Genesis of the Brazin Iron Skarn Deposit, NW Iran: Analysis of formation conditions of calc-silicates and the evolution of fluids responsible for the massive magnetite precipitation

Abstract

The Brazin iron skarn occurs along the contact zone between Late Cretaceous flysch-type sequences and an Oligocene-Miocene granodiorite. The prograde skarn stage is identified by the formation of garnet and clinopyroxene, whereas the retrograde stage is dominated by epidote, scapolite, calcite, and quartz. Metal minerals include magnetite + pyrite + chalcopyrite ± pyrrhotite. Garnet with andradite-grossularite-almandine solid-solution is ubiquitous throughout the skarn zones. Two principal types of garnet were identified in the skarn zones: (I) weakly-altered euhedral coarse-grained isotropic garnet exhibits minor variation in the overall composition (And25.95–39.41 Grs54.95–62.25 Alm + Sps + Py5.39–13.04); (II) strongly-altered anhedral to subhedral isotropic to anisotropic garnet shows a wide variation in the overall range of composition (And25.91–55.25 Grs41.37–61.15 Alm + Sps + Py1.48–12.85) in which FeOt increases, whereas Al2O3 decreases rimward.Clinopyroxene with a salitic-ferrosalitic composition is homogenous, has low to relatively high Fe/(Fe + Mg) ratios, and is poor in TiO2, MnO, and Na2O. It displays a decrease in the diopside content with increasing distance from the causative intrusion, whereas the ferrosalite content is enriched near the marble and hornfels fronts.The fluid system evolves from high temperatures (>580 °C), saline fluids (>53 wt% NaCl equivalent) in the late crystallization stage under lithostatic pressure of ∼750 bar at a depth of ∼3 km. Fluid in the later retrograde stage has lower temperatures (<430 °C) and salinities (<40 wt% NaCl equivalent). Mixing and dilution between the initial hydrothermal and external fluids might cause a decrease in fluid temperature and salinity causing the formation of the magnetite.