Genesis Of Quartz Porphyry And Its Relationship With Mineralization In The Obulag Copper Gold Deposit,Inner Mongolia

The study area of Inner Mongolia obulage copper gold deposit is located in the north of the west section of Langshan.The quartz porphyry in the deposit is closely related to the mineralization of copper gold orebody,but there is little research on it.The ore-forming control factors and metallogenic mechanism of the deposit have not been solved.Based on the study of petrology,mineralogy and geochemistry of quartz porphyry,this paper discusses the genesis and metallogenic mechanism of quartz porphyry in the area.The quartz porphyry in the obulag copper gold mining area mostly occurs as veins.Quartz porphyry samples are light yellowish brown to light gray white,phenocrysts are feldspar,quartz and a small amount of muscovite,accessory minerals are rutile,apatite,etc.,matrix is felsic,alteration mineralization phenomenon is more common.The EPMA analysis shows that the feldspar phenocrysts in quartz porphyry are diorthosite and albite,and the matrix is diorthosite.The content of fluorine in apatite is very high,which is a typical fluorapatite,indicating that the formation of quartz porphyry in this area was affected by fluid action.The quartz porphyry is rich in Si O₂,Na₂O and K₂O,poor in Ca O,Fe O and Mg O,rich in LREE,K,Rb,Ba and Th,poor in HREE,Nb,Ta,Ti,P and Sr,and A/CNK>1.1,showing the characteristics of peraluminous high-K calc alkaline granite.The chondrite normalized REE distribution curve of quartz porphyry shows a"right dip",enriched in LREE and depleted in HREE.In general,LREE fractionation is obvious,while HREE fractionation is weak,with moderate negative Eu anomaly(&Eu=0.46-0.84)and slight negative Ce anomaly.The quartz porphyry is enriched in U,K and Pb and depleted in Cr,CO and Ni with Mg^#ranging from 31.78 to 42.58,indicating that the quartz porphyry is the product of partial melting of the crust and may also be affected by mantle derived magma;The REE patterns and Sr/Yb diagrams show that the residual facies in the source area of quartz porphyry is granulite facies,and the source rocks may be garnet granulite or hornblende garnet granulite.Finally,it is speculated that the quartz porphyry is formed by the partial melting of the upper crust caused by the thickening and warming of the crust or regional crustal uplift and decompression caused by the collision orogeny,and its provenance is mainly composed of metamorphic greywacke.The quartz porphyry in the study area shows that plagioclase and quartz are strongly transformed by fluid,resulting in fluid crystal mineral assemblage;Quartz phenocrysts are strongly broken and interpenetrated by calcite veins,which indicates that fluid overpressure occurred in the magmatic channel system in this area,and also shows that quartz porphyry is closely related to mineralization.The metallogenic model is as follows:the sulfide slurry formed by the reaction between magma and sulfur rich carbonate rock wall rocks accumulates at the bottom of the magma chamber,and the fluid rich in Na,K,Ti and P enters into the magma chamber,resulting in fluid overpressure in the deep magma chamber,forming copper rich gold melt fluid,rising along the magma channel,and finally emplacement mineralization in the weak structural zone.