Biotite compositions and geochemistry of porphyry-related systems from the central Urumieh Dokhtar Magmatic Belt, western Yazd, Iran: Insights into mineralization potential

Abstract

Commonly biotite occurs as a minor rock-forming mineral in a wide range of felsic to intermediate intrusive rocks. It can record the physicochemical conditions of the parent magma due to its complex crystal structure that allows multiple site substitutions of a wide range of elements. Geochemical studies of whole rocks and biotite (using EPMA and LA-ICP-MS methods) with petrographic analysis of biotite provide distinction between barren and fertile granitoids in western Yazd. Fertile suites are characterized by Si- and Mg-rich (3.47–4.35, ave. 3.75 a.p.f.u.) and total REE-poor biotite. In contrast, Fe (2.66–3.15, ave. 3.08 a.p.f.u.), Al, Ti, and REEs (in particular HREE+Y) increase in barren rocks. On the basis of biotite geobarometry, the fertile rocks (Darezereshk Porphyry Copper Deposit (PCD)) solidified at low pressure (ca. 0.6 kb, i.e., subvolcanic); in contrast, the barren rocks (Galvic and Shirkouh granodiorites) crystallized at higher pressures (1.1 to 3.6 kb). The temperature of the fertile granitoids is inferred to be about 750 °C, whereas T's of 625–675 °C for barren granitoids. The fertile suites have redox characteristic near the Ni-NiO buffer, whereas the barren suites are more reduced, near the quartz-fayalite-magnetite (QFM) buffer. Therefore, biotite major-element compositions, fertile magma bodies are emplaced at higher temperature and redox to lower pressures (subvolcanically) than the barren intrusive bodies. The whole-rock geochemical signatures show that both barren and fertile suites are represented by peraluminous to metaluminous granite to diorite that have calc-alkaline characteristics with an I-type volcanic arc (VAG) tectonic setting. However, fertile suites originated from the mantle-crust mixed magma exhibiting calc-alkaline affinity, whereas the barren suites are derived predominantly from recycling of continental crust.