A study of millerite from Cu-Ni-PGE footwall veins, Sudbury, ON: crystal-chemistry, morphology, & geological implications

Abstract

Millerite (-NiS) is a common accessory mineral in magmatic Cu-Ni-PGE deposits, such as those occurring in Sudbury, Ontario, Canada. In such localities, millerite develops as platy crystals, often forming large, cleavable masses. This is in stark contrast to the more widespread occurrence of millerite as a hydrothermal precipitate within vugs in carbonate basins, where it develops an acicular morphology, with crystals often exhibiting exaggerated aspect ratios (l:w > 20). The reason for these contrasting morphologies is attributed directly to the genesis of magmatic millerite that initially crystallizes as the high-temperature mineral crowningshieldite (-NiS), later inverting to millerite upon further cooling. The morphology exhibited by platy millerite is thus representative of crowningshieldite via a paramorphic relationship. Millerite of magmatic origin also frequently exhibits secondary twinning on {011̅2}, resulting from a postcrystallization increase in pressure. Development of these twins produces a variable response under BSE microscopy that is attributed to orientation contrast.