Distinctive source and hydration state of gold-ore–forming arc magmas

Abstract Whole-rock trace-element analyses contain previously unutilized evidence of mantle source fertility, degree of partial melting, and silicate melt hydration state that are applicable to arc magmatism globally. Spidergram arrays of gold-fertile mafic melts have steeper slopes than gold-infertile ones, characterized by higher Ba/Zr, Nb/Y, and U/Yb. Spidergrams of gold-fertile mafic magmas indicate relatively low percentage melting of lithospheric mantle metasomatized by migrating silicate melts as well as aqueous fluids. Gold-fertile and -infertile melts retain distinctively high Ba/Zr, Nb/Y, and U/Yb across the mafic-to-felsic differentiation series. Plots of V/Y versus Ba/Zr discriminate relative hydration states during magmatic differentiation. The ability of these ratios to discriminate gold-mineralizing magmas is demonstrated by comparison of barren reference suites with our compilation of the compositions of Phanerozoic magmas that generated 138 magmatic-hydrothermal gold deposits worldwide. The distinction between mafic melt compositions in mineralized and barren igneous complexes demonstrates that gold metallogenic fertility and infertility are primary properties of melts generated in different mantle source regions. Magmatic gold fertility typically pervades gold ore provinces for the duration of the regional metallogenic epoch, until changing geodynamics modifies the balance of mantle melt sources.