Mineralogy of polymetallic nodules and associated sediments from the Central Indian Ocean Basin

151

Marine Geology, 74 (1987) 151-157

Elsevier Science Publishers B,V., Amsterdam - - Printed in The Netherlands

MINERALOGY OF POLYMETALLIC NODULES AND ASSOCIATED SEDIMENTS FROM THE CENTRAL INDIAN OCEAN BASIN V. P U R N A C H A N D R A R A O National Institute of Oceanography, Dona Paula, Goa 403 004 (India)

(Received February 26, 1986; revised and accepted May 26, 1986)

Abstract Rao, V.P., 1987. Mineralogy of polymetallic nodules and associated sediments from the Central Indian Ocean Basin. Mar. Geol., 74: 151-157. Todorokite is the dominant mineral phase in the nodules of the northern Central Indian Ocean Basin. These nodules are characterised by a rough surface texture, are relatively rich in Mn, Cu and Ni, and are associated with radiolarian sediments rich in montmorillonite, chlorite and illite. $-MnO2 is the dominant mineral phase in the nodules of the southern Central Indian Ocean Basin. These nodules have a smooth surface texture, are relatively rich in Fe and Co, and are associated with pelagic clay-sized sediments comprising keels of foraminifera, zeolite and the clay minerals chlorite and illite. The formation of these mineral phases and their composition in the nodules may have been controlled by the properties of associated sediments and the nucleus of the nodule does not influence the oxide layer mineralogy.

Introduction B u s e r a n d G r u t t e r (1956) c o m p a r e d t h e X-ray p a t t e r n of m a n g a n e s e m i n e r a l s o c c u r r i n g in p o l y m e t a l l i c n o d u l e s w i t h t h o s e of s y n t h e t i c c o m p o u n d s a n d d e s i g n a t e d t h e s e as 10/k m a n ganite, 7/k m a n g a n i t e a n d $-MnO 2. B u r n s a n d B u r n s (1977) r e c o m m e n d e d t h e n a m e s of t h e c o m m o n l y o c c u r r i n g m a n g a n e s e m i n e r a l s tod o r o k i t e ( S t r a c z e k et al., 1960), b i r n e s s i t e ( J o n e s a n d Milne, 1956) a n d 5-MnO 2 as equival e n t s of 10/k m a n g a n i t e , 7/k m a n g a n i t e a n d 5MnO2; this h a s b e e n followed in this paper. I r o n m i n e r a l s h a v e n o t b e e n studied in detail, as t h e i r o n oxide in t h e n o d u l e s a p p e a r s to be a m o r p h o u s (Glasby, 1972). O t h e r a c c e s s o r y m i n e r a l s r e p o r t e d a r e quartz, f e l d s p a r a n d p r o d u c t s of v o l c a n i c a c t i v i t y s u c h as zeolite and clay minerals. C r o n a n a n d T o o m s (1969), G l a s b y (1972) a n d Siddiquie et al. (1978) r e p o r t e d t o d o r o k i t e f r o m 0025-3227/87/$03.50

n o d u l e s of t h e deep-sea b a s i n a n d ~-MnO2, a h i g h l y oxidised form, in n o d u l e s on s e a m o u n t s a n d in r e l a t i v e l y s h a l l o w w a t e r depths. L y l e et al. (1977) s u g g e s t e d t h a t the c h e m i s t r y of t h e nodules determines the mineralogy and that n o d u l e s r i c h in Mn, Cu a n d Ni c o m p r i s e todorokite. H a l b a c h a n d O z k a r a (1979) a n d C r o n a n a n d M o o r b y (1981) c o n c l u d e d t h a t t o d o r o k i t e rich nodules are associated with radiolarian sediments. A f t e r studies of n o d u l e s f r o m t h e Pacific a n d I n d i a n O c e a n s m a n y w o r k e r s h a v e c o n c l u d e d t h a t t h e t o p o g r a p h y , depth, r e d o x p o t e n t i a l a n d t y p e of s e d i m e n t s on t h e sea floor p l a y a m a j o r role in t h e f o r m a t i o n of m i n e r a l s in p o l y m e t a l l i c nodules. T h i s p a p e r r e p o r t s t h e m i n e r a l o g y of p o l y m e t a l l i c n o d u l e s a n d t h e i r r e l a t i o n to o t h e r p a r a m e t e r s s u c h as m o r p h o logy, c h e m i s t r y , n u c l e u s of t h e n o d u l e s a n d with the associated sediments from the Central I n d i a n O c e a n Basin.

© 1987 Elsevier Science Publishers B.V.

152

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153 layers were c a r e f u l l y peeled off from the nucleus. T h e oxide layers and the n u c l e u s were s e p a r a t e l y g r o u n d to - 6 2 pm size. T h e subsamples were s c a n n e d at 1° 2 0 min -1 on a Philips X-ray d i f f r a c t o m e t e r using nickelfiltered Cu-K~ r a d i a t i o n from 8 ° to 68 ° 2 0 (Figs.2 and 3). T h e subsamples were digested in a q u a regia and the chemical analysis has been c a r r i e d out on Atomic A b s o r p t i o n Spectrophot o m e t e r for Mn, Fe, Cu, Ni and Co. R e p r e s e n t a t i v e subsamples of e a c h grab sample were oven-dried at 60°C, and the s t a n d a r d m e t h o d of F o l k (1965) was adopted to determine the t e x t u r e of the sediments. T h e compon e n t s of the coarse f r a c t i o n were identified u n d e r a b i n o c u l a r m i c r o s c o p e and the < 2 pm f r a c t i o n was used for clay-mineral study. Orie n t e d samples were p r e p a r e d by p i p e t t i n g on a glass slide and these samples were s c a n n e d

Materials and m e t h o d s P o l y m e t a l l i c nodules and associated sedim e n t s were collected in the C e n t r a l I n d i a n O c e a n Basin at 1° spacing by P e t e r s o n and free-fall grabs. T h e cruise was p l a n n e d by the N a t i o n a l I n s t i t u t e of O c e a n o g r a p h y , Goa, to explore for and s t u d y the e n v i r o n m e n t of the f o r m a t i o n of p o l y m e t a l l i c nodules. Samples a l o n g a n o r t h - s o u t h t r a n s e c t o v e r a d i s t a n c e of a b o u t 1220 k m were selected to s t u d y r e g i o n a l variations. T h e s e comprised (1) nodules from s t a t i o n s 1-12; (2) b o t h nodules and sediments from s t a t i o n s 13, 14, 23 and 24; and (3) sediment samples r e c o v e r e d d u r i n g the cruise (Fig.l). R e p r e s e n t a t i v e nodules of a p p r o x i m a t e l y 4 cm size from e a c h s t a t i o n were w a s h e d with fresh water, oven-dried at 60°C, and t h e oxide

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Fig.2. X-ray diffractograms of the polymetallic nodules. R S = rough surface texture; S S = smooth surface texture. The number below the station indicates the depth at which nodules were collected. T = todorokite; D = MnO2; P h = phillipsite; Q = quartz; P L = plagioclose; m = montmorillonite.

Fig.1. Map showing sample locations and sediment distribution in the Central Indian Ocean basin.

154

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from 2° to 30 ° 2 0 at 2° 2 0 m i n - 1 on a Philips Xray diffractometer using nickel-filtered Cu-K~ radiation. Identification of the clay minerals follows the methods described by Carrol (1970) (Fig.4). Results The mineralogy, morphology and chemistry of the nodules is described for three distinct geographic areas. Area 1 nodules are from the siliceous ooze sediments and includes the n o r t h e r n portion of the n o r t h - s o u t h transect from Stations 1-8 and 13, 15, 16, 17, 23 and 24. Area 2 nodules are from pelagic clay sediments and consist of the southern part of the transect from stations 9-12, 14 and 25. Area 3 comprises stations 18-22 and contains the carbonate sediment samples recovered in the Peterson grab. Area 1 nodules are characterised by a rough surface texture and e a r t hy black colour. Wellcrystallised todorokite reflection and 5-MnO: reflection are p r e s e n t in the X-ray diffractograms. The intensity and crystallinity of todorokite increases with increasing depth (Fig.2). The accessory minerals present are quartz,

phillipsite and feldspar. High concentrations of Mn, Cu and Ni and the average Mn/Fe ratio of the nodules in this area is about 3.91. Area 2 nodules are characterised by smooth surface t ext ure and a brown colour. 5-MnO2 shows the highest reflection followed by very fragments from the ocean floor excepting the variation in relative mineral abundance (Fig.3). Following Hekinian (1971), it is concluded that the nucleus is a highly altered basic volcanic rock, probably basalt. There appears to be no difference in composition of the nucleus of todorokite and 5-MnOz-bearing nodules. The sediments and nodules in Area 1 are from greater depth (5100-5460m). They are yellowish-brown in colour. The coarse fraction contains siliceous radiolarians, sponge spicules, glassy flakes and micronodules. The latter are found at only one station (1.3). The clay poor reflection of todorokite. Quartz continues to be present in minor amounts as an accessory mineral. Fe and Co concentrations are higher and Mn, Ni and Cu concentrations are less as compared to Area 1 and the average Mn/Fe ratio of the nodules in this area is 1.6.

155 Q-I-I

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Fig.4. X-raydiffractogramof the 2-#msize clay. m= montmorillonite;I = illite; P h Q= quartz. The nucleus of the nodules in all the samples is a greyish-white rock fragment. The minerals in the nucleus identified from the X-ray diffractogram are plagioclase feldspar, chlorite, hematite and augite. This assemblage is similar to the one observed for fresh basaltic fragminerals present in the <2 #m fraction are montmorillonite, chlorite and illite. Montmorillonite in this region is the Fe-rich variety and occurs at 17.7 A on glycolation indicating an authigenic origin. The sediments and nodules in Area 2 are from relatively shallower depths (42004900 m). The sediments are brown in colour and contain keels of foraminifera, indicating their dissolution, and a high percentage of micronodules relative to Area 1. Illite, chlorite and zeolite are present in the <2 ~m-sized sediments. The sediments in Area 3 are from shallower depths and are light brown in colour. They contain calcareous oozes with a very little content of fine fraction which consists of chlorite. Discussion The formation of mineral phases in polymetallic nodules depends on the flux of metals and

=

philipsite;p l = plagioclase; C h

=

chlorite;

the redox potential of the environment. The redox potential is controlled by the rate of sedimentation and accumulation of organic carbon and these depend on the depth of the water and the biological productivity in the surface waters. Todorokite-rich nodules form at a lower Eh (+0.465V than ~-MnO2 (+0.562V)-rich nodules (Glasby, 1972). The most important prerequisite for the formation of todorokite (Mn-rich phase and poor in Fe) is the supply of Mn 2 + ions which are mobilised in the early diagenesis. The crystal lattice of todorokite can take up further divalent ions of transition metals such as Cu 2 +, Ni 2 ÷ and Zn 2÷ which have suitable ionic radii to occupy the exchangeable cation positions in the 10 A interlayer spacing of todorokite (Piper et al., 1984). Area 1 lies in the region of equatorial currents where the biological productivity is comparatively higher and influences the rate of deposition of organic carbon on the sea floor. The sediments in this region are siliceous ooze (present study and Udintsev, 1975). The water depths in this region are greater than 5100m, i.e., well below the CCD. The CCD plays a major role in the distribution of elements, because metals such as Fe, Mn and some Cu and Ni scavenged in surface waters by being adsorbed on carbonate surfaces were

156

released to the water column below the CCD, and these elements increase in decreasing carbonate flux (Greenslate et al., 1973; Halbach and Puteanus, 1984). Siliceous skeletons like radiolarians may store Cu and Zn (Marchig and Gundlach, 1981), and Ni is associated with opaline silica (Sclater et al., 1976). Micronodules which are important sources of Mn, Cu and Ni (Verges and Clinard, 1983) are observed in one of the stations (13). Their absence in other stations may be because of dissolution due to negative redox potentials. Therefore, Mn, Cu and Ni in the pore waters of sediments and the slight gradient in the redox potential caused by the decomposition of organic matter may be responsible for the mobilisation of metals and the formation of todorokite-rich nodules in Area 1. The Mn/Fe ratio plotted on the triangular diagram of Halbach et al. (1981) (modified from Bonatti et al., 1972) indicates that the nodules were formed in the early diagenesis. Well-crystallised montmorillonite in this region indicates that it was formed by the reaction between silica and iron (Lyle et al., 1977). Manganese nodules are poor in Fe, because the iron in the sediments was utilised for the formation of Fe-rich montmorillonite thus leaving free manganese to form todorokite. The earthy black colour of the nodules may be due to the enrichment of Mn, Cu and Ni. The nodules could develop a rough surface texture in the early diagenesis because they may be formed in the thin peneliquid layer (Halbach and Ozkara, 1979) of the sediments and the precipitated particles adding to the nucleus of the surface of the nodules under early diagenetic conditions may be larger. Halbach et al. (1975) calculated the particle sizes of nodules and found that the particle sizes of diagenetic nodules are larger ( > 700 A) than in hydrogenetic nodules. Area 2 nodules are comprised of only $-MnO2 and very poor todorokite reflections. The 5MnO2 phase consists of a mixture of colloidal particles of ~-MnO2 and amorphous iron hydroxide. The sediment in this region is brown pelagic clay and the depths are comparatively shallower (< 5000 m). The clay minerals iden-

tiffed are chlorite, illite and montmorillonite in order of abundance. The Mn/Fe ratio of 1.6 in the nodules of this region indicates that they were formed in hydrogenetic conditions (Halbach et al., 1981). The nodules may have accumulated by the precipitation of colloidal particles of ~-MnO 2 and amorphous iron hydroxide phases in contact with near bottom sea water. The colloidal particles of 5-MnO 2 distributed in the water column are able to enrich Co by specific absorption (Halbach et al., 1981). Some of the iron phase may have derived from the decomposition of skeletons and volcanic products. The Fe enrichment may give a brown colour to the nodules and the smooth surface of the nodule may be the result of the accumulation of smaller particles ( < 300 .~) (compared to diagenetic nodules) of colloidal nature to the nucleus or surface of the nodule from nearbottom sea water. The absence of nodules in Area 3 may be due to the high sedimentation rate and shallower water depths. Basaltic fragments occur in the nuclei of both the nodules containing todorokite and ~MnO2-rich phases implying that the nucleus does not influence the oxide mineralogy.

Acknowledgements The author is indebted to Dr. H.N. Siddiquie, Director, and R.R. Nair for their critical review of the manuscript and valuable suggestions. Grateful thanks are due to Department of Ocean Development, Government of India, for permitting me to publish these data.

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157 Carrol, D., 1970. Clay minerals: A guide to their X-ray identification. Geol. Soc. Am., Spec. Pap., 126:80 pp. Cronan, D.S. and Moorby, S.A., 1981. Manganese nodules and other ferromanganese oxide deposits from the Indian Ocean. J. Geol. Soc. London, 138: 527-539. Cronan, D.S. and Tooms, J.S., 1969. The geochemistry of the manganese nodules and the associated sediments from the Pacific and Indian Oceans. Deep-Sea Res., 16: 335 359. Folk, R.L., 1968. Petrology of Sedimentary Rocks. Hemphill, Austin, Texas, 177 pp. Glasby, G.P., 1972. The mineralogy of manganese nodules from a range of environments. Mar. Geol., 13: 57-72. Greenslate, J.L., Frazer, J.Z. and Arrhenius, G., 1973. Origin and deposition of selected transition elements in the sea bed. In: M. Morgenstein (Editor), Papers on the Origin and Distribution of Manganese Nodules in the Pacific and Prospects for Exploration. Inst. of Geophysics, Hawaii, pp.45-70. Halbach, P. and Ozkara, M., 1979. Morphological and geochemical classification of deep sea ferromanganese nodules and its genetic interpretation. In: La G~n~se des Nodules de Manganese. Coll. Int. C.N.R.S., 289: 77-88. Halbach, P. and Puteanus, D., 1984. The influence of carbonate dissolution rate on the growth and composition of Co rich ferromanganese crusts from the Central Pacific Seamount areas. Earth. Planet. Sci. Lett., 68: 73-87. Halbach, P., Ozkara, M. and Hense, J., 1975. The influence of metal content on the physical and mineralogical properties of pelagic manganese nodules. Miner. Deposita, 10:397 411. Halbach, P., Scherhag, C., Hebisch, U. and Marchig, V., 1981. Geochemical and mineralogical control of different genetic types of deep sea nodules from the Pacific Ocean.

Miner. Deposita, 16: 59-84. Hekinian, R., 1971. Chemical and mineralogical differences between abyssal hill basalts and ridge tholeites in the eastern Pacific Ocean. Mar. Geol., 11: 77-89. Jones, L.H.P. and Milne, A.A., 1956. Birnessite, a new manganese oxide mineral from Aberdeenshire, Scotland. Min. Mag., 31: 283-288. Lyle, M., Dymond, J. and Heath, G.R., 1977. Copper-nickel enriched ferromanganese nodules and associated crusts from Bauer Basin, Northwest Nazca Plate. Earth. Planet. Sci. Lett., 35:55 64. Marchig, V. and Gundlach, H., 1981. Separation of iron and manganese and growth of manganese nodules as a consequence of diagenetic aging of radiolarians. Mar. Geol., 40: M35-M43. Piper, D.Z., Basler, J.R. and Bischoff, J.L., 1984. Oxidation state of marine manganese nodules. Geochim. Cosmochim. Acta, 48: 2347-2355. Sclater, F.R., Boyle, E.A. and Edmond, J.M., 1976. On the marine geochemistry of nickel. Earth. Planet. Sci. Lett., 31: 119-128. Siddiquie, H.N., Dasgupta, D.R., Sengupta, N.R., Srivastva, P.C. and Mallik, T.K., 1978. Manganese-iron nodules from the Indian ocean. Ind. J. Mar. Sci., 7: 239-253. Straczek, J.A., Horen, A., Ross, M. and Warshaw, C.M., 1960. Studies of the manganese oxides: Todorokite. Am. Mineral., 45: 1174-1184. Udintsev, G.B., 1975. Geological-Geophysical Atlas of the Indian Ocean. IOC UNESCO, Pergamon, New York, N.Y., 151 pp. Verges, E.L. and Clinard, C., 1983. Ultra thin section study of the mineralogy and chemistry of manganese micronodules from the south Pacific. Mar. Geol., 52: 267-280.