| Metalliferous sediments are one of the products of the hydrothermal process.They were formed by a mixture of metal-bearing matter from hydrothermal sources with background pelagic sedimentary materials.Mineralogical and geochemical investigations on metalliferous sediments are essential for the reconstruction of the history,position,intensity,and evolution of hydrothermal activity because they can give a reliable insights on the dynamics of hydrothermal activity.The identification of hydrothermal particles in the distal sediments from the ocean basin can further act as a useful tool for prospecting hydrothermal mineral deposits in the oceans.Carlsberg Ridge(CR),which separates the Indian Ocean and Somalian tectonic plates in the northwest Indian Ocean is classified as a slow-spreading ridge with the full-spreading rate of 22-32mm/yr.Unlike the slow-spreading Mid-Atlantic Ridge,the CR has received limited exploration;active hydrothermal activities and polymetallic sulfides had not been confirmed,until the discovery of basalt-hosted Wocan hydrothermal Field(WHF),on the NW slope of an axial volcanic Ridge.A preliminary investigation showed that the field consists of two hydrothermal sites,Wocan-1.which showed indications for recent active high-temperature hydrothermal activity at a water depth of 2970-2990 m,and Wocan-2 site,with features of an inactive toppled sulphide chimney located at a water depth of 3100m,and?2.7km northwest to Wocan-1.Recharge systems are generally invisible on the sea floor compared to the dynamic activity at high-temperature discharge systems.Based on limited information on the newly discovered Wocan hydrothermal field,the geochemical investigations on metalliferous sediments can give reliable insight into the new venting sites,and shed more light on the hydrothermal process in Carlsberg Ridge,and relative locations of the sampling stations to hydrothermal upflow zones and the recharge zones in Wocan hydrothermal systems.In this thesis,morphological,mineralogical and geochemical investigations were conducted on surface sediments samples from the active(Wocan-1)and inactive(Wocan-2)site,and subsurface sediment samples in the Indian ocean basin(?450km west of the Carlsberg Ridge segment).The goal of this study is to use the surface metalliferous sediments to locate the probable existence of new venting sites in the WHF,and to understand the intensity of hydrothermal circulations,primary and post-depositional process in the Wocan hydrothermal systems.This study further aims to understand the impact of the newly discovered venting sites from the Carlsberg Ridge segments on the deep-sea sediments.The bulk chemical compositions of the surface metalliferous sediments of Wocan-1 site when normalized with average values of the surface sediments from the Ridge flanks are depleted in Mn concentration and U/Fe ratio with a higher concentration of Iron(Fe),Zinc(Zn),lead(Pb)and Fe/Mn ratio.The bulk sediment mineralogy showed a distinct abundance of pyrite,anhydrite and altered volcanic glass.The morphology of the isolated sulfides(e.g.,pyrite,chalcopyrite and sphalerite)grains from the surface sediment samples are ma.jorly euhedral,while,the mineral chemistry of isolated sulfide grains is enriched in selenium(Se,0.01-0.05wt.%).cobalt(Co,0.02-0.06wt.%).copper(Cu.0.01-0.36wt.%),arsenic(As,0.02-0.3 lwt.%),antimony(Sb,0.01-0.03wt.%)and lead(Pb,0.05-0.12wt.%).The isolated Fe-oxyhydroxide grains are reddish-brownish,and the mineral chemistry of the Fe-oxyhydroxide mineral-grains showed distinct enrichment in lead(Pb,0.05-0.18wt.%),barium(Ba,0.01-0.10wt.%),strontium(Sr,0.01-0.04wt.%)and arsenic(As,Bdl-0.1 1 wt.%),with increased values of transition metals(?(Cu+Co+Zn+Ni)of up to?1.19 wt.%.The bulk chemical compositions of the surface metalliferous sediments of Wocan-2 site when normalized with average values of surface sediments from the Ridge flanks showed a higher concentration of copper(Cu),manganese(Mn),uranium(U),and enriched U/Fe ratio at station TVG-10.The bulk sediment mineralogy showed a distinct abundance of chalcopyrite,secondary Cu-sulfides(e.g.,bornite and covellite),Fe-oxyhydroxides,and distinct peaks of goethite and manganese oxide(?-MnO2)minerals from the X-ray diffraction(XRD)studies.The morphology of the isolated sulfide grains from the surface sediments is non-euhedral,while,the mineral chemistry of the sulfide grains are enriched in Zinc(Zn,0.01-0.06wt.%),copper(Cu,0.01-0.17wt.%),Tin(Sn,0.02-0.03wt.%)and Thallium(T1,0.02-0.04wt.%).The isolated Fe-oxyhydroxide grains are reddish-yellow,while the mineral chemistry of the Fe-oxyhydroxide separates showed enrichment in copper(Cu,0.42-1.12wt.%)and zinc(Zn,0.32-1.06wt.%)at station TVG-10,with transition metals(?(Cu+Co+Zn+Ni)of up to?1.23 wt.%and less than 0.01 wt.%,in the two sampling stations(TVG-10,and TVG-05 respectively).Seawater-fluid-basalt interaction modelling(?34S-mix= X*?34Sseawater+(1-X)*?34Sbasait)of the ?34S values of surface sediments were used to determine the relative contribution of seawater and magma derived sulfur in the hydrothermal circulation of Wocan-1 and Wocan-2 hydrothermal systems.The surface sediments of Wocan-1 showed lower ?34S(3.0‰ and 3.6‰)values in the two sampling stations(station TVG-07,and TVG-08 respectively),with calculated seawater-derived components of less than 20%,at both stations.The surface sediment samples of Wocan-2 showed ?34S values of 4.1-4.5 ‰ and 6.4-8.7‰,with calculated seawater-derived components of up to?20‰ and?41‰ at station TVG-10 and TVG-05 respectively.The subsurface sediment samples from the Ridge flank revealed a preserved geological records of the enrichment in major and trace elements(Fe,Mn,Cu,Zn,Ni,and Co),hydrothermally altered minerals(e.g.,clinochlore),and hydrothermal plume derived Fe-oxyhydroxides at 34-36cmbsf layer,with a corresponding low sedimentation rate of?1.85cm/kyr,at an estimated age of?17.12kyr.The enriched bulk sediment geochemical compositions(Fe and Fe/Mn ratio),mineral chemistry of isolated sulfides and Fe-oxyhydroxides grains(Se,Co,As,Sb and Pb)and lower bulk sediment ?34S values(3.0-3.6‰)indicate the closer proximity of the surface sediment samples of Wocan-1 site to the high temperature(e.g.,>300?)vent discharge environments.The closer proximity is also supported by the<20%of the calculated seawater-derived components of the Wocan-1 sediment samples and the enriched transition metals(?(Cu+Co+Zn+Ni)of up to?1.19 wt.%)in the Fe-oxyhydroxides grains of Wocan-1 sediments.The enriched transition metals further indicate sulfide minerals as the precursor of the Fe-oxyhydroxide deposits of the Wocan-1 stations.The lower concentration of Fe/Mn ratio(<600),enriched U concentration(>1 Oppm)and U/Fe ratio.together with the depleted metal contents(Se,Co<0.0lwt.%)from the mineral chemistry of the sulfides grains suggest the closer proximity of Wocan-2 stations to the recharge zone environments.These assumptions are supported by the calculated seawater-derived components(up to 41%),and the depleted transition metals(?(Cu+Co+Zn+Ni)<0.01 wt.%)in the isolated Fe-oxyhydroxide grains of the station TVG-05.The depleted transition metals further indicate a combination of hydrothermal plume and seawater in the mode of precipitation of the Fe-oxyhydroxide deposits of station TVG-05 in the Wocan-2 stations.The ?34S values and the calculated magma and seawater-derived components and mineral chemistry of the isolated sulfide and Fe-oxyhydroxide grains further indicate increased hydrothermal contributions to station TVG-08 relative to TVG-07,and stronger seafloor weathering at station TVG-05 relative to TVG-10.The observed isolated grains of Fe-oxyhydroxides and clinochlore in the 34-36m layer in the subsurface core sediment samples of the ocean basin indicates the geological records of the incorporation of the hydrothermal plume and hydrothermally altered particles into the deep sea sediments from the Carlsberg Ridge segment at?17.12ka(during the late Pleistocene). |
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