Massive sulfide-late diabase relationships, Home mine, Quebec: genetic and chronological implications

Abstract

At the Home mine, Noranda, Quebec, evidence of penetration and apparent replacement of the 'Late Diabase' dikes by massive sulfides formerly led several workers to believe that these dikes were older than the orebodies. However, a glassy selvage of the diabase against a sulfide wall, in contrast with a microcrystalline one against adjoining rhyolite, demonstrates a different rate of cooling against materials of different thermal diffusivity. Also, dissociation of pyrite to magnetite, increase in the monoclinicity of pyrrhotite, copious development of chalcopyrite blebs in sphalerite, and some interesting sulfide-sulfide and sulfide-silicate reactions in the contact zone record unmistakable imprints of thermal metamorphism of the sulfide wall. Microscopic evidence indicates that, an early stage of dissociation of Fe-sulfides to magnetite was followed by pyritization of pyrrhotite and ferromagnesian silicates, and migration of chalcopyrite along and into the contact zone. These suggest a persistent pressure gradient towards the 'hot-wall' side of the reaction zone, created presumably by the fracturing of the chilled edge and by early dissociation of pyrite. Uninterrupted extensions of tabular, 'vein-like', chalcopyrite bodies from the massive sulfides protrude well into the diabase. Absence of chilling of the diabase along the walls and terminations of these protuberances, and the deformed and recrystallized fabric of these bodies point to their late emplacement by plastic flowage along post-intrusion shear planes. A similar mode of origin is proposed for the very small sulfide veins in 'D' dikelets because the two types of veins are similar. Sulfide patches within the diabase wall, with intensive but localized wall-rock alteration, retain evidence of more thorough dissociation of iron sulfides and other high-temperature mineralogical reconstitution. These masses are therefore inferred to represent clusters of detached or semi-detached portions of the sulfide wall that lost their identity due to the action of deuteric fluid. The fluid, enriched in sulfur, accounted for pyritization and other replacement and alteration features in the surrounding diabase that impart the deceptive appearance of an encroaching sulfide front. Galena-marcasite veinlets on either side of the contact in places indicate a much later solution activity along a potentially weak zone.