Genisis Study Of Mo-Cu Polymetallic Deposit From The Bangpu, Tibet

The Bangpu porphyry-type Mo-Cu polymetallic deposit is a typical large-scaledporphyry deposit was found between the years from2007to2009,which lies ineastern Gangdese metallogenic belt,Tibet.Its geotectonic position is in Gangdese andNyenchen Tanglha Range terrane Tibetan Plateau,Tibet.On administration, which wascame within the jurisdiction of Nimajiangre County, Maizhokunggar County.Thispaper mainly through detailed study of geological characteristics, geochemistry of theelements, isochronology of the magma emplaced into the crust and formed theore-bearing porphyries,fluid inclusions,sulfur and lead isotope and so on,deeplyanalyzes important problems of its genesis, built up a metallogenic evolutionmodel,and throuth the study of prospeeting and evaluation,pointed out the maintargets of exploration guide.The exposed strata in the Bangpu ore filed from old to young are Luobaduiformation（P1l） of Lower Permian,the Dianzhong formation（E1d） of the Eogene andQuaternary（Q）.Study shows that its mother rock is monzonite granite-porphyry.Detailed petrographic and geochemical studies show that it belongs to theshoshonite series and high-K calc-alkaline series,which high in SiO2（67.06%～73.70%）、K₂O （3.61%⁷.642%）， poor in CaO （0.14%⁴.40%）and Na₂O（0.140%⁴.40%）.The monzonite granite-porphyry have high content of∑REE andvary grealy from22.01×10^-6to107.606×10^-6,rich in LREE,low in HREE. A weaknegative to normal Eu anomaly vary from0.592～1.064.Traceelements arecharacterized by LILE-rich（enrichments in Rb、U、Cs） and HFSE-depletion（depletments in Ba、Sr、P、Ti and Y）.The forming environment of monzonitegranite-porphyry is complex,but mainly in syn-collisional tectonic setting whichbelongs to the Indian-Asian Continental Collision,to which full of crust extensionaltectonic and refer to many stages of Gangdese Regional Plate subduction.Ore bearing monzonite granite-porphyry was mainly came from the crust’s reworking granitewhich belongs to S-types,but does not rule out of A-types of mantle oringin granite.The ore body maily be made up of Mo and Cu ore body and Pb and Zn ore body.TheMo and Cu ore body are located within the monzonite granite-porphyry（ηγπ） whichbelongs to the late Himalayan stage and wall rock of Dianzhong formation volcanicrock.In the plane of the horizon,the ore body is hollow-oval which like a bean orenormous size equiaxed dendrites in shape,while in section,it likes aciculate types orvertical body.ore body occurrences along their extension are stable.The Pb and Zn orebody yields in the contact zone and nearby where rock contact with limestone belongsto Luobadui Formation in Lower Permian（P1l）,the distribution of ore body are nests,lentil,vein and layer-like-shaped.The structure of ore are impregnation,string,crumb, breccoated and massivestructure et al.The main testure are self-crystals to semi-self crystals,xenotopic,inclusion and coterminal texture et al. Dominanent metallic minerals are molybdenite,chalcopyrite,pyrite, and galenite,sphalerite et al.Nonmetallic minerals arequartz,potassium feldspar,plagioclase,sericite,calcite,chlorite etc.Through electronicprobe test, Mo, S main components content in molybdenite is similar to its theoreticalvalue（40.06%）,calculate the molecular formula of molybdenite as Mo0.99S2-Mo1.02S2.In addition of the main elements such as Mo,Cu,Pb,Zn can be used, elementsof S,Au and Ag can also be comprehensive utilizated.This deposit have typical wall-rock alteration,which alteration zoningcharacteristic are similar to the model of lowell and Geibert.Zoning in plane or insection are like the same,from the inner to outer,K-silicate zone, sericite altered zone,argillic zone and aropilitic zone.The mineralization of Mo and Cu are mainly inK-silicate zone,secondly in sericite altered zone,while Pb and Zn are distributed inaropilitic zone.Isochronology study of rock forming and ore forming was applicated in thisarticle The zircon SHRIMP U-Pb technique was applied to measure the age of themonzonite granite-porphyry and granite-porphyry from the Bangpu porphy deposits,It was obtained that the206Pb/238U mean age respectively are13.9±0.3Myrs （MSWD=3.05）and14.2±0.2Myrs（MSWD=0.79）.The isochron of whole rock Rb-Sr ofmonzonite granite-porphyry is13.88±0.38Myrs,which are all consistent with theirmodel ages.The isochron of Re-Os of molybdenite is14.92±0.18Myrs（MSWD=1.3）,weighted mean age is14.56±0.18（MSWD=6.0）,which are consistentwith their errors.While associated with the former researched data,full of nine point,gained an isochron age of Re-Os are14.67±0.54Myrs,weighted mean age as14.54±0.093Myrs,these ages are so closed to each other.So the age of14.67±0.54Myrs presented the age of ore-forming of Mo and Cu from Bangpu porphyrydeposit.Continuous laser microprobe40Ar-39Ar technique has been taken to carry outin stiu analysis on to the ore-bearing vein quartz,an40Ar-39Ar isochron which givesout an isochrons age of13.9±0.9Myrs（MSWD=29） and reverse isochrones age of13.8±1.6Myrs（MSWD=30）.A pyrite Rb-Sr Isochron age of11.0±1.5Myrs withMSWD=3.5was obtained from ore-bearing Mo and Cu ores which represents the ageof Mo-Cu’s later episode mineralization.A sphalerite Rb-Sr Isochron age of3.6±1.5Myrs（R=0.9942）was obtained from ore-bearing lead and zinc ores,thise ageis lower than the age of zircon U-Pb,Re-Os age and Ar-Ar age, shows that the leadand zinc deposit is the activity of porphyry-after the term hydrotherMyrsl,whichrepresents the age of Pb and Zn’s later episode mineralization.The isochron age of thisdeposit is from l5.32±0.79Myrs to11.0±1.5Myrs.In this deposit,the Mo-Cu Myrsinlymineralized from14.67Myrs to11.0Myrs, ore-beard porphyry age of mineralizationwas from14.4Myrs to13.5Myrs,shows that its rcok-forming and ore-forming is acontinuous process,which belongs to a rapid outbreak of large-scale mineralizationprocess.Study of the temperatures,pressures,salinities,compositions and oxygen andhydrogen isotopes of fluid inclusions in quartz and calcite in various mineralizationstages of the hydrothermal ore-forming process indicates that the ore fluids in everyperiods belong to high and medium temperature （172²39℃）,medium sality（23.96⁵0.85wt%NaCleq）,high and medium density（1.0696～1.0849g/cm3）onesand the（Cl^--）SO₄（2-）Na⁺-（K⁺）hydrochemical type and formed in a low-pressure（36.9×10⁵Pa¹10.23×10⁵Pa）and ore-forming pressure（23.4×10⁵Pa⁴1.9×10⁵Pa）,ore-forming depeth is0.094⁰.184km which belongs to super hypabyssal oreforming process.Three periods of the ore fluids may have been derived frommagmatic-hydrothermal and subsurface hydrothermal water,while subsurfacehydrothermal water prodorminent.Through the study of sulfur, lead and noble gas isotopes, it show that34S fromore-bearing magmatic and sulfide mineral are in accordance with magma sulfurcomposition （34S=0‰） which vary from-0.48‰¹2.2‰（mean=3.80‰） and-1.5‰～+4.3‰（mean=0.98‰）respectively,and34S∑Sis-1.903‰.The lead isotopeof206Pb/204Pb,207Pb/204Pb,208Pb/204Pb from ore-bearing magmatic is vary from38.857³9.857,15.620¹5.704,18.684¹8.768respectively, while the lead isotopefrom galena is vary from18.752¹8.825,15.712¹5.718,39.348³9.463, it shows thenormal lead characteristics with the same homologous evolutionary history or the source.The eigenvalue and lead isotopic diagram shows they mixed with crustalmaterials and mantle materials, but majored in crust origin, mainly comes from theupper crust in orogenic belts.Sr isotopes and Re isotopes indicate that themetallogenic substance may be derived from the mantle, or mainly from mantle.Through these studies above,the genesis of this deposit was confirmed, it is avolcano hydrothermal deposit.while basised on the ore-bearing rock,the ore of Moand Cu is a porphyry copper type deposit,while the lead and zinc ore is carbonatetype.According to the analysis of ore-forming process, the evolution model of ore-forming in the area being established.Through alteration zones characteristics,geological prospecting line section,fluid inclusions and other aspects for explorationguide, We suggest that the west area have metallogenic potential of Mo-Cu deposit,especially at depth,the emphasis may be on the west and depth for later work.