Typomorphic Characteristics And Geological Significance Of Biotites In Jiama Porphyry Deposit System, Tibet

Jiama deposit is located in the east of Gangdese metallogenic belt, Tibet. It has been the important achievements of the combination of nonprofit with commercial prospecting. Proven and controlled resource of copper is more than 7 million tons, associating with molybdenum, lead, zinc, gold and silver resources. Each of these metals is of over a large-scale deposit currently. Jiama porphyry deposit is the "four types in one deposit" classic porphyry system, composed of porphyry molybdenum-copper orebody, skarn copper polymetallic orebody, hornfels copper-molybdenum orebody and independent gold orebody in fracture zones.Based on the fully compiled regional geological datas, predecessors’ research results and the latest exploration results, through detailed geological investigation and drilling geological logging etc., this paper systematicstudiescharacteristicsandgeological significance ofbiotites by using the microscopic identification, electronmicroprobe analysis.Detailed determination of genetic type of biotite in Jiama copper polymetallic deposits, the biotite can be distinguished into two categories:one is protogenic biotite and belongs to the pre ore; the other is hydrothermal biotite of magmatic hydrothermal formation, and belongs to mineralization. In rock mass,protogenic biotite is magmatic biotite, and hydrothermal biotitesare hydrothermal metasomatic biotite and new hydrothermal biotite.Inhornfels, protogenic biotiteis formed in hornfelsic thermal metamorphic process, divided into punctate biotite and banded biotite; hydrothermal biotite is formed in inside or cracks in hornfelsin magmatic hydrothermal differentiation process, and according to the symbiosis of the typical mineral assemblageit can be divided into 4 types: A-biotite veinlets, B-quartz+biotite, Cquartz+biotite+chlorite,D-quartz+biotite+albite.Study geochemical character of two types of biotite. Protogenic biotiteand hydrothermal biotite belong to eastonite and phlogopite, and biotite in rock mass bias in eastonite with rich magnesium features. Inhornfels, compared with protogenic biotite, the content of Mg and Cu rise, Fe3+-Fe2+differentiation degree increase, and Fe decreased in hydrothermal biotite; In rock mass, compared with protogenic biotite, hydrothermal biotite has characteristic of low titanium high aluminum-TiO2<3%, Al2O3>15%, the content of Mg and Cu rising, and Fe decreasing.There is no significant difference between the five types of hydrothermal biotite composition. In D hydrothermal biotite, TFeO content is lower than others;Inhydrothermal biotite of rock mass, TFeO content is higher than otherhydrothermal biotite.Determine the physical and chemical conditions of two types of biotite formation. The crystallization temperature(T) and oxygen fugacity(logfO2) of the protogenic biotites are 720℃~ 750℃ and-11.5~-13.0 in granite porphyry. Granite porphyry, with the characteristic of causes of crust-mantle mixed source, belong to orogen calc-alkaline rock series.In hornfels, compared with Protogenic biotite, the ratio of Fe2+/(Fe2++Mg2+) from hydrothermal biotite is low homogeneous degree, and the ratio of Fe3+/Fe2+ increase significantly, and even some hydrothermal biotite does not contain Fe2+. Indicating the magmatic hydrothermal fluid system gradually transforming to oxidation state, it is conducive to the formation of porphyry copper deposit.Explore the relationship between the formation of two types of biotite and mineralization. Compared to protogenic biotite, Cu tends to enrich in hydrothermal biotite; but Mo is common in protogenic biotite and hydrothermal biotite, and does not have characteristics of selective occurrence. For rock-ore samples of hydrothermal biotite, mineralization intensity is positively related to the content of Cu in biotite.