Abstract

Amphibolites (hornblende–plagioclase rocks with Tpeak metamorphism = 650–700 °C) are abundant near the old Calumet Zn–Pb–Ag mine and Au prospect 90 km northwest of Ottawa. They lack primary structures and their original petrochemical character is obscured by metamorphism and profound alteration manifested by a variety of phlogopite–biotite-, garnet-, cummingtonite-, gahnite-, clinopyroxene-, carbonate scapolite-, and calcite-bearing assemblages. This alteration has modified the SiO2, alkali, and most alkaline earth distributions. Trace and minor element chemistry demonstrates that the amphibolites are metaigneous and bimodal, with two suites, one derived from differentiated arc tholeiites and the other from relatively undifferentiated rocks having transitional arc basalt – boninite chemistry. The complex rock package enclosing mineralization includes metatholeiites, whereas the boninitic metabasalts occur in a discrete interval in the structural hanging wall of the mineralization. Although these rocks represent a new geochemical assemblage in the Allochthonous Monocyclic Belt (Central Metasedimentary Belt), their implied geotectonic setting is comparable to those previously inferred for several other areas, including that near Montauban, Quebec, where there are very similar styles of mineralization.

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