Descussion On The Mechanism Of Seperation Of Copper And Molybdenum Of Jima Copper-polymetallic Deposit, Tibet

Jiama porphyry-skarn copper polymetallic deposit, located at the eastern sectionof Gangdese metallogenic belt in Tibet, has the highest level of prospecting, and it isthe first supper gaint deposit mined specificly. Through fully collecting and studyingprevious data in detail, as well as a large number of field geological work, puttingforward that Jiama copper-polymetallic deposit is a “quaternity “type porphyrymineralization system which is mainly comprise of molybdenum ore body bearing inupper Linzongbu formation, Cu polymetal ore body bearing in medi-interlayertectonic zone and porphyry contact zones, Cu-Mo ore body bearing in bathy-hiddenporphyry and Au ore body bearing outside of structure fracture zone. In the skarn orebody, molybdenum mineralization mainly developed at the upper near the contactzone between honfels and skarn; while copper mineralization mainly at the lower nearthe contact zone between skarn and marble, thus forming a spatial zonation ofmolybdenum and copper. However, in the honfels and porphyry ore bodies, thephenomenon that “copper upper, molybdenum lower” and “copper earlier,molybdenum latter”.Detailed microthermometry result of fluid inclusions in the quartz veins withcopper and molybdenum mineralization showed that: on the whole, the temperature ofcopper mineralization(roughly190~483℃, mainly between340~380℃) ishigher than that of molybdenum mineralization (roughly160~500℃, mainlybetween310~360℃). Since the temperature of forming copper-minerals is mostlyhigher than that of quartz precipitation(about350℃), this leads to less quartz+chalcopyrite/bornite viens in the copper phase. While, in the process of molybdeniteprecipitation, the overall temperature of metallization is lower than350℃, so that a large number of quartz and molybdenite precipitated at the time, forming vast scale ofquartz+molybdenite veins.The salinities of fluid inclusions in in the quartz veins with copper andmolybdenum mineralization are broadly similar, mainly located in <18wt%NaCleqvand>30wt%NaCleqv the two intervals, lack of an intermediate salinity zone, whichis due to metallogenic fluid, under P-T conditions, is unstable, leading to theoccurrence of phase separation. The phase separation of metallogenic fluid makes Cu,Au, Fe, Mn, S, As and other elements enter into the volatile gas phase stongly andselectively, being migrated. But Mo is mainly concentrated in the residual liquidphase and migrated.The result of Laser Raman microscopy analysis showed that fluid inclusions inthe veins with copper mineralization commonly contain the sub-minerals with highoxygen fugacity, such as magnetite, gypsum, in the meanwhile, the component ofvolatile in the gas phase is mostly CO2, absence of reducing volatile, such as CH4. Itmeans that the earlier copper metallogenic fluid is oxidative with neutral pH, and thehigh oxygen fugacity environment is conducive Cu precipitation, making the lowsulfuration Cu-Fe sulfides prior to precipitation. Cu, Mo mineralization areconsuming S element, while a large number of copper minerals precipitation hadconsumed a large amount of sulfur, so it inhibits the formation of MoS2. When thefluid continuously came out from the magma chamber with fractional crystallization,Mo continue to be enrichment, and S continue to be supplied largely. As thecontinuous evolution of the fluid, the acidity and reduction of fluid are enhanced infavor of large number of MoS2precipitation.The Cu-rich hydrotherm, evoluted from quartz diorite(the earliest emplacedporphyry), was migrated upward along the fracture system in the banded biotitehornfels, formed by metasomatic alteration. At the high place, because of the suddendrop of temperature and pressure, that hydrothermal fluid precipitated gradually.Granite porphyry, emplaced lately, can release high silicon, molybdenum-richmagmatic hydrothermal fluid. In the process of upward migration along the fissuresystem of hornfels, with the passage of magma evolution and the increasing degree ofthat, and the temperature decreasing, so that the Mo-rich hydrothermal fluidprecipitated together with the quartz, forming quartz-molybdenite (net) veins.Thus,copper mineralization mainly distributed in the upper part of hornfels and porphyry,and the lower places mainly molybdenum mineralization copper and molybdenumseparation. The results of comprehensive study and analysis, the preliminary view that thecopper-molybdenum separation in the skarn ore-body of Jiama deposits, owing to orebearing magmatic hydrothermal liquid, along the interlayer structures betweenLinbuzong formation and Duodigou formation for nappe structrure, and escaped bythe lateral places. However, copper-molybdenum separation in the hornfels andporphyry ore-bodies are mostly restricted by the nature, emplacement sequence anddepth of ore-bearing magma, resulting in the differences of the reduction-oxidationand the content of sulfur in the metallogenic fluids, as well as the different physicaland chemical properties of copper and molybdenum, thus forming the phenomenon of“copper upper, molybdenum lower” and “copper earlier, molybdenum latter”.