Abstract

SHRIMP-RG U-Pb zircon geochronologic data together with published U-Pb ages from the succession of quartz diorite porphyry intrusions in the western porphyry Cu-Au centers at Reko Diq, western Balochistan, Pakistan, indicate the emplacement of four spatially distinct porphyry Cu centers over ~1 m.y. in the Miocene between ~12.9 and 11.9 Ma. Intrusive activity in the western porphyry cluster began with emplacement of hornblende-rich quartz diorite porphyry associated with the northern and uneconomic H79 porphyry Cu center with U-Pb ages of 12.87 ± 0.27 and 12.72 ± 0.3 Ma. To the south of the H79 porphyry Cu center, successively younger porphyry intrusive centers formed along a south-southwest trend. The economic H15 center has U-Pb zircon ages for four quartz diorite porphyry intrusions between 12.58 ± 0.29 and 12.18 ± 0.19 Ma with three of the porphyry intrusions that span the intrusive sequence in the H15 center having essentially identical ages between 12.58 ± 0.29 and 12.49 ± 0.20 Ma. The adjacent economic H14 center to the south has U-Pb ages on zircons for five quartz diorite porphyry intrusions between 12.22 ± 0.15 and 12.00 ± 0.16 Ma with four of the porphyry intrusions spanning the intrusive sequence having ages between 12.14 ± 0.14 and 12.00 ± 0.16 Ma. The southern end of the trend is the subeconomic H13 complex where mineralized quartz diorite porphyry has a U-Pb age on zircon of 11.93 ± 0.14 Ma. Two new Re-Os ages on molybdenite reported herein agree with the magmatic ages for the H15 (12.43 ± 0.05 Ma) and H14 complexes (11.95 ± 0.05 Ma), whereas previously published Re-Os ages on molybdenite are younger (H15: 11.73 ± 0.04 Ma; H14: 11.82 ± 0.04 Ma) than the youngest dated intrusion in those centers. Published K-Ar ages on hydrothermal biotite from the H79 center (13.0 ± 0.2 Ma), H14 (12.0 ± 0.2 Ma), and H13 (12.0 ± 0.2 Ma) indicate the individual system cooled rapidly. Overall the combined U-Pb geochronology dataset for the four porphyry intrusive centers suggests short-lived magmatic systems characterized each center. However, the available Re-Os ages on molybdenite suggest the potential for complexity in associated hydrothermal systems at the H15 and H14 porphyry Cu-Au centers.

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