The subduction-related Saindak porphyry Cu-Au deposit formed by remelting of a thickened juvenile lower crust underneath the Chagai belt, Pakistan

Abstract

The subduction-related Saindak porphyry Cu-Au deposit lies in the western segment of the Chagai belt, Pakistan. This deposit consists of three orebodies (south, east, and north), and each orebody is centered around ore-forming tonalite and crosscut by diorite porphyry dikes and andesite dikes. The three orebodies show similarities in the alteration zones, mineralization styles, and metallic associations. K-silicate, sericite-chlorite, quartz-sericite, and chlorite-epidote alteration assemblages are zoned around the porphyry intrusions. Laser ablation-inductively coupled plasma–mass spectrometry (LA-ICP–MS) U-Pb zircon geochronology yields ages between 22.2 ± 0.2 and 21.8 ± 0.3 Ma for the ore-forming tonalite, 22.1 ± 0.2 Ma for the diorite porphyry, and 22.2 ± 0.4 Ma for the andesite. Molybdenite re-Os dating yields an age of 22.2 ± 0.2 Ma for mineralization. The new geochemical compositions of magmatic rocks indicate that the andesite and K-rich diorite porphyry originated from partial melting of the asthenospheric mantle and metasomatized lithospheric mantle underneath the Chagai belt, respectively. The adakite-like ore-forming tonalites were derived from partial melting of a thickened juvenile lower crust, implying that the ore-forming materials in the Saindak deposit may be closely related to the lower crust. The injection of melts represented by the K-rich diorite porphyry would have increased the oxygen fugacity and water content of melts derived from the juvenile lower crust, making them favorable for the formation of the Saindak deposit. Combined with previously published data, we suggest that large porphyry Cu-Au deposits in the Chagai belt all developed in the thickened crust.