Ore-forming Fluids And Ore Geneses Of The Typical Magmatic-hydrothermal Deposits In Southern Anhui Province

As an important type of magmatic-hydrothermal deposit, porphyry-type deposit supplies a large proportion of multi-metal mineral resources in the world. Numerous magmatic-hydrothermal deposits have been formed in Tongling Ore Distinct, the most famous area for mineral resources in Anhui Province and even in the Middle and Lower Yangtze River Metallogenic Belt. However, some issues including whether or not the Paleozoic exhalative sedimentary mineralization exists are still debated. In addition,although the southern Anhui Province is located in the Qin-Hang Metallogenic Belt,the mineral resources are scantier than the other areas. The corresponding studies are also deficient. Therefore, this study conduct detailed studies on the most typical Xinqiao deposit in the Tongling Ore Distinct and then on the newly-discovered Dongyuan W deposit and Zhuxiling W deposit in southern Anhui Province. The ore geneses are strongly constrained and the ore-forming regularities are summarized,which could provide the theoretical basis for further geological exploration.In Xinqiao Re-Os isotopic analyses indicat that the pyrites were formed at two distinct ages which are 138 ± 26Ma and 393 ± 40Ma. The former have high initial 187Os/188Os, indicating significant contribution from crust. Whereas the later show more contribution from mantle. Furthermore, the Paleozoic pyrites have higher Li, Cu, Ga,Rb, Sm, Pb, U and lower Sc, Co, Ni, Zr, Ba content than the Mesozoic ones. The different ages and different composition of the pyrites demonstrate that there are two mineralization events in the Xinqiao deposit. The mineralization includes magmatic-hydrothermal type in the Yanshanian and exhalative sedimentary mineralization in the Paleozoic. In addition, the Paleozoic pyrites have high content of ore-forming metals like Cu and Pb, indicating that the Paleozoic mineralization brought significant ore-forming materials and should doubtless be an important mineralization event. The ore-bearing magmatic rock (the Jitou Pluton) is similar with island arc magmatic rocks in composition and does not belong to adakite rock, which demonstrates that the Jitou Pluton should originate from partial melting of the mantle wedge. AFC (Assimilation and Fractional Crystallization) process also plays an important role in the formation of the Jitou rock mass.The studies on fluid inclusion indicate that the ore-forming fluid in Xinqiao mainly comes from magmatic fluid. It belongs to the NaCl-H2O system and contains no CO2.The salinity of fluid inclusions varies from 4.2 to 50.7 wt.% and the homogenization temperature from 140 to 432?. Fluid-boiling and fluid-mixing between magmatic fluid and meteoric water are the key processes controlling the precipitation of ore-forming materials, which happened at around 250?. Furthermore, the lack of CO2 in the fuids is consistant with the source characteristics of the Jitou Pluton, which both indicate that the mineralization is closely related with the subduction of the Pacific Plate.The sapmles from the Dongyuan deposit are all mineralized and contain high W content. Whereas in Zhuxiling, only three sample are mineralized. All the mineralized rocks in both Dongyuan and Zhuxiling show similar characteristics with respect to petrology and mineral CL. They are strongly altered and contain numerous secondary minerals and secondary textures which result from potassic alteration, sericitization,chloritization, silicification and calcilization. The non-mineralized rocks in Zhuxiling are weakly altered and contain fresh plagioclase and quartz crystals. The ore-bearing magmatic rocks are granodiorite porphyry in composition. Mineralized rocks contain higer K, Pb and lower Na, Sr, Ti content than the non-mineralized ones. Furthermore,the content of fluid-mobile elements are either positively or negatively correlative to that of W, which indicate that the modification by fluid alteration play a key role in mineralization.The two deposits share the similar fluid inlusion characteristics with each other.Inside quartz phenocryst and quartz vein, five types of inclusions are recognized, i.e.,i.e., aqueous-carbonic (WC-type), aqueous with minor CO2 (Wm-type), aqueous without CO2 (Wn-type), carbonic (pure CO2, C-type) and late secondary aqueous inclusions (LW-type). Among them, WC-type inclusion is the earliest and represent the early magmatic fluid. LW-type inclusion represents the latest stage meteoric water. The others represent the evolved magmatic fluids. With the fluid evolving from WC- to Wm- and then to Wn-type inclusions, the homogenization temperature dereases from 250?391? to 220?320 ? and finally to 175?300 ?, the CO2 content decreases to 0 and the salinity increases from <5 wt.% to 3-7 wt. % and finally to 4-10.7 wt.%. By contrast, LW-type inclusion has the lowest temperature (< 250 0C) and salinity (< 4 wt.%). Fluid-immiscibility and CO2 effervescence are the main factors which give rise to the above changes. Through these processes, CO2 and steam escape from the fluid,decreasing the temperature and pH and increasing the salinity of the residual fluids.Moreover, extensive mineral alteration is closely associated with fluid evolution and scheelite mineralization. On one hand, the decomposition of plagioclase and the formation of K-feldspar and sericite change both the quality and quantity of the solute in fluids. On the other hand, the relase of Ca provides important ore-forming material for scheelite mineralization. Furthermore, the injection of numerous Ca into the fuids triggers and promotes scheelite precipitation. This process is also promoted by the pH increase which results from the loss of CO2 in the evolving fluids. The ore-forming fluids of the two deposits have the same characteristics (CO2-rich and low salinity) as those of deposits from the Nanling region, which indicates both deposits formed in the same intracontinental extension setting as those in the Nanling region. Thus, the Yanshanian granites from southern Anhui Province may have a great potential for W mineralization, especially the blind ones in deep levels.