Abstract: Microscopic investigations have provided further evidence that pyrrhotite in the massive sulphide ores of the Norwegian Caledonides was mainly formed during sedimentation and diagenesis and has undergone polyphase metamorphism, deformation, recrystaliization and annealing. A minor portion of the pyrrhotite was formed during diaphthoretic and metahydrothermal events. No pyrrhotite was found in the present study that appeared to have been derived from pyrite by progressive metamorphism. Observations using a magnetic colloid revealed that intimate intergrowths of the hexagonal and monocilnic varieties can be exhibited by either sedimentary-diagenetic, retrogressive or metahydrothermal pyrrhotite. The intergrowths are classified morphologically in to crystallographically controlled lamellar patterns and fissure- or grain boundary-controlled irregular ones. Those two Patterns were form ed either by exsoiution from the hexagonal solid solution or by replacement of the hexagonal variety by sulphur-rich fluids. Recent research indicates that volcanogenic massive sulphide deposits can form at temperature higher than 300°～350℃, and even above 450℃. Such temperatures may be attained particularly in the case of proximal deposits and are favourable for the crystallization of hexagonal pyrrhotite (hpo), which will be able to exsolve moncolinic pyrrhotite (mpo) during cooling if the bulk composition plots in the hpo+mpo region. Monoclinic pyrrhotite will he absent in the case of ores that were formed under sulphur-poor sea-floor conditions or at temperatures be low 254℃. Metamorphism at temperatures exceeding that of the mpo-hpo transition will erase earlier exsolved monoclnic pyrrhotite by homogenization during reocrystallization and annealing. Such a proof of homogenization will account for the fact that the monoclinic variety is normally absent in recrystallized and annealed pyrrhotite. Well-preserved moneclinic lamellae in retrogressive and metahydrotharmal pyrrhotites may indicate that these pyrrhotites have fiever been affected by post-crystallization and high-temperature events. Most of the volcanogenic massive sulphide deposits in old orogenic belts have, to various extents, been subjected to metamorphism, recrystallization and annealing. This could be considered as one of the most important reasons for the absence of monoclinic pyrrhotite in their ores.