Geochemistry and geology of spatially and temporally associated calc-alkaline (I-type) and K-rich (A-type) magmatism in a Carboniferous continental arc setting, Pataz gold-mining district, northern Peru

Abstract

The Pataz–Parcoy gold-mining area, in the Eastern Andean Cordillera of northern Peru, is located on the western margin of the Amazonia craton, in a transtensional jog on the Cordillera Blanca Fault. Episodic subduction, accretion and rifting have taken place in the Eastern Andean Cordillera since the Mesoproterozoic. In the Pataz district, the Cambrian–Ordovician Vijus and Atahualpa formations comprise dacitic and rhyodacitic volcaniclastic rocks that formed by fractionation and/or crustal assimilation of a regionally more abundant andesitic parent melt. Mississippian magmatism began with enriched tholeiitic magmas that lack a Ti anomaly and formed by melting undepleted metasomatised asthenosphere. These magmas assimilated variable amounts of continental crust and were emplaced as Vista Florida Group volcaniclastic rocks, and dioritic plutons of the Pataz batholith. Ponding of these mantle-derived magmas in andesitic lower crust caused partial melting, which generated the volumetrically dominant, granodioritic component of the calc-alkaline (I-type) Pataz batholith. Within a few million years but following rifting, and uplift of the Pataz batholith, relatively K-rich (A-type) magmas formed by melting of a mid-crustal tonalitic source, and were emplaced in the upper crust as the latitic to rhyolitic Lavasen Volcanics and Esperanza subvolcanic complex. Anatexis in the mid crust was promoted by a second batch of magma derived from the undepleted metasomatised asthenosphere, evidence of which is preserved as mafic components of the Esperanza subvolcanic complex and post-Esperanza dolerite dykes. A K-rich magma chamber, the source of the Lavasen Volcanics and the Esperanza subvolcanic complex, is proposed as a possible source of the ore fluid, which deposited gold in veins hosted by the Pataz batholith.