The Study Of Geological Characteristics And Ore-Forming Fluid Of Rongga Molybdenum Deposit In The Bangonghu-Nujiang Metallogenic Belt,Tibet

Rongga molybdenum deposit is located in Ge Ji,A Li,Tibet,which is the first single molybdenum deposit in the Bangonghu-Nujiang metallogenic belt.To find out the geologic features of Rongga deposit,analysis the evolutionary process and the metallogenic model,this paper combined observation of core samples and EPMA experiment to study the characteristics of altered mineral and metallic mineral assemblage of the region in detail,classified the alteration zone and the metallogenetic periods and stages,then measured the temperature of fluid inclusions.Finally,coming into being the following conclusions:The ore-bearing rock of Rongga Mo deposit is monzonitic granite porphyry,formed in the early stage of late cretaceous,with the wall rocks of terrigenous clastic rock of K₁q.The alteration can be divided from inside to outside into:strong silicified zone,weak potassium feldspathic zone,intense sericitization zone and weak prophylitic zone.The main ore body of Mo forms in sericitization zone and weak prophylitic zone.The metallogenetic periods and stages can be divided into 3 stages:quartz-silicate stage;Mo-Fe sulfide stage;Fe-Cu-Mo sulfide stage.After metallogenetic period,the deposit has the stage of carbonite-hypergen.The all stages include 22 types'vein in general.In Mo-Fe sulfide?2?stage,most molybdenite precipitates.The component classification of some minerals:The plagioclase belong to albite as a whole;all biotite are magnesium biotite,and MgO of it is inversely proportional to FeO^T;the chlorite are mafic in the mass;the muscovites and sericites can be divided into 2 types with the features of white and green respectively,but most muscovites are white and most sericites are green,they have no classification in components,but only green sericites have the lowest TiO2 and Na2O.There are 3 types of fluid inclusions:LV type,S type and C type.Each stage's homogenization temperature respectively are:300?-437.3?;245.8?-366.9?;212.5?-354.1?;172.2?-275.1?.The temperature and salinity decrease gradually.The ore-forming fluid of early stage has the features of high temperature,middle salinity,high oxygen fugacity,K rich,Na rich,deficient F and CO₂,evolving to low temperature and low salinity gradually.In?2?stage,the precipitation of molybdenite are cooling of fluid,boiling because of stress declining,decreasing of oxygen fugacity and pH.In?3?stage,the precipitation of polymetallic sulfide are cooling of fluid,escaping of the volatile,increasing of pH.Rongga Mo deposit formed in the collision period of north Lasa terrane and south Qiangtang terrane,its ore-formed fluid has very high oxygen fugacity and comes from mixing of crust and mantle.This paper has a view that the formation of deep magma chamber may be relate to the mixing substance of crust and mantle in subduction period,and formed the deposit with the magma emplacement when collision.Finally the porphyry Mo deposit gets the special geologic features with a collision background.