The Mineralization Mechanism And Geochemistry Of The Tonglushan Skarn Cu-Fe Deposit, Southeastern Hubei, China

Tonglushan deposit is a most typical, large-scale skarn Cu-Fe in the southeast of Hubei province. Base on the pervious researchers and detail field investigation in Tonglushan deposit, focusing on the analysis of the ore-forming geological background, the deposit.characteristics, ore types and minerals sequences, some work including the electron microprobe analysis, fluid inclusions, stable isotopes, and diagenesis and mineralization chronology were carried out. We studied the source and evolution of ore fluids of this representative Cu-Fe deposit, analyzed the geochemical characteristics of the intrusion related, and finally discussed the metallogenic mechanism. Main results obtained are as follows:1. The deposit was temporally and spacialy related to the Tonglushan quartz diorite. A geochemical and Sr-Nd-Pb isotopic studies were carried out for the quartz diorite. Its compositions of plagioclases are oligoclase (An=21-31). Amphiboles belong to magnesihomblend, have characteristics of low Ti (<0.2)and high Mg/(Mg+Fe)(>0.5). MF values of biotites vary from 0.58 to 0.66, suggesting biotites are Mg-riched. The rocks are characterized by SiO2 and alkaline compositions between 58.86% and 67.71%, and from 5.67%to 9.63%, respectively, enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE), but relative depletion in Nb,Ta,Ti elements.They have initial Nd(eNd(f)=-3.27--7.57), Sr((87Sr/86Sr)i=0.7055~0.7069) and Pb((206Pb/204Pb)i=17.66-18.00, (207Pb/204Pb)i=15.49~15.56, (2O8Pb/2O4Pb)i=37.73~38.19). The analysis demonstrates that the magma was from the enrich mantle of about 40km deep. Amphiboles and biotite thermometers estimate magma crystallized at 650 ℃~800 ℃ and 500 ℃~630 ℃ temperatures, under 1.49kbar pressure, corresponds to the emplacement depth of about 4.9km.2. Calcium and magnesium complex skarn mineral assemblage occurred dependent on the compositions of the wall-rock, including garnet, pyroxene, hornblende, epidote, phlogopite et al. We describe the characteristics and occurrences of minerals from every mineralization stage, then analyze their compositions using electron microprobe. The results show garnet is dominated by grossular and andradite, their end member varies from grossular to andradite. In single garnet and epidote, Fe tends to increased from core to margin. There is also a garnet with grossular core and andradite margin. These suggest the concentration of Fe in fluid increased during the process of minerals formation. Pyroxene belongs to diopside, with minor hedenbergite. Amphiboles consist of tremolite, pargasite and actinolite. Minerals of Tonglushan deposit are basically consistent with the typical skarn Cu-Fe deposits in the word. The compositions of garnet and pyroxene may be related to mineral metals. Magnesium or calcium with some Mg composition skarn are more conducive to mineralization relative to pure calcium skarn.3. Ore-forming fluid of Tonglushan deposit belonged to the NaCl-H2O system. It contains two different groups fluid:the high saline fluid and low saline fluid. The early skarn minerals were formed at high temperatures (>550℃) and in saline (> 66.57 wt% NaCl equiv.) fluid, and probably from metal-bearing magmatic fluid under lithostatic pressure of about 20.1-37.9 MPa. Continued depressurization and cooling (405℃~567℃) might result in both decrease of high salinity (48.43-55.36 wt% NaCl equiv.) and deposition of magnetite, but low saline fluid had the higher salinity (7.17-11.81 wt% NaCl equiv.) than that in skarn stage(6.74-10.36 wt% NaCl equiv.). In the quartz-sulfide stage, boiling took place, which led to abundant sulfide ores precipitated with an extensive range of salinity (4.96~50.75 wt% NaCl equiv.) and temperature focused on (240℃~350℃), under a pressure of 11.6~22.2 MPa. Calcite was formed in the late carbonate stage, accompanied by non mineralization, and has relatively low temperature (174℃~284℃) and salinity (1.57-4.03 wt% NaCl equiv.), which might result from mixing of early magmatic fluids with meteoric water.4. The hydrogen and oxygen isotopic analysis of every mineralization for Tonglushan deposit were performed. The skarn stage have ore-forming fluid with δ18OH2O in 6.68‰-9.67‰ range and 8D between-67‰ and-92‰. The δD values of ore-forming fluids from-67‰ to-92‰ and the δ18OH2O values vary from 2.26‰ to 3.74‰ in magnetite stage. Quartz from the quartz-sulfide stage has 4.98%o of δ18OH2O and-66‰ of δD.The results suggested that the ore-forming fluids could be produced by principle magmatic water. Some meteoric water was minxed into since the magnetite stage.5. Calcites in quartz-sulfide stage have different carbon and oxygen isotopic compositions with that in carbonate stage,δ13CV-PDB values are between-2.9‰ and-6.3‰, and δ18OV-smow between 9.6 to 12.6‰. Two values increased obviously in carbonated stage, with δ13Cv-PDB varied from-0.9 to 1.3‰, and δ18Ov_SMOW in the 15.2-17.3‰ range. This shows the carbon was mainly from the deep magma in quartz-sulfide stage, but produced the isotopic exchange reaction with the carbonate strata in the late carbonate stage.6. Sulfides have δ34SV-CDT values in the narrow range (+0.71‰~+3.8‰), with an average of 2.0‰, completely fall within the scope of the mantle fluid source. Histogram of sulfur isotope was significantly normal distribution, different from sulfur isotope component effected by strata, indicating the sulfur in Tonglushan deposit from the deep source of magma.7. Several isotopic dating methods including SHRIMP zircon U-Pb, molybdenite Re-Os and phlogopite 40Ar-39Ar are utilized in order to determine the timing of the mineralization and its related intrusive rock. We obtained the SHRIMP zircon U-Pb weighted mean age of (140±2) Ma for the Tonglushan quartz diorite. The molybdenite Re-Os isochron age is (137.3±2.4) Ma and the 40Ar-39Ar plateau age of phlogopite related to magnetite is (140.3±1.1) Ma.These imply that Tonglushan intrusion formed simultaneously with minerlization, both occurred in early Cretaceous, in response to basaltic underplating, which was followed by lithospheric thinning.