SEDIMENTARY MANGANESE
by E. R. Force¹, S. Paradis² and G.J. Simandl³

Force, E.R., Paradis, S. and Simandl, G.J. (1999): Sedimentary Manganese; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines.

IDENTIFICATION

SYNONYMS: "Bathtub-ring manganese", "stratified basin margin manganese", shallow-marine manganese deposits around black shale basins.

COMMODITY: Mn.

EXAMPLES (British Columbia (MINFILE #): Canada/International): Molango (Mexico), Urcut (Hungary), Nikopol (Ukraine), Groote Eylandt (Australia).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION:  Laterally extensive beds of manganite, psilomelane, pyrolusite, rhodochrosite and other manganese minerals that occur within marine sediments, such as dolomite, limestone, chalk and black shale. The manganese sediments often display a variety of textures, including oolites and sedimentary pisolites, rhythmic laminations, slumped bedding, hard-ground fragments and abundant fossils. "Primary ore" is commonly further enriched by supergene process. These deposits are the main source of manganese on the world scale.

TECTONIC SETTING:  Interior or marginal basin resting on stable craton.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:   These deposits formed in shallow marine depositional environments (15-300 m), commonly in sheltered sites around islands along some areas of continental shelf and the interior basins. Most deposits overlie oxidized substrates, but basinward, carbonate deposits may be in reducing environments. Many are in within transgressive stratigraphic sequences near or at black shale pinchouts.

AGE OF MINERALIZATION:  Most deposits formed during lower to middle Paleozoic, Jurassic, mid-Cretaceous and Proterozoic.

HOST/ASSOCIATED ROCK TYPES: Shallow marine sedimentary rocks, such as dolomites, limestone, chalk and black shales, in starved-basins and lithologies, such as sponge-spicule clays, are favourable hosts. Associated rock types are sandstones, quartzites, and a wide variety of fine-grained clastic rocks

DEPOSIT FORM:  Mn-enriched zones range from few to over 50 m in thickness and extend from few to over 50 km laterally. They commonly have a "bathtub-ring" or "donut" shape. Some deposits may consist of a landward oxide facies and basinward reduced carbonate facies. Ore bodies represent discrete portions of these zones

TEXTURE/STRUCTURE: Oolites and sedimentary pisolites, rhythmic laminations, slumped bedding, hard-ground fragments, abundant fossils, fossil replacements, and siliceous microfossils are some of commonly observed textures.

ORE MINERALOGY [Principal and subordinate]: Manganese oxides: mainly manganite, psilomelane, pyrolusite; carbonates: mainly rhodochrosite, kutnohorite, calcio-rhodochrosite.

GANGUE MINERALOGY [Principal and subordinate]:   Kaolinite, goethite, smectite, glauconite, quartz, biogenic silica; magnetite or other iron oxides, pyrite, marcasite, phosphate, ± barite, carbonaceous material, ± chlorite, ± siderite, manganocalcite.

ALTERATION MINERALOGY:  N/A.

WEATHERING:  Grades of primary ore are relatively uniform; however, supergene enrichment may result in a two or three-fold grade increase. The contacts between primary ore and supergene-enriched zones are typically sharp. Mn carbonates may weather to brown, nondescript rock. Black secondary oxides are common.

ORE CONTROLS:  Sedimentary manganese deposits formed along the margins of stratified basins where the shallow oxygenated water and deeper anoxic water interface impinged on shelf sediments. They were deposited at the intersection of an oxidation-reduction interface with platformal sediments. Sites protected from clastic sedimentation within transgressive sequences are most favourable for accumulation of high grade primary deposits.

GENETIC MODELS:  Traditionally these deposits are regarded as shallow, marine Mn sediments which form rims around paleo-islands and anoxic basins. Manganese precipitation is believed to take place in stratified water masses at the interface between anoxic seawater and near surface oxygenated waters.. The Black Sea and stratified fjords, such as Saanich Inlet or Jervis inlet, British Columbia (Emerson 1982; Grill, 1982) are believed to represent modern analogues. Extreme Fe fractionation is caused by a low solubility of iron in low Eh environments where Fe precipitates as iron sulfide. A subsequent increase in Eh and/or pH of Mn-rich water may produce Mn-rich, Fe-depleted chemical sediments. The manganese oxide facies is preserved on oxidized substrates. Carbonate facies may be preserved either in oxidized or reduced substrates in slightly deeper waters.

ASSOCIATED DEPOSIT TYPES: Black shale hosted deposits, such as upwelling-type phosphates sediment-hosted barite deposits, shale-hosted silver-vanadium and similar deposits and sedimentary-hosted Cu  may be located basinward from the manganese deposits. Bauxite and other laterite-type deposits (B04), may be located landward from these manganese deposits. No direct genetic link is implied between sedimentary manganese deposits and any of these associated deposits.

COMMENTS:  A slightly different model was proposed to explain the origin of Mn-bearing black shales occurring in the deepest areas of anoxic basins by Huckriede and Meischner (1996).

Calvert and Pedersen (1996) suggest an alternative hypothesis, where high accumulation rate of organic matter in sediments will promote the development of anoxic conditions below the sediment surface causing surface sediments to be enriched in Mn oxyhydroxides. When buried they will release diagenetic fluids, supersaturated with respect to Mn carbonates, that will precipitate Ca-Mn carbonates.

Sedimentary manganese deposits may be transformed into Mn-silicates during metamorphism. The metamorphic process could be schematically represented by the reaction:

Rhodochrosite + SiO₂ = Rhodonite + CO₂.

Mn-silicates may be valuable as ornamental stones, but they are not considered as manganese metal ores under present market conditions.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE:  Mn-enriched beds. Mn/Fe ratio is a local indicator of the basin morphology that may be reflecting separation of Mn from Fe by precipitation of pyrite. Some of the large manganese deposits, including Groote Eylandt, coincide with, or slightly postdate, d ¹³C positive excursions. These d ¹³C anomalies may therefore indicate favorable stratigraphic horizons for manganese exploration.

GEOPHYSICAL SIGNATURE:  Geophysical exploration is generally not effective. Supergene cappings may be suitable targets for the self potential method.

OTHER EXPLORATION GUIDES:  These deposits occur within shallow, marine stratigraphic sequences Black shale pinchouts or sedimentary rocks deposited near onset of marine regression are particularly favourable for exploration. High Mn concentrations are further enhanced in depositional environments characterized by weak clastic sedimentation. Manganese carbonates occur basinward from the manganese oxide ore. Many sedimentary manganese deposits formed during periods of high sea levels that are contemporaneous with adjacent anoxic basin. If Mn oxides are the main target, sequences containing shellbed-biogenic silica-glauconite are favorable. Evidence of the severe weathering of the land mass adjacent to, and contemporaneous with the favourable sedimentary setting, is also considered as a positive factor. In Precambrian terrains sequences containing both black shales and oxide-facies iron formations are the most favorable.

ECONOMIC FACTORS

TYPICAL GRADE AND TONNAGE:  The average deposit contains 6.3 Mt at 30% MnO, but many deposits exceed 100 million tonnes. There is a trend in recent years to mine high-grade ores (37 to 52% Mn) to maximize the output of existing plants. The countries with large, high-grade ore reserves are South Africa, Australia, Brazil and Gabon.

ECONOMIC LIMITATIONS: On the global scale the demand for manganese ore, siliconmanganese, and ferromanganese depends largely on the steel industry. The 1996 world supply of manganese alloys was estimated at 6.6 Mt. Partly in response to highly competitive markets, in the western world much of the manganese ore mining is being integrated with alloy production. As a result, the bulk of manganese units for the steel production is now being supplied in form of alloys. There is also a new tendency to have the ore processed in China and CIS countries. The high cost of constructing new, environment-friendly plants and lower costs of energy are some of the reasons.

END USES:  Used in pig iron-making, in upgrading of ferroalloys, in dry cell batteries, animal feed, fertilizers, preparation of certain aluminum alloys, pigments and colorants. Steel and iron making accounts for 85 to 90% of demand for manganese in the United States. Increasing use of electric-arc furnaces in steel-making has resulted in gradual shift from high-carbon ferromanganese to siliconmanganese. Natural manganese dioxide is gradually being displaced by synthetic (mainly electrolitic variety). There is no satisfactory substitute for manganese in major applications.

IMPORTANCE:  Sedimentary marine deposits are the main source of manganese on the world scale. Some of these deposits were substantially upgraded by supergene enrichment (Dammer, Chivas and McDougall, 1996). Volcanogenic manganese deposits (G02) are of lesser importance. Progress is being made in the technology needed for mining of marine nodules and crusts (Chung, 1996); however, this large seabed resource is subeconomic under present market conditions.

REFERENCES

Calvert, S.E., and Pedersen, T.F. (1996):  Sedimentary Geochemistry of Manganese - Implications for the Environment of Formation of Manganiferous Black Shales: Economic Geology, Volume 91, pages 36-47.

Cannon, W.F. and Force, E.R. (1983):  Potential for High-grade Shallow Marine Manganese Deposits in North America, in Unconventional Mineral Deposits; W.C. Shanks, Editor, Society of Mining Engineers, pages 175-189.

Chung, J.S. (1996):  Deep-ocean Mining-Technologies for Manganese Nodules and Crusts, International Journal of Offshore and Polar Engineering, Volume 6., pages 244-254.

Dammer, D., Chivas, A.R. and McDougall, I. (1996):  Isotopic Dating of Supergene Manganese Oxides from the Groote Eyland Deposit, Northern Teritory, Australia, Economic Geology, Vol.91, pages 386-401.

Emerson, S., Kalhorn, S., Jacobs, L., Tebo, B.M., Nelson, K.H. and Rosson, R.A. (1982):  Environmental Oxidation Rate of Manganese (II), Bacterial Catalysis; Geochimica et Cosmochimica Acta, Volume 6, pages 1073-1079.

Frakes, L. and Bolton, B. (1992): Effects of Ocean Chemistry, Sea Level, and Climate on the Formation of Primary Sedimentary Manganese Ore Deposits, Economic Geology, Volume 87, pages 1207-1217.

Force, E.R. and Cannon W.R.(1988):  Depositional Model for Shallow-marine Manganese Deposits around Black-shale Basins, Economic Geology, Volume 83, pages 93-117.

Grill, E.V. (1982):  The Effect of Sediment-water Exchange on Manganese Deposition and Nodule Growth in Jervis Inlet, British Columbia, Geochimica et Cosmochimica Acta, Volume 42, pages 485-495.

Huckriede, H. and Meischner, D. (1996):  Origin and Environment of Manganese-rich Sediments within Black-shale Basins, Geochemica and Cosmochemica Acta, Volume 60, pages 1399-1413.

Jones, T.S., Inestroza, J. and Willis, H. (1997): Manganese, Annual Review-1996, U.S. Geological Survey, 19 pages.

Laznicka, P. (1992):  Manganese Deposits in the Global Lithogenetic System: Quantitative Approach, Ore Geology Reviews, Volume 7, pages 279-356.

Morvai, G. (1982):  Hungary; in Mineral Deposits of Europe, Volume .2, Southeastern Europe, F.W. Dunning, W. Mykura and D. Slater, Editors, Mineral Society, Institute of Mining & Metallurgy, London, pages 13-53.

Okita, P.M. (1992): Stratiform Manganese Carbonate Mineralization in the Molango District, Mexico, Economic Geology, Volume 87, pages 1345-1365.

Polgari, M., Okita, P.M. and Hein, J.M. (1991): Stable Isotope Evidence for the Origin of the Urcut Manganese Ore Deposit, Hungary. Journal of Sedimentary Petrology, Volume 61, Number 3, pages 384-393.

Polgari, M., Molak, B. and Surova, E. (1992):  An Organic Geochemical Study to Compare Jurassic Black Shale-hosted Manganese Carbonate Deposits, Urkut, Hungary and Branisko Mountains, East Slovakia; Exploration and Mining Geology, Volume 1, Number 1, pages 63-67.

Pracejus, B. and Bolton, B.R. (1992):  Geochemistry of Supergene Manganese Oxide Deposits, Groote Eylandt, Australia; Economic Geology, Volume 87, pages 1310-1335.

Pratt, L.M., Force, E.R. and Pomerol, B. (1991):  Coupled Manganese and Carbon-isotopic Events in Marine Carbonates at the Cenomanian-Turonian Boundary, Journal of Sedimentary Petrology, Volume 61, Number 3, pages 370-383.

Robinson, I. (1997):  Manganese; in: Metals and Minerals Annual Review,, Mining Journal London, page 59.