Comparative study of mineral chemistry of four biotite types as geochemical indicators of mineralized and barren intrusions in the Sungun Porphyry Cu-Mo deposit, northwestern Iran

Abstract

The Sungun Porphyry copper-molybdenum deposit, in the northwestern part of the post-collisional Urumieh-Dokhtar magmatic arc, developed as a consequence of hydrothermal alteration related to the emplacement of the Sungun quartz monzonite porphyry stock into Eocene volcanic rocks. Based upon petrographic characteristics, back-scattered electron images, and compositional attributes, biotite from the Sungun Porphyry copper-molybdenum deposit was grouped into least-altered magmatic biotite (LA-Mbt), least-altered equilibrated magmatic biotite (LA-Eq Mbt), equilibrated magmatic biotite (Eq-Mbt), and secondary hydrothermal biotite (S-Hbt) types. The LA-Mbt from the mineralized zone have high TiO2 (3.92–4.33 wt%) compared to LA-Eq Mbt (3.81–4.20 wt%), Eq-Mbt (2.75–3.85 wt%), and S-Hbt (0.27–3.00 wt%). The LA-Mbt of the barren dike is also characterized by high TiO2 (4.17 wt% on average). Na2O, SiO2, and MnO (wt%) of biotite are systematically different between mineralized intrusions and barren ones. The highest amounts of Na2O (0.39–0.58 wt%) and MnO (0.11–0.18 wt%) are found in the LA-Mbt of the barren dike, which contrast with LA-Mbt of the mineralized zone having 0.08–0.15 wt% Na2O and 0.03–0.14 wt% MnO. Low SiO2 values (36 wt% on average) are characteristic of LA-Mbt from the barren dike in comparison with LA-Mbt of the mineralized zone (37 wt% on average). The SO3 contents in LA-Mbt of the mineralized zone (0.03–0.12 wt%) are much higher than those for LA-Mbt of the barren dike (0.01–0.04 wt%). The fluctuations in SO3 content of S-Hbt reflect a complex history of formation of S-Hbt in two stages. One group of S-Hbt has SO3 > 0.04 wt% and are principally related to the potassic (medium–high)/phyllic (low) assemblage, forming with magnetite at high oxygen fugacity, whereas the second group has SO3 < 0.04 wt% and is related to the phyllic (high)/argillic (low) assemblage, having formed during a stage of sulfide mineralization. The calculated oxygen fugacities (ƒO2) of 10−15.4–10−13.8 for the mineralized zone of the Sungun Porphyry Cu-Mo deposit lie completely within the domain of logƒO2 > ΔFMQ+2 (where FMQ is the fayalite–magnetite–quartz oxygen buffer), typical of oxygen fugacities associated with Cu-Mo porphyry mineralization. Altered biotite types have higher Cl contents than the least altered varieties, indicating that Cl may be used as an indicator of post-magmatic hydrothermal processes. The range of F contents (0.336–0.621 wt%) for the LA-Mbt of the mineralized zone is higher than those of the LA-Mbt types of the barren dike (0.219–0.347 wt%). Fluorine in biotite may be used as an exploration vector to characterize mineralized versus subeconomic porphyry Cu-Mo systems.