Researches On Metallogenic Series And Metallogenic Regularities In Laojunshan Metallogenic Belt, Yunnan Province

Laojunshan Metallogenic Belt is in the Southeast of Yunnan province, it is famous for the abundant ore resources of tin, zinc, ungsten and silver. In which one giant large scale (Man jiazhai) deposit, three large scale(Tongjie, Xinzhai and Nanyangtian) deposits, five medium scale(Lazizhai, Shuidongchang, Nandangchang, Gel ing, and Laozhai) deposits, and more than twenty other ore deposits (points) were proven. Laojunshan Metallogenic Belt is an important part of the southeastern metallogenic belt(Gejiu, Bozhushan and Laojunshan)of Yunnan.This dissertation was guided by the theories of exhalative sedimentary mineralization and metallogenic series, the petrology, petrochemistry and geothemistry of the stratum, skarn and granite were analysised; the geological characteristics of the deposits, the mineralogy, texture and structure of the ores were studied; the microelement, rare earth element and stable isotope of the rocks and ores have been studied. On the basis of examining the relationship between the metal mineralization and strata, structure, granites and geophysics, geochemical, metallogenic regularities were summarized. Based on analyses of metallogenic mechanism, metallogenic series were formed. Ore-forming Geological Conditions were analysed, the metallogenic model and prospecting model were established, ore formation prospect evaluation was given. The main research findings and new viewpoints achieved in this dissertation are as follows:(1)Laojunshan metallogenic belt with the central of Laojunshan mountain, as a front seabasin of Yangtze platform during Caledonian epoch, with thicker sedimentary cover, During late Caledonian epoch the belt was gradually uplift, and was integral uplift during Hercynian, only shallow sea left in the belt. The sequence of Early and Middle Devonian strata, mainly carbonate, sandstone and mudstone. Crust remelting granites formed in the central of the uplift belt during Yanshanian, and the dome like shape was formed during the same period.(2)The region has a complex geological structure and strong metamorphism, metamorphic lithologies include greenschist, amphibolite and migmatite. Based on the petrographic and mineralogical characteristics of metamorphic rocks, and the petrochemical characteristics, that the metamorphic rocks are marine volcanic exhalation-sedimentary origin. There are two kinds of skarns:complex skarn and simple skarn. Complex skarns are close to the granite, that were formed by volcano-exhalative-sedimentation-superimposition and reformation. Simple skarns are far from the granite, that were formed by volcano-exhalative-sedimentation-reformation. According to the isotopic dating, Indo-chinese epoch was the peak metamorphic period of Laojunshan Metallogenic Belt.(3)There are some evidences of volcanic exhalation-sedimentary:①Polymetallic sulfide orebodies are beded, have a gradual transitional contact and interbeded with the wall rocks, the ore-bearing strata occur steadily, and the along strike can be length up to more than 10 kilometres.②Blastoporphyritic texture, intergranular texture, crystalloblastic texture and vesicular structure, amygdaloidal structure, oolitic structure, nodule structure are widespread in the belt.③Isotopic dating of ore-forming events showed that Early caledonian and Indosinian period were the main mineralization stages in Laojunshan Metallogenic Belt, earlier than Yenshanian granite intrusion period, which is in contradiction with granitic genesis.④REE patterns and La/Yb—REE diagram showed the marine volcanic exhalation-sedimentary origin of the skarns and ores. The volcanic exhalation-sedimentary deposits are mainly occurred in Lower cambrian chongzhuang formation and middle cambrian tianpeng formation. (4)The ore generation process is experienced multi-period metamorphism and complex geological evolution process, that could mainly divide into three mineralization stages:Volcanic exhalation-sedimentary stage→Regional metamorphic stage→magmatic hydrothermal superimposition stage. During volcanic exhalation-sedimentary stage, source beds were formed in the sedimentary strata, and most of the source beds were skarnized in regional metamorphic stage, Laojunshan granite belongs to aluminium supersaturation and continental crust remelting type. The granites are rich in ore-forming elements, not only formed many pegmatite deposits nearby, but also the magmatic pneumato-hydrothermal effects are superimposed on preexisting polymetallic sulfide deposits.(5)Three metallogenic series are classified in Laojunshan metallogenic belt. From center to perimeter:Ⅲ. Granitic superimposed metallogenic series of the late Yanshanian epoch,Ⅱ.Volcanic exhalation sedimentary in early Caledonian—regional metamorphism in Indo-chinese epoch—granitic superimposed in late Yanshanian epoch metallogenic series,Ⅰ. Volcanic exhalation sedimentary in early Caledonian—regional metamorphism in Indo-chinese epoch metallogenic series. The deposit types devided, the occurrence of ore bodies, texture and structure of the ore bodies and the wall rocks, and geochemical characteristics of each metallogenic series was studied.(6)The metallogenic models of Laojunshan metallogenic belt were mainly multiple sources and multistage superposition, the formation of ore belt may result from the combination of the volcanic exhalation sedimentary of early Caledonian, the regional metamorphism of Indo-chinese epoch and the granitic superimposed of late Yanshanian epoch.(7)The ore-controlling regularities:①Almost all of the polymetallic sulfide deposits were formed in the Cambrain stratum, lower Cambrian chongzhuang formation, middle Cambrian tianpeng formation and middle Cambrian longha formation were the main ore-hosting stratas in the belt, Lithofacies changes zone from clastic to marble had the favorable condition for the ore formation.②Nearly all of the deposits are restricted between Wenshan-Malipo fault and Maguan-dulong fault, and fault system around the tectonic dome always have good condition for the mineralization of vein ore bodies.③Ore-bearing rocks of the belt always associated with high-grade metamorphism, greenschist is one of the most important ore-bearing facies belts, skarn is one of the most important ore-bearing rock types.④Yenshanian granite was the source of rich metal-bearing fluids that formed hypothermal deposits in the belt, and also important for the polymetallic sulfide deposit.