Abstract

Archaean felsic metavolcanic rocks in the Superior Province of the Canadian Shield may be divided into three major groups on the basis of trace-element abundances and ratios. (1) FI felsic metavolcanic rocks are dacites and rhyodacites characterized by steep REE patterns with weakly negative to moderately positive Eu anomalies, high Zr/Y, low abundances of high-field-strength elements (e.g., HREE, Y, Zr, Hf), and high abundances of Sr. Examples occur in the Bowman Subgroup and Skead Group in the Abitibi Belt, in the Kakagi Lake, Lake of the Woods, Shoal Lake, and Sturgeon Lake areas of the Wabigoon Belt, and in the Confederation Lake area of the Uchi Belt. None of these horizons, as known, hosts base-metal sulphide deposits. (2) FII felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping REE patterns with variable Eu anomalies, moderate Zr/Y, and intermediate abundances of HFS elements and Sr. Examples occur in the Misema Subgroup of the Abitibi Belt, in the Wabigoon Lake and Sturgeon Lake areas of the Wabigoon Belt, and in the Confederation Lake area of the Uchi Belt. Of these horizons, only those in the Sturgeon Lake area host base-metal sulphide deposits, and they exhibit the most pronounced negative Eu anomalies of this group. (3) FIII felsic metavolcanic rocks are rhyolites and high-silica rhyolites characterized by relatively flat REE patterns, which may be subdivided into two types. FIIIa felsic metavolcanic rocks exhibit variable negative Eu anomalies, low Zr/Y, and intermediate abundances of HFS elements and Sr. Examples occur in the Noranda mining district of the Abitibi Belt. FIIIb felsic metavolcanic rocks exhibit pronounced negative Eu anomalies, low Zr/Y, high abundances of HFS elements, and low abundances of Sr. Examples occur in the Kamiskotia, Kidd Creek, Matagami, and Noranda mining districts, the Garrison Subgroup in the Abitibi Belt, and at the South Bay mine in the Confederation Lake area of the Uchi Belt. All of these FIII horizons, with the exception of Garrison, host important base-metal sulphide deposits.These geochemical variations are interpreted to reflect differences in the petrogenesis of the felsic magmas, specifically, their formation or degree of modification in high-level magma chambers, which also influenced the formation of massive base-metal sulphide deposits. Most massive base-metal sulphide deposits in the Superior Province are underlain by subvolcanic magma chambers, which have been interpreted to have supplied heat to drive the ore-forming hydrothermal systems. FIII and some FII felsic volcanic rocks are interpreted to have been derived from these high-level magma chambers, accounting for their distinctive geochemical signatures and their association with massive base-metal sulphide mineralization. In contrast, FI felsic volcanic rocks are interpreted to have been derived from a deeper source and are considered to have escaped significant high-level modification, accounting for their distinctive geochemical signatures and the lack of associated base-metal sulphide mineralization. With certain limitations, the geochemistry of felsic metavolcanic rocks therefore may be used as a guide to identify prospective horizons for massive base-metal sulphide exploration in the Superior Province.

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