This post was inspired by a SEG (Society of Economic Geologists) field trip to northern Chile. A week was spent visiting numerous IOCG type mineralization occurrences between Antofagasta and Copiapo.
The Coastal Cordillera of northern Chile and Peru is home to the youngest known IOCG belt (Jurassic to Early Cretaceous; ca 180 to 100 Million years ago), characterized by an arrangement of deposit styles including: polymetallic veins, breccia pipes, mantos and skarns. The Andean IOCG belt is a major contributor to Chile’s copper production. Deposits range in size from 0.5 to 500 Million tonnes with grades varying from 0.4 to 10 percent copper and gold values generally below 0.5 grams per tonne (Sillitoe, 2003).
Andean type IOCG deposits generally display the following characteristics:
• Structurally Controlled: Hosted within fault or shear zone systems parallel to main orogenic zones (e.g. Atacama Fault Zone) that have undergone regional extension.
• Associated with magmatic intrusions of gabbrodiorite to diorite composition.
• Copper mineralization hosted within veins (e.g. Julia), breccia pipes (e.g. Manto Verde), mantos (e.g. Atacama Kozan) and skarns (e.g. Las Pintadas).
• Volcanic Host: Host lithology is within a suite of volcanic assemblages including basalt to andesite, tuffs and minor sedimentary successions. (e.g. La Negra Formation and the Punta del Cobre Group)
Regional extension along orogen parallel fault or shear zones allows for the uprising and emplacement of mantle derived intrusions mainly of gabbrodiorite to diorite composition. The intrusive bodies create hydrothermal convection cells allowing for mineralized fluids to precipitate copper sulphides and iron oxides along structures and permeable lithologies. Regional potassic and/or sodic-calcic alteration zones are commonly associated with large IOCG deposits.
Iron oxide mineralization commonly occurs as hematite (specularite) and/ or magnetite; with a transition of hematite to magnetite with increased depth.
Copper mineralization is characterized by chalcopyrite, bornite and chalcocite; with a transition towards increased chalcopyrite at deeper paleodepths. Secondary copper oxide mineralization such as malachite, azurite, chrysocolla and atacamite are common at or near surface.
To keep this post short, that is all I will cover for now. So with what we know about these types of IOCG deposits, how do we explore for more? Where will the next IOCG belt be discovered?
Sillitoe, R.H., 2003, Iron oxide-copper-gold deposits: An Andean view: Mineralium Deposita, v. 38, p. 787–812.
For a great review on IOCG deposits check out:
Groves, D.I., Bierlein, F.P., Meinert, L.D., and Hitzman, M.W., 2010, Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits: Economic Geology, v. 105, pp. 641-654.