Intermediate-Acid Intrusive Rocks Of Tongling Ore District And Copper-Polymetallic Deposits Of Shizishan Ore-Field,Anhui Province

The hydrothermal deposits related to magmatism, such as porphyry deposits and skarn deposits, are important deposits which produced Cu, Mo, Au and polymetals. So they have been becoming the hotspot and keystone of the economic geology and very high achievements have been reached. Tongling is one of the important Cu ore-concentration districts in the Middle-Lower Reaches of Yangtze River structural-magmatic-metallogenetic belt, which have long been listed as an important metallogenetic belt for exploration of mineral resources, and attracted many geological workers and researchers, especially economic geolgists in China to focus on. Abundant research reports and papers have been made and many new theritical ideas broght forth from the Tongling district, while some geological controversies over differing opinions for a long time are remained yet.Mesozoic intrusive rocks, predominated in intermediate-acid, are widespread in Tongling ore-concentration distric. The lithological, lithogeochemical and geochemical characteristics of the intrusive rocks and the enclaves inside the rocks were studied by the predecessors. And the generation and evolution, the geotectonic background of the magmas were also discussed. Based on the previous research data and achievements, for the Mesozoic magmatic rocks in Tongling district, the characteristics of the spatial distributions were systemtically summarized, the formation ages collated accurately, the types and the series divided exactly. According to the geochemical characteristics of major elements, trace elements, rare-earth elements, and Pb-Sr-Nd-O isotopes of the intrusive rocks, the geodynamic processes of the magma and the mechanisms of rock-forming were discussed. It is concluded that the intermediate-acid intrusive rocks in Tongling ore-concentration district was formed in late Jurassic to early Cretaceous (132～147Ma), and the duration of magmatic activities was approximately10to15Ma. The intrusive rocks distributed zonally along with E-W direction deep faults and formed hypabyssal intrusions mainly controlled by faults in different directions. The mineral compositions of intrusive rocks changed widely, but dominated by plagioclase. According to Q-A-P diagram, the intrusive rocks can be divided into pyroxene diorite, quartz diorite (or monodiorite) and granodiorite. All of the intrusive rocks show medium contant of SiO2, slightly acidic or basic, high K2O+Na2O and richer in Na2O than K2O, high K and metaluminous, which belong to high-K calc-alkaline series. The three types of the intrusive rocks show similar geochemical characteristics of trace elements, rare-earth elements and Pb-Sr-Nd-0isotopes, and these geochemical characteristics are similar to those of the adakite, suggesting that the magma was derived from enriched lithospheric mantle and the petrogenesis process went pass the mixing action of mantle-derived basaltic magma with adakite-like magma which formed in lower crust by underplating of the mantle-derived magma. Those three types of intrusive rocks interpenetrated with each other in spatial and overlapped with each other in temporal, indicating that the magma was likely derived from different layers of the magma chamber which was zonally distributed in the lower crust and that the magma took place connecting with deep faults induced by tectonic movements, randomly ascending; pulse intruding and then crystallizing. Due to the convection and diffusion which caused by temperature gradient, zonal magma layers in the deep magma chamber may occurred a certain degree of liquation and differentiation. The magma derived from the melting of the thickened lithosphere mantle and lower crust in geodynamics background from compression to extension. The magmatism was formed in the continental plates, closely relevant to subduction of the Paleo-Pacific plate, and also restricted by the fault depression in Hercynian-Indochina period and pre-Mesozoic basement which formed by the collisional orogenesis between north and south China blocks.Tongling ore-concentration district is mainly existed at the middle part of Tongling-Shatanjiao tectonic-magmatic belt which is distributed along the near E-W direction basement fractures. The ore deposits are concentrated in5ore-fields those are Tongguanshan, Shizishan, Xinqiao, Fenghuangshan, and Shatanjiao. The ore bodies are occurred in the stratahorizons from the Fentou formation and Maoshan formation of middle-upper Silurian System to the Ma’anshan formation of middle Triassic. The most important ore-bearing layers are dolostone and limestone of the Huanglong formation and Chuanshan formation of middle-upper Carboniferous System. Mineralization in the vertical section show a zonal distribution phenomenon, i.e., the gold (silver) mineralization found in the upper layer and copper (molybdenum) in the lower layer, and, epithermal vein-like mineralization in the upper part, skarn-type mineralization in the middle part and porphyry mineralization in deep part. The types of deposit genesis are diversity, mainly skarn type, then porphyry and vein type. And there are several mineralization forms of skarn type those are fracture-fissure form, contact-belt form, interlayer form and layer-controlled form. The porphyry ore deposit newly found provide a beneficial inspiration for deeper and outer prospecting of ore deposits. The isotope chronology proves that the mineralization is closely related to the Yanshannian magmatism and its related hydrothermal activity. Whether there was any volcanic eruption or volcanic exhalation (spout) deposition mineralization in Hercynian sedimentary events, and its contributions to the mineralization, needs further exploration and reserch.The Shizishan ore-field was chosed to discuss the space zonal regularities and the genetic mechanism of the copper polymetallic ore deposits in Tongling ore district, based on systemical geological and geochemical studies. The result shows that although the mineralization characteristics of Tongling ore district and Shizishan ore-field is skarn type, the hydrothermal polymetallic sulfide mineralization took placed in late stage is very intensive, so that most minerals formed in early skarn stage in the deposits was strong reformed, even part of the minerals disappeared. The metallogenesis of the deposit in Shizishan ore-field can be divided into (early+late) silicate (skarn) stage, oxide stage,(early+late) sulfide stage and carbonate stage. The copper polymetallic mineralization mainly concentrated in the early and late sulfide stage, and copper mineralization of some deposits may concentrate in silicate stage and gold mineralization in carbonate stage. In the ore-field, the primary inclusions in the ores of the main deposits are mainly3types inclusions:gas-rich inclusions, liquid-rich inclusions and contaning daughter mineral multi-phase inclusions. The types of the fluid inclusions in different ore-forming stage vary slightly, but the gas-rich inclusions are always inter-grown with liquid-rich inclusions. The ore-forming fluids are high salinity, middle temperature, weak acidic to slightly alkaline. In the same ore-forming stage, such as sulfide stage, the ore-forming temperatures of the gold or gold (copper) deposits are often lower than those of copper (gold) deposits, reflecting that gold deposit in slightly lower temperature. According to thermodynamic calculations and analysis, in the evolution process of the ore-forming hydrothermal fluid, the coexisted copper and gold in ore-forming fluids precipited in different space and time because of the differences for the types and solubilities of gold and copper complexes and the respone to the physicochemical condition changes, which lead to the metal elements spatial and temporal separation. At the same time, the multi-stage superposition of the regional structure-magmatism and the related hydrothermal activity, the successive evolution of the ore-forming hydrothermal fluid, and the undulate change of the physicochemical condition of the mineralization, often lead to the coexistence between copper mineralization and gold mineralization. H-O isotope geochemical characteristics reflect that the ore-forming fluids come mainly from magma, and atmospheric precipitation increased in the ore-forming fluids from early to late stage. Lead isotopic compositions show that the ore-forming metal elements mainly come form the magmatism, and the metal lead come from the sedimentary rocks cannot be eliminated but is secondary undoubtedly. The sulfur-sulfide isotopic compositions of Dongguashan deposit are obviously different from those of Sedex deposit, VHMS deposit, and same as those of porphyry deposit by drawing a simple analogy of sulfur isotopic composition. Thermodynamics calculations show that the sulfur in the ore-forming solution came from hydrothermal fluids which were differentiated from high potassium calc-alkaline magmatic melt, and the ores did not save sulfur isotope evidence of Hercynian sedimentary mineralization, although sulfur isotope composition of regional sedimentary rock shows the diagenesis process had been undergone obviously sea deposation and sulfate bacteria reduction. Combined with geological features, it is regarded that the copper polymetallic deposits in Shizishan ore-field are belong to porphy-stratabound skarn-epithmal vein type deposits those are controlled by the unified Yanshanian magmatic hydrothermal system.