Abstract

The Falcon Island complex is an ultrapotassic pluton with a U-Pb age of 2695+ or -3 Ma intruding the Lake of the Woods greenstone belt in the Wabigoon Subprovince of the Superior Province, in western Ontario. It consists of nepheline-bearing alkali feldspar syenite and alkali feldspar melasyenite with lesser amounts of clinopyroxenite, monzodiorite, and peraluminous alkali feldspar syenite. The magma evolved with increasing P(H 2 O), as indicated by clinopyroxene cores mantled by hornblende, and hypersolvus perthitic alkali feldspar phenocrysts followed by subsolvus interstitial plagioclase, alkali feldspar and nepheline. An "aegirine-enrichment trend" is evident in the clinopyroxene as progressive depletion in Mg and Ca, and enrichment in Fe and Na (both as zoning in individual grains and in the successively fractionated units). Decreasing Fe (super 3+) /(Fe (super 2+) +Fe (super 3+) ) accompanied by increasing Fe/(Fe+Mg), Ti, and IV Al contents of the biotite of the fractionated rock units indicates progressive reduction and Al enrichment of the magma with fractionation. A spatial and genetic linkage of gold mineralization to oxidized alkaline intrusions has been proposed on the basis of mineral chemical studies from the Abitibi Subprovince. The lack of spatially associated gold mineralization with the progressively reduced Falcon Island pluton, despite its proximity to a potentially favorable zone of deformation, supports this hypothesis.

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