Metallogenic implications of biotite chemical composition: Sample from Cu-Mo-Au mineralized granitoids of the Shah Jahan Batholith, NW Iran

Abstract Igneous biotite has been analyzed from three I-type calc-alkaline intrusives of the Shah Jahan Batholith in NW Iran, which host several Cu-Mo-Au prospects. The XMg (Mg/Mg+Fe) value of biotite is the most significant chemical factor and the relatively high value of XMg corresponds to relatively high oxidation states of magma (estimated fO2 is mostly 10−12.5 to 10−7.5 bars), which is in good agreement with their host intrusions' setting and related ore occurrences. Based on criteria of AlIV and AlVI values, all studied biotites are primary (AlVI = 0), and based on Altotal values (2.23–2.82 apfu) are in distinctive ranges of mineralized granitoid (Altotal=3.2 apfu). The maximum F content of biotite from the Shah Jahan intrusions is moderately higher than those from some other calc-alkaline intrusions related to Cu-Mo porphyries in the world, and in contrast, Cl content is relatively lower. It is likely a result of primary magmatic vs. secondary hydrothermal origin, as well as the Mg-rich characteristics of the biotite in Shah Jahan. XMg values do not correlate with F and Cl contents of biotite, suggesting that biotite records changes in the F/OH and Cl/OH ratios in coexisting melt/fluids. It is consistent with F-compatible and Cl-incompatible behavior during fractional crystallization of wet calc-alkaline I-type granitoid magma generated at subduction related arc settings. The fugacity ratios of (H2O/HF), (H2O/HCl) and (HF/HCl) magmatic solutions coexisting with biotite illustrate similar trends in the three intrusions, which can be due to parental magma sources and/or indicate occurrence of similar magmatic processes prior to or contemporaneous with exsolution of fluids from melt. The observed trends caused F-depletions and Cl-enrichments within developed magmatic-hydrothermal systems which are one of the essential characteristics of potential Cu-Mo-Au mineralized I-type granitoids