The Canadian Mineralogist
Volume 34, pages 503-512 (1996)

COBALTITE-GERSDORFFITE SOLID SOLUTION AS A PRIMARY MAGMATIC PHASE IN SPESSARTITE, LACAUNE AREA, MONTAGNE NOIRE, FRANCE

DIDIER BÉZIAT, PIERRE MONCHOUX and FRANCIS TOLLON
Laboratoire de Minéralogie et Cristallographie, URA 067, Université Paul Sabatier, 39 Allées Jules Guesde, 31000 Toulouse, France

Abstract

A cobaltite-gersdorffite solid solution occurs in spessartite sills from the Lacaune area, Montagne Noire, southern France, enclosed as inclusions within phenocrysts and found in the matrix. It commonly encloses rounded grains of pyrrhotite with lamellae of pentlandite. Compositional variation of cobaltite-gersdorffitess is represented mainly by a Co-for-(Ni + Fe) substitution; the core is cobaltiferous, and the rim, nickeliferous. Atomic proportions in the structural formula, as obtained from over 80 analyses, vary within the following ranges: (Co0.05-0.59Ni0.27-0.66Fe0.14-0.32)As0.97-1.07S0.89-0.97. The Ni/Fe value is near 2, which could indicate the presence of a (Ni0.66Fe0.33)AsS component with a Ni-Fe order in the Ni site. Textural evidence and compositional data support an igneous rather than secondary origin. Cobaltite-gersdorffitess can crystallize by separation of an early immiscible sulfarsenide liquid from a calc-alkaline lamprophyric magma. These sills could represent an attractive target for Au prospecting in the Montagne Noire area.