Manganese nodules: controversy upon controversy D. S. Cronan

Manganese nodules have been the subject of controversy ever since they were first discovered on the deep ocean floor during the Challenger Expedition just over one hundred years ago and described in 189 1 by J. Murray and A. F. Renard. Some of the early disputes have now been settled, but others have arisen to take their place and provide a stimulus for continuing debate on the deposits.

Distribution Nodules occur in shallow near-shore areas and lakes, as well as over much of the ocean floor, but it is only in certain parts of the deep ocean that they are sufficiently abundant to form potential ore deposits (figure 1). Their distribution is governed by a number of factors. Perhaps most important is the need for low rates of sedimentation of land-derived material and organic remains from the surface layer of the oceans, in order that the concretions can grow to appreciable size without being buried. It is for this reason that they are most abundant in the central areas of the oceans far from land, and are perhaps best developed in the northeastern equatorial Pacific (figure 2) where mining company interest has focused to date. However, even within these areas their distribution is very patchy, and is influenced by local environmental factors such as bottom current activity which can lead to the erosion and redistribution of sediments, causing alternate burial and exposure of nodules. The presence of nuclei suitable for ferromanganese oxide precipitation to be initiated is also an important factor in determining nodule distribution. Most nodule nuclei are composed of fragments of volcanic rock and thus nodules tend to be abundant in submarine volcanic areas. However, in the area of mining company interest in the Pacific, fragments of pre-existing nodules often serve as nuclei for new concretionary growth, thus increasing the overall grade of the deposits relative to those with barren volcanic nuclei.

Sources of elements One of the earliest controversies concerning manganese nodules was on the source of the elements that they contain. J. Murray[ 11 thought that they were derived from the alteration of submarine volcanic rocks, while his co-author, A. Renard, proposed the neptunist argument that they were formed from elements in seawater which were ultimately derived from the continents. Other suggested sources have been submarine volcanic springs and diagenesis (figure 3). Debate on this subject continued for several decades with the neptunists 12, 31 and vulcanists [4] alternately propounding their arguments. However, as a result of detailed study of the large numbers of nodules collected during the post-war expansion’ in oceanography [5, 6, 71 it became apparent that the metals in nodules do not accumulate from just one source, but that manganese and associated elements on the deep ocean floor from any source are potential contributors to forming manganese nodules. The problem has thus now become to determine which of the possible sources is most important in supplying elements to any particular suite of nodules. This has become an additional subject of controversy in its own right.

Morphology Nodules exhibit a bewildering array of shapes, sizes, and forms [81. Most tend towards sphericity or oblateness, but much more complex forms are common. Potato, saucer, grape, and

D.S.Cronan,Ph.D..M.I.M.M.

Is a graduate in geology of Durham University and after postgraduate studies in marine geology and geochemistry at Oxford was awarded his Ph.D. at Imperial College, London, in 1967 for a thesis on manganese nodules. After further research in this field in North America. he returned to Imperial College in 1973 to take up direction of marine geochemistry in the Applied Geochemistry Research Group at Imperial College, where he has initiated research on a wide range of geological problems related to undersea mineral deposits.

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hamburger shaped are terms that have sometimes been employed to describe nodule morphology in the absence of more geometrically explicit descriptions. Some nodules vary in morphology from one side to the other. This feature is common in discoidal nodules from the area of mining company interest in the northeastern equatorial Pacific I91. These deposits appear to be immobile within the sediments, and have a smooth upper side in contact with seawater, and a coarse lumpy underside in contact with the underlying sediments (figure 4). There are compositional differences between the tops and bottoms of these nodules with Ni, Cu, and Mn highest on their undersides but depleted on their uppersides, where Fe, Co, and Pb are more abundant. W. Raab [91 attributed the compositional differences between the tops and bottoms of such nodules to the formation of the deposits by submarine volcanic activity followed by the dissolution of their uppersides in contact with seawater undersaturated in the depleted elements. Other authors such as S. Calvert and N. Price 1101 have suggested a more plausible mechanism to explain the differences, in which growth processes are different on the tops and bottoms of the nodules, the former being precipitated from seawater and the latter from the interstitial waters of the underlying sediments. However, this explanation comes up against a major problem in that the differences between the tops and bottoms of the nodules noted by Raab do not apparently extend to their interior layers, which are more uniform in composition all round [Sl. If the tops and bottoms of the nodules are accreting materials from different sources at the present time, the relative homogeneity of their interiors would suggest either that the most recent phase of deposition is atypical or that the interiors have undergone a post-depositional chemical reorganisation which has smoothed out original compositional variations.

Internal structure Under the reflected light microscope, as well as showing typical depositional and erosional features such as concentric layering and unconformities 1111, the interiors of manganese nodules display a great variety of small-scale structures. The latter have been variously interpreted as also being original depositional features, or the result of post-depositional reorganisation, or the remains of organisms. A commonly occurring structure is columnar cusps orientated with their long axes in the radial plane of the nodules (figure 5). D. Cronan and J. Tooms [ 121 considered this structure to result from the post-depositional reorganisation of nodule interiors with the migration of metals to new centres of nucleation. The generally better development of the cusps towards the interiors of the nodules described by Cronan and Tooms, coupled with independent microscopic evidence of recrystallisation and replacement of nodule interiors I 11, 131, would tend to support this hypothesis. The possibility of post- depositional migration and segregation of metals within nodules is of great importance because it means that certain elements could become concentrated within parts of nodules high enough to be of ore grade, even though the nodule as a whole might not be.

Growth rates Growth rates of nodules have caused a great deal of controversy. Evidence built up over a number of years, based on the decay of radioactive isotopes, tends to suggest that most nodules grow

very slowly, in the order of a few millimetres per million years [ 141. However, D. Heye 1151 has shown that this slow growth is not uniform and that periods of more rapid growth (up to 50 mm/IO6 years) alternate with periods of no growth at all. More recently, the whole concept of growth in the order of millimetres per million years has been challenged by several workers. C. Lalou [ 161 has criticised the interpretation of radioactive decay patterns in nodules in terms of rapid growth. Furthermore, Lalou and others have reported excess *‘OTh in the nuclei of some nodules which led them to suggest that these nodules grew

Figure 1 Manganese nodules in ‘ore’ abundance on the ocean floor.

rapidly. However, such thorium could have been introduced from seawater through cracks, and may not therefore be an original feature of these nuclei. Nevertheless, in some cases there is unequivocal evidence that nodules can grow rapidly. Nodules which have accreted around naval shells, spark plugs, and iron nails have been found on the sea floor, and in lakes nodules have been found to form around beer cans. It is evident, therefore, that a rapid rate of growth of nodules is quite feasible, and would depend on the rate of supply of the elements that they contain.

rich in Co but low in Ni and Cu; those from continental margins are alternately rich in Mn or Fe and low in most other metals; those from mid-ocean ridges are rich in Fe; and those from the abyssal sea floor are rich in Ni and Cu but are low in Co (Table 1). It is the last group, therefore, that is of potential economic value, for there is general agreement that Ni and Cu are the two elements in nodules of greatest commercial interest. However, within the abyssal environment ( > 3500 m) nodule composition varies considerably. Large areas of the deep ocean floor contain nodules with less than one per cent or so each of Ni and Cu and thus are unlikely to be of any economic interest, at least in the foreseeable future. It is only in a few widely scattered areas that the Ni and Cu exceed about 2-3 per cent combined, the minimum value required for the deposits to be a worthwhile economic proposition. These areas include parts of the northern tropical Pacific, the south Pacific, and the central Indian Ocean (figure 2), with by far the greatest abundance of proven ore-grade nodules occurring in the northern tropical Pacific.

TABLE I. AVERAGE CONCENTRATIONS OF Mn, Fe, Ni, Co AND Cu

(in wt per cent). IN MANGANESE NODULES FROM DIFFERENT PHYSIOGRAPHIC SElTINGS, from D. S. Cronan [17].

Seamounts Continental Mid-Ocean Abyssal Margins Ridges Sea Floor

Mn 14.62 38.96 15.51 16.78

Fe 15.81 1.34 19.15 17.27

Ni 0.351 0.121 0.306 0.540

co 1.15 0.011 0.400 0.256

cu 0.058 0.082 0.08 1 0.370

Origin of ore grade nodules A full understanding of the origin of ore grade nodules has not yet been achieved, but is of considerable importance as it would help us to predict in what areas of the deep sea floor as yet not fully explored, additional ore grade nodules might be found. Some workers believe submarine hydrothermal activity to be important

r,yura L UI~~~IUULIV~ UT nnown ore-graae manganese noaules In tne oceans.

Compositional variability It has been evident for some time that manganese nodules are not uniform in composition throughout the oceans. Regional

in the genesis of ore grade nodules 181. Others consider the extraction of metals from seawater to be of greater importance 118, 191.

variations in the abundance of the economically important elements Ni, Cu, Co, and Mn have been reported by several authors, together with variations in the concentrations of other elements too. For example, nodules from seamounts tend to be

Data collected recently by the writer on manganese concretions formed in association with submarine hydrothermal activity in the Galapagos region suggest that metals supplied by hydrothermal processes are largely precipitated very close to the

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hydrothermal vents, and are strongly fractionated from each chemical reactions in the uppermost layers of the sediments, other over short distances on the sea floor. Thus, hydrothermal leading to the formation of manganese nodules. The enrichment

from buried sediments

sources of metals from submarine volcanism

Figure 3 Sources of metals added to manganese nodules.

activity would be unlikely to be responsible for the relatively uniform enrichment of elements such as Ni and Cu in nodules over wide areas of the ocean floor such as occurs in the northeastern equatorial Pacific. A more plausible mechanism to account for this enrichment involves the selective extraction of metals from seawater. It is of significance that the two main occurrences of ore-grade nodules reported to date, those in the northeastern equatorial Pacific and the central Indian Ocean (figure 2) have certain features in common. Firstly, they occur on the margins of the equatorial zone where high biological productivity, and thus large-scale sinking of organic remains, is

I I Fe,Co from seawater

Figure 4 Morphological and compositional differences between the tops and bottoms of manganese nodules in the northeastern equatorial Pacific ‘ore zone’.

thought to occur. Secondly, they are associated with siliceous ooze sediments below the depth at which organisms composed of calcium carbonate dissolve. Thirdly, they contain todorokite, one of the two principal minerals in nodules, as their principal mineral phase and which has a structure suitable to accommodate enriched Ni and Cu. Such nodules may form as a result of the extraction of metals from the surface waters by organisms and their transport to the sea floor where they are liberated into the bottom waters and interstitial waters of the sediments as the organisms dissolve. The metals are then available to take part in 82

manganese nodule (approx. 100 times magnification).

of Ni and Cu in the deposits would come about firstly as a result of an enhanced rate of supply of these metals to the sea floor via

the sinking organisms, and secondly, from the uptake of these metals into the manganese nodules in an environment conducive to the formation of tokorokite in the nodules, the mineral phase which can accommodate them best.

Economic potential The potential economic value of deep sea manganese nodules is one of the most controversial aspects of these deposits at the present time. J. Mero [201 considered that they contain sufficient supplies of many metals to last for thousands of years at present rates of consumption, and furthermore, that the deposits are forming faster than they could be mined. However, other estimates suggest that under present conditions only a fraction of the nodules on the sea floor contain economically mineable concentrations of the metals of interest. For example, the ‘French Group’ [ 2 11 have calculated that potentially recoverable nodules in the part of the North Pacific that they have surveyed contain

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to support a mining operation, although the 2.27 per cent and 1.8 per cent respectively of Ni and Cu proposed by J. T. Kildow et al. I221 is gaining acceptance. On the basis of figures such as these, J. Frazer 1231 estimated that there are between 14 and 56 first generation mine sites which can currently be. identified; A. Archer [24] estimated 44; and A. Holser 1251 estimated 80. D. Pasho and J. McIntosh [26] have estimated that there is a 50 per cent chance of there being more than 30 mine sites in the northeastern equatorial Pacific alone and D. S. Cronan and S. A. Moorby [271 suggested that the central Indian Ocean also contains sufficient nodules of ore grade to warrant more detailed exploration. From estimates such as these, Archer [24] has concluded that the quantities of Ni and Cu that would become available from manganese nodule mining are likely to be neither enormously greater nor enormously less than remain to be mined on land.

Estimates of numbers of possible mine sites must take into account potential problems in actually mining the deposits, such

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A Single sromountr Figure 6 Distribution of manganese nodules and major topographic features in the northeastern equatorial Pacific ‘ore zone’.

only 100 times the current annual world consumption of Ni and 8 times the annual world consumption of Cu. Several other workers have attempted to assess the economic potential of manganese nodules on the ocean floor on the basis of data in the public domain, and, although these estimates differ somewhat from each other, they are all more conservative than those proposed by Mero.

There are several problems in calculating reserves of manganese nodules. Firstly, many of the data on which resource estimates are made were collected for other purposes, and certain assumptions have to be made before they can be used. Secondly, there is no universal agreement as to what grade and abundance of nodules actually constitutes a mine site. Many workers have taken the minimum abundance of nodules required to support a mining operation to be 10 kg/m*. However, the ‘French Group’ consider that this value is too high. Similarly, there is no firm agreement on average and cut-off grades of Ni and Cu necessary

as the degree of ruggedness of the sea floor. Much of the area of greatest economic interest in the northeastern equatorial Pacific lies between the Clarion and Clipperton Fracture Zones (figure 6) where a rolling abyssal hill topography prevails. However, within this region there are some quite hilly areas which would make mining difficult [281. Furthermore, the future of nodule mining rests not only on technological problems being solved, but on the legal regime under which the mining will take place being clarified. This has been the subject of successive meetings of the Law of the Sea Conference, as well as discussions on unilateral action within the United States, but no agreement had been reached at the time of writing.

There can be little doubt that manganese nodule mining will take place at some time in the future, but when is uncertain. Some estimates put 1983-84 as the earliest possible date. In the meantime, however, it is of the utmost importance to refine our knowledge of manganese nodules by more detailed exploration

83

and evaluation in all oceans, so that when mining does start the true extent of the resource can be gauged.

Acknowledgment Original work by the writer and co-workers referred to in this review was financed by the Natural Environment Research Council.

111 Murray, J. and Renard, A. F. Deep Sea Deposits, Report of the Scientific Results of H.M.S. Challenger, 1873-76. Her Majesty’s Stationery Office, London. 189 1.

[21 Goldberg, E. D.J. Geol, 62,249,1954. 131 Manheim, F. T. ‘Manganese-iron accumulations in the shallow

marine environment.’ Symposium on Marine Geochemistry, University of Rhode Island Occasional Publication No. 3. 1965.

141 Bonatti, E. and Nayudu, Y. Am. J. Sci., 263,17, 1965. 151 Skomyakova, N. S., Andrushchenko, P. F., and Fomina, L. S.

Okeanologiya, 2, 1962. 161 Arrhenius, G., Mero, J., and Korkisch, J. Science, 144,170,1964. [71 Cronan, D. S. ‘The geochemistry of some manganese nodules and

associated pelagic deposits.’ Ph. D. thesis, University of London. 1967.

[81 Raab, W. and Meylan, M. Morphology, Chapter 5 of Marine Manganese Deposits, in G. P. Glasby (ed.), Elsevier, Amsterdam. 1977.

[91 Raab, W. Physical and chemical features of Pacific deep sea manganese nodules and their implications to the genesis of the nodules. In D. R. Horn (ed.) ‘Ferromanganese Deposits on the Ocean Floor’, National Science Foundation, Washington, pp. 3 l-49,1972.

1101 Calvert, S. E. and Price, N. B. Marine Chem., 5,43,1976. 1111 Sorem, R. K. and Fewkes, R. H. Internal Characteristics, Chapter

6 of Marine Manganese Deposits, G. P. Glasby (ed.). Elsevier, Amsterdam. 1977.

[I21 Cronan,D.S.andTooms, J. S. DeepSeaRes., 15,215,1968. [131 Dugolinsky, B. ‘Chemistry and morphology of deep sea

manganese nodules and the significance of associated encrusting protozoans on nodule growth.’ Ph. D. thesis, University of Hawaii. 1976.

1141 Ku, T. L. (1977). Rates of Accretion, Chapter 8 of Marine

Manganese Deposits, G. P. Glasby (ed.). Elsevier, Amsterdam, 1977.

1151 Heye, D. Wachstumsverhiiltnisse von Manganknollen. Geol. Jb. E5,3,1975.

1161 Lalou, C. (1978). ‘Rates of accumulation of manganese nodules and associated sediment from the equatorial Pacific.’ Internat. Monograph on Geol. and Geochem of Manganese. I Varentsov (ed.). Hungarian Acad. Sci. (in press). -

1171 Cronan, D. S. (1977). Deco-sea nodules: distribution and geochemistry. Chapter 2 of Marine Manganese Deposits. G. P. Glasby (ed.). Elsevier, Amsterdam. 1977.

[181 Greenslate, J. L., Frazer, J., and Arrhenius, G. Origin and deposition of selected transition elements in the seabed. In Papers on the origin and distribution of manganese nodules in the Pacific and prospects for exploration. M. Moraenstien (ed.). Hawaii Institute OfGeophysics: 1973.

,

[ 191 Cronan, D. S. ‘Geological and geochemical factors determining the variability of marine manganese nodules.’ Proc. 3rd Oceanology International, Brighton, England. 1975.

[201 Mero, J. L. ‘The Mineral Resources of the Sea’. Elsevier, Amsterdam. 1965.

1211 Bastien-Thiry, H., Lenoble, J. P., and Rogel, P. Energy and Mining Journal 178,86, 1977.

[221 Kildow, J. F., Bever, M. B., Dar, V. K., and Capstall, A. E. ‘Assessment of Economic and Regulatory Conditions Affecting Ocean Minerals Resource Development’. Report to the U.S. Debartment of the Interior. 1976. M.I.T. (Unpublished).

[231 Frazer, J. Z. Marine Mining, 1. 103, 1977. [241 Archer, A. A. ‘Resources and’potential reserves of nickel and

copper in manganese nodules’. Group of experts meeting on sea bed mineral resource assessment, United Nations, 1977 (in press).

[251 Holser, A. F. ‘Manganese nodule resources and mine site availability’. Professional Staff Study, Ocean Mining Administration, U.S. Department of the Interior, Washington D.C. 1976 (Unpublished).

[261 Pasho, D. W. and McIntosh, J. A. ‘Recoverable nickel and copper from manganese nodules in the northeast equatorial Pacitic- preliminary results’. C.I.M. Bulletin 69, No. 773, 15, 1976.

[271 Cronan, D. S. and Moorby, S. A. ‘Preliminary results of renewed investigations on manganese nodules and encrustations in the Indian Ocean’. UNESCAP Tech. Bull. 2. 118. 1976.

[281 Andrews, J. E. ‘Aspects of the geologic .setting of manganese noduledeposits’. UNESCAP Tech. Bull. 2, 1, 1976.

Endeavour, New Series Volume 2, No. 2.1978 (C Pergamon Press Ltd. Printed In Grwt Britain)

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