The Thirty Mile Lake area is underlain by upper amphibolite to granulite grade para- and orthogneiss and by Archean megacrystic alkali feldspar megacrystic granite. Paleoproterozoic gabbro, lamprophyre, and feldspar porphyry dykes intrude basement rocks. A thick hematitic saprolite was developed in the gneiss prior to deposition of South Channel Formation sharpstone conglomerate. The lamprophyre dykes, equated to the Christopher Island Formation of the Baker Lake Group, are of two types. The first contains phlogopite phenocrysts in a groundmass consisting of phlogopite, alkali feldspar, and apatite; the second contains phlogopite±amphibole in a groundmass consisting of amphibole, alkali feldspar, and apatite. The lamprophyre dykes intrude along a northwest-trending joint set and east-trending mylonite zones. Oblate xenoliths of granitoid and less abundant mafic-ultramafic compositions occur in the dykes; some xenoliths display recessive rims indicating reactions with the host rock. The lamprophyres are being thoroughly studied in order to evaluate their diamond potential.
Abstract Oriented inclusions of monazite occur in the dark core of apatite porphyroblasts in a muscovite schist from the Archaean Hemlo gold deposit, Ontario, Canada. The monazite inclusions are elongated along the b-axis and parallel to the c-axis of the apatite host; the complete orientation relationship of the monazite/apatite intergrowth is bMnz//cAp , cMnz//aAp . From analysis by SIMS and EMP, the dark core of the apatite porphyroblasts is depleted in LREE (La N /Yb N = 0.56). The monazite inclusions are correspondingly enriched in LREE , but markedly depleted in HREE , compared with monazite grains in the rock matrix and cross-cutting veins. The monazite inclusions precipitated by oriented reaction through rock-fluid interactions during a late hydrothermal alteration. Their unusual REE composition is probably related to both a preferential leaching of LREE from the dark core and a selective transfer of HREE out of the apatite porphyroblasts.
The recently discovered Ni-Cu-Co Suluk occurrence is interpreted to be Paleoproterozoic and represents the first recognition of a gabbro-associated magmatic sulphide deposit type in the Western Churchill Province. Archean tholeiitic metavolcanic rocks of the Gibson-MacQuoid greenstone belt that host the occurrence were reworked during Paleoproterozoic thermotectonism. A recrystallized and altered composite gabbroic intrusion is spatially associated with and inferred to be genetically related to the nickel-copper occurrence. Massive sulphide is irregular in form, concordant with the foliation in the enveloping tholeiitic metavolcanic rocks and is composed of massive nickeliferous pyrrhotite-pentlandite that contains abundant wallrock xenoliths. This magmatic nickel sulphide event in the Western Churchill Province is correlated with extensive mafic magmatism and associated Ni-Cu-Co and Fe-Ti deposits, circa 2.2-1.9 Ga, in the Fennoscandian and eastern Baltic shields.
A study has been made of the petrology and geochemistry of a 30-mile (48.3 km) segment of a belt of ultramafic-gabbroic igneous bodies extending past the south side of Lake Abitibi, Ontario. In general, the bodies are sill-like and are differentiated into major layers of peridotite, clinopyroxenite, and gabbro, the layers generally being in that stratigraphic order. In detail, the intrusions fall into four groups: (1) complex sills in which there is a cyclic repetition of layers; (2) simple differentiated sills showing only one sequence of the above rock layers; (3) bodies composed only of peridotite and dunite; and (4) bodies composed wholly of gabbroic rocks.One of the group (2) bodies has a chilled margin equivalent in composition to a tholeiitic basalt. The general structure of the intrusions and their petrographic and chemical features indicate that they are differentiated from basaltic magma by gravity-controlled fractionation. However, it appears that while solidifying, some of the intrusions were open to periodic addition or subtraction of magma. Thus, in the intrusions showing cyclic repetition of layers, it is apparent that the magma was altered prior to the formation of each cyclic unit such that the original order of mineral crystallization was repeated. For other intrusions, it can be inferred that large amounts of partly crystallized liquid were expelled such that each of these intrusions is now largely or wholly represented by ultramafic rocks. The bodies composed wholly of gabbro may be derived from the expelled magma.
The Garland Peak Syenite (GPS) of the Red Hill complex, NH, consists predominantly of amphibole, oligoclase, perthite, and quartz; amphiboles are zoned from kaersutite cores to hastingsite-hornblende rims. The association of kaersutite with quartz indicates that the GPS magma experienced substantial changes in silica activity during its crystallization history. Camptonites are also associated with the Red Hill complex, and in order to elucidate the camptonite-GPS, and the GPS kaersutite-quartz relationships, amphiboles in these rocks were analyzed by electron and ion microprobe techniques. Kaersutites in the camptonites and GPS are very similar in terms of major and minor elements. REE concentrations in the camptonite's kaersutite are slightly less than the GPS kaersutite; LREE abundances are 100 times chondrites, La/Yb values are 4--5. GPS kaersutitic cores have LREE abundances between 200--300 times chondrites with La/Yb values between 6--8. Compared to the cores, the hastingsite rims are preferentially enriched in REE with LREE between 1,000 and 2,000 times chondrites, La/Yb values range between 16--18, and the patterns have large negative Eu anomalies. Although complicated by the certainty of changing partition coefficients during crystallization, the enrichment in total REE from camptonite--GPS kaersutite--GPS hastingsite is consistent with a differentiation origin of the suite. Rimward depletionsmore » in Sr, Eu, V, and Ti concentrations, and the increase in La/Yb values suggest that parental camptonites fractionated amphibole, magnetite, and feldspar to produce silica-oversaturated GPS liquids. The significance of amphibole and magnetite fractionation on camptonite-GPS silica activity is also indicated by bulk-rock, major element modeling.« less
