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Tracing metallic pre-concentrations in the Limousin ophiolite-derived rocks and Variscan granites (French Massif Central)

El Korh, A. ; Boiron, M.C. ; Cathelineau, M. ; Deloule, E. ; Luais, B., Lithos

Tracing metallic pre-concentrations in the Limousin ophiolite-derived rocks and Variscan granites (French Massif Central)

El Korh, A. ; Boiron, M.C. ; Cathelineau, M. ; Deloule, E. ; Luais, B.

Lithos, 2020, 356-357

Abstract :

The French Massif Central (FMC) is part of the West European Variscan belt. The economic and sub-economic ore deposits fromthe Limousin area are the consequence of intensive fluid-rock interactions related to a lithospheric delamination process that occurred during the Neo-Variscan Period (320–290Ma). This study focuses on the fingerprints of fluid-rock interactions in the Limousin ophiolite-derived rocks and granitic massifs, in order to trace the source of trace elements transported by fluids involved in the formation of the Variscan ore deposits (including Sn, W, Au and U deposits). A series of compatible and incompatible metallic trace elements (Ag, As, Au, Bi, Li, Co, Cr, Cu, Ga, Ge, Ni, Pb, Sb, Sc, Sn, Ti, V,W) were analysed in a series of basic (metagabbros, basic dykes), ultrabasic (serpentinites) and felsic (granites and gneisses) rocks, as well as in the mineral assemblages, in order to determine their relationships with the Limousin ore deposits. Serpentinites and theirmineral assemblages (serpentine, olivine, amphibole, chlorite and spinel) show high concentrations in compatible transitionmetals (Sc, V, Cr, Co, Ni, Zn, Cu) and low abundances of incompatible metals (Ge, Ga,Mo, Sn, Sb, W). Associated amphibolites (metagabbros and metamorphosed dykes) show high contents of compatible metals, though lower than ultrabasic rocks. Amphibole and, to a lesser extent, titanite, are the main hosts for metallic trace elements. Abundances in incompatible metals are generally low. Amphibole has low Sn and Sb contents, while higher Sn contents were measured in titanite. In a UHP zoisite-eclogite cropping out close to one of the ophiolite-derived massifs, zoisite, rutile, omphacite and retrograde amphibole showhigh concentrations of incompatible elements (Sn, Sb,W, As, Ga, Ge, Mo and Bi). However, it has been shown that high-temperature and low-temperature hydrothermal alteration that have affected the basic and ultrabasic protoliths is c. 100Ma older than those that were responsible for the formation of ore deposits. For this reason, and as well because of their low Wand Sn abundances, the ultrabasic and basic rocks cannot be considered as source for the LimousinW-Sn ore deposits. In granite and gneisses, the metallic trace elements are mainly hosted by biotite and muscovite. Quartz and feldspars only show minor amounts of Sc, Ga, Ge, Sn and Pb. The micas contain low amounts of Ge, As, Sb, Bi and variable amounts of Sc, Cu and Ga. Besides, micas display significant Sn and W contents. Variations in Sn and W in micas allow tracing the pre-mineralisation concentrations in the granitic massifs : there is a clear geographical correlation between the Sn and W composition of micas and the importance of the W-Sn ore deposits, arguing for a local source of pre-concentrations. Granites from the Vaulry W-Sn ore deposit display the highest W and Sn concentrations, while granites hosting the Au deposits in the southern part of the Limousin (St-Yrieix, Cheni, Lauriéras) show the smallest concentrations.

Voir en ligne : https://doi.org/10.1016/j.lithos.20...



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publié lundi 20 janvier 2020