Geochemistry and origins of lacustrine ferromanganese nodules from the Malawi Rift, Central Africa

0016-7037/92/$5.00

Geochimica d Cosmochimica Ann Vol. 56, pp. 2703-27 I2 Copyright 0 1992 Pergamon Press Ltd. Printed in U.S.A.

Geochemistry

and origins of lacustrine ferromanganese from the Malawi Rift, Central Africa

+ 03

nodules

T. M. WILLIAMS’ and R. B. OWEN* ‘British Geological Survey, Applied Geochemistry Unit, Keyworth, Nottingham, UK *University of Malawi, Department of Earth Sciences, Box 280, Zomba, Malawi (Received March 12, 199 I ; accepted in revised form April 22, 1992)

Abstract-Ferromanganese nodules recovered from lOO- 130 m depth near Likoma Island, eastern Lake Malawi, have been analysed for some thirty-four elements by DC-arc optical emission spectrometry. The concretions routinely hold in excess of 50% Fe + Mn, although actual Fe/ Mn ratios appear to vary inversely with nodule size. Subcrustal values are recorded for Mg, Al, Si, K, Ca, and Ti. The abundance of these major elements is considered to primarily reflect the amount and mineralogy of detrital impurities within the nodule structures. Of the twenty-six analysed trace elements, Zn, Co, Pb, Ba, Y, La, V, Zr, Ag, Be, and Nb are present at levels exceeding their average crustal abundances, while subcrustal or subdetection limit values are recorded for Sr, Ni, Cu, Cr, SC, Rb, Ga, Li, B, MO, Cd, Bi, Sn, Ce, and Nd. The high enrichment factor noted for Ba (2 1.95), the limited enrichment of Co (5.53) and Zn (2.06), and depletion of Ni (0.62) and Cu (0.09 ) are characteristic of most lacustrine ferromanganese deposits and adequately distinguish the Lake Malawi nodules from their deep-ocean counterparts. While the ferromanganese deposits of the Malawi rift are predominantly of diagenetic origin, hydrothermal exhalations may significantly control the supply of elements such as Fe, V, and Be. This implies a need to extend existing classification systems to include nodules formed through the simultaneous precipitation of metals from two or more sources. The nodule sequences in the vicinity of Likoma Island are physically suited to economic exploitation, but fail to meet prescribed chemical criteria for Mn or Ni-Co-Cu ores. INTRODUCIION

as late as the Pliocene (CROSSLEY and CROW, 1980; CROSSLEY,1982). The geothermal flux through the lake floor is considerable, maximum values of ca 2.9 rrcal/cm*/s-’ having been recorded in the central basin area between Nkham Bay and Likoma Island ( VON HERZEN and VACQUIER, 1967). Hydrothermal brine exhalations are also common and exert a primary control on the supply of elements such as Si and F to the lake waters (KIRKPATRICK,1969). Syn-rift sedimentation of ca 1 mm/y ( SCHOLZand ROSENDAHL, 1988) has induced the accumulation of a diverse range of basin-floor sediments in Lake Malawi, including turbidites, diatomites, varved pelagic muds, homogeneous clays, and quartzo-feldspathic sands. The precise distribution of these lithofacies has been subject to considerable scrutiny (e.g., CROSSLEY,1984; JOHNSONand DAVIES,1989; OWEN et al., 1990; SCHOLZ et al., 1990) and appears to reflect the complex interaction of structural (Fig. 1) and palaeoclimatic controls. Documented ferromanganese deposits occur around Likoma, Mbenje, and Nkhotakota (Fig. 1) and consist mainlv of oolitic aeothites overlying qua&o-feldspathic sands (WILLIAMsand OWEN, 1990). Nodular nontronite sequences have also been observed over relatively pure diatomites in the southeast arm of the lake ( MULLER and FORSTNER,1973; OWENet al., 1992). In all instances, the deposits are confined to depths of 80-160 m, leading WILLIAMSand OWEN ( 1990) to conclude that their authigenesis requires ( 1) proximity to sub-littoral muds which act as a reservoir for mobile metals, (2) proximity to shallower, coarse-grained facies for the provision of suitable precipitation nucleii, and (3) oxic conditions immediately above the sediment/water interface to induce the precipitation of Fe, Mn, and trace elements held in the interstitial porewaters.

THE OCCURRENCEOF ferromanganese nodules in lake Malawi has been widely documented (e.g., MULLER and FORSTNER, 1973; WILLIAMSand OWEN, 1990; OWEN et al., 1992), but no comprehensive investigation of their geochemistry has previously been made. Such deposits offer a potential source of economically important trace metals (MERO, 1952, 1959; AMOS et al., 1973; ROTHSTEIN and KAUFMAN, 1973), and their evaluation may contribute to our knowledge of the regions natural resource base. In addition, the Malawi rift basin constitutes an unusual environment for freshwater nodule formation, its depth (>700 m) and tectonic setting in many ways appearing more analogous to ocean-ridge systems than to most lacustrine basins. The geochemical affinities between the Lake Malawi concretions and those from other sedimentary environments may, therefore, prove pertinent to the refinement of existing systems of nodule classification (e.g., PRICE and CALVERT, 1970; CRONAN, 1977). Here, we present data for thirty-four elements and make preliminary comparisons with ferromanganese deposits elsewhere. ENVIRONMENTAL SEITING

SAMPLING AND ANALYTICAL PROCEDURE

The 560 km long by 70 km wide waters of Lake Malawi occupy the southernmost trough of the great East African rit? system, forming the world’s fifth largest freshwater body by volume. The lake’s surface lies at an altitude of ca. 475 m, but with a maximum depth of 704 m, a considerable proportion of its floor lies below sea level. The basin’s origin is attributable to the most recent major episode of crustal rupturing and subsidence in eastern Africa, responsible for the formation of the Kenyan and Western rifts during the Miocene (GAUTIER, 1967; EBINGER,1989) and possibly influencing Malawi

During the period 1984-1989, approximately 200 gravity cores and 50 grab samples were collected from Lake Malawi as part of a basin-wide lithofacies mapping programme (OWEN and CROSSLEY, 1990). The corer consisted of a 2 m long by 5 cm diameter plastic tube weighted with a 50 kg drum about 50 cm from the upper end. This permitted retrieval of cores averaging 1.4 m in length. The survey highlighted the presence of oolitic and pisolitic hydrous oxide se2703

2704

T. M. Williams and R. B. Owen

FIG. 1. Relationships between rift structure and lithofacies distribution in Lake Malawi, as determined by a combination of coring, grab sampling, echo-sounding, and seismic methods. Structural cross sections are not to scale and relate only to surhcial sediment deposits. Inset (a) shows the location of sampling stations 207 and 208 for which geochemical data are presented. Inset (b) shows the regional position of Lake Malawi.

quences at numerous locations (Fig. I ). the purest of which lie at water depths of lOO- 130 m over the fault-controlled platform of the Likoma and Chisumulu islands. It is from this location that the nodules described here were collected. Nodules from the surlicial strata at sampling stations 207 and 208 (Fig. 1) were combined to form a single sample which was shaken and sieved mechanically to remove loosely attached elastic material. The sample was then graded into fine (~4 mm diameter) and coarse (>4 mm diameter) fractions for subsequent analysis by direct reading DC-arc optical emission spectrometry (DR). Approximately I g of material was used for each analysis. Sample excitation was carried out in an atmosphere of 25% oxygen/75% argon to minimise arc instability. A Hilger-Analytical E 1000 Polyvac spectrometer (incorporating some forty simultaneous monitoring channels with a first order wavelength range of 250-850 nm) was used to characterise the emission spectrum. Trace metal detection limits for the method have been calculated as between 0.01-l .O ppm for Li, Be, Rb, Ag, and Cd; l-10 ppm for B, V, Cr, Co, Ni, Cu, Zn, Ga, Sr, Y, MO, Sn, Ba, and Bi; lo-25 ppm for Zr, La, and Pb and >25 ppm for Nd, SC,and Ce. Precision was evaluated from a series of duplicate analyses in accordance with the methods of FLETCHER (1981). Of the elements present at concentrations exceeding the analytical detection limit, onlyV(27.2%),Zr(36.2%),La(49.1%),Cu(50.7%),andAg(41.2%) yielded variability indices of above 25% (quoted for the population average at 95% confidence).

With the exception of Mn, all major elements were determined as oxides and were subsequently corrected to allow the presentation of data in elemental form. RESULTS Physical Characteristics Individual nodules recovered from stations 207 and 208 range from 0.4- 14.0 mm in diameter. Physical dissimilarities are evident between the fine (<4 mm) and coarse (>4 mm) size fractions, the former appearing darker, more polished, haematitic, less brittle and best resembling the Lake Malawi ‘nontronites’ described by MULLER and FORSTNER ( 1973 ). Both size fractions were typically extracted simultaneously from well-mixed, heterogeneous deposits. There is thus no evidence that variations of nodule size/morphology are related to localised differences of depositional environment or stratigraphic position. In cross-section, all nodules show concentric lamination and are centred by quartzo-feldspathic nucleii of 0. I - 1.O mm diameter. The nodules do not conform to any single morphological group within pre-existing clas-

Mn nodules from Lake Malawi, East Africa sifications for freshwater concretions PRICE, 1977).

(e.g., CALVERT and

2705

growth is clearly inferred. The average Mn-enrichment factor ( 166) is analogous to the global mean for nodules from deep ocean (ca. 170) and continental shelf environments (ca. 159), but is much higher than that observed in most lacustrine systems. The corresponding value for Fe (6.5 ) is high relative to the average for any other depositional setting. Values for Mg, Al, Si, K, and Ca show accordance with those for ferromanganese deposits from virtually all other sedimentary environments, remaining at subcrustal levels throughout. These elements are predominantly incorporated into nodule structures in association with detrital impurities such as clay minerals ( CALVERTand PRICE, 1977 ) . Given a sufficient knowledge of local sediment mineralogy, the size of this clay fraction can be estimated from the abundance of Al and Si ( CALVERTand PRICE, 1970). A total aluminosilicate presence of ca. 25% is indicated by the peak values of 1.7% and 7.5%, respectively, reported here. Of the major cations, only K is held at high levels relative to other lacustrine and marine nodules. However, the maximum values of K, Mg, and Ca (2.09%, 0.86%, and 2.24%, respectively) are all greater than could be held solely in silicates. Lattice substitution by these elements in 10 8, man-

Major Element Composition Geochemical data for two <4 mm and two >4 mm diameter nodule samples are presented in Table 1, from which the mean values for eight major elements have been extracted, normalized to upper crustal abundance (TAYLOR, 1964)) and plotted alongside comparative data for deep ocean, nearshore marine, and mid-latitude lacustrine concretions (Fig. 2). The most abundant elements, Fe and Mn, constitute over 50% of the total mass of all samples: a much greater proportion than the ca. 20-404 generally recorded elsewhere (Tables 2 and 3). Ratios of Fe/Mn for <4 mm nodules are high (>2.9) relative to the >4 mm phases (<1.8), consistent with the more “haematitic” appearance previously noted for the finer fraction. No mechanistic explanation for this inverse relationship between Fe/Mn and nodule size has yet been provided, but a progressive adjustment of the relative adsorption /precipitation rates of the two metals during nodule

Table 1: Abundance and average enrfchment factors for 8 major elements (X) and 26 trace elements (ppm) in ferromanganese nodules from Lake Mafawf. Results are given for the ~4 mm and > 4 mm size fractffs of two i~ndent subsamples (A and B). taken folfowfng the amafgamation of ‘pure’ nodufe material from stations 297 and 206. The elemental mtks Fe/M and CorNi are also given for each sample. Average crustal values are derived from Taylor (1964).

c

f=e Mn

2 Bi K Q Tl Li Bs B V Cr co Ni (Ir 2-n & Fb Sr Y Zr hb Mo Ag Cd Sn Ba La Fb Bi SC Ce Nd Fe/M-r

4mm (A) 39.69 13.66 0.48 1 .oo 4.30 1.34 0.84 0.03 2.4 9.6 n.d 262 20 141 26 2 222 10 13 197 83 142 56 sd Ad nd 9217 76 48 n-d n.d n-d nd 2.90 5.04

+ 4mm (B)

> 4mm (A)

> 4mm (B)

39.69 11.17 0.76 1.50 7.50 2.09 1.35 0.14

34.02 19.19 0.74 1.10 5.40 1.04 2.24 0.13

33.62 19.20 0.66 1.70 5.50 1 .oo 2.24 0.09

3.0 14.6 n-d 304 60 96 34 4 150 13.40 14 263 97 364 59 nd 0.1

4.9 6.0 n-d 263 2 127 56 6 106 16.60 24 462 123 199 38 rid 1.4 nd nd 9221 114 56 nd n-d

5.2 6.3 n-d 220 0 76 55 6 96 10 24 454 137 122 50 n-d n.d n-d n-d 9224 112 34 n.d n-d n.d nd 1.73 1.41

nd nd 9216 75 46 n.d nd nd nd 3.54 2.62

nd

nd 1.77 2.26

Crustal Ave. 5.63 0.10 2.33 8.23 28.15 2.09 4.15 0.57 20 3 10 130 100 20 70 50 70 16 90 370 30 160 20 1.5

0.07 0.15 0.20 420.. 30 10 0.17 20 70 30

Ave. Enrfchmt. 6.52 166.42 0.30 0.16 0.20 0.65 0.40 0.17 0.19 3.06 2.02 0.20 5.53 0.62 0.09 2.06 0.73 0.21 0.95 3.67 1.29 2.56

8.92 21.95 3.10 4.60

2706

T. M. Williams and R. B. Owen

ganites has been observed frequently (e.g., FRONDELet al., 1960; BROWN, 1971; CALVERTand PRICE, 1970) and the co-variability of Mg and Ca with Mn is consistent with similar influences here. Some Ca may also be incorporated in the form of manganoan calcite and calcareous shell debris (e.g., CALVERTand PRICE, 1977). The limited enrichment of Si ( ~7%) is significant as it precludes any analogy with the Lake Malawi “nontronites” described by MULLERand FORSTNER ( 1973 ). The Ti enrichment factor (0.17 1) is typical of that for lacustrine nodules (e.g., MANHEIM, 1965; VARENTSOV, 1972) but is an order of magnitude lower than the average for oceanic phases (Table 2 )

ative of enrichment by coprecipitation rather than by adsorption ( ROSSMAN, 1973), authigenic psilomelane ( BaMn fr”O,s[OH], ) probably constituting the principal BaMn phase. Mean values of ca. 9200 ppm are five times greater than reported for most deep ocean nodules (Table 2), but are exceeded by some nearshore marine and lacustrine concretions (e.g., VARENTSOV,1972; ROSSMAN,1973 ). Enrichment of Y, La, and associated REEs is uncommon in freshwater ferromanganese deposits (Table 3), although high combined Y-REE values (ca. 1000 ppm) are common in primary (nondiagenetic) marine precipitates ( GLASBY, 1973). Substitution of Fe by Y has also been observed in authigenic phosphates ( FOMINAand VOLKOV, 1969). In the Malawi Rift, selective adsorption by Mn-oxides provides the best explanation for the observed Y enrichment, with highest values consistently prevailing in the Mn-rich (>4 mm) fraction (Table 1) . Enrichment of V and Zr is rare in lacustrine nodules. Here, both elements show some tendency to concentrate in Fe-rich fractions, although significant amounts of Zr may also reside in resistate impurities (NICHOLLSand LORING, 1962). Adsorption of Ag by poorly crystalline Mn oxides has been widely reported (e.g., ANDERSONet al., 1973; CHAO and ANDERSON,1974) but enrichment in ferromanganese nodules has previously only been observed in the marine environment (e.g., RILEY and SINHASENI,1958; MERO, 1965). The maximum Ag value recorded here ( 1.4 ppm) may, therefore, be considered highly significant, exceeding the crustal average by a factor of 20. While a zero Ag value is reported for one subsample, the metal may plausibly be present at concentrations too low for detection by DR methods. No comparable data for Be and Nb have formerly been published.

Enriched Trace Elements

Depleted Trace Elements

Eleven analysed trace elements show enrichment relative to mean upper crustal values (Fig. 3). Substantial accumulation of Zn, Co, and Pb has been noted in concretions from most environments (Tables 2 and 3). Their average values in the samples described here ( 144 ppm, 110 ppm and 46 ppm, respectively) are typical of coastal shelf and lacustrine nodules (average 310 ppm Zn, 125 ppm Co, 65 ppm Pb), but appear low relative to most deep ocean deposits (average 7 10 ppm Zn, 2980 ppm Co, 860 ppm Pb). Highest Co levels occur in those samples most enriched with Fe, consistent with accumulation by Fe-controlled sorption (e.g., PIPER et al., 1979) rather than lattice substitution in 10 A manganites as proposed by CALVERT and PRICE (1970, 1977) and CRONANand THOMAS ( 1972). The covariation of Zn with Fe is consistent with the preferential sorption of Zn*+ to ferric oxides (e.g., ROBINSON, 1981). The high valency Pb4+ ion may substitute for Mn4+ in bMnOz (GOLDBERG, 1963) and also for Fe3+ in FeOOH (BURNS, 1965). There is little evidence of preferential Pb accumulation in the most Fe-rich sample fractions as noted by CALVERTand PRICE ( 1977). At least 20-fold enrichment of Ba is recorded for all analysed samples. The limited variation of Ba content with nodule size (and hence specific adsorption surface) is indic-

Some seven analysed trace elements occur at abundances falling below their respective crustal means, while a further eight proved completely undetectable by DR methods (Fig. 4). Marginal depletion of Sr is apparent over the sample population as a whole (0.95 X crust), but it is actually enriched (to 1.3 X crust) in the Mn-rich (>4 mm) fraction. The affinity of Sr to Mn in hydrous oxides is well documented (e.g., CALVERTand PRICE, 1970) and, like Ba, it probably enters the 10 A manganite lattice by substitution for Mn, Ca, or Mg ( FRONDEL et al., 1960; STRACZEKet al., 1960). A similar bias towards Mn oxides has been noted for Ni (CALVERTand PRICE, 1977)) which yields an average enrichment factor of 0.62 in the Malawi samples, rising to 0.80 if the Mn-rich (~4 mm) fraction is considered independently. However, the maximum value of 56 ppm (Table 1) remains low compared to the >5000 ppm levels recorded in most oceanic nodules (e.g., MERO, 1965; AHRENS et al., 1967) and several freshwater deposits (e.g., ROSSMANand CALLENDER, 1969; CRONANand THOMAS, 1970 ). Values for Cu in the range 2-6 ppm (Table 1) are among the lowest ever recorded in ferromanganese nodules (see Tables 2 and 3 ) , depletion being especially pronounced in the most Fe-rich fractions.

-

Cont shelf

-

Mid-lat lakes

-

Deepsea

.1 Fe

Mn

Mg

Al

Si

K

Ca

Ti

Element

FIG. 2. Average crust-normalized abundance of eight major elements in ferromanganese nodules from Lake Malawi sampling stations 207 and 208, with comparative data for deep sea, continental shelf, and mid-latitude lacustrine environments. See Tables 2 and 3 for sources of comparative data.

Fe

Includes 1977).

Bl Sc Ce Nd Fe/Mn Co/N1

Fb

La

BP

sn

Ag cd

MO

Mg Al SI P K Q TI LI Bo B V Cr Cc Nl Qi al cg Rl Sr Y Zr N,

Mn

Element

data

(1970),

1.31 0.96

1270 6

4960

of Glasby

0.86 0.46

70 14

1620

Mero

Ahrens

1.06 3.16

2610

5030

460

600 10 10150 3210 560

1.06

15.61 14.62

Sea-mounts

(1965),

0.60 0.52

440 6 7 2.7 2760 160 840 6 9

490

290 11

650 310 520

930

660

27 53 13 3350 6340 3920 660 10

11.96 19.76 1.71 3.06 6.32 0.23 0.75 1.96 0.67

Paclflc

630 70 316C 3260 1160 640

20.76 15.76 1.69 3.27 9.56 0.10 0.67 2.96 0.42

Atlantic

440 14 2420 5070 2740 610

3.18 0.63

3.60 11.40

13.30 15.12

IndianOcean

et al (1967)

0.04

0.69 _ _.

347c 6410 670

11.87 17.17

Plateaux

and

Cronan

1.23 0.47

4000 3060 610

19.15 15.51 1.62 3.10 8.62 0.22 0.64 2.53 0.64

(1976,

1.02 0.47

2560 5400 3700

17.27 16.76

0.96 0.61

410 6 9 2 2010 160 860 6.3 10

620 310 640

270 550 10 2960 4680 2560 710 10

15.61 16.17 1.62 3.10 8.62 0.22 0.64 2.53 0.64

Active ridges Abyssal Plains GlobalAve.

Table 2: Abundance and average enrichment of selected elements in the worlas principal oceans and submarine settings. Values are given in % for major elements, and in ppm for trace elements. Average crustal values are derived from Taylor (1964).

1.5 0.07 0.15 0.2 420 30 10 0.17 20

370 30 160

10 130 100 20 70 50 70 16

5.63 0.10 2.33 6.23 26.15 0.10 2.09 4.15 0.57

CruslalAve

274.60 65.71 60.00 13.50 4.76 5.33 66.00 46.23 0.50

2.20 10.33 4.00

27.00 4.23 0.10 149.36 69.10 51.20 10.10 0.55

2.77 170.25 0.76 0.36 0.31 2.13 0.31 0.61 1.13

Ave Enrlchmt.

2.30 1.31

24

98 23 84 47 17 135

22.78 9.90 0.43 1.69 10.90 0.89 1.45 1.38 0.29

Gulf of Riga

3.91 0,29

18

0.12 2.87

43

3090

55

42

230 77 17 60

3.92 30.19 1.67 2.26 2.26 0.35 1.03 5.57 0.21

LochFyns

40 770 26 55

166 18 64 261 37

4.45 0.10

28.84 6.79 1.04 1.65 5.56 1.14

Black Sea

7.54 2.00

27

1000

10

30

300

10 10 60 40 40 50

35.63 4.73 0.45 1.32 7.65 0.29 0.17 1.21 0.09

Swedish Lakes 36.15 2.68 0.23 1.11 8.68 0.22 0.06 0.37 0.06

FinnishLakes

2.06 1.30

2912 29 2551

10

72 59 34 69

58 34 34 26 12 1112

0.30

0.26 1.82

15.14 7.25

UK Lake Dist.

and Volkov (1973).

13.39 3.25

50

10 10 130 40

and

for trace

marine

in ppm

in shallow

and

of Manheim (1965) Varentsov (1972, 1973) Sevast’yanov and Price (1970) Gorham and Swaine (1965) and Rossman

1.80 0.21

38

2500

130

150 10 180 750 48 80

22.47 14.03 0.56 1.54 7.88 0.89 0.75 1.22 0.13

Baltic Sea

Includes data (1967) Calvert

Ag Cd Sn Bs La Ftl Bi Sc ce Nd FelMn ColNi

MO

Fe Mn Mg Al Si P K Ca Ti Li Be B V Cr CY Ni Q m cg Fb Sr Y Zr r&

Average

elements.

Element

settings.

lacustrine

and average enrichment of selected elements Values are given in % for major elements, crustal values are derived from Taylor (1964).

Abundance

Table 3:

2.27 0.46

10328

38

148

24 116 239 26 324

0.14 1.17

20.78 9.15 0.25

L. Michigan

1.06 9.10

12

10300

8 2 110 12 4 161

20.00

420 30 10

1.5

10 130 100 20 70 50 70 16 90 370 30 160

5.83 0.10 2.33 8.23 26.15 0.10 2.09 4.15 0.57

23.33 21.94 0.08 1.15 4.89 0.33 0.97 0.44

Cruslal Ave.

Eningi-Lampi

21.00 45.11

3.50

14.60 0.96 88.30

0.60 0.45 1.02 0.44 0.44 2.08 0.93 0.31

6.66

0.17 0.16 4.70 1.02 0.41 5.95

4.72 98.69 0.11 0.14 0.23 2.51 0.22 0.22 0.34

1.13 0.15 6.72 4.14 0.60 1.30

3.35 159.47 0.42 0.22 0.23 8.63 0.51 0.76 0.32

Ave Mar. Enr. Ave. Laks Enr.

2709

Mn nodules from Lake Malawi, East Africa

ppm Sn), plus the prevalence of very weakly mineralized porewaters. The DR detection limits for Ce and Nd (500 ppm and 200 ppm, respectively) are well in excess of the levels generally associated with hydrous oxide deposits (Tables 2 and 3). DISCUSSION Mechanisms of Nodule Growth Morphologically similar ferromanganese concretions may form through numerous mechanisms including direct precipitation from seawater (e.g., RILEY and CHESTER, 1971), BeVCo~YZrbbAgbF!bBa Element

RG.3. Average crust-normalized abundance of elements showing enrichment in ferromanganese nodules from Lake Malawi sampling stations 207 and 208.

The elements Cr and SC are not systematically enriched in ferromanganese nodules from any environment (e.g., MANHEIM,1965; VARENTSOV,1972) and the average Cr enrichment factor reported here (0.205 ) is actually higher than that for many analogous deposits elsewhere (Tables 2 and 3). Notably, Cr values are up to 30 times greater in the Ferich (~4 mm) fraction than in the larger, Mn-rich phases (Table 1 ), suggesting either preferential sorption by ferric oxides ( KRAUSKOPF,1956) or the derivation of Fe, Cr and other Fe-related elements such as V from a common (possibly hydrothermal) source (e.g., BOSTROMet al., 1971). Of the remaining detectable elements, Ga (0.73 X crust) and Rb (0.21 X crust) are probably held in detrital impurities such as clays and feldspars. While the Ga values vary markedly between subsamples, the maximum of ca. 19 ppm (approximating the crustal mean) appears high in comparison to the < 14 ppm levels previously observed in oceanic and lacustrine ferromanganese deposits (e.g., MANHEIM, 1965; MERO, 1965). Ratios of K/Rb are high (ca. 1000) relative to the underlying sediments at stations 207 and 208, apparently confirming the presence of K in authigenic oxides as well as in detrital material. Comparative data for Li have not been published, but the prevalence of low values (2-5 ppm), coupled with undetectable B, is consistent with a strictly limited clay mineral impurity within the analysed samples. Several of the ‘undetectable’ elements are typically enhanced in concretions from other environments, notably MO which is more enriched than any other element in oceanic nodules (average 274.6 X crust) and is reported at up to 50 ppm in freshwater concretions from Scandinavia ( MANHEIM, 1965 ) . The average Cd enrichment factor is 60 in deep sea nodules (Table 2) and levels of up to 10 ppm (ca. 66 X crust) have been noted in Swedish lakes (MANHEIM, 1965). While no reliable data are available for B, Bi and Sn in lacustrine nodules, values of 290 ppm (29 X crust), 26 ppm (ca. 47 X crust), and 2.7 ppm (ca. 13.5 X crust), respectively, are reported for Pacific and Indian ocean samples ( MERO, 1965; AHRENS et al., 1967; CRONAN, 1976). Null or zero values for these elements may partially reflect their high DR detection limits (5 ppm MO, 2 ppm Cd, 10 ppm B, 2 ppm Bi, 5

diagenesis of Fe and Mn in surlicial sediments (e.g., WILLIAMS and OWEN, 1990), leaching of metals from tholeiitic or basaltic extrusions at sites of submarine volcanism (e.g., CORLISS, 1971; BERTINE, 1972; BOSTROMet al., 1972), and the precipitation of metals from hydrothermal fluids (e.g., ZELENOV,1964; DEGENSet al., 1973; MULLERand FORSTNER, 1973). In the Malawi Rift, the principal model of nodule genesis has been subject to considerable contention. MULLER and FORSTNER( 1973 ) and CALVERTand PRICE( 1977 ) have intimated that Fe may precipitate in the open waters of the lake due to the presence of a marked redoxycline in the permanently stratified water column (see ECCLES,1974). More recently, WILLIAMSand OWEN( 1990) have proposed a simple diagenetic model, relating the formation of goethitic nodules to the postdepositional reduction, diffusion, and near-surface precipitation of Fe and Mn in sediments overlain by oxic waters. From the data presented here, the extensive enrichment of both Fe and Mn (as opposed to just Fe) appears consistent with the dominance of the latter process. While open-water Fe precipitation has been invoked to explain the occurrence of numerous oolitic deposits elsewhere (e.g., MANHEIM, 1961; CALVERT and PRICE, 1977), it lacks a mechanism for enriching Mn relative to Fe such as occurs during early sedimentary diagenesis ( KRAUSKOPF, 1957;

Li B Cr Ni CuChWSrMoCdBn

Bi ScCeM

Element RG, 4. Average crust-normalized abundance of elements present at subcrustal or subdetection limit concentrations in ferromanganese nodules from Lake Malawi sampling stations 207 and 208. Elements in the latter class include MO, Cd, Bi, SC, Ce, and Nd (detection limits 5 ppm, 2 ppm, 2 ppm, 25 ppm, 225 ppm and >25 ppm respectively), for which the precise degree of enrichment/depletion relative to continental crust cannot be reliably determined.

2710

T. M. Williams and R. B. Owen

STRAKHOV, 1966; SEVAST’YANOVand VOLKOV, 1967). Consequently, the concretions formed in this manner are characteristically much poorer in Mn (~5%) than those reported here (e.g., MANHEIM, 1965; GORHAM and SWAINE, 1965). Controls on Nodule Distribution In addition to the controls ofdepth and sediment lithology outlined by WILLIAMSand OWEN ( 1990), the distribution of ferromanganese nodules in Lake Malawi may be closely determined by ambient rates of sedimentation. In marine systems, relatively pure nodule sequences are generally associated with sites of unusually low (x1-3 mm/ 10 3y) detrital deposition (e.g., EWING et al., 1971; HORN, 1972; CRONAN, 1976). Around Likoma Island, the quartzo-feldspathic sediments underlying much of the ferromanganese sequence are interpreted as fluvial and colluvial sands, probably deposited during the last major ‘low stand’ of Lake Malawi between ca. 500-350 BP (OWEN et al., 1990). Since submergence, further sedimentation over these sands has been inhibited by locally rapid currents (ca. 1 km/h), in a manner possibly similar to that causing low elastic deposition and concomitant ferromanganese nodule development in the Antarctic Circum-polar Current Zone (WATKINS and KENNETT, 197 1). The submerged topography around Mbenje is equally conducive to the intensification of currents. Local scouring influences may therefore explain the presence of analogous nodule deposits at this location (WILLIAMSand OWEN, 1990). Influence of Tectonic Setting The role of tectonism and attendant hydrothermal activity has been stressed in several studies of rift-hosted authigenic metal deposits (e.g., BOSTROMet al., 197 1; DEGENS et al., 1973; RENAUT et al., 1986), occasionally prompting comparisons with the larger geothermal systems of the Red Sea (e.g., DEGENSand Ross, 1969; BISCHOFF,1972) and Pacific Ridge Province (e.g., BISCHOFFand SAYLES,1972). In the Malawi rift, MULLERand FORSTNER( 1973 ) equated the distribution of nontronite sequences with the location of anomalous basin-floor heat flows ( VON HERZEN and VACQUIER, 1967) and proposed that hot springs may constitute the source of Si for the subsequent coprecipitation of Fe-Si phases. Most available data indicate that tectonic setting exerts only a second order control on the composition of Lake Malawi’s ferromanganese deposits. For example, levels of Ti, B, and chalcophiles such as Ni, Co, Zn, and Cu (all of which are enriched in many hydrothermal precipitates) are an order of magnitude lower than reported for nodules from the Red Sea and Pacific Ocean fracture zone (e.g., BACKER and SCHOELL,1972; SKORNYAKOVA,1979). However, a limited tectonic control is indicated by the prevalence of unusually high ratios of Fe/Mn ( 1.73-3.54), exceeded in other freshwater nodules only under circumstances of very low (~5%) Mn assimilation (e.g., MANHEIM, 1965). Analogous ratios in Mn-rich (> 10% Mn) samples are most common in the vicinity of active ocean ridge systems, where the excess Fe is almost certainly derived from volcanism (BERTINE, 1972; CRONAN, 1976). A similar explanation may therefore plau-

sibly apply to the very high (ca. 40%) Fe levels in some Malawi rift concretions. Hydrothermal influences are further signified by the unusually high levels of V (220-304 ppm) and Nb (38-59 ppm) in all analysed samples. The former is known to be strongly enriched in active ocean ridge deposits in association with a broad group of elements with strong Fe affinities such as Ti, Sr, Ba, Zn, and Co (e.g., GLASBY, 1970; GOODELL et al., 1971; BISCHOFF and PIPER, 1979; SCOTT et al., 1972; CRONAN, 1977), but is characteristically held at subcrustal levels in lacustrine ferromanganese nodules (e.g., GORHAM and SWAINE, 1965; MANHEIM, 1965; ROSSMAN and CALLENDER,1969). The occurrence of F-rich brines at, or very near, sites of nodule accumulation (KIRKPATRICK, 1969; MULLER and FORSTNER, 1973) may be instrumental in the unusual (ca. 3-fold) enrichment of Be, inducing the conversion of poorly reactive phases such as Be(OH)* to more mobile fluoroberyllate. This interpretation is consistent with the prevalence of locally enhanced (>2 pcal/cm2/s-‘) lake-floor heat fluxes (VON HERZENand VACQUIER,1967) in the vicinity of the Likoma nodule province. Economic Significance The potential of ferromanganese nodules as an economic source of Mn has been recognised for at least four decades ( MERO, 1952). More recently, observations of concretions containing percentage levels of Co, Ni, and Cu has prompted considerable interest amongst mining companies seeking alternative ores for these metals (e.g., MERO, 1972; AMOS et al., 1973; ROTHSTEIN and KAUFMAN, 1973). In industry, the large specific adsorption surfaces and unusual charge characteristics of ferromanganese nodules have proved valuable for the removal of heavy metals from crude oil prior to refinement ( WEKZ and SILVESTRI,1973), and the high porosity (50-60% pore space) of certain concretions has led to their use as a matrix for gas absorption ( ZIMMERLY,1967). It should be emphasised that the present study incorporates limited data for only one locality, and that conclusions regarding the economic potential of rift-hosted ferromanganese deposits are, at best, tentative. Economically significant deposits in deep ocean environments also display marked physical and compositional diversity ( MERO, 1977) , making the selection of “standard” criteria for comparison highly problematic. Basic physical criteria for establishing the viability of ferromanganese nodule deposits have been outlined by KAUFMAN (1974) and MERO (1977), and include a basinfloor slope angle of < lo%, a nodule yield of >5 kg/m* and a gangue impurity of ~20%. These conditions are met in at least two localities (totalling ca. 40 km2) of the Likoma Island nodule province (where oolithic sequences attain 40 cm thickness and 95% purity) and possibly over western lakefloor areas near Mbenje and Nkhotakota (WILLIAMS and OWEN, 1990). Samples from stations 207 and 208 routinely fail to meet the threshold chemical assays of 20% Mn, 1% Ni, 0.8% Cu, and 0.2% Co proposed for the identification of ore-grade sequences (KAUFMAN, 1974). BENNETT( 1987 ) has suggested

Mn nodules from Lake Malawi, East Africa

that restricted enrichment of first row transition elements may result from very rapid nodule growth which, while exerting little control on the incorporation of predominantly coprecipitated elements such as Ba (as seen here), serves to reduce the unit mass concentration of all adsorbed metal species. Around Likoma, a rapid nodule growth rate is evident from the presence of concretions of up to 14 mm in diameter over sands which were submerged by a major transgression as recently as 500-150 y BP (OWEN et al., 1990). For comparison, average accumulation rates for Ni-Co-Cu rich deep sea nodules have been calculated at ca. 2.0-28.0 mm/ lo6 y (Ku, 1977). SUMMARY

AND CONCLUSIONS

1) The geochemical data presented confirm earlier reports (WILLIAMS and OWEN, 1990) that the ferromanganese deposits around Likoma Island have formed primarily through early Fe-Mn diagenesis in a similar fashion to concretions from most other lacustrine and marine shelf environments (e.g., CALVERT and PRICE, 1970; VARENTSOV,1972 ) . 2) The analysed nodules are clearly distinguishable from those of tectonically stable lakes (e.g., MANHEIM, 1965; CORHAMand SWAINE,1965; ROSSMANand CALLENDAR, 1969; ROSSMAN,1973 ) by their partial derivation of certain elements (e.g., Fe, V, Be) from nonsedimentary sources. This hydrothermal influence is likely to recur in other intracontinental rift basins such as Lakes Tanganyika and Baikal. Existing nodule classification systems may therefore require modification to account for concretions with a “dual” diageneticlhydrothermal origin. 3) The Likoma setting appears inadequate for the formation of economic Mn or Ni-Co-Cu ores, but the occurrence of higher grade deposits in areas of slower nodule growth cannot be discounted. The value of the Lake Malawi ferromanganese nodules as a source of very rare elements such as Ag may also warrant further consideration.

Acknowledgments-Thanks are due to the Malawi Government Department of Fisheries for the use of the research vessel Ethelwyn Trewuvas during sampling. Financial support was provided by the University of Malawi and Amoco Oil Co. Ltd. Editorial handling: J. I. Drever

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