A missing link between ancient and active mafic-hosted seafloor hydrothermal systems – Magmatic volatile influx in the exceptionally preserved Mala VMS deposit, Troodos, Cyprus

Abstract

Reconciling observations between ancient volcanogenic massive sulfide (VMS) and actively forming seafloor massive sulfide (SMS) deposits is critical for understanding the sources and processes that govern metal enrichment in marine hydrothermal systems. For a mafic VMS deposit, the Mala VMS mound located within the Troodos ophiolite, Cyprus, is unusual as pyrite is enriched in magmatic volatile elements (Au, Cu, Te and Se), sulfide δ34S values average − 3.8‰ ± 1.9‰ (1σ, n = 28), and gypsum averages +14.5‰ ± 2.0‰ (1σ, n = 26) - in stark contrast to the bulk of Troodos VMS pyrite, which averages +4.6‰ ± 2.8‰. To date, this combination of features has only been observed in actively forming SMS deposits in immature, subduction-influenced environments and rarely in ancient VMS deposits hosted in felsic environments. Traditionally, the leaching of igneous rocks is considered as the primary source of metals in mafic VMS deposits. However, at Mala, and perhaps other active SMS deposits in mafic environments, we suggest that Au, Cu, Te and Se were initially sourced from the direct contribution of a magmatic volatile phase where SO2 underwent disproportionation, a signature that is later overprinted by reacted seawater during deposit maturation and is therefore not usually preserved in ancient analogues. Thus, the exceptional preservation of Mala provides evidence of a magmatic volatile contribution in the early stages of mafic VMS deposit formation.