Mineral paragenesis and pegmatite formation associated with the Eden Lake syenite complex, northern Manitoba

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Arden, Kyla M.
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The Eden Lake syenite complex is a morzonitic intrusion located in the Trans Hudson orogenic terrane. It intrudes granitic rocks between the Lynn Lake and Leaf Rapids greenstone belts in northern Manitoba. Although the complex has not yet been dated, geochemical, textural and mineralogical characteristics support its interpretation as a postorogenic, metaluminous pluton with alkaline characteristics. The syenite complex comprises mafic and felsic monzonite phases and four types of associated pegmatites. The Type I mafic pegmatites comprise aegirine-augite, magnetite, plagioclase and hornblende; the Type II granitic pegmatites contain quartz, K-feldspar + - plagioclase. Neither of these are rare element pegmatites and neither contain any REE-bearing accessory minerals. The Type III granitic pegmatites are rare element pegmatites; they contain REE-bearing accessory minerals including apatite, titanite and zircon, as well as andradite, aegirine-augite, K-feldspar, plagioclase, quartz and fluorite. The REE-bearing phases are among the earliest minerals to crystallize in the Type III pegmatites; they display a similar stratigraphy of mineral growth, reflected in zoning patterns. The Type IV radioactive pegmatites are significantly REE-enriched; they contain REE-bearing minerals including apatite, titanite, zircory allanite and britholite, as well as aegirine-augite, K-feldspar, plagioclase, quartz and fluorite. In the Type IV pegmatites, allanite and britholite are interpreted to be the last minerals to crystallize, after the other REE-bearing phases. Britholite has a number of unusual characteristics that suggest that it has been amorphized, annealed and altered by a hydrothermal fluid. Apatite, titanite and zircon display the same sequence of mineral growth as the crystals in the Type III pegmatites. The Type I and Type II pegmatites represent crystallization of a liquid derived from a melt that had not undergone significant differentiation; this melt was not related to the syenite complex melt, but to a younger intrusion in the Eden Lake area. The Type III pegmatites were produced by crystallization of a residual liquid slightly enriched in incompatible trace elements formed by the fractional crystallization of the mafic and felsic monzonite phases of the syenite complex. The lack of significant REE-enrichment in the Type III pegmatites is the result of separation of a REE-, F-enriched immiscible vapour phase during the crystallization of the main phases of the syenite complex. The Type IV pegmatites are genetically related to the Type III pegmatites; they represent Type III pegmatites that have been altered by reaction with REE-enriched fluids. These fluids were introduced along shear zones and reacted with the pegmatite minerals before crystallizing first allanite, then britholite. The most likely source for these fluids is the REE -and F-enriched immiscible vapour phase that separated from the syenite melt during crystallization. Alteration of minerals within the Type III and IV pegmatites indicates both have been affected by late meteoric or magmatic fluids at elevated temperatures. The Eden Lake syenite complex and the associated REB-bearing pegmatites have undergone a long and complex history of fluid movement and alteration, similar to other REE deposits.