Journal of Economic Geology (Jun 2019)
Physicochemical Attributes of Parental Magma in Collisional Porphyry Copper Systems; Using Biotite Chemistry, Case Study: Chahfiruzeh Porphyry Copper Deposit
Abstract
Introduction Almost all of the well-known porphyry copper deposits in Iran occur within the Kerman Cenozoic magmatic arc (KCMA) (Fig 1) in the southeastern part of Cenozoic Urumieh–Dokhtar magmatic belt (Hassanpour et al., 2015). The Miocene Chahfiruzeh porphyry copper deposit as an example of collisional porphyry intrusion is located in the Kerman Cenozoic magmatic arc (Fig 2) (Einali et al., 2014). In this research, we attempt to characterize the physicochemical attributes of parental magma in the Chahfiruzeh porphyry deposit, using chemistry of magmatic biotite. Materials and methods Samples from various rocks were collected from drill cores belonging to 123.1-667.1m depths. The chemical compositions of magmatic biotites were obtained by analyzing the carbon coated polished thin sections using electron probe microanalyzer (EPMA). All samples were prepared and analyzed in the Montanuniversität Leoben, Austria using a superprobe Jeol JXA 8200 instrument. The analyses were conducted with 15 kV accelerating voltage and 10 nA beam and beam size of about 1 μm. The counting times (upper and lower) were 100 and 20 s, respectively. Results Chahfirouzeh porphyry deposit containing 100 Mt ore reserves (Mohammaddoost et al., 2017), and 0.4-0.8% Cu is located at the 95 km NW of Sarcheshmeh deposit and 35 km NE of Shahr-e- Babak in the Kerman province. (Einali et al., 2014). The compositions of analysed magmatic biotites from the Chahfiruzeh porphyry copper deposit are summarized in Table 1. According to (Mg–Li) vs. (Fetot + Mg + Ti–AlVI) and Mg–(Fe2+ + Mn)–(AlVI + Fe3+ + Ti) diagrams, the biotite from Chahfiruzeh deposit are Mg-rich (Fig. 5-A,B). Chemical compositions of biotites on the ternary diagram of Beane (1974) shows a magmatic origin for the analysed samples (Fig 7). Magmatic biotites are characterized by high SiO2 values ranging from 37.44-44.71 (Wt. %). Also, MgO and FeO vary between 12.54-14.36 (Wt. %) and 14.94-16.3 (Wt.%), respectively. Moreover, TiO2, K2O, and Na2O range from 4.53-5.97, 8.3-9.38, 0.15-0.29 (Wt.%), respectively (Fig 6-A-D). Fluorine and Cl contents in biotite range from 0.3 to 1.52 wt.% and 0.03 to 0.04 wt.%, respectively. Discussion Chemical compositions of selected biotites on the classification diagrams of Abdel-Rahman (1994) indicate that magmatic biotites from the Chahfiruzeh porphyry copper deposit belong to calc-alkaline (C) series (Fig. 8). Also, according to ternary Xpdo-Xan-Xph diagram (Wones and Egster , 1965), Oxygen fugacities of Chhfiruzeh and other pre-collisional porphyry deposits (e.g., Reagan) occur in NNO distinct (Fig 9). It is evident that granitic rocks of both studied deposits are formed in relatively similar oxidiant conditions. Moreover, the preformed geothermometry on the magmatic biotites in the Chahfiruzeh porphyry the copper deposit shows a range of temperatures between 478-632 ° C (average 565.3 °C). Moreover, XMg/XFe values confirm that Mg is enriched in Chahfiruzeh (collisional porphyry) compared to that of Reagan (pre-collisional porphyry; Fig 10-A). Also, fluorine has the highest concentration in collisional porphyry copper deposits. The plot of the Chahfiruzeh biotites on the log (XMg/ XFe) versus log (XF/XOH) discrimination diagram of Brimhall and Crerar (1987) represent that the intrusion crystallized from a weakly to strongly crustal-contaminated, I-type granitic magma (Fig 11). The log fH2O/fHF and fH2O/fHCl ranges between 4.69-4.84 and 4.09-4.28 having an average value of 5.14 and 4.14, respectively (Table 1). According to XFe vs. XF/XOH and XCl/XOH, Cl fugacities in Chahfiruzeh are analogous to that of Reagan porphyry (Fig 12). The calculated halogen fugacity ratios (log fH2O/fHCl vs. log fH2O/fHF and fHF/fHCl) and log fH2O/fHCl vs. IV(Cl) of magma in equilibrium with biotite for Chahfiruzeh porphyry and comparison with Reagan and other known porphyry of the world show that Chafiruzeh and Ragan deposits are analogous to those of Sarcheshmeh and Bingham porphyry deposits (Fig 13 and 14). Finally, Chahfiruzeh deposit as collisional porphyry has higher IV(F) than Reagan deposit (pre-collisional). In comparison with collisional porphyry copper systems (Chahfiruzeh), poor mineralization in the Reagan pre-collisional deposit may be due to lower Cl content of the magma in Reagan deposit. Acknowledgements This research was made possible by the help of the office of vice-chancellor for Research and Technology, Shahid Chamran University of Ahvaz. We acknowledge their support. Authors highly appreciate the efforts of Prof. Johann. Raith and Dr. Federica Zaccarini for EMPA analysis. We also gratefully acknowledge the staff of the National Iranian Copper Industries Company (NICICO), for helping us in sampling. References Abdel-Rahman, A.M., 1994. Nature of biotites from alkaline, calc-alkaline, and peraluminous magmas. Journal of Petrology, 35 (2): 525–541. Beane, R.E., 1974. Biotite stability in the porphyry copper environment. Economic Geology, 69(1): 241–256. Brimhall, G.H. and Crerar, D.A., 1987. Ore fluids: magmatic to supergene. Reviews in Mineralogy and Geochemistry, 17(1): 235–321. Einali, M., Alirezaei, S. and Zaccarini, F., 2014. Chemistry of magmatic and alteration minerals in the Chahfiruzeh porphyry copper deposit, south Iran: implications for the evolution of the magmas and physicochemical conditions of the ore fluids. Turkish Journal of Earth Sciences, 23(1): 147–165. Hassanpour, Sh., Alirezaei, S., Selby, D. and Sergeev, S., 2015. 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