The Study On Metallogenic Model And Prospecting Pattern For Jiama Polymetallic Copper Deposit, Tibet

At present, Gangdese metallogenic belt is the largest tectonic-magmatite belts which makesthe highest level of prospecting and studying, discovering ore deposits, setting up the mostexploration zone and proving through investigation mineral resources reserves. Jiama copperpolymetallic ore deposit is the highest level of prospecting, possessing a variety of ore-formingelements and ore body types of porphyry-skarn deposit in middle east of Gangdese metallogenicbelt, which has investment in producing and taking economic benefit for Tibetan. Throughstudying on ore-controlling conditions, metallogenic model、prospecting model and explorationmarkers etc, all of them are to get a clearly prospecting direction in ore field and outside. Duringsix years based on geological field work and clearing up previous work, we adopting moderndeposit research method, combining with multidisciplinary knowledge, applying advanced testmethod and technology. At last, we obtaining some understandings as below：1. Ore field mainly comprise of Linbuzong formation sandy slate, hornfels and Duodigouformation limestone, marble. Linbuzong formation offer metallogenic materials and hoststructures for upper garnet skarn and hornfels, expect that it also offer lithology entrapmentconditions for copper polymetallic ore deposit; Duodigou formation took calcium for skarnforming. There are five types of magma in the ore field, and they are mainly high-K calc-alkaline,forming from post-collision orogenesis condition; magma migmatization is obvious, the base unitsmainly are gabbroic magma; the sequence of magma invasion is: quartz diorite porphyrite�'granite porphyry�'monzonite granite porphyry�'granodiorite-porphyry, and Cu mineralizationhas close relation with quartz diorite porphyrite, while Mo mineralization has close relation withmonzonite granite-porphyry; UST structure, erosion of quartz phenocryst and melting-fluidmixing inclusion all offer evidence for magma-hydrothermal transition. Nappe structure controlthe occurrence of mother-lode and sliding structures control the squats occurrence in eastern orefield.2. Making a detailed study on geology of deposits, includes:①Putting forward Jiamadeposit is a “quaternity “type porphyry-skarn deposits which is mainly comprise of molybdenumore body bearing in upper Linzongbu formation, Cu polymetal ore body bearing in medi-interlayertectonic zone and porphyry contact zones, Cu-Mo ore body bearing in bathy-hidden porphyry andAu ore body bearing outside of structure fracture zone;②The skarn ore yield by dense disseminated structure, cloddy, vein structure, hornfels and porphyry ore structure mainly isdisseminated-stringer, and they forming because of metasomatism and solid solution segregation;③The mainly ore mineral are chalcopyrite, bornite, molybdenite, cyprite, tetrahedrite, galena,sphalerite and native gold etc. gangue mineral mainly are skarns mineral（Garnet, pyroxene,Wollastonite） and quartz, including a bit of anhydrite, calcite, fluorite etc;④Improvingore-forming elements banding pattern in skarn: From the centre to middle of rock mass till topre-margin of skarn and marble contact zones. Mineral assemblage in skarn homologousore-forming elements zonation is Mo+Cu±Au±Ag�'Cu±Mo±Au±Ag�'Pb+Zn+Cu±Au±Ag±Mo�'Au±Ag±Cu;⑤setting up zoning model of mineral assemblage and composition in skarn; as awhole, wollastonite skarn is rather independent relative to pyroxene skarn, it is mainly located inrock mass contact zones and middle-lower to marble contact zones on the vertical of centralsection. While garnet and pyroxene skarn located in rock mass contact zones�'middle�'remotezones front margin of skarn, Garnet/pyroxene＞10:1�'about5:1�'about2:1; Colour andcomposition of garnet is red-brown andradite（Ad＞80）�'brown-yellow andradite（Ad50-80）�'atrovirens-light yellow grossularite（Gr＞50）;⑥Dividing the alteration zoning of Jiamadeposit, rock plant includes elements mainly are K and Si. Above the hornfels appear argillizationzones-beresitization zones-propylitization zones that are located in nugget region, the transitionregion and the periphery;⑦Summing up three minerogenetic stages, among of them Gaswater-hydrothermal mineralization stage includes early stage skarn-silicate stage, late degradationaltered stage, quartz-copper sulfide stage, quartz-copper molybdenum sulfide stage, quartz-copperlead and zinc sulfide stage and quartz-gold-forming stages.3. A detailed geochemical study on the four ore body type which are classified at this time.On the one hand, a study was made on the formation of skarn and marble skarn duringmineralization process, garnet-based barren skarn, wollastonite-based non-ore skarnization,garnet-based ore skarn and mainly wollastonite-based skarn ore silicon in terms of major and traceelements and REE characteristics. An overall analysis of the geochemical behavior of elementsduring the alteration and mineralization process has been concluded to supporting the research onthe skarm mineralization process and reasons; on the other hand, the comparative study of thegeochemical characteristics has been made on the ZK4504other drillings which containing quartzdiorite porphyry and non-containing quartz diorite in the mining area. It is pointed out that the latehydrothermal gold mineralization was caused by external structure of the porphyry system,impacted by temperature and pressure control, but not closely related to the surrounding rock.4. The characteristics of sulfur magma source has been showed that sulfide in the major metalin the deposits and sulfur in the sulfate with a high degree of homogeneity, the value is near zero;isotopic composition of206Pb/204Pb, generally is18‰¹9‰;²⁰⁷Pb/²⁰⁴Pb is15.8‰to15.4‰;²⁰⁸Pb/²⁰⁴Pb is38.4‰to39‰, same as porphyry rock lead isotope compositions in the miningarea. The isotope composition of lead has the characteristic that is the orogenic belt andcrust-mantle mixing relating to magmatism. isotope show orogenic belt and magmatic activity andthe shell curtain mixed lead source characteristics the Pb isotopic composition between theYarlung Zangbo ophiolites and Himalayan gneiss and granite, and as same as the lead isotopecomposition in the magmatic rocks from Lhasa block; comparing the characteristics of ore siliconisotope composition from the different deposits formed differently, it is confirmed that the siliconisotope composition of Jiama skarn is as exactly same as mine magmatic, but is far away from thetypical spray jet rocks. hydrogen and oxygen isotope study confirmed that the fluids originated from magmatic-hydrothermal ore-forming system in the Jiama, with the participation ofatmospheric precipitation in its late period, with inclusions in salinity-temperature variationJiama the evolution of the ore-forming fluid model with different mineralization stages of fluid.5. The study on the deposit chronology, it is concluded that the mineralization age of Jiamadeposit is±16Ma and15Ma±. This important geological fact negates view of the genesis ofsedimentary mineralization in the late Jurassic to early Cretaceous jet, and supports theunderstanding of the porphyry-skarn mineralization.6. On the basis of the research of the deposit geology and the geochemical characteristics,this paper has made a analogy research between Jiama deposits at laboratory and comparing withtypical porphyry-skarn deposits abroad, to analyze their similarities and differences. Accordingly,it is believed that the magmatism offer metallogenic material for mineralization, structure offerpositioning space for ore body, and the lithology provides a good trap for the formation of the orebody. The coupling effect of the magma-Construction-lithologic traps formed a very largedeposit of Jiama, besides, it made a deposit ore-forming fluid lateral escape and metallogenicdynamics vertical escape mechanism, so as to point out the direction to find different types of orebodies at different spatial location and to establish a metallogenic model; On this basis, combiningwith the integrated exploration geophysical, geochemical exploration, and other information, arelatively new element of porphyry copper zoning prospecting model and the traditionalgeological-geophysical-geochemical prospecting model have been established. Moreover,combined with the practice in prospecting mining area peripheral, initially, the paper confirmedthe practicality of prospecting model.7. In short, through the " quaternity " prospecting model, the drilling up to350in more thanfour years, not only the skarn mineralization prospecting has been achieved, on the0line to40line, the copper-molybdenum ore body produced in the hornsfels has been found, also found indepth porphyry molybdenum （copper） ore body. So far, found out copper resources is more than6million tons, molybdenum resource of over600thousand tons, Pb and Zn resources is more than500thousand tons, associated gold over140tons, associated silver over8000tons, the equivalentcapacity copper resources of over15million tons. Jiama copper polymetallic has beenbreakthrough in the prospecting, and became the largest porphyry-skarn-type deposit, in Tibet.