TIMING AND RELATIONS OF MAGNETITE AND COPPER ORE MINERALIZATION IN IOCG AND MTAP DEPOSITS: COASTAL CORDILLERA, COPIAPO REGION CHILE,

The Punta del Cobre Formation, in the Coastal Cordillera of the Andes, hosts both Iron Oxide Copper Gold (IOCG) and Magnetite Apatite (MtAp) deposits. Similar yet distinct deposit classifications have been proposed that suggest a genetic link despite differing ore mineralogy; with IOCG deposits containing magnetite/hematite with chalcopyrite +/- Au, and MtAp deposits containing primarily magnetite and apatite. Several models exist explaining IOCG and MtAp ore genesis including a magmatic, hydrothermal and magmatic-hydrothermal mix, but the relationships between both styles of mineralization remains unclear.

The ore deposits in the Punta del Cobre Formation are generally hosted in andesite-dacite that have undergone pervasive alkali-calcic metasomatism, caused by the infiltration of hydrothermal fluids, with the majority of ore deposition associated with the replacement of the host volcanic rocks by feldspar, actinolite, biotite and chlorite. The metallic assemblage is composed primarily of veins and breccias supported by magnetite, chalcopyrite and pyrite. The MtAp deposits in the area, similar to many MtAp deposits elsewhere, have ore consisting of massive magnetite with actinolite and apatite. Previous work in deposits of the Punta del Cobre Formation obtained ages using K/Ar and ⁴⁰Ar/³⁹Ar dating methods to constrain ages between 130-110 Ma for both ore rock and host rock using amphibole and biotite. Problems from low closure temperatures and fluid related alteration causes ages obtained from K/Ar and ⁴⁰Ar/³⁹Ar dating to commonly reflect later thermal events or metasomatism rather than mineralization.

Analyses of zircon grains from IOCG deposits (Mantos de Cobre, Las Pintadas, and La Tigresa), and a MtAp deposit (Los Colorados), and the Copiapo Batholith, provide: 1) a more precise age determination of the emplacement age of the host, the hydrothermal alteration, and mineralization; 2) Lu-Hf isotope ratios provide constraints on the source of host rock and the ores, either originating from the depleted mantle or reworked continental crust; and 3) Trace and minor element analyses of zircon provide insight into the oxidation state during crystallization, and have been proposed to predict economic potential (sulfide mineralization) at the time of zircon crystallization.