The Magmatism And Fluid Mineralizationin Shizishan Copper-gold Ore-field Of Tongling, Anhui Province

Shizishan ore-field of Tongling is a huge representative and typical Cu-Au ore-field in the metallogenic belt of the middle and lower reaches of Yangtze River. The tectonic activity and magmatism as well as mineralization in the ore-field are complicated. The tectonic evolution went through three stages from the active stage (the basement forming) to the stable stage (the cover forming) and to the re-active stage (the within-plateform deformation). Especially after middle Triassic period, the within-plateform deformation led to take place the cosmical middle-acid magmatic intrusion and volcano, sub-volcano movement, and formed so called "multiplayer floor" deposit distributing pattern.Based on the modern analysis technique such as the SHRIMP isotopic precision age determination, the ICP-MS composition analysis of fluid inclusions for minerals and the MS analysis ofδ¹³C_CO2 andδ¹³C_CH4 of fluid inclusion and so on, it is gained for the diagenetic era of magma, the gas and liquid composition of fluid inclusion, the composition of metallogenic elements and trace elements in mineralization fluids, as well as the isotopic compositions of C, H, O and Pb of ore deposits. According to these results and the studies of the setting of the region mineralization, using the modern theory of the diagenetic and metallgenic geology and geochemistry, it is discussed for the magmatism and magmatic evolution, the relation between mineralization and diagenesis, the space-time evolution of mineralizing fluid, the space-time coupling of mineralization physico-chemical field and the location of deposit ore-body. The following main results and innovative views are obtained in this paper:1. Magmatic rock bodies in Shizishan ore-field present as dyke and stock, which are grano-diorite, quartze-monzo-diorite, and pyroxene-monzo-diorite three main petrological types. According to the study of chemistry and geochemistry of intrusive rocks in Shizishan ore-field, they are mainly calc-alkalic series enriching K. The characters of low Cr, Ni and high Rb, Th, Ba, K and enriching Sr indicate that the magma derived from mantle and the magmatic rocks are formed by mantle-crust contamination. Connecting with the study of isotopic geochemistry of Nd and Sr, it is concluded that the magmatic rocks of this region originated from partial melting within upper mantle and lower crust. The magmatism took place in the geological setting from extrusion to spread. The geochemical study of main elements and rare earth elements of magmatic rocks indicates that they have same magmatic source and evolution, and the fractionation crystallization of femic minerals play an important role in diagenetic process. The primary magma originated from magma of upper mantle or lower crust, went up to middle and upper crust and come to magmatic chamber and then took place some extent fractional crystallization. The magma took place some zonal distributing in the composition randomly ascending, emplacing along the tectonic crack and then crystallizing.2. Zircon SHRIMP of precise isotopic age determination shows that the ages of the magmatic emplacement in the ore-field are 133.3Ma～142.9Ma, that is late-Jurassic period to early Cretaceous period. The similarity and difference of isotopic ages indicate that the intrusive rocks are formed by many emplacements in the same magmatic movement. There are two magmatic intrusive activities began from 140Ma and 136Ma respectively. The time for the magmas ascending and emplacing to cooling and crystallizing is generally rather short. But the cooling history of pyroxene monzo-diorite of Baimangshan is longer, the latter went through the fractional crystallization in deep magma chamber, magma ascending and emplacement with tectonic pulsation, heated due to pressure reducing, the re-dissolution of early crystals as well as cooling and crystallizing process.3. Many kinds of mineral deposits distribute in the ore-field, mainly including porphyry deposit in the deep and stratabound skam deposit, interstratified skarn deposit in the middle and breccia pipe or contact skarn deposit in the shallow. The geochemistry study of fluid inclusion shows that there are many kinds of fluid inclusions, but the main types are rich-liquid inclusions and rich-gas inclusions. The homogenization temperature measured reveals that the mineralization temperature is 411℃～600℃in the skarn stage, 173℃～440℃in the quartz-sulfide stage, and 117℃～280℃in the carbonate stage. The changes of mineralizing temperature have characters of continuity and relatively concentration. The salinity of mineralizing fluid has two ends of high salinity and low salinity on the skarn stage and the quartz sulfide stage. Moreover the homogenization temperatures of the inclusions of different types containing high salinity and low salinity are basically identical, which reflects that mineralizing process is related to fluid boiling. In addition, for the mineralizing fluid of every deposits the values of p, pH, Eh, f_O2 reduced with mineralization evolution from early stage to late stage, but the values of p, Eh, f_O2 reduced and the value of pH increased from deep deposit to low deposit in space.4. Gas component of all deposits are mainly H₂O and CO₂, secondly are reduction gases such as CH₄, C₂H₆, N₂, H₂S, Ar. Reduction parameter and H₂S concentration reduces from the deep to the shallow in the same quartz-sulfide stage of different deposit The ratio of CO₂/H₂O denotes that the depressurization space of mineralizing fluid is from -520m to -730m in the middle part And the boiling of fluid caused by depressurization is the dominant mechanism of ore precipitation. In the fluid inclusions, the anions in liquid are mainly Cl^-, SO₄^2- and F, and the cations mainly K⁺, Na⁺, Ca²⁺, Mg²⁺, K⁺/Na⁺, F^-/Cl^- and other relative figures show that the mineralizing solution mainly came from the magmatism. The stratigraphic component and the meteoric water may mix in ore-forming fluids in the later mineralization stages. In space, K⁺/Na⁺, F^-/Cl^- totally submits reducing trend with the depth declining, but the ratios of K⁺/Na⁺, F^-/Cl^- changed obviously from -500m to -730m in the middle part of the section, which reveals that the ore-forming fluids mixed strongly, and this space is according to the mineral enriching position in ore-field.5. The minor element characteristics of fluid inclusions in quartz indicate that the minor element characteristics of the ores are determined by the characteristics of the minor element in mineralizing fluid. The metallogenic element abundance in the fluid instructs that the mineralizing fluids are copper-rich fluids. Comparing with skarn stage and carbonate stage in ore-forming process, Cu, Sr, Ba intensely enriched in quartz-sulphide stage, the main mineralization stage. thus ore-forming processes are more intense. According to the zonal distribution of ore and the characteristics of fluid component of deposits, the zone containing Cu pyrite and pyrrhotite ore belt which have high Cu contents perhaps is the zone of mineralization fluid confluence and unloading mineralization material. Generally, the enrichment coefficients of Cu decrease, but the enrichment coefficient of Pb, Zn increase from the deep to the shallow in space for different ore deposits. Characteristic elements and their ratios show that ore-forming hydrothermal solutions have characteristic of deep sources.6. The comparison studies of REE show that REE geochemical characteristics in fluid inclusions, in magmatic rocks, and in minerals such as quartz and garnet are identical, but different from those in ores, which indicates that ore-forming environment of the deposits may have an important effect for ore deposition. The REE geochemical characteristics of the fluid inclusion in quartz are more representative than those in ores and in minerals. The REEs of fluid inclusion in copper and gold deposits have a right-inclination pattern, weak differentiation between LREE and HREE and less 8Eu abnormality, which shows the mineralization fluids come from deep source. The differentiation between LREE and HREE and SEu abnormality indicate that the mineralization is controlled by the evolution of the mangmatic fluid. The fluids of deposits in the deep and the middle have 8Eu negative abnormality, while those in the shallow have 8Eu positive abnormality. These phenomena perhaps are relative to environment changes and the complex action of F^-, Cl^-, CO₃^2-.7. The C, O isotope composition of calcite indicates that C possibly originated from the deep magma, and water-rock reaction is the main factor of hydrothermal depositing. The values ofδ¹³C_CO2 in fluid inclusion in quartz totally are within the range ofδ¹³C in magmatic system, but perhaps parts of carbon root from wall-rocks, andδ¹³C_CH4 values show the mixture of organic carbon and inorganic carbon. All of these reflect that mineralizing material comes from magmatism, but the stratigraphic component and the meteoric water may add to ore-forming fluids in the later mineralization stages. The values ofδ¹³C_CH4 reduce from the deep deposit to the shallow deposit perhaps reveal that CH4 is the main factor for separating copper and gold in mineralization process.8. Hydrogen and oxygen isotopes of mineralizing fluids show that the ore-forming fluid is mainly magmatic hydrothermal solution, and has the addition of stratum water and the meteoric precipitation in the later mineralization stages. In spatial changes of the composition of hydrogen and oxygen isotopes are regular from the inside to the outside, and from the bottom to the upper, which imply that ore-forming fluids flow from the center of the magmatic body to the outward and the upper. In time, magmatic hydrothermal in the ore-forming fluids reduced gradually and the atmospheric precipitation increased gradually from the early stage to the later stage. These phenomena reveal that magmatism plays the most important role in the mineralization process.9. Re-Os age determination for molybdenite in the two different types of ores in Dongguashan copper deposit shows that the ore-forming ages are 139.1Ma and 139.4Ma. The identity of the diagenetic age and the metallogenic age indicates that the mineralization is closely related to the magmatism. Lead isotope studies show that the lead isotopic compositions of the sulphide in ores are consistent with those of the magmatic rocks, and different from those of the sedimentary rocks, which shows that the mineralization material is also mainly from the deep source.10. Synthesizing above geological and geochemical studies, the magmatism mineralization process can describe as follows: The underplating of the upper mantle-derived basalt magma melted the lower earth crust and formed diorite and monzonite magmas and the metallogenic element enriched firstly. The magma raised, mixed and assimilated the middle or upper earth crust and formed the monzo-diorite magma and the metallogenic element secondly. The magma in chamber took place fractional crystallization of femic minerals and differentiation of melt and solution, and the metallogenic element enriched again. The fluid diffused by the way of infiltration along the favorable structural positions, and the heat of the magmatic bodies cause meteoric precipitation circulated and leached the metallogenic material from the wall-rocks (water-rock reaction). The metallogenic element enriched many times and formed the rich-Cu fluid. Because of the physical and chemical conditions changed, the wall-rocks and structure changed, and different mineralization fluids mixed, the ores deposited.