Metallogenesis Of Paleoproterozoic Copper Deposits In The Northern Zhongtiaoshan Mountains, Shanxi Province

Precambrian strata are widely exposed in the northern section of Zhongtiao Mountain,Shanxi province. From bottom to top, the precambrian strata-tectonic units can be dividedinto: Sushui multi stage granites-supracrustal rocks complex, flysch-volcanic formation ofJiangxian group, marina-shallow marine epicontinental clastic-carbonate formation ofZhongtiao group, molasse formation of Danshanshi group and volcanic formation ofXiyanghe group. The latter four are connected by unconformities which mark themovements of Jiangxian, Zhongtiao stage I and Zhongtiao stage Ⅱ, respectively.Tongkuangyu super-large porphyry copper (molybdenum) deposit occurred in the volcaniccomplex of Jiangxian group and Hu-Bi-type stratabound copper (cobalt) deposit in theterrigenous clastic–carbonate of Bizigou group are the most important types of copperdeposits, and also, the main objects of this study.In Zhongtiao Mountain, there mainly exist three events: the granitoids intrusion eventin2.55~2.35Ga, bimodal volcanic rocks eruption and the later calc-alkaline hypabyssalintrusion of Jiangxian group in2.2~2.15Ga and volcanic eruption of Xiyanghegroup/Xionger group in1.8~1.75Ga. Stage Ⅰ (1.88-1.85Ga) of Zhongtiao movement isthe main metamorphism stage, response to continent-continent collision between the eastand west blocks of North China Craton. The metamorphic event led to the most strata inZhongtiao Moutain metamorphism into the granulites-greenschist facies and the formationof a series EW-striking closed syn-oblique folds and reverse ductile shear zones in thedeep strata. In1.8~1.75Ga, because of post-orogenic extension, mafic dikes were widelydeveloped as well as basaltic andesitic volcanic eruption of Xiyanghe group in ZhongtiaoMountain. The regional rocks were widely retrograde and the strata developed a series ofextensional tectonics.Over the years, scholars have been identified Early Proterozoic tectonic setting ofZhongtiao Mountain for the rift zone, under the influence of this theory, many scholarsbelieve that the evolution of the "rift" control the Early Proterozoic mineralization in Zhongtiao Mountain.“Alkaline bimodal volcanics” of Tongkuangyu subsets, Jiangxiangroup is often considered to be formed by Early Proterozoic rift, mafic volcanic rocks ofJiangxian group are generally influenced by the later fluid activities and have thecharacteristics of abnormal high potassium. Features of fluid incompatibility elementssuggest the protolith may be calc-alkaline basalt (or some tholeiitic basalt), rather thanalkaline volcanic rocks.The geochemical characteristics of subduction zone rocks showthe tectonic setting of the “rift” in Early Proterozoic is not a continental rift, but thepartial extension under subduction setting. Geological features of flysch-calc-alkalinevolcanics of Jiangxian group show it may be formed in an extensional tectonic settingsimilar to back-arc basin, while the Zhongtiao area at the same time is an active continentwith trench-arc-basin property. The granodiorite porphyry intruded in about2.15Ga andthe later Jiangxian movement show the conversion tectonic setting from extension tocompression and eventually closed. Tongkuangyu porphyry copper (molybdenum) depositis exactly formed in this conversional tectonic setting.Tongkuangyu porphyry copper (molybdenum) deposit develops in the volcaniccomplex of Jiangxian group, mineralization types are mainly disseminated veinlets,closely related with granodiorite porphyry (2.15Ga) in spatial and temporal. Alterationzones have been deformed, in banded and stratiformed distribution, banded redalbitization (calcification?)-biotitization alteration zone is limited in a more widelyphyllic alterated zone, temporal sequences is: albitization (calcification?)�'biotitization/early stage phyllic�'main stage phyllic alteration. Fluid inclusion study shows: the earlymineralization stage occurs in the devolatilization process after biotitization, and themineralization temperature ranges from340to390°C, pressure from100to120Mpa, thebiotitization process may provides certain minerals for mineralization, and lower salinityof the main mineralization stage may be due to the involving of meteroric water. Isotopestudy has shown that S, H-O isotopes of the fluid in Tongkuangyu deposit display themagma source. Although H-O isotopes mainly fall into the range of magmatic water, itmay cover up the characteristics of meteroric-involved O isotopic, which caused bywater-rock reactions and metamorphic processes that lead to fluid O isotope exchange with that of the volcanic wall rock.Hu-Bi type stratabound copper (cobalt) deposit is located in the exposed Zhongtiaogroup, on the wing of Shangyupo duplex anticline. The ore field is generally controlled byShangyupo closed syn-oblique fold system and the subsequent extensional structuresystem, that formed in the stage I of Zhongtiao movement, but the influence ofinterlaminar shear rheological structure to the deposit can not be ignored, the all threecontrol the entire structural style of the whole ore field/deposit. Interlaminar shearrheological structure is a structural style of partial interspersed-contained with each otherof the rocks due to differences in the physical properties, which led to the different strainresponse among the adjacent layers because of the competent differences in the stress field.The mining area widely developed of interlaminar shear rheology structures, by structuralrecovery, the native ore-bearing formation of Hu-Bi-type stratabound copper (cobalt)deposit is biotite marble/siliceous albitite-impure marble-carbonaceous schist combination,the main mineralization types are vein-type and veinlet disseminated type, butmineralization is basically limited to the terrigenous clastic-carbonate strata in the upperpart of the Bizigou group. Geochemical characteristics of the ore-bearing siliceous albititeshow that it is not metavolcanic rocks and the detrital zircons of the rocks indicate theore-bearing siliceous albitite may be the albitization sedimentary rocks.Zhongtiao group is a stable shallow water clastic-carbonate construction formed afterJiangxian movement, while Yuxiayuan group may be formed in the lagoon environment.Marble of Yuxiayuan group and muddy-clastic of the lower Bizigou group are composedof a small transgressive sequence, while the pelitic-carbonate construction of the upperBizigou group and marble of Yujiashan group are generally composed of a smallregressive sequence, the detritus material of Bizigou group may provides certain materialsfor mineralization. No evidence shows the existence of magmatism contemporary withdeposition times of Bizigou group, currently. In Cu/Co vs Mo diagram, it showssignificantly linear correlation between Hu-Bi-type stratabound copper (cobalt) depositand Tongkuangyu porphyry deposit, distribution of detrital zircon ages also shows thedetritus material in ore-bearing formation is the volcanic rocks of Jiangxian group, suggesting the erosion part of Tongkuangyu copper deposit provides Cu, Co, Moore-forming materials for Hu-Bi-type stratabound copper (cobalt) deposit. The mainmineralization temperature of Hu-Bi-type stratabound copper (cobalt) of deposit rangesfrom160to200℃, the ore-forming fluids may be the construction water/metamorphicwater, the main mineralization stage may occur in the diagenesis process to the dewateringprocess of low-grade metamorphism, generally high salinity of the ore-forming fluidsshows some ore-bearing layers may exist evaporite minerals. C-O isotope study shows thewater-rock reaction of mineralization process is similar to that of the sediment-hostedstratiform copper-cobalt deposit in Central African copper belt, compared with typicalCu-Co stratabound deposits, mineralization process of Hu-Bi-type stratabound copper(cobalt) is similar to, only slightly different from that of the sediment-hosted stratiformcopper-cobalt deposit in Central African copper belt.