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 (original) (raw)

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 TiO 2 (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 TiO 2 (4.17 wt% on average). Na 2 O, SiO 2 , and MnO (wt%) of biotite are systematically different between mineralized intrusions and barren ones. The highest amounts of Na 2 O (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% Na 2 O and 0.03-0.14 wt% MnO. Low SiO 2 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 SO 3 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 SO 3 content of S-Hbt reflect a complex history of formation of S-Hbt in two stages. One group of S-Hbt has SO 3 > 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 SO 3 < 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 (ƒO 2) of 10 −15.4-10 −13.8 for the mineralized zone of the Sungun Porphyry Cu-Mo deposit lie completely within the domain of logƒO 2 > 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. 1975; Parry and Jacobs, 1975); and (iii) investigating the physicochemical features of ore fluids, as well as their evolution through the entire mineralization process (Munoz, 1984; Lentz, 1992, 1994; Selby and Nesbitt, 2000). Several previous studies of biotite chemical compositions from porphyry Cu systems focused on the calculation of F and Cl contents, with the objective of identifying mineralized and barren plutons (e.g.,