Geology And Metallogenic Geochemistry Studies On Typical Gold Deposits In The South Anhui Province

Ore-forming geological condition and mineral resource are superior and abundant in Anhui Province, which stretches across three I level geotectonic elements, with 12 kinds of mineral resources ranking top 10 in China. However, gold is not the advantage resources in Anhui, with a total resource of 321 tons (including associated gold), mainly distribute in the south area (over 60 percent in the Tongling region). Gold is mainly coexistence (and/or associated) with copper-iron-sulphur deposit. Recently, more separate gold deposits are discovered as prospecting breakthrough, like Paodaoling deposit with more than 30 tons of gold in Chizhou city, it is necessary to have a systematic understanding about separate gold deposits in the south Anhui area, to provide important reference and guidance role for further prospecting. In this study, geological and geochemical characteristics of three mid-large gold deposits (i.e., Yangchongli gold mine in Toling, Paodaoling gold mine in Chizhou and Tianjingshan gold mine in Huangshan) are studied, metallogenic model is built, providing important significance for the geodynamics in this area.(1) Yangchongli gold mine in TonglingYangchongli medium sized gold mine is an altered rock type gold deposit, which is a new type of gold deposit fist discovered in recent years in Tongling region. The gold ore-body occurred in the strata or fracture zones in diorite, and the major ore is cataclastic rocks with pyritization, which is different from other types of gold mine in the Tongling. Deposit associated magmatic rocks are monzodiorite and deeply concealed syenogranite (differ from magmatic rock association in Tongling), with zircon U-Pb ages of 140.7±1.8Ma and 126.4±1.2Ma, respectively. Monzodiorite has the similar geochemical features as Early Cretaceous adakite in the middle and lower reaches of the Yangtze River, like high HREE, Sr and Ba contents, depleted in Y and Yb, and high Sr/Y and La/Yb ratios with no Eu anomaly. Geochemical features indicate that magma of monzodiorite was possibly due to ascend and emplacement of magmatic evolution derived from lithospheric mantle with metasomatism caused by subducted oceanic slab, during which the AFC process occurred. In contrast, syenogranite in this region has similar diagenetic age and geochemical characteristics as A-type granite in the Tongling and surrounding areas, indicating a basaltic magma source with AFC process derived from enrich lithospheric mantle in a back-arc extension tectonic setting.Monzodiorite has high oxygen fugacity, with Ce4+/Ce3+ of zircons ranging from 176 to 1292 (with an average of 533), indicating the magma of monzodiorite may be metasomatized by fluids with high oxygen fugacity caused by dehydration of subducted oceanic crust, which may be advantageous to mineralization. Crystallization temperature of monzodiorite and syenogranite calculated by Ti-in-zircon thermometer ranges from 625 to 716℃ and 652 to 849 ℃, with an average of 662 ℃ and 751℃, repectively.834S values of pyrite in ores have a range of 4.08-5.33‰, limited composition indicate a single source.206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb values of pyrites are 18.188-18.306,15.508-15.584 and 38.161-38.398, respectively, limited Pb isotopic composition also indicate a single source. Moreover, all samples are dropped in MORB area in 207Pb/204Pb-206Pb/204Pb and 208Pb/204Pb-206Pb/204Pb diagram, which are obviously different from adakite formed by partial melting of thickening or delamination lower crust. This indicates that the ore-forming fluids of Yangchongli gold deposit may derived from mantle source with metasomatism caused by subducted ocean crust, with continent crust contamination.Subduction at-140Ma and back-arc extension at-125Ma were both important in the ore-forming process of Yangchongli gold deposit, and it was the response of the structure transformation and large-scale diagenetic and mineralization events in Yanshanian in the East China.(2) Paodaoling gold mine in ChizhouPaodaoling gold deposit in Chizhou is an Au only porphyry-type deposit, thick As-Sb-Au-Ag-Zn-Pb-Bi-Mo anomaly was discovered in the drill hole at depth of 550-700m, the abnormal superimposition of front halo, ore halo and rear halo, as well as argillization, indicating the ore-body may extend to the depth. Combine with other geological features, it is proposed that this gold mine has the potential to be super large. This deposit and Shimenan vein type Au-polymetallic sulfide deposit, as well as Tongshanpai skarn copper deposit, formed a large porphyry-type metallogenic system.Magma activities were strong in the Paodaoling, which were close related to Au-polymetallic deposit. Geochronological and geochemical studies of Paodaoling porphyry, Wushi porphyry, Huayuangong rock-mass and quartz syenite rock veins in west Wushi were carried out, results showed that there were two different ages of rocks,140-141 Ma for ore-bearing porphyry and 120-125Ma for ore-barren rock-mass. Their geochemical features were different too:ore-bearing porphyry was rich in LILE and depleted in HFSE, with weak Eu negative anomaly, similar to island arc magmatic rocks, in contrast, ore-barren rocks were richer in alkali and HFSE, with strong Eu negative anomaly, similar to A-type granite. Ce4+/Ce3+(528 in average) and Eu/Eu* (0.58 in average) values of zircons in porphyry were higher than alkaline rock, and temperature calculate by Ti-in-zircon thermometer showed that porphyry formed at 652 ℃, lower than alkaline rocks of722℃, indicating the magma of porphyry had high oxygen fugacity and water content.δ34S values of ore-bearing dacite porphyrite ranged from 6.49 to 7.17, mainly came from magma sulphur contaminated with few formation sulphur.It is proposed that Paodaoling deposit constituted to the Mesozoic diagenetic and metallogenic system of the middle and lower Yangtze mineral belt, and ore-bearing porphyry in Paodaoling region ascended and emplaced in a subduction tectonic setting at-140Ma, and alkaline rocks were form in the followed subduction exhumation process at-120Ma.(3) Tianjingshan gold mine in HuangshanTianjingshan gold mine is located in the southeast margin of the Jiangnan orogenic belt, east part of the Qinhang metallogenic belt, and is the most representative gold deposit in Huangshan region, south Anhui. Important gold deposit in the east part of the Qinhang metallogenic belt including:Jinshan gold deposit in Jiangxi, Tianjingshan gold deposit in Anhui and Hengshan gold deposit in Zhejiang, compared to the other two deposit, little studies have been carried on Tianjingshan gold mine. In this study, researches on mineral deposits, geochronology and geochemistry were carried out, as well as comparative study to other gold deposit in the east part of the Qinhang metallogenic belt, in order to provide constrains to metallogenic background.Zircon U-Pb ages of Lingshan and Hanjia rock-mass in Tianjingshan deposit were 794.7±5.2Ma and 765.9±3.7Ma, respectively, formed in Jinning period. Ar-Ar age of chlorite in gold-bearing mylonite was 331.5±3.2Ma, indicating mineralization process occurred in Hercynian.δ34S values of pyrites in gold-bearing quartz vein range from 5.28 to 9.21‰, together with H and O isotopic features of fluid inclusions, it is considered that the ore-forming fluids had complicated sources, mainly were metamorphic fluids, mixed with magmatic water and meteoric water. Tianjingshan gold deposit was an orogenic type gold deposit, formed in the long-term collosion-assemblage process between Yangtze and Cathaysian paleo-plate, gold was first enriched in Jinning period and mineralized in Hercynian.Typical gold deposits in the east part of the Qinhang metallogenic belt had similar mineralized process as Tianjingshan deposit. Ore-forming materials were first provided by Neoproterozoic volcanism, and then regional metamorphic fluid mixing with magmatic water and meteoric water formed CO2 rich and low salinity ore-forming fluids, then mineralization occurred in secondary thrust brittle-ductile shear zone during long-term collosion-assemblage process between Yangtze and Cathaysian paleo-plate. Altered rock type gold mine formed in Jinning period, while quartz vein gold mine in Hercynian, both were orogenic type gold deposit. The orogenic type gold deposits found in the east part of the Qinhang metallogenic belt provide important clues to the deep and regional prospecting, as well as constraints to the evolution of Jiangnan orogenic belt.In summary, the independent gold-deposits in the northern margin of the south Anhui province (Tongling-Chizhou belt) are the products of two stage magmatism at ca.140 Ma and ca.125 Ma, respectively. The first stage igneous rocks (arc rocks or adakitic rocks) which was formed in the subduction environment provided amount of ore-forming metals and experienced initial enrichment. Then the emplacement of the second stage magmatic rocks provided sufficient heats and some metals, promoting the thermal cycling of meteoric water, and caused not only the further enrichment of the ore-bearing fluids, but also the incorporation of metals from the strata. Finally, the deposition of these highly metal-enriched fluids formed tectonic-altered rock type Au deposits (quartz-bearing veined type Au deposits) in the NE trending fault systems, or high grade Au deposits on superposition of early independent porphyry gold deposit.The Au deposits in the northern margin of the south Anhui province were dominated by the tectonic transition of eastern China in the Yanshanian, closely related to the subduction and roll-back of the paleo-Pacific Plate. In contrast, the Au mineralization in the southern part of the south Anhui province is not obviously related to the Yanshanian magmatism, probably due to the thick sedimentary covers in the area. The geological characteristics of the Tianjingshan Au deposit and other Au deposits in the adjacent area show that the mineralization are mainly occurred in the Jingningian and Hercynian. Quartz veined type Au deposits were mainly formed in the Hercynian, and the Yanshannian mineralization in the area need more studies. The independent Au deposits in the Jiangxi-Zhejiang-Anhui area are the long-term orogeny products of the Yangtze and Cathaysia plates, showing typical characteristics of orogenic Au deposits.