Manganese Crusts Research Articles

Manganese nodule project economics: Factors relating to the Pacific region

This paper discusses and compares the main elements that determine the overall economics of nodule projects. The rarely examined issue of the impact on costs of processing nodules in different locations in the Pacific region is investigated. The authors then move on to an analysis of the sensitivity of nodule economics to changes in major cost and revenue elements. The paper then shifts from the quantitative to the more qualitative issues that may influence future seabed mining developments, including: the implications of the recently established 200 nautical mile Exclusive Economic Zones; the discovery of cobalt-rich manganese crusts; the prospects for nodule or crust mining for strategic reasons.

Marine Metallic Mineral Resources of the Pacific Basin

In the 1970s and 1980s, the ocean became a focus of attention for mineral producers and consumers. This paper surveys that trend with particular emphasis on several specific metals and types of resource deposit. In particular, the known and speculative details of manganese nodules, cobalt-rich manganese crusts, and polymetallic massive sulfides are discussed and analyzed in an economic context.

Potential of Pacific Ocean Nodule, Crust, and Sulfide Mineral Deposits

This paper summarizes the main physical and economic features of manganese nodules, cobalt‐rich manganese crusts and marine polymetallic sulfide deposits in the Pacific region. The most favourable areas for deposits of commerical interest are shown on a map of the Pacific. The gross metal sales values of hypothetical mines are compared to estimated size of world metal markets in the year 2000. It is suggested that a prudent policy for the major developed‐country governments is to support research on those marine minerals that appear to have the potential to become a major source of minerals in the next century.

Nd–Sr isotopic and REE constraints on the genesis of hydrothermal manganese crusts in the Galapagos

Manganese oxide crusts from the Galapagos spreading centre, interpreted to be of hydrothermal origin, have Nd isotopic compositions and rare-earth element (REE) concentrations that approach seawater values as the age increases. Numerous authors1,2 have cited hydrothermal processes along the mid-oceanic ridges to explain the composition of ‘metalliferous’ sediments. Other authors believe that these sediments, whether now located on a ridge axis or in some deep, were derived from hydrothermal activity near ridges and were modified during removal from these ridges3–6. Nd and Sr isotopic compositions as well as concentrations of REE and Cu, Ni and Co in hydrothermal Mn crusts of different ages from the Galapagos spreading area were determined to provide new information regarding the problem of the alteration behaviour with time of oceanic sediments. Our data suggest that hydrothermal crusts react with seawater and ultimately acquire the trace-element characteristics of hydrogeneous crusts.

Chemical weathering of Mn-garnets under lateritic conditions in northwest Ivory Coast (West Africa)

Lateritic profiles developed on Mn 2+-bearing metamorphic rocks in the Ziemougoula area (northwest Ivory Coast) permit the observation of chemical weathering of garnets, essentially spessartites. Two main stages are found in the process of weathering. In the lower part of profiles, spessartites alter principally to birnessite and the Al content of the parent garnet is leached out of the zone. Higher in the profile, garnets alter directly to lithiophorite; in this geochemical environment Al is no longer mobile. In such a layer, the earlier formed birnessite is transformed into nsutite along with minor cryptomelane. These latter minerals can locally evolve into ramsdellite or pyrolusite. The associated nsutite and lithiophorite are the principal phases of the hard manganese crust capping the hillrocks of the lateritic landscape of the Ziemougoula area.

10Be-dating of a manganese crust from Central North Pacific and implications for ocean palaeocirculation

One of the most promising applications of the new 10Be detection technique using accelerator mass spectrometry is the determination of growth rates of Mn-nodules and crusts1–5. Because the half life of cosmogenic 10Be (1.5 Myr) is of the same order as the time needed for a few millimetres of nodule material to grow, 10Be is a useful tool for unravelling such evolution during the late Tertiary, when most nodules are assumed to have started their growth6. Mass spectrometric measurement of a 10Be profile across the manganese crust VA 13-2 from the Central Pacific, using a tandem van de Gr£ a ff accelerator, yields growth rates of 2.7 and 4.8 mm Myr−1 for the layers of the crust accumulated between Recent and 6 Myr BP and between 6 and 11 Myr, respectively. Using these measurements as well as 230Th dating, we have been able to distinguish boundaries between zones in the crust with different structures and chemical compositions and to assign ages to them. These ages are 0.12, 2.9–3.4, 5.7–6.7, 7–9, 10–12 and 13–16 Myr BP. All of these boundaries apparently coincide with the times reported for Quaternary and late Tertiary palaeoclimatic events, suggesting that the crust growth has been strongly influenced by palaeoclimate.

Assessment of Cobalt‐Rich Manganese Crusts in the Hawaiian, Johnston and Palmyra Islands' Exclusive Economic Zones

Cobalt‐rich manganese crusts occurring within the 200 mile Exclusive Economic Zones (EEZs) of Pacific island nations may be of long‐term strategic importance to the major industrialized nations and represent an important long‐term development option for Pacific island nations.A resource assessment methodology is described and used to estimate the resource potential of cobalt, copper, nickel and manganese within the EEZs of the Hawaiian Archipelago, Johnston and Palmyra Islands. Results of this study indicate a resource of roughly 10 million tons of cobalt, 6 million tons of nickel, 1 million tons of copper and 300 million tons of manganese within the study area.Available data on manganese crusts and ocean bottom topography is inadequate for undertaking a full assessment of the resource potential of crusts within the EEZs of most Pacific island nations. Therefore it is recommended that to evaluate the potential of these nations a comprehensive research programme should be developed and implemented.

10Be-dating of a manganese crust from the central North Pacific and implications for ocean palaeocirculation : Segl, M. et al., 1984. Nature, Lond., 309(5968):540–542

10Be-dating of a manganese crust from the central North Pacific and implications for ocean palaeocirculation : Segl, M. et al., 1984. Nature, Lond., 309(5968):540–542

Constancy of oceanic deposition of 10Be as recorded in manganese crusts

Recent developments in the use of nuclear accelerators as high-energy mass spectrometers1,2 has renewed interest in the study of cosmogenic 10Be (half life 1.5 Myr) as a geophysical time tracer. The accelerator techniques enable us to measure 10Be with a sensitivity of 107 atoms or less. The potential age dating span with 10Be is extended back to at least 15 Myr BP, providing that the 10Be production and deposition rates remain constant or nearly constant over that time period. A knowledge of the long-term constancy in the 10Be deposition rate is thus very important. We report here the first detailed measurements of 10Be and 9Be in two ferromanganese oxide crusts from the sea floor of the equatorial Atlantic (sample no. K-9-21) and the North Pacific (SCHW-1D), each representing >10 Myr of accumulation history. We have found in these crusts that the deposition of 10Be and 10Be/9Be during the past 7–9 Myr has been constant. Averaged over time intervals of ∼1 Myr, the variation is within ±6%. Before that time, both crusts show similar significant deviations. The 7–9 Myr demarcation may be related to the reported late Miocene global abyssal circulation change in the ocean3.

Ferromanganese deposits off northeast Brazil (S. Atlantic)

Ferromanganese deposits, mostly manganese crusts, are common in elevations along the northeastern Brazilian continental margin. Association of the deposits with more or less altered basaltic rock can be observed. On the Pernambuco Plateau and Ceará Guyot, ferromanganese deposits occur associated with phosphatic material and nodules. The mineralogical composition of the ferromanganese deposits indicates a predominance of the manganese oxide phase δ MnO 2. Low contents of Mn and Cu are characteristic of their chemical composition. Fe and Mn in the deposits probably precipitated from the sea water.

The nature and composition of the acid-insoluble residue and hydrolysate fraction of manganese nodules from selected areas in the equatorial and S.W. Pacific

The composition of the acid-insoluble residue and hydrolysate fraction of manganese nodules along an equatorial Pacific transect between 16°30′N and 20°S at 134°W (including the Aitutaki Passage) as well as the Peru Basin has been determined. Variations in the average % acid-insoluble residue of the nodules along the transect are observed but are not sufficient to account for the variations in the bulk element compositions of the nodules. Variations in nodule composition between the equatorial Pacific high productivity zone and the S.W. Pacific low productivity zone therefore reflect genuine differences in the composition of the authigenic phase (hydrolysate fraction) of the nodules and not merely dilution by lithogenous material. X-ray diffraction analyses of the acid-insoluble residue of the nodules are not incompatible with a volcanic origin for this material. Barite is found in the acid-insoluble residue of nodules from the equatorial high productivity zone and is a host mineral for barium in these nodules. In the hydrolysate fraction of the nodules, high Mn/Fe rations and high Ni, Cu and Zn contents of the nodules are most pronounced in a fairly narrow band about 500 km wide on the margins on either side of the equatorial high productivity zone. The influence of biogenic processes (i.e. thein situ dissolution of siliceous tests in the sediment column) appears to be the main factor controlling this process. The enrichment sequence of elements in nodules due to this process lies in the approximate sequence Cu>Ni>Zn> Mn. Cu is therefore the most biogenically enriched element in nodules in the equatorial regions. The biogenic supply of Cu is about 5.5 times that of seawater in the zone of maximum productivity. Both the Mn/Fe and Cu/Ni ratios of the nodules vary systematically across the equator. Examinations of pre-existing data show that nodules from beneath the low productivity subtropical anticyclonic gyres of both the central North and South Pacific are characterized by Mn/Fe ratios of the order of unity and low Ni+Cu contents. The compositions of nodules in the North and South Pacific therefore appear to some extent to be mirror images of one another. Manganese crusts appear to be quite distinct from nodules in both their % acid-insoluble residue and composition of the hydrolysate fraction.

Carbonate nodules with manganese crust found in the Neogene Tertiary of theKameda oil field, Northeast Japan

Carbonate nodules with manganese crust found in the Neogene Tertiary of theKameda oil field, Northeast Japan

海成マンガン団塊の鉱石殻の厚さ

The diameters of ferromanganese nodules and their cores show different distribution patterns corresponding to their localities, i. e. marginal seamount, oceanic seamount, and deepsea basin. The ore-shell of nodule from the marginal seamounts is the thickest among them. This thick shell seems to be formed by the deposition of a large volume of materials supplied from the continent. The nodule from the deepsea basin has slightly a thick ore-shell as compared with that from the oceanic seamount. This facts may suggest that the elements of the manganese crust are supplied not only from the sea water but also from the deepsea sediments. The distribution of ore-shell thickness of the nodules consisting mainly of 10 A manganite coincides with the error function. On the basis of this nature, the total volume of ore-shells can be estimated within the error less than 10%.

Isotopic evidence for uranium exchange during low-temperature alteration of oceanic basalt

Measurements of uranium concentration and the 234U/ 238U activity ratio in oceanic basalts which have undergone low-temperature seafloor alteration indicate that uranium uptake is a pervasive occurrence but that the various phases involved behave differently with respect to this process. Palagonite exhibits uranium contents 8–20 times higher than unaltered glass coupled with low 234U/ 238U, suggesting ongoing preferential leaching of 234U. Altered crystalline interiors of several old basalts have 234U/ 238U > 1, indicative of recent uranium exchange with seawater. The data also provide evidence for uranium sources with 234U/ 238U higher than the seawater value of 1.14. Manganese crusts on basalts of a variety of ages have isotopic ratios indicating that they either are recent deposits or also have experienced continuing uranium exchange with seawater.

Determination of mercury in manganese nodules and crusts by cold-vapor atomic absorption spectrometry

Determination of mercury in manganese nodules and crusts by cold-vapor atomic absorption spectrometry

Deep-tow observations at the mounds abyssal hydrothermal field, Galapagos Rift

Small 5–20 m high mounds of lithified sediment were discovered in 1972 during a deep-tow survey 20–30 km south of the spreading axis of the Galapagos Rift, at a depth of 2.7 km. They were reexamined with an improved deep-tow instrument in 1976. The features are 20–50 m wide, conical in section and generally elongate in plan, and arranged in parallel chains that are commonly 1–2 km long, and aligned parallel to the rise flank's fault-block relief. Their rough sides, sloping at 35–45° from sharp crests, are encrusted with thick deposits of manganese oxide. Manganese crusts also occur on the sea floor in 10–20 m wide stripes of high acoustic reflectivity which extend discontinuously for several kilometers, generally on the crests of low rises (sometimes linking the cones and ridges of a chain). Although they are structures of the 30 m thick sediment blanket rather than of the volcanic bedrock, many chains and stripes overlie minor bedrock faults, and their plan patterns mimic those of minor faults and fissures exposed and mapped at the present spreading axis. These landforms and bands of mineralization are interpreted as the result of precipitation around the discharge vents of an active hydrothermal circulation in the oceanic crust, though no plumes of modified bottom water were found above them. They seem to form only after the basaltic crust has been completely buried with sediment, even along the scarps at fault-block boundaries which are the likely sites of discharge on younger crust. Their existence demonstrates the ability of localized discharges to vent through a thin blanket of pelagic sediment, and their shapes and distribution patterns emphasize the importance of zones of fracturing for controlling the site and scale of circulation in the bedrock.

Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N

Fifty acoustically positioned samples of fresh basalt were collected by the submersible Alvin from the median valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (FAMOUS) in the summer of 1974. The samples show regular compositional variations from the center of the rift valley (central lava flows) out to the rift valley walls (flank lava flows). The central lava samples show higher ratios of olivine relative to clinopyroxene and plagioclase phenocrysts and contain chrome spinel. Glasses of the flank lava samples are enriched in SiO 2 , TiO 2 , K 2 O, H 2 O, and FeO/MgO relative to central lava samples. Studies of the thickness of palagonite and manganese crusts indicate that the flank lava flows are considerably younger than the inferred spreading age of the crust on which they occur. Flank lavas are generally older than central lavas, but notable exceptions occur. The composition of the flank lava glass can be derived by the removal of approximately 29 wt percent of analyzed phenocrysts (in the ratio 5.7 plagioclase, 2.5 olivine, 1.8 clinopyroxene) from the central lava glass. In addition, other processes (possibly involving volatile transfer) must enrich the flank lavas in K 2 O, TiO 2 , and H 2 O. A model is proposed whereby this crystal fractionation occurs in a shallow, narrow (6-km-wide) magma chamber underlying the median valley. The chamber is compositionally zoned, and central lavas are fed from dikes tapping its hotter axial zone, whereas flank lavas are fed from the cooler, differentiated melt on the margins. The nature of the chemical variations in the lavas permits an estimate of the composition and thickness of the cumulates forming at the base of the chamber.

Surface densities of manganese nodules in the southern sector of the South Pacific

Abstract The surface density distribution of manganese nodules in the southern sector of the South Pacific Ocean has been recorded by reference to the complete underwater photographic collection of the U.S.N.S. Eltanin. High concentrations of manganese nodules (75–100% coverage of sea floor) are shown to occur in five main areas: the Southwestern Pacific Basin; the Tasman manganese pavement; the basin centred on 60° S, 100° W; the central region of the Drake Passage; and the crest of the Pacific-Antarctic Ridge (as manganese-encrusted boulders). High surface densities of manganese nodules on the sea floor result from a combination of factors such as low sedimentation rates and high bottom-current velocities. Sea-floor spreading has played an important role in controlling the distribution of massive manganese crusts and manganese nodules across the Pacific-Antarctic Ridge.

The distribution of U and Th in growth zones of manganese nodules

Growth zones and individual sublayers from one manganese nodule and three manganese crusts from an area south of Hawaii were analysed for U and Th by the delayed-neutron counting technique. The concentrations of uranium and thorium in the manganese nodule are highest in the outermost zone on top of the nodule, being the surface last exposed to sea water. In this zone U varies from 6.3 to a maximum of 8.3 ppm in different sublayers, decreasing to 5.0 ppm in the inner zones and 3.4 ppm in the outer zone last exposed to the sediment. Ferromanganese material scraped from the zone last exposed to the sediment which has low concentrations of Fe, relative to the zone last exposed to sea water, has also low U contents (2.7 ppm). Th concentrations are higher in the outermost zone on top of the nodule (40 to a maximum of 130 ppm) than in the zone last exposed to the sediment (about 20 ppm Th). Manganese crusts contain up to 9 ppm U in the outermost zones last exposed to the sea water. They also have higher concentrations of Th (up to 64 ppm) relative to the inner zones of the crust growing on altered andesitic rock, which contains about 8 ppm U and about 26 ppm Th as an average.

Hydrothermal manganese crusts from two sites near the Galapagos spreading axis

Manganese oxide crusts similar to those reported from the Mid-Atlantic Ridge rift valley by Scott et al. (1974) were dredged at two sites near the Galapagos spreading axis on ocean floor estimated from magnetic anomalies to be 2.4 and 0.3 m.y. old. Compared to the typical ocean-floor manganese deposits attributed to precipitation from seawater, the 2–6 cm thick manganese crusts reported here exhibit very low Fe/Mn and low 232Th/ 238U ratios, as well as lower transition metal and higher manganese concentrations. The manganese crusts were deposited several orders of magnitude faster than the more common hydrogenous nodules; this fact together with other geochemical characteristics and the geophysical environment suggests the manganese deposits reported here are of hydrothermal origin.