Nickeliferous Pyrrhotite Deposits, Knox County, Southeastern Maine
Published:January 01, 1976
Nickeliferous ores (pyrrhotite, pentlandite, and chalcopyrite) in the Harriman peridotite and Warren gabbro-diorite of Knox County, southeastern Maine, occur as lenticular bodies in the Ordovician Penobscot Formation. The ore bodies were emplaced in Late Ordovician time, most likely synchronously with the Taconic orogeny; they were later modified by Acadian dynamothermal and Permian thermal metamorphism.
The Harriman body is a feldspathic lherzolite peridotite with limited serpentinization in the central zone. The amount of serpentine, amphiboles, talc, and carbonate minerals increases markedly away from the central zone, thus defining a crude zoning pattern. Sulfides occur interstitially or, less commonly, as thin massive layers. The Warren body is composed predominantly of andesine-labradorite, hornblende, cummingtonite, and biotite. Sulfides occur interstitially, as irregular disseminations, and in massive segregation bodies.
The textural intergrowths of sulfides are generally simple, although locally they are more complex as a result of metamorphism. The interstitial nature and macroscale and microscale segregation drops of the sulfides indicate an orthomagmatic origin. This is supported by the high combined concentrations of nickel, cobalt, and copper n i pyrrhotite (approximately 0.7 wt percent) and more than 2 wt percent cobalt plus copper in all varieties of pentlandite. These values are consistent with values for similar minerals in deposits of presumably early magmatic origin, such as those of Sudbury, Canada; Pecenga, USSR; and Insizwa, South West Africa.
Lack of primary pyrite, as well as higher than stoichiometric metal to sulfur ratios of the major sulfides, indicates relatively low sulfur fugacities. These features, along with the virtual absence of primary magnetite and the presence of as much as several volume percent of graphite, distinguish the Harriman and Warren deposits from other nickeliferous pyrrhotite deposits. The occurrence of chromian ulvospinels somewhat similar to those from lunar samples, the dominant ilmenite-spinel assemblage, and the absence of hematite and primary magnetite indicate that low oxygen fugacities prevailed during the development of the ore deposits.