Stratiform Manganese Deposits Research Articles

Earth observation approach for targeting stratiform deposit of manganese in central India

The stratiform manganese deposits, hosted within the Sausar Group of rocks, in Madhya Pradesh state of India, has undergone polyphase deformation and metamorphism. The region is an enterprising geosite to understand the lithological and tectonic control on manganese mineralization. We have carried out a conjugate remote sensing based analysis using Visible, Near Infrared and Short Wave Infrared bands (VIS-NIR-SWIR) of optical imagery from multiple sensors supplemented with analysis of L-band microwave data over the Balaghat district of Madhya Pradesh to delineate prospective areas for manganese exploration, supported by the lab and field-based evidence.The enhanced image products (i.e.band ratios, principal components) and spectral anomaly map derived from Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) and Sentinel-2B multispectral bands are used to delineate the lithological contrast of the host rock and indicate key locales of mineralization. The geological structures interpreted using the merged product of Advanced Land Observation Satellite Phased Array L-band SAR (ALOS PALSAR −1), Landsat 8 PC bands (2 & 3) and the processed Horizontal Gradient Magnitude (HGM) from advanced satellite gravity model (EIGEN-6C4), illustrates the structural control on the mineralization. The petrological details derived from the host rock samples shows microstructures like micro-folding, foliations, quartz grain elongation etc., thus representing the microscale deformations within the rock units occurring in the area.The lab-based spectral signatures of manganese-bearing host rock was resampled to image spectra and spectral anomaly map using ConstrainedEnergyMinimization(CEM), which shows good correlation with the existing mine locations.The information from remote sensing and lab analysis has been integrated thematically using the knowledge-based Weighted Sum Method (WSM) with existing mine locations as the training dataset. The derived mineral prospect map indicates several new areas of manganese prospectivity which can serve as a guide for detailed future explorations in the region .

Mohammadabad Manganese deposit, southwest Sabzevar basin, Iran: Evidence of sea-floor exhalation and geochemical studies in the late Cretaceous volcano-sedimentary sequence

The southwest Sabzevar basin in northeast Iran is one of the most important areas for volcanogenic stratiform manganese deposits. The Mohammadabad Mn deposit, found in this basin, is hosted by the upper late Cretaceous marly tuffs in a region that has an active history of tectonism. Their geology, mineralogy and geochemistry, including major, trace and rare earth elements (REE), are described in this work. The main ore minerals include pyrolusite, psilomelane, cryptomelane, with minor braunite and manganite. Four styles of manganese mineralization are recognized: (1) layered; (2) nodular; (3) disseminated; and (4) replacement to massive. Regardless of style, pyrolusite, psilomelane, and cryptomelane usually tend to parallel the stratification of sediments, while braunite veins occur in high angle fractures. The replacement of radiolarian and foraminiferal fossils by manganese minerals during diagenesis near the seawater-sediment interface suggests that mineralization occurred in oxygen-rich conditions under the influence of low-temperature hydrothermal fluids. The geochemistry of trace elements of manganese oxides and rare earth elements gives clear evidence of hydrothermal-exhalative origin for Mohammadabad deposit. In general, ore minerals have Co/Ni ratios above 0.12, and U/Th ratios vary from 0.13 to 11.29. The presence of negative Eu anomalies in manganese minerals shows that the ore-forming fluids were generally low-temperature and oxic during mineral formation. In-situ mineralogy studies based on SEM-EDS have distinguished syngeneic and diagenetic Mn systems during the formation of manganese mineralization. The Mohammadabad deposit is considered a typical case of volcanic hydrothermal exhalative-diagenetic Mn mineralization, in which minerals deposited when hydrothermal fluid was released from oxic to dysoxic water columns.

Conodont and radiolarian biostratigraphic age constraints on Carnian (Upper Triassic) chert-hosted stratiform manganese deposits from Panthalassa: Formation of deep-sea mineral resources during the Carnian pluvial episode

Numerous stratiform manganese deposits occur in the Permian to Jurassic bedded chert successions of the Jurassic accretionary complexes of the Chichibu, Mino–Tamba, and North Kitakami belts in Japan. These accretionary complexes consist of deep-sea sediments accumulated in a pelagic, open-ocean setting within the Panthalassa Ocean. Although the depositional ages and processes of the Permian and Jurassic manganese deposits in these accretionary complexes have been studied, the Triassic bedded chert-hosted stratiform manganese deposits are poorly understood. We investigated the occurrence and stratigraphic distribution of Triassic stratiform manganese deposits in Japan, constraining their ages by means of integrated conodont–radiolarian biostratigraphy. On the basis of the conodont and radiolarian biostratigraphies, the depositional age of the Triassic manganese deposits was constrained to the latest Julian to earliest Tuvalian (mid-Carnian). This age indicates that the formation of the Triassic manganese deposits occurred contemporaneously with the last eruptive phase of the Wrangellia Large Igneous Province (LIP) volcanism and subsequent changes in the deep-sea redox conditions in the Panthalassa Ocean during the Carnian pluvial episode (CPE). Therefore, these manganese deposits may have been formed with the hydrothermal activity associated with the eruption of the Wrangellia LIP and/or the subsequent changes in marine redox conditions in the CPE.

Chemical composition of magnetite and chlorite from the stringer zone of the Nudeh volcanogenic massive sulfide (VMS) deposit, Iran: geological implications

The southwest Sabzevar basin situated in the Sabzevar zone is considered to be an attractive metallogenic province in Iran that hosts both volcanogenic massive sulfide (VMS) and stratiform manganese deposits. The Nudeh Besshi-type VMS deposit is located in the Lower Late Cretaceous volcano-sedimentary sequence. The ore mineralization in this deposit is hosted in the alkali olivine basalt flow and tuffaceous silty sandstone rocks. The Nudeh VMS deposit consists of 2 million metric tons of Cu-Zn massive sulfide overlying a Cu-Fe-rich stringer. The massive sulfide orebody consists dominantly of pyrite, chalcopyrite, friedrichite, magnetite, and sphalerite, together with minor quartz, chlorite, and sericite. Chloritization, silicification, and sericitization are the main wall-rock alteration types; alteration intensity increases towards the stringer zone. Chloritized footwall rocks extend up to 20 m below the stringer zone. The quartz-bearing stringer veins also contain pyrite, chalcopyrite, magnetite, and bornite. Magnetite crystals from the stringer ores show variable contents of many elements, such as MgO (0.05 wt%), Al2O3 (0.63 wt%), TiO2 (0.07 wt%), V2O3 (0.045 wt%), SiO2 (0.65 wt%), CoO (0.10 wt%), NiO (0.009 wt%), ZnO (0.023 wt%), and CaO (0.03 wt%). The moderate to high V contents are interpreted to result from relatively reduced, seafloor hydrothermal activiy. Compositional variations of magnetite are possibly related to variations in oxygen fugacity, temperature, and water/rock interaction. Within the stringer zone, chlorite 2 (Chl-2) in the vein-veinlets and chlorite 1 (Chl-1) in the chloritized alkali olivine basalt rock are chemically indistinguishable, with 26.92–34.67 wt% FeO and 5.99–14.01 wt% MgO. Chlorite geothermometer studies indicate crystallization formation temperatures of 414 °C (Chl-1) and 303 °C (Chl-2), respectively.

Radiolarians and geologic age of Neogene stratiform manganese deposits in the Fukaura District, Aomori Prefecture, Japan

Radiolarians and geologic age of Neogene stratiform manganese deposits in the Fukaura District, Aomori Prefecture, Japan

Geological and geochemical constraints on the Cheshmeh-Frezi volcanogenic stratiform manganese deposit, southwest Sabzevar basin, Iran

The Cheshmeh-Frezi Mn deposit belongs to the southwest Sabzevar basin to the north of the Central Iranian microcontinent. This basin, which hosts abundant mineral deposits including Mn exhalative and Besshi-type Cu-Zn volcanogenic massive sulfide deposits, followed an evolution closely related to the subduction of the Neo-Tethys oceanic crust beneath the Central Iranian microcontinent. Two major sedimentary sequences are recorded within this basin: (I) the Lower Late Cretaceous volcano-sedimentary sequence (LLCVSS) and (II) the Upper Late Cretaceous sedimentary dominated sequence (ULCSS). The Cheshmeh-Frezi Mn deposit is hosted within red tuff with interbeds of green tuffaceous sandstone of the LLCVSS. Mineralization occurs as stratiform blanket-like and tabular orebodies. Psilomelane, pyrolusite and braunite are the main minerals of the ore, which display a variety of textures. Such as layered, laminated, disseminated, massive, replacement or open space fillings. The footwall and hanging-wall volcanic rocks are predominantly andesite and trachyandesite rocks. Footwall and hangingwall volcanic rocks at Cheshmeh-Frezi are enriched in light rare earth elements (LREEs) compared to chondrite, have steep REE patterns, and generally show Ta and Nb depletions relative to chondrite which are characteristic of back-arc environments. The significant geochemical characteristics of ore such as high Mn content (12.41–33.14 wt%; average 19.41 wt%), low concentration of Fe (0.64–2.27 wt%; average 1.63 wt%), high Ba (49.7–9901ppm, average 2728.67ppm), LREE>HREE, and negative Ce and Eu anomalies reveal a primary distal hydrothermal-exhalative source for mineralization. Cheshmeh-Frezi deposit, in comparison with different types of volcanogenic manganese deposits shows broad similarities with the Cuban-type Mn deposits such as tectonic, host and associated rock types, geometry, textures, structures, mineralogy and lithogeochemistry.

Stratigraphy and radiolarian ages of stratiform manganese deposits from the Chichibu Belt in the Saiki area, eastern Oita Prefecture, Japan

Stratigraphy and radiolarian ages of stratiform manganese deposits from the Chichibu Belt in the Saiki area, eastern Oita Prefecture, Japan

Stratigraphy and age of stratiform manganese deposits in Jurassic accretionary complex in the Chichibu Belt, eastern Kyushu, Japan.

Stratigraphy and age of stratiform manganese deposits in Jurassic accretionary complex in the Chichibu Belt, eastern Kyushu, Japan.

Thematic Issue: Rare Earth Resources: Are They Sufficient or Deficient?

This thematic issue is composed of the papers presented at the symposium “Rare Metals–Rare Earth Resources” of the annual meeting of the Society of Resource Geology held in Tokyo, Japan, on 19 July 2007. The symposium was organized by Shunso Ishihara, Hajime Hishida and Yasushi Watanabe. Seven of the 11 topics presented in the symposium were related to rare earth elements (REE), reflecting their recent increasing demand and prices (Fig. 1). The Japanese REE market and industry expect new REE suppliers to develop outside China, because of the decreased availability of rare earth materials for import from China. Production of rare earth elements (REE) and prices of dysprosium (Dy), neodymium (Nd) and neodymium-praseodymium (Nd-Pr) in the last two decades. CIF, cost, insurance and freight; REO, rare earth oxides. The articles in this thematic issue include a review of rare earth deposits in North America (Castor, 2008), a recent interpretation of the genesis of the world’s largest Bayan Obo deposit in China (Wu, 2008), and two papers and a note that describe the relationship between rare earth mineralization and granite (Ishihara, 2008; Ishihara et al., 2008; Murakami & Ishihara, 2008). This issue also includes a review article on the rare earth magnet by Minowa (2008), and an article on rare earth enrichment in stratabound manganese deposits in India (Moriyama et al., 2008). Minowa (2008) provides basic information on recent demand for neodymium and dysprosium, which is caused mainly by the production of dysprosium-bearing neodymium magnets in China and Japan. He estimates that 16,000 t of neodymium and 2600 t of dysprosium will be necessary to produce alloys for the magnets in 2010 for the world market. The review of rare earth deposits of North America by Castor (2008) concludes that world reserves are sufficient to meet international demand for most REE commodities in the 21st century and the development of new REE deposits in North America is unlikely in the near future because Chinese producers, especially those at Bayan Obo, are capable of large REE production at low prices. Details of Bayan Obo are reviewed by Wu (2008), who suggests a hydrothermal origin related to carbonatite magmatism for the deposit. Ishihara et al. (2008) describe the mineralogical and geochemical characteristics of the granites related to ion-adsorption REE deposits in southern China, which are the current source for heavy REE. Ishihara (2008) also discusses the occurrence of REE-enriched zircon in the Naegi granite in Japan. Murakami and Ishihara (2008) investigate REE abundance in the weathered crust and clay sediment above granitic rocks in Japan and China, and describe the ore grade and composition of the currently mined Dingnan deposit in southern China. Moriyama et al. (2008) explore manganese deposits for a new heavy REE source. They have found REE enrichment up to 975 ppm in stratabound or stratiform manganese deposits in Orissa, India, and suggest that this enrichment was caused by hydrothermal activity during deposition. The papers in the present issue conclude collectively that heavy REE such as dysprosium will become scarce in the future despite the large reserves of total REE in the world, because the current supply source of heavy REE is limited to ion adsorption deposits in China. I am convinced that the studies of the geneses of the ion adsorption deposits and related granites, some of which are presented in this issue, will be utilized for future world exploration of this type of deposit.

Geochemistry and Origin of Ananai Stratiform Manganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

Abstract. Major and trace element contents are reported for Permian manganese ore and associated greenstone from the Ananai manganese deposit in the Northern Chichibu Belt, central Shikoku, Japan. The manganese deposit occurs between greenstone and red chert, or among red chert beds. Chemical compositions of manganese ore are characterized by enrichments in Mn, Ca, P, Co, Ni, Zn, Sr and Ba, and negative Ce and positive Eu anomalies relative to post-Archean average Australian Shale (PAAS). Geochemical features of the manganese ore are similar to those of modern submarine hydrother-mal manganese deposits from volcanic arc or hotspot setting. In addition, geochemical characteristics of the greenstone closely associated with the Ananai manganese deposit are analogous to those of with-in plate alkaline basalt (WPA). Consequently, the Ananai manganese deposit was most likely formed by hydrothermal activity related to hotspot volcanism in the Panthalassa Ocean during the Middle Permian. This is the first report documenting the terrestrially-exposed manganese deposit that was a submarine precipitate at hotspot.

Radiolarian Age of Manganese Ore and Red Chert from the Ananai Stratiform Manganese Deposit in the Northern Chichibu Belt, Central Shikoku, Japan

Abstract. We obtain radiolarian fossils such as Follicucullus monacanthus, Pseudoalbaillella aff. globosa, Pseudoalbaillella cf fusiformis, and Pseudoalbaillella spp. belonging to the Pseudoalbaillella globosa and Follicucullus monacanthus Zones from manganese ore and associated red chert of the Ananai stratiform manganese deposit in the Northern Chichibu Belt, central Shikoku. The depositional age of the Ananai ore deposit is constrained as Guadalupian (Middle Permian) that is between 270.6 and 260.4 Ma in the updated geologic time scale.

Chemical Profiles across a Jurassic Stratiform Manganese Deposit at Katsuyama in the Mino Terrane of Central Japan: Implications for Depositional Environment, Diagenetic Metal Redistribution and Paleoceanography

Abstract. Inorganic chemical compositions are determined for a series of rocks crossing an Early Jurassic stratiform manganese ore deposit in a chert-dominant sequence at Katsuyama, in the Mino Terrane of central Japan. The lithology in the vicinity of the manganese ore bed is classified into lower bedded chert, black shale, massive chert, manganese ore and upper bedded chert, in ascending order. The rocks surrounding the manganese deposit are anomalously high in certain elements: Pb (max. 29 ppm), Ni (1140) and Co (336) in the lower bedded chert, Mo (438), As (149), Tl (29) and U (12) in the black shales, V (210) and Cr (87) in the massive chert, and MnO and W (24) in the manganese ore. The aluminum-normalized profiles reveal a distinct zonation of redox-sensitive elements: Pb-Zn, Ni-Co-Cu(-Zn) and U-Cr in the lower bedded chert, Mo-As-Tl in the black shale, V(-Cr) in the massive chert, and Mn-Fe-Ba-W in the manganese ore, in ascending order. The lower and upper bedded cherts and manganese ore generally exhibit flat rare earth element patterns with positive Ce anomalies, whereas the uppermost part of the lower bedded chert, the black shale and massive chert have flat patterns with weak or nonexistent negative Ce anomalies and weak positive Eu anomalies. The strong enrichment in Ni, Co, W, Tl and As detected in the Katsuyama section is not recognized in other sediments, including those of anoxic deposition origin, but is identified in modern ferromanganese nodules, suggesting that metal enrichment in the Katsuyama section is essentially due to the formation of ferromanganese nodules rather than to deposition in an anoxic environment. The observed elemental zonation is well explained by equilibrium calculations, reflecting early diagenetic formation and associated gradual reduction with depth. The concentration profiles in combination with litho- and biostratigraphical features suggest that formation of these bedded manganese deposits was triggered by an influx of warm, saline and oxic water into a stagnant deep ocean floor basin in Panthalassa at the end of the middle Early Jurassic. Paleoceanographic environmental controls thus appear to be important factors in the formation and preservation of this type of stratiform manganese deposit.

Chemistry and Sulfur Isotopes in a Chert-dominant Sequence around the Stratiform Manganese Deposit of the Noda-Tamagawa Mine, Northern Kitakami Terrane, Northeast Japan: Implication for Paleoceanographic Environmental Setting

. Chemistry and sulfur isotopes are analyzed for a series of rocks in the chert-dominant sequence around the stratiform manganese ore deposit of the Noda-Tamagawa mine in the northern Kitakami Terrane, northeast Japan. The sequence is litholog-ically classified into six units in ascending order: lower bedded chert, lower black shale, massive chert, manganese ore, upper black shale, and upper bedded chert. The rocks around the manganese ore deposit exhibit anomalous enrichment in Ni (max. 337 ppm), Zn (102) and U (30) in the upper part of lower bedded chert, Mo (122), Tl (79) and Pb (33) in the lower black shale, MnO, Cu (786) and Co (62) in the manganese ore, and As (247) and Sb (17) in the upper black shale. The aluminum-normalized profiles reveal zonal enrichment of redox-sensitive elements around the manganese bed: Zn-Ni-Fe-Mo-U(-Co), Tl-Pb(-Mo), Mn-Fe-Cu-V-Cr-Co(-Zn) and As-Sb in ascending order. The uppermost part of the lower bedded chert and black shale exhibit negative Ce/Ce* values, whereas the massive chert, manganese ore and lower part of the upper bedded chert display positive values. The isotopic δ34S values are 0±6 % in the lower part of the lower bedded chert, -19 to -42 % in the upper part of the lower bedded chert, -36 to -42 % in the lower black shale, -28 to -35 % in the massive chert, manganese ore and upper black shale, and -23±5 % in the upper bedded chert. Thus, there is a marked negative shift in δ34S values in the lower bedded chert, and an upward-increasing trend in δ34S through the manganese ore horizon. The present data provide evidence for a change in the paleoceanographic environmental resulting from inflow of oxic deepwater into the stagnant anoxic ocean floor below the manganese ore horizon. This event is likely to have triggered the precipitation of manganese oxyhydroxides. The redistribution of redox-sensitive elements through the formation of metalliferous black shale and manganese carbonate ore may have occurred in association with bacterial decomposition of organic matter during early diagenesis of initial manganese oxyhydroxides.

Classification, Genesis, and Exploration Guides for Nonsulfide Zinc Deposits

Nonsulfide deposits, popularly but incorrectly termed zinc oxide deposits, are becoming attractive exploration targets owing to new developments in hydrometallurgy. They are divided into two major geologic types—supergene and hypogene deposits. Supergene deposits are the most common type of nonsulfide deposit and are distributed worldwide. The vast majority occur in carbonate host rocks owing to the high reactivity of carbonate minerals with the acidic, oxidized, zinc-rich fluids derived from the oxidative destruction of sphalerite-bearing sulfide bodies. Formation of these deposits depends upon the size and mineralogy of the preexisting occurrence, vertical displacement of the water table, rate of water table descent through tectonic uplift and/or arid climatic conditions, wall-rock fracture density, and a suitable neutralizing trap site. Weathering of Mississippi Valley-type and high-temperature carbonate replacement-type deposits may generate significant supergene nonsulfide deposits, but the weathering of pyrite-rich, sedimentary exhalative, and volcanogenic massive sulfide deposits is much less likely to form economic supergene deposits. Three subtypes of supergene nonsulfide deposits are recognized—direct replacement, wall-rock replacement, and residual and karst-fill deposits. Hypogene nonsulfide deposits are more poorly known owing to the paucity of examples; however, two major subtypes are recognized: structurally controlled, replacement bodies and manganese-rich, exhalative(?) stratiform bodies. The structurally controlled bodies contain willemite and variable amounts of sphalerite, are hematitic, and are generally associated with hydrothermal dolomitization. Stratiform, manganese-rich, nonsulfide deposits appear to be end members of a spectrum of deposits that include base metal-poor stratiform manganese deposits and sulfide-dominant Broken Hill-type deposits. Hypogene nonsulfide deposits appear to have formed owing to the mixing of a reduced, low- to moderate-temperature (80°–200°C), zinc-rich, sulfur-poor fluid with an oxidized, sulfur-poor fluid.

Rare earth, major and trace elements in Jurassic manganese shales of the Northern Calcareous Alps: hydrothermal versus hydrogenous origin of stratiform manganese deposits

¶The rare earth, major and trace element geochemistry of Jurassic deep marine manganese shales allow insight into their environment of deposition. We present data of 24 samples from the Northern Calcareous Alps (Eastern Alps), collected from Late Jurassic strata of the Tennengebirge and from Early Jurassic strata of the Karwendel Mts. Whereas major and trace element geochemistry is controlled by the detrital input, rare earth element data and the relationship between total organic carbon and (pyritic) sulfur can be used to constrain the origin of the manganese accumulation. The data of this study are compatible with a hydrogenous precipitation of manganese by strongly varying redox-conditions at the sediment–water interface of a slope basin.

Deep-water manganese deposits in the mid- to late Proterozoic Penganga Group of the Pranhita-Godavari Valley, South India

Abstract Facies analysis of the unmetamorphosed sediments enclosing the stratiform manganese oxide deposits of the mid- to late Proterozoic Penganga Group identifies the base of slope of a distally steepened deep-water ramp as their site of accumulation. The interpretation is based on their close association with a variety of mass flow deposits ranging from limestone conglomerates to calcarenites and deep-water, plane-bedded micritic limestone devoid of current- or wave-generated structures as well as detritus coarser than fine silt. These deposits occur within a major transgressive succession. The base of slope origin of stratiform manganese deposits is uncommon in the rock record and their origin is to be constrained against the background of base of slope depositional setting.

Paleogeography and geochemistry of the “Cenomano-Turonian” formations in the manganese district of Imini (Morocco) and their relation to ore deposition

Detailed geochemical and microfacies analyses in the carbonate sequence of the Cenomano-Turonian in the Moroccan High Atlas Mountains indicate the possible mode of formation of the high-grade manganese ores in the Imini and Tasdremt district, south of the High Atlas. Regional distribution and stratigraphic setting of manganese and the associated metals suggest that the ores were formed in a zone of mixing between oxidized groundwater of meteoric origin and reducing seawater near the coast. The ore-forming metals were provided by continental weathering solutions and by remobilization processes in anoxic deep ocean basins. Giant stratiform manganese deposits of the Imini-type are formed in periods of high sea-level stand, warm climate and global anoxic conditions in the oceans. Large regional halos of associated minor metals in the surrounding carbonates can be used for the exploration of Imini-type manganese deposits.

Manganese occurrences in the Al Hammah Range — Wahrah Formation, Oman Mountains

Abstract The host rocks of the manganese in the Al Hammah Range (125 km S of Muscat) are red radiolarian cherts of Tithonian to Neocomian age, continuously exposed over 1000 km 2 . The stratigraphic sequence starts with fine-grained turbiditic limestones followed by a transition zone of coloured silt-mudstones and cherts which grade into almost CaCO 3 − free cherts, topped by 1–5 m of silicified limestones. The base of the Wahrah Formation is unknown. Sedimentological and geochemical features indicate that the cherts are of biogenic, non-hydrothermal origin. The stratiform manganese deposits are the result of sedimentary and tectonic enrichment processes. The single Mn-layers formed during periods of condensed sedimentation at the sediment-water interface. The different Mn-chert types (disseminated and layered ore and black Mn-rich cherts) can be explained by varying sedimentation rates of radiolaria, clay and manganese precipitation. Geochemically, the manganese deposits are characterized by high Mn/Fe ratios and by an extremely low minor element (e.g. Ni, Cu, Co) content. Using geochemical discrimination diagrams, the manganese deposits of the Wahrah Formation would be of hydrothermal origin. This contrasts with the sedimentary characteristics of the manganese-bearing cherts and with the field observations. Because of the Tithonian to Neocomian age of the Wahrah Formation cherts, the manganese enrichment can not be related to the formation of the Semail ophiolite. It is proposed that they might relate to a long distance discharge from the Owen fracture zone during the formation of the Masirah ophiolite.

Isotopic evidence for organic matter oxidation by manganese reduction in the formation of stratiform manganese carbonate ore

Unlike other marine-sedimentary manganese ore deposits, which are largely composed of manganese oxides, the primary ore at Molango (Hidalgo State, Mexico) is exclusively manganese carbonate (rhodochrosite, Mn-calcite, kutnahorite). Stable isotope studies of the carbonates from Molango provide critical new information relevant to the controversy over syngenetic and diagenetic models of stratiform manganese deposit formation. Negative δ 13C values for carbonates from mineralized zones at Molango are strongly correlated with manganese content both on a whole rock scale and by mineral species. Whole rock δ 13C data fall into three groups: high-grade ore = −16.4 to −11.5%.; manganese-rich, sub-ore-grade = −5.2 to 0%.; and unmineralized carbonates = 0 to +2.5%. (PDB). δ 18O data show considerable overlap in values among the three groups: +4.8 to −2.8, −5.4 to −0.3%., and −7.4 to +6.2 (PDB), respectively. Isotopic data for individual co-existing minerals suggest a similar separation of δ 13C values: δ 13C values from calcite range from −1.1 to +0.7%. (PDB), whereas values from rhodochrosite are very negative, −12.9 to −5.5%., and values from kutnahorite or Mn-calcite are intermediate between calcite and rhodochrosite. 13C data are interpreted to indicate that calcite ( i.e. unmineralized carbonate) formed from a normal marine carbon reservoir. However, 13C data for the manganese-bearing carbonates suggest a mixed seawater and organic source of carbon. The presence of only trace amounts of pyrite suggests sulfate reduction may have played a minor part in oxidizing organic matter. It is possible that manganese reduction was the predominant reaction that oxidized organic matter and that it released organic-derived CO 2 to produce negative δ 13C values and manganese carbonate mineralization.

Geology and genesis of manganese oolite, Chiatura, Georgia, U.S.S.R.

The Oligocene stratiform manganese deposits in the Chiatura region (Georgia, Soviet Union) were deposited on stable crystalline basement in a restricted arm of the Paratethys during a transgressive-regressive cycle. Transgression was characterized by, in the early stages, the development of a fining-upward gravel and sand sequence and, with increasing water depth, deposition of organic-rich, fine-grained sediments in waters increasingly depleted in oxygen. Manganese, derived from weathering of recks surrounding the basin, was probably concentrated in these reduced waters to levels well above those of normal sea water. Manganese carbonates were formed in offshore areas subject to reducing conditions; in well-aerated, nearshore parts, manganese oxides were formed. Landward migration of the oxidation/reduction interface during transgression led to further concentration of manganese in nearshore regions and eventually to extensive precipitation and accumulation of manganese oxide ooliths and pisoliths, occasionally in graded units, during peak transgression and early regression when basin waters were reoxygenated. Late regression was characterized by the deposition of sands and spongiolite. Petrographic studies of manganese-oxide ooliths and pisoliths indicate a concentrically laminated structure in which constituent pyrolusite microcrystals show a characteristic radial and, less frequently, random orientation with respect to the grain center. Ooliths and pisoliths, which also exhibit micro-unconformities and entrapped detrital material, are considered to be analogous to some present-day calcite and aragonite ooliths and were probably formed in a low energy, shallow-marine environment. Graded bedding in oolite and pisolite is comparable to similar beds documented in the Cretaceous stratiform manganese deposit at Groote Eylandt in northern Australia. Grading was probably formed as a result of changes in energy and/or oxygen levels during small-scale fluctuations in sea level.