The Behavior Of Magmatic Volatiles In The Evolution Of Mineralized Porphyry Systems-Example From Dexing Porphyry Deposit

The Dexing ore deposit, which is located in the Circum-Pacific ore belt, is the largest porphyry copper deposit in eastern China and is composed of 3 separate mines(Zhushahong, Tongchang, Fujiawu). Studying the behavior of magmatic volatiles in the evolution of this system is significant for understanding the formation of Dexing porphyry deposit. This paper presents electron microprobe analyses of biotite, apatite, amphibole and rehomogenized glassy melt inclusions of the ore-forming porphyritic granodiorite of Tongchang mine. In addition, we analyzed the bulk rock composition of the ore-forming pluton.Heating the quartz crystals separated from ore-forming porphyritic granodiorite(of Tongchang) in the internally heated pressure vessel is an effective method for homogenizing silicate melt inclusions from mineralized porphyry systems. In this study, we determine that 850℃and 8h are appropriate temperature and time for homogenizing melt inclusions from Tongchang system.The Tongchang porphyry varies from dioritic to granodioritic composition and calc-alkaline to high-potassium calc-alkaline series. In relative to the whole rock composition, the compositions of melt inclusions are much highly evolved. These porphyry is enriched in LREE in relative to HREE. The characteristics of trace elements show that the porphyry enriched in LILES and depleted in HSFES. In addition, the Tongchang porphyry is less evolved to medium evolved and is high oxidized. The parental magma for Tongchang porphyry stems from the melting of juvenile crust which formed after the subduction of the Cathaysia block to Yangtze block.All the studied micas are Mg-rich biotite. The biotites are separated into altered primary-magmatic and secondary types based on their petrographic and geochemical characteristics. The phlogopite components of the secondary biotites are typically higher than those of the altered magmatic biotites, and the XMg values of all biotites correlate negatively with their Cl contents showing that they conform to the Mg-Cl avoidance principle. The XMg values also correlate positively with the concentrations of K2O+Na2O+Ba O, Fe O and Ti O2 for all generations of biotites. The calculated log(f H2O/f HCl) values of the coexisting potassic fluids, which are determined from the altered magmatic biotite compositions, ranging from 4.45 to 4.61, are very similar to those of other porphyry deposits. However, the log(f H2O/f HF) and log(f HF/f HCl) values computed for the same hydrothermal fluids are significant higher and lower than those of other porphyry deposits, respectively. All these results show that Dexing hydrothermal system is extremely chlorine-enriched and fluorine-deficient.The Cl concentrations of amphiboles and melt inclusions range from 0.18 to 0.32 wt.% and 0.15 to 0.44 wt.%, respectively. Most apatites trapped in biotite and plagioclase phenocrysts display a bimodal Cl distribution: 0.19 to 1.35 wt.% and 1.48 to 3.73 wt.%. Similarly, the S contents of the apatite also show a distinct bimodal distribution reflecting the effects of variable anhydrite saturation during evolution of the Tongchang melt and variable dissolution of anhydrite by saline aqueous fluids. The Cl contents of the apatites from the Tongchang system are typically higher than those of other studied porphyry deposits. Furthermore, the Cl contents of the melt inclusions are at or very near the Cl saturation levels(0.36 to 0.46 wt.% at 850°C and 50 MPa and 0.42 to 0.54 wt.% at 850°C and 200 MPa) for these melt compositions at shallow crustal pressures. These findings suggest that the area of the granodioritic magma represented by our samples, and perhaps the bulk of the Tongchang granodioritic magma was rich in Cl. The melt inclusion compositions are consistent with a high-salinity, hydrosaline liquid being exsolved directly from the granodioritic melt. This high-salinity hydrosaline liquid was likely very efficient at dissolving, transporting and precipitating ore metals in the mineralizing magmatic-hydrothermal system.