in theNujiang-Lancangjiang-Jinshajiang metallogenic belt. The collision between India andEurasia resulted in the huge Jinshajiang-Ailaoshan-Honghe River strikeslip-faultsystem, where locates a northwest strike Himalayan alkali-rich porphyries. Moreover,the magmatic action and crust-mantle interaction are frequent; therefore this region ischaracterized by complex tectonics and obvious mineralization.By studying the geological characteristics of Machangqing Deposit,we noticethat ore body shows gradient features from rock,contact zone to wall rock,with amineralization change from Molybdenum ore,copper-molybdenum ore to gold oreand temperature changes from high,high-medium,medium-low to low. According toresearch on wall rock alteration , 3 zones are recognized which include strongsilicification in the core,quartz K-feldspar in the center,quartz sericite in the margin .In the contact zone,there is contact Metasomatic(Skarn) alteration while in the wallrock there are silicification,dolomitization and calcilization alteration,associated witha black opaque micro-crystal proved to be an indication of mantle fluid mineralizationand reason for wall rock alteration and crust-mantle overlapping mineralization.Based on trace elements geochemical and REE researches of ores,veins andmineralized wall rocks, we discover that REE patterns show remarkable enrichedLREE as well as slight negative Eu abnormal. The general patterns are rich in largeion lithophile elements(LILE) and high field strength elements(HFSE).Further more,isotope geochemistry suggests that ore-forming fluid and magma originated frommantle. In Machangqing molybdenum(copper)ore block,it is mainly mantle source;In Luantongshan copper ( molybdenum ) ore block , it is mainly crust-mantleoverlapping source ; In Jinchangqing gold ore block , it is mainly crust source.Therefore,conclusion can be made that mantle source effect becomes weaker while crust source effect gets stronger.Besides,systematic geochronology research of rocks and ores forming process inMachangqing deposit supplies the Zircon LA-ICP-MS U-Pb data of ganite-porphyryand molybdenite Re-Os data:the Zircon U-Pb age of Machangqing ore-bearing rockis 36.17±0.36Ma , which is the rock-forming age of alkali-rich porphyries ; Themolybdenite Re-Os age is 34.7～35.1Ma,represents ore-forming epoch. Comparewith formers'results , we believe the Metallogeny of study area is related withmagmatism of late alkali-rich porphyries.Combined with trans-magmatic fluid Metallogeny theory, we believe that theore-forming fluid is mantle source and derives from the alkali-rich magma. During theupwelling of ore-forming fluid , it is separated with the alkali-rich magma andcontaminated with crust , consequently a series of Metallogenies developed. Indetails , molybdenite block of orthomagmatic Metallogeny system is formed inalkali-rich porphyries ; copper ( molybdenum ) block of contact MetasomaticMetallogeny system is found in rock-wall rock contact zones;and the gold block is inwall-rock. Some of the gold blocks occurring in porphyries are often controlled bypost-faults that cut porphyries. With the develop of ore forming,the volatile of mantlefluid escaped,and the characteristics of mantle fluid varied from magma,supercriticalfluid to hydrothermal fluid due to different depths and environments. Meanwhile,activated ore-forming materials were migrated to proper host rock areas and enrichedthere,which finally led to the series of ore-forming belts of molybdenum-copper-golddeposit in Machangqing Area. |
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