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Abstract

Background and Objective: The Lar Cu-Mo prospect is located in Sistan suture zone, southeast of Iran. In this study, the chemical composition of biotite is determined using electron probe micro-analyzer in shoshonitic igneous rocks. Shoshonitic rocks are syenitic to monzonitic in composition that can be divided into the granular and porphyroid groups. Methods: Studying biotites generally occurs as primary and secondary types. The primary biotites are large poikilitic phenocrysts associated with Fe-Ti oxides, giving the rocks a spotted texture. In addition, secondary biotites show varying size, color, and shape. All of the Lar analyzed biotites are Mg-and Ti-rich and F-and Cl-poor. Analyses of the Lar biotites suggested that crystallization took place at an average temperature of 731°C and 640°C for primary and secondary samples, respectively. Results: Estimation of the oxygen fugacity, based on chemical composition and Fe3+ content of biotite, shows that the oxygen fugacity was limited in fayalitemagnetite quartz to middle of HM buffers in quality and was about 10 −12 to 10 −14 bar in quantity for primary and secondary type, respectively. Halogen fugacity ratios in biotite show that the Lar porphyroid rocks (ore-bearing) are affected mainly by meteoric water than granular type (barren). Calculated halogen fugacity values for biotite in the Lar Cu-Mo prospect are consistent with many other porphyry copper-forming systems. Conclusion: The Lar Cu-Mo prospect was formed by the hydrothermal activity related to the syenitic to monzonitic porphyroid igneous rocks with shoshonitic nature which introduces the Lar igneous complex volcanic sequence. The hydrothermal alterations are limited and occurred as silicic, potassic, phyllic, propylitic, and argillic. Hypogene sulfide minerals mainly occur as disseminated and sulfidic and silicic veins and veinlets consisting of chalcopyrite, bornite, molybdenite, and minor pyrite. Biotite as main ferromagnesian mineral of host rocks occurs as primary in less altered rocks and secondary in the alteration areas.