THERMAL AND EXHUMATION HISTORY OF THE PARRA KOH PORPHYRY CU-AU DEPOSIT, PAKISTAN: INVERSE NUMERICAL MODELING OF U-PB-HE DATA

The Parra Koh porphyry Cu-Au prospect is one of several mineralized deposits of the Reko Diq copper complex located in western Pakistan. The prospect is associated with the emplacement of quartz and feldspar-phyric intrusions within interbedded sandstone, shale and limestone units of the Eocene to Oligocene Saindak Formation. We report on a zircon and apatite U-Pb-He triple-dating study on designed to determine the thermal and exhumation history of the Parra Koh deposit. An aspect of the study was to determine whether (U-Th)/He systematics could be used to detect the “thermal footprint” of the porphyry formation event in detrital apatite and zircon in the sedimentary host formation.

A cross-sectional series of outcrop and drill core samples were collected from the igneous stock and adjacent sandstone units for apatite and zircon (U-Th)/He thermochronology and zircon U-Pb dating. Preliminary results for igneous apatite and zircon constrain over 700°C of thermal history and reveal that the porphyry was emplaced at about 12.5 Ma and exhumed to the upper 1-3 km of the crust at around 5-6 Ma. Inverse numerical modeling of the U-Pb-He triplet geochronology data indicates that the Parra Koh porphyry was emplaced at a depth of 5.6~6.6 km, reached thermal equilibrium with country rock at 12.1~11.8Ma and was exposed to the surface at 4.4~5.2 Ma. Calculated erosion rates range from 0.20 - 0.79 mm/yr for the Parra Koh prospect and therefore, between 350-1000 meters of potentially mineralized rock has been eroded since its exposure to the Earth's surface. The cooling rate was 1800 ºC/My for the intrusion's early history (hydrothermal-magmatic stage) and 20 ºC/My during the later exhumation stage. The duration of hypogene ore formation for the prospect was about 110 Ky with an average cooling rate of 1600 ºC/My. The modeling results have been further refined by applying constrains of U-He thermochronology data from the ambient country rocks and of the present exposure states of the porphyry system. Zircon U-Pb dating of zircons from the host sediments indicates that they were derived from orogenic belts formed at 40 ± 5 and 110 ± 5 Ma, whereas their U-He ages have all been reset to 11 ± 2 Ma. Further work is underway to determine whether the thermal resetting of the U-He ages of the detrital zircons is a local or regional event.