MINERALOGICAL FEATURES OF THE NEAR SULFIDE MOUND SEDIMENTS - MIR ZONE, TAG HYDROTHERMAL FIELD (MID-ATLANTIC RIDGE, 26-DEGREES-N)

The major components forming the hydrothermal part of the near sulfide mound sediments at the TAG field are yellow ochrous lumps, green coll oform crusts and rusty red crusts. Partly oxidized sulfides (pyrite, m arcasite, chalcopyrite, pyrrhotite), silica phases (quartz, opal-CT, c ristobalite; often etched), atacamite, hematite, Mn-oxide crusts and m icronodules are present throughout the sediment profile. Metalliferous sediment horizons and dispersed hydrothermal precipitates have formed as a result of direct precipitation from the hydrothermal fluids, col lapse of active chimneys and mass wasting of the gossans. Oxidation, d issolution, remobilization, precipitation and recrystallization of the phases as well as bacterial action have played an important role in t he formation of near sulfide mound sediments. Cyclic volcanic activity at discrete volcanic centers has contributed specific material in the sedimentary cycle and formed distinct layers rich in basaltic compone nts (olivines, plagioclases, glass, magnetite).