Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities

International audienceThe Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar + biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz + hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behaviour of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2 < δ18O (vs. SMOW) < 8.5‰ and -4.7 < δ13C (vs. PDB) < -5.4‰ in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (δ18OZr = 4 to 4.7‰, δ18OIlm = -4.3 to -1.5‰ respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high δ18O values (δ18O = 8.6 to 21.8‰), show evidence of a medium- to low-temperature hydrothermal alteration event by an aqueous fluid. Calcitic veins in the carbonatite record another alteration event, outlined by the co-variation of δ18O and δ13C values (δ18O = 16.3 to 24.7‰ and δ13C = -4.7 to 0.2‰), implying the involvement of a mixed H2O-CO2 fluid. As a whole, the circulation of fluids in the URAPC was initiated during magmatic emplacement and the geometry of this circulation was controlled by the syn-emplacement crustal scale shear zones. Element mobility, one expression of which being the mineralization features described here, follow the same scheme

Trace-element imaging at macroscopic scale in a Belgian sphalerite-galena ore using Laser-Induced Breakdown Spectroscopy (LIBS)

Laser-Induced Breakdown Spectroscopy (LIBS) is a fast in-situ analytical technique based on spectroscopic analysis of atomic emission in laser-induced plasmas. Geochemical mapping at macroscopic scale using LIBS was applied to a decimetric Zn-Pb ore sample from east Belgium, which consists of alternating sphalerite and galena bands. A range of elements was detected with no or minimal spectral correction, including elements of interest for beneficiation such as Ge, Ag and Ga (although the detection of gallium could not be confirmed), and remediation, especially As and Tl. The comparison between LIBS and Energy Dispersive Spectroscopy (EDS) analyses showed that LIBS intensities reliably relate to elemental concentration although differences in spot size and detection limits exist between both techniques. The elemental images of minor and trace elements (Fe, Cu, Ag, Cd, Sb, As, Tl, Ge, Ni and Ba) obtained with LIBS revealed with great detail the compositional heterogeneity of the ore, including growth zones that were not visible on the specimen. In addition, each mineral generation has a distinct trace-element composition, reflecting a geochemical sequence whose potential metallogenic significance at the district scale should be addressed in further work. Although qualitative and preliminary, the obtained LIBS dataset already produced a wealth of information that allowed to initiate discussion on some genetical and crystallochemical aspects. Above all, LIBS appears as a powerful tool for screening geochemically large samples for the selection of zones of particular interest for further analysis.LIBS Scree

(U-Th)/He dating of supergene iron (oxyhydr-)oxides of the Nefza-Sejnane district (Tunisia):new insights into mineralization and mammalian biostratigraphy

The mining district of Nefza-Sejnane (Tunisia) encloses numerous ores and raw material deposits, all formed in relation with successive Fe-rich fluids of meteoric and/or hydrothermal origins. Here, for the first time in Tunisia, (U-Th)/He ages were obtained on supergene goethite from various localities/deposits of the district highlight direct dating of significant weathering episodes during late Tortonian and late Pleistocene. These weathering events are most likely associated with favorable conditions that combine (i) wet climate displaying sufficient meteoric water/fluid; and (ii) regional exhumation, due to large-scale vertical lithospheric movements enhancing the percolation of fluids. Matched with previous works, these results refine the stratigraphic frame for the polymetallic mineralization and clay deposits in the district, confirming the influence of meteoric fluids circulation during the late Cenozoic. As a consequence of the new (U-Th)/He data, we moreover propose a taxonomic and stratigraphic revision of the well-known mammalian fauna from the Fe-rich Douahria locality, suggesting an early Tortonian age for the fossils, i.e., prior to the first episode of meteoric event in the area

Artificial weathering of an ordinary chondrite: Recommendations for the curation of Antarctic meteorites

Meteorites are prone to errestrial weathering not only after their fall on the Earth’s surface but also during storage in museum collections. To study the susceptibility of this material to weathering, weathering experiments were carried out on polished sections of the H5 chondrite Asuka 10177. The experiments consisted of four 100-days cycles during which temperature and humidity varied on a twelve hours basis. The first alteration cycle consisted of changing the temperature from 15 to 25 °C; the second cycle consisted of modifying both humidity and temperature from 35 to 45% and 15 to 25 °C, respectively; the third cycle consisted of varying the humidity level from 40 to 60%; and the fourth cycle maintained a fixed high humidity of 80%. Weathering products resulting from the experiments were identified and characterized using scanning electron microscopy–energy dispersive spectroscopy and Raman spectroscopy. Such products were not observed at the microscopic scale after the first cycle of alteration. Conversely, products typical of the corrosion of meteoritic FeNi metal were observed during scanning electron microscope surveys after all subsequent cycles. Important increases in the distribution of weathering products on the samples were observed after cycles 2 and 4 but not after cycle 3, suggesting that the combination of temperature and humidity fluctuations or high humidity (>60%) alone is most detrimental to chondritic samples. Chemistry of the weathering products revealed a high degree of FeNi metal corrosion with a limited contribution of troilite corrosion. No clear evidence of mafic silicate alteration was observed after all cycles, suggesting that postretrieval alteration remains limited to FeNi metal and to a lesser extent to troilite

Caractérisation géochimique et isotopique dans un système d'altération complexe, du protolithe magmatique à la minéralisation Fe-Pb-Zn: le cas de la mine de Tamra (N. Tunisie)

L’objectif principal de cette thèse est de décrire et interpréter la mise en place des différents types de minéralisations présents dans le district minier de Nefza (NO de la Tunisie), qui constitue une région complexe et particulière du point de vue géologique dans ce pays.En effet, outre la présence de diapirs triasiques (commune à tout le Nord de la Tunisie), le district minier de Nefza est caractérisé par :(1) la mise en place d’épaisses nappes au Néogène qui forment le substratum sédimentaire régional, et (2) la présence d’une grande variété de roches magmatiques tant plutoniques que volcaniques, d’âge miocène (12,9 à 6,4 Ma),-\Doctorat en Sciencesinfo:eu-repo/semantics/nonPublishe

Mineralogical and geochemical study of the rare earth element mineralization at Gakara (Burundi)

The rare earth element (REE) mineralization of Gakara (Burundi) has first been discovered in 1936 and has periodically been the subject of geological studies, at times when the exploitation of bastnäsite-(Ce) and monazite-(Ce) was economically interesting. This study focuses on the establishment of a mineral paragenesis for Gakara, with special attention to the REE-bearing phases, to understand the formation history of the deposit. The paragenesis can be subdivided into 3 stages: primary ore deposition, brecciation stage and supergene alteration. Evidence for fenitization processes (i.e. pinkish-red cathodoluminescence of K-feldpar, brecciation stage) and the strong enrichment of light REEs in bastnäsite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity. This likely confirms the association of the Gakara REE deposit with the Neoproterozoic alignment of alkaline complexes and carbonatites along the present-day Western Rift. It suggests a direct link with a – currently unidentified – carbonatitic body at depth, possibly derived from a predominantly metasomatized lithospheric mantle

Poly-phase alteration history of the kaolinitized `Cava di Caolino' volcanics (Lipari Island, southern Italy)

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An Antarctic chondrite story: from the field to the lab

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Petrogenesis of the Mairupt microgranite: A witness of an Uppermost Silurian magmatism in the Rocroi Inlier, Ardenne Allochton

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