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., Ashadze, 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 (delta Zn-66), Cu (delta Cu-65) and Fe (delta Fe-56) 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 (<250 degrees C) and (2) the subsequent high temperature (>350 degrees 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 parts per thousand <= delta Zn-66 <= +0.47 parts per thousand; -0.93 parts per thousand <= delta Cu-65 <= +0.24 parts per thousand; -0.15 parts per thousand <= delta Fe-56 <= +0.25 parts per thousand). 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 delta Zn-66 (+0.30-+0.47 parts per thousand) in peridotites relative to primitive mantle (Zn similar to 55 ppm, Cu similar to 20 ppm, delta(66)zn similar to +0.16 parts per thousand). Striking correlations between delta Zn-66 and indices of serpentinization (LOI and Fe3+/Sigma Fe) 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 delta Fe-56 and indices of peridotite fertility (e.g., Al2O3/SiO2). (2) The mineralization of serpentinites near the Rainbow stockwork is accompanied by an increase in Fe3+/Sigma Fe (>0.7), FeO (up to 19.8 wt%), Zn (>> 50 ppm) and Cu (>> 20 ppm) concentrations. The delta Zn-66 and delta Cu-65 values progressively decrease with indices of serpentinite mineralization (e.g., Zn, Cu, Fe3+/Sigma Fe), while several samples display abnormally high delta Fe-56 (up to 0.25 parts per thousand) relative to primitive mantle (delta Fe-56 similar to 0.025 parts per thousand), 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 Feat-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. (C) 2018 Elsevier B.V. All rights reserved.