Genetic Model Of The Jiama Copper-Polymetallic Ore Deposits,Tibet

Jiama polymetallic copper deposit locates in the middle-east of the Gangdesemetallogenetic belt which belong to the world’s second lagest porphyry metallogenicdomain-Tethys-Himalayan giant prophyry metallogenic domain. It’s a typicalrepresentative of porphyry copper polymetallic ore in “post collision” tectonicenvironment and an important part of the Miocene intermediate-acid hypabyssalmagmatic formation related to submetallogenic series of copper, molybdenum, leadand zinc, wolfram,gold and silver diposits. Based on the geology, geochemistry,fluid/melt inclusions, isotopic composition and chronology, we disscussed the originand evolution of magmatic rocks, fluid exsolution,origin and enrichment mechanismof the mineral source, origin and evolution of gas-liquid alteration and the mechanismof mineral precipitation, established a preliminary complete ore deposit genetic mode,Mainly the following understandings:1、Detailed field geological investigation, geochemistry, isotopic compositionsand the chronology make clear that the hypabyssal intrusive rocks in Jiama depositincluding minettes, spessarite, diorite porphyrite, monzo-granite porphyry, grano-diorite porphyry and granitic porphyry.Their emplacement age between17Ma-13.2Ma and their evolution sequence express granite porphyry->diorite porphyrite->minettes->monzogranite porphyry->spessartite->granite diorite porphyry->spessartite.The minettes and spessartite which belong to potassic calc-alkaline and sodiumcalcium alkaline lamprophyres severally are isogenesis and origin from meta-somatized mantle wedge by the flow （molten） from subducting slab;The dioriteporphyrite are metaluminous high-K calc-alkali rocks with the characteristics ofI-type granites and adakites origin from the new lower crust;The granite porphyry arealuminum shoshonite, and with the characteristics of the S-type granites origin frommetamorphic sandstone in lower crust;The monzogranite porphyry and the granitediorite porphyry are I-type high-K calc-alkali rocks, both are the mixing result of thethree magmatic melt.2、Determine the alteration types, alteration zoning and their relationship to themin-eralization. Porphyry mineralization system in Jiama deposit develop sodium silication （tourmaline）, potassium silication, skarnization, sericitization, propylitiz-ation and argillization, and superposed different types of iron ore. Epithermal systemin Jiama deposit develop quartz-carbonatization, argillization, chalcedonization,fluorite-zation and manganesation. Sodium silication （tourmaline） are related to thechalco-pyrite-molybdenite mineralization capsule in the interior pluton and cryptoexplosive chalcopyrite-molybdenite mineralization in the top of pluton. The mainlyquartz-biotite potassium silicattion are related to chalcopyrite mineralization. Theskarni-zation and potassium silication which developed simultaneously are related tocopper mineralization; Sericitization are related to chalcopyrite and molybdeniteminerali–zation. Propylitization are related to copyrite mineralization. Epithermalminerali-zation related to Cu,Au,Pb-Zn mineralization.3、Ascertain the characters of the fluid/melt inclusions and the source of the fluidby the H-O and inert gases isotopes. The fluid inclusions in the Jiama deposits aremainly gas phase rich inclusions, liquid phase rich inclusions and daughter crystalscontaining multiphase inclusions. The homogenization temperature of the the silicatephase fluid inclusions change in437～650℃,salinity changes in2～46.2%NaCl;Thehomogeniz ation temperature of potassium silicate phase fluid inclusions changes in315～457℃, salinity changes in1.0～47.1%NaCl;The homogenization temperatureof the skarn stage fluid inclusions changes in220～413℃, salinity changes in2.7～47.3%NaCl;The homogenization temperature of phyllic stage fluid inclusionschanges in230～378℃, salinity changes in the4.1～43.8%NaCl;Thehomogenization temperature propylitic rock stage fluid inclusions changesin380～410℃and salinity changes in the1.7～5.7%NaCl;The homogenization temperatureof shallow epithermal stage fluid inclusions changes in170～253℃, salinity changesin2.8～5.5%NaCl;Each alteration phase ion composition of fluid inclusions LordK++Na++Ca+,the gas phase composition is H2O-CO₂-CH₄-N₂type. Fe, Cu, Zn, Pb andAu in magmatic hydrothermal transition stages are all enriched in the vapor phase ofthe fluid inclusions, but the metal element concentration in the liquid phase isrelatively lower,suggesting a strong gas-liquid fractionation of metallogenicmaterials.Noting that the I-2type fluid inclusions in sodium silicate salinization stagemay be supercritical fluid captured with temperature of625℃and9.8%NaClsalinity.The alteration and mineralization fulids mainly come from deep crust andmixed some deposition or precipitation fulid.4、Summarize the porphyry genetic model and alteration genetic model of theJiama deposit.18Ma before, the upwelled asthenosphere leaded to convection anddelamin-ation of the thinned lithosphere,.cause partial melting of the different sourcearea of the lithosphere, foming basaltic melts containing Cu-Mo, water and alkali richand high fO₂, granodioritic and granitic melt containing Cu-Mo-Au-Pb-Zn,water andalkali rich and high fO₂.During18～14Ma, In the extrusion and extension structureconversion process, the three types of melt migrated and emplaced along the EW andNS-trending strike-slip fracture fromed lamprophyre、diorite porphyrite and graniteporphyry respectively, the monzogranite porphyry and the granite diorite porphyry arethe mixing result of the three magmatic melt. The supercritical fluid formed inimmiscible phase and separate into low-density gas and high salinity fluid during the magmatic hydrothermal transitional stage. Low-density gas formed the earliestscattered Na-K silication alteration with Cu-Au mineralization and later fromed theepidotization by addition of the meteoric water. High salinity fluid caused sodiumalteration is the earlier ore-forming fluid. The potassium alteration fluid origin fromthe fractionation of the sodium and potassic of the high salinity fluid, in the later, thesulfur fugacity of the potassic fluid reduced with the magnetite and gypsum crystaledcaused quartz+biotite potassium silication associated chalcopyrite; Meanwhile, theskarnizationwhich origined from the fulid interacted with hornfels and marble caucedthe activation of the Ca2+and induced content of the sulfur,mainly fromed thebornite.After then, the fluid pressure turn lithostatic pressure to hydrostatic pressure,the increasion of atmospheric precipitation decreased the H+/K+ratio, and formedsericitization associated molybdenum and chalcopyrite mineralization. In the heatconvection of the geothermal system, the mineral （Cu, Au, Pb-Zn） recombinated andformed a series of low temperature alteration, including chalcedony, calcite, fluoriteand pyrolusite, etc. Finaly, owing to the uplift and denudation of the Tibet plateau, theprimary ore upraise to the surface, Cu and Au enrichment again under supergeneoxidation and secondary enrichment, formed chalcocite+limonite+hematite+chryso-colla+covellite+gold mineralization.