THE MAGNETITE CRISIS IN EVOLVING ARC MAGMAS

The spatial relationship between Au-Cu-rich sulfide deposits and convergent margin magmatism is well established, however the reason for this association is unclear. The study of Au ores themselves has given little clue to the earliest stages of the concentration process. Previous studies have shown that during melt evolution in a crustal magma chamber, the concentrations of Cu and Au increase with increasing SiO₂ until 60 wt. % SiO₂, at which point they suddenly decrease (Sun et. al., 2004). The cause was suggested to be Au loss in a fugitive volatile phase triggered by magnetite saturation and accompanying redox changes in an evolving crustal magma chamber, but this hypothesis remained untested by other observations. We present a more comprehensive geochemical data set for volcanic glasses from the Eastern Manus back-arc basin, including Ag, Pt, and the key element Se and compare this information with our data base of MORB analyses.

Se, a proxy for S, whose magmatic concentrations are not disguised by syn-eruption, late-stage degassing, shares the abrupt decrease with Au, Cu, and Ag. Petrologic modelling reveals the amount of magnetite fractionation is sufficient to convert most of the S originally dissolved in the magma as sulfate to sulfide, triggering saturation in a Cu-rich crystalline sulfide mineral, probably bornite (ideally, Cu₅FeS₄). The Cu-rich sulfide mineral sequesters Au and Ag, elements with the same valence as Cu in sulfides, but not other traditionally chalcophile elements such as Ni, Re and Pt. This mechanism of Au concentration requires specific conditions, the occurrence of which provides a pre-enrichment step to the formation of economic Au-Ag-Cu provinces. The ore-metal abundances in the mantle source of the parental Eastern Manus basalts are similar to the sources of mid-ocean ridge basalt (MORB), so the Cu-Au-rich characteristics of the province requires no enrichment from subducted material. Instead, the association of major Cu-Au deposits with convergent-margin magmatism results from a specific enrichment event during magmatic evolution under oxidizing conditions.

Sun, W, Arculus, RJ, Kamenetsky, VS and Binns, RA, 2004, Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallisation. Nature, v. 431, p. 975-978.