Geochemistry and origin of cordierite-orthoamphibole gneiss and associated rocks at an Archaean volcanogenic massive sulphide camp: Manitouwadge, Ontario, Canada

Abstract

Cordierite-orthoamphibole gneiss, amphibolites, quartz-biotite schist and calc-silicate rocks occur in spatial association with Archaean metamorphosed volcanogenic massive sulphide (VMS) deposits at Manitouwadge, Ontario. Trace-element characteristics, particularly those of the high field strength elements (HFSE), support field relationships that most of the cordierite-orthoamphibole gneisses represent metamorphosed equivalents of chlorite-rich rocks that were derived hydrothermally from tholeiitic basalts as an integral part of the syngenetic ore-forming processes on the Archaean seafloor. However, a garnet-rich cordierite-orthoamphibole dike, that is characterized by low TiO 2 content and high concentrations of Zr, Y, Nb and REE, and has single-zircon Pb-evaporation 207 Pb 206 Pb ages of 2715–2743 Ma, may have been derived from a synvolcanic felsic dike. Evidently, the seafloor hydrothermal alteration at Manitouwadge resulted in similar alteration products from contrasting protoliths and, therefore, was a fluid-dominant system. The amphibolite lenses within the cordierite-orthoamphibole gneiss also have been affected by the seafloor hydrothermal alteration, and some of them most likely represent metamorphosed equivalents of calcsilicate alteration products formed on the seafloor. Mass-balance calculations indicate that HFSE and REE were mobilized during the seafloor hydrothermal alteration. The mobility of HFSE and REE was controlled by both their host minerals and the hydrothermal fluids. In particular, a significant portion of igneous zircon in the garnet-rich dike has survived the seafloor hydrothermal alteration and the subsequent high-grade regional metamorphism. The regional metamorphism did not change the bulk-rock REE characteristics but was important in redefining the mineralogy of REE. Trace-element geochemistry of mafic metavolcanic rocks at Manitouwadge are similar to those of the adjacent Schreiber-Hemlo greenstone belt and are consistent with a volcanic island arc setting. However, further work is required to determine whether the Manitouwadge and adjacent greenstone belts of the Wawa Subprovince were originally a larger, continuous terrane.