Abstract
The oxidation state of the Earth’s mantle and its partial melting products exerts a fundamental control on the behavior and distribution of sulphur and chalcophile and siderophile elements between the mantle and crust, underpinning models of ore deposit formation. Whether the oxidized nature of magmas in inherited from the asthenospheric mantle source or acquired during ascent and differentiation is vigorously debated, limiting our understanding of the mechanisms of extraction of sulphur and metals from the mantle. Here, we focused on the redox-sensitive behavior of sulphur in apatite crystallized from quenched alkaline basaltic melts preserved within a peridotite xenolith from the El Deseado Massif auriferous province in southern Patagonia. We took advantage of this unique setting to elucidate the redox evolution of melts during their ascent through the subcontinental lithospheric mantle, and grasp the inner workings of the Earth’s mantle during gold metallogenesis. Our data reveal that an initially reduced silicate melt (ΔFMQ -2.2 to -1.2) was oxidized to ΔFMQ between 0 and 1.2 during percolation and interaction with the surrounding peridotite wall-rock (ΔFMQ 0 to +0.8). This process triggered changes in sulphur speciation and solubility in the silicate melt, boosting the potential of the melt to scavenge ore metals such as gold. We suggest that large redox gradients resulting from the interaction between ascending melts and the surrounding mantle can potentially modify the oxidation state of primitive melts and enhance their metallogenic fertility. Among other factors including an enriched metal source and favorable geodynamic conditions, redox gradients in the mantle may exert a first-order control on the global-scale localization of crustal provinces endowed with gold deposits.
Highlights
The oxidation state of the Earth’s mantle is a fundamental parameter on models that attempt to explain the formation of metallogenic provinces on a lithospheric scale (Mungall, 2002; Sillitoe, 2008; Richards, 2015)
We examine the effects of melt–mantle interaction on the f O2 and metal fertility by measuring directly the formal oxidation state(s) of sulfur in magmatic apatite contained within quenched silicate glass entrained in a peridotite xenolith from Patagonia, Argentina, which represents a silicate melt that ascended through the subcontinental lithospheric mantle (SCLM) below the El Deseado Massif auriferous province
By combining micro-X-ray absorption nearedge structure (μ-XANES) spectroscopic analysis of sulfur in apatite with petrological and mineralogical data, we show that originally reduced silicate melts that infiltrated a relatively oxidized SCLM interacted with the surrounding peridotite wallrock, triggering changes in sulfur speciation and solubility
Summary
The oxidation state of the Earth’s mantle is a fundamental parameter on models that attempt to explain the formation of metallogenic provinces on a lithospheric scale (Mungall, 2002; Sillitoe, 2008; Richards, 2015). Relatively little attention has been paid to melt–rock interaction processes occurring in the subcontinental lithospheric mantle (SCLM) that might lead to changes in the oxidation state and composition of ascending magmas after partial melting in the asthenosphere and before they reach crustal levels. Tollan and Hermann (2019) found that arc magmas oxidize during ascent and reaction with surrounding peridotite before reaching crustal levels. Melt–SCLM interaction processes might have a pivotal impact on the metal fertility of ascending magmas
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