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Manganese orebodies occur only at the base of the succession of basic volcanic and siliceous carbonate rocks in the aulacogen. The olistolith shows disorderly deposition with slump folding and irregularly shaped blocks, some within the orebodies. Black mudstones at the base of the orebodies formed in deep water with low rates of deposition and have a relatively high, though not commercial, Mn content. The thickness of the known orebodies varies greatly; their length is 5-8 times greater than their width, as in the case of the No.1 orebody in Xiaotianjin where the outcrop is less than 100 m wide but the extent westward along strike is about 800 m. Orebodies at the base of the volcanic rocks are characterised by oxidised manganese and iron-manganese, but those in the siliceous carbonates consist mainly of carbonate ores. The main minerals are pyrolusite, psilomelane and rhodochrosite. Secondary deposits are of carbonate surrounded by oxide ores, with the main minerals manganite, braunite and magnetite. It appears that volcanic exhalations brought the metals into the water during the graben stage, to be deposited in low-lying areas or depressions on the sea floor whose shapes were controlled by the E-W striking Runanshao fault. The relatively high oxygen fugacity near the vent favoured the formation of Mn and Fe-Mn oxides and the reducing environments in the depressions Mn carbonates. High grade ore formed entirely by diagenesis: ore grade is 10-15% lower near the vents than away from them., Manganese orebodies occur only at the base of the succession of basic volcanic and siliceous carbonate rocks in the aulacogen. The olistolith shows disorderly deposition with slump folding and irregularly shaped blocks, some within the orebodies. Black mudstones at the base of the orebodies formed in deep water with low rates of deposition and have a relatively high, though not commercial, Mn content. The thickness of the known orebodies varies greatly; their length is 5-8 times greater than their width, as in the case of the No.1 orebody in Xiaotianjin where the outcrop is less than 100 m wide but the extent westward along strike is about 800 m. Orebodies at the base of the volcanic rocks are characterised by oxidised manganese and iron-manganese, but those in the siliceous carbonates consist mainly of carbonate ores. The main minerals are pyrolusite, psilomelane and rhodochrosite. Secondary deposits are of carbonate surrounded by oxide ores, with the main minerals manganite, braunite and magnetite. It appears that volcanic exhalations brought the metals into the water during the graben stage, to be deposited in low-lying areas or depressions on the sea floor whose shapes were controlled by the E-W striking Runanshao fault. The relatively high oxygen fugacity near the vent favoured the formation of Mn and Fe-Mn oxides and the reducing environments in the depressions Mn carbonates. High grade ore formed entirely by diagenesis: ore grade is 10-15% lower near the vents than away from them.

The modern depositional setting can be directly applied to the post-Miocene sediments, in which indurated siderite layers are underlain by clays with a mineralogical composition much like those of the modern analogue. Deferrification of onshore sedimentary continental formations by pedological podzolisation processes can be inferred from such features as bleached quartz sands and organic hardpans. The iron was probably removed from the soils by "black" waters rich in iron-complexing compounds, while its deposition as iron carbonate is likely to have occurred hundreds of kilometres away laterally, in the brackish reducing environment of tidal lagoons., The modern depositional setting can be directly applied to the post-Miocene sediments, in which indurated siderite layers are underlain by clays with a mineralogical composition much like those of the modern analogue. Deferrification of onshore sedimentary continental formations by pedological podzolisation processes can be inferred from such features as bleached quartz sands and organic hardpans. The iron was probably removed from the soils by "black" waters rich in iron-complexing compounds, while its deposition as iron carbonate is likely to have occurred hundreds of kilometres away laterally, in the brackish reducing environment of tidal lagoons.

The metal distribution varies with stratigraphy as well as organic carbon content and shows increases for certain metals in areas of tectonism. In non-metamorphosed areas, U, Mo, V and Pb are enriched by factors of 2-4 relative to other black shales. The organic-rich (50% C) kolm is enriched by 0.5% U, 200 ppm radiogenic Pb, 800 ppm Y and heavy rare earth elements (e.g. 45 ppm Yb) but depleted in Mo, V and Ni. Normalising to Al accounts for the major and rock-forming elements that are the same as in the surrounding shale, but not for U, Pb, Y and HREE. Increases in content of trace elements are often not correlated with the contents of host rocks or sites for non-metamorphosed alum shale or kolm. A combination of unusual source rocks rich in U and other elements, selective element transport and efficient concentration by organic matter is suggested as the means for enrichment. In the metamorphosed areas the carbon content of the shales is similar to that in non-metamorphosed areas but there are increases to 0.2% V, 500 ppm Ni and Zn, 10 ppm Cd and 1 500 ppm Ba. Epigenetic addition of metals from external sources may have been the cause, rather than internal redistribution., The metal distribution varies with stratigraphy as well as organic carbon content and shows increases for certain metals in areas of tectonism. In non-metamorphosed areas, U, Mo, V and Pb are enriched by factors of 2-4 relative to other black shales. The organic-rich (50% C) kolm is enriched by 0.5% U, 200 ppm radiogenic Pb, 800 ppm Y and heavy rare earth elements (e.g. 45 ppm Yb) but depleted in Mo, V and Ni. Normalising to Al accounts for the major and rock-forming elements that are the same as in the surrounding shale, but not for U, Pb, Y and HREE. Increases in content of trace elements are often not correlated with the contents of host rocks or sites for non-metamorphosed alum shale or kolm. A combination of unusual source rocks rich in U and other elements, selective element transport and efficient concentration by organic matter is suggested as the means for enrichment. In the metamorphosed areas the carbon content of the shales is similar to that in non-metamorphosed areas but there are increases to 0.2% V, 500 ppm Ni and Zn, 10 ppm Cd and 1 500 ppm Ba. Epigenetic addition of metals from external sources may have been the cause, rather than internal redistribution.

The lacustrine facies in Serbia contain sedimentary zeolites, bentonite, oil shale with authigenic minerals, magnesite and borates. Some of the deposits formed by diagenesis, including the alteration of volcanic glass. In some cases hot waters of volcanic origin migrated through serpentinites that underlie the lake basins, bringing magnesium to the lake water. Boron was derived by the leaching of volcanic and pyroclastic rocks by meteoric waters. The marine shallow-water facies mined at Tajmiste in western Macedonia include oolitic ironstones, mostly chamositic clays, which were deposited within a clastic sequence composed of clays, silts and sands which were subsequently metamorphosed. All primary structures were destroyed by metamorphism and/or tectonic activity. The internal structures of the chamosite ooids are mostly tangential, although in some cases they are radial. The iron had a multiple origin, including streams that drained weathered terrains and possibly upwelling deep basin waters., The lacustrine facies in Serbia contain sedimentary zeolites, bentonite, oil shale with authigenic minerals, magnesite and borates. Some of the deposits formed by diagenesis, including the alteration of volcanic glass. In some cases hot waters of volcanic origin migrated through serpentinites that underlie the lake basins, bringing magnesium to the lake water. Boron was derived by the leaching of volcanic and pyroclastic rocks by meteoric waters. The marine shallow-water facies mined at Tajmiste in western Macedonia include oolitic ironstones, mostly chamositic clays, which were deposited within a clastic sequence composed of clays, silts and sands which were subsequently metamorphosed. All primary structures were destroyed by metamorphism and/or tectonic activity. The internal structures of the chamosite ooids are mostly tangential, although in some cases they are radial. The iron had a multiple origin, including streams that drained weathered terrains and possibly upwelling deep basin waters.

Enrichment in metals can develop in several stages in the geological evolution of organic matter from living tissue through kerogen and liquid hydrocarbons to solid hydrocarbon products. Possible scenarios include: uptake by organic matter from groundwater flowing through ore deposits; organic matter caught up in metal-rich hydrothermal systems; and organic matter with a signature of low-level synsedimentary metal concentrations which were subsequently remobilised into ore deposits. Some solid bitumens are so enriched in metal that they constitute the actual orebody. Uranium especially is found enriched within organic materials, including bitumen which precipitates around uranium minerals due to radiation-induced polymerisation, but many other metals also form enrichments., Enrichment in metals can develop in several stages in the geological evolution of organic matter from living tissue through kerogen and liquid hydrocarbons to solid hydrocarbon products. Possible scenarios include: uptake by organic matter from groundwater flowing through ore deposits; organic matter caught up in metal-rich hydrothermal systems; and organic matter with a signature of low-level synsedimentary metal concentrations which were subsequently remobilised into ore deposits. Some solid bitumens are so enriched in metal that they constitute the actual orebody. Uranium especially is found enriched within organic materials, including bitumen which precipitates around uranium minerals due to radiation-induced polymerisation, but many other metals also form enrichments.

Mn-Fe oxyhydroxide encrustations overgrow and impregnate hydrothermally phosphatised Middle Eocene limestones which blanket the seamount. In chemical composition they are transitional between hydrothermal and hydrogenetic types. The major minerals are iron vernadite and manganese feroxyhyte. The presence of goethite, mixed layered asbolane-buserite, magnesium asbolane and birnessite in the crusts is interpreted as resulting from postdepositional transformations of the initial Mn-Fe oxyhydroxide material., Mn-Fe oxyhydroxide encrustations overgrow and impregnate hydrothermally phosphatised Middle Eocene limestones which blanket the seamount. In chemical composition they are transitional between hydrothermal and hydrogenetic types. The major minerals are iron vernadite and manganese feroxyhyte. The presence of goethite, mixed layered asbolane-buserite, magnesium asbolane and birnessite in the crusts is interpreted as resulting from postdepositional transformations of the initial Mn-Fe oxyhydroxide material.

A pronounced change from Late Cambrian oceanic conditions is particularly evident in the Northern Head Group. A marked increase in bioturbation, anomalously high manganese and barium content and changes in C:S ratios all suggest a transition to a new depositional regime characterised by ventilated depositional and early diagenetic conditions. This is much more subtly reflected in the Cow Head Group. The contrast is ascribed to along-strike differences in margin morphology and extends to sulphides within shales throughout the section. In the Northern Head Group, anomalous concentrations (up to 300 ppm) of zinc, copper and lead occur within Lower Ordovician shales, with disseminated sulphides apparently remobilised and replacive. In the Cow Head Group, on the other hand, localised metal concentrations are scattered throughout the entire stratigraphic interval from late Middle Cambrian upwards and are generally associated with organic carbon. Sulphur isotope profiles also suggest a marked Lower Ordovician change from stratified to ventilated conditions, immediately followed by a temporary return to stratified conditions. These circulation changes may have controlled redox conditions and thus governed metal mobilisation and precipitation., A pronounced change from Late Cambrian oceanic conditions is particularly evident in the Northern Head Group. A marked increase in bioturbation, anomalously high manganese and barium content and changes in C:S ratios all suggest a transition to a new depositional regime characterised by ventilated depositional and early diagenetic conditions. This is much more subtly reflected in the Cow Head Group. The contrast is ascribed to along-strike differences in margin morphology and extends to sulphides within shales throughout the section. In the Northern Head Group, anomalous concentrations (up to 300 ppm) of zinc, copper and lead occur within Lower Ordovician shales, with disseminated sulphides apparently remobilised and replacive. In the Cow Head Group, on the other hand, localised metal concentrations are scattered throughout the entire stratigraphic interval from late Middle Cambrian upwards and are generally associated with organic carbon. Sulphur isotope profiles also suggest a marked Lower Ordovician change from stratified to ventilated conditions, immediately followed by a temporary return to stratified conditions. These circulation changes may have controlled redox conditions and thus governed metal mobilisation and precipitation.

Mn-rich facies in Algoma, Lake Superior and Rapitan types of iron formation are an important part of the group of siliceous metalliferous sediments related to cherty iron formation, or stratafer group. Mn oxide and carbonate facies associated with iron formation, cherts, carbonates, shales, turbidites, tuffs and lavas are up to 30 m thick and have Fe/Mn ratios ranging from 0.2 to 2. The major and minor element contents of stratafer Mn sediments are compared with typical oxide facies of iron formation and with modern protolithic facies on the sea floor that formed by hydrothermal effusive and hydrogenous processes. Cherty manganiferous facies and their gondite metamorphic equivalents occur throughout the geological record, provide major resources of manganese and are the most common protore for high-grade Mn deposits formed by secondary enrichment., Mn-rich facies in Algoma, Lake Superior and Rapitan types of iron formation are an important part of the group of siliceous metalliferous sediments related to cherty iron formation, or stratafer group. Mn oxide and carbonate facies associated with iron formation, cherts, carbonates, shales, turbidites, tuffs and lavas are up to 30 m thick and have Fe/Mn ratios ranging from 0.2 to 2. The major and minor element contents of stratafer Mn sediments are compared with typical oxide facies of iron formation and with modern protolithic facies on the sea floor that formed by hydrothermal effusive and hydrogenous processes. Cherty manganiferous facies and their gondite metamorphic equivalents occur throughout the geological record, provide major resources of manganese and are the most common protore for high-grade Mn deposits formed by secondary enrichment.

Copper minerals in the Um Bogma region are of two contrasting genetic types: stratabound to stratiform malachite confined within a certain stratigraphic level of Cambro-Ordovician clastic sediments, and stratabound Cu carbonates, chlorides, silicates, sulphates and phosphates confined within a buried Carboniferous karst profile. The geometric distribution patterns of the first type represent a high degree of congruence with the primary, deformational and biogenic sedimentary structures of the enclosing rocks. The malachite was formed syngenetically during the accumulation of the host sediments in fluviodeltaic environments. In the second type, the geometric distributions of the Cu minerals and the geochemistry and mineralogy of the host karst reveal that the minerals are of pedogenic origin, deposited from solutions in a palaeosol developed during the karst event. Variation in pH and Eh conditions of the palaeosol environment and related biogenic processes are suggested as the main factors controlling the leaching of Cu and its migration and redeposition within the subsoil and topsoil horizons of the palaeokarst profile., Copper minerals in the Um Bogma region are of two contrasting genetic types: stratabound to stratiform malachite confined within a certain stratigraphic level of Cambro-Ordovician clastic sediments, and stratabound Cu carbonates, chlorides, silicates, sulphates and phosphates confined within a buried Carboniferous karst profile. The geometric distribution patterns of the first type represent a high degree of congruence with the primary, deformational and biogenic sedimentary structures of the enclosing rocks. The malachite was formed syngenetically during the accumulation of the host sediments in fluviodeltaic environments. In the second type, the geometric distributions of the Cu minerals and the geochemistry and mineralogy of the host karst reveal that the minerals are of pedogenic origin, deposited from solutions in a palaeosol developed during the karst event. Variation in pH and Eh conditions of the palaeosol environment and related biogenic processes are suggested as the main factors controlling the leaching of Cu and its migration and redeposition within the subsoil and topsoil horizons of the palaeokarst profile.

The deposits occur in an algal reef carbonate development with the two main horizons, an algal dolomite and a black bed, are the products of two transgressive cycles. Trace element data show that the ore materials were derived from the host rocks and underlying beds and that in addition Bi, Pb and Zn were introduced through deep fractures. The ore-forming fluid was a hot connate brine. Sulphur was derived from sulphate-reducing bacteria and carbon from marine carbonates. Organic matter played an important role in metallogenesis. In the Kangdian Axis, two deep fractures controlled the sedimentation of a cupriferous algal reef limestone. Copper was subsequently mobilised into vein deposits., The deposits occur in an algal reef carbonate development with the two main horizons, an algal dolomite and a black bed, are the products of two transgressive cycles. Trace element data show that the ore materials were derived from the host rocks and underlying beds and that in addition Bi, Pb and Zn were introduced through deep fractures. The ore-forming fluid was a hot connate brine. Sulphur was derived from sulphate-reducing bacteria and carbon from marine carbonates. Organic matter played an important role in metallogenesis. In the Kangdian Axis, two deep fractures controlled the sedimentation of a cupriferous algal reef limestone. Copper was subsequently mobilised into vein deposits.

The stratabound iron-manganese umber, extending for 2 km along strike, overlies a tholeiitic pillow lava. It is divided into lower metalliferous and upper argillaceous segments, changing gradually upwards to ferruginous, spicule-rich shale. The metalliferous umber contains 41-54 wt% Fe2O3 and 8.0-16.8 wt% MnO2+MnO, comprising massive aggregates of very fine goethite, amorphous calcic Mn oxides and pyrolusite. Direct evidence of formation during submarine hydrothermal activity following basaltic volcanism is an umber-filled pipe within the pillow lavas beneath the stratabound deposits. Away from this conduit the MnO:MnO2 ratio in the umber decreases, reflecting the redox potential of formation, and pyrolusite content increases. There are high REE concentrations and a strong negative Ce anomaly, while C and O delta values for calcite in the pipe umber suggest precipitation from heated sea water at 55-68 degrees C; the metal-carrying hydrothermal water was probably much hotter. Abundant spicules in the overlying shale are thought to represent sponges that fed on the nutrient-rich water., The stratabound iron-manganese umber, extending for 2 km along strike, overlies a tholeiitic pillow lava. It is divided into lower metalliferous and upper argillaceous segments, changing gradually upwards to ferruginous, spicule-rich shale. The metalliferous umber contains 41-54 wt% Fe2O3 and 8.0-16.8 wt% MnO2+MnO, comprising massive aggregates of very fine goethite, amorphous calcic Mn oxides and pyrolusite. Direct evidence of formation during submarine hydrothermal activity following basaltic volcanism is an umber-filled pipe within the pillow lavas beneath the stratabound deposits. Away from this conduit the MnO:MnO2 ratio in the umber decreases, reflecting the redox potential of formation, and pyrolusite content increases. There are high REE concentrations and a strong negative Ce anomaly, while C and O delta values for calcite in the pipe umber suggest precipitation from heated sea water at 55-68 degrees C; the metal-carrying hydrothermal water was probably much hotter. Abundant spicules in the overlying shale are thought to represent sponges that fed on the nutrient-rich water.

The method described assists in the quantification of oxide Mn minerals and associated materials from the deposits. The ores are commonly composed of very fine-grained minerals intergrown with lateritic components like kaolinitic clays and Fe oxyhydroxides, while many Mn phases are poorly ordered structures which are difficult to identify. Although FTIR spectroscopy has produced dependable data for a limited range of processed ores it failed with rocks containing a mixture of ore minerals and gangue phases, as did other analytical techniques. The normalisation is based on the same principles as other mineral norms and the norm minerals themselves were developed according to the mineralogical conditions in the Groote Eylandt deposits, though the list could easily be extended and adjusted to slightly different environments such as that of bauxites. Minerals that can be quantified by the normalisation technique are romanechite, todorokite, cryptomelane, pyrolusite, anatase, quartz, kaolinite, gibbsite, goethite for haematite-free and haematite-containing samples, haematite and excess water., The method described assists in the quantification of oxide Mn minerals and associated materials from the deposits. The ores are commonly composed of very fine-grained minerals intergrown with lateritic components like kaolinitic clays and Fe oxyhydroxides, while many Mn phases are poorly ordered structures which are difficult to identify. Although FTIR spectroscopy has produced dependable data for a limited range of processed ores it failed with rocks containing a mixture of ore minerals and gangue phases, as did other analytical techniques. The normalisation is based on the same principles as other mineral norms and the norm minerals themselves were developed according to the mineralogical conditions in the Groote Eylandt deposits, though the list could easily be extended and adjusted to slightly different environments such as that of bauxites. Minerals that can be quantified by the normalisation technique are romanechite, todorokite, cryptomelane, pyrolusite, anatase, quartz, kaolinite, gibbsite, goethite for haematite-free and haematite-containing samples, haematite and excess water.

The black shales are of lacustrine origin and form several complexes of varying lithology. They represent sedimentary environments varying from oxidised and unoxidised parts of the pelagic zone to the littoral and coastal plain. Horizons with uranium mineralisation occur within beds of organic-rich pelagic sediment. Facies analysis and studies of organic matter suggest that mineralisation was syndiagenetic and occurred in rocks below the wave base. The presence of migrated bitumen on fracture surfaces and a correlation between uranium and organic carbon suggests the possibility of reconcentration of uranium within migrated bitumen., The black shales are of lacustrine origin and form several complexes of varying lithology. They represent sedimentary environments varying from oxidised and unoxidised parts of the pelagic zone to the littoral and coastal plain. Horizons with uranium mineralisation occur within beds of organic-rich pelagic sediment. Facies analysis and studies of organic matter suggest that mineralisation was syndiagenetic and occurred in rocks below the wave base. The presence of migrated bitumen on fracture surfaces and a correlation between uranium and organic carbon suggests the possibility of reconcentration of uranium within migrated bitumen.

Throughout the Mesozoic, deposition in the eastern area of Mexico was in fault-bounded basins with considerable relief. Both clastic and carbonate sediment was derived from adjacent highs. The manganese ores were deposited in the slope facies of a shelf-basin transition in water deeper than storm wave-base. Rocks below the ore were deposited in a euxinic basin; those above the ore in a more oxidising but still suboxic basin. Manganese was mobilised in deeper, low-oxygen water, then precipitated as manganese oxide on contact with shallower, oxygen-rich water. Manganese carbonate formed diagenetically from the oxides via reduction by organic matter and iron sulphide. Because organic matter was in excess, no primary manganese oxide survived early diagenesis., Throughout the Mesozoic, deposition in the eastern area of Mexico was in fault-bounded basins with considerable relief. Both clastic and carbonate sediment was derived from adjacent highs. The manganese ores were deposited in the slope facies of a shelf-basin transition in water deeper than storm wave-base. Rocks below the ore were deposited in a euxinic basin; those above the ore in a more oxidising but still suboxic basin. Manganese was mobilised in deeper, low-oxygen water, then precipitated as manganese oxide on contact with shallower, oxygen-rich water. Manganese carbonate formed diagenetically from the oxides via reduction by organic matter and iron sulphide. Because organic matter was in excess, no primary manganese oxide survived early diagenesis.

Abundant microfossils found in deposits in eastern Guizhou, western Hunan, western Liaoning and Gansu, together with radiometric dating, indicate deposit ages around 1 200-750 Ma. A number of fossils of Sphaerocongregus (Bavlinella) have been found in rhodochrosite; their different preservation in relation to the Fe and Mn contents of the ores suggest that cyanobacteria were directly or indirectly implicated in the genesis of the deposits., Abundant microfossils found in deposits in eastern Guizhou, western Hunan, western Liaoning and Gansu, together with radiometric dating, indicate deposit ages around 1 200-750 Ma. A number of fossils of Sphaerocongregus (Bavlinella) have been found in rhodochrosite; their different preservation in relation to the Fe and Mn contents of the ores suggest that cyanobacteria were directly or indirectly implicated in the genesis of the deposits.

Organic matter in carbonate rocks of Cambrian to Devonian age is distributed inhomogeneously. Some metal deposits are associated with two asphalt belts occurring in northern and southern China. From the characteristics of soluble organic matter, reflectance and the structure of the asphalts it is inferred that most of the vanadium, nickel, molybdenum and uranium deposits were formed in stable pressure and temperature conditions, while arsenic and mercury deposits were formed in open system conditions near to natural gas pools. Comparing the metallic element content of asphalts, black shales and carbonate shows that most metals in the asphalts could have been derived from black shales but some lead, tin, silver, zinc and manganese were possibly contributed by hydrothermal solutions. The metals associated with organic matter are mostly chalcophile with variable valencies. Experiments indicate that organic matter and metals combine during two particular stages, when reflectance is less than 1.2% and greater than 2%., Organic matter in carbonate rocks of Cambrian to Devonian age is distributed inhomogeneously. Some metal deposits are associated with two asphalt belts occurring in northern and southern China. From the characteristics of soluble organic matter, reflectance and the structure of the asphalts it is inferred that most of the vanadium, nickel, molybdenum and uranium deposits were formed in stable pressure and temperature conditions, while arsenic and mercury deposits were formed in open system conditions near to natural gas pools. Comparing the metallic element content of asphalts, black shales and carbonate shows that most metals in the asphalts could have been derived from black shales but some lead, tin, silver, zinc and manganese were possibly contributed by hydrothermal solutions. The metals associated with organic matter are mostly chalcophile with variable valencies. Experiments indicate that organic matter and metals combine during two particular stages, when reflectance is less than 1.2% and greater than 2%.

In the Atlantis II Deep stratified brines determine different physico-chemical conditions. Todorokite is formed mainly in the upper layers, where an alkaline environment with high oxygen content gives rise to hydrated thixotropic macrostructures of nMnO2.mH2O, which can adsorb cations. Favourable conditions for manganite formation are found in the lower layers where oxygen is deficient and there are large amounts of manganese and low pH. The amorphous Mn hydroxides are less hydrated and contain no adsorbed cations besides Mn. The bulk of goethite and haematite is also formed in the lower part of the brines or in the sediments during diagenesis. Ferrihydrite, present in the upper sediments, is formed through microbiological processes in the layer of brines transitional to sea water. In the Thetis Deep there is no stratification of bottom water and the Mn content is considerably lower. MnO2 gel coagulates slowly to form asbolane with layer structures. The main factor affecting iron ore mineralogy is the kinetics of crystallisation in the amorphous phase, which in turn depends on the Fe/O2 ratio: magnetite is formed during active supply of hydrothermal iron and lepidocrocite when hydrothermal activity diminishes., In the Atlantis II Deep stratified brines determine different physico-chemical conditions. Todorokite is formed mainly in the upper layers, where an alkaline environment with high oxygen content gives rise to hydrated thixotropic macrostructures of nMnO2.mH2O, which can adsorb cations. Favourable conditions for manganite formation are found in the lower layers where oxygen is deficient and there are large amounts of manganese and low pH. The amorphous Mn hydroxides are less hydrated and contain no adsorbed cations besides Mn. The bulk of goethite and haematite is also formed in the lower part of the brines or in the sediments during diagenesis. Ferrihydrite, present in the upper sediments, is formed through microbiological processes in the layer of brines transitional to sea water. In the Thetis Deep there is no stratification of bottom water and the Mn content is considerably lower. MnO2 gel coagulates slowly to form asbolane with layer structures. The main factor affecting iron ore mineralogy is the kinetics of crystallisation in the amorphous phase, which in turn depends on the Fe/O2 ratio: magnetite is formed during active supply of hydrothermal iron and lepidocrocite when hydrothermal activity diminishes.

The formation, located in a rift belt on the margin of the Yangtze Craton, extends for over 600 km and for 150 km perpendicular to strike in eastern Guizhou and western Hunan. It is interbedded between two sequences that have been interpreted as glacial diamictites, and comprises manganese ore and Mn-rich black shales which show fine, graded laminae with ABC and ABE Bouma sequences and contain algae, fungi, acritarchs and radiolaria. These features suggest deposition in relatively deep water. The Mn ore bodies include lenses and pillow-shaped structures with a concentric internal fabric, and are cut by fine barite and quartz veinlets. The deep-water environment may have been established by a marine transgression following glacial melting. Hydrothermal exhalation at the sea bed could then have resulted in stratification of the water column, with an anoxic bottom layer in which syngenetic manganese deposits were precipitated as successive concentric layers around nuclei., The formation, located in a rift belt on the margin of the Yangtze Craton, extends for over 600 km and for 150 km perpendicular to strike in eastern Guizhou and western Hunan. It is interbedded between two sequences that have been interpreted as glacial diamictites, and comprises manganese ore and Mn-rich black shales which show fine, graded laminae with ABC and ABE Bouma sequences and contain algae, fungi, acritarchs and radiolaria. These features suggest deposition in relatively deep water. The Mn ore bodies include lenses and pillow-shaped structures with a concentric internal fabric, and are cut by fine barite and quartz veinlets. The deep-water environment may have been established by a marine transgression following glacial melting. Hydrothermal exhalation at the sea bed could then have resulted in stratification of the water column, with an anoxic bottom layer in which syngenetic manganese deposits were precipitated as successive concentric layers around nuclei.