The Chagai Porphyry Copper Belt, Baluchistan Province, Pakistan

Abstract

Although porphyry copper-gold mineralization in the region has been known for approximately 40 years, the Chagai belt has only recently revealed its true potential with the discovery of a world-class deposit at H14-H15 at Reko Diq. The ~300-km-long, east-west–trending Chagai belt comprises several superimposed magmatic arcs and corresponding porphyry copper- (gold, molybdenum) mineralization generated in consecutive events at 43 to 37 Ma (middle-late Eocene), 24 to 22 Ma (early Miocene), 18 to 16 Ma (early Miocene), 13 to 10 Ma (middle Miocene), and 6 to ~4 Ma (late Miocene-early Pliocene) The tectonomagmatic evolution of the region was marked by major, continental-scale events associated with (1) the northward drift of India and its interaction and final collision with the southern margin of Eurasia, and (2) the collision of Arabia with central Iran resulting in final closure of the Neo-Tethyan Ocean. Initial, precollisional stages of arc evolution between the Middle Jurassic and Paleocene formed kilometer-thick submarine volcanic sequences with important flyschlike packages and intercalations of biohermal limestone. The subalkaline basalt and basaltic andesite of this stage possess a tholeiitic lineage of oceanic island-arc affiliation. Small-scale, submarine volcanogenic copper mineralization of manto- and Kuroko-type formed during the late, mature stages of this arc just prior to collision. Early contact of the leading edge of India and the island arcs in the late Paleocene (~55 Ma) resulted in a reorganization of the arc in the Chagai belt, with the consequent extrusion of subaerial andesite followed by emplacement of magnetite-series, calc-alkaline composition batholiths during the late Eocene. This plutonism accompanied the first manifestations of porphyry-type alteration and mineralization at ~43 Ma, in a rapidly emerging arc of Andean type. The belt is defined by 48 porphyry systems with copper-(gold, molybdenum) mineralization associated with calc-alkaline, biotite- and amphibole-bearing porphyry stocks of predominantly quartz diorite to granodiorite composition. Hydrothermal alteration includes potassic, propylitic, sericite-clay-chlorite, sericitic, and advanced argillic assemblages, the last in transitional epithermal environments. Calcic-potassic assemblages are developed locally at the expense of more mafic, commonly dioritic, rocks. Widespread porphyry copper and copper-gold mineralization during the Miocene, at 24 to 22 and 18 to 16 Ma, formed in a subaerial subvolcanic environment during moderate tectonic uplift, most notably during the early Miocene at Saindak and Tanjeel. Faster rates of regional uplift and exhumation, in conjunction with a period of conspicuous volcanic quiescence, characterized large-scale porphyry copper-gold mineralization at H14-H15 at Reko Diq during the middle Miocene between 13 and 10 Ma. Regional, tectonically triggered uplift and consequent exhumation may have been the result of collision of Arabia with central Iran. In general, mature, cumulative supergene enrichment and oxidation zones seem to be absent in the porphyry copper deposits of the Chagai belt, primarily due to the high neutralization potentials and low pyrite contents of the ore-related potassic alteration. Nevertheless, a supergene chalcocite blanket formed at Tanjeel during the Pliocene, at ~4 Ma, in sericitic alteration that accompanied the hypogene mineralization. The tectonomagmatic characteristics of the Chagai porphyry copper belt, including the multimillion-year history of subduction along an Andean-type margin, together with conspicuous short-lived contraction events and concomitant volcanic quiescence during arc construction and evolution are similar to those in contractional belts at convergent margins containing large-scale porphyry copper deposits elsewhere. Superposition of the Chagai magmatic arcs and corresponding porphyry copper events occurred during the last 55 m.y., a situation that implies that the focus of magma generation above the subducting slab remained essentially stationary. Possible mechanisms responsible for this include seaward migration of the trench axis during much of the Tertiary, especially since the Oligocene, and consequent flattening of the subducted slab.