2-PHASE SEPARATION OF FOSSIL HYDROTHERMAL FLUIDS IN THE MID-INDIAN RIDGE OPHIOLITES

Rocks of the ophiolite suite were dredged from the axis of the Mid-Ind ian Ridge at 1-degrees-N. Three stages of their hydrothermal alteratio n were identified: (1) actinolite-epidote-albite (420-450-degrees-C), (2) epidote-chlorite-quartz, and (3) chlorite-smectite (190-degrees-C) . Epidote-chlorite-quartz veins cutting basaltic breccias include an o paque, amorphous, Na-bearing silica rock that grades into cryptocrysta lline quartz. Sometimes it contains rare transparent microcrystalline inclusions, which may be inferred from their high Na, Cl, and Si conte nts to be quartz intergrown with halite. The specific bulk composition , structure, and texture of the matter allow the interpretation that i t is a lithified fossil colloid, precipitated from a highly saline hyd rothermal solution as a result of phase separation in the Na-Si-Cl-H2O system at elevated PT-conditions. The phase separation of hydrotherma l fluids is proposed as a mechanism generating supersaturated salt-sil icate solutions with simultaneous precipitation of ore components. The latter is confirmed by the negligible Fe and Mn concentrations in bot h the colloidal matrix and the microcrystalline inclusions in the pres ence of disseminated ore mineralization in the examined basalt breccia s.