Distinctive composition of copper-ore-forming arcmagmas

Abstract

Magmatic fertility to generate porphyry- and high-sulfidation copper ore deposits generally develops, not at the scale of individual igneous complexes, but rather at the scale of petrochemical provinces spanning hundreds of kilometres of arc length, and containing many igneous complexes. The extent and duration of copper metallogenic provinces and epochs are apparently controlled by geodynamically imposed compressive stress that commonly lasts ca 5–20 Ma and fosters entrapment of mantle-derived basaltic magmas in chambers near the Moho where they cool very slowly and are likely to last long enough to experience intermittent replenishment during fractional crystallisation of layered ultramafic–mafic cumulate complexes on the chamber floor. Accumulation of dissolved H2O through multiple cycles of replenishment and crystallisation in high-pressure magma chambers can explain the distinguishing chemical features of copper-ore-forming calc-alkalic arc magmas. The distinguishing features include unusually high contents of Sr and V, and unusually low contents of Sc and Y. The distinction from ordinary andesitic, dacitic and rhyolitic arc magmas can be enhanced by using ratios of enriched elements to depleted elements. The ratios Sr/Y and V/Sc are shown here to be effective in discriminating least-altered samples of ore-forming intrusions from ordinary, metallogenically unproductive arc magmas.