Solid bitumen reflectance and authigenic clay mineralogy in relation to the Irankuh ore deposition: An innovative approach to basin analysis

Abstract

This research investigates the relationship of petroleum generation and authigenic clay development in function of the paleo geodynamic setting and ore genesis of the Irankuh Zn–Pb deposit, Iran. Core samples from seven boreholes from the northern flank of the ore field, originating from the Gushfil, Rowmarmar, and Tappehsorkh mines, were studied by macro- and micro-petrography. Three important paleo-indicators of temperature, i.e. homogenization temperature of fluid inclusions (Th), Kübler Index (KI), and solid Bitumen Reflectance (BR) were used to constrain the geodynamic setting of the Irankuh deposit. The paleo-geothermal gradient is determined based on the correlation between KI and the inferred vitrinite reflectance (VR), and the maximum heating time is derived from a simulation study of VR versus Th. Results show that the ore mineralization occurred when Irankuh was located in a fore-arc setting during the Upper Cretaceous or Paleocene. The time of increased heat flux has been inferred to be more than one and likely below 10 million years. Zn–Pb mineralization is concentrated in zones enriched in residue solid hydrocarbon (RSH) which is a pyrobitumen type. Pyrobitumen represents a secondary byproduct of primary-oil solid bitumen reacting with hot hydrothermal mineralizing fluids. These interactions in an oil-water-rock system created secondary porosities that were filled by ores and authigenic clay minerals. Our evidence supported an epigenetic origin for ore formation in Irankuh. This study gives new insights into defining the tectonic setting and timing of ore genesis.