Formation, Evolution And Metallogenesis Of Granite-Greenstone Belt In Qingyuan, Liaoning Province

As the typical Archean granite-greenstone belt in China, Qingyuan is not only an important copper-producing area, but also has many iron and gold ore deposits in it.In this paper, on the basis of the regional metallogenic theory, geodynamic evolution of Qingyuan granite-greenstone belt, metallogenic during the Archean period and the Mesozoic period, and analysis of typical deposits and experimental test, the author summarized the characteristics of deposits in Qingyuan area, and mineralization conditions, times and space. Further more, regional metallogenic model is established, which is helpful to make further exploration wok.According to the study of geologic setting and geodynamic evolution, it is suggested that the Qingyuan granite-greenstone belt had gone through two stages, which are:(1) the formation, metamorphism and deformation of the Precambrian crystalline basement; and (2) mobilization of the block during Phanerozoic Eon.According to the zircon U-Pb dating and geochemical analysis of supracrustal rocks and granitic rocks in the Qingyuan area, the author discussed the tectonic setting, and further more, established geodynamic evolution during the Precambrian period.-2550Ma, Qingyuan granite-greenstone belt was mainly formed during this period. TTG granite rocks were formed in the island arc environment; while the supracrustal rocks were formed in the back-arc basin environment, and were the production of the bimodal volcanic eruptions on the sea floor. During the volcanic intermittent periods, the VMS copper-zinc deposit and BIF ore deposits were formed.-2520Ma, regional metamorphism-tectonic events occurred because of the collision in Qingyuan area, together with the occurrence of upper amphibolite facies to granulite facies metamorphism and ductile deformation on supracrustal rocks and granite. Charnockite and diorite-granodiorite batholith invaded in the early formed TTG granite and supracrustal rocks. During the Uplift stage after the collision, potassic granitic magma emplaced in about2500Ma. During the period when regional metamorphism and deformation occurred, the metamorphic hydrothermal lode gold deposit was formed together with the remobilization and locally enrichment of the VMS and BIF deposits.-1850Ma, by the influence of the closure of "Liao-Ji Ocean", greenschist facies metamorphism and large-scale brittle fracture formation occurred on the supracrustal rocks and granitic rocks. To Meso-Proterozoic period, Qingyuan region has experienced intense stretching and mafic magma emplacement events (eg. diabase dikes implacement in1256Ma), which is related to Columbia (Nuna) supercontinent cleavage.Based on the results of zircon U-Pb dating, geochemical analysis and Lu-Hf isotope of the Mesozoic intrusions in Qingyuan area, the author discussed the nature of the magma source and tectonic setting, established geodynamic evolution of the Mesozoic era in Qingyuan area.By the influence of the closure of the paleo-asian ocean, during the stretch stage after the collision (231-217Ma), Acid rocks, together with basic-ultrabasic rocks invading in Qingyuan area, developed Cu, Ni mineralization. During the period of early Cretaceous, by the influence of subduction of the ancient Pacific plate, Qingyuan area was in the compression to extension conversion tectonic setting, and thus formed the mesothermal lode gold deposits. In the end of the early Cretaceous, K-feldspar granite invaded, forming a series of related magmatic hydrothermal Cu, Au mineralization.In this paper, Hongtoushan group and Shipengzi group volcanic rocks were classified according to Hart et al.(2004). Shipengzi group was classified as FI type, which are mainly alkaline dacite and rhyolite-dacite, and have no deposits in them. Hongtoushan group was classified as FII type, which are mainly calc-alkaline rhyolite-dacite and rhyolite, and may have some VMS deposits in them. FI type to FII type shows that the depth of the magma source turns shallower. And it can explain why to the south of Hunhe fault, no VMS deposits occurred.In this paper, according to the field occurrence, ore morphology, phases of ore remobilization and the distribution of alteration zones, the stages of ductile deformation were reclassified. Instead of three stages which proposed by other researchers, two stages were raised in this paper. The metamorphism and deformation process can mainly summarized as superimposed folds+inner layer shear rheology (dynamic), leading to the changes in morphology and spatial distribution of the ore bodies, and the formation of two types of ores, which are’ore pillar’along the vertical hinge of the synformal fold and "rope knot-like " orebodies in the wings of the fold.In VMS deposits, polymetamorphism has resulted in changes in the composition and textures of minerals in the deposit, along with remobilization. During metamorphism, the original alteration minerals that formed with the ore minerals, such as chlorite and sericite, were transformed into cordierite, anthophyllite, and phlogopite. After further remobilization, new minerals, such as gahnite and actinolite, were formed. In this process, the original textures were destroyed and new textures were formed, including recrystallization and growth textures, brittle and ductile deformation textures, durchbewegung textures, replacement textures, chalcopyrite disease, and retrograde textures.In BIF deposits, by comparison of two iron deposits which have different metamorphic grade (upper amphibolite facies and granulite facies), the author concluded that:as metamorphic grade increases,(1) quartz-magnetite-amphibole is replaced by quartz-magnetite-grunerite-hypersthene-chalcopyrite-pyrrhotite;(2) the size of mineral particles getting bigger, which has size ranging of0.1-0.4mm in the Xiadianzi and0.25～1mm in the Xiaolaihe iron deposits;(3) a lot of CO2-rich fluid inclusions appear in the Xiaolaihe iron deposit while little can be seen in the Xiadianzi, Hongtoushan and Shujigou, which can further confirm the origin of hypersthene in Xiaolaihe (mantle-derived CO2fluid leads to magma dehydration).Fluid inclusions in Mesozoic mesothermal lode gold deposits indicate that the hydrothermal was middle-low temperature, low salinity and low density, CO2-rich NaCl-H2O-CO2system, suggesting that mineralization depth was at about5.87-8.52km. S34S values and Pb isotopes imply that the ore-forming material mainly comes from the mantle or deep crust. While the hydrogen isotope exhibit an character of mantle water, with a little precipitation mixed in. And it is concluded that ore-forming material comes from the upper mantle.