Geological, Geochemical Characteristics And Genesis Of The Yangla Copper Deposit, Deqin County, Yunnan

Yangla copper deposit is in the center of Jinshajiang tectonic belt, as an essential part of Paleo-Tethys Orogenic Belt, Jinshajiang tectonic belt has undergone the complicated evolution of arc-basin systems and has occurred a various metallogenic types. The metallization is paralleled with the orogenic evolution, several deposits has been discovered include one large-sized and two medium-sized copper deposits and two medium-sized lead-zinc deposits. As the only large-sized copper deposit so far, Yangla deposit has attracted a great many researchers for its important commercial value and particular tectonic setting, although a lot of achievements has been accumulated, there are different viewpoints on some crucial issues such as tectonic setting, genetic type and ore-searching direction. The paper makes the comprehensive study of stratum, magmatic rocks and geological characteristics of Yangla copper deposit and then goes deep into geochemistry and ore-forming chronology of representative Linong Block, on this basis, metallogenic condition, tectonic setting and genesis and metallogenic evolution are discussed, since the deposit is occur in Jinshajiang suture zone, each part of the discussion is tried to around the evolution of arc-basin systems and polyphase mineralization. The main research findings and new viewpoints achieved in this paper are as follows:1. In terms of tectonic slice, a new lithostratigraphic subdivision for Yangla Deposit was built, this study described 23 lithologic layers and make a comprehensive analysis on conodonts, chronology and petrographic, the stratum, which used to be define as the Permian Gajinxueshan Group is departed into several massifs from Devonian to Permian, the main age scale of the thick volcanic-sedimentary rock series in ore district is inferred from Middle Devonian to Carboniferous, its formation is related to the cracking-spreading of Jinshajiang ocean.2. Basalt of Beiwu Formation in Lower Carboniferous(C1b) is widespread and approach to those occurring in mid oceanic ridge, It has formed in spreading centre and similar with those from ophiolite units of Baimangxueshan snow mountain and Gongka, trace element indicate that the basalt originated from an enriched source very likely. 3. Indosinian granitic rocks in ore district are characterized by high K, calc-alkali, metaluminous and similar with Cordillera I type granite occurred in island arc, REE and trace element also show that the magma have some characteristics of that originated from subduction zone, the magma source is dominated with crustal substance mixed with mantle ingredient.zircon U-Pb dating of granodiorite obtained some data as follows:238-239Ma of Linong and Lunong pluton,228Ma of Jiangbian pluton and 214Ma of Beiwu pluton, the magma intrusion occurred in collision orogenic background, it restricts the timescale of mineralization which related to the magma activity.4. Lithology, mineralogy and REE geochemistry are applied to representative bedded orebodies and wall rocks, the result shows the ore have syndepositional characteristics, ore bearing skarn is Fe-rich and iron content is far more than general contact metasomatism skarn, it is concluded that skarnization effected on various rock types of Linong formation and Jiangbian formation so the iron could be supplied from pyrite and greigite of initial ore layers. Positive correlation ofδEu and Cu grade may indicate varying degrees of superposition and enrichment result from skarnization. REE reveals that clastic rocks, silicolite and metamorphic clastic rocks in ore bearing rock series are homologous. Silicolite in Yangla deposit is probably sediment in a transitional condition of continent margin to ocean basin, the study shows the ore bearing volcanic-sedimentary formations are more likely formed in Bathyal environment at initial rift-ocean basin stage.5. Mineral assemblage of ore bearing skarn in bedded orebodies is close to mineralization assemblage of Fe-Cu type, pyroxene and garnet which related to orebodies are iron rich, besides, there is obvious relationship between skarn and mineralization, as product of hydrothermal activity, abundant of metal sulfides formed after pyroxene and garnet, the skarn is related to Indosinian magmatism.6. Fluid inclusions of bedded copper deposit are usually dense and multidirectional, the ratio of gas and liquid in these Fluid inclusions is about 10%, the homogenization temperature is between 165℃and 302℃and averages 193℃, the main mineralization was occurred in meso-epithermal condition. sulfur isotopic compositions of skarn ores indicates that metallogenic substance deposited in a unbalance fractionation, the most high frequency of 834S value is between-0.75‰～0‰and it is similar with mantle sulfur or magmatic sulfur. Lead isotopic compositions of bedded deposit shows the lead is originated from crust.7. It is considered that the initial syndepositional ore layers formed in late Devonian to Carboniferous through comprehensive study. Furthermore, Molybdenite Re-Os dating is applied to determine the age of superimposed mineralization,228.3±3.8Ma represent the age of sulfide formation, Molybdenite and Linong pluton and Jiaren pluton were formed in same tectonic-magmatic-thermal event. On this basis, the complicated genesis of Yangla copper deposit can be concluded as follows:Hercynian syndepositional mineralization related to seabed volcanism, Indosinian magmatic hydrothermal fluid reformed and upgraded primary sulphide orebodies more or less, post magma fluid brings about metasomatism-filling mineralization, accordingly Yangla beddeded deposit is compound genesis.