Geochemistry of igneous rocks associated with mineral deposits in the Tarom-Hashtjin metallogenic province, NW Iran: An analysis of the controls on epithermal and related porphyry-style mineralization

Abstract

Cenozoic magmatism in Iran is associated with many epithermal-porphyry Cu-Au ore systems, which mainly occur within the Alborz Magmatic Arc and Urumieh-Dokhtar Magmatic Arc systems. The Tarom-Hashtjin metallogenic province in the NW part of Iran is located in the western Alborz Magmatic Arc, and is host to numerous LS-IS and HS-type epithermal deposits within Eocene volcano-plutonic rocks. Whole-rock geochemical data indicate that volcanic rocks vary extensively and continuously from subalkaline basaltic to rhyolitic compositions. Granitoid rocks are characterized by subalkaline, metaluminous to peraluminous, I- to A-type magmatism. Geochemical and geochronological studies of the igneous rocks associated with epithermal deposits, particularly the enrichment of large-ion lithophile elements, depletion of high field strength elements, including Nb-Ta depletion and their Eocene ages, are consistent with extensional back-arc tectonic settings, suggesting that primary source magmas were generated by partial melting of the mantle-wedge (garnet-bearing depleted lithospheric upper mantle) and were subsequently affected by both fractional crystallization and crustal assimilation during their magmatic evolution. The regional magmatism likely contributed to the heat advection and composition of metal-bearing hydrothermal fluids in the formation of the epithermal deposits. Considering strong plagioclase fractionation relative to hornblende fractionation, an extensional back-arc tectonic setting, the lack of any skarn mineralization, and a considerably shallower depth of magma generation, the Tarom-Hashtjin metallogenic province is proposed to be prospective for precious metal epithermal mineralization. By contrast, the Urumieh-Dokhtar Magmatic Arc and Ahar-Arasbaran zone systems are probably more eroded and thus more prospective for porphyry Cu-Au exploration.