| Gangjiang copper-molybdenum deposit of Nimu,Tibet,is located in the Gangdese epicontinental volcano magma arc and the central part of Gangdese metallogenic belt.Gangjiang deposit is mainly composed of a series of platelike copper-molybdenum ore bodies,which is distributed in the monzogranite porphyry.In this paper,based on the Cu-I ore body of Gangjiang deposit,the author research on the morphological variations of orebody,diagenetic and metallogenic chronology,Mineralization enrichment regularity,metallogenic mechanism and so on.Used EH-4 and high precision magnetic survey,combined with the reprocessing of romote sensing,soil geochemical survey and IP measurement,to carry on the prospecting prediction and evaluation work in Gangjiang mining area and periphery.The main sesults obtained as follows:(1)In general,the Cu-I ore body of Gangjiang deposit is composed of a series of platelike secondary ore bodies,which is nearly N-S trending,and inclining to the west steeply.Quantitative analysis is made on the grade and thickness of the ore body,and the result shows that Cu-Mo grade has a single peak distribution characteristic,the northern section of ore body thickness is greater than the southern section of ore body,but the grade of southern ore body is slightly higher than the northern ore body.As a whole,Cu-I ore body shows the characteristic of ?upper copper and lower molybdenum?.(2)Based on the mineralogy,major elements,REE elements and trace elements analysis of the main intrusive rocks of Gangjiang mining area,the magma source,geological backgroud and ore-bearing ability are discussed: the Gangjiang intrusive rocks belong to the high–K calc-alkaline slightly pearaluminous I-S type granite.The intrusive rocks are commonly rich in large-ion lithophile elements such as Th,U and Sr,but in depletion of HFS elements such as Nb,Ta and Zr,and in strong depletion of HREE,Y element and Yb element.The REE distribution patterns are characterized by LREE enrichment and sligtly negative Eu anomalies.And formed in post-collisional extension setting of India-Asia continental collision orogenic belt.Relationship between rock and mineralization is evaluted from the angle of element of enrichment,monzogranite porphyry is the most important ore bearing rock.Gangjiang mineralized and non mineralized rocks have similar distribution patterns,and with the enhancement of mineralization intensity,REE,LREE/HREE,(La/Yb)N of mineralized rocks show a continuous downward trend.The above shows that the REE and trace element are moved out with proportion during the hydrothermal activity.(3)The author carried out a study of LA-ICP-MS zircon U-Pb ages of the monzogranite porphyry,granodiorite porphyry and tonalite porphyrite in Gangjiang mining area.It is shown that the zircon U-Pb weighted isochron ages are 16.6±0.3Ma(MSWD=0.94,n=10),16.1±0.2Ma(MSWD=1.07,n=12),14.4±0.4Ma(MSWD=1.12,n=7).Meanwhile,twelve molybdenite samples separated from quartz sulfides in Gangjiang copper orebodies were used for Re-Os dating,the model ages obtained range from 13.24±0.20 Ma to 13.55±0.22 Ma,with weighted average 13.4±0.1Ma(MSWD=0.65)and isochron age 13.6±1.6 Ma(MSWD=1.2).On the basis of geological studies,and conbine with the results mentioned above and previous research data,it is proposed that the rock mass intrusion sequence is monzogranite porphyry?granodiorite porphyry?tonalite porphyrite,and petrogenetic and metallogenic is a continuous process of magmatic evolution.Gangjiang porphyry copper-molybdenum deposit was formed in post-collisional extension setting of India-Asia continental collision orogenic belt during the Miocene.(4)The vertical primary halo pattern of Gangjiang ore body has been made: Mn,Co,Sb enrichment mainly occurs in upper part of the ore body,while Cu,Mo,Ni,Bi,W in the central section and Zn,Ag,Pb in the bottom.The quantitative evaluation model has been built,chosing the zoing index as the(Cu×Mo×Sb)D/(Pb×Zn×Ag)D as the indicator of the model.The walkrock alteration has obvious zonation,from inner to outer,K-silicate zone,phyllic zone,argillic zone,propylitic zone.The mineralization of Cu and Mo are mainly in K-silicate zone,secondly in phyllic zone.A variety of alteration zones superimposed are favorable sites for mineralization.The mineralization has been divided into three phases: magmatic phase,hydrothernmal phase and hypergene phase.The formation of Gangjiang deposit is one of the tectonic magmatic metallogenic events in the post collision extension of India-Asia continent.Cu and Mo mineralization is closely related to the two periods hydrothermal activity in Miocene.Two ore-bearing hydrothermal superimposed along the rock crack penetration,and multi period crystallization diffentiation is the key to mineralization.Finally,the fine grade veinlet-disseminated copper-molybdenum ore body has formed.The ore controlling structure,denudation depth and caprock property lead to the difference of the grade and ore body scale between Gangjiang and Tinggong,Bairong,Qulong deposit finally.(5)Based on prospecting indicator and criteria of Gangjiang mining area and periphery,according to the research degree and geological features of different areas,the different prospecting techniques have been selected in prospecting prediction.The forecast bases have been put forward and prospecting area has been delineated.The D1 low resistivity anomaly on the west side of Gangjiang EH-4 section has good continuity,which is more likely to ore induced anomaly.The southern Bairong has potential to become the second east-west ore(mineralized)zone.There are obvious remote sensing alteration in northwest Ronggangmeng,limonitization with large scale has been found by field reconnaissance,which shows great ore prospecting potential.A new idea for similar to Tangbala skarn copper polymetallic deposit in the northern Bairong area,the deep abnormal position,which is reflected by EH-4 and high precision magnetic profile,is agreement with theoretical calculation depth by Bairong-Tangbala geological prediction model.The delineation of prospecting area needs further exploration and research,in order to achieve the purpose of improving quality and storage. |
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