The Magnetite Crisis in the Evolution of Arc-related Magmas and the Initial Concentration of Au, Ag and Cu

Abstract

The association of large Au-Cu-Ag ore deposits with convergent margins is commonly attributed to the higher content of chalcophile elements in the parental magmas generated at subduction zones compared with mid-ocean ridges. We present new geochemical data for arc-like, relatively oxidized mantle-derived basalt to rhyolite magmas from the Pual Ridge and vicinity, Eastern Manus Basin, which show that the initial abundances of base and precious metals in the parental basalts are similar to those of mid-ocean ridge basalt (MORB). The contents of Au, Cu, and Ag are built up in the evolving Pual Ridge liquids to levels considerably in excess of those in MORB because, unlike MORB, they are not saturated in a sulfide phase, which is a consequence of their being more oxidized than MORB. The behaviour of S during the evolution of the Pual Ridge magmas is obscured by late-stage SO2 loss during eruption, but we show that it may be inferred by using Se as a proxy, because this element follows S closely during magmatic evolution except it is not lost during low-pressure (near sea-floor) degassing. The onset of magnetite fractionation at ~60 wt % SiO2 and an Mg-number of ~40 is accompanied by an abrupt decrease in the contents of Au, Cu and Ag, previously attributed to separation of Cu-Au-rich fluid, which is also shown by Se, implying that magnetite fractionation triggers sulfide saturation. Petrological modelling reveals that the amount of magnetite fractionation involved is sufficient to convert most of the S originally dissolved in the magma as sulfate (SO42-) to sulfide (S2-), triggering saturation in a Cu-rich sulfide phase, tentatively identified as bornite (Cu5FeS4). This sulfide phase sequesters Au and Ag, elements with the same valence as Cu in sulfides, but not other potentially chalcophile elements such as Ni, Re, and Pt, which suggests that the phase is crystalline rather than an immiscible sulfide melt. The relatively high contents of Cu and Au characteristic of evolved convergent margin magmas requires no enrichment from subducted material. Instead, the association of major Cu-Au deposits with convergent-margin magmatism results specifically from the process of magmatic evolution under oxidizing conditions. This same property also leads to early magnetite fractionation, triggering the abrupt saturation in the Cu-rich sulfide. Hence the easily recognizable trend of magmatic evolution under oxidizing conditions (i.e. the sharp drop in chalcophile element concentrations) may be an exploration guide to economic Au-Ag-Cu provinces, or a crucial pre-enrichment step in the formation of such deposits. The decrease in P2O5 and Sr at the same stage in the fractionation sequence (~ 60 wt % SiO2) indicates that saturation in apatite is concomitant with magnetite-sulfide saturation in the Eastern Manus Basin.