Ore component mobility, transport and mineralization at mid-oceanic ridges: A stable isotopes (Zn, Cu and Fe) study of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Ultramafic hosted hydrothermal deposits are ubiquitous along slow-spreading ridges such as the Mid-Atlantic Ridge (MAR; e.g., Ashadzé, Rainbow, Lost City) where they exert a major control on the cycling of economically important elements (e.g., Zn, Cu, Ni). However, the origin of metal mobility in these environments remains unclear. Here we use Zn (Zn), Cu (Cu) and Fe ( Fe) stable isotopes to explore the mobility of metals during (1) the serpentinization of the Rainbow massif basement in a seawater dominated system at low temperature (350 °C) mineralization of serpentinites through seawater-derived fluids that interacted with gabbro prior to interacting with serpentinite near hydrothermal sites (stockworks). The Rainbow samples display among the largest range of isotopic variations ever reported for ultramafic rocks (−0.10‰ ≤ Zn ≤ +0.47‰; −0.93‰ ≤ Cu ≤ +0.24‰; −0.15‰ ≤ Fe ≤ +0.25‰). These variations reflect a two-stage process. (1) Serpentinization of the ultramafic basement is accompanied by a decrease in Zn (26–41 ppm) and Cu (1–13 ppm) concentrations and an increase of Zn (+0.30–+0.47‰) in peridotites relative to primitive mantle (Zn ∼ 55 ppm, Cu ∼ 20 ppm, Zn ∼ +0.16‰). Striking correlations between Zn and indices of serpentinization (LOI and FeFe) show preferential leaching of isotopically light Zn by fluids during the serpentinization of the massif. This isotopic fractionation is controlled by the dissolution of both mantle sulfides and/or spinels and Zn complexation with chlorine in fluids. At this stage, Fe seems to be immobile as attested by correlations between Fe and indices of peridotite fertility (e.g., Al2O3/SiO2). (2) The mineralization of serpentinites near the Rainbow stockwork is accompanied by an increase in FeFe (>0.7), FeO (up to 19.8 wt%), Zn (≫50 ppm) and Cu (≫20 ppm) concentrations. The Zn and Cu values progressively decrease with indices of serpentinite mineralization (e.g., Zn, Cu, FeFe), while several samples display abnormally high Fe (up to 0.25‰) relative to primitive mantle (Fe ∼ 0.025‰), suggesting a high mobility of Zn, Cu and Fe in high temperature hydrothermal fluids. These isotopic fractionations can be explained by the local oxidation of sulfur bearing fluids in contact with seawater. This process enhances metal precipitation as well as the formation of Fe3+-bearing phases, such as magnetite, beneath the stockwork, explaining the presence of magnetic anomalies below the Rainbow hydrothermal field. Our study shows that the mobility of metals in hydrothermal fluids can be enhanced by both peridotite interaction with seawater or with fluid that interacted with deeper mafic bodies and then flowed to the surface. These processes may generate hydrothermal deposits with distinct metal signatures