Manganese metallogenesis: A review

Ore Geology Reviews, 4 (1988) 155-170 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

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MANGANESE METALLOGENESIS: A REVIEW SUPRIYA ROY Centre of Advanced Study in Economic Geology, Department of Geological Sciences, Jadavpur University, Calcutta 700 032 (India) (Received March 2, 1987; revised and accepted August 27, 1987)

Abstract Roy, S., 1988. Manganese metallogenesis: a review. Ore Geol. Rev., 4: 155-170. Metallogenesis of manganese in space and time is reviewed here in the light of the progressive development of the atmosphere, the hydrosphere and the lithosphere attendant with the varied styles of tectonism. Economic deposits of manganese first appeared c.3000 Ma ago, postdating by at least 800 million years the oldest known geological sequence containing iron-formation and base-metal sulfide ores. The development of manganese deposits in the Archean as a whole, vis-h-vis that of iron-formation and stratiform massive sulfides, was minor. This is possibly a reflection of the composition of the then endogenic exhalations and/or the character of the atmosphere and the hydrosphere. The geologic setting of the manganese deposits of this age was always atypical of the Archean period. Deposition of manganese was intensified with the advent of the Proterozoic with the changing tectonic style leading to stabilization of the cratons and oxygenation of the hydrosphere and the atmosphere. Large to superlarge deposits were formed mainly through terrigenous input during this period. The Mesozoic era ushered in the supremacy of manganese deposition and the peak was reached in Cenozoic time. This was largely due to the formation of giant shallow-water deposits in areas of marine transgression as well as deposition of manganese-rich nodules and crusts in deep-sea environments. Three major aspects of manganese metallogenesis stand out as most important but enigmatic. These are: (a) the extreme fractionation of iron and manganese in nature and their reverse trend of metallogenic development; (b) the universal record of shallow-water deposition of manganese in land-based deposits in contrast with the deep-sea milieu observed in modern basins; and (c) the common evidence of biological activity in close association with manganese deposition which could be either causal or casual. All these aspects merit further in-depth study and metallogenic analysis in a broad spectrum.

Introduction As the general understanding of the formation of ore deposits of different metals and their localization in space and time becomes more and more clear, it is evident that despite the apparent diversity, the varying ore-types are not freaks, nor were these formed in isolation in respect of the contemporary geological environments. Rather, these were produced in optimum conducive geological situations at specific pe-

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riods during the evolution of the global geological system. Therefore, attempts have been made during recent years to evaluate the role of the various ore-forming processes and to integrate these episodes into the broad panorama of development and evolution of the earth's crust from the initial stages to the present. While the ore deposits of other metals have often been discussed at considerable length in terms of metallogenic evolution, those of manganese did not receive adquate attention till re-

156 cently when the vast resources on modern ocean floors became known and were assessed. In this paper an attempt will be made to tie the global metallogeny of manganese to the variety of earth processes in different environments during the evolution of the lithosphere, atmosphere, hydrosphere and biosphere spanning the entire geological history of this planet.

Archean inception The formation of manganese deposits in Archean time was distinctly inhibited, in contrast with the steady development of iron-formation during this period starting from the Isua Supracrustals, Greenland, dated at 3800 Ma (Gole and Klein, 1981; J a m e s and Trendall, 1982). Economic manganese deposits first appeared only around 3000 Ma in the Iron Ore Group, India (3200-2950 Ma), after a substantial period of ongoing evolution of the crust, the hydrosphere, the atmosphere and the biosphere. Only five economic manganese deposits of Archean age have so far been identified, of which three occur in India, two in Brazil (Table 1) and none in otherwise well developed Archean terrains elsewhere in the world. Thus, the meagre metallogenesis of manganese during the Archean was also highly selective in space. Subeconomic manganese concentrations of Archean age are recorded in limestone at SaganZaba, U.S.S.R. (Varentsov and Rakhmanov, 1980 ) and in metamorphosed silicates in Khapchensk Series, Anabar Massif, U.S.S.R. (Var-

entsov et al., 1984). Only two economic deposits occur in greenstone belts and even these are restricted to the upper sedimentary cover (Rio das Velhas Series, Brazil and Chitradurga Group, India). These apparently are not related to volcanics in time or space. The deposits of Chitradurga Group show evidence of deposition in shallow water in an Archean intracratonic basin of "Proterozoic character" (cf. Kaapvaal Basin, South Africa: Anhaeusser, 1981; Condie, 1981; Radhakrishna, 1983). An Archean high-grade region hosts the manganese deposits of the Khondalite sequence, Eastern Ghats, India ( > 2 6 0 0 Ma). These deposits were originally deposited in shallow-water intracratonic aulacogens in a sandstone-shale-limestone-evaporite sequence and were metamorphosed to granulite facies (Sarkar, 1980). The Archean high-grade region of Brazil in Bahia State (2800-2600 Ma) also hosts sedimentary manganese deposits metamorphosed to granulite facies (Marau, Santo Antonio de Jesus, Coaraci, Cachoeira; Valarelli et al., 1976). The orebodies of Iron Ore Group, India (3200-2950 Ma), along with the associated litho-units, were deposited in shallow-water stable shelf environments and are only incipiently metamorphosed. The Iron Ore Group sequence is rather different from the typical Archean formations that are represented by high-grade regions and greenstone belts (Anhaeusser, 1981; Sarkar, 1984). It is evident, therefore, that while manganese metallogenesis was initiated in the Archean, the

Dr. Supriya Roy, Professor of Geologyat Centre of Advanced Study in Economic Geology,Jadavpur University, Calcutta, received Ph.D. and D.Sc. degreesin 1957and 1963from the University of Calcutta. He is recipient of several academic awards including the National Mineral Award (1969) of the Government of India for his research contributions on metallogenesis and ore-petrology, mainly in respect of manganese ore deposits. He was elected a Fellow of the Indian National Science Academy in 1972. He served as the Leader of the IGCP Project No. 111: Genesis of Manganese Ore Deposits during 1978-1985 and is currently Joint Leader of the IGCP Project No. 226: Correlation of Manganese Deposits to Palaeoenvironments. Professor Roy has been serving as Vice-President of the IAGODCommission on Manganese for the last several years.

157 TABLE 1 Archean deposits of stratiform manganese ores, iron-formation and massive sulfide ores Age (Ma)

Iron formation

Massive sulfide deposits

c.

3800

Isua Complex, Greenland

Isua Complex, Greenland

c.

3700-3000

Substantial development in Africa, Australia and South America

Yilgarn and Pilbara Blocks, Australia {c. 3500 Ma)

3200-2950

Iron Ore Group, India

Iron Ore Group, India

> 2700

Rio das Velhas Series, Brazil

Rio das Velhas Series, Brazil; Bahia State, Brazil (28002600)

c.

2700

Michipicoten - Vermillion Deposits, Canada and U.S.A.

Major Canadian volcanogenic sedimentary deposits (2730-2655)

c.

2600

Chitradurga Group (? = Sandur Belt), Dharwar Supergroup, India

Chitradurga Group, Dharwar Supergroup, India

deposits were all concentrated in shallow-water intracratonic basins which were more of Proterozoic rather than Archean in character. While most stratiform ore deposits of Archean age are considered to have been derived from endogenic sources (cf. volcanogenic/hydrothermal massive sulfides and iron-formations: Franklin et al., 1981; Gross, 1980, 1983; James and Trendall, 1982 ), the manganese deposits do not indicate such a derivation. Thus, even the minor metallogenesis of manganese of this period is atypical of the Archean style. To explain the general paucity of manganese deposits in the Archean and their total absence in the earlier part, the following possibilities may be considered: (a) In the early stages of the development of this planet, volcanic/hydrothermal exhalations were too deficient in manganese for economic concentration. While most of the volcanogenic/hydrothermal massive sulfide deposits of Proterozoic and Phanerozoic age demonstrate a prominent

Manganese deposits

Khondalite sequence, Eastern Ghate, India ( > 2600) Chitradurga Group (? ---Sandur Belt), Dharwar Supergroup, India

manganese halo (Stumpfl, 1979; Roy, 1981 ) indicating significant presence of manganese in the exhalations, no such association of manganese is reported from any of the Archean volcanogenic massive sulfide deposits. This suggests a deficiency of manganese in endogenic exhalations of Archean time. The observation of Sangster (1976) that Archean oceanic sediments and magma were poor in manganese (and lead) is relevant in this context. (b) The deposition of manganese was inhibited in the Archean, in the earlier stage in particular, in an anoxic or oxygen-poor environment (Lambert and Groves, 1981 ). Owing to differences in solubility, manganese could not accompany iron during the latter's deposition in a limited oxygenated environment. The development of manganese deposits only in late Archean basins of"Proterozoic type" indicates that the deposition of this metal was dependent on a particular tectonic condition related to basin

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development and an optimum build up of oxygen in the atmosphere and hydrosphere.

Proterozoic growth Consequent to the increasing stabilization of the thin, dominantly volcanic Archean crust with time, the onset of the Proterozoic was marked by the development of large shallow sinking basins that acted as repositories of thick sediment piles interlaced with volcanics. By this time, substantial free oxygen was produced by photosynthesizing prokaryotes and the hydrosphere (and atmosphere) was sufficiently oxygenated to support significant biota and was capable of triggering deposition of iron and manganese in amounts much greater than so far permitted. Incidentally, iron-formation attained its peak of development in the early Proterozoic. During Proterozoic time manganese deposits started to develop on a substantially larger scale (Table 2). The early Proterozoic Kalahari manganese field, South Africa

(Transvaal Supergroup: 2500-2100 Ma ), representing deposits in the Hotazel Formation, is enormous and is reportedly estimated at about 7500 million tons of plus-30% ore. This is perhaps the largest single land-based deposit in the world (Beukes, 1983, 1984). These deposits show interstratification with iron-formation, occasional association with hyaloclastites and andesitic pillow lavas and mineralogical assemblages that exhibit variation in different beds from bottom to top (Beukes, 1983; Roy, 1981). Based on evidence of interfingering of pillow lavas with ore horizon in the Hotazel Formation, Beukes ( 1983, 1984) assigned a volcanogenicsedimentary origin to these manganese deposits in contrast to the earlier ideas of their terrigenous-sedimentary character (De Villiers, 1970; SShnge, 1977; Roy, 1981). However, Beukes (1983) admits that "the exact geographical relationship of the volcanic source to the manganese deposits has as yet not been fully established" (p. 187). The source of metal for this giant deposit is thus possibly not yet un-

TABLE 2 Proterozoic deposits of stratiform manganese ores, iron-formation and massive sulfide ores Age (Ma)

Iron-formation

2650-2350 2600-1900 2600-1700 2500-2100

Major deposits in W. Australia and U.S.S.R.

2500-1850

Transvaal Supergroup, S. Africa Superior-type Deposits, U.S.A. and Canada

Decline in iron-formation

c. 1350 c. 900 850- 550 720- 590 800- 500

Minas Series, Brazil Morro do Urucum, Brazil Rapitan Group, Canada Damara Supergroup, Namibia Maliy Khingan, U.S.S.R.

Massive sulfide deposits

Major initiation of sedimenthosted massive sulfide deposits

Massive sulfide deposits hosted in sediments and volcanics

Manganese deposits

Transvaal Supergroup, S. Africa. Graphite System, Malagasy (2420) Birrimian System, W. Africa (2100-1800 ) Aravalli Supergroup (c. 2000), Sausar Group (c. 2000), Gangpur Group (c. 2000-1700), India. Lukoshi Complex, Zaire ( > 1845 ). L~ngban, Sweden (1900-1750) Franceville Formation, Gabon (c. 2140) Wafangzi, China (Middle Proterozoic) Minas Series, Brazil Morro do Urucum, Brazil Damara Supergroup, Namibia Maliy Khingan, U.S.S.R. Penganga Beds, India (c. 775) Xiangtan-type, China (Sinian)

159 equivocally determined. Beukes (1983) also observed that the amount of manganese associated with iron-formation increases upwards in the Transvaal Supergroup and the lower level Mn+2-bearing carbonates change to Mn +4 and Mn ÷~ oxides in the upper stratigraphic levels, obviously in response to changing Eh and pH of the basinal water. This specifically demonstrates the control of manganese deposition and its fixation in different valency states by the quantum of oxygenation achieved by the hydrosphere and atmosphere with progress of time. Manganiferous stromatolites have been described from Botswana in a sequence correlatable to the Transvaal Supergroup (Litherland and Malan, 1973). A manganese orebearing horizon older than that of the Kalahari manganese field, but within the Transvaal Supergroup, occurs in karst topography in the Campbellrand dolomites in the PostmasburgSishen area, South Africa (Beukes, 1983; De Villiers, 1983; Roy, 1981 ). Manganese deposits of the Graphite System (2420 Ma), Malagasy, are rather small and the primary features have been obliterated by metamorphism. Many of the major Proterozoic deposits were formed between 2100 Ma and 1700 Ma. These include the volcanogenic-sedimentary deposits of the Birrimian System (2100-1800 Ma) of West Africa (Ghana, Ivory Coast, Upper Volta, Eastern Liberia and Guinea) where the orebodies are conformably related with siliceousshaly formations in a mafic volcanic setting. The small but interesting deposits of Svecofennian age (1900-1700 Ma) at Langban, Harstigen and other areas in Central Sweden are restricted to the volcanogenic-sedimentary metamorphosed leptite formation. It has been suggested that these deposits represent oreconcentration either at a spreading centre or at a subduction zone in an island-arc basin during Precambrian time (BostrSm et al., 1979). The early Proterozoic deposits of India (Aravalli Supergroup: c. 2000 Ma; Sausar Group: c. 2000 Ma and Gangpur Group: c. 2000-1700 Ma) were all deposited in shelf environments. They

are terrigenous-sedimentary in origin and were later metamorphosed. The metasedimentary deposit of Lukoshi Complex (> 1845 Ma), Zaire, consists of stromatolitic (Collenia) M n carbonate formation (shallow-water) interbedded with Mn-silicate rocks and graphite schists (Doyen, 1973). A large terrigenoussedimentary manganese deposit was formed in a transgressive, somewhat euxinic basin at Moanda, Gabon (Franceville Series: c. 2140 Ma) consisting of diagenetic manganese carbonate associated with carbonaceous black shales and dolomites (Leclerc and Weber, 1980 ). The middle Proterozoic terrigenous-sedimentary manganese deposits of Minas Series (youngest age c. 1350 Ma), Brazil, are associated with iron-formation and dolomite (Dorr, 1973). Important sedimentary manganese deposits of China are also middle Proterozoic in age. The deposit at Wafangzi in the Tieling Formation occurs in a sequence of black shale and calcareous siltstones and consists of manganite and Mn-carbonates. In one part of the deposit (Baoshemiro area), the ores show metamorphic changes. Boron-bearing manganese carbonate ores occur in muddy and silty dolostones in the Gaoyuzhuang Formation. All these deposits occur in stable platform areas and were formed in shallow-marine environments (Yeh et al., 1986). Many of the late Proterozoic manganese deposits of significant size similarly occur interstratified with iron-formation. The deposit at Morro do Urucum, Brazil (c. 900 Ma) is interbedded with iron-formation and was laid down at the transition stage of continental to marine regimes of sedimentation. In Damara Supergroup, Namibia (720-590 Ma) and Maliy Khingan, U.S.S.R. (800-500 Ma), manganese deposits are also associated with iron-formation. Conversely, the most characteristic late Proterozoic iron-formation, Rapitan Group, Canada, does not contain any manganese deposit. All these deposits have been assigned terrigenous-sedimentary origin (Chebotarev,

160 1960; Dorr, 1973; Roper, 1956). The iron-formations associated with these deposits are stratigraphically related to glaciogenic formations and the possibility of their deposition in glacial environment has been suggested (Young, 1976; James and Trendall, 1982 ). The Xiangtan-type stromatolitic Mn-carbonate deposits of Sinian age in China occur in tillite beds (Yeh et al., 1986). On the other hand, late Proterozoic manganese oxide deposits of Penganga Beds, India, occur in a transgressive sequence and are not associated with iron-formation. The orebodies are interstratified with chert and included in limestone in a sequence of sandstonelimestone-shale, devoid of volcanism or other evidence of endogenic activity. The presence of phosphatic cement and suspected organic forms in interlayered chert and ore suggest contemporary biological activity. Volcanogenic-sedimentary manganese deposits were also formed in the late Proterozoic period such as those of Tiouine, Idikel, Migouden, Oufront (1050-900 Ma) in the Anti Atlas region of Morocco. These are related to felsic volcanics on platforms and are associated with sediments of continental type. Development of manganese deposits in the Proterozoic, therefore, was in substantially larger scale than in the Archean. Contrasting genetic models for the giant Kalahari manganese deposits, South Africa have been suggested and the derivation of these ores from a terrigenous or volcanogenic source is as yet conjectural. The important observation, however, is that in the Transvaal Supergroup manganese exhibits a continuous enrichment as well as a progressively higher oxidation state with correspondingly higher stratigraphic level and with the progress of time (Beukes, 1983). Such features indicate an overall progressive increase in Eh and pH with time suggesting transition from Proterophytic to Paleophytic type (cf. Cloud, 1973). The manganiferous stromatolite from a sequence correlatable to the Transvaal Supergroup in Botswana also indicates biological assistance in precipitation of

manganese (cf. SShnge, 1977). Biological influence in manganese deposition is also apparent in orebodies of the Lukoshi Complex, Zaire, Franceville Series, Gabon; and possibly in the Penganga Beds, India. Most of the significant manganese deposits of this period are terrigenous-sedimentary, deposited in shallow water in shelf conditions (cf. deposits in India; Moanda, Gabon; Minas Series and Morro do Urucum, Brazil etc.). Some of these (cf. Morro do Urucum, Brazil; Moanda, Gabon; Penganga Beds, India) were formed during marine transgression. Unequivocal volcanogenic-sedimentary manganese deposits were relatively minor in this period (cf. Birrimian System, W. Africa; L~ngban-type deposits, Sweden; Anti Atlas deposits, Morocco) and many of them are quite small in size. In other words, the source for manganese in the Proterozoic was both endogenous and exogenous, the latter apparently predominating over the former. All major deposits were formed in shallow-water stable shelf environments. The formation of manganese deposits on a substantial scale in the Proterozoic was thus triggered by a combination of progress in basinal development and oxygenated atmosphere and hydrosphere and was casually or causally attended by biological activity. Volcanic episodes were not the overpowering factor in Proterozoic metallogeny of manganese. Phanerozoic supremacy

The Phanerozoic metallogeny encompasses diverse types of ores concentrated in varied environments (Table 3). Plate motions largely controlled the tectonic regime and played an important role in the formation of ore deposits of different types and their specific localization. A variety of ores are produced by volcanism and hydrothermal activity in diverging and converging plate boundaries as evidenced by the presently forming deposits (cf. Mn-oxide crusts at TAG field, 26°N on Mid-Atlantic Ridge: Rona, 1984; Zn-Fe-Cu sulfide deposits formed by black smokers at 21°N and other areas on

161 TABLE 3 Phanerozoic deposits of stratiform manganese ores, iron-formation/ironstone and massive sulfide ores Age

Iron formation/iron stone Massive sulfide deposits (IF) (IS) (only significant deposits )

Manganese deposits

Paleozoic

Harlech Dome, Wales; Mazulskii, Durnovso (U.S.S.R.)

Cambrian

Ordovician

Minor IF

Bathurst, Canada

Timna Dome, Israel

Silurian

Captains Flat, Tasmania; Buchans, Takhta Karacha, Dautash, U.S.S.R. Canada

Devonian

Rammelsberg, Meggen, Germany

Carboniferous Last IF

Rio Tinto, Spain & Portugal; Tyn- Huelva Province, Spain & Portugal; Glibagh, Ireland en-Nam, Morocco Besshi, Japan (late Paleozoic) Ulu Telyak, U.S.S.R.

Permian

Magnitogorsk, Atasu (Dev.-Carb.); U.S.S.R.

Mesozoic

Jurassic

Northern Apennine Ophiolite Complex, Italy

Northern Apennine Ophiolite Complex, Italy; Bou Arfa, M'koussa, Tiaratine, Morocco; Molango, Mexico; Tethyan Alps, Germany-Austria-Sicily; Urkut, Hungary; Franciscan Formation, U.S.A.

Cretaceous

Troodos Massif, Cyprus; Semail Nappe, Oman; Nicoya Complex, Costa Rica; Madenkoy, Turkey

Troodos Massif, Cyprus; Semail Nappe, Oman; Nicoya Complex, Costa Rica; Eastern and Western Timor; Groote Eylandt, Australia; Imini-Tasdremt, Morocco

Cenozoic

Eocene

Olympic Peninsula, U.S.A.; Oriente Prow ince, Cuba; Vitilevu, Fiji

Oligocene

Solomon Islands

Solomon Islands; Nikopol-Chiatura type deposits, U.S.S.R.; Varna, Bulgaria

Miocene

Kuroko deposits, Japan

Kokko, Pirika, Japan

Pliocene

Lake Mead, U.S.A.; Lucifer, Mexico

Pleistocene

Tarapaca Province, Chile

Holocene

En Kafala, Ethiopia Recent ocean floor deposits Rare iron oxide/hydroxRed Sea: Zn-Cu-Fe ( -2-_Pb) Hydrothermal crusts on mid-oceanic ridge ide-rich crusts and nodules Juan de Fuca Ridge: Zn-Cucrests and ferromanganese nodules in intraFe( + Pb) plate areas of the world oceans East Pacific Rise 21 ° N: Zn-Cu-Fe Galapagos Spreading Centre: CuZn-Fe Mid-Atlantic Ridge (TAG Field): Cu-Zn-Fe

162

the East Pacific Rise: Rona et al., 1983). Similar ore-forming processes acted in the geologic past as recorded in ophiolitic complexes (base metal sulfides, iron and manganese oxides ) and well defined island-arc suites (base metal deposits of Kuroko and porphyry copper types). In intra-plate pelagic parts of the modern oceans, hydrogenous and early diagenetic (oxic) ferromanganese crusts and nodules, often en-' riched in Ni, Cu, Co and occasionally noble metals, form extensive deposits (Cronan, 1980; Heath, 1981; Roy, 1981). Present day shallow seas, lochs and freshwater lakes also show occurrences of ferromanganese nodules. The geologic record on land, as well, demonstrates the occurrence of "fossil" ferromanganese nodules associated with ancient pelagic sediments that were thrust over the continents (Jenkyns, 1977; Margolis et al., 1978). The generally oxygenated alkaline basinal water and the direct or indirect action of specific biological communities, triggered the development of oxide ores in the Phanerozoic, though stagnant euxinic basins also served as depositional sites of specific oretypes of black shale facies (cf. Paleozoic Kupferschiefer-type and Fe-sulfides in the central part of the modern Black sea). This eon witnessed the final exit of iron-formation in early Paleozoic. Iron, however, continued to f o r m sedimentary ironstone deposits throughout the Phanerozoic, but was not necessarily related to manganese deposition. Manganese deposits were moderately developed in the Paleozoic era. Most of these (cf. Mazulskii, Durnovso, Magnitogorsk, Atasu: U.S.S.R.; Huelva Province, Spain-Portugal; Glib-en-Nam, Morocco) are volcanogenic-sedimentary in character, occurring in association with both mafic and felsic volcanic rocks. A few terrigenous-sedimentary deposits were also located in variable geologic settings. The concretionary manganese ores of the Ordovician Timna Dome deposit, Israel, are associated with terrigenous sediments, particularly red shales and these concretions were considered by Jenkyns (1977) as pelagic "fossil" nodules. The

early Carboniferous Um Bogma deposit (Sinai) was formed in tidal pools in flat intertidal environments by sedimentary-diagenetic concentration (Mart and Sass, 1972). The Ulu Telyak deposit, U.S.S.R. (early Permian) was formed in shallow water during a period of marine transgression (Varentsov and Rakhmanov, 1980). By and large, the manganese deposits of the Paleozoic era are much less developed than those in the rest of the Phanerozoic eon. The Mesozoic era ushered in large-scale development of sedimentary manganese deposits. Important deposits at Tiaratine and Bou Arfa (Jurassic) and Imini-Tasdremt (Cretaceous) in Morocco are hosted in dolomite. Most of the earlier workers (see Roy, 1981 ) considered these deposits as terrigenous-sedimentary, though Pouit (1982) suggested a distal volcanogenic origin. Rakhmanov and Tchaikovsky (1980) also noted the absence of evidence of volcanic activity in the Imini-Tasdremt deposits, suggested the participation of microorganisms in the process of accumulation of the ores but rather inexplicably concluded that the source of metals was endogenous. In any case, the opinions were unanimous that these deposits were developed on platforms during marine transgression-regression cycles. The late Jurassic large terrigenous-sedimentary manganese carbonate deposit of Molango, Mexico, occurring as thin rhythmic interbeds with limestone, was also formed on a platform during marine transgression and under a strongly reducing environment (Tavera and Alexandri, 1972). The early/middle Cretaceous giant manganese oxide deposit of Groote Eylandt, Australia, has been considered by all as shallowmarine sedimentary/diagenetic concentration of nonvolcanic derivation in a shelf environment (Smith et al., 1965; Slee, 1980; Ostwald, 1981; Frakes and Bolton, 1984). However, the precise mechanism of ore formation suggested so far varies considerably in specific modes and details. A number of workers (cf. Slee, 1980)

163 considered that the source of manganese was the neighbouring terrestrial rocks and that the dissolved manganese was carried to the basin giving rise to ore concentration either by direct (inorganic) sedimentation or by diagenetic remobilization from pre-formed manganesebearing members. Ostwald (1981) concluded that biological participation was responsible for manganese deposition and suggested various organic structures as evidence. While this mechanism is plausible in the background of the widely accepted idea of biologically induced precipitation of manganese, the evidence presented, as in other similar cases (cf. LaBarge, 1973 ) may be either causal as suggested or may be argued as simply casual. The most interesting mechanism for concentration and deposition of manganese has been suggested by Frakes and Bolton (1984) drawing critically from the concept of high sea-level stand and attendant anoxia during marine transgression leading to manganese concentration, and interpreting field data on graded units of Mn-oxide pisoliths and ooliths of Groote Eylandt deposit. In this deposit, both normal and inverse graded units are present and the former have been correlated to deposition during transgression and the latter to precipitation by oxygenation during regression. Thus, at Groote Eylandt, Frakes and Bolton (1984) suggested build up of dissolved manganese at high sea-level stand and anoxia during transgression with minor deposition (normal graded units) and precipitation in bulk due to oxygenation during shallowing or regression (inverse graded bedding). During a short visit to this deposit in 1986, the present author could observe these important features. Through this admittedly limited exposure, he is inclined to accept the Frakes-Bolton model of manganese ore deposition at Groote Eylandt. Varentsov (1982) suggested a supergene origin of the manganese deposits of Groote Eylandt in the weathering zone, a contention that may perhaps be related only to a part of the ores that suffered supergene modification post-dating sedimentary deposition.

On the basis of the observation that many large and moderate-sized manganese deposits of Precambrian (Moanda, Gabon; Morro do Urucum, Brazil), Mesozoic (Molango, Mexico; Imini-Tasdremt, Morocco; Groote Eylandt, Australia) and Cenozoic ages (Nikopol-Chiatura, U.S.S.R. ) are typically related to marine transgression episodes, Cannon and Force (1983) constructed a model interrelating transgression-high sealevel stand-anoxia with manganese accumulation. Concurrently Frakes and Bolton (1984; at Groote Eylandt) and Bolton and Frakes (1985; at Chiatura) demonstrated the application of a slightly different model and identified regression with shallowing and oxygenation as an important factor for manganese ore deposition. Manganese deposits formed by hydrothermal process in the oceanic spreading centres and later obducted on land as part of ophiolitic suites are common in the Mesozoic (cf. Apennine Ophiolitic Complex, Italy: late Jurassic; Franciscan assemblage, California, U.S.A.: late Jurassic to early Cretaceous; Troodos Massif, Cyprus; Semail Nappe, Oman; Nicoya Ophiolitic Complex, Costa Rica; Las Bela Ophiolites, Pakistan and Buena Vista, Solomon Islands: late Cretaceous). In these ophiolitic complexes, small manganese deposits occur in the upper stratigraphic level, either in direct contact with the volcanics or enclosed in sediments just overlying the volcanics (Crerar et al., 1982; Robertson and Boyle, 1983; Roy, 1981). Massive volcanic-hosted sulfides usually underlie the manganiferous formations. "Fossil" ferromanganese nodules derived by hydrogenous deposition on pelagic ocean-floor without much volcanic/hydrothermal input, have also been recorded from many Mesozoic formations (cf. Northern Limestone Alps, Germany-Austria; Rocca Argentaria and Rocca Busambra, W. Sicily: Jurassic; Eastern and Western Timor: Cretaceous). These "fossil" nodules occur in limestone, radiolarian chert and red clay which were originally deposited in deep-sea pelagic areas and were later raised or thrust onshore

164

(Jenkyns, 1977; Margolis et al., 1978; Roy, 1981 ). Small to moderate-size volcanogenic-sedimentary/hydrothermal manganese deposits are common in the geologic record of the Cenozoic era (cf. Oriente Province, Cuba; Olympic Peninsula, U.S.A.; Viti Levu, Fiji; Kokko, Pirika, Inakuraishi, Japan; Tarapaca Province, Chile; En Kafala, Ethiopia). Some of these deposits, such as Olympic Peninsula (Garrison, 1973) and those in Oriente Province, Cuba (Mitchell and Bell, 1973) are considered to have formed originally on the ocean-floor and later been inserted on land. The Cenozoic era is particularly characterized by giant manganese deposits all of which are terrigenous-sedimentary in character. These deposits are all concentrated in the Oligocene Epoch in U.S.S.R. (cf. Chiatura, Nikopol, Bol'shoi Tokmak, Laba etc.) and are regarded to represent together the largest onshore repository of manganese ores (Maynard, 1983 ). These were all deposited in a near-shore region of shallow-water marine basins (Varentsov and Rakhmanov, 1980) and show general commonalities in respect of host rocks and mineral zonation (Sapozhnikov, 1970). There has been disagreement, however, on the source of the metal and the specific mechanism of ore concentration. While the majority view is in favour of a terrigenous source for the metals (cf. Varentsov and Rakhmanov, 1980; Roy, 1981), a few workers have suggested volcanogenic/hydrothermal input for the Chiatura deposit (Dzotsenidze, 1966, 1974; Khamkhadze, 1980; Makharadze, 1973; Mstislavskiy et al., 1984). Further, the oxide-carbonate zonation at Chiatura, Nikopol and other deposits has been attributed to offshore variation o f E h (Betekhtin, 1946; Sokolova, 1964; Sapozhnikov, 1970) as well as to partial transformation by diagenesis (Strakhov and Shterenberg, 1966; Varentsov and Rakhmanov, 1980). Bolton and Frakes (1985) discovered normal and inverse graded units of pisoliths and ooliths of Mn-oxides in the Chiatura deposit and emphasized the role

of transgression-regression cycles on ore deposition. They suggested extensive concentration of manganese in solution (fed by terrigenous rocks) in high sea-level stand and anoxia during transgression and precipitation of some Mn-oxide in peak period giving rise to normally graded units. Subsequent regression, concomitant with oxygenation, led to major precipitation of manganese showing inverse graded units of pisoliths and ooliths. In this respect, the Chiatura deposit is comparable to that of Groote Eylandt, Australia, discussed earlier. The geological record in the world oceans testitles to deposition of manganese nodules and crusts since the Cretaceous period, gradually increasing in intensity with time. These deposits occur as blankets on the floor of the pelagic part of the oceans and are also found buried at different stratigraphic levels in the sediments (Cronan, 1980; Heath, 1981; Roy, 1981). The nodules and crusts are largely formed in intraplate areas by hydrogenous a n d / o r early diagenetic processes. In equatorial high organic productivity zones (cf. Clarion-Clipperton fracture zone, NE equatorial Pacific and Central Indian Ocean Basin), these are enriched in Ni, Cu and other trace metals, for which a direct or indirect biological contribution is indicated. The total reserve of manganese in these nodules and crusts is enormous, exceeding the entire reserve on land perhaps several times over. At this moment, however, these deposits are in the category of submarginal resource potential in respect of manganese. Deposition of ferromanganese nodules and crusts in modern shallow seas and gulfs (Manheim, 1965; Varentsov, 1973) as well as in freshwater lakes (Callender and Bowser, 1976) has also been recorded but such deposits are minor in comparison with their deep-sea counterparts. During the Phanerozoic eon, therefore, a large variety of manganese deposits were formed in diverse geologic settings such as shallow-water shelf zones of intracratonic basins, deep-sea intraplate areas and in the plate boundaries. The stupendous scale of manganese deposition,

165

combining the onshore and ocean-floor occurrences, is also unprecedented. Deposits of hydrothermally derived manganese in oceanic spreading centres and hydrogenous and early diagenetic ferromanganese nodules and crusts are retained in the obducted sequences on land. Most of the onshore deposits of large size and formed in shallow-water domains are related to marine transgression-regression cycles that permitted accumulation of manganese in solution during transgression (high sea-level stand and anoxia) and the deposition of the metal partly during peak transgression and perhaps mainly during regression (lowering of sea-level and oxygenation). Such large manganese deposits formed during transgression-regression cycles abound in the Phanerozoic (cf. Groote Eylandt, Australia; Molango, Mexico; IminiTasdremt, Morocco; Ulu Telyak and NikopolChiatura, U.S.S.R.) though their presence in the Proterozoic has also been noted (cf. Morro do Urucum, Brazil; Moanda, Gabon; Penganga Beds, India). General r e m a r k s

The study of the entire spectrum of manganese metallogenesis must take into account certain important aspects that require proper assessment and explanation. Biological participation in manganese deposition is one of them. It has been demonstrated that deposition of manganese in marine and freshwater milieu is triggered by microorganisms through oxidation. Various types of Mn-oxidizing microbes have been studied in this context (cf. Dubinina, 1980; Dugolinsky et al., 1977; Greenslate et al., 1973; Ehrlich, 1972, 1975; Marshall, 1979; Perfil'ev and Gabe, 1965; Rosson and Nealson, 1982) and the presence of many of these in modern marine and lacustrine deposits have been recorded. A Mn-oxidizing microorganism (MetaUogenium) has been described from Cretaceous chert associated with "fossil" manganese nodules of Timor (Crerar et al., 1980) and from the Oligocene Chiatura deposit,

U.S.S.R. (Sokolova-Dubinina and Deryugina, 1966). As earlier stated, manganiferous stromatolites have been described from early Proterozoic deposits of Botswana (Litherland and Malan, 1973) and Zaire (Doyen, 1973) as well as the Sinian Xiangtan-type deposit of China (Yeh et al., 1986). All these suggest a strong probability of active biological participation in the formation of manganese deposits though the extent of such participation has not been assessed so far. Organic activity also indirectly facilitates concentration of manganese in suitable environments. At high sea-level stand during transgression, organic accumulations cause anoxia and thereby concentration of manganese in solution for subsequent deposition. It is, therefore, vital to establish the precise cause and effect relationship of biological activity and manganese deposition. The unprecedented concentration of manganese in modern deep-sea areas in contrast with the almost universal restriction of ancient deposits to shallow-water locales is an enigma. This difference in depositional milieu can perhaps be explained either by assuming consumption of all earlier deep-sea deposits in subduction zones (but for the minor ones obducted on land) or by attributing it to the absence of deep (bottom) oxidizing polar currents in earlier geological history. In the course of analysis of metallogenic evolution vis-a-vis the development of the lithosphere, it is clearly brought out that manganese and iron, neighbours in the periodic table, are generally fractionated in respect of each other during ore formation. This paradox is explicitly expressed by the diametrically opposite development of ore deposits of these two metals in respect of time. While deposition of iron ores (principally in iron-formation) dominated the Precambrian scenario and declined thereafter, the concentration of manganese in ore deposits started developing in a very modest scale in the late Archean, somewhat increasing during the Proterozoic and reached its zenith only in the later part of the Phanerozoic. The iron-forma-

166 tions themselves are poor in manganese and most major manganese deposits are poor in iron, indicating near-complete fractionation of the two metals. This puzzle also awaits a suitable solution.

S u m m a r y and conclusions The salient features of metallogeny of manganese through time are summarized below: (1) The first several hundred million years of available geological record is totally barren of economic manganese deposits. Metallogenesis of manganese was indeed poor in the whole of the Archean period. This is in contrast to the substantial and consistent development of ironformation that started in the oldest dated sequence (the Isua Supracrustals, Greenland: 3800 Ma). Stratiform massive sulfide deposits are also well developed in the Archean geologic record. Only five economic manganese deposits of Archean age are known of which three occur in India and two in Brazil. The other well-developed Archean sequences of Africa, Australia and North America are totally devoid of manganese deposits. Thus, limited as it was, manganese deposition during this period was also highly selective in space. But for the incongruous geologic setting of Archean Iron Ore Group, India, all other deposits occur in greenstone belts and high-grade regions characteristic of this period. In all cases, however, the Archean manganese deposits are hosted in shallow-marine sediments away from volcanic activity in space and time. Thus, these deposits are atypical in the broad canvas of Archean metallogeny. The rarity of manganese deposits of Archean age may be attributed to the possible paucity of manganese in volcanic exhalations of that time and it is also suggested that the relative anoxic environment of early Archean time inhibited deposition of manganese, the solubility of which is much greater than that of iron. (2) The Proterozoic period was marked by the formation of large, shallow, sinking sedi-

mentary basins, oxygenated atmosphere and hydrosphere and a significant biological community. Such an environment promoted deposition of manganese in greater magnitude. The early Proterozoic giant deposit of Kalahari manganese field of South Africa shows upward enrichment of manganese in the Transvaal Supergroup and also gradual increase in the oxidation state of manganese from lower to upper stratigraphic levels with progress of time. This trend of progressively increasing deposition and oxidation of manganese has been interpreted to be related to an overall gradual increase in Eh and pH during transition from Proterophytic to Paleophytic type. This observation further attests to the crucial role of increasing free oxygen in manganese deposition. The early Proterozoic deposits of India (Aravalli Supergroup, Sausar Group, Gangpur Group ) are all shallowwater terrigenous-sedimentary in character, deposited in shelf environments. Manganiferous stromatolites have been described from Botswana in a sequence correlatable to the Transvaal Supergroup. The deposits of Lukoshi Complex, Zaire (early Proterozoic) and the Xiangtan type of China (Sinian) are also constituted of shallow-marine stromatolitic manganese carbonate. At Moanda, Gabon, in the Franceville Series (c. 2140 Ma), manganese carbonates were formed in euxinic condition with organic-rich black shale in an epicontinental basin. Thus, there are indications of direct or indirect biological participation in deposition of manganese ores even in the Proterozoic period. In the late Proterozoic, terrigenously derived manganese oxide deposits were mainly formed interstratified with iron-formation (Morro do Urucum, Brazil; Damara Supergroup, Namibia; Maliy Khingan, U.S.S.R. ). In Penganga Beds, India, however, Mn-oxide ores are interstratified with chert and included in a thick limestone sequence. Some of these Proterozoic non-volcanogenic deposits (cf. Moanda, Gabon; Morro do Urucum, Brazil; Penganga Beds, India) are related to transgression-regression cycles that are shown to have

167 acted as suitable locales for deposition of large manganese deposits later in the Phanerozoic eon. Volcanogenic-sedimentary deposits are subordinate in the Proterozoic. These include those of Birrimian System, W. Africa and L~ngbantype deposits of Sweden of early Proterozoic age. The late Proterozoic manganese deposits of volcanogenic derivation were exclusively related to felsic volcanism, associated with continental-type sediments and were deposited on platforms (cf. Tiouine, Migouden, Oufront, Idikel; Morocco). (3) An impressive array of geological processes and situations were responsible for the formation of manganese deposits in the Phanerozoic. This eon witnessed the development of giant manganese deposits both in shallowmarine locales and in deep-sea pelagic areas. Much smaller deposits were also generated by volcanic exhalations and hydrothermal fluids in oceanic spreading centres. Shallow-water, onshore sedimentary manganese deposits of large reserves have mostly been correlated to transgression-regression cycles and related anoxia and oxygenation respectively. The deepwater deposits in the oceans were and are being formed by hydrogenous/early diagenetic deposition attended by direct or indirect biological participation. (4) The absence of deep-water manganese deposits in the older geological record and their dominance since the Mesozoic has been a dilemma. This had been explained bY assuming consumption of earlier deep-sea deposits in the subduction zone. Another reason may be the absence of deep (bottom) oxidizing polar currents (cf. Antarctic Bottom Current) that contribute largely to the formation of the modern deep-sea manganese deposits. (5) The important role of Mn-oxidizing microorganisms in deposition of manganese has been demonstrated in laboratory culture, and Mn-specific microbes have been recorded not only in modern deposits but also in ancient orebodies. Stromatolitic manganese deposits of

Precambrian age have also been described. Indirect assistance of biota in concentration and deposition of manganese has also been indicated. Therefore, a proper assessment of the extent of biological control on manganese deposition is required. (6) Manganese and iron, though neighbours in the periodic table, behave rather differently in their fixation in nature. The metallogenic development of these two metals follow opposite trends in respect of time. Iron deposits (mainly in iron-formation) abound in the Precambrian and decline in the Phanerozoic whereas manganese deposits are scarce in the Archean, moderately developed in the Proterozoic and are extensive in late Phanerozoic. Such fractionation of iron and manganese in nature has not been explained adequately so far.

Acknowledgements

This paper was presented at the Field Workshop organized by the IGCP Project No, 226; Correlation of Manganese Sedimentation to Palaeoenvironments at Groote Eylandt, Australia in September, 1986. Grateful thanks are due to the Groote Eylandt Mining Company Pty. Ltd. for hosting the workshop and extending excellent hospitality. Constructive review of this paper by Larry A. Frakes and Igor M. Varentsov is gratefully acknowledged.

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