Mineralogical and geochemical study of pseudocoticule from the Stavelot Massif, Ardennes (Belgium), and redefinition of coticule

Abstract

A petrographic investigation of pseudocoticules from the Salm Valley, Belgium, shows that this type of rock is characterized by a quartz + spessartine + hematite assemblage. Spessartine grains can reach ca. 200 μm in diameter. The rocks directly in contact with the pseudocoticules are very rich in chloritoid and spessartine, thus showing that metasomatic processes affected the pseudocoticules at the end of the Variscan orogeny. Geochemical data indicate that Fe is higher in the host rocks than in the pseudocoticules, whereas Mn shows the inverse behaviour. Cu shows a significant increase in pseudocoticules when compared to the host rocks, thus indicating that pseudocoticules may be the source for the enrichment in exotic trace elements (Cu, Te, Mo, Bi, and Pb) observed in the quartz veins of the Salm Valley. The term coticule is redefined on an historical basis, as a yellow rock characterized by small spessartine grains (≥85 mol. % Sps, 5–20 μm) included in a matrix containing large amounts of phyllosilicates. Whole-rocks analyses indicate (K 2 O + Na 2 O) ≥ 1 wt% and Al 2 O 3 ≥ 15 wt%. Such a rock can be used as whetstone, whereas pseudocoticules, which contain larger amounts of quartz, do not show the same technical properties. Most samples from the literature, considered as coticules, rather correspond to pseudocoticules or to almandine-bearing quartzites.