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A neutron activation analysis procedure for the simultaneous determination of Au, Pd and Ir, and separate determination of Os is described and applied to the determination of these metals in the Mount Albert ultramafic pluton, Quebec. The Mount Albert is an alpine-type ultramafic body whose petrography, geology and geochemistry has been discussed in some detail by MacGregor (1962, 1964). It consists mainly of serpentinized dunite and peridotite. However, it is distinguished by the presence of an eastern lobe which is essentially unserpentinized. In addition to ferromagnesians, two varieties of spinel including disseminated accessory chrome spinel and massive, discordant chromite veins or schlieren are important mineralogical constituents. Twenty samples were analyzed including 5 fresh dunites and peridotites, 8 serpentinized equivalents, 4 disseminated chrome spinels, 1 clinopyroxenite, and 2 massive chromite. The concentrations (in p. p. b.) of average fresh ultramafics and their serpentinized equivalents are:-------------------------------------- ---------------------Pd----Os----Ir----Au------------------------------------------- Fresh rocks------------7.7----7.2--2.5--0.99------------------------------------------- Serpentinized rocks---10----6.6--2.2---2.4-------------------------------------------- The piuton was intruded as a nearly solid mass of olivine and some orthopyroxene with about 15 weight per cent melt. A small concentration of the precious metals, a factor of 2 to 3, in the melt relative to crystalline phases appears to have occurred. The disseminated chrome spinel and massive chromite appear to concentrate the precious metals. In particular, Os and Ir content of the massive chromite is approximately 10 times that of fresh or serpentinized whole rocks. There is little change in average precious metal content with serpentinization. Pd shows a slight increase, and Ir and Os slight decreases in concentration with degree of serpentinization. Only Au appears significant to be enriched in serpentinized rocks where a twofold increase occurs. The variation in precious metal content of all four metals increase when the rocks are serpentinized: When normalized to chondrite meteorites, the precious metal data give fractionation trends similar to that of average chondrite. These trends are compatible with a model in which alpine ultramafics are regarded as residua from the partial melting of parental material of approximately chondritic composition. Thesis Master of Science (MSc)

An intrusive body of ultramafic rocks, which crops out over an area of 7 square miles at Union Bay, southeast Alaska, has been mapped and studied petrographically. The ultramafic rocks are probably Jurassic or Cretaceous in age, and constitute a spoon-shaped lopolith with a dunite feeder plug at the east end. The rocks of the lopolith are composed of various proportions of diopside and forsteritic olivine, with some magnetite and hornblende, and no feldspar. The most basic rocks, dunite and peridotite, occur in the central portion of the lopolith, and are enveloped by olivine-pyroxenite, pyroxenite, and hornblende-pyroxenite in that order. The peridotite rocks in the central portion of the complex are composed of olivine-pyroxenite intruded by sills and irregular bodies of dunite. Attitudes of the dunite sills conform to the spoon-shaped structure of the logolith. Large areas of pyroxenite containing 10% to 25% of primary magnetite crop out around the periphery of the lopolith. An intrusive body of gabbro adjoins the south side of the ultramafic lopolith. The gabbro is earlier than, and has been intruded by, the ultramafic rocks. The structure and petrology of the ultramafic lopolith are explained by an hypothesis of successive injections of increasingly basic magmas composed essentially of: (1) a cotectic blend of diopside and magnetite; (2) a cotectic blend of diopside and forsteritic olivine; and (3) forsteritic olivine. The intrusion of the magmas in that order, the reverse order of crystallization of the component minerals, is a reflection of the differential melting of the ultramafic source in the mantle of the earth. Each arriving magma must have found the hot, still crystallizing, central portion of the preceding magma the most accessible locus of intrusion. The lack of a very high grade of contact metamorphism in the pelitic sediments adjacent to the ultramafic rocks is explained by the assumption that the ultramafic magmas contained very little or no water, and, therefore, expelled no significant amounts of volatile phases during crystallization.