An experimental study on cubanite irreversibility; implications for natural chalcopyrite-cubanite intergrowths

Isocubanite, CuFe (sub 2) S (sub 3) , was synthesized at 700 degrees C by reaction of the component sulphides and sulphur in evacuated silica tubes; EPMA showed a nearly stoichiometric composition. Annealing at 185 degrees C for 4, 8, 12, 24 and 32 days produced equant grains of pyrrhotite and chalcopyrite lamellae and spindles parallel to (111) and (110) within the isocubanite matrix. The formation of pyrrhotite shows that the chalcopyrite is not produced by simple exsolution, but is due to the breakdown reaction CuFe (sub 2) S (sub 3) = CuFeS (sub 2) + FeS. Repeated homogenization and annealing resulted in thicker chalcopyrite lamellae. The possible mechanism for the intergrowth of orthorhombic cubanite within chalcopyrite, characteristic of ancient hydrothermal deposits, from the intergrowth of chalcopyrite within isocubanite that is exclusively encountered in modern sea-floor hydrothermal deposits, is discussed.