ALBERT

Description: The combination of structural, geochemical and paleotopographic data proves to be an efficient tool to understand fluid transfers in the crust. This study discriminates shallow and deep fluid reservoirs on both sides of the brittle-ductile transition under an extensional regime, and points out the role of major transcurrent fault activity in this paleo-hydrogeological setting. Paleo-fluids trapped in quartz and siderite-barite veins record the transfer of fluids and metal solute species during the Neogene exhumation of the Sierra Almagrera metamorphic belt. Ductile then brittle-ductile extensional quartz veins formed from a deep fluid reservoir, trapping metamorphic secondary brines containing low-density volatile phases derived from the dissolution of Triassic evaporites. During exhumation, low-salinity fluids percolated within the brittle domain, as shown by transgranular fluid inclusion planes affecting previous veins. These observations indicate the opening of the system during Serravalian to early Tortonian times, and provide evidence for the penetration of surficial fluids of meteoric or basinal origin into the upper part of the brittle-ductile transition. During exhumation, synsedimentary transcurrent tectonic processes occurred from Late Tortonian times onwards, while marine conditions prevailed at the Earth's surface. At depth in the brittle domain, quartz veins associated with hematite record a return to high-salinity fluid circulation suggesting an upward transfer fed from a lower reservoir. During the Messinian, ongoing activity of the Trans-Alboran tectono-volcanic trend led to the formation of ore deposits. Reducing fluids caused the formation of siderite and pyrite ores. The subsequent formation of galena and barite may be related to an increase of temperature. The high salinity and Cl/Br ratio of the fluids suggest another source of secondary brine derived from dissolved Messinian evaporites, as corroborated by the δ 34 S signature of barite. These evaporites preceded the main sea-level drop related to the peak of the salinity crisis (5.60-5.46 Ma). This article is protected by copyright. All rights reserved.

Description: Advances in the quantification of rare earth elements (REE) at micrometric scale in uranium oxides by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are described. The determination of the best analytical conditions is tested using a uranium oxide (Mistamisk) for which the concentrations of REE were previously estimated by other techniques. Comparison between the use of U or Pb as internal standard clearly shows a diameter-dependent fractionation effect related to Pb at small crater diameters (16 and 24 μm), which is not found for U. The quantification of REE contents in a uranium oxide samples using both matrix-matched (uranium oxide) and non-matrix-matched (NIST SRM 610 SRM certified silicate glass) external calibrators display no significant difference, demonstrating limited matrix effect for REE analysis by LA-ICP-MS. Moreover, no major interferences for REE have been detected. The proposed methodology (NIST SRM 610 as external calibrator and U as internal standard) is applied to samples from worldwide uranium deposits. The results show that LA-ICP-MS is a suitable analytical technique to determine REE concentrations, down to μg g −1 level, in uranium oxides at the micrometre scale and that this technique can provide significant insights into uranium metallogeny. © 2012 The Authors. Geostandards and Geoanalytical Research © 2012 International Association of Geoanalysts

Description: Uranium deposits form in a wide range of geological settings, including deep magmatic to surficial conditions, and range in age from Archaean to recent. These temporal and spatial variations have given rise to an extreme diversity of ore deposits. However, understanding their conditions of formation has remained challenging. This article reports rare earth element (REE) abundances, measured by microbeam methods in uranium oxides, for a series of worldwide uranium occurrences. The REE patterns are very specific to each deposit type and directly reflect the conditions of their genesis. We propose an evaluation of the first-order parameters controlling the REE behaviour in each mineralised system. This study demonstrates that the REE pattern is the most efficient tool for constraining the geological models of uranium deposits and for genetically discriminating new uranium discoveries. This approach may form the starting point for a new procedure in the fight against nuclear trafficking. Terra Nova, 00, 1–6, 2011

Description: Giant uranium deposits formed from exceptionally uranium-rich acidic brines Nature Geoscience 5, 142 (2012). doi:10.1038/ngeo1338 Authors: Antonin Richard, Christophe Rozsypal, Julien Mercadier, David A. Banks, Michel Cuney, Marie-Christine Boiron & Michel Cathelineau Giant uranium deposits were formed during the Mesoproterozoic era, 1.6–1.0 Gyr ago, in both Canada and Australia. The deposits are thought to have formed from large-scale circulation of brines at temperatures of 120–200 °C that percolated between sedimentary basins and underlying crystalline basement rocks. However, the precise conditions for transport of the uranium in these brines are poorly understood. Here we use mass spectrometry to analyse the uranium content of brines preserved in naturally occurring fluid inclusions in ore deposits from the Athabasca Basin, Canada. We measure concentrations of uranium in the range 1.0×10−6–2.8×10−3 mol l−1. These concentrations are three orders of magnitude above any other common crustal fluids. Experimentally, we measure the solubility of uranium as a function of NaCl content and pH, in mixtures that are analogous to ore-forming brines at 155 °C. To account for the high uranium content observed in the Athabasca deposits, we find that the brines must have been acidic, with a pH between 2.5 and 4.5. Our results strongly suggest that the world’s richest uranium deposits formed from highly concentrated uranium-bearing acidic brines. We conclude that these conditions are a necessary requirement for the formation of giant uranium deposits in relatively short periods of time of about 0.1–1 Myr, similar to other world-class deposits of lead–zinc and gold.

Description: The formation of most fluorite deposits in northern Coahuila (NE Mexico) is explained by MVT models, and is a part of the metallogenic province of northeastern Mexico. However, fluorite skarn deposits also occur in the same region, and there is evidence for late hydrothermal manifestations with no clear origin and evolution. The latter are the main focus of this study; in particular, F–Be–U–Mo–V–P stringers in the Aguachile-Cuatro Palmas area that overprint preexisting fluorite mantos. The region experienced the emplacement of several intrusives during the Eocene and the Oligocene that are collectively grouped into the East Mexico Alkaline Province (EMAP) and postdate MVT-like deposits. Some of these intrusives have associated skarn deposits; most of them are polymetallic, but the unusual El Pilote deposit contains fluorite mineralisation that was remobilised from MVT-like deposits. The formation of the Aguachile deposit (and, collectively, part of the Cuatro Palmas deposit) has been attributed to a shallow retrograde skarn model. The Cuatro Palmas and Las Alicias fluorite deposits consist of MVT-like deposits overprinted by late hydrothermal fluorite mineralisation rich in Be–U–Mo–V–P, and the Aguachile deposit consists entirely of the latter type. The systematic fluid inclusion study of MVT-like, skarn, and late hydrothermal fluorite deposits reveals a very different distribution of temperature and salinity data that allows the discrimination of mineralising fluids for the type of deposit. MVT-like deposits were formed by fluids with temperatures of homogenisation that range between 50 °C and 152 °C and salinities between 5 and 15.5 wt.% NaCl equivalent. The El Pilote fluorite skarn was formed by fluids with temperatures of homogenisation that range between 78 °C and 394 °C and salinities between 5 and 34 wt.% NaCl equivalent, and include CaCl2-rich brines with salinities that range between 24.5 and 29.1 wt.% CaCl2. Late shallow fluorite–Be–U–Mo–V–P hydrothermal deposits were formed by fluids with temperatures of homogenisation that range between 70 °C and 180 °C and salinities between 0.9 and 3.4 wt.% NaCl equivalent; the sole exception to the above is the La Fácil deposit, with salinities that range between 7.9 and 8.8 wt.% NaCl equivalent. While temperatures of homogenisation are similar between MVT-like and late hydrothermal deposits, and both even have hydrocarbon-rich fluid inclusion associations, the salinity of late deposits is similar to that of retrograde skarn fluids, although further diluted. However, homogenisation temperatures tend to be higher in late hydrothermal than in MVT-like deposits, thus making them more similar to retrograde skarn fluids. Although this characteristic cannot solely establish a genetic link between a retrograde skarn model and late hydrothermal deposits in the study area, the characteristics of fluids associated with the latter separate these deposits from those ascribed to an MVT-like model. Assuming that mineralising fluids for late fluorite–Be–U–Mo–V–P hydrothermal deposits may correspond to a retrograde skarn (or “epithermal”) deposit, the source for fluorine may be either from (A) the dissolution of earlier formed MVT-like deposits, (B) the entrainment of remaining F-rich basinal brines, or (C) hydrothermal fluids exsolved from highly evolved magmas. Possibilities A and B are feasible due to a hypothetical situation similar to the El Pilote skarn, and due to the occurrence of hydrocarbon-rich fluid inclusions at the La Fácil deposit. Possibility C is feasible because intrusive bodies related to highly evolved magmas would have provided other highly lithophile elements like Be, U and Mo upon the exsolution of their hydrothermal fluids. Such intrusive bodies occur in both study areas, and are particularly conspicuous at the Aguachile collapse structure.

Description: Germanium is considered a critical element, with a demand that has sharply increased due to booming high-technology industries. To understand Ge incorporation mechanisms in natural systems, we investigate Ge speciation in Cu-bearing sulfide minerals using synchrotron X-ray fluorescence (XRF) chemical mapping and Ge K-edge µ-X-ray absorption near-edge structures (µ-XANES) spectroscopy. The samples investigated include (i) a homogeneous chalcopyrite from the Kipushi polymetallic deposit (Central African copperbelt, D.R. Congo) and (ii) a zoned Ge-rich chalcopyrite from the Barrigão Cu deposit (Iberian pyrite belt, Portugal). First, our spectroscopic analysis supports the occurrence of tetrahedrally-coordinated Ge4+ in chalcopyrite, independently from origins or zoning patterns observed for these minerals. Then, based on statistical analyses of XRF chemical maps, we demonstrate that tetravalent germanium most likely incorporates chalcopyrite through the Fe crystallographic site via coupled substitutions with the following form: (2x + 3y)Fe3+ ⟷ (x + 2y)(Ge,Sn)4+ + x(Zn,Pb)2+ + y(Cu,Ag)+, although the presence of lattice vacancies cannot be completely excluded.