environments and processe osf manganese deposition 92_roy

Econom ic Geology Vol. 87, 1992, pp. 1218-1236

Environments and Processes of Manganese Deposition SUPRIYA ROY

Department of Geological Sciences, Jadavpur University, Calcutta - 700032, India

Abstract

Concentration of manganese in solution and its deposition takes place by redox-controlled processes in a variety of modern and ancient geologic and geochemical environments. Mod- ern Mn deposition occurs predominantly in deep-sea areas rather than shallow-water domains. Although deep-sea sedimentary deposits dominate, hydrothermal contribution of Mn to the ocean system may be substantial. Mn deposition from hydrothermal solutions at or near sea-floor-spreading centers and less commonly in island-arc areas is known. In addition, near- and far-field dispersion of Mn from vent sites is also substantial. Such distributions are controlled by the flow rate and egress temperature of the solution and the residence time of Mn in seawater. Thus, even in sedimentary deposit domains, at least partial derivation of Mn from a hydrothermal source is possible. Sedimentary Fe-Mn crusts on older volcanic substrates on seamounts form by hydrogenous deposition of metal concentrated from terrigenous sources in the mid-water column, oxygen-minimum zones. Thus, the presence or absence of volcanic rocks is not a clear indication of whether sedimentary Mn deposits, particularly in the ancient geologic record, are the result of a totally terrigenous or a totally volcanogenic source. Abys- sal Fe-Mn nodules are considered to form from a basin water (hydrogenous) and/or pore water (early diagenetic) supply of metals, but in most cases the extent of supply from either of the sources is unknown. The metal incorporation mechanisms of free-moving nodules is little understood and it is possible that in most cases both sources contribute to the nodule composition. Therefore, no nodule should be considered as totally hydrogenous or totally early diagenetic based only on its bulk composition. The determined growth rate giving only an average value cannot by itself reveal the growth history of the nodules. Biological participation, directly or indirectly, controls Mn deposition. The stratified Black Sea demonstrates the concentration of Mn in solution in an anoxic zone, its advection toward the redox interface, and its precipitation in an oxygenated condition. Similar stratified basins are contemplated for ancient Mn deposition in shallow-wate_r basin-margin areas. Geologic and geochemical signatures indicate that during sea-level highstands, stratified basins formed in which Mn was concentrated in solution in the anoxic part. Corresponding transgression led to the impinge- ment of the redox interface on the continental shelf, and precipitation of Mn oxides could take place across the interface during transgression-regression cycles. Offshore, in anoxic or dysaerobic conditions, Mn carbonate could form by early diagenetic reaction of Mn +2 with CO2 or HCOj produced by organic carbon oxidation. Critical Mn deposits occurring in transgressive, glaciogenic, and black shale-bearing ancient sequences support this palcoenviron- mental model for Mn deposition.

Introduction

DEPOSITION of manganese on different scales took place through much of geological history. It was sub- dued during the Archcan, developed considerably in the Proterozoic and the Paleozoic, proliferated in the Mesozoic, and reached its peak in the Cenozoic (Roy, 1988). An earlier attempt to synthesize the information available up till the late 1970s on the mechanism of formation of modern and ancient manganese deposits was made by Roy (1981). Since then the data base has expanded substantially through experiments, more detailed studies on modern depositional environments, and in-depth interpretation of geologic and geochemical signatures of ancient deposits. Consequently, the spectrum of knowledge has broad- ened considerably: genetic models are now more re-

fined and the commonalities and dissimilarities be-

tween modern and ancient processes of Mn deposition are better understood. A critical assessment of

these models based on updated information will be presented here. Manganese deposits produced by supergene enrichment are not included in this discussion.

Geochemical Constraints: The Basic Tenet

This discussion will be limited only to the geochemical controls of primary Mn deposition and the nature of the initial precipitates. These controls have been investigated both experimentally and in the natural conditions of modern depositional sites. Krauskopf (1957) emphasized the control of Eh-pH in the deposition of Mn and its fractionation with Fe in inorganic aqueous systems. However, solution and precipita-

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ENVIRONMENTS OF Mn DEPOSITS 1219

tion of Mn and Fe may be additionally affected by the presence of HCO•, SO• 2, HPO• 2, and dissolved organic matter in the system (Hem, 1963; Stumm and Morgan, 1970). Phase equilibria studies on the manganese-water system at room temperature and pressure by Hem (1963, 1972) confirmed that Mn is much more soluble than Fe. In this system the stability fields of MnO•, Mn•O3, Mn304, and Mn(OH)• in different Eh and pH were demarcated (cf. Bricker, 1965; quoted by Roy, 1981). It was also shown that the stability of MnCOa is a function of the presence of bicarbonate species. Johnson (1982) predicted that in the presence of a large amount of calcite, rhodochrosite is likely to be precipitated directly. When a sulfate complex is additionally introduced, the MnCOa stability range persists and a niche for MnS at very high pH and very low Eh is created. However, in modern basins the occurrence of MnCOa is rare and its formation is attributed to early diagenetic reactions (Lynn and Bonatti, 1965; Pederson and Price, 1982; Manheim, 1982). Early diagenetic formation of MnCOa has also been postulated for ancient deposits (Hein et al., 1987a; Force and Cannon, 1988). The formation of Mn sulfide in sedimentary-diagenetic conditions is unexpected.

Mn oxide-hydroxides undergo phase changes from the initial transient precipitates to more stable species. The solid phase in equilibrium with seawater was shown as Mn304 (hausmannite; Klinkhammer and Bender, 1980) and also as 3'-MnOOH (mangan- itc; Grill, 1982). Experimental studies showed that Mn +z oxidation produces MnaO 4 and /•-MnOOH (Stumm and Giovanoli, 1976), MnaO 4 (Murray et al., 1985) or MnaO4,/•-MnOOH, and •,-MnOOH (Hem, 1978; Hem and Lind, 1983). Either M%O 4 and/•- MnOOH (feitknechtite) together or Mn304, through an intermediate stage of•-MnOOH, convert by aging to 3'-MnOOH which is the most stable species formed by Mn +2 oxidation. The oxidation number of Mn in all these phases is not greater than +3. However, the minerals (todorokite, •-MnOz, birnessite) in modern Fe-Mn nodules and crusts, considered as initial precipitates, show an oxidation number approaching +4 (Murray et al., 1984; Piper et al., 1984). This has been explained by disproportionation reactions involving manganite that can produce Mn +4 oxides (cf. Hem, 1978; Hem and Lind, 1983). No relict mangan- itc, however, has so far been reported from Fe-Mn deposits in modern basins. In ancient unmetamor- phosed deposits, the initial mineralogy is modified by late diagenesis (Roy et al., 1990a). Through metamorphism the mineralogy may undergo a radical change through reactions between Mn rich and other phases controlled by P-T, fo and fco erasing much of the original signatures •f the d•positional processes (Roy, 1981; Dasgupta et al., 1989, 1990).

Biotic mediation of Mn and Fe chemistry may in-

volve changes in their oxidation states leading to dissolution or precipitation (for details, see Roy, 1981, and Baturin, 1988). Direct enzymatic oxidation by Mn-specific microorganisms is known (cf. Arthro- bacter, Metallogenium personaturn, Pedomicrobium (bacteria), Chadosporium, Cephalosporium, Coniothy- rium fuckelii (fungi), and Chlorococcum humicola (al- gae); Ehrlich, 1963; Perfil'ev and Gabe, 1965; Mar- shall, 1979). Direct microbial oxidation of manganese is known from cold and hot springs, fresh-water lakes and streams, soils, desert varnish, and deep sea. Even off-axis hydrothermal plumes in oceans show effects of bacterial scavenging of Mn, with or without Fe (Cowen et al., 1986). Stromatolitic Mn oxides from a Proterozoic sequence in Botswana contain Mn-encrusted microstructures, morphologically similar to stalked or budding Mn oxidizing bacteria (Lith- erland and Malan, 1973; Nagy, 1980). Rosson and Nealson (1982) recognized microbial action in oxidizing and reducing Mn in laboratory conditions. They cautioned, however, that even in cases of modern Mn precipitates causative bacterial involvement cannot always be unequivocally proved by study of bacterial morphology. Nevertheless, indirect biotic activity inducing changes in Eh and pH in the environment plays a major role in Mn accumulation in solution or precipitation. This aspect needs careful assessment in all cases.

Processes of Primary Manganese Deposition

Hydrothermal and sedimentary processes of primary manganese deposition are well recognized. Both processes occur in a variety of tectonic and geochemical regimes. Hydrothermal manganese deposits are usually small whereas the sedimentary deposits may attain large size. Traditionally, the sedimentary deposits have been classified into volcanogenic (or exhalative) and nonvolcanogenic (terrigenous) types, based on the presence or absence of volcanic rocks proximally or distally in the sequence. Modern sediment-hosted hydrothermal Mn deposits crusts, mounds, etc., at or near spreading centers or island arcs, however, are often indistinguishable from volcanogenic sedimentary deposits in their attributes and this dual nomenclature can be confusing. In addition the spatial association of volcanic rocks, in the absence of additional compelling evidence, does not establish exclusively either a volcanic or a hydrothermal source for the manganese (cf. modern oceanic hydrogenous Fe-Mn crusts on older volcanic substrates on seamounts). Similarly, only in certain specific locales (e.g., fresh-water lakes and shallow seas) can a totally terrigenous source for Mn deposition be identified. In large open basins such as oceans, the total Mn flux and the resultant deposits may have multiple sources for metals. To avoid confusion, therefore, the mechanisms of Mn deposition will be dis-

1220 SUPRIYA ROY

cussed under the broad heads of hydrothermal and sedimentary processes. Elaboration, when necessary, will be made within this framework. The supergene concentration process is beyond the scope of this study.

Models for Hydrothermal Deposition of Manganese Hydrothermal manganese deposits have been

characterized from both modern and ancient geologic settings. For modern settings the depositional processes are much better understood through direct assessment of the involved parameters, often with quantitative estimation. For ancient occurrences, the geochemical signature in the Mn deposits them- selves, and/or in the host rocks, and the tectonic framework provide clues for interpreting the pro- eesses.

Modern settings Modern examples of hydrothermal Mn deposition

are known both from continental and deep-sea realms. Ocean-floor hydrothermal deposits occur mainly at or around the axial region of spreading centers, subordinately at transform faults with large offsets (>50 km) and in off-axis midplate volcanic centers and island ares. At sea-floor-spreading centers, hydrothermal mineralizations have been modeled based on field data from the products and on laboratory experiments on seawater-rock interactions at different water/rock ratios and varying tem- peratures (Rona, 1978, 1984; Seyfried, 1987, and references therein).

Downward circulation of seawater through frac- tured oceanic crust (layers 2 and 3) leads to progressive warming and reduction (through volcanogenic heat sources) and an increase in acidity (through precipitation of Mg hydroxide, Mg hydroxy aluminosili- cates, and Mg hydroxysulfate hydrate and resulting protonation) that convert ordinary seawater to low- intensity (•<200øC) and high-intensity (>200 ø to ca. 400øC) hydrothermal solutions which are capable of leaching different elements from volcanic rocks (Rona, 1984). The metal-enriched hydrothermal solution is ultimately convected upward to the sea floor at or near the ridge crests. Deposition of different metals as sulfides or oxide-hydroxide is dependent on the decreased pressure and temperature and increased Eh and/or pH of the solution. Cooling of the hydrothermal solution may occur by conduction and/ or by subsurface admixture with ambient seawater (-•2øC) . during upwelling. Though modern deposits of Mn oxide and Cu-Zn-Fe sulfide often form from

the same hydrothermal system regardless of the spreading rate, these are in most cases spatially removed owing to the greater solubility of manganese.

Hydrothermal crusts with variable Mn/Fe ratios are reported from sea-floor-spreading centers. In the

TAG field at 26 ø N on the mid-Atlantic ridge (Mn/Fe -- 848, Scott et al., 1974; Mn/Fe -- 211-315, Toth, 1980) and the Galapagos spreading center, Pacific Ocean (Mn/Fe -- 553-5,000, Moore and Vogt, 1976; Mn/Fe = 202-493, Toth, 1980). Mn is strongly fractionated from Fe. In both cases, birnessite dominates in the crust. The hydrothermal crusts on the Juan de Fuca Ridge, are entirely composed of vernadite (& MnO2) and show a dominance of Fe over Mn (Mn/Fe -- 0.60-0.69 in five samples, Toth, 1980). All these crusts consistently show low concentrations of Ni, Cu, and Co in respect to sea-floor Fe-Mn nodules and hydrogenous crusts because of much faster growth, and therefore, less scavenging time. The variation in the Mn/Fe ratio in the different crusts can be explained either by the original ratio in the solution or by the relative efficiencies of the competing processes of precipitation in situ and advection of the metals away from the vents. The precipitation in situ of oxides with high Mn/Fe ratios indicates venting of low-temperature solutions with low flow rates. Hy- drothermal discharge in sediments and fixation of Fe in other phases may also affect Mn/Fe ratios in oxides (cf. Fe fixed in nontronite in the Galapagos hydrothermal mounds, in the Gulf of Aden, and the FAMOUS area on the mid-Atlantic ridge; Cann et al., 1977; Corliss et al., 1978; Hoffert et al., 1978; all references cited by Roy, 1981).

Substantial Mn is introduced into seawater by hydrothermal solutions in restricted oceans in the early stage of opening (the Red Sea) and in intermediate to fast-spreading centers (East Pacific Rise). In such cases, Mn deposition mainly takes place distally with respect to the hydrothermal vents. In stagnant, poorly oxygenated deeps in the Red Sea, Mn is not precipitated and it moves upward to form particulates only at the interface of oxygenated seawater and brine. Such Mn oxide particulates are only preserved on the sea floor beyond the deeps where the interface intersects sediment substrate. Thus, a dispersion halo of Mn (with scavenged Cu-Zn-Hg) is created around the Atlantis II deep in the Red Sea (Bignell et al., 1976). Such an Mn-dominated dispersion halo has been accepted as an effective pathfinder for hydrothermal base metal deposits (Stumpfi, 1979, and others).

In the case of slow-spreading centers in the open ocean (TAG field on the mid-Atlantic ridge), in addition to direct precipitation, some Mn is removed to middepth in the water column and advected at least 750 km west of the mid-Atlantic ridge, showing a far- field signal of hydrothermal source (Rona, 1984). In the intermediate- to fast-spreading center of the East Pacific Rise at 21ø N, Mn-rich crusts are practically absent on the ridge crest though substantial hydrothermal introduction of Mn (with aHe) is evident in buoyant hot water plumes --•200 m above the


•350øC hydrothermal vents (Lupton et al., 1980). Such a plume, on reaching density equilibrium with seawater, spreads laterally leading to its dispersal far and near field depending upon the ocean currents and the scavenging residence time of Mn (•50 yr; Weiss, 1977).

Such far-field dispersal of Mn in solution up to even thousands of kilometers to form distant metalliferous sediments (Heath and Dymond, 1977; Klinkhammer and Hudson, 1986; Klinkhammer et al., 1986) indicates substantial input of hydrothermal Mn in the ocean water-sediment system. This hydrothermal supply of oceanic Mn has been quantified variably from as low as 1.5 to 1.7 percent (Strakhov, 1974) and •10 percent (Elderfield et al., 1977) to as high as almost 90 percent (Bruland, 1983; Lisitsyn et al., 1985; cited by Glasby, 1988a). If the higher esti- mates are true, the hydrothermal component of the ocean Mn budget, since the Jurassic, could have been dominant relative to that from terrigenous supply. It must be realized, however, that the descending seawater could originally contain Mn derived from terrigenous sources which was further overprinted by the hydrothermal process.

Hydrothermal Mn oxide crusts were recovered from the island-arc setting of the Tonga-Kermadec Ridge (Cronan et al., 1982, 1984) and the Bismarck archipelago, southwest Pacific; the Ogasawara (Bonin) arc, northwest Pacific, and the Mariana arc, west Pacific. Glasby (1988b) and Hein et al. (1988a)

have presented excellent reviews of subduction-related Mn crusts in island-arc situations.

Recent evidence indicates that a large volume of fluid is released at •100øC during dewatering of subducting sediments through fractures in landward- dipping seismic reflectors (cf. Cloos, 1984; Shipley and Moore, 1986; ODP leg 110 Scientific Party, 1987; yon Huene et al., 1987; all cited by Glasby, 1988b). Fe-Mn crusts of restricted thickness and ar- eal extent could be precipitated from such hydrothermal solutions. The chemical composition of these crusts (Table 1), however, shows ranges of the Mn/Fe ratio and trace metal contents intermediate between those typical of hydrothermal crusts related to spreading centers and midplate hydrogenous Fe-Mn crusts and nodules. Such a composition has been attributed to derivation of the island-arc crusts by hydrothermal processes in concert with hydrogenous deposition (Hein et al., 1988a).

Mn oxides, often with fluorite, calcite, barite, and travertine, are being currently deposited by land- based hot springs. Active hot springs in the United States (0.14-3.4 ppm Mn) and in Japan (Komaga- dake, Tokati-dake, Tarumac, Akan; 2.8-4.7 ppm Mn) are depositing Mn oxides on the surface. Hariya (1980) concluded that hydrothermal solutions leached metals from basic or intermediate volcanic

rocks and that CO2 pressure in the transporting solution promotes concentration of Mn with respect to Fe. Mn oxide deposits in the apron of the active Akan

TABLE 1. Chemical Composition (wt %) of Modern Hydrothermal Manganese Crusts

Mn/Co + Setting and location Mn Fe Mn/Fe Co Ni Cu Ni + Cu

Spreading center TAG field, mid-Atlantic

ridge at 26 ø N (1) 39.04 0.046 847 Galapagos spreading

center, Pacific Ocean (2) 54.61 0.062 881

Island arc North Mariana arc,

west Pacific (3) 40.00 1.50 26.66 Ogasawara arc-trench,

northwest Pacific (4) 48.00 0.60 80.00 Bonin arc, northwest

Pacific (5) 34.00 3.83 8.87 Tonga-Kermadec

Ridge, southwest Pacific (6) 41.00 0.82 50.00

Seamount hydrogenous crusts (<2,000 m) Average Pacific crusts

(3)

0.0019 0.0350 0.0043 947

0.0034 0.0181 0.0051 2,053

0.0100 0.0200 0.0100 1,000

0.0015 0.1120 0.0373 318

0.0080 0.0430 0.0220 465

0.0033 0.0310 0.0120 885

22.00 15.00 1.46 0.6300 0.4400 0.0800 10.43

References: (1) Scott et al. (1974); (2) Moore and Vogt (1976); (3) Hein et al. (1988a); (4) Yuasa and Yokoto (1982); (5) Usui et al. (1981); (6) Moorby et al. (1984)

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hot spring are interlayered with CaCOa. Hydrogen isotope study of recent Mn oxides deposited by hot springs in Japan showed meteoric affinities (Hariya, 1986). The Komanoyu spring, Hokkaido, Japan, demonstrates bacterially mediated deposition of Mn in fa- vor of Fe (Hariya and Kikuehi, 1964).

Ancient settings

Paleohydrothermal Mn concentration led to the formation of strata-bound deposits and epithermal veins. In certain eases, the tectonic framework, litho- logic association, and chemical, including isotopic and REE, signatures permitted correlation of these strata-bound Mn ores with those forming at present at or near sea-floor-spreading axes or in subduetion- related island-are situations. Other strata-bound Mn

deposits have been related to hot springs emitted in shallow basins on land. Hydrothermal vein deposits are hosted in a variety of rocks of all ages but occur predominantly in voleanies ranging in composition from those of rhyolites to basalts. Mineralogieally the veins are often similar to the products of active land- based hot springs.

Ophiolitie complexes hosting Mn deposits (Mn- rich crusts and Fe-Mn umbers) occur in the Mesozoic Tethyan realm (e.g., Mesozoic passive continental margin, Antalya complex, Turkey, Robertson and Boyle, 1983; Late Jurassic northern Apennine ophiolitic complex, Italy, Bonatti et al., 1976; Late Cre- taceous Troodos massif, Cyprus, Robertson and Boyle, 1983; Late Cretaceous Semail nappe, Oman, Fleet and Robertson, 1980) as well as in the eastern Pacific margin (e.g., Late Jurassic-Early Cretaceous Franciscan assemblage, California, Crerar et al., 1982; Chyi et al., 1984; Eocene Olympic Peninsula, Washington State, Park, 1946; Lee, 1982). The above workers concluded that these deposits, except for those in the Antalya Complex, originated in sea- floor-spreading centers and were later obducted on land.

Other contrasting views regarding the original lo- cale and even the mode of genesis for some of these deposits have been expressed. For example, there has been a controversy on the spreading center ver- sus subduction-related origin of the volcanics of the Troodos massif and the Semail nappe. Hein et al. (1987a) and Hein and Koski (1987) even rejected a hydrothermal origin of the Mn deposits of the Francis- can assemblage. The Tethyan ophiolites, hosting Mn oxides and base metal sulfides, were reaffirmed as of sea-floor-spreading center origin, and the Mn halo around the sulfide deposits of the Troodos massif and the Semail nappe asserts their hydrothermal origin (Robertson and Boyle, 1983).

Ancient strata-bound and vein-type hydrothermal Mn deposits in recognized island-arc settings are generally hosted in andesite, dacite, and rhyolite. These

deposits occur in the Neogene Green Tuff belt of Ja- pan and are temporally (middle Miocene) and spatially coincident with the subduction-related kuroko deposits (Hariya and Tatsumi, 1981). The ophiolitic complex of the Florida Group (Late Cretaceous to Miocene) of Buena Vista, Solomon Islands, southwest Pacific, and the lower middle Miocene Tonumea deposit, Tonga Island, hosting hydrothermal Mn deposits, have island-arc affinities (Taylor, 1976; Cronan et al., 1984). The middle Proterozoic Lingban Mn deposit associated with acid volcanic rocks and tuffs of extremely sodic to extremely potas- sic composition has also been related to island-arc exhalative sedimentary (strata-bound hydrothermal?) type (Bostr6m et al., 1979; L6fgren, 1979).

Besides the deposits that can be correlated to sea- floor-spreading centers and island-arc settings, ancient hydrothermal Mn deposits have also formed in other situations. Vein-type base metal sulfide deposits with Mn (and Ag) halos have been described from several localities in the United States (Butte and Phillipsburg, Montana; Tombstone, Arizona, etc.), South Korea, and Japan. The Golconda deposit (Pleis- tocene), Nevada, is overlain by a thick travertine bed, and the upper Pliocene Burmister deposit, Arizona, is interlayered with travertine and volcanic ash. These are similar to those produced by modern land- based hot springs (for details, see Roy, 1981). The San Francisco Mn oxide deposit occurring as lenses in a Tertiary volcano-sedimentary sequence in Mexico was formed by hydrothermal deposition in a lake in a continental setting (Zantop, 1981). Therefore, it is evident that hydrothermal deposition of Mn is characterized by a variety of geologic settings through space and time.

Models for Sedimentary Deposition of Manganese Manganese deposits of sedimentary origin are wide-

spread in space and time. The geologic and geochemical environments of their formation are discussed below.

Modern environments

Sedimentary deposition of manganese is taking place in a variety of modern geologic and geochemical environments. These settings vary from deep-sea and midocean water column to shallow seas, fjords, and fresh-water lakes where different parameters in- teract to produce Mn deposits of variable nature. Some of these products and their depositional milieus can be traced back in the geologic record.

Deep-sea ferromanganese nodules occur exten- sively on the floors (water depth >• 4,000 m) of the world's oceans in areas where sediment accumulation rates are sufficiently low ('"7 m/m.y.-1). The crucial factor controlling Fe-Mn deposition in the deep sea is the oxidizing Antarctic bottom water.

ENVIRONMENTS OF Mn DEPOSITS 12 23

Glasby (1986, 1988a) correlated a major hiatus in the ocean sedimentary record in the Miocene (12 Ma) to increased circulation of Antarctic bottom water and to widespread formation of nodules that has contin- ued to the present. However, he did not advocate a total absence of Antarctic bottom water prior to the Miocene because deep-sea "fossil" nodules of Juras- sic and Cretaceous age are known. Modern Fe-Mn nodules lie on pelagic oxidized siliceous and carbonate oozes and red clay as well as on hemipelagic (reduced) sediments. In the northeast Pacific these sediments have been classified into MANOP sites S (siliceous ooze and clay), C (calcareous ooze), R (red pelagic clay), and H (hemipelagic clay). The compositions of the nodules are variable in respect to different elements (Fe, Mn, Cu, Co, Ni, Zn, etc.). This is largely related to the nature of the host sediments.

Morphologically the deep-sea Fe-Mn nodules show wide variation. The majority are rounded to el- liptical in shape with smooth to rough surfaces. Some- times, the nodules partially buried in the sediment show a smooth top and a rough bottom surface often with an equatorial girdle (Raab, 1972). The Fe-Mn oxide layers, accreted around variable nuclei, are very fine grained. The Mn-rich phases identified are todorokite and minor buserite (10• manganite), bir- hessitc (7• manganite), and vernadite (8-MnOz). The 7• phase (birnessite) could be an artifact and may actually represent Mn-rich and Cu-Ni-poor todorokite (Dymond et al., 1984). The Fe hydroxides present are goethite, ferrihydrite, ferroxyhyite, and minor akaganeite and lepidocrocite. The tunnel structure todorokites can admit Mn +a, Mg +z, Ni +z, Cu +2, and Co +z in the M2-type oetahedral sites and Ca +z, Ba +z, Na +, K +, and HzO in tunnel sites (Burns et al., 1983, 1985). Vernadite shows high Fe and moderate Si and A1 contents. The high Fe content is due to fine-scale epitaxial intergrowth of Fe- and Mn-rich

phases (Burns and Burns, 1977). A1 and Si possibly represent detrital components. Co is positively correlated to vernadite.

Hydrogenous (deposition from basin water) and early diagenetic (deposition from sediment pore water) processes produce deep-sea sedimentary Fe- Mn nodules. The latter is further subdivided into oxic and suboxic types depending on the oxidized and reduced nature of the host sediments. All these processes are probably directly or indirectly biologically mediated. Hydrogenous deposition of deep-sea nodules and crusts involves supply from the basinal bottom waters. Well-defined near-bottom maxima of dissolved trace metals are shown by vertical seawater profiles. The Mn/Fe ratio in these nodules is --, 1 and Cu + Ni + Co values are moderate (Table 2). Scav- enging through adsorption on active surfaces possibly attended by autocatalytic growth could only lead to incorporation of metals to these nodules (Crerar and Barnes, 1974; Murray, 1975; Varentsov et al., 1979). Microbial oxidation ofMn +z triggering precipitation of MnOz may also be a viable complementary mechanism (cf. Ehrlich, 1963, and others cited by Baturin, 1988). Hydrogenous growth rates determined radiometrically are very slow (•<5 mm/m.y.-1). Vernadite intergrown with amorphous FeOOH. xHzO is dominant with only minor todorokite which accounts for the low Mn/Fe bulk ratio in the nodules. Such nodules rest on red clay substrates which have a low biogenic component compared to siliceous oozes. Thus, early diagenetic metal supply to the nodules is also restricted. However, the bottom parts of the partially buried nodules do show an increase in Mn/Fe ratios and Cu-Ni contents, indicating an early diagenetic signature (Table 2). Therefore, bulk compositions suggesting a hydrogenous source may actually represent average values of inhomogeneous composition dictated by different sources.

TABLE 2. Chemical Composition (wt %) of Oxide Layers from Top and Bottom Parts of Ferromanganese Nodules from Pelagic and Hemipelagic Sites in the Northeast Equatorial Pacific Ocean

Siliceous ooze substrate MANOP

site S (avg 5 samples) Red clay substrate MANOP

site R (avg of 7 samples) Hemipelagic clay substrate MANOP

site H (avg of 27 samples)

Top Bottom Bulk I Top Bottom Bulk 2 Top Bottom Bulk 3

Mn 27.12 31.2 29.1 17.5 22.4 20.6 34.7 43.8 38.8 Fe 7.76 4.47 4.80 15.6 10.8 11.5 3.8 1.1 2.9 Ni 1.30 1.77 1.52 0.56 1.01 0.86 0.806 0.531 0.742 Cu 0.786 1.270 1.187 0.303 0.672 0.498 0.437 0.172 0.371 Co 0.364 0.243 0.214 0.328 0.240 0.304 0.0216 0.0081 0.0195 Zn 0.136 0.211 0.169 0.056 0.107 0.076 0.243 0.203 0.234 Mn/Fe 3.5 7.3 6.1 1.1 2.1 1.8 9.1 39.8 13.4

Data from Dymond et al. (1984), averaged by Baturin (1988) • Early diagenetic (oxic) 2 Hydrogenous 3 Early diagenetic (suboxic)

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In pelagic oxidized biogenic sediments (e.g., siliceous ooze-clay in MANOP site S) the pore waters are significantly enriched in transition metals (Cat- lender and Bowser, 1980; Ktinkhammer, 1980) through supply from the dissolving biotic tests. The biogenic components initially scavenged these trace metals from seawater (Martin and Knauer, 1973; Fowler, 1977; Bostr/Sm et at., 1978). The maximum concentration of trace metals in the sediments, pore water, and the nodules, therefore, is recorded in the equatorial zones of high biological productivity.

The early diagenetic pore water from oxic sediments enriches the nodules at the sediment-water in-

terface in Mn, Cu, and Ni, leading to their economic grade. The bulk Mn/Fe ratio and the Cu + Ni + Co contents of these prime nodules show much higher values than those typical for the hydrogenous nodules (Table 2). The major mineral is todorokite formed either by fractionation of Fe in nontronite or through control of the pH of the nodule surface by precipitation of authigenic silicates (Lyte et at., 1977; Bischoff et al., 1981; both cited in Dymond et at., 1984). Where these nodules are partially buried, the smooth top and rough bottom parts differ substantially in chemical composition suggesting different sources of metal supply (Table 2). The early oxic diagenetic growth rate (ca. 16 mm/m.y. -•) is much faster than the hydrogenous rate (Moore et at., 1981). Study of metal diagenesis in the sediments on site by Ktinkhammer et at. (1982), however, showed that Mn concentration in oxidizing pore water is very low (<3 times bottom water) attended by relatively low Ni contents whereas regeneration of dissolved Cu in the boundary layer produces a flux to the nodules 30 to 40 times higher than that in ambient seawater. Thus the Mn accretion rate of todorokite-rich nodules as determined at MANOP site S is not sustained

by steady-state molecular diffusion through pore water. To explain this enigma of Mn and Ni fluxes, Lyte et at. (1984) proposed an adsorption-bioturba- tion model of transfer of an additional amount of transition metals sorbed on sea-floor surface sediment

particles to the growing nodules. Calvert and Piper (1984) further observed that oxic early diagenetic nodules may be derived from an immediate as well as a remote metal source of sediment pore water. It is evident, therefore, that while metal enrichment in Fe-Mn nodules on pelagic siliceous biogenic sediments is grossly related to oxic diagenesis of the sediments, no specific mode of transfer of the metals to the nodules is established.

Suboxic diagenesis of hemipetagic organic-rich reduced sediments can produce Mn nodules in the uppermost part of the sediment profile or at the sediment-water interface. At MANOP site H, for example, primary productivity in overlying surface water

is higher by a factor of five than at either site S or R (Dymond et at., 1984). A further increment of organic matter from terrestrial sources is also possible. Consequently, the composition and behavior of the sediment pore water are different from the pelagic areas. Reduction of Mn (and other metals) occurs at subsea-floor depths of 10 to 15 cm (Bonatti et at., 1971; Ktinkhammer, 1980), but mainly Mn is transferred to the sediment-water interface whereas other

metals are fixed in the sediments to a large extent either as organometattic complexes or as sutfides. He- mipetagic nodules, thus, show unusually high Mn/Fe and Mn/minor metal ratios (Table 2). High Zn contents have been recorded in MANOP site H nodules, reflecting its enrichment in planktons (cf. Martin and Knauer, 1973). It was suggested that tabitc organic matter at the sediment-water interface released Zn

by decomposition which then accreted to the nodules (Dymond et at., 1984). The growth rate of these nodules is 100 to 200 mm/m.y. -• (Huh, 1982; Reyss et at., 1982; Finney et at., 1984; all cited by Dymond et at., 1984). From growth rate data and normative mod- eling, Dymond et at. (1984) proposed that pulses of high organic productivity produce transient Mn reduction in the uppermost part of the sediments, re- suiting in episodic suboxic accretion.

Todorokite, the major Mn phase in hemipetagic nodules, is much depleted in Cu and Ni and correspondingly enriched in Mn +2 or Mn +a (cf. Peru basin, Halbach et at., 1981; MANOP site H, Dymond et at., 1984), reflecting the metal flux of suboxic diagenetic pore water. The generalized geochemical profile of interstitial water (Crerar et at., 1972) shows an increased CO2 content with depth indicating oxidation of organic matter possibly coupled with reduction of Mn. A part of the Mn +• diffusing upward as bicarbonate may lose its mobility in the upper sediment layers, depositing Mn carbonate below the anoxic- oxic boundary in hemipetagic areas (Zen, 1959; Fo- mina, 1962; Lynn and Bonatti, 1965; Skornyakova, 1965; Bostr/Sm, 1967; all references cited in Roy, 1981). Such early diagenetic formation of Mn carbonate is also reported from other modern basins (e.g., Baltic Sea).

The processes of abyssal Fe-Mn nodule growth as discussed so far are perhaps too simplistic and they are obviously restricted by gaps in knowledge. For example, several studies indicate direct microbial oxidation of Mn +•, thus facilitating Mn deposition (see geochemistry section), but this is yet to be proved viable for nodule growth. Similarly the action of Mn- reducing microbes in dissolving Mn in the sediments requires further assessment. The pathway of the early diagenetic metalliferous pore water to the nodule accretion site is not fully understood. In certain cases, metal may have been supplied congruently


from both the sediment column and the surface sediments (including labile organic matter, Dymond et al., 1984; Lyle et al., 1984).

The question remains open as to whether pore- water metal supply is directly and entirely incorporated into the nodules or whether the metal enriches the bottom water before its eventual accretion onto

the nodules. Only nodules partially buried in sediment receive metals supplied by the hydrogenous source at the top and pore water at the bottom, but the nodules showing subrounded grossly uniform concentric growth on all sides probably moved freely (pushed by bottom currents). In such cases, particularly in the siliceous ooze province, dual supply in variable time domains of nodule growth can hardly be discounted. The remote early diagenetic metal flux suggested by Calvert and Piper (1984) obviously enriches the bottom water initially and then the nodules. The internal oxide layers in the nodules on a fine scale show variations of chemical composition corresponding to both hydrogenous and diagenetic fields (Roy et al., 1990a). Such nodules obviously received metal supplied by both basinal and pore waters at different times and any one source could dominate over the other. Krishnaswami and Cochran (1978) suggested episodic growth of nodules from accretion rate data. Changes in growth rate at different stages of nodule accretion have also been documented

(Krishnaswami et al., 1982). Dymond et al. (1984) proposed episodic suboxic accretion. Considering all these it may be suggested that sediment pore water episodically enriches the bottom seawater in metals that are transferred to the nodules, producing an early diagenetic signature whereas in intervening periods nodule growth from unadmixed bottom water (hydrogenous) takes place. The bulk composition of the nodules should not be used to categorize them into totally hydrogenous and totally early diagenetic types. At best such classification indicates the dominance of one process over the other.

It has been claimed as a general rule that the mineralogy is related to the microstructures in the nodules and these two expressions characterize hydrogenous (vernadite: laminated) and early diagenetic (todorokite: dendritic) growths (Halbach et al., 1981; Moore et al., 1981; Marchig and Halbach, 1982). Such a claim is untenable because todorokite can form by diverse processes (Dymond et al., 1984; Ostwald, 1986) and neither todorokite nor vernadite is restricted to any specific microstructure (Roy et al., 1990a).

It is commonly assumed that Fe-Mn oxide in the abyssal nodules accreted uniformly through their en- tire growth history, layer by layer, and that their pris- tine character is retained. Neither of these assumptions is true, at least for all nodules. Curiously, petro-

graphic studies of nodules are few, but all of them show several types of microstructures (massive, mottled, columnar-cuspate-dendritic, nondirectional collomorphic, laminated) that show random distribution and often depositional hiatus (Sorem, 1973; Sorem and Fewkes, 1977, 1979; Heye, 1978; Hal- bach et al., 1981; Marchig and Halbach, 1982; Roy et al., 1990a). Therefore, the assumption that all nodules grew by uniform and constant accretion of Fe- Mn oxide layers is not valid.

Burns and Burns (1978) demonstrated that nodules underwent postdepositional intranodule diagenetic reactions involving clay, biogenic silica, vernadite, and amorphous FeOOH. xH20 to produce Ni- and Cu-rich todorokite, FeOOH. xH20, and phillipsite. Ni and Cu were supplied by siliceous plankton through their degradation and production of organically complexed cations. Piper and Williamson (1981) indicated postdepositional recrystallization of todorokite in the interior parts of the nodules. De- tailed petrological studies and mineral analyses of some nodules established that todorokite has been recrystallized in places and has also formed by partial transformation of vernadite (Roy et al., 1990a). The primary and recrystallized todorokites have different chemical compositions. Partial dissolution of biotic remains and development of phillipsite with clusters of Mn oxide on the surface have been recorded in these nodules (cf. Burns and Burns, 1978). There- fore, postdepositional reorganization of the minerals, biotic remains, and their chemical components have been observed in at least certain nodules and there is a distinct possibility that such intranodule diagenesis has taken place in others. This aspect has significant implications for the growth rate determined radiometrically under several constraints and the growth history of the nodules.

The growth rate of nodules is very slow (mm/ m.y. -1) as determined by radiometric methods using radionuclides such as •aøTh, •'a•pa, •'aøTh/•'a•'Th, 2aøTh/ zaXpa, and 1øBe. However, there are several assumptions on which these determinations are based. Ku (1977) listed all these assumptions as follows: (1) long-term (millions of years) accumulation ofradionu- clides at a constant rate on all points of the nodule surface; (2) constant and uniform layer by layer accretion of Fe-Mn oxides on nodules; (3) nodules represent a closed system; (4) nodules retain their pris- tine features in all respects and were not subjected to postdepositional changes; and (5) the uncertainty of the sampling method is ___20 percent.

Critical examination of these assumptions shows that in the case of (1) the constant rate of incorporation of radionuclides may not be sustained as shown by the measured inventories of •'aøThexc and •'aXpaexc on a scale of centimeters only on Fe-Mn crusts from

1226 SU?RIYA ROY

MANOP site S and on an Atlantic seamount (Sharma et al., 1984). Assumption (2) is contested strongly by the microstructures of Fe-Mn oxides which record

complex growth histories and depositional hiatuses (Sorere, 1973; Sorem and Fewkes, 1979; Glasby, 1986). Dendritic and nondirectional collomorphic structures can also lead to sampling artifacts (Heye, 1978; Lalou et al., 1980). Unfortunately, no petrographic characterization has been reported for any of the nodules dated so far. Assumption (3) that the nodules act as closed systems is also not beyond criticism in view of the porosity of the nodules, syn- and postdepositional fractures, and radioactive disequilib- rium inside the nodules (Lalou et al., 1980). Many believe that the radionuclides incorporated on the surface of the nodules diffused to the inner parts and created an exponential decrease in concentration with depth that mimics radioactive decay (Lalou et al., 1980, and references therein). This concept cannot be negated by the proponents of the decay model (cf. Ku et al., 1979; Krishnaswami et al., 1982). As- sumption (4) is not sustained by the observations of postdepositional rearrangement of mineralogy, mineral chemistry, and texture discussed above. Postde- positional burrowing by benthic organisms can also disorganize the primary internal microstructures of the nodules (Furbish and Schrader, 1977). In assumption (5) the uncertainty of the sampling method reckoned as _+20 percent may be accentuated many- fold if the microstructural features of the nodules are

not preexamined. Episodic as well as multisource metal flux to the

nodule has been indicated. If a metal supply from different sources generates the nodules at different stages in their growth, along with depositional hiatus(es), radiometrically determined accretion rates will produce only an average value. That such growth rates indeed represent only average values has been confirmed by Krishnaswami and Cochran (1978) and Krishnaswami et al. (1982). The growth history of the nodules, thus, cannot be established by growth rate determination alone.

Sedimentary ferromanganese crusts of different thickness occur in modern oceans at variable water depths (800-4,900 m) in midplate regions. These crusts generally form on substrates of volcanic rocks of much older age. The crusts occurring in shallower water depths (•800-2,000 m) and those formed in a deep-sea situation (>2,000 m) can be distinguished by their thickness and specific chemical characteristics.

The deep-sea crusts form on or at the flanks of seamounts and may be up to 24 cm thick. These show almost constant Mn/Fe ratios at •1 (Aplin and Cronan, 1985) with a low content of the manganophile elements, particularly Co, mirroring the chemistry of abyssal hydrogenous nodules. Vernadite and

FeOOH. xH20 are the main components, both derived by hydrogenous accretion.

The Fe-Mn crusts in shallower water depths (<2,000 m) in the Pacific are usually thin (2-4 cm). Thicker crusts ( '-• 8-10 cm) consist of at least two gen- erations of Fe-Mn oxides separated by a hiatus marked by a thin phosphoritc layer (carbonate-fluor- apatite; Halbach et al., 1982; Hein et al., 1988b). Vernadite is dominant, intergrown (possibly epitax- ially) with amorphous FeOOH. xH20, only minor todorokite, goethite, and rare manjiroite (Halbach et al., 1982; Hein et al., 1987b). These shallow-water crusts show a higher Mn/Fe ratio (1:3), a significant Co content (1-2%, max 2.5%; Manheim, 1986), and a low nickel (•1%) and minimal Cu content (0.04- 0.15%). The outer layers are grossly more enriched in Mn, Co, and Ni (Halbach et al., 1982, 1983; Aplin and Cronan, 1985; Manheim, 1986). However, the crusts from the Marshall Islands showed that no such general trend for manganophile elements occurs on a fine scale. The inner layers of the crusts are substantially enriched (max 1.3 ppm) in Pt (Halbach et al., 1984; Hein et al., 1988b, 1992).

Radiometrically determined growth rates of the Fe-Mn crusts vary between 0.8 to 2.7 mm/m.y. -1 in the outermost part (Halbach et al., 1983), up to 5 mm/m.y. -1 in the inner layers (Segl et al., 1984). Hein et al. (1992), on the other hand, showed a random variation of growth rate (SVSr/86Sr method) at different depth intervals of a single Pacific crust from 1.6 to 8.8 mm/m.y. -•. Such slow growth rates dis- count the possibility of hydrothermal or early diagenetic metal flux for these crusts. Hydrothermal input is further negated by the Co enrichment in the crusts (Manheim and Lane-Bostwick, 1988). The degree of Co enrichment in these crusts is inversely propor- tional to their growth rate (Halbach et al., 1983).

The enrichment of the crusts in Co, Mn, and Ni is negatively correlated to water depth (Halbach et al., 1982; Manheim, 1986). The maxima for these metals in the crusts are attained at water depths correlatable to those of the midwater column oxygen-minimum zones (Halbach et al., 1982; Halbach and Puteanus, 1984; Manheim, 1986). Latitude is negatively correlated to the thickness and enrichment of the Fe-Mn crusts in metals (Manheim, 1986; Hein et al., 1987b). This correlation reflects the profound effect of equatorial high organic productivity on the development of the midwater oxygen-minimum zone and the origin and chemical composition of the crusts.

Mn concentrations in the oxygen-minimum zone (800-1,200 m) in the Pacific have been explained by three possible models: model A--lowering of pH and/or pE by which the equilibrium is shifted away from particulate to dissolved Mn; model B--libera- tion of Mn from near-shore reducing sediments and its lateral advection to the oxygen-minimum zone;


and model C--consumption of organic matter and release of Mn in which it was bound (Klinkhammer and Bender, 1980). In situ regeneration of metals within the oxygen-minimum zone in the equatorial northeast Pacific for supply to the crusts was suggested by Halbach et al. (1983) and Halbach and Pu- teanus (1984). Model B was, however, proposed for the same oxygen-minimum zone (Martin and Knauer, 1982). Martin and Knauer (1984) eventually esti- mated that about 30 percent of Mn in the oxygen- minimum zone resulted from in situ regeneration and the remaining 70 percent was derived by lateral advection. Considering the similar vertical distribution of Co and Mn demonstrated in the northeast Pacific

water column, Knauer et al. (1982) supported lateral advection for additional supplies of Mn and Co to the oxygen-minimum zone which were finally incorporated in the crusts (cf. Aplin and Cronan, 1985; Chave et al., 1986). The initial concentration of Mn and Co in the near-shore sediments was derived al-

most totally from terrestrial sources. The Co-rich Fe-Mn crusts were deposited hydro-

genetically from dissolved metals (Halbach and Pu- teanus, 1984) and/or oxide riocs suspended in the water column (Aplin and Cronan, 1985). The major element concentration in the crusts reflects both particulate and dissolved seawater chemistry in the oxygen-minimum zone. The crusts can form at the redox interface just beneath the oxygen-minimum zone where the metals would tend to diffuse (Halbach and Puteanus, 1984). Surface catalysis by Fe hydroxide species in the water column may initiate Mn +2 oxidation and precipitation of vernadite on seamount substrate leading to the ubiquitous finely intergrown mixture of vernadite and FeOOH.xH20 (Halbach and Puteanus, 1984). Preferential enrichment of Co is due to the presence of vernadite, which has a large surface area where Co += is initially adsorbed and then oxidized by the strong electrical field of Mn +4, leading to its incorporation in the vernadite lattice (Burns, 1976). The processes are autocatalytic. Co- rich Fe-Mn nodules, also reported from shallow- water depth on topographic highs (Cronan, 1975; Halbach et al., 1982), were formed by the same process in the presence of suitable nuclei.

Substantial concentration of platinum is reported from these crusts (Halbach et al., 1984; Hein et al., 1988b). Pt correlates positively with Mn in the crusts as well as in the dissolved state in the water column

(Jacinto and van der Berg, 1989). By contrast with Co, the Pt concentration increases in the deeper parts of the crusts. Halbach et al. (1984) suggested that Pt += can be absorbed onto hydrous Mn oxide and that it may also be oxidized to Pt +4 to form PtO=. nH=O at PO2 > 0.1 atto which can be reached several hundred meters below the oxygen-minimum zone. Alternatively, Pt may be enriched in MnO= in

the crust in the elemental form by reduction of Pt +2 according to the reaction Mn += + PtC1j = + 2H=O -• Pt ø + MnO2 + 4C1- + 4H + (Halbach et al., 1984; Hein et al., 1988b, quoting Hodge et al., 1985, and Halbach, 1986). Halbach et al. (1989) considered that in seawater dissolved Pt mainly occurs as PtC1j 2 species which cannot be adsorbed on the surface of negatively charged hydrous MnO=, thereby prevent- ing oxidation of Pt += after surface adsorption. There- fore, the latter process involving coupled redox reactions was supported by them.

Although modern oceans are largely oxygenated with only localized development of midwater oxygen-minimum zone, restricted extant basins demon- strate widespread anoxic-oxic stratification. The Black Sea presents a classic example of such a modern stratified basin (Ross and Degens, 1974) where the anoxic H=S-producing zone has been rising in the water column (Deuser, 1974). At present, in the central basin, the anoxic-oxic interface lies at about a 150-m water depth and the anoxic zone extends for about 2,000 m to the sea floor. H=S is being produced in the anoxic water column (Sweeny and Kaplan, 1980) forming FeS both in the water column as well as at the sediment-water interface (Leventhal, 1983). The dissolved Mn content of the anoxic water column

is greater by a factor of 500 than that in the oxic zone (Brewer and Spencer, 1974) and it reaches the maximum value just below the anoxic-oxic interface. This is caused by the diffusion ofMn +2 toward the oxygenated zone, which on crossing the interface precipitates as Mn oxide-hydroxide particulates that sink back to the anoxic zone. However, in basin-margin areas where the redox interface intersects the shelf, Mn oxide is precipitated and retained on shelf substrates (Sevast'yanov and Volkov, 1966; Georgescu and Lupan, 1971).

The epiric and brackish Baltic Sea also shows density and redox stratification (Winterhalter, 1980). The oxygenated sea floor (depth •-' 80 m) permits deposition of Fe-Mn oxide nodules on a sandy clay substrate. These nodules are Fe dominant (Mn/Fe < 1). Discrete anoxic troughs (deeps, up to 460 m) occur, where the redox interface coincides with the sea

floor. These anoxic deeps are sites of Mn carbonate formation (Suess, 1979; Manheim, 1982). The •i•aC values of the carbonate and its pseudomorphism after calcareous bacteria suggest that it is formed by oxidation of organic matter during early diagenesis. These modern stratified basins serve as models for those

produced in the past and provide an insight into ancient concentration-deposition processes of manganese.

Fresh-water lakes in the northern hemisphere high latitudes (Callender and Bowser, 1976) show deposits of Fe-Mn nodules and crusts which are chemi- cally different from the oceanic deposits, both in

1228 S UPRIYA ROY

their enhanced Fe content and highly depleted trace metals. Fe and Mn were supplied from terrigenous sources through acidic surface and ground water. Most of these deposits are early diagenetic in origin and formed by pore water supply from the sediments (Strakhov, 1966; Crerar et al., 1972; Callender and Bowser, 1976); the mobility of Mn may have been biologically mediated (Perfil'ev and Gabe, 1965).

Concentration of organic carbon in landlocked fresh-water lakes is much higher than that in the open ocean. The sediment pore water shows that Mn +2 is saturated with respect to MnCOa (e.g., Lake Michigan; Rossmann and Callender, 1969) and early diagenetic rhodochrosite has formed (Callender, 1973). Strakhov (1966) showed that lakes receiving organic detritus exhibit a progressive increase of tro- phicity with time (from oligotrophic to eutrophic). As

a result lake waters became increasingly stratified, leading to the shift of Fe-Mn oxide depositional sites from the deep central part of oligotrophic basins to the sublittoral and littoral marginal zones with time.

Ancient environments

Sedimentary Mn deposition of an economic scale was initiated ca. 3,000 m.y. ago, though such deposition was typically inhibited throughout the Archean (Table 3). This lack can be interpreted as being due to adverse geochemical environments for Mn deposition in the Archcan. With the onset of the Protero-

zoic, corresponding to substantial oxygenation of the atmosphere-hydrosphere, Mn deposits of large to moderate size were formed hosted in banded iron- formations, carbonates, black shale, and sandstone (Table 3). The Phanerozoic scenario includes the

T•,BLE 3. Precambrian Sedimentary Manganese Deposits

Shallow-water basin margin

Sandstone-claystone Black shale Carbonate Iron-formation

Late Proterozoic

Early Proterozoic

Archean

Xiangtan, Tangganshan, China (G, S); Datangpo, China (E)

Wafangzi deposit, Teiling Formation, China (ca. 1200 Ma)(TR)

Gangpur Group, India (ca. 2000 Ma); Sausar Group, India (ca. 2000 Ma) (TR); Kgwakgwe Hill, Botswana (ca. 2000 Ma)(S)

Aravalli Supergroup, India (2500- 2000 Ma)

Amapa Series, Brazil (ca. 2000 Ma) (TR)

Franceville Series, Gabon (ca. 2140 Ma) (TR)

Graphite Group, Malagasy (2420 Ma)

Chitradurga Group, India (ca. 2600 Ma); Eastern Ghats, India (>2600 Ma)

Serra do Jacobina, (3100-2700 Ma); Iron Ore Group, India (3200-2950 Ma)

Rio das Velhas Series, Brazil (>2700 Ma)

Penganga Group, India (ca. 775 Ma) (TR)

Gaoyuzhuang Formation, China; Lukoshi Complex, Zaire (>184,5 Ma) (S)

Sausar Group, India (ca. 2000 Ma) (TR)

Eastern Ghats, India (>2600 Ma)

South Bahia, Brazil (3100-2700 Ma)

Damara Supergroup, Namibia (720- 590 Ma) (G)

Morro do Urucum, Brazil (ca. 900 Ma) (G)

Minas Series, Brazil

Transvaal

Supergroup, South Africa

(2500-•S50 Ma) (TR)

E = evaporitic, G = interglacial, S = stromatolitic, TR = transgression-regression cycle


TABLE 4. Phanerozoic Sedimentary Manganese Deposits

Shallow-water basin margin

Deep sea Sandstone-claystone Black shale Carbonate

Cenozoic Pacific, Atlantic, and Indian Oceans

(Paleocene- Recent)

Mesozoic

Paleozoic

Olympic Peninsula, U.S.A. (Eocene); Oriente Province, Cuba (Eocene)

Timor Island

(Cretaceous); Nicoya Complex, Costa Rica

(Cretaceous) Tethyan alps

(Jurassic)

Nikopol, Ukraine; Chiatura, Georgia (Oligocene) (TR)

Groote Eylandt, Australia (Cretaceous) (TR)

Timna Dome, Israel

Zunyi, China (Permian) (TR)

Taojiang, China (Ordovician) (TR)

Imini-Tasdremt, Morocco (Cretaceous) (TR)

Molango, Mexico (Jurassic) (TR)

Falang Formation, China (Triassic) (TR, E)

Ulu Telyak, C.I.S. (Permian) (TR, E)

Um Bogma, Sinai (Carboniferous) (E)

Xialei, China (Devonian) (E)

Usinsk, C.I.S. (Cambrian)

E = evaporitic, TR = Transgression-regression cycle

most prolific development of land-based sedimentary Mn deposits in the Mesozoic and Cenozoic eras (Ta- ble 4). All Precambrian and most Phanerozoic deposits are sediment-hosted shallow-water types developed near basin margins. The exceptions to this general rule are those hosted in obducted deep-sea sediments (fossil nodules; Jenkyns, 1977; Margolis et al., 1978; and others, all cited by Roy, 1981). These deposits are usually small.

Sea-level changes and formation of stratified basins served as major factors for deposition of Mn as sedimentary orebodies in the past (Cannon and Force, 1983; Frakes and Bolton, 1984; Force and Cannon, 1988). This concept is based on the mechanisms of formation of midwater oxygen-minimum zones in modern oceans and oxygen-density stratification in today's Black and Baltic Seas and their relation to Mn concentrations.

At a sea-level highstand and corresponding warmer climate, the pole to equator thermal gradient was affected, resulting in diminished deep-sea circulation and a decrease in the rate of oxygen supply to the deeper parts of the oceans. Concomitantly, an increase in temperature led to decreased solubility of

oxygen in seawater. The sea-level highstand resulted in marine transgression on the cratonic shelf overrun- ning areas of high biological productivity (Cannon and Force, 1983; Force and Cannon, 1988). The coincident episodes of oxygen consumption (by degrad- ing organic matter) and its nonrenewal led to anoxia through much of the water column. Only a shallow layer, oxygenated by atmospheric interaction, formed at the surface. Thus, in contrast with the restricted oxygen-minimum zone in present-day open oceans, stratified oceans could form in the past through expansion of the oxygen-minimum zone toward the ocean floor.

Evidence of transgression-regression and atten- dant ocean anoxic events (recorded by black shales) is not rare in the geologic record. These episodes were possibly common as far back as the Precambrian (cf. Degens and Stoffers, 1976). More incisive investigations on the Phanerozoic sequences documented re- peated episodes of sea-level change and anoxic events (Hallam and Bradshaw, 1979; Haq et al., 1987; Jenkyns, 1988). Sedimentation of organic matter leading to the creation of anoxia has been quantified by (•laC values in the Phanerozoic (cf. Scholle

1230 SUPRIYA ROY

and Arthur, 1980; Jenkyns, 1988) and Precambrian sequences (Moanda deposit, Gabon, J. R. Hein, pers. commun., 1989; Oehler et al., 1972; Goodwin et al., 1976). That mid-Cretaceous strong Oa minima in the Pacific (Thiede et al., 1982) was limited to palcoequa- torial setting attests to high organic productivity as a causative factor in creating anoxia (compare modern oxygen-minimum zones in equatorial Pacific).

In a manner similar to the oxygen-minimum zone in today's marine basins, Mn +a was concentrated in solution in the anoxic part of the ancient stratified oceans (Pomerol, 1983; Pratt et al., 1991). Its presence signals biological mediation in creating anoxia, at least in certain cases, as shown by the positive correlation of negative •13C values with Mn content in carbonates (Okita et al., 1988). Mn +a concentrated in stratified marine reservoirs was precipitated only in the optimum presence of oxygen. Where the anoxic seawater was charged with HaS in the water column, Fe sulfide precipitated, fractionating Fe from Mn (cf. modern Black Sea). The same effect is realized by early diagenetic processes within reduced sediments in modern hemipelagic ocean environments. Mn +a did not form sulfides in such situations

because of geochemical constraints. Mn oxide deposition took place only by crossing the anoxic-oxic interface (Cannon and Force, 1983; Frakes and Bolton, 1984; Force and Cannon, 1988). The tendency of dissolved Mn +a (___Fe +a) to advect to this interface is recorded in today's Black Sea (Brewer and Spencer, 1974).

The prerequisites for marine shallow-water Mn deposition in the geologic record, therefore, were two- fold: a high concentration of dissolved Mn attained in anoxic parts of stratified basin margins, followed by its precipitation across the redox interface through oxygenation; and an intersection of the anoxic-oxic interface by cratonic shelf substrate. The first condition could be achieved either through upwelling of anoxic water from the deeper part of a stratified sea charged with dissolved Mn (_+Fe) to the basin margin across the interface (cf. Holland, 1973; Drever, 1974) or by transgression of the redox interface over the cratonic shelf where additional Mn could be taken into solution from the shelf sediments by the anoxic bottom water (Cannon and Force, 1983; Frakes and Bolton, 1984). Theoretically, the processes could act independently or might coincide. In- tersection of the anoxic-oxic interface with the shelf substrate was essential for the preservation of the precipitated manganese. The original source of Mn in the basinal water is immaterial in respect to this model. Single or multiple sources such as hydrothermal, terrestrial, or submarine weathering inputs were possible.

Generally Mn oxide-hydroxide was deposited on the cratonic shelf above the redox interface (cf. Groote Eylandt, Chiatura, Nikopol: Table 4; Bolton

and Frakes, 1985; Bolton et al., 1988). However, Mn carbonate may also occur above this interface due to extremely slow precipitation of Mn oxide (cf. Cham- berlain deposit; Force and Cannon, 1988). Mn carbonate was widely formed in ancient black shale facies below the redox interface or in dysaerobic carbonate sequences (Tables 3 and 4). These different facies are contemporaneous, but whether the oxide and carbonate facies will be spatially adjacent (cf. Ni- kopol, Chiatura, Georgia) or separated (cf. Groote Eylandt, Australia) will depend on the rate of Mn precipitation and lateral seawater circulation.

Sedimentary Mn carbonate forms by early diagenesis. This may be achieved by several pathways. Just below the seaward redox interface, dissolved Mn +a may react with substrate carbonates to produce MnCO3-rich deposits (Force and Cannon, 1988). Al- ternatively, in pyrite-rich black shale, COa produced by oxidation of organic carbon coupled with reduction of sulfates, may react with Mn +a below the interface to form Mn carbonate (Jenkyns, 1988). The presence of Fe oxide minerals in the Mn carbonate ore

zone in Molango, Mexico, with or without minor pyrite, has been explained by invoking initial Mn oxide deposition at the redox boundary. This Mn oxide was possibly reduced later by microaerophylic bacteria, coupled with the oxidation of organic matter, producing Mn +a and isotopically light HCO•, respec- tively, that reacted to form xaC-enriched MnCO3 (Okita et al., 1988).

Thus, during a sea-level highstand, transgression of the redox interface across the cratonic shelf can lead to precipitation of Mn oxide at the interface and well within the oxic zone near the feather edge of the transgressive wedges because of the sluggish Mn oxidation reactions (Cannon and Force, 1983; Force and Cannon, 1988). During the same or a similar event, Mn carbonate may form in dysaerobic or, more commonly, in anoxic conditions. Transgression-related shelf-regime Mn oxide and Mn carbonate deposits hosted in variable lithologies have been documented through much of geologic history (Tables 3 and 4). That these deposits are indeed shallow marine is at- tested by the dominant primary pisolitic-oolitic character of the ores (e.g., Groote Eylandt, Chiatura, Ni- kopol) besides other attributes.

Mn deposition during marine regression was modeled at the Groote Eylandt deposit, Australia (Frakes and Bolton, 1984). This model envisages concentration of dissolved Mn in anoxic intracratonic basin

water during transgression followed by precipitation of Mn during regression. The veil effect (fiocculent fallout from river water) in the estuarine area and the shoreward broom effect (tidal lag driving Mn particulates) concentrated the precipitated Mn to form ore deposits. Inverse graded bedding of Mn pisolite and oolite indicated Mn precipitation during regression triggered by oxygenation of basin water. Normal and


inverse graded bedding of Mn oxide pisoliths and oo- liths in the Chiatura deposit, Georgia, has also been interpreted as due to precipitation of Mn during peak transgression and succeeding regression, respec- tively (Bolton and Frakes, 1985), following the Groote Eylandt model.

Manganese deposits occur in distinctively glaciogenic sequences of late Proterozoic age. The litholog- ical associations and the character of the ores are, however, not unique. Mn oxide ore is interbedded with banded iron-formation in the transgressive sequence of the Urucum deposit, Brazil, and in the Da- mara Supergroup, Namibia. Beukes (1989) considered that these banded iron-formations (and included Mn deposits) were related to a period of deglaciation following almost global freezing of the oceans and suggested that major transgressions took place in the interglacial stage. The presence of Mn oxide ores in the Morro do Urucum deposit has been explained by the build up of dissolved Mn +2 in anoxic stagnant water below the ice cover (Urban and Stribrny, 1985). This could also happen during interglacial warming and sea-level highstand leading to transgression. Deposition of Mn oxide could take place above the redox interface. Iron was fractionated and

accommodated in banded iron-formation. In interglacial late Proterozoic Xiangtan-type deposits (Xiang- tan, Minle, Datangpo, Tangganshan), China (Ye et al., 1988), lenses of Mn carbonate are conformably enclosed in pyritiferous black shale that denotes an anoxic event concomitant with peak sea level during deglaciation. As before, this situation was favorable for Mn concentration in solution. It is possible that, below the redox interface, organically derived CO2 produced by sulfate reduction led to Mn carbonate formation.

Considering the very widespread distribution of banded iron-formation, commercial Mn deposits hosted in it are rare. Besides those in glaciogenic sequences, the most remarkable occurrence is the giant Kalahari Mn deposit (Hotazel Formation, Transvaal Supergroup, South Africa; Table 3) where two major ore types, braunite-kutnohorite-Mn calcite-hausmannite-jacobsite (oxide-carbonate) and braunite- bixbyite-hausmannite-hematite (oxide), occur (Beukes, 1973, 1983). Evidence of marine transgression in the sequence and data on trace element and REE concentrations in the ores led Beukes (1989) to conclude that Mn+•-enriched anoxic water from a stratified ocean produced these deposits in the shelf area. The 81•C values of the Mn carbonate suggest its formation by early diagenetic organic carbon oxidation and coincident Mn reduction (Beukes, 1989).

The formation of deposits hosted in carbonate rocks in shelf regions (Tables 3 and 4) has mostly been attributed to transgression events and stratified oceans. Their shallow-water deposition is further confirmed by the presence of stromatolites (Lukoshi

Complex, Zaire; Table 3) and evaporites (deposits of the Falang Formation and Xialei, China; Ulu Telyak, U.S.S.R.; Um Bogma, Sinai; Table 4). These deposits generally consist of Mn carbonate, though in the Sau- sar and Penganga Groups (India), Imini-Tasdremt (Morocco), and Um Bogma (Sinai), Mn oxides consti- tute the ores. These Mn oxides were formed by primary deposition on the shelf above the redox interface using the carbonate rocks merely as substrates (Roy, 1981; Roy et al., 1990b). For the Imini deposit a ground-water mediation model has been suggested (Force et al., 1986; Thein, 1990).

The temporal and spatial spread of black shale- hosted Mn carbonate deposits throughout the Proter- ozoic (Table 3) indicates that recurrent ocean anoxia and stratification with transgression during sea-level highstand were responsible for Mn deposition (e.g., Moanda, Franceville Series, Gabon; Serra do Navio, Amapa Series, Brazil; Tangganshan deposit, China; Table 3). The Wafangzi deposit in the transgressive Telling Formation (ca. 1200 Ma), China, shows a facies variation from rhodochrosite to manganite, hosted correspondingly in black shale and sandstone- claystone (Ye et al., 1988) and indicating Mn deposition from stratified sea above and below the redox

interface intersecting the cratonic shelf during transgression (cf. "zoned" deposit of Force and Cannon, 1988). Even in the Archean, the Mn carbonate deposit (now metamorphosed to rhodochrosite-rock and Mn silicate-carbonate rock) at Morro do Mina, Rio das Velhas Series, Brazil, is hosted in graphitic schist and suggests deposition in anoxic (or dysaerobic) conditions (Dorr et al., 1956). A similar depositional milieu for the Mn silicate-carbonate rock in the

Archean Eastern Ghats sequence, India, is suspected. Most of the original depositional signatures in these deposits have been erased by later deformation and high-grade metamorphism.

Phanerozoic black shale-hosted Mn carbonate de-

posits occur in the Middle Ordovician Modaoxi For- mation of Hunan Province, China. Mn carbonate beds are intercalated with black shale and Fe-Mn-

bearing limestone in the Taojiang deposit. The sequence shows evidence of transgression and regression and the ore-bearing horizon is located in a zone between peak transgression and beginning of regression (Fan Delian, pers. commun., 1988; Ye et al., 1988). The 81•C/pDB / values of--5 to --18 per mil in Mn carbonate indicate that a significant proportion of carbon was derived from organic matter through SO• or MnO• reduction during early diagenesis in dysaerobic to anoxic environments (Okita and Shanks, 1988).

Giant- to moderate-sized manganese deposits, temporally and spatially widespread, occur in shallow- water sandstone-claystone formations (Tables 3 and 4). Most deposits of this type consist of Mn oxide formed in stratified marine basins above the redox

1232 SUPRIYA ROY

interface (e.g., Iron Ore Group, Eastern Ghats, Chi- tradurga Group, Aravalli Supergroup, Gangpur Group, India; Serra do Jacobina, Brazil; Groote Ey- landt, Australia). In Nikopol, Ukraine, and Chiatura, Georgia, deposits, however, primary oxide and carbonate facies formed in shoreward and basinward

areas controlled by redox potential (Varentsov and Rakhmanov, 1980; Bolton and Frakes, 1985). Evi- dence of marine transgression has been recorded from many of these ore-bearing sequences.

Summary and Conclusions Following the foregoing discussions, certain im-

portant aspects of manganese deposition in modern and ancient environments may be reiterated:

1. Primary Mn deposits are forming at present by low-temperature hydrothermal precipitation (submarine or on land) or by sedimentary processes in the deep-sea, midwater column of oceans and shallow- marine and lacustrine environments. Mn is concen-

trated in solution in reducing and acidic conditions and is precipitated by optimum oxygenation in suitable pH. In both these processes, direct or indirect biological participation is important.

2. Hydrothermal input of Mn from modern sea- floor-spreading centers is independent of spreading rates, but deposition at vent sites takes place only with low flow rates. During high temperature and a high flow rate of emission of hydrothermal solution, Mn is rarely, if at all, precipitated in situ and is mostly advected far and near field, depending on ocean circulation and residence time. Far-field transfer of Mn

may enrich the sediments away from the domain of hydrothermal activity.

3. The source of Mn for sedimentary deposition in modern basins may be multiple, such as terrigenous input (as indicated for concentration in oxygen-minimum zone) and far-field advection from hydrothermal discharge. No dependable budget for either is known.

4. The abyssal Fe-Mn nodules should not be cate- gorized as totally hydrogenous or totally early diagenetic based on bulk composition because in most cases the compositions of finer laminae indicate that both sources were operative through their complex growth history. The pathway of transfer to the nodules of metals which were concentrated in early diag- netic sediment pore water is also not clear. It is possible that instead of direct and total transfer of the

metals to the nodules, as generally visualized, sediment pore water supplies most of its metals to the bottom water episodically. The episodes of such enrichment of the bottom water in metals may be re- flected in the early diagenetic chemistry of the then growing nodule surfaces. In intervening periods of cessation of pore-water metal supply, the unenriched bottom water may act as the sole contributor of metals to the nodules of hydrogenous composition.

5. The pitfalls in radiometric determination of growth rates of deep-sea Fe-Mn nodules indicate that these rates are at best average values and cannot be used to deduce the growth history of the nodules. Petrographic study (including mineral chemistry) of the nodules is essential to the understanding of their growth history. The aspect of intranodule diagenesis is important and needs further documentation. Miner- alogy and microstructure do not per se indicate any specific genetic process. An average slow growth rate is not contested at present in view of the agreement of results obtained from different methods but cer-

tainly the basic tenets need reassessment. 6. The practice of denoting ancient sedimentary

Mn deposits as volcanogenic (and/or exhalative) and nonvolcanogenic (terrigenous source) is not precise. The deposits should simply be classified into hydrothermal and sedimentary types based on solid geological-geochemical attributes. To call sedimentary deposits volcanogenic owing to mere spatial proximity of volcanic rocks is too simplistic and may be errone- ous (cf. hydrogenous Fe-Mn crusts on volcanic substrates in modern oceans).

7. The midwater column oxygen-minimum zone in modern oceans and the anoxic-oxic stratification in

today's Black Sea show that anoxic water columns act as major reservoirs of Mn, Fe, and other elements. Mn (and Fe) advects toward the redox interface and on crossing it oxidizes and precipitates on impinging substrates. These modern situations serve as models

for the genesis of ancient Mn deposits. The anoxic- oxic stratification in ancient oceans was related to

sea-level changes. During a sea-level highstand the anoxic zone could expand to the sea floor, with the redox interface near the surface, and transgression on the cratonic shelf could precipitate Mn as oxide-hydroxide above the redox interface on basin-margin shallow-shelf substrate independent of the host-rock type. Early diagenetic-coupled organic matter oxidation and sulfate reduction led to the formation of Mn

carbonate by reaction between dissolved Mn +2 and CO2 in the anoxic domain.

8. The extensive occurrence of black shales host-

ing Mn carbonate deposits far back in the Precam- brian suggests development of stratified basins at that time. This was perhaps an expression of the CO•- enriched atmosphere, warm climate, presence of biota, a sea-level highstand, and corresponding transgression. Transgressive sequences hosting Mn deposits abound from the early Proterozoic to the Ce- nozoic suggesting stratified basins as the major source for ancient Mn deposits.

Acknowledgments

This study is based on original investigations by fel- low participants in IGCP Project 226 "Correlation of Manganese Sedimentation to Palaeo-environments" and by others. Thanks are due to my colleagues S.


Dasgupta, P. Sengupta, and H. Banerjee for constructive criticism during the preparation of this paper. Constructive reviews by Economic Geology referees helped much in preparing the final draft.

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