Geology and genesis of ultrabasic nickel-copper-pyrrhotite deposits at the Pacific Nickel property, southwestern British Columbia

Abstract

This paper deals with geology and genesis of some pipelike nickel-copper-pyrrhotite deposits on the property of Pacific Nickel Mines in southwestern British Columbia. These deposits occur almost exclusively within a stocklike ultrabasic mass one and a half miles across. The ultra-basic body consists essentially of pyroxenite with cores of peridotite, with hornblendic replacement phases, and with a remarkable reaction margin of pegmatitic hornblendite. The mineralization consists of disseminated and massive pyrrhotite with subordinate pentlandite and chalcopyrite among fresh olivine, bronzite, augite, and hornblende. The ultrabasics intermingle with, and partly cut, a larger batholithic mass of genetically related diorites of Late Mesozoic age. Both rocks and ore are cut by small ultrabasic and gabbroic dikes, and by veins and alteration zones. Some of the ore occurs as sulfidic olivine-rich cores or shells with bronzitic borders, forming remarkably zoned, steeply plunging pipe- or parsnip-shaped magnesian ultrabasic structures 100 feet or more in diameter. These features suggest replacement origin. The remainder of the ore forms massive sulfide-silicate bodies similar in size, attitude, and mineralogy to the zoned ones but more irregular in cross section and showing flow lines, banding, drag folds, sharp contacts, inclusions, and hornblendic reaction rims--all suggestive of injection origin.Genetic characteristics of other similar nickel deposits and rocks of the world are summarized and compared, then two theories are presented for genesis of the ore at Pacific Nickel. Magmatic segregation is suggested with injection of sulfides in an incipiently molten state (perhaps 700-800 degrees C) followed by minor replacement and reconstitution. Alternatively, a new twist is added to the hydrothermal replacement theory by suggesting that ascending water vapor above 650 degrees C could deposit the sulfides and, by removal or addition of silica, convert bronzite to olivine or vice versa to produce the zoned structures and other features.