Adjacent Iron Oxide-Apatite and Iron Oxide Copper-Gold Mineralization in the Acarí District, Peru: The Magmatic to Hydrothermal Transition

Abstract

Abstract Hierro Acarí and Cobrepampa are adjacent, Lower Cretaceous iron oxide-apatite (IOA) and iron oxide copper-gold (IOCG) districts, respectively, emplaced in the Coastal Cordillera of southern Peru, separated by a regional NW-trending fault. Mineralization in both districts is hosted mainly by Lower Cretaceous plutons of the Coastal batholith that intrude sedimentary and volcanic sequences of the inverted Cañete basin. The deposits form part of a region that includes the much larger Marcona and Pampa del Pongo IOA/skarn and the Mina Justa IOCG deposits, although Marcona is 60 m.y. older than the others. Hierro Acarí contains fracture-controlled steeply dipping, shallow-plunging N- and NE-trending magnetite veins in a 101.7 ± 1.4 Ma dioritic intrusive, and hematite mantos in a quartzite roof pendant. The magnetite ore shoots show colloform and microporous textures and grade downward within 150 m to actinolite and carbonates, which replace the magnetite. The Cobrepampa district comprises numerous NW-striking chalcopyrite-pyrite ± hematite veins and oxidized equivalents in a 99.7 ± 0.4 Ma monzonitic intrusive. Fluid inclusions indicate that the copper mineralization was deposited at 166° to 266°C from a fluid with low to moderate salinity of 3.8 to 16.5 wt % NaCl equiv. Colloform and microporous textures in magnetite are interpreted in comparable Chilean deposits to indicate colloidal transport of magnetite microlites, crystallized from a magma, which attach to bubbles in the magmatic-hydrothermal fluid in a process resembling froth flotation. Magnetite textures at Hierro Acarí suggest that magnetite was transported by this late magmatic process and deposited in the roof zone of the host intrusive. The hydrothermal fluid may have evolved to deposit hematite as mantos in the overlying sediments under more oxidizing conditions and may then have deposited the Cobrepampa copper and hematite veins in distal NW-trending faults. Recent studies into the geochemistry of Cañete basin volcanic rocks suggest that a mid-ocean ridge was subducted at the latitude of the Raúl-Condestable IOCG deposit, 350 km northwest of the Acarí district, at approximately 130 Ma, predating current Andean subduction below southern Peru. That situation could result in a configuration resembling the Gulf of California, where a series of fault-terminated pull-apart basins focus hydrothermal fluids at intervals along the main trough. IOCG and IOA/skarn mineralization in the Acarí district occurred during the early stages of Late Cretaceous basin inversion and intrusion of the Coastal batholith. However, the Marcona IOA deposit in the same districts was emplaced during an earlier, unrelated Jurassic to Early Cretaceous mineralizing episode.