Sulfide-impregnated and pure silica precipitates of hydrothermal origin from the Central Indian Ocean

This is the first report about silica-rich hydrothermal precipitates which were sampled together with hydrothermal sulfides (chimney fragments) in an extinct vent field in the Central Indian Ocean. There are two kinds of silica-rich rocks: a jasper which is impregnated and replaced to various degrees mainly by sphalerite, and to a lesser extent by barite, pyrite and traces of chalcopyrite, and an opalite which is an almost pure silica-phase without any sulfide or sulfate impregnations, but which is sometimes covered by manganese crusts. ernal concentric zoning indicating typical chimney structures can be recognized in the jasper and/or opalite samples, the textures rather suggest a sedimentary silica and/or iron deposition from diffuse, low-temperature (±60 °C) vent fluids, partly with still visible indications of former bacterial mats and synsedimentary deformation structures; the sphalerite- and barite-impregnations within the jasper, however, are considered to have precipitated from white-smoker-type fluids since they were deposited under intermediate temperatures between 155 and 265 °C, according to fluid inclusion studies. lfur isotopic composition (δ34S) of our sulfide samples has mean values of 6.1% for sphalerite and 5.7% for pyrite indicating a mixture of predominantly basaltic sulfur with subordinate amounts of reduced seawater sulfur. The oxygen isotope signals of some pure jasper concentrate samples indicate that the mean formation temperature calculated from these values lies at 63.2 °C. lationship between the massive pyrite- and chalcopyrite-ores from the extinct chimney structures and the silica-rich precipitates can be explained by different cycles of hydrothermal activity: one high-temperature (above 300 °C) cycle dominated by pyrite and chalcopyrite formation and one later epithermal (below 300 °C) cycle which resulted in sphalerite- and silica-dominated precipitates. Furthermore, zonation and zone-refining processes are part of the evolution of the mineralized field.