Magmato-hydrothermal transport and deposit of W(Sn, Nb (Ta) in magmatic-metamorphic fluids around plutons
Transport and deposition of W (Sn, Nb (Ta) ores from in magmatic-metamorphic fluids around plutons
The objective of the study of the magmatic-hydrothermal transition is of primary importance to understand the current mineralogical expression of the rare metal ore deposits and to discriminate the strictly magmatic processes (issued from partial melting, fractionated crystallization, assimilation, immiscibility) from the hydrothermal processes in which host formations (granites) could also have played a major role as source of metals. An important point is to determine if metals such as W-Nb-Ta-Li-Sn result from a same source or if several sources including non-magmatic ones are involved. All the significant Sn-W-Nb-Ta deposits have already been studied, mainly during the 80s-90s. Since this time, methodological progresses have been made, allowing (i) better constraining of the P-T-t-D path through improvement of the fluid inclusion characterization and petrology and coupling with deformation events, (ii) micron-scale analysis of the ore minerals), and (iii) in-situ measurement of the fluid composition from the fluid inclusions. Consequently, renewed studies have to be conducted on selected deposits, with a particular focus on the determination of the paleo-ore fluid chemistry (including metal contents), and the detailed characterization of metals in ores in order to identify the behaviour of Nb and other elements as traces associated in wolframite, cassiterite, and Ti-minerals.This work will be based both on field works, in particular in France and Portugal (in coll. Porto and Lisboa Univ, and Panasqueira mine), and on laboratory experiments.
Field and analytical Work:
The characterization of hydrothermal alteration haloes and mineral phases in open veins will be investigated from the scale of the deposit to the micrometer scale using EPMS techniques wherever appropriate. With this instrument we hope to gather reliable data on the chemistry of each mineral phase, on its chemical zonation and also on its contents in rare metals whenever their concentration is above the EPMA detection limits. This data, coupled with similar data collected on the unaltered host rocks and on the unaltered granite, will enable us to constrain the chemical reactions which took place during the geological processes leading to metal accumulation. The analysis of the location of metals in major minerals using LAICP-MS and in some instances synchrotron radiation will be conducted in order to obtain a detailed mapping of metals distribution with the ores. Due to the complex mineralogy and paragenesis of the deposits, analytical techniques with application at micrometre scale and in selected spatial zones are required to carefully study such complex and polyphase mineral assemblages and thus to determine the concentrations of trace elements (< 1000 ppm: metals, rare earth elements, Li, Nb, Ta, Sn, W, B, Br …) in fluid inclusions and minerals for each stage of mineral deposition and associated alteration.
A part of the thesis will be finally devoted to a better understanding of the solubility and incorporation of Nb-(Ta) in main mineral phases (wolframite in particular). Specifically, a set of experimental and theoretical studies will be conducted to determine the composition and stability constants of the Nb-and Ta-containing species in hydrothermal fluids (CO2 and F-rich fluids) at conditions close to natural (300 – 600°C, 0.5 - 2 kbar). It should provide quantitative geochemical modeling of Nb- and Ta-bearing fluids transport properties in a wide range of temperature, pressure, and composition of hydrothermal fluids. The experiments at temperatures 300 to 500°C and pressures up to 1 kbar will be performed using high TP hydrothermal flexible cell reactors. Core test equipped with a special sampling cell allows multiple and ultrarapid extraction of a small part of the vapor and liquid phases without loss of volatile component (CO2 – H2S- H2). Some dedicated experiments will be performed in glass capillary coupled with Raman spectroscopy to provide a structural description of the solvent properties (H2OCO2) under hydrothermal conditions.
Michel Cathelineau- (leader of the projet ERAMIN Newores): email@example.com,
Laurent Truche (Georessources): firstname.lastname@example.org
Isabelle Abildtrup (Labex RESSOURCES21): email@example.com