Skarn Diagenesis And Metallogenesis In Jiama Copper-polymetallic Deposit,Tibet

As a giant pophyry-skarn deposit located in Gangdes belt,Tibet,with great economic value and scientific importence,the Jiama Cu polymetallic deposit has preserved the imformation of skarn deposit's formation completely.Despite our group members have spent nearly 8 years on the exploration and research of this deposit,have gotten a lot of important achievements and accumulated a abundant of research date,there are still some weaknesses in the research of the deposit on the important scientific issues of the diagenesis and metallogenic mechanism.This study has researched the tectonic setting,geologic feature,the formation and evolution of skarn and mineralization of this deposit in the method of microscopic examination and other advanced testing approachs,with mainly focusing on,formation and evolution and mineralization of the skarn.This study has also established a skarn metallogenetic model of Jiama after analyzing the type of skarn,mineral composition,mineral zonation,source of ore fluid and it's developing features which means a lot for prospecting in outside and the whole region.Through this study we achieved the following research progress and innovation achievements:1.Definited the type of the ore body and identified the ore characteristics.It was concluded that the Jiama Cu polymetallic deposit is consist of four parts which could be consided as a model of four-in-one.The first part of this deposite is a concealed porphyry Mo(-Cu)ore body in the deep.The second part arounding a porphyry mass and occurencing in a structural belt between sand slate-hornstone from Linbuzong Fm(lower Cretaceous)and limestone-marble from Duodigou Fm(upper Jurassic)proved to be a porphyry Cu-Mo-Pb-Zn(-Au-Ag)body.An other part is a hornfels Cu-Mo(-Au-Ag)ore body distributed in fracture system of upper porphyry.The last part distributed in the periphery structural fracture,and it's an independent gold deposite.The structure of skarn are mainly presented as dense disseminated,cloddy,veined and veinule-disseminated.And when it comes to the hornfels and porphyry,it's typical veined-disseminated structure.There are three main types of the ore texture which are crystallization,metasomatism and exsolution.In ore minerals,chalcopyrite,bornite,molybdenite,chalcocite,tetrahedrite,gelenite,sphalerite and native gold are major.The gangue minerals are mainly quartz,feldapar and skarn minerals and modicum anhydrite,calcite,fluorite.2.Identified a kind of typical calcium skarn mineral assemblage and found out the zonation of skarns.We identified a set of mineral assemblage of typical calcicskarn containing garnets,diopsides,grammites,idocrases from prograde metamorphism stage and chlorites,epidotes,hornblendes from retrograde metamorphism stage.There are some apparent zonation about lithologic properties,mineral association and composition of skarn in both vertical and lateral direction.As for the vertical direction,the rock distribut as black massive biotite hornfels?banded silicified biotite hornfels?skarnized hornfels?garnet skarn?wollastonite-garnet skarn?garnet-wollastonite skarn?wollastonite skarn?skarnized marble?marble from the top down and the content of andradite garnet trends to increase(And48.36?And79.36).The color of garnet go with a spatial variation rule as tawny?green(brown)?dark reddish brown from shallow to deep.And for lateral direction around the contact zone,the granet/pyroxene ratio go as above 20:1?Jn10:1?Jn 5:1 from near-end of porphyry contact zone to distante-end and the content of andradite garnet intendes to decrease.The color of garnet turns as reddish brown?breen?green-canary.The performance of garnet which close to the hornstone(the top plate)and the marble(the bottom plate)are different: And54.87?And46.71?And41.87 and And88.76?And77.47?And66.86.3.Have systematic research on the results of skarn's lithogeochemistry,mineral chemistry,isotope geochemistry and fluid inclusion.The ore-forming fluid comes from magma originally and mix with meteoric water in the later period.After the ore-bearing hydrothermal sexsolution from magma chamber the magma intruding upwardly and the mineral elements migrat with gas and fluid along with the escaping of high salinities,high temperature fluid and high temperature gas.The fluid migrits laterally in the enlarged space of Linbuzong Fm-Duodigou Fm caused by tectonic activities and vertically in craks of porphyry and hornstone which formed in different phases.The driving forces come from tectonic movement,temperature gradient and pressure gradient.Mineralization goes through a process of four steps including exsolution of volatiles and fluid,formation of supercritical fluid,fluid boiling and fluid mixture.The boiling phrase is an important mechanism for mental deposite.4.Completed the zoning pattern of metallic mineral and ore-forming elements in skarn.It's indicated that there is a obviouse correlation between mineral zonation and mineralization.The wollastonite skarns distribut near contact zone of magmatic rock and bottom of middle contact zone and contact zone of marble.Copper mineralization is the major mineralization.The metallic minetal are mainly bornites,tetrahedrites and chalcocites,barely no galenas,sphalerites and molybdenites.The Mo-Cu mineralization develop mainly in garnet-diopside skarn,the Cu mineralization manifest as presenting of chalcopyrites and few bornites.Most of the ore minerals develop between garnet crystals and partially assemble into massive ores,these all ores contributes to the formation of ore shoots.Garnet-diopside-(tremolite-actinolite)skarns and garnet-epidote-(diopside)skarn lies on subsurface with mainly Pb-Zn mineralization.These types of mineral is consists of large sum of gelenites andsphalerites which present as symbiont and small sum of chalcopyrites,associate with Au-Ag mineralization.For near-end of porphyry contact zone skarn?central-end of porphyry contact zone skarn? distante-end skarn,the ore-forming elements which is corresponded with the mineral assemblage is Mo+Cu±Au±Ag?Cu±Mo±Au±Ag?Pb+Zn+Cu±Au±Ag±Mo?Au±Ag±Cu.5.Identified the magmatism and mineralization of the skarn.The metallogenic progress of skarn in Jiama deposit is divided into three metallogenetic stages:magmatic stage,hydrothermal stage and exogenous stage.The hydrothermal stage which is after magmatic stage is the main mineralization period.Further more,according to the difference in ore-forming temperature,the hydrothermal stage is futher divided into evolution metasomatic stage,degradation altered stage,quartz-copper sulfide stage,quartz-copper-molybdenum sulfide stage,quartz-lead-zinc-copper sulfide stage,quartz-gold ore forming stage and quartz-carbonate stage.6.Summarized and identified the metallogenic model of Jiama skarn deposit.In Miocene langhian stage,a large-scale of acid magma emplacement in Jiama area leading to the thermal alteration that transformed sand-slates and limestones into hornstones and marbles respectively.This emplacement also caused the formation of barrel-shaped crack system in the overlying Linbuzong Fm hornfels.Typical alteration zoning and copper and molybdenum mineralization in the porphyry copper system formed due to the vertical escape of magmatic hydrothermal fluid.Lateral escaping fluid finally run into the overlying hornstones of Linbuzong Fm and underlying marble of Duodigou Fm,afterwards,the forceful dimetasomatism produced a large scale of skarn.In the retrograde alteration peroid,the evolution of hydrothermal fluid metasomatism progradation skarn transformed into degradation skarn under the metasomatism caused by hydrothermal fluid,and along with Cu,Mo,Pb,Zn(Au,Ag)mineralization.