Intergrowths of hexagonal and monoclinic pyrrhotites in some sulphide ores from Norway

Pyrrhotites in polished sections from more than twenty stratabound massive sulphide and magmatic nickel-copper deposits in Norway were studied under the microscope using the magnetic olloid method. In both types of deposits, two distinct styles of intergrowths between monoclinic and hexagonal pyrrhotites were found: crystallographically-controlled lamellar intergrowths and fissure-controlled irregular intergrowths. Lamellar intergrowths consist of crystallographically oriented monoclinic lamellae occurring in a hexagonal matrix and were produced originally by exsolution from hexagonal pyrrhotite on cooling. Irregular intergrowths comprise blades and patches of monoclinic pyrrhotite occurring along fissures and grain boundaries of hexagonal pyrrhotite, and were formed by interactions between hexagonal grains and sulphur-rich hydrothermal solutions. Increase in lamella thickness and spacing, development of lamella zonations, wedge-shaped composite ends, boxworks and composite lamellae were caused by progressive lamellae coarsening and maturation during natural annealing, which could have been promoted by anisotropic stress. Metamorphic recrystallization a d annealing tend to homogenize pyrrhotite and erase preexisting exsolution lamellae. I~vwoaos: annealing, exsolution, magnetic olloid, metamorphism, pyrrhotite