Preliminary Study Of Genetic Mineralogy And Prospecting Mineralogy From Jiama Polym Etallic Copper Deposit, Tibet

Jiama polymetallic copper deposit locates in east of Gangdese volcano - magmatic arc,which is an important component of the Gangdise metallogenic belt. Jiama deposit is the most presentative super-mall porphyry-skarn-epithermal polymetallic copper deposit, which is relating to post-collision.The stratums of deposit are mainly Limbuzong group of the lower Cretaceous and duodigou group of the upper Jurassic. The Ore body is located between the two stratums as like-layered. The main types of medium-acid porphyry in Jiama deposit are granite porphyry, granodiorite porphyry, monzogranite porphyry and (quartz) diorite porphyry.Through the intensive search for the minerals of Jiama polymetallic copper deposit, This paper has identified the magmatic evolution and the P-T alteration of magmatic, and proved the earth dynamics of the magma mixing, established.the process of magma mixing.We also have found the evidences of magmatic to hydrothermal transition, controlled over the physics and chemistry conditions of the exsolution fluid. Forthemove, we have dentified the geological records of hydrothermal evolutionary modification, established the evolutionary modification of ore-forming fluid. At last, we have discussed the processes of the origin, transport and precipitation of ore-forming matter.The results show that,Cathodoluminescence is an effective technical method on studying microstructure of quartz. According to the difference of growth patterns in quartz CL, there are three generations and six types of quartz phenocrysts which form the Jiama Porphyry Copper Deposit in Tibet . The microscropic structure of quartz phenocrysts indicate that primary magma have gone through two times of magma mixing events. After two resorption,the recrystallization temperature of quartz phenocrysts were raised by 110℃and 80℃respectively, according to the content changing of Ti in zoned quartz. In addition, element concentration steps in feldspar phenocrysts ( Ba, Sr, Fe), plagioclase-mantled K-feldspars etc. indicate mixing of silicic magma with a more mafic magma for several magmatic phases of the porphyry in the Jiama Cu-polymetalic deposits. Based on the result, the process of magma mixing has been established. At 16Ma, as a result of asthenosphere up swelling and lithosphere dismantling and subsiding, lower crust remelted and formed potassic magma containing Mo, forming primary magmatic chamber in where the first generation quartz crystals. During the post-collisional crustal extension period , shallow emplacement and fluid exsolution of the newly-born adakitic magma , resulting from the lower crust and rich in metals , water and high fO₂ , and mixed with primary magma which contain Mo , formed the shallow crustal magma chamber. Owing to the first mixing with mafic magma, resorption surfaces appeared around the core of quartz phenocrysts. And then, the quartz has experienced stable growth conditions, forming a steady growth zone with low CL contrasts. Gangdise orogen belts was in a extensional condition. At these condition, amount of normal faults have be made. The pressure of magma reservoir has reduced rapidiy, magmatic reemplacemented. Owning to the suddenly empty of magma chamber, the second magma mixing occurred.Information about the magmatic to hydrothermal transition is preserved in porphyry of Jiama copper-polymetallic ore deposit. Include: 1) quartz phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz cathodoluminescence banding. 2) Pods of saccharoidal quartz are connected by graphic quartz–alkali feldspar intergrowths and ragged biotite. 3) Fluorite as an accessory mineral in igneous rocks, high F in hydroxyl sites in igneous biotite and amphibole. 4) Fluid inclusions and silicate melt inclusions are present in the porphyry of Jiama. We present detailed petrographic observations of primary igneous features, combined with silicate-melt and fluid inclusion studies, and found physical models of exsolution. Metallic elements are preferentially partitioned into the associated magmatic volatile phase.As the volatile concentration increase, the vapor pressure leads to sudden failure of the carapace and adjacent wallrock– this occurs once the vapor pressure is greater than the confining pressure. Magmatic fluids escape as cracking of the wallrock continues; these fluids then hydrothermally alter the rock they pass through. At the same time, caused by the rapid decreased in pressure, hypersaline fluid was boiling again, and it have caused Cu-Mo sulfides to deposit from the fluid.The combination of scanning electron microscope–cathodoluminescence (CL), and LA-ICP-MS microanalysis of Al, Ti, K and Fe in vein quartz has yielded results permitting a greater understanding of the complex mineralisation of the JiaMa porphyry-style deposits, These data demonstrate the relationship between quartz precipitation, dissolution and ore deposition as the mineralising fluid chemistry changed through time. Five major quartz generations are identified. Using the Ti-in-quartz geothermometer and fluid inclusion analysis, crystallisation temperatures for JMi to JMv is between 823°C and 130°C. The CL and trace element signatures of the JMi to JMv stages of the porphyries show similar features to those observed in porphyry-style deposits from other regions. This suggests that a common sequence of quartz crystallisation occurs during the formation of early veins in many porphyry copper systems.