Synorogenic copper mineralization during the Alpine–Himalayan orogeny in the Zafarghand copper exploration district, Central Iran: petrogrography, geochemistry and alteration thermometry

Abstract

The Zafarghand porphyry copper exploration district is located in the NE of Isfahan (Central Iran), about 15 km SW of the city of Ardestan. This area consists of Eocene extrusive and Miocene intrusive rocks. The Eocene (ca. 56 Ma) units include andesites, dacites and rhyodacites spatially associated with pyroclastic materials (ignimbrites and pyroclastic tuff breccias), which are intruded by a Miocene (26–19 Ma) microgranular I-type granodiorite. The entire igneous complex is intersected by NW–SE to E–W-trending andesitic and dacitic dykes and N–S-trending andesitic basalt dykes. Both the Miocene granodiorite and Eocene extrusives have undergone several hydrothermal alteration stages: (i) potassic, (ii) phyllic, (iii) argillic and (iv) propylitic alteration. The geochemical data of these formations are consistent with (i) the modal mineralogy derived from petrographic analysis of the samples and (ii) the regional tectonic setting of the samples having been emplaced in an active subduction-related continental volcanic arc setting during the Zagros Orogeny. Copper mineralization in the Zafarghand area shows an average grade of 0.4% Cu, 200 ppm Mo, 0.2 ppm Ag and 0.01 g/t Au and a maximum age of 21 Ma, which implies that copper mineralization was synorogenic, coeval with the Alpine–Himalayan orogeny in this region. Copper mineralization occurs as chalcopyrite and bornite minerals as disseminated and stockwork mineralization associated with remobilized late epithermal vein mineralization. Chlorite geothermometry for the potassic and propylitic alteration zones yields a temperature range of 361–471 and 196–266 °C, respectively, which implies copper mineralization in a cooling synorogenic magmatic arc environment. Chlorite grains in these alteration zones indicate a classification of chlorites mainly as ripidolite, pycnochlorite, brunsvigite and diabantite. Copyright © 2016 John Wiley & Sons, Ltd.