The Geological Characteristics And Discussions On The Genesis Of Porphyry Copper Deposit In Changdagou

Changdagou porphyry copper deposit locates at the northern section of Yidun Island-Arc, which is the Dege-Yi Dun-Shangri-la porphyry copper metallogenic belts northern part too. The strata are late Triassic clasolite, thus forming a NWW-NNW arc mutual type syncline. In the scope of the mining area and the 140 km2 wide periphery land, there are total 126 little acid Intrusive rocksand its ultra shallow branch which mainly composed by granite, quartz diorite, dacites emerge, rhyoli ticeuch. In the small scope the deposits are distributed in the form of group, aganist the form of bonded in the big scope. The deposits has a certain horizon(equal to the gachun volcanic cycle) Chanda ditch‘s copper deposit ore is mainly composed of two copper granite diorite(spot) rock mass. Ore’s shape are mainly Veinlet disseminated shape, few Infection and veinlike, rare lump. Metal mineral are pyrite,pyrrhotite, copper pyrite. The average grade of copper ore change between 0.31 to 1.58%, the majority is 0.5%, low grade lean ore; Gold grade < 0.1 x 10-6 a small amount is 0.1 to 0.4(x10-6), individual has the grade26x10-6, deposit is small scale. Wall rock alteration appears in the rock mass itself and the contact zone, mainly are potassium, silicification, sericitization and chloritization, zoning is not obvious. The ore deposit belongs to porphyry copper deposits’ inferior class.Its ore-forming phase is hydrothermal stage.In this paper, the main researching contents include: regional geological background, regional magmatic rock characteristics, ore deposit geological feature, chronology, ore deposit geochemical characteristics and origin, etc. Through the above research, the following results were got:(1)Changdagou’s granodiorite-porphyry belongs to the high potassium calc-alkaline series; Rock’s aluminum content is very high, belongs to the typical type aluminum rock; Ore-bearing porphyry’s potassium content is high too,belongs to the typical potassic rock series.Some to the large porphyry mass which belongs to the south part of Yidun Island-Arc.(2)Changdadou’s granodiorite-porphyry’s geochemical characteristics suggest that the source material of the ore-bearing porphyry source area is shallow. The porphyry may mainly formed by remelting crustal material mix with a little Oceanic crust material. The diagenetic temperature is low. Its a shallow-sourced low temperature ore deposit.(3)The constant elements’ characteristics show that the deposit may be formed in the arc environment(island arc or continent arc) or the collisioning environment, but most closely to the continental arc environment. Same to the orogeny. Trace element’s characteristics show that ore-bearing porphyry may be formed in the high maturity of volcanic arc or continental arc environment. Comprehensive analysis thought the Chanda ditch’s copper ore mining area granite diorite porphyry most likely formed in the arc-tectonic environment,close to the continent arc environment. From the tectonic direction Chanda ditch’s porphyry copper ore are arc tectonic environment same to the south part of Hongzuo, Xujiping’s porphyry copper deposits. such as red zhuo deposits are basically identical. In the macro Yi Dun island-arc’s northern zone also have a high potential on looking for porphyry copper.(4)The Determination of zircon dating the forming period for the deposit shows it was 216 Ma old belong to the moment called Indosinian instead of Yanshanian which was thought been. Its age was the same to the south part of Pulang, Xujiping’s porphyry copper deposits.According to the living part of Yi-dun Island-Arc were 250-208 Ma,the peak period of magmatic activity should be the 215 masome to the Changda ditch’s. The south and the north part rock mass from the Yidun Island-Arc were forming in the same age which prove that some well mineralization of the porphyry copper deposits had been exist in the north part’s rock mass. The north and the south’s porphyry body has the similar property which shows that the north part might had some big prospecting potential of Porphyry copper.(5)For a long time, predecessors thought the porphyry body of Yi Dun arc section in Changdagou is yanshan period, which exists much difference among the magma activity and structural environment of southern section of Yi Dui arc, and don’t have large porphyry southern formation conditions of the geological background. The study found that the porphyry body of the north of Changdagou region is indo-china period, geochemical characteristics of rock mass and the age of formation is similar to the south of ore-bearing porphyries, at the same time, it is similar to the mineralization geological background of the south of porphyry body. Therefore, the north of Yidun island arc also exists potentials for prospecting large-scale porphyry copper. However, larger breakthrough is not obtained, which is related to the lower level of work.