Geological Characteristics And Metallogenic Model And Prospecting Prediction Of Copper Deposits In YunXian Section Of Lancangjiang Volcanic Arc, Yunnan Province, China

South Lancangjiang arc volcanic rock belt belongs to part of SanjingTethyan tectonic metallogenic domain, which is one of the most potentialcopper polymetal exploration prospective areas. Based on the study ofregional metallogery, geological setting and its evolution in detail and thegeneral gathering of geology, remote sensing, geophysical prospecting,geochemical exploration, prospecting and production data of the oredeposits(spot) in the study area, and guided by regional metallogeny andarc volcanic rock mineralization theory, this paper has studied the typicalore deposit——Guanfang copper deposit and representative granitoidsmall rock body and middle-late Triassic potassium-high volcanic rocksin the study area intensively and systematically for the fisrt time. Themain results and some new viewpoints can be summarized as following:(1) Late Triassic Xiaodingxi Formation basic volcanic rocks belongto potassium-high cacl-alkali basalt series-shoshonite series with theirRittmann index varying in the range of 2.23～9.24 (σ=6.27 on average)in the study area, which are enriched in LREE and potassium(K₂O=2.68% on average). The REE distribution pattern is slightly rightoblique with an average∑REE of 171.83×10^-6, and they exist no orvery weak negative Eu anormalies. The rocks show evident LILE like K,Rb, Ba, Th positive anomalies and HFSE such as Nb, Ti, Ta negativeanormalies in the trace element distribution pattern. According to a lot oftectonic setting discrimination diagrams of the rocks as well as geologicalcharacteristics of the rock assemblage, it is suggested that the late TriassicXiaodingxi Formation basic volcanic rocks along South Lancangjiangbelt should be composed of active epicontinental arc-volcanic rocks. The characteristics of radiogenic Sr, Nd isotope show that XiaodingxiFormation basic volcanic rocks have a relatively high value of ⁸⁷Sr/⁸⁶Srwith an average of 0.708, which is higher than the (⁸⁷Sr/⁸⁶Sr)_UR presentvalue 0.7045 of chondrite uniform reservoir. The values ofε(Nd) arenegative, which vary from -2.48 to -1.21, and the values of¹⁴³Nd/¹⁴⁴Nd are obviously lower than the present value of chondriteuniformless reservoir. The characteristics of geochemistry and isotopicgeochemistry infer that the source of late Triassic Xiaodingxi Formationbasic volcanic rocks has the feature of crust-mantle mixed material,which is consistent with EMâ…¡. So the potassium-high basic volcanicrocks in the study area were not directly from the remelting of normalmantle or crust material, but it's from the partial melting of mantle source,enriched in large-ion lithophile elements and LREE, in which continentalcrust material and oceanic sediments are largely involved and inevitablyrelated to the subduction eastward of Tethyan Lancangjiang oceanic plate.(2) Middle Triassic acid volcanic rocks of Manghuai Formationbelong to potassium-rich calc-alkali rhyolitic volcanic rock series, whichis characteristic of high silica(SiO₂=75.49% on average), medium Al₂O₃,CaO＜1%, ALK＜8%, rich potassium(K₂O=3.38% on average), lowtitanium(TiO₂=0.136% on average), Rittmann index (σ)＜3.3, K₂O＞Na₂O and A/CNK higher than 1.1(1.53 on average). The rocks also havehigh LILE like K, Rb, Ba, Th and LREE concentrations with obvious Euanomalies but poor in HFSE such as Nb,Ti, Ta and siderophile element.The ⁸⁷Sr/⁸⁶Sr values of these acid potash-rich volcanic rocks are higherthan 0.710 (0.755 on average) andε(Nd) values of them are all less thanzero but far and away greater than the average of the crust(-15), whichvary from -3.63 to -1.66. Based on a comprehensive analysis of thetectonic setting and geochemical characteristics, it is concluded that thesepotash-rich volcanic rocks of Manghuai Formation are mainly originatedfrom the remelting of continental crust materials, accompanied with theparticipation of subduction zone materials, which made them have thecharacteristic of collision type arc volcanic rocks.(3) The chemical composition of siliceous rocks among late TriassicXiaodingxi Formation basic volcanic rocks is mainly SiO₂ in the range of 96.28%～97.92, secondly is FeO and Al₂O₃, other ingredient is very low.The siliceous rocks mostly belong to organic origin and formed in thecontinental margin according to some discrimination diagrams, which isin accord with the conclusion that the tectonic setting of middle-lateTriassic volcanic rocks in the study area is epicontinental arc.(4) The Laomaocun rock body in South Lancangjiang belt is mainlymonzonite granite-porphyry. Chemical composition of the rock body isSiO₂=77% on average, Al₂O₃=12.08%～14.33%, A/CNK (molecule ratio)＞1.1(1.68 on average), K₂O/Na₂O=4.54 on average, Rittmann index(σ)=0.75～1.34 (1.14 on average), DI=84.0～87.7,∑REE=204.27×10^-6～274.17×10^-6,∑LREE/∑HREE=3.43～5.44 andδEu=0.47～0.60.The rock body shows obvious negative Sr, P, Ti and Eu anomalies as wellas positive K, Rb, Ba, Th, U anomalies in the primitivemantle-normalized trace element spider diagrams, suggesting evidentfractional crystallization. According to its lithofacies and petrological andgeochemical characteristics, the Laomaocun rock body is silica-high,potassium-rich and belongs to peraluminous calc-alkaline S type granite,which is similar to collision type acid volcanics of Manghuai Formationin the middle Triassic period. Rb-Sr age of the rock body is 169Ma±5Ma.Various oxide and trace element diagrams for discrimination of tectonicsetting reveal that the Laomaocun rock body is post-orogenic granite(POG) formed in a post-orogenic extensional structural environment.(5) The ore bodies of Guanfang copper deposit are strictly controlledby the faults that radiate toward the outside and lithologic characters ofpotassium-high volcanic rocks, the vein orebodies cut off the strata ofdifferent sub-section of Xiaodingxi Formation basic volcanic rocks,brecciated structure and vein structure are widespread in the ore. Thementioned features of Guanfang copper deposit show that the forming eraof the ore deposit is posterior to that of Xiaodingxi Formation basicvolcanic rocks. Each subsection of Xiaodingxi Formation has no initialcopper enrichment in the regional section and the REE characteristic ofthe ore is quite different from that of the host rocks. Therefore theore-forming materials of Guanfang copper deposit should have comefrom the lower crust and upper mantle instead of Xiaodingxi Formation basic volcanic rocks in the process of mineralization. The zone of highpermeability is the best position for mineralization composed ofvolcanicclastics, breccias, amygdules and fractures on the top of eacheruption gyration of subaerial flood basalt, which is one of the major orecontrols of orebodies, especially quasi-strata-bound ore bodies.(6) Compared to the typical 'Volcanic redbed copper deposit' (VRC),Guanfang copper deposit is characterized by its low Cu background valueand its absence of low greenschist facies metamorphism of the hostvolcanic rocks although they have some common points. Coppermineralization is close related to silicification and pyritization, which iswidespread in the ore deposit. It is suggested that the ore-formingmaterials mostly derived from the lower crust and uppermantle, themetal-bearing fluid is H₂O-NaCl-CO₂ system with low-middle salinity,the ore-forming fluid is of hybrid type that is composed of magma waterand meteoric water according to the study of trace elements, fluidinclusions and isotopes of the copper ores and ore-beating rocks. Thesulfur isotope composition of the main sulfides is within the range of-11.88‰～-5.67‰δ³⁴S_V-CDT. In combination with tectonic setting,wallrock alteration type and ore control element, it is concluded thatGuanfang copper deposit is hydrothermal Cu ore deposit that is ofgenetically low to medium temperatures and formed in a relatively opensystem and closely related to the evolution of the magmatic activity ofthe concealed intermediate-acidic rock or Indosinian sub-volcanic rock inthe volcanic apparatus.(7) The evolution history of volcanic rocks in South Lancangjiangbelt indicates that late Triassic epoch is a turn time of regional stress field,and the massive extensional activity and the eruption of potassium-highcacl-alkali basalt series-shoshonite series volcanic rocks and thesubsequent mineralization event are not insular and occasional, which isconstrained by some mechanism, and lithosphere delamination maybe is arational deep mechanism. The remobilization of the thrust-nappe andstrike-slip and extensional structure such as Lancangjiang major faultwithin the continental on a large scale that incise down to mantle owingto Indian plate's northward impact and extrasion after late Triassic epoch perhaps result in the partial melting of the oceanic plate relic (maybe havehigh background value of the mineralization elements of Cu, Ag and soon) leaving in the mantle in the tectonic zone and intrude upwardsfollowing the dotty structure position such as volcanic mechanism and theconvergence of two major faults. Thus the formative magma withabundant initial Cu element and volatile flux and potassium became theintermediate-acidic fusant that is rich in fugitive constituent such as Cl^-and HS^-- and metallogenic elements such Cu²⁺, Pb²⁺, Ag⁺ and so on in thelate stage after ceaselessly differential evolution, the ore-forming fluidbeing drived by the power of structure and volatile flux through thefracture zone deposited in favorable place of Xiaodingxi Formation basicvolcanic rocks. So the evolution of tectonic setting of South Lancangjiangbelt and magma activity, the magmatic evolution and the mineralizationactivity is the all and the one, and the mineralization activity is the onlylast link among them.(8) Based on the study and summary of the geological features andmineralization law of Guanfang copper deposit, the relation of spacetimeand genesis between the copper orebodies and potassium-rich volcanicrocks is expounded, the metallogenic model of Guanfang copper depositand the comprehensive prospecting model of copper deposits in YunXiansection of Lancangjiang volcanic arc are set up, the prospecting signs aredesignated, the prospecting perspective is confirmed and the prospectingtargets are optimized in the study area. In the meantime the prospectingwork of Guanfang copper deposit obtained a great success and createdhuge economic and social benefit.