Cobaltite-Gersdorffite Solid Solution As A Primary Magmatic Phase in Spessartite, Lacaune Area, Montagne Noire, France

A cobaltite-gersdorffite solid solution occurs in spessartite sills from the l,acaune area, Montague Noire, southem France, enclosed within phenocrysts and found in the matrix. It commonly encloses rounded grains of pyrrhotite with lamellae of pentlandite. Compositional variation of cobaltite-gersdorffiteo is represented mafuly by a Co-for-(Ni + Fe) substitution; the core is cobaltjferous, and the rim, nickeliferous. Atomic proportions in the structural formula as obtained from over 80 analyses, vary witlin the following ranges: (Co6.s5-o5y'.{ie.27-a.66Fes.1o-a.3)As6.e7-1.67S0.se-o.sz. The Ni/Fe value is near 2, which could indicate the presence of a (1.{io.*Feo.u,)Ais component witl a Ni-Fe order in the Ni site. Textural evidence and compositional data suppofi an igneous rather rhaa secondary origin. Cobaltite-gersdorffite* can crystallize by separation of an early immiscible sulfaxsenide liquid from a calc-alkaline lamprophyric magma. These sills could represent an atffactive target for Au prospecting in the Montagne Noire area.