SPESSARTITES IN THE MONTAGNE NOIRE, FRANCE - MINERALOGICAL AND GEOCHEMICAL DATA

Spessartites occur as sills in the Lower Cambrian formations on the no rthern slope of the Montagne Noire, France. They show an unusual miner alogy with phenocrysts of amphibole (magnesio-hastingsite to pargasite ) and phlogopite, both including chromian rutiles (up to 3.5 wt.% Cr2O 3), zincian chromites and Ni-Co-Fe sulfarsenides with pyrrhotite. The lack of magnetite and ilmenite together with the presence of sulphides and sulfarsenides would suggest high fS2 for fO2 values slightly abov e the NNO buffer. The magmatic evolution is firstly marked by a partia l cumulation process of early magmatic phases (olivine and associated chromite and sulfarsenides) as suggested by the very high concentratio ns of MgO (15.5-16 wt%), Cr (1400-1500 ppm), Ni (400 ppm) and Co (75-1 10 ppm) measured in these rocks. As crystallization proceeds, amphibol e and phlogopite become more and more Mg-rich; olivine is destabilized to phlogopite. The latest phases to crystallize are plagioclases and quartz. These rocks are calc-alkaline, SiO2 saturated with a K/Na rati o higher than 1. They display a strong enrichment of LFS (low-field-st rength) and light rare earth elements, with very low concentrations of heavy REE, Ta and Ti. Their chemical and mineralogical features sugge st that the primary magma can be generated by relatively small degrees of partial melting from a metasomatically enriched phlogopite + garne t +/- amphibole lherzolite. The occurrence of these sills seems to ind icate transcrustal fractures allowing a rapid ascent without magma dif ferentiation, consistent with an extensional continental zone. Their m agmatic sulfarsenides suggest the possible occurrence of deep-level go ld-bearing As-S mineralizations such as found in the Salsigne district .