Spessartites in the Montagne Noire, France: mineralogical and geochemical data

Abstract

Spessartites occur as sills in the Lower Cambrian formations on the northern slope of the Montagne Noire, France. They show an unusual mineralogy with phenocrysts of amphibole (magnesio-hastingsite to pargasite) and phlogopite, both including chromian rutiles (up to 3.5 wt.% Cr 2 O 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 fS 2 for fO 2 values slightly above the NNO buffer. The magmatic evolution is firstly marked by a partial cumulation process of early magmatic phases (olivine and associated chromite and sulfarsenides) as suggested by the very high concentrations of MgO (15.5-16 wt%), Cr (1400-1500 ppm), Ni (400 ppm) and Co (75-110 ppm) measured in these rocks. As crystallization proceeds, amphibole 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, SiO 2 saturated with a K/Na ratio higher than 1. They display a strong enrichment of LFS (low-field-strength) and light rare earth elements, with very low concentrations of heavy REE, Ta and Ti. Their chemical and mineralogical features suggest that the primary magma can be generated by relatively small degrees of partial melting from a metasomatically enriched phlogopite + gamet ± amphibole lherzolite. The occurrence of these sills seems to indicate transcrustal fractures allowing a rapid ascent without magma differentiation, consistent with an extensional continental zone. Their magmatic sulfarsenides suggest the possible occurrence of deep-level gold-bearing As-S mineralizations such as found in the Salsigne district