2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 13
Presentation Time: 11:25 AM


FU, Frank Q.1, MCINNES, Brent I.A.2, EVANS, Noreen J.2, DAVIES, Peter J.1, SCHLODERER, John3, GRIFFIN, William L.4 and SAEED, Ayesha4, (1)School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia, (2)CSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6151, Australia, (3)Tethyan Copper Co. Ltd, PO Box 1810, West Perth, 6872, Australia, (4)GEMOC Key Centre, Macquarie University, NSW 2109, Australia, qifu6346@mail.usyd.edu.au

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.