Comparative mineralogy, geochemistry, and conditions of formation of two metasomatic talc and chlorite deposits; Trimouns (Pyrenees, France) and Rabenwald (Eastern Alps, Austria)

Abstract

The Trimouns (French Pyrenees) and Rabenwald (Austrian Alps) talc and chlorite deposits occur within Paleozoic metamorphic series (Hercynian metamorphism) and are primarily of sedimentary origin. They were produced by metasomatism in zones of intense deformation (thrust planes, surfaces of tectonic slicing) of late Hercynian (Trimouns) and Alpine (Rabenwald) age. The study of the mineral assemblages and the mineral chemical composition (chlorites, phengites) in the various metasomatic zones demonstrates a near attainment of local equilibrium and points to a contrast in crystallization conditions: approximately, T = 400 degrees C and P = 1 kbar at Trimouns; T = 500 degrees to 550 degrees C and P = 8 to 9 kbars at Rabenwald, where the talcphengite assemblage is characteristic.Talc-rich ores originate from carbonates (dolomite, magnesite), whereas Mg chlorite-dominant ores result from the alteration of the silicoaluminous rocks (mica schists, gneisses, and associated felsic igneous rocks). In the latter case, large quantities of Mg are added while Na, K, and Ca are entirely leached, as documented by extensive data on the chemical composition of the rocks. Quartz is totally absent from the ores due to alteration into talc and by dissolution. At Trimouns (low P) Al is a quasi-inert component, whereas a relatively high mobility is evidenced at Rabenwald (high P).A fluid inclusion study (microthermometry and leachate analyses) of quartz from recrystallized pods in zones adjacent to the talc-chlorite ores shows very typical solutions with similar compositions in both deposits--high salinity with halite often present at room temperature, relatively high Ca and Mg concentrations, and low CO 2 content--leading to the alteration of the carbonates at relatively low temperatures. In other respects, the respective isochores confirm the pressure contrast between the two localities.All these data provide constraints regarding the physical and chemical conditions of the hydrothermal alteration. It is noticeable that the high salinity of the solutions seems to be an important factor in explaining the crystallization of talc and the absence of tremolite.