Geometry, conditions and timing of off-axis hydrothermal metamorphism and ore-deposition in the Solea graben

Abstract

The recent discovery of fossil spreading axes along the northern margin of the Troodos ophiolite¹ provides a unique opportunity to compare a fossil seawater-hydrothermal system with systems of modern oceanic spreading centres^2–6. Whereas previous accounts of ophiolite hydrothermal systems^7–13 including Troodos^14–16have elucidated many aspects of sea floor hydrothermal metamorphism, earlier workers have not placed ophiolite hydrothermal features within any specific spreading-centre structural framework. Most ophiolites exhibit complex histories of post-accretion deformation that mask or obliterate early structures related to seafloor spreading. The Troodos ophiolite is apparently still in the process of being 'emplaced'¹⁷ and most structural elements at Troodos appear to have originated beneath a Cretaceous sea floor¹. We have discovered an extensive area of intense seawater-hydrothermal alteration within the Sheeted Dyke Complex of the Solea graben (Fig. 1). The metamorphic petrology and oxygen-isotope geochemistry of >100 dyke samples from this graben reveal the size and three-dimensional geometry of the hydrothermal system and yield estimates of temperature, salinity, oxygen-isotope composition and recharge-discharge geometry for fluids that transported the constituents of massive sulfide orebodies. Geometric discordance between the trends of the hydrothermally altered area and of earlier, seafloor spreading structures compels us to consider a post-spreading, off-axis origin for the orebodies of the Solea graben.