Geology and genesis of the Chomalu polymetallic deposit, NW Iran

Abstract

The vein-type Chomalu polymetallic epithermal deposit, located in the Tarom-Hashtjin Metallogenic Province (THMP), western Alborz, formed in two separate mineralized zones controlled by fault zones within andesite and olivine basalt host rocks. The deposit occurs adjacent to the Varmarziar quartz monzonite, quartz monzodiorite, and monzosyenite intrusions. Ore mineralization consists of three stages; (i) an early stage dominated by gold-bearing breccia veins mainly consisting of quartz, pyrite, arsenopyrite, chalcopyrite, minor magnetite and bornite assemblages, (ii) a main stage, which is divided in three sub-stages I, II and III. Sub-stage I is dominated by Pb-Zn-Cu-Ag-rich mineral assemblages including galena, Fe-poor sphalerite, minor chalcopyrite, tetrahedrite, and pyrite. Sub-stage II is only represented by galena and Fe-poor sphalerite. Sub-stage III is defined by Fe-Mn-Zn-Ca-Mg carbonate minerals. The late supergene stage (iii) is characterized by malachite, covellite, chalcocite, cerussite, and Fe-oxide and hydroxides. Microthermometry of primary fluid inclusion assemblages indicate homogenization temperatures (Th) of 253° to 305 °C and salinities of 3.5–4.82 wt% NaCl equivalent in the early stage. The main stage indicates homogenization temperatures (Th) of 200° to 280 °C and 180° to 235 °C with salinities of 2.18–4.96 and 0.53–2.90 wt% NaCl equivalent for the sub-stages I and II, respectively. Ore precipitation was possibly caused by simultaneous dilution and cooling of hydrothermal fluids. The epithermal mineralization in the Chomalu deposit may be classified as an E-type intermediate sulfidation epithermal deposit in the extensional back-arc regime.