Study On Geological Characteristics And Genesis Of Songjianghe Gold Deposit,dunhua City,Jilin Province

The Songjianghe gold deposit of Dunhua city, Jilin Province, is located in the south of Tianshan-Xingmeng orogenic belt, the northeastern edge of the North China plate and the central region of Jiapigou-Haigou gold and polymetallic metallogenic belt. It is the favorable metallogenic area with the superior metallogenic geological conditions for exploring gold and nonferrous metals in Jinlin Province.Exposed strata in the ore district are mainly the Middle Proterozoic Seluohe group. The main rock types are greenschist-amphibole facies with high-content of gold, its protoliths are basalt, andesite, rhyolite and argillaceous-arenaceous terrigenous elasticsedimentary rocks containing shingle. In this region, the fault structure develops well. The territorial NE trending Qingchaguan-Baishuitan fault zone across the area obliquely is the primary ore- and rock-controlling structure, but the NS-trending secondary fractures control the orebodies. The ore district is characterized by the widely distribution of magmatic rocks as frequent magmatic activites, including the magmatic rocks of clinopyroxene hornblendite, early Indosinian biotite plagioclase granite and early Yanshan feldspar granite.Ten economic ore bodies numbered as Ⅱ-1 ~Ⅱ-10 have been discovered in Songjianghe gold district, Ⅱ-1 is the major orebody among them and the other are secondary. Ore types in Songjianghe gold deposit are mainly altered with quartz vein types in local. The ore minerals consist mainly of pyrite, chalcopyrite, ilmenite, pyrrhotite and natural gold. The ores are mainly euhedral-hypautomorphic texture, xenomorphic granular texture, including texture and cataclastic texture, with disseminated-scattered disseminated structure and flaky-banded structure. The wall rock alterations are developed and composed of silicification, potassic, epidotization, chloritization and star-like pyritization etc. The metallogenic episode is disparted in four stages: Ⅰquartz-oxide stage, Ⅱpyrite-sericite-quartz stage,Ⅲ quartz-carbonate-multimetalsulfide stage, Ⅳcarbonate stage the second and third stages are very important to the mineralization of gold.The regional metallogeny of Songjianghe gold deposit is related to large scale sinistral strike slip movement of Dunhua-Mishan fracture zone in early Yanshanian. The SN trending ore-controlling fault in major metallogenic stage is characterized by sinistral compresso-shear structure, indicating partial oppening positions in the shallowly inclined part and the left of fault strike have a better ore potentiality.Three types of metallogenic fluid inclusions have been found in Songjianghe depsosit, they are gas-liquid two-phase inclusions, gas liquid and CO2 three-phase inclusions and liquid inclusions, showing the characteritics of low salinity(5 ~ 9wt%NaCl.eqv), low density(0.66 ~ 0.72g/cm3) and middle temperature(173.7 ~ 345.2 ℃). Estimated mineralizing pressure are 84~106Mpa with mineralizing depth 7.50~8.52 km.The mineralization of Songjianghe gold deposit is closely related to the Wudaoliuhe granite that occurred stock-shaped body in early Jurassic and originated from the lower crust, and it formed in the transformational tectonic setting of compression to extension during the subducting of Paleo-Pacific plate. The orebodies controlled by the compresso-shear fault structure, simple metallic mineral assemblage of the gold ore, the ore-forming fluid of middle temperature, low salinity, low density and typical rich-CO2 inclusions, all these characteristics are similar with typical orogenic gold deposit. The deposit belongs to middle depth orogenic gold one according to the classification of metallogenic geodynamics background.From the aspect of fluid evolution, it was the partial high temperature oxidizing milieu in the first mineralization stage. But in the second mineralization stage, the fluid changed to reduction, occurring major sulfides such as pyrite and pyrrhotite with the precipitation of gold. In the third mineralization stage, due to decreasing temperature, overflow of gas phase and boiling action, major polymetallic sulfides precipitated and Au(HS)2 resolved, which resulted in the gold precipitated in the quartz, carbonate, fracture and grain gap of sulfides. In the fourth mineralization stage, it turned into neutral and slight-alkaline as an addition of atmospheric water, developing carbonate. Combining with the characteristics of deposit geology and fluid evolution, the geneic type is middle temperature hydrothermal vein gold deposit.