Two-phase separation of fossil hydrothermal fluids in the Mid-Indian Ridge ophiolites

Abstract

Rocks of the ophiolite suite were dredged from the axis of the Mid-Indian Ridge at 1 °N. Three stages of their hydrothermal alteration were identified: (1) actinolite-epidote-albite (420–450°C), (2) epidote-chlorite-quartz, and (3) chlorite-smectite (190°C). Epidote-chlorite-quartz veins cutting basaltic breccias include an opaque, amorphous, Na-bearing silica rock that grades into cryptocrystalline quartz. Sometimes it contains rare transparent microcrystalline inclusions, which may be inferred from their high Na, Cl, and Si contents to be quartz intergrown with halite. The specific bulk composition, structure, and texture of the matter allow the interpretation that it is a lithified fossil colloid, precipitated from a highly saline hydrothermal solution as a result of phase separation in the NaSiClH 2O system at elevated PT-conditions. The phase separation of hydrothermal fluids is proposed as a mechanism generating supersaturated salt-silicate solutions with simultaneous precipitation of ore components. The latter is confirmed by the negligible Fe and Mn concentrations in both the colloidal matrix and the microcrystalline inclusions in the presence of disseminated ore mineralization in the examined basalt breccias.