A new scenario for related IOCG and Ni-(Cu) mineralization: The relationship with giant mid-crustal mafic sills, Variscan Iberian Massif

Abstract

Both magmatic Ni-(Cu) and hydrothermal iron oxide-copper-gold mineralization coexist in ancient belts but their relationship remains poorly known. Geochronology and field geology evidence show that in SW Iberia both styles of mineralization were coeval with widespread metaluminous to peraluminous Variscan magmatism (350-330 Ma). We propose that mineralization was probably related to a hidden large layered mafic-ultramafic layered complex, recently inferred from geophysics, that was emplaced in a mid-crustal decollement zone. Contamination of a primitive mantle-derived magma with continental crust in the layered complex led to fractional crystallization accompanied by high-T-low-P metamorphism and the incorporation of volatiles into the melt. Hot hypersaline CO2-CH4-bearing brines were subsequently released and focused along major thrusts and strike-slip faults to produce the IOCG mineralization. Assimilation of continental crust also led to the formation of sulphide magmas that were tectonically injected high into the crust, leading to the formation of pipe-like breccia-hosted Ni-(Cu) ore bodies. All these processes took probably place as a consequence of oblique ridge- and/or continent-continent collision. © 2005 Blackwell Publishing Ltd.