Polymetallic Crusts Research Articles

Mineralogy, geochemistry and genesis of the polymetallic crusts from the Chagos-Laccadive Ridge, Arabian Sea, India

Recent offshore exploration by the Geological Survey of India (GSI) located ferromanganese crusts (FeMn) along the Chagos-Laccadive Ridge (CLR) offshore western continental margin of India. CLR FeMn crusts grew on phosphorite substrate rock (phosphorite cemented sand/silt with moderate carbonate fluorapatite (CFA) cement and goethite). Phosphorite precipitation is the product of intense upwelling and phosphorus regeneration due to organic matter degradation and fish debris dissolution. CLR FeMn crusts are of hydrogenetic origin precipitated in an oxic environment. As a result of hydrogenetic processes under oxic conditions, Fe-vernadite dominated the mineral phase of FeMn crusts. CLR FeMn crusts formed when the North Indian deep-water layer mixed with oxygen-depleted waters from the Arabian Sea, Persian Gulf, and Red Sea. Three genetic layers are identified in the FeMn crusts; Type-I, Type-II, and Type-III developed in response to fluctuation in dissolved oxygen during periods of intensified suboxic bottom waters and monsoon maxima associated with past climatic events in the Arabian Sea. Type-I layers are columnar showing low Mn/Fe ratio (∼1), reflectivity, Mg (average 1.24%), and Ni (average 0.29%). Dendritic Type-II layers show high Mn/Fe ratios (average 12.29), reflectivity, Mg (average 3.51%) and Ni (average 1.69%). Type-III layers show very low Mn/Fe ratios (average 0.21), reflectivity, Mg (average 1.4%) and Ni (average 0.01%) and contain a substantial amount of terrigenous debris of aeolian origin. Elements of economic interest such as Mn, Co, Ni, Cu, Mo, Li, Te, and REY in CLR FeMn crusts are depleted owing to terrigenous contribution.

Nano-mineralogy and growth environment of Fe-Mn polymetallic crusts and nodules from the South China Sea

Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS) consist of submarine ferromanganese (Fe-Mn) oxide precipitates, and represent important marine mineral resource with substantial economic and scientific research value. Previous studies on the SCS polymetallic crusts and nodules were mainly focused on their bulk mineralogy and geochemistry, whilst research on their nanomineralogy is still lacking. In this study, transmission electron microscopy (TEM), Raman spectroscopic mapping, and in-situ micro X-ray diffraction (XRD) analysis were conducted on the nano-mineralogy of the SCS polymetallic crusts and nodules. It is found that the SCS polymetallic crusts and nodules consist mainly of layered/columnar/mottled nano-phase Fe-Mn minerals and detritus such as quartz, feldspar, and clays. Also, an independent Ti mineral phase has been documented, and the mineralogical analysis reveals the transformation from vernadite to birnessite and todorokite. Titanium forms colloidal minerals in seawater and precipitates into the crusts and nodules with other colloids, such as FeOOH and Si-Al. Vernadite and birnessite can be transformed to todorokite with stable structure under sub-oxic conditions. Therefore, the SCS polymetallic crusts and nodules were formed in a short period of sub-oxic environment and diagenetic process, and the transformation can influence the enrichment of Ni and other metals during the crust/nodule growth.

Morphologies Characteristics and Anomaly on Os Isotope Curve and the Comparison Dating Method for Polymetallic Crusts

During the partial melting of the mantle, due to the compatibility difference, Re tends to enter the melt and concentrate in the earth crust, while Os tends to remain in the residual phase, thereby enriching in the mantle. This isotope fractionation caused differences in the distribution of Re and Os isotopes between the crust and the mantle, which lead to the change in the isotopic composition of 187Os/188Os in the ocean. The change is recorded in the growing of the hydrogenetic marine authigenic mineral such as polymetallic crusts, and by comparing the layered Os isotope composition curve of crusts with the seawater standard curve, the dating can be realized. K/T (Cretaceous/Tertiary) boundary low values, E/O (Eocene/Oligocene) boundary low values and Miocene “deflection” are the only three “anchor points” for curve comparison before, which causes the uncertainty of the dating result. In this study, by comparing the Os isotopic composition curves of six polymetallic crust samples from the central and western Pacific, four new curve characteristics were recognized, providing reference for the accurate chronology work. Meanwhile, the regional and periodic difference of the curve are studied. It can be preliminarily explained as the regional earth-mantle derived materials supply events but it didn’t break the overall pattern of the Pacific system.

Impact of ferromanganese ore pollution on phytoplankton CO2 fixation in the surface ocean

Because ferromanganese polymetallic crusts can become a global resource of valuable elements the ecological impact of seafloor crust mining requires evaluation. Whilst the detrimental impact on deep-ocean benthos is established, experimental evidence about the mining hazard to surface-ocean is sparse. When retrieved, mined crusts can leach elements potentially harmfull to the core oceanic CO2-fixers – phytoplankton. To directly assess the magnitude of this potential hazard at ocean-basin scale, we examine the impact of ore slurry on phytoplankton CO2 fixation along a meridional transect through the South Atlantic Ocean. Within 12 h crust slurry additions caused a 25% decrease of CO2 fixation in the subtropical region and 15% in the temperate-polar region. Such moderate susceptibility of phytoplankton indicates limited release of harmful elements from tested polymetallic powder. Although this implies that environmentally sustainable seafloor mining could be feasible, longer-term complex studies of the mining impact on the surface ocean are required.

Relationship between geoacoustic properties and chemical content of submarine polymetallic crusts from offshore Brazil

Marine mining is the ocean’s new exploration frontier, and polymetallic crusts (PMCs) and ferromanganese nodules are considered a strategic resource for the future. Acoustic geophysical methods are a valuable tool for oceanic research and have been employed for several decades in the exploration of marine resources and environmental evaluations. The main objective of this work was to investigate the correlation between the chemical composition of PMCs from three different areas along the Brazilian continental margin and their physico-acoustic attributes (P-wave velocity, density and acoustic impedance). The results show that the geoacoustic properties of PMCs are correlated to their chemical composition. Measurements showed positive and negative trends between acoustic impedance and studied elements (Fe, Mn, Co, Cu, Zn, W and Ni) according to the direction of measurement, indicting high anisotropy levels. Our study sheds initial light on the correlation between acoustic properties and metal content of PMCs. The study facilitates assessments of the acoustic responses of PMCs allowing more efficient prospection and exploitation compared to ship-board geophysical techniques that are too qualitative to identify PMCs. The results can contribute to determining the best procedures and techniques for more efficient future exploration of this resource.

Helium and Argon Isotopes in the Fe-Mn Polymetallic Crusts and Nodules from the South China Sea: Constraints on Their Genetic Sources and Origins

In this study, the He and Ar isotope compositions were measured for the Fe-Mn polymetallic crusts and nodules from the South China Sea (SCS), using the high temperature bulk melting method and noble gases isotope mass spectrometry. The He and Ar of the SCS crusts/nodules exist mainly in the Fe-Mn mineral crystal lattice and terrigenous clastic mineral particles. The results show that the 3He concentrations and R/RA values of the SCS crusts are generally higher than those of the SCS nodules, while 4He and 40Ar concentrations of the SCS crusts are lower than those of the SCS nodules. Comparison with the Pacific crusts and nodules, the SCS Fe-Mn crusts/nodules have lower 3He concentrations and 3He/4He ratios (R/RA, 0.19 to 1.08) than those of the Pacific Fe-Mn crusts/nodules, while the 40Ar/36Ar ratios of the SCS samples are significantly higher than those of the Pacific counterparts. The relatively low 3He/4He ratios and high 40Ar concentrations in the SCS samples are likely caused by terrigenous detrital input with high radiogenic 4He and 40Ar contents. The SCS crusts and nodules have shorter growth periods, implying that in situ post-formation radiogenic 3He, 4He and 40Ar produced by decay of U, Th and K have no effect on their isotope compositions. Thus, the SCS crusts/nodules inherited the noble gases characteristics of their sources. Helium and Ar isotope compositions in the SCS Fe-Mn crusts and nodules reflect the product of an equilibrium mixture between air-saturated seawater and radiogenic components during their growth, while the partial 3He excess in some SCS samples may represent a little mantle-derived origin. The different He and Ar isotope compositions of the Fe-Mn crusts and nodules between the South China Sea and the Pacific Ocean are due to their different sources and genetic processes. The characteristics of He and Ar isotope compositions in the SCS polymetallic crusts and nodules are similar to the properties of hydrogenetic Fe-Mn oxide/hydroxide precipitates, which reflects mainly the product of an equilibrium mixture between air-saturated seawater and radiogenic components.

Deep-sea video technology tracks a monoplacophoran to the end of its trail (Mollusca, Tryblidia)

Originally known as fossils from the Cambrian to the Devonian, the finding of a living monoplacophoran mollusc in 1952 was one of the great zoological discoveries of the twentieth century. Now, over 35 living species have been documented from deep-sea locations around the world, mainly from samples collected with trawls. Encountering these animals is extremely rare, and in situ observations are scant. Here, we report two new sightings and ecological data for a probable undescribed species of Neopilina including the first ever high-definition close-up video of these monoplacophorans in their natural environment, obtained while exploring seamount environments in American Samoa. Extensive trackways, similar to those associated with the monoplacophoran siting, may be evidence of a larger population at both seamounts. Living monoplacophorans are important to understanding the recent evolution of deep-sea fauna, yet their habitat, on polymetallic nodules and ferromanganese crusts, is under rapidly increasing pressure for deep-sea mineral extraction.

Rare Earth Elements Composition and Constraint on the Genesis of the Polymetallic Crusts and Nodules in the South China Sea

AbstractThe rare earth elements (REE) composition of the polymetallic crusts and nodules obtained from the South China Sea (SCS) were analyzed through inductively coupled plasma mass spectrometry. Results revealed great differences in the REE abundances (ΣREE) of the SCS polymetallic crusts and nodules; the crusts show the highest ΣREE, whereas the nodules exhibit the lowest ΣREE. The similarity in their NASC–normalized patterns, the enriched light REE (LREE), the markedly positive Ce anomaly (δCe), and the non– or weakly positive Eu anomaly (δEu), suggest that the polymetallic crusts and nodules are of hydrogenetic origin. Moreover, the REE contents and their relevant parameters are quite different among the various layers of the crusts and nodules, which probably results from the different marginal sea environments and mineral assemblages of the samples. The growth profiles of the SCS polymetallic crusts and nodules reveal the tendency ΣREE and δCe to slightly increase from the outer to the inner layers, suggesting that the growth environments of these samples changed smoothly from an oxidizing to a relatively reducing environment; in addition, the crust ST1 may have experienced a regressive event (sea–level change) during its growth, although the REE composition of the seawater remained relatively stable. On the basis of the regional ΣREE distribution in the SCS crusts and nodules, the samples collected near the northern margin were influenced by terrigenous material more strongly compared with the other samples, and the REE contents are relatively low. Therefore, the special geotectonic environment is a significant factor influencing the abundance of elements, including REE and other trace elements. Compared with the oceanic seamount crusts and deep–sea nodules from other oceans, the SCS polymetallic crusts and nodules exhibit special REE compositions and shale–normalized patterns, implying that the samples are of marginal sea–type Fe–Mn sedimentary deposits, which are strongly affected by the epicontinental environment, and that they grew in a more oxidative seawater environment. This analysis indicates that the oxidized seawater environment and the special nano property of their Fe–Mn minerals enrich the REE adsorption.

Mineralogy, geochemistry and genesis of the polymetallic crusts and nodules from the South China Sea

Recent marine mineral investigations in the South China Sea (SCS) have found increasingly significant abundances of Fe–Mn polymetallic crusts and nodules with variable substrates and nuclei. There is, however, no systematic and complete study of the SCS crusts and nodules. The lack of mineralogical and geochemical studies affects our explicit understanding of the mineralization characteristics and genesis of the crusts and nodules. In this study, we analyzed a set of SCS polymetallic crusts and nodules obtained from ten sites to determine their mineralogy, chemistry, and Fe and Os isotopes. The SCS crusts/nodules are composed predominantly of vernadite (δ–MnO2) and amorphous Fe oxides/hydroxides. The nodules also contain minor todorokite. TEM imaging and electron diffractions show that the Fe–Mn minerals are nano–sized minerals with very poor crystallinity. Nitrogen adsorption/desorption analysis indicates an average specific surface area of 257.128 m2/g for the SCS crusts and nodules, and main apertures of 2–10nm for their nanopores in the Fe–Mn minerals. It is noteworthy that the SCS crusts/nodules contain substantial amount of terrigenous detritus, suggesting that marginal sea–type Fe–Mn deposits are strongly influenced by terrigenous sedimentary input. Furthermore, geochemical analysis indicates that the SCS crusts/nodules have relatively lower Co (1482ppm), Cu (556.2ppm), Ni (3132ppm), Mo (276.2ppm), and Zn (598.3ppm) compared to open oceanic crusts/nodules, but relatively high concentrations of REY (1647ppm), PGE (234.0ppb), Ba (1056ppm), Pb (1878ppm), Sr (995.9ppm), Te (28.81ppm), and Tl (67.87ppm). Iron isotopic compositions suggest that the Fe sources of the SCS crusts/nodules may have been Fe dissolved in terrigenous (fluvial) inputs. Osmium isotopes indicate that the main ore–forming elements came directly from the ambient seawater, which was mainly influenced by terrigenous inputs, although there may have been mixing with minor mantle–derived signatures.Evidence from mineralogy, major element geochemistry, shale–normalized rare earth element (REE) patterns, and the Fe and Os isotopes suggest that the SCS crusts/nodules are mainly hydrogenetic, whilst the formation of some nodules may have been contributed by minor pore waters of their surrounding sediments. The SCS crusts/nodules had probably much higher growth rates (mean 19.20mm/Ma) than most hydrogenetic Fe–Mn deposits in other open oceans, which are typical of marginal sea Fe–Mn deposits that contain sufficient supply of ore–forming terrigenous detritus.Grades of some critical metals of economic interest (notably Co, Cu, Ni, Mo, and Zn) are slightly lower in the SCS crusts and nodules than in the crust and nodules from Pacific prime mineralization zones. Concentrations of Mn, REY, Pt, Mo, Co, Te, and Bi, however, show relatively high concentrations relative to the continental crust. The SCS crusts and nodules could thus represent a strategic REE, Co and Pt resource, and will likely catalyse additional investigation of seabed resources in the South China Sea.

The effect of Fe-Mn minerals and seawater interface and enrichment mechanism of ore-forming elements of polymetallic crusts and nodules from the South China Sea

Ferromanganese crusts and nodules are important submarine mineral resources that contain various metal elements with significant economic value. In this study, polymetallic crusts and nodules obtained from the South China Sea (SCS) were determined by using X-ray power diffraction (XRD), Raman spectroscopy (RS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to systematically investigate and analyze the mineralogical and spectral characteristics of the Fe-Mn minerals. XRD measurements revealed that the SCS polymetallic crusts and nodules were composed of vernadite, quartz, and plagioclase. The nodules also contained todorokite. The Fe-phase minerals of the SCS crusts and nodules were composed of amorphous Fe oxide/hydroxide, and the Mn- and Fe-phases minerals exhibited relatively poor degrees of crystallization. FTIR results showed that the Fe-Mn minerals in the crusts and nodules included a large number of surface hydroxyl groups. These surface hydroxyl groups contained protons that could provide reactive sites for complexation of ore-forming elements in seawater. XPS results indicated that the surfaces of the Fe-Mn minerals mainly contained Fe, Mn, and O. Fe was present in the trivalent oxidation state, while Mn, which may contain several bivalent oxidation state, was present in the tetravalent and trivalent oxidation states. The SCS polymetallic crusts and nodules were compared with Pacific seamount crusts, and results showed that the surface hydroxyl (-OH) groups of the SCS crusts and nodules numbered more than the lattice oxygen (O2-). But the lattice oxygen of Pacific seamount crusts numbered more than the surface hydroxyl groups. This characteristic indicated that the degree of crystallization of Fe-Mn minerals from the Pacific Ocean was higher than that of minerals from the South China Sea. Comprehensive studies showed that ore-forming elements in the interface between seawater and the Fe-Mn minerals in the submarine ferromanganese crusts and nodules employed the following enrichment mechanisms: (1) the metal ion complexed with the surface hydroxyl of Fe-Mn minerals to form hydroxyl complexes, which were connected by coordination bonds or stable inner-sphere complexes that exchanged protons on the mineral surfaces; (2) the charged surfaces of the minerals and metal cations formed outer-sphere complexes, which made up the electrostatic double layer, through electrostatic adsorption; and (3) the metal cations isomorphously exchanged the Mn and Fe ions of the mineral lattice structure.

Fe Isotope Characteristics of Polymetallic Crusts and Nodules from South China Sea and its Geological Implications

Fe Isotope Characteristics of Polymetallic Crusts and Nodules from South China Sea and its Geological Implications

Corrosion of calcite crystals by metal-rich mud in caves: Study case in Crovassa Ricchi in Argento Cave (SW Sardinia, Italy)

Unusual orange ochre crusts were recently discovered in Crovassa Ricchi in Argento Cave (San Giovanni Mine, SW Sardinia). These speleothems appear covering the cave walls on hydrothermal calcite spars as well as filling widened spaces between calcite crystals. Planar crusts display geometrical forms following the boundaries between the calcite spars. EDX–SEM microanalyses reveal that these deposits comprise substances of Fe, Mn, Pb, Zn and O that occur as solid inclusions in pits on the surface of altered calcite microcrystals. Micro-Raman spectroscopy analyses suggest the presence of calcite and ferromanganese oxides with a low degree of crystallinity. The genetic mechanism proposed for these speleothems describes an initial stage of precipitation of euhedral calcite crystals from warm water under subaqueous conditions. The crystal surfaces were eroded and corroded by colder aggressive water that smoothed the surfaces of the crystals and slightly widened the spaces between calcite spars. Metal-rich mud coming from alteration of bedrock and ore bodies filled the cave, also penetrating along the spaces between the calcite spars. When the water table fell below the cave level, part of the sediments was eroded but the cave walls remained covered with metal-rich clayey sediments. Under aerobic conditions, metals – which were reduced in previous stages – oxidized to oxides, lowering the pH and thus the crystal surface and the calcite planes between the spars were corroded. Subsequently, the polymetallic crusts became harder through evaporation within the cave, “fossilizing” the products of this process within the planes between spars. Finally, the exposed calcite surfaces continued to be altered due to CO2 diffusion into condensation water, while the boundaries between crystals were preserved against corrosion thanks to the crust coating. As a result, the external crystal edges protrude by several centimeters from the current cave wall, while the crystal surfaces are depressed, giving rise to calcite “ghosts”.

Assessing the distribution and abundance of seabed minerals from seafloor photographic data in the Central Indian Ocean Basin

Distribution characteristics of deep-sea mineral resources such as polymetallic nodules and ferromanganese crusts are often influenced by local seafloor features such as the topographic undulations and sediment thickness. Qualitative as well as semi-quantitative estimation of these resources have been attempted earlier by using seafloor photographic data. This study analyses the distribution of these minerals in relation to their associated seafloor surface features and develops empirical formulae for abundance estimation of a deposit from photographic data in conjunction with sounding and sampling data in the Central Indian Ocean Basin. Data from >20,000 photos were analysed and correlated with geological features such as bathymetry, sediment thickness, and sediment physical properties. The study shows that nodule populations are negatively correlated with the thickness of the acoustically transparent layer (ATL), which in turn is controlled by the topographic undulations. The variable burial of nodules under the ATL, as indicated by sampling data, leads to underestimation of nodules from photographs. In order to improve the accuracy of such estimations, different constant factors (C-factor) are proposed for use in empirical formulae depending upon the distribution characteristics of the deposits.

Contribution of biomineralization during growth of polymetallic nodules and ferromanganese crusts from the Pacific Ocean

The ocean hosts inorganic raw materials to a magnitude, which surpasses the resources of these materials available on land. Those mineral resources include industrial minerals, metalliferous oxides, hydrothermal metalliferous sulfides, and dissolved minerals. Hence, a significant source of minerals for sustainable recovery in the future may be ocean waters. Among of those mineral resources, there are two kinds of very important minerals which are consolidated on the seabeds of ocean basins in polymetallic nodules and on the surface of seamounts in polymetallic crusts. Until now, the (bio)- chemical processes that result in the formation of metal deposits in the form of nodules or crusts are not understood. In the present review, we concentrate on the (potential) biogenic origin of nodule and crust formation.

Extreme Enrichment of Tellurium in Deep-Sea Sediments

Tellurium is a sort of scattered rare element on the earth. Its concentration is very low in earth’s crust, only 1.0 ng/g. However, it has extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments and aerolites. To find out the origin of tellurium enrichment in deep-sea sediments, we analyzed and compared tellurium concentrations and helium isotope compositions in the magnetic parts and those in the bulk parts of deep-sea sediments. The result indicates that the helium content, 3He/4He ratio and tellurium concentration are obviously higher in the magnetic parts than those in the bulk parts. The 3He abundance varies synchronously with the tellurium concentration. 3He and Te have a distinct positive correlation with each other. It is the first time that the paper brings forward that the extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles (IDPs). Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs.

Les depots d'enduits manganoferriferes en environnement marin littoral; l'exemple de grottes sous-marines en Mediterranee nord-occidentale

Abstract Black coating of hard substrates by Mn and Fe oxides has long been reported from shallow, dark, submarine caves. However, these littoral metallic deposits have never been studied in detail, despite expected analogies with deep-sea polymetallic crusts. Submarine caves are characterized by darkness and low rates of exchanges with the open sea. Lack of primary production and confinement of inner water bodies result in marked oligotrophy and extremely reduced biomass, i.e. conditions close to those prevailing in deep-sea habitats. Field evidences suggested that the formation of Mn-Fe coatings was closely tied to these particular environmental conditions. The goal of this study was to examine the detailed features of Mn-Fe coatings from dark caves with different local conditions, and to try to identify the processes responsible for their deposition. Study sites and methods: Three sublittoral, single-entrance, caves were sampled by scuba diving along the coasts of Provence (France, Mediterranean Sea). The first site is a large karstic cave (Tremies Cave, 16 m depth at entrance floor, 60 m long; Marseille-Cassis area) with an ascending profile which results in a buffered thermal regime and markedly oligotrophic conditions due to warm water trapping in its upper part. Wall fragments were sampled at 30 m (medium confinement: zone B) and 60 m (strong confinement: zone C) from the cave entrance. The second site is a large tubular cavity open in conglomerate formations (3PP Cave, 15 m depth at entrance floor, 120 m long; La Ciotat) with a descending profile which results in relative permanence of winter temperatures within the inner parts, complex water circulation and presumed greater input of sedimented particles than in the preceding cave. Wall samples were taken at 25 m, 70 m and 100 m from entrance. The third site is a small, horizontal, cave open in quartzite formations (Bagaud Cave, 7 m depth at entrance floor, about 10 m long; WNW of Port-Cros Island, bay of Hyeres). Sampling was performed on walls of a narrow corridor between an anterior room and a smaller inner room. A sporadic outflow of continental waters is located in the inner room. The samples were preserved in 50% ethylic alcohol or studied soon after their sampling. Before carbon coating and SEM examination, or microanalyses with SEM-associated spectrometers, they were treated in a 33% Chlorox solution and thereafter washed in demineralized water and dried. Micromorphology: At low-medium magnification (<20,000), the aspect of coatings varies between caves and, especially, between inner-cave locations. All the described structures are made up of Mn and Fe oxides. In Tremies Cave, coatings of walls from zone B are composed of irregular erected constructions (height: 10s to 100s mu m) formed by the aggregation of roughly ovoid primary concretions of about 10 mu m. The surface of those primary concretions displays numerous lacunose to reticulate films (pores, about 0.5 mu m in diameter, are often subrounded). Remnants of these films and organomorphic corpuscles occur also within the primary concretions. On younger substrates (broken wall exposed since 1970), primary concretions are poorly developed and no prominent construction is visible. In more confined conditions (zone C), the erected constructions of ancient coatings are smaller and less numerous than in zone B but are well individualized. In this zone C, besides some remnants of lacunose to reticulate films, there is an appearance of filaments and ovoid corpuscles (height/width: 10-30/5-15 mu m), which seem to be linked to filaments by a short stalk. In 3 PP Cave, at 25-70 m from entrance, wall coatings present porous heaps of primary concretions. The surface and the inside of the latter comprise remnants of lacunose to reticulate films that evoke those observed in Tremies Cave. On younger substrates (hard parts of sessile invertebrates), coatings are restricted to micrometric organomorphic corpuscles with some remnants of lacunose or fibrous films. At 100 m from the entrance, coatings are shaped by numerous erected constructions, more or less coalescing. Besides remnants of lacunose films, the primary concretions contain interlacing filaments (diameter: 0.2-0.3 mu m) forming cords or veils. In Bagaud Cave, the primary concretions are aggregated in irregular heaps. Lacunose films are particularly frequent and tend to form three-dimensional mamillated structures that were not observed in the other caves. In particular, there is an appearance of tubular structures and of numerous hemispheroidal structures (diameter: 4-5 mu m) with an upper orifice. At higher magnification (20,000), whatever the cave and inner-cave location, the aspect of oxide deposits is rather smooth or, especially, microgranular. Mineral composition: The composition of coatings is different between caves and according to their inner-cave location. In both large caves (Tremies and 3 PP), the Mn/Fe ratio increases with the distance from the cave entrance, i.e. when exchanges with the open sea diminish. This trend is particularly clear in Tremies Cave, where the confinement gradient is strongly marked.