Element mobilities during serpentinization: Insights from the Ronda Massif, Spain

Serpentinization is a widespread hydrothermal alteration of ultramafic rocks, particularly peridotites, resulting in the formation of serpentinite through hydration processes. While the behavior of fluid-mobile elements (FME; e.g. B, Li, Cl, As, Sb, Pb, U, Th, Cs, Sr, Ba) during serpentinization is well documented, the mobility of other elements (e.g. base metals) remains poorly constrained. We analyzed bulk rock compositions of pairs of poorly and highly serpentinized peridotite rocks from the Ronda Massif (Spain), complemented by in situ LA-ICP-MS mineral analyses. The Loss On Ignition (LOI) content of whole rock represents a good proxy for serpentinization intensity. Ca, Al, Cu, V, and Ti are increasingly mobilized during the initial stages of pervasive serpentinization. In a second stage corresponding to total serpentinization along cm-wide, several meters-long corridors, a systematic loss in Ca, Al, Rb, Sr, REE Hf, Ti, Cu, and V was observed. In contrast, during the third stage of serpentinization corresponding to late faulting only Ca and Cu were leached. The trace element composition of serpentine minerals, including Ti, V, and Zn, is influenced by the primary mineral assemblage, particularly olivine and pyroxenes. In contrast, the mobility of Al and Ca from millimeter to meter scale is notably linked to the bastite-pyroxene and serpentine mesh texture-olivine transformation. Co, Ni, Zn, and Cr depletion in serpentine within veins is compensated by their significant incorporation into magnetite. Copper is systematically leached from serpentinized rocks, suggesting that serpentinization likely serves as a significant source for Cu-rich hydrothermal metallogenic systems hosted in ultramafic rocks. • LOI values in peridotites are a reliable proxy for serpentinization degree. • Serpentinization in the Ronda Massif is clearly non-isochemical. • Ca, Al, Ti, Sr, V, and Cu are leached during serpentinization. • Serpentinization may be a key source term in ultramafic-hosted deposits.