ALBERT

Description: Many occurrences of mineral resources in ocean water, on the ocean floor and in its deeper parts have long been well-known. This is readily understandable, because numerous ancient mineral products which are exploited on land were originally formed in marine milieus as far back as the Precambrian , e.g., marine sedimentary iron ores, rock salt, potassium, phosphate and manganese , as well as petroleum and natural gas. This book deals with ores in sediments, and in sedimentary and volcanic rocks. According to the Concise Oxford Dictionary (Fowler and Fowler , 1964), ore is defined as "solid native mineral aggregate from which valuable constituents not necessarily metal may be usefully extracted". Therefore , in tllis chapter only mineral resources of inorganic origin in ocean water and on the ocean floor are discussed and not those of organic derivation, although the existence of petroleum, natural gas and, to some extent, of coal in the subsurface of the ocean shelves is of greater economic importance. The amount of oil and gas produced in 1969 represented more than 90% by value of all mineral resources obtained from the oceans and ocean floors. Ore deposits in bed-rock formations of the shelf region, such as those which are exploited near Cornwall (England) and Newfoundland for example, are also irrelevant, as these are merely extensions of discoveries on the nearby mainland. The purpose of this chapter is to give a general review of the (inorganic) mineral resources of the oceans and the ocean floors . In this respect, not only are the economically important products which are already in use discussed, but also those materials which, in their oceanic environment, can become of economic value in the near or more remote future. For a better understanding of the existence of such materials, some deposits have also had to be reviewed which will not become of economic value . Since some contributions in this book are specifically dedicated to Recent marine ferromanganese deposits and Recent phosphorite deposits, these mineral resources are only briefly discussed in tllis chapter. (For details on Recent marine and lacustrine manganese deposits see Chapters 7 and 8 by Glasby/Read and by Callender/Bowser, respectively, in Volume 7.) The litarature on the mineral (inorganic) resources of the oceans and ocean floors is extremely extensive, so that a choice had to be made and, therefore, the bibliography added to this chapter consists mainly of recent publications.

Description: Thick sediments (maximum of at least 4900 m) infill the Natal Valley, which lies between the coast of South Africa/Mozambique and the Mozambique Ridge. Eight physiographic provinces are recognised in the valley, and their boundaries can be related to well-defined sediment thickness or facies discontinuities within the basin infill. The acoustic stratigraphy of the area is established by reference to two regionally developed reflecting horizons, which, on the basis of previously published borehole and seismic data, are tentatively identified as mid-Cretaceous (McDuff) and mid-Cainozoic (Angus) hiatuses or important facies boundaries. Sedimentation began in the Natal Valley before mid-Cretaceous times and since then sediment dispersion has been strongly influenced by the disposition of large basement (? volcanic) highs: the approximately NE—SW Almirante Leite and Naudé ridges, and the larger N—S Mozambique Ridge. There is abundant evidence that sedimentation in the vicinity of these ridges has been current-controlled since at least mid-Cainozoic (Angus) times. The two main terrigenous sediment input points have been the Tugela and Limpopo rivers, which have large sediment cones adjacent to their mouths.

Description: Five separate exposures of oceanic basalts were dredged in the vicinity of the Peru-Chile Trench between 9° and 27°S latitude. Each dredge is dominated by abundant pillow basalts. Approximately ten of the most unaltered, glassy and fine-grained samples were selected for detailed chemical and petrographic analyses from each dredge area. All basalts recovered in the Peru-Chile Trench are olivine and quartz-normative tholeiites that are believed to have formed at the now extinct Galapagos Rise 30–50 m.y. ago. Detailed chemical analyses of the basalts, including major and selected trace and rare earth elements, indicate that considerable compositional variability exists both within each of the dredged areas as well as between areas. Most of the inherent chemical variability observed within particular basement sections appears consistent with the concept of temporal evolution of magma bodies at a former spreading center by shallow-level fractional crystallization involving primarily plagioclase and olivine. In contrast, important chemical differences between the dredged areas suggest compositional heterogeneities in the mantle source regions. Our results indicate that although shallow-level fractionation has brought about large changes in composition of basalts in each area, compositional trends are distinct and appear to reflect original mantle-derived compositional differences.

Description: Thirty-four ash layers of Pleistocene and Pliocene age from DSDP Site 192, northwestern Pacific Ocean, have been subjected to detailed chemical and optical study to evaluate: (1) the chemical and optical variability in glass shards from deep-sea ash layers, and (2) secondary changes brought about by prolonged exposure to seawater. Glass shards from approximately half of the ash layers studied were found to have uniform compositions which approach the precision of the microprobe chemical analyses, whereas the remainder are compositionally diverse (e.g., SiO2, variations of 5–15% among shards from the same ash layer) and appear to be the eruptive products of compositionally zoned magma chambers. Optical studies of glass shards confirm the absence of devitrification or the formation of pervasive secondary alteration products. By contrast, chemical studies suggest that the glass shards have experienced progressive hydration with possible minor ion exchange of K, Mg, Ca and Si. The hydration occurs rapidly and leads to a rather uniform water content of 4.5–5% after several hundred thousands of years exposure to seawater. Step-wise heating dehydration experiments, optical effects, and published'oxygen isotope studies indicate that the water of hydration is incorporated uniformly within the glass. Systematic chemical differences between electron microprobe analyses of glass shard interiors and corresponding bulk chemical study by atomic absorption lead us to postulate that glass shard margins have undergone a minor chemical exchange with major cations in seawater. They have gained 0.10–0.20 wt. % K20, MgO, and CaO while losing a corresponding amount of Si2O. Although the glass shards from DSDP Site 192 are hydrated and may have experienced subtle, surficial ion exchange, we stress that they are the most chemically representative samples available of magmas that were explosively erupted from volcanic arcs.