The Marbridge No. 3 and 4 deposits are associated with lens-like bodies of serpentinized ultramafic rock that occur within a sequence of interbanded felsic volcanic rocks and graywackes. Both deposits consist of sulfide disseminated within the host peridotite, together with smaller amounts of stringer ore. Ore at the No. 3 deposit occurs towards the top of its associated serpentinite while that at the No. 4 occurs towards the base.The major sulfide minerals at the No. 4 deposit are pyrite and pentlandite together with minor amounts of millerite. At No. 3, the ore is zoned parallel to the contact with the host volcanics. The major part consists of pyrite, pentlandite and millerite, but close to the contact the assemblage pyrite-pentlandite-pyrrhotite is developed. Small amounts of pentlandite and heazlewoodite occur at the center of the serpentinite lens at the No. 3 deposit. Pentlandite is extensively altered to violarite throughout much of this deposit.Electron microprobe analyses of pentlandite from the Marbridge deposits, together with samples from the Alexo mine, the Texmont mine and the Sudbury ores indicate that the Fe:(Ni + Fe) ratio of pentlandite varies directly with the minerals with which it is associated, ranging from 0.518 for that in association with troilite and intermediate hexagonal pyrrhotite to 0.341 in association with heazlewoodite. This relationship indicates that equilibrium has been established between pentlandite and its associated minerals and is supporting evidence for the existence of a stable tie-line between pentlandite and pyrite at low temperatures in the Fe-Ni-S system.