A cloritização na Mina Uruguai, Minas do Camaquã, RS, Brasil

The area of Camaquã Mines is part of the camaquã basin, which has NE-NW directions and is filled out by silliciclastics sediments interbedded with volcanic rocks. Chlorite is the most abundant clay mineral in the area and appears in high amount in the halos of hydrothermal alteration present in the host's mineralization rocks. This work consists on the chlorite's petrologic, chemical and structural characterization, supplying important information on different processes and formation conditions of the hydrothermal environment. The following procedures were made: optical petrography, X-ray diffraction, difractograms modeling through the Reynolds Newmod© program, and scanning electron microscopy ( secondary electrons and microanalysis - EDS) in representative samples of the different identified hydrothermal alteration zones. Chlorites are present under three different tendencies: Chlorite I takes place with a pervasive aspect over the matrix rocks located near the mineralized veins; it is classified as Fe-chlinoclore, presents fraction ˂1 µm predominance of plytype IIb and a Mg2+  enrichment in the fraction of  ˂10 µm predominance of polytype Ib (90°) and Fetotal enrichment. Chlorite II occurs as veins filling out small fractures; it is classified as Chamosite and presents structural polytype IIb. Chlorite III occurs altering detrital minerals; it is classified as Mg-chamosite. The variation on the amount of Fe of the chlorites generated over the hydrothermal processes ( Chlorite I ˂10 µm and Chlorite II) indicates the occurrence of at least two pulses in the process of the hydrothermal alteration: one is responsible for the great matrix and clasts alternation and the other one for the generation of the late veins. The variation in the amount of Fetotal of the two different responsible fluids for the crystallization of the chlorites is was quite more enriched in iron than the early fluid that crystallized Chlorite I ˂10 µm.A região das Minas do Camaquã é a parte constituinte da Bacia do Camaquã, a qual possui direção NE-NW e é preenchida por sedimentos siliciclásticos intercalados com rochas vulcânicas. A clorita é o argilomineral mais abundante da área, ocorrendo em grande quantidade nos halos de alteração hidrotermal presentes nas rochas encaixantes nas mineralizações na Mina Uruguai. Este trabalho consiste na caracterização petrológica, química e estrutural das cloritas, que fornece importante informações sobre diferentes processos e condições de formação do ambiente hidrotermal. Para isso se realizou petrografia óptica, difratometria de raios X, modelamento dos difratogramas através do programa Reynolds Newmod © e  microscopia eletrônica de varredura (elétrons secundários e micro-análises por EDS) em amostras representativas de diferentes zonas de alteração hidrotermal identificadas. As cloritas se apresentam com três diferentes tendências: a Clorita I ocorre com aspecto pervasivo sobre matriz das rochas localizadas próximos aos filões mineralizados; é classificada como Fe-clinocloro, apresenta na fração ˂1 µm predominância do politipo IIb e um enriquecimento em Mg2+  e na fração ˂10 µm predominância do politipo Ib (90°) e enriquecimento em Fetotal .  A Clorita II ocorre com veios preenchendo pequenas fraturas; é classificada como Chamosita e apresenta politipo estrutural IIb. A Clorita III ocorre alterando minerais detríticos sendo classificada como Mg-Chamosita. A variação na quantidade de ferro das cloritas geradas por processos hidrotermais ( Clorita I ˂10 µm e Clorita II) fornece indícios da ocorrência de pelo menos dois pulsos no processo de alteração hidrotermal: um responsável pela intensa alteração da matriz e dos clastos das rochas e outro responsável pela geração de veios tardios. A variação na quantidade Fetotal  dos dois diferentes fluidos responsáveis pela cristalização das cloritas fica evidenciada pela associação de co-genecidade da Clorita II com a hematita, mostrando que o fluido final foi muito mais enriquecido em ferro que o fluido precoce cristalizou a Clorita I ˂10 µm

Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites.

12 pagesInternational audienceMicrobialites are widespread in modern and fossil hypersaline environments, where they provide a unique sedimentary archive. Authigenic mineral precipitation in modern microbialites results from a complex interplay between microbial metabolisms, organic matrices and environmental parameters. Here, we combined mineralogical and microscopic analyses with measurements of metabolic activity in order to characterise the mineralisation of microbial mats forming microbialites in the Great Salt Lake (Utah, USA). Our results show that the mineralisation process takes place in three steps progressing along geochemical gradients produced through microbial activity. First, a poorly crystallized Mg-Si phase precipitates on alveolar extracellular organic matrix due to a rise of the pH in the zone of active oxygenic photosynthesis. Second, aragonite patches nucleate in close proximity to sulfate reduction hotspots, as a result of the degradation of cyanobacteria and extracellular organic matrix mediated by, among others, sulfate reducing bacteria. A final step consists of partial replacement of aragonite by dolomite, possibly in neutral to slightly acidic porewater. This might occur due to dissolution-precipitation reactions when the most recalcitrant part of the organic matrix is degraded. The mineralisation pathways proposed here provide pivotal insight for the interpretation of microbial processes in past hypersaline environments

Recent mobilization of U-series radionuclides in the Bernardan U deposit (French Massif Central)

Several nuclides of the ;238;U (;234;U, ;230;Th, ;226;Ra, ;210;Pb) and ;235;U (;231;Pa, ;227;Ac) decay series have been measured through gamma spectrometry in the U-bearing episyenites of the Bernardan mine (northwest Massif Central, France), in order to study recent mobilizations in the U-deposit and their time-scales. (;230;Th/;238;U) and (;231;Pa/;235;U) ratios show that U has been little affected during the last 350 ky, especially in the U-rich samples. In contrast, ;226;Ra was subjected to a general redistribution, with a clear tendency for Ra to be lost from the U-rich samples and added to U-poor samples. The highest Ra excesses are present in low-U samples, and are related to adsorption onto iron hydroxides. Ra appears to have been redistributed within the mineralised episyenitic bodies, which seem to remain in ;226;Ra;238;U radioactive equilibrium, a result consistent with the earlier study of Leroy [1984. Episyénitisation dans le gisement d'uranium du Bernardan (Marche): Comparaison avec des gisements similaires du Nord-Ouest du Massif Central français. Miner. Depos. 19, 2635]. As shown by the ;226;Ra and ;210;Pb data, the Ra redistribution was very active between 8 and 0.1 ky, and this might be a consequence of the hydrological changes in groundwater circulation accompanying the Holocene climate warming

Clay minerals in basalt-hawaiite rocks from Mururoa Atoll (French Polynesia). II. petrography and geochemistry

The clay minerals formed in chilled margins and massive crystallized inner parts of three basalt-hawaiite bodies of Mururoa Atoll (French Polynesia) exhibit contrasting textures. Glass alteration textures are observed around fractures crosscutting the quenched margins: Fe-rich clays grow inward into the glass (retreating surface) while Mg-rich clays grow outward (open space). The textures of clay deposits filling the diktytaxitic voids (mesostasis) in the massive inner parts of the three volcanic bodies are different: unoriented clay matrix with embedded euhedral apatite and pyroxene microcrysts (submarine flow); pallisadic clays coating the void walls and the crystal surfaces of apatite and K-feldspar microcrysts (subaerial flow); and clay muffs covering all the apatite needles, with the central part of the void remaining empty (dike). The unoriented texture could result from the alteration of a glass precursor concomitant with the olivine phenocrysts (clay pseudomorphs). However, such an alteration implies important chemical transfers which are not observed. The pallisadic and muff textures form through heterogeneous nucleation on the solid surfaces and crystal growth from a saline solution. No glass precursor existed. As the center of the diktytaxitic voids in the dike is empty, the residual liquid was probably boiling. The amounts of light rare earth elements (LREE), Sr, and the most incompatible elements are greater in clays from diktytaxitic voids relative to the amounts formed in the altered glass of the chilled margins. Thus, diktytaxitic clays formed from a residual liquid which gave either an evolved glass or a saline solution after cooling (fractionation process). The δO variationvs. loss on ignition (LOI) indicates that sea waterwas involved either in rock alteration or magma contamination. This is confirmed by the Rb/Sr ratio of bulk rocks and clay fractions from the quenched and massive inner parts of the three volcanic bodies which do not fit with the 11.5 Ma isochron indicating that the Rb-Sr system was not closed at any stage during the magmatic history

Timing of geothermal activity in an active island-arc volcanic setting: preliminary 40Ar/39Ar data from Bouillante geothermal field (Guadeloupe, French West Indies),

International audienceMineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias, newly discovered in the active Bouillante geothermal field (Guadeloupe archipelago), and investigated by 40Ar/39Ar geochronology in order to constrain the timing of geothermal activity in this part of the active Lesser Antilles island arc. The inverse isochron diagram indicates an age of 248±50 ka (2σ) for all adularia from one breccia sample (n=8), with an initial 40Ar/36Ar ratio of atmospheric composition (309±12 (2σ)) attesting that this age is valid. This age is concordant with the weighted mean age of 290±40 ka for the same sample. Adularia from other samples yields concordant ages. The obtained 40Ar/39Ar ages can be related either to the magmatic activity of the Bouillante Volcanic Chain (c. 850–250 ka ago) or to the initiation of the volcanic activity of the active Grande Découverte–Soufrière system (200 ka ago–present day). Our results demonstrate that the Bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area. The possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed

Clay minerals in basalt-hawaiite rocks from mururoa atoll (French polynesia). I. mineralogy

Clay minerals in chilled or brecciated margins (altered glass) and massive inner crystalline parts (mesostasis) of three basalt-hawaiite bodies from Mururoa Atoll (French Polynesia) have been studied in order to compare their chemical and mineralogical compositions. Polyphase assemblages comprise di- and trioctahedral phases, both of which consist of non-expandable layers (chlorite, celadonite) and two types of expandable layers (saponite and Fe-rich smectite or 'nontronite-like' material). The presence of the Fe-rich clays is supported by the presence of the X-ray diffraction 060 peak at 1.51-1.52 Å and of the infrared absorption bands at 875 and 822 cm (Fe-Al-OH and Fe-Fe-OH groups, respectively). The chemical composition of the Fe-rich smectites does not fit with the theoretical nontronite field. The layer charge averages 1 per SiO making these Fe-rich smectites close to 'celadonite-type' clays. This could explain the presence of mixed-layer celadonite-smectite. Plotted in an M/4Si vs. Fe/sum octahedral cations diagram, the chemical compositions of clay minerals in the mesostasis form a continuous field limited by the celadonite high-charge nontronite-like smectite and chlorite end-members. The clay assemblages are different from those formed in hydrothermal systems or low-grade metamorphic conditions which are characterized by the sequence: saponite → randomly ordered chlorite-smectite mixed-layered minerals (MLMs)→ corrensite → chlorite. The systematic presence of Ferich clays either in the altered chilled margins or in the massive inner parts of the basalt-hawaiite bodies (high-charge nontronite-like smectite and mixed-layer nontronite-celadonite) makes the Mururoa seamount a potential terrestrial analogue for Mars surface exploration

Quantitative evolution of the petrophysical properties of andesites affected by argillic alteration in the hydrothermal system of Petite Anse-Diamant, Martinique

International audienceThe evolution of the petrophysical properties of rocks induced by hydrothermal alteration is often considered qualitatively by sorting the hydrothermal alterations encountered from low to high intensity, depending on the identified mineral paragenesis. In this paper, we studied the evolution of three different petrophysical properties (connected porosity, permeability and electrical conductivity) as a function of the argillization degree of andesites affected by argillic alteration identified in the caprock formation of the Petite Anse-Diamant hydrothermal system in Martinique. These petrophysical measurements were supplemented by quantitative mineral evaluation via scanning electron microscopy (i.e., the Quantitative Evaluation of Minerals by Scanning electron microscopy or the QEMSCAN® method), connected porosity mapping using 14C-PMMA method and measurements of methylene blue (MB) values and structural water proportions (H2O+). In the series of rocks investigated, an increasing trend of clay mineral abundances (montmorillonite and kaolinite) from 5.68 to 37.56% was revealed by the QEMSCAN® method. Based on the quantitative results, the structural water proportions were used as a proxy of argillic alteration progression. Comparison between the mineral maps provided by the QEMSCAN® system and connected porosity mapping observed in autoradiographs using 14C-PMMA method revealed a good correlation with clay mineral phases, dissolution vugs and fractures. The connected porosities evaluated with the triple weight method range from 3.74 to 35.35% and show a good correlation (R2 = 0.8835) with the H2O+ values, revealing that porosity development is mainly due to the replacement of primary phases by clay minerals. In contrast, the crystallization of silica, carbonates and, to a lesser extent, iron oxides tends to clog connected pores, inducing a local decrease in connected porosity, as revealed in autoradiographs. The Darcian permeability ranges from 7.66∙10−20 to 5.36∙10−17 m2 and shows a moderate correlation with the connected porosity. The bulk electrical conductivity measured as a function of the conductivity of the saturating solution reveals the significant contribution of surface conduction arising from montmorillonite. At a pore fluid salinity of 2 wt% (the salinity of the Eaux Ferrées thermal spring), the bulk electrical conductivity shows a relatively good correlation with the H2O+ values (R2 = 0.8591) and an even better correlation with the MB values (R2 = 0.9579). The contribution of montmorillonite to the bulk electrical conductivity was estimated with the use of the isoconductivity point, showing an increase in the BM values and a strong correlation (R2 = 0.9806)

Relating unconformity-type uranium mineralization of the Alligator Rivers Uranium Field (Northern Territory, Australia) to the regional Proterozoic tectono-thermal activity: An illite K-Ar dating approach

International audienceIllite particles of <0.4, 1-2 and <2 mu m size were separated from Kombolgie sandstones and basement rocks of the Alligator Rivers Uranium Field (Northern Territory, Australia) at the Jabiluka and Nabarlek deposits and at nearby exploration sites, on the basis of a previous X-ray diffraction and scanning electron microscope study. Dated by the conventional K-Ar method to constrain the regional tectono-thermal evolution, illite crystallized during successive Proterozoic tectono-thermal episodes at 1774 +/- 40 Ma for one separate of a basement rock, and at similar to 1656, 1600, 1554, 1457, 1425, 1371, 1355 and 1318 Ma (mean-age peaks). Most of the studied host-rocks underwent hydrothermal processes, mainly from similar to 1650 to 1300 Ma, that monitored large-scale basin-derived brine flows initiated by contemporary tectonic pulses. Comparison with ages obtained before by U-Pb, Pb-207/Pb-206, Sm-Nd and Rb-Sr on UO2, and Rb-Sr and Ar-40/Ar-39 on illite and other silicates of deposits comparable to those studied here, suggests that illitization was not always linked to U precipitation. However, significant amounts of illite crystallized between similar to 1680 and 1520 Ma (K-Ar individual ages) at the time U was deposited. This indicates that the unconformity-type U deposits formed probably during three major regional events between 1680 and 1520 Ma (at 1656, 1600 and 1554 Ma). Only detected away from U concentrations between similar to 1457 and 1318 Ma, the younger illitization episodes resulted probably from U-devoid hydrothermal fluids, although probably of basinal origin, during minor tectono-thermal events. Further U-Pb ages from 1300 to 500 Ma obtained previously on UO2 are also considered as isotopic resets. Not linked to illitization, they were probably not connected to tectono-thermal activity and large-scale brine circulations. These late events appear to have played only a minor role in the U endowment of the region. This study demonstrates also that only specific tectono-thermal events of the 1680-1520 Ma period in the ARUF region induced regional mineralizing brine circulations and subsequent U deposition, while younger illitization episodes until similar to 1300 Ma caused restricted fluid circulations and illitization episodes without U supply, as records of the further history of this U deposition. At a broader scale, this study confirms also the critical importance of the geodynamic and subsequent tectono-thermal conditions during this 140-Ma long period during which a variety of world-class metal deposits (U, but also Cu, Pb and Zn) formed in the Paleo-Proterozoic sedimentary basins of the North-Australian Craton