Manganese In Sediments Research Articles

A genetic model for the H.Y.C. Deposit, Australia; based on regional sedimentology, geochemistry, and sulfide-sediment relationships

The fine-grained, laminated, stratiform zinc-lead ores of the McArthur River (HYC) ore deposit have been the subject of continued debate in regard to the timing of zinc-lead mineralization. Early workers (e.g., Croxford and Jephcott, 1972; Lambert, 1976) considered the ores to be sedimentary-exhalative (sedex), while more recent researchers (e.g., Williams, 1978a; Eldridge et al., 1993; Hinman, 1996) have argued for a syndiagenetic replacement origin. Several lines of evidence, including studies on the regional Sedimentology, sediment geochemistry, nature of sulfide layering, paragenesis, and chemistry of base metal transport and deposition, are used here to demonstrate that a synsedimentary origin is likely for the HYC deposit.Studies on the regional Sedimentology and geochemistry of the Barney Creek Formation have shown that the host rocks to the ore deposit, the HYC Pyritic Shale, represent a period of water deepening. Water depth was probably in excess of several hundred meters, with the deepest water euxinic facies corresponding to the ore position at the base of the HYC Pyritic Shale. A widespread manganese halo, developed in the W-Fold Shale Member below the ore horizon, is considered to relate to the start of the basin-deepening event, with manganese-bearing connate brines released into the basin along marginal rift faults. Manganese deposition occurred in the transitional dolomitic shale facies, where shallow-water, oxidized basin fluids intermittently mixed with deeper-water, reduced fluids along a redox interface. Sedimentary manganese deposition was immediately followed by zinc-lead mineralization in the deep-water, reduced facies of the HYC Pyritic Shale Member.The intimate relationship between the eight stratiform ore lenses and inter-ore sedimentary breccias indicates that sedimentary breccia deposition and ore formation are genetically linked. It is suggested that deep seismic events led to the following ore cycle: fault movement; sediment breccia deposition sourced from faults; episodic release of hot metalliferous brines along faults accompanied by sedimentary turbidites; and metal sulfide deposition in muds and turbidites adjacent to faults.The three modes of base metal sulfide occurrence in the ores, each considered by Eldridge et al. (1993) to have formed by replacement, are shown to have separate origins. The fine-grained sphalerite-galena bands (Mode 1) are intricately interlayered with pelagic organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as load casts and flames, indicative of a synsedimentary origin. Coarser-grained Mode 2 base metal sulfides show complex intergrowths with nodular dolomite bands, and are considered to form by replacement during diagenesis. The discrete patches of coarse, Mode 3 base metal sulfides are interpreted to represent clastic input from turbidity currents sourced partly by the erosion of vein-style mineralization from the faulted margins of the basin.The detailed nature of the layering in the ores suggests that the introduction of metalliferous brines into the basin was not continuous but occurred as a series of pulses interspersed with pelagic sedimentation and turbidite deposition. Preliminary calculations suggest that On the order of 10,000 discrete pulses of brine were required to form each ore lens, and it is suggested these pulses were related to microseismic activity following each major seismic event in the basin.Solubility considerations indicate that the mineralized brines were most probably sulfate-rich (SO (super 2-) 4 > H 2 S) and capable of transporting up to several thousand parts-per-million zinc and lead at moderate temperatures (150 degrees -250 degrees C), and near-neutral pH. Zinc-lead deposition was the result of two possible processes: (1) thermochemical reduction of brine sulfate by organic matter in anoxic basin water, and (2) diffusion of H 2 S from the anoxic water column into the metalliferous brine layer. Sulfur isotope data are best explained if both processes operated together to deposit galena and sphalerite.

Ocean manganese nodule and sediment reference materials

In this article, sampling locations, sample types, sample processing methods, and certified values of ocean manganese nodule and sediment reference materials available throughout the world are briefly described. Recommended or information values of major, minor, and trace constituents in those samples have also been collected, and a review of the main characteristics of their property values is presented.

Cycling of iron, manganese, and phosphate in a meromictic lake

Lake Nordbytjernet (Southeast Norway) had up to 40 mg liter−1 dissolved iron and 67 mg liter−1 dissolved manganese in a stagnant layer. The ratio between iron and manganese loading was around 3:1, while the inventory of manganese was six times higher than for iron. High concentrations of dissolved iron occurred solely under strictly anoxic conditions, while dissolved manganese was unaffected by intrusion of oxygen during the autumnal partial circulation. Mixed potentials caused by oxidation of ferrous iron and reduction of manganic oxide or O2 controlled the redox potential values, while dissolved manganese appeared to have no influence. The following main processes controlled the sedimentation rates: fluvial supply of suspended material, precipitation of ferric hydroxide and manganese oxide from the water column, and plankton production. Sorption to ferric hydroxide in the oxic zone and reductive desorption in the anoxic zone controlled the deep water stratification of phosphorus, dissolved organic carbon, and some major cations. No similar effects were seen for manganese sedimentation. The iron sedimentation rate was enhanced in the anoxic zone due to redox coupling of iron oxidation and manganese reduction. Phosphorus was depleted in the anoxic zone during periods of substantial ferric hydroxide sedimentation. Sedimentation of manganese under anoxic conditions seemed to be governed by precipitation of carbonate and phosphate. Residence times in the lake were 0.2 yr for iron and phosphorous vs. 3.3 yr for manganese and 1.4 yr for water.

Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO2 and SiO2 from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness.

210Pb, manganese and carbon: indicators of focusing processes on the northwestern Mediterranean continental margin

Organic carbon contents, 210Pb data, and organic and oxidised forms of manganese in sediments are used for reconstructing the shelf and deep-sea transport of particulate matter and for evaluating early diagenesis conditions. The latter are used as indicators of focusing processes on the northwestern Mediterranean continental margin. Twelve different physiographical sites were sampled between 400 and 2000 m in the Pyrenean and Golfe du Lion canyon axes and on their open-slopes. In the canyon axes, which indent the Pyrenean slope, 210Pb activities show a regular exponential decrease with sediment depth: computed sedimentation rates range from 170 to 198 cm 10 −3 yr. For an identical regular exponential decrease in the Golfe du Lion canyon, the sedimentation rate is lower (119 cm 10 −3 yr). On the lower open slopes and deep-sea fans, rapid deposition (turbidites and surficial sliding) occurs onto sediments that were deposited regularly at a rate of about 175 cm 10 −3 yr. A deposit enriched in organic carbon is identified at mid-slope between 500 and 1300 m. In this area, surface sediments of the canyon axes are richer in organic carbon than at the adjacent open slope sites (0.9% against 0.6% dry weight). Canyon axes are more active than open-slopes as channels for organic matter transfer. In the axial channel of the Pyrenean canyon, organic carbon decreases exponentially in the uppermost 12 cm. In the Golfe du Lion canyons, only the upper 2 cm of sediment are enriched in organic carbon. This is related to lower sedimentation rates in the Golfe du Lion area as compared to the Pyrenean area. On the upper and lower open slopes, the organic carbon contents decrease rapidly within 1–2 cm of sediment. There is no exponential evolution profile and the residence times at the sediment-water interface are longer than in the canyons. The highest surface Mn concentrations are also found in the middle of the continental slope. The average is 64 μg g −1 for Mn associated with the organic phase (Mn(org)) and 1400 μg g −1 for (Mn(Ox)). As for organic carbon contents, canyons are areas of high concentrations. The lowest values (4 μg g −1 for Mn(org) and 495 μg g −1 for Mn(Ox)) are found for the lower open-slope (1200–1700 m) where there is active surficial remobilisation and gravity mass movements. Moreover, the vertical distribution of Mn has allowed us to identify sites located at depths of about 1000 m where the redox discontinuity is located in the first few centimetres and where the (Mn(Ox)) and the (Mn(org)) profiles display a positive gradient from the deepest layers to the sediment-water interface. The distribution of the various parameters was caused by three different mechanisms: (1) bottom advection of particulate matter transferred from the continental shelf deposits to the slope along the canyon axes; (2) scavenging of the dissolved phases of metals and (3) sedimentary focusing of the particles in a “favoured” area of the continental slope—a mid-slope depocenter. These data contribute to our knowledge of the role of continental margins in the global recycling of carbon and other materials.

Sediment manganese and biogenic silica as geochemical indicators in estuarine salt marshes of coastal Georgia, USA

Salt-marsh estuarine sediments are not homogeneous. It is obvious that a steady state cannot be assumed if the depositional environments under which salt marshes accumulate change from subtidal through non-vegetated intertidal to vegetated intertidal state during their formation. In addition to these, the supply of sedimentary material depends on the tidal prism which changes as salt marshes are formed. Based on the study of cores collected in the estuarine region of Georgia, USA, it was found that the Mn/Al ratios in sediments of marsh cores change from subtidal through non-vegetated intertidal to vegetated intertidal. The relative variation of biogenic silica preserved in sediments of marsh core reflects the relative supply rate of organic carbon (phytoplankton) produced in the water column to the sediment surface at the time of deposition. In this paper it is proposed that sediment manganese and biogenic silica may be applied as geochemical indicators of changing depositional environments and organic carbon originating from phytoplankton in the estuarine salt marsh sediments.

Geochemical-focusing of manganese in lake sediments ? An indicator of deep-water oxygen conditions

The lateral distributions of Mn concentrations in the sediments of two Swiss lakes under varying oxygen conditions have been determined. The comparison of Mn distribution patterns with oxygen in the deep-water provides strong evidence for a geochemical-focusing effect, which is driven by the redox cycle of manganese. Conditions essential for this process to occur are anoxic sediments in contact with oxic deep-water. Average sedimentary manganese concentrations determined for different water-depth ranges are directly proportional to the area of shallower sediments. This result indicates that geochemical-focusing of manganese in lake sediments is a promising proxy indicator for the reconstruction of oxygen conditions during deposition.

Manganese ores of the Ghoriajhor-Monmunda area, Sundergarh District, Orissa, India: geochemical evidence for a mixed Mn source

Abstract The Proterozoic sedimentary manganese ores of the Gangpur Series, Orissa, India are of gonditic type and are variable in size, depth and grade of ore. The host sediments were deposited in a shallow shelf environment, as reflected in the variable clastic input into the sequence. Braunite and hollandite are the principal ore minerals, with bixbyite, rare jacobsite and rare vredenburgite. Manganese minerals in the gondites are spessartine and rhodite, together with quartz and aluminosilicates. Supergene weathering and enrichment of the ore horizon and gondite produced secondary ores composed of cryptomelane and pyrolusite. The geochemistry of the deposits shows Mn/Fe≈4, low trace element concentrations and arsenic enrichments that exceed 3000 mg kg −1 . These are interpreted as indicating a mixed hydrogenetic-hydrothermal source for the manganese oxides of the original precurser (pre-metamorphism) sediments. Regional metamorphism to amphibolite grade transformed this variable oxide-clastic sequence into manganese ores, sharply interbedded with gondites.

Redox cycling of iron and manganese in sediments of the Kalix River estuary, Northern Sweden

Iron and manganese redox cycling in the sediment — water interface region in the Kalix River estuary was investigated by using sediment trap data, pore-water and solid-phase sediment data. Nondetrital phases (presumably reactive Fe and Mn oxides) form substantial fractions of the total settling flux of Fe and Mn (51% of Fetotal and 84% of Mntotal). A steady-state box model reveals that nondetrital Fe and Mn differ considerably in reactivity during post-depositional redox cycling in the sediment. The production rate of dissolved Mn (1.6 mmol m−2 d−1) exceeded the depositional flux of nondetrital Mn (0.27 mmol m−2 d−1) by a factor of about 6. In contrast, the production rate of upwardly diffusing pore-water Fe (0.77 mmol m−2 d−1) amounted to only 22% of the depositional flux of nondetrital Fe (3.5 mmol m−2 d−1). Upwardly diffusing pore-water Fe and Mn are effectively oxidized and trapped in the oxic surface layer of the sediment, resulting in negligible benthic effluxes of Fe and Mn. Consequently, the concentrations of nondetrital Fe and Mn in permanently deposited, anoxic sediment are similar to those in the settling material. Reactive Fe oxides appear to form a substantial fraction of this buried, non-detrital Fe. The in-situ oxidation rates of Fe and Mn are tentatively estimated to be 0.51 and 0.16–1.7 μmol cm−3 d−1, respectively.

Facies Associations and Depositional Environment of the Proterozoic Carbonate-hosted Microbanded Manganese Oxide Ore Deposit, Penganga Group, Godavari Rift Basin, India

The late Proterozoic Penganga Group, Pranhita-Godavari valley, South India, is a sequence of siliciclastic-carbonate sediments that was deposited in an intracratonic rift basin. The nonvolcanogenic sedimentary succession containing listric basin-marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies. The cratonic sediments of the basin are mature siliciclastics with clear imprints of tide-dominated, shallow-subtidal deposition. The siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment-gravity-flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part. The limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale. Uniformly thick, laterally persistent, parallel-sided and thin-bedded limestone-shale rhythmites and parted limestone rhythmites characterize the ribbon limestones. Except for widespread thin parallel lamination, structures formed by waves, tidal currents, and storm-induced currents are totally absent. The limestone is brown and argillaceous in the basal part, and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone. The physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep-water slope and basinal environment of carbonate deposition under continued transgression. The bedded manganese oxide ore interstratified with chert, simulating iron formation, typically shows micron-scale to millimeter-scale thin parallel laminae of almost uniform thickness, texture, and composition. The manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size. Todorokite and birnessite are the most abundant sedimentary manganese oxides; they were locally transformed to braunite and bixbyite during diagenesis. The centimeter-thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz. The sedimentary attributes of the ore-chert sequence suggest deposition from suspension fallout in a low-energy environment. Penecontemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor. The sedimentology and stratigraphy of the Penganga Group suggest a sediment-gravity-flow-influenced deep-water, base-of-slope/basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting. On the basis of sedimentological evidence and parallel studies done on iron formations, a possible hydrothermal source of manganese is also considered. This represents a significant departure from the prevalent views of high-energy, shallow-water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits, and thus it has important implications for genesis of manganese ores.

Surficial Distribution of Chemical Constituents in Accumulated Reservoir Sediments in Relation to Observed Water Quality Patterns

ABSTRACT Surficial sediments from the main stem of a reservoir were examined to determine the distribution of selected chemical constituents in accumulation zones. Accumulated sediments were apparent along a longitudinal axis, coincident with maximum depth, for approximately 54 km of the 63 km long reservoir. Within this zone, concentrations of total phosphorus, nitrogen, and iron were observed to increase from the headwater region to the dam. Concentrations of total phosphorus, nitrogen, iron, and total organic carbon were also significantly correlated, (p < 0.05). Total manganese concentrations were variable and longitudinal gradients were not observed. Water quality patterns observed for the reservoir included seasonal hypolimnetic anoxia, increased solubilization of sedimentary manganese in the downstream region, and longitudinal gradients in selected water quality constituents. Comparisons of surficial distribution patterns in accumulated sediments to water quality patterns suggest sediment quality i...

Fault-controlled metasomatic alteration of early Proterozoic sedimentary manganese ores in the Kalahari manganese field, South Africa

Low-grade Mamatwan-type sedimentary manganese ore has been upgraded to high-grade Wessels-type ore in the Kalahari manganese field. The alteration appears to be of hydrothermal (metasomatie) origin closely related to zones of ferruginization (hematitization) developed along major north-south- and minor east-west-trending normal faults and fractures. The faults and fractures appear to have acted as channelways for the hydrothermal fluid eausing metasomatie alteration of the manganese ore.Manganese ores display a very consistent zonation from zones of ferruginization along faults to unaltered sedimentary manganese ore in the cores of fault blocks. This zonation begins with least altered Mamatwantype sedimentary ore, finely laminated, and composed of finely intergrown braunitc and Ca-Mn carbonates with minor hematite and abundant carbonate ovoids. Parallel to normal faults and zones of ferruginization the ore has been sequentially altered to brannite II-brannite (new), bixbyite-hansmannite, and hausmannite-rich ore. Immediately adjacent to zones of ferruginization a second stage of brannite-rich ore has been developed. The high-grade ore appears porous and massive and original sedimentary bedding is obliterated.The formation of high-grade ore appears to be related to a process of silica and carbonate leaching from sedimentary protore by hydrothermal fluids. In proximity to the feeder channels for the hydrothermal fluids the manganese ore was replaced by hematite. Some of the manganese was reprecipitated to form very high grade (> or =50 wt % Mn) manganese ore immediately adjacent to zones of ferruginization. During waning stages of hydrothermal fluid flow, silica and some calcium carbonate leached from sedimentary manganese ore apparently reprecipitated close to and within the fluid channelways. This led to the formation of a secondary stage of braunitic manganese ore developed as a transition between zones of ferruginization and high-grade hansmannite-rich ore. At the same time zones of ferruginization became calcified and some rare silicate minerals were formed.

Occurrence and genetic implications of hyalophane in manganese-rich iron-formation, Cuyuna Iron Range, Minnesota, USA

AbstractThe recent discovery of hyalophane [(K,Ba)Al1−2Si3−2O8] on the North range segment of the Early Proterozoic Cuyuna Iron Range of east-central Minnesota has shed new light on the depositional environment of these rocks. This Ba-feldspar occurs in a 10 m thick interval within the main iron-formation and typically contains between 8 and 26 mol.% celsian (BaAl2Si2O8). Its occurrence in several textural settings suggests that barium was being deposited at various stages in the paragenetic history of the iron-formation. Some of the hyalophane grains occur as the cores of micronodules, which are structurally similar to oolites or oncolites, but mineralogically are very complex. The hyalophane also occurs as rims on core grains of diverse mineral composition and as discrete phases in late crosscutting veins.Hyalophane, like other Ba-silicates, has a very restricted paragenesis. They are associated typically either with sedimentary manganese and ferromanganese deposits, or with Cu-Pb-Zn-Ba deposits. The presence of hyalophane in the Early Proterozoic manganiferous iron ores of east-central Minnesota casts doubt on the historic interpretation of these deposits as typical Superior-type sedimentary iron-formations and instead supports the view that these deposits, at least in part, consist of chemical sediments from a hydrothermal fumarolic system. The suggested involvement of a hydrothermal system is also supported by the occurrence of aegirine within the hyalophane-rich layer, and the occurrence of tourmalinites and Sr-rich baryte veins elsewhere in the Cuyuna North range.

Primary braunite in Triassic sedimentary manganese deposits of Dounan, Yunnan, China

This paper is concerned with the genesis of Dounan braunite. The study includes the precise determination of the mineral by: (1) transmitted and reflected light microscopy, (2) cathodoluminescence and scanning electron microscopy, (3) X-ray diffraction, (4) X-ray fluorescence and (5) electron microprobe. Besides determinations of the mineral, the petrology and geologic environment of the samples are reviewed for an interpretation of its mode of origin. In many descriptions, even during recent years, braunite has not been positively identified and the criteria by which the modes of origin are interpreted leave some doubt that conclusions are correct. The study indicates that Dounan braunite is primary sedimentary in origin although braunite elsewhere has usually been interpreted as either a metamorphic or hydrothermal, or rarely a diagenetic product. Dounan braunite as accretionary ooliths/pisoliths occurs conformably in the terrigenous clastic-carbonate sediment in the transgressive/regressive Middle Triassic marine sequence of the Falang Group, southeastern Yunnan, China. This sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed. Examination of the mineralogical character and chemical composition of the important phases as well as their textures suggest that braunite occurs in two generations. Primary braunite probably grew as accretionary ooliths/pisoliths precipitated as primary sediment within lagoonal and intertidal zones during sedimentation. Coarse or polycrystalline braunite, resulting from recrystallization and replacement of earlier braunite, is considered as a diagenetic product of primary manganese ores.

Comparison between microwave and conventional heating procedures in Tessier's extractions of calcium, copper, iron and manganese in a Lagoon sediment

The results of Tessier's extraction of calcium, copper, iron and manganese in sediments using conventional and microwave heating were compared. The metals were determined by atomic absorption spectrometry with flame and electrothermal atomization. Magnesium chloride (extractant of exchangeable fraction) was replaced with ammonium acetate. Sediments collected from the Venice lagoon were analysed. The results showed that microwave heating produces results comparable to those of conventional heating with a much shorter operating time.

The biogeochemistry of manganese and iron reduction in marine sediments

Manganese and iron reduction in marine sediments are known to play important roles in the biogeochemical cycles of many elements, including carbon, sulfur, phosphorus and several trace elements. These reduction reactions affect these cycles on a variety of time scales, ranging from those as short as seasonal time scales (e.g., nutrient cycling in coastal ecosystems), to those as long as thousands to tens of thousands of years (e.g., glacial—interglacial transformations in deep sea sediments). In this review article I will briefly summarize the results of laboratory studies on the types of manganese and iron reduction that are known to occur in marine sediments, and then discuss the occurrence of these processes in different sedimentary environments. Particular efforts will be given to examining the rates and mechanisms of sedimentary manganese and iron reduction in relationship to other biogeochemical processes in sediments.

Mineralogy and geochemistry of two metamorphosed sedimentary manganese deposits, Sierra Nevada, California, USA

Laminated to massive rhodochrosite, hausmannite, and Mn-silicates from the Smith prospect and Manga-Chrome mine, Sierra Nevada, California were deposited as ocean floor sediments associated with chert and shale. The principal lithologies at Smith are chert, argillite, rhodochrosite-, hausmannite- and chlorite-rich layers, and relatively uncommon layers of jacobsite. The Manga-Chrome mine also contains layers rich in manganoan calcite and caryopilite. Tephroite, rhodonite, spessartine, and accessory alleghanyite and sonolite formed during metamorphism. Volcaniclastic components are present at Manga-Chrome as metavolcanic clasts and as Mn-poor, red, garnet- and hematite-rich layers. There is no evidence, such as relict lithologies, that Mn was introduced into Mn-poor lithologies such as chert, limestone or mudstone. Replacement of Mn-poor phases by Mn-rich phases is observed only in the groundmass of volcanic clasts that appear to have fallen into soft Mn-rich mud. Manganiferous samples from the Smith prospect and Manga-Chrome mine have high Mn Fe and low concentrations of Ni, Cu, Zn, Co, U, Th and the rare-earth elements that are similar to concentrations reported from other ancient Mn deposits found in chert-greenstone complexes and from manganiferous sediments and crusts that are forming near modern sea floor vents. The Sierra Nevada deposits formed as precipitates of Mn-rich sediments on the sea floor, probably from mixtures of circulating hydrothermal fluids and seawater. The composition of a metabasalt from the Smith prospect is consistent with those of island-arc tholeiites. Metavolcanic clasts from the Manga-Chrome mine are compositionally distinct from the Smith metabasalt and have alkaline to calc-alkaline affinities. A back-arc basin is considered to be the most likely paleoenvironment for the formation of the Mn-rich lenses at the Manga-Chrome mine and, by association, the Smith prospect. Layers of rhodochrosite, hausmannite and chert preserve the composition and some textures of the sedimentary protoliths at both Sierra Nevada deposits. Jacobsite-rich layers probably represent a Fe-rich protolith. Caryopilite and manganoan calcite represent additional protoliths at the Manga-Chrome mine. The metamorphic assemblage prehnite-chlorite-epidote-calcite in a metabasalt from the Smith prospect constrains regional metamorphic conditions to a maximum temperature of 325°C and a pressure of 2 kbar. Slightly higher temperatures are indicated by the presence of actinolite in another metabasalt. Compositions of Mn-rich minerals in Smith samples are consistent with these metamorphic conditions.

Effects of ocean chemistry, sea level, and climate on the formation of primary sedimentary manganese ore deposits

The mineralogy, geochemistry, and in some cases, the mode of origin of many Phanerozoic sedimentary manganese orebodies are fairly well known, but the effects of the important variables represented by changing ocean chemistry, sea level, and climate on ore formation are only recently being considered. The high degree of mobility of manganese, particularly through redox processes, insures that it can change phase during weathering, transport, deposition, and diagenesis.During chemical weathering, manganese is solubilized in acid, reducing conditions and carried in surface and subsurface waters to the coastal zone, where many of the exploitable deposits can form in slightly reducing (carbonate ores) to oxidizing conditions (oxide ores). Rainfall controls ground-water acidity through fostering vegetation growth. Major deposits at Groote Eylandt, Australia, and the extensive deposits of the Paratethys seaway in eastern Europe and the Soviet Union accumulated as a result of large-scale delivery of dissolved Mn to restricted black shale basins during marine transgressions. The deposits show compositional zoning and display evidence of having been deposited in shallow-marine environments following accumulation of a dilute ore solution in the basin interiors. Manganese was precipitated when the environments of the basin became oxidized, apparently during the early stages of marine regressions.During the Phanerozoic the concentrations of dissolved oxygen and carbon in the oceans varied significantly, in response to large changes in the burial rate of organic carbon. When the rate was high, there was a drawdown of atmospheric CO 2 , which led to global reverse-greenhouse cooling. Later, upon oxidation of sea-floor organic matter, CO 2 was released and greenhouse warming eventuated. Manganese is now known to have precipitated rapidly as crusts in the oceans late in the glacial and early in the interglacial stages of the Quaternary, probably as a result of extensive sea-floor oxidation accompanying the release of CO 2 . This same set of processes, operating over geologic time, may have played a significant role, first, in generating large reservoirs of dissolved manganese in intracratonic basins and in the oceans, and second, by providing the metal for the formation of ore deposits when conditions changed.

The process of formation of manganese carbonate deposits hosted in black shale series

The two most important types of manganese deposits in China are sedimentary and supergene, which account for 71 and 15 percent of the total manganese reserves, respectively. According to the host-rock associations, sedimentary manganese deposits can be classified into (1) a mud-rock type, (2) a black shale series type, (3) a carbonate rock type, and (4) a chert-limestone-mudstone type. The black shale series-type manganese deposits account for 48 percent of the total manganese reserves and are large- or medium-sized deposits.The black shale series or dark-colored sedimentary association consists of dark-colored mudrock-chert-carbonate rocks rich in sulfides and organic matter (C organic > or =1%). The term, black shale series-type manganese deposits, is a general name for all manganese deposits hosted in black shale, black chert, black siliceous shale, dark gray limestone, etc.Black shale series-type manganese deposits are distributed in 13 stratigraphic horizons from middle Proteozoic to Early Triassic. They can be divided into three subtypes: (1) a black shale subtype, (2) a black chert-siliceous shale subtype, and (3) a dark gray limestone subtype. Subtype 1 consists mainly of primary Mn carbonate ores; subtypes 2 and 3 consist mainly of Mn-containing limestones. Weathering and leaching processes can change the Mn limestones into Mn oxide ores which form large and high-quality supergene deposits, for example, the Mugui and Dongxiangqiao deposits.Manganese carbonate ores hosted in the black shale series consist mainly of rhodochrosite, calcio-rhodochrosite, kutnohorite, manganocalcite, Mn-containing calcite, Mn-containing dolomite, etc. The ores are black, dark gray, light gray, yellowish, and pink in color and have banded, laminated, massive, oolitic, spotted, and brecciated structures.Commonly, changes in the ore-bearing sequences show, from bottom to top, (1) black shale series --> (2) Mn carbonate ore bed --> (3) light-colored rocks. This indicates that ore beds formed in the transition from reduced to slightly oxidized and from weakly alkaline to alkaline conditions. The evolution of the ore-bearing sequence indicates that manganese mineralization occurred during the early stage of regression.The important Mn carbonate deposits hosted in the black shale series resulted from three main processes during formation: concentration in seawater, precipitation from seawater, and enrichment in the sediments. Anoxic conditions are a basic factor for manganese concentration in seawater; a weak oxic and alkaline environment is favorable for Mn carbonate precipitation, and microbiological activities play an important role for early diagenetic enrichment of manganese in the deposits.

Glauconite formation as a factor in sedimentary manganese deposit genesis

Concentrations of in the crust are dominated by a single ore type described as the orthoquartzite-glauconite-clay association or as manganese giants. Current research suggests that these large accumulations of oxide and/or carbonate are associated with relatively rapid marine transgressions and regressions across older basement rocks. Some of the larger deposits of this type, such as Nikopol and Chiatura, USSR, and Groote Eylandt, Australia, occur in petrologically simple terrigenous formations, generally sandstones and shales, interpreted as paleoshoreline sediments. A major problem lies in the very high Mn/Fe ratio of these deposits (30-50) as against that of ordinary sediments (0.02). Research on the Groote Eylandt Cenomanian age deposit suggests that the high Mn/Fe of the primary ore (15-20) was associated with enrichment in coastal waters, which in part was related to glauconitization (iron-trapping) reactions. Mineralogical evidence indicates a sequence from a well-defined glauconite phase in deeper water, off-shore sediments (with whole-rock Mn/Fe of 0.3-1.2) to a glauconite-smectite in near-shore estuaries and rivers, as a minor component of pisolitic oxides of Mn/Fe ranging from 15 to 20. Data on these glauconite minerals, including X-ray diffraction, infrared spectroscopy, microanalyzes, textural occurrences, and genesis are presented. Geologically glauconite is a product of a postoxic environment, possibly related to fluctuating water depth during transgression-regression, seaward from the site of oxide precipitation.