The Werner Lake Co-Cu-Au deposit, Ontario: Evidence for pre-metamorphic hydrothermal origin

Abstract

The Werner Lake Co-Cu-Au deposit in the Late Archean English River metasedimentary subprovince of the Superior Province, Ontario, is hosted by pyroxene-bearing amphibolites, and is characterized by association with metamorphosed alteration assemblages including garnetiferous biotite schists, garnetiferous quartzites, and calc-silicate rocks. Mineralization includes disseminated chalcopyrite and massive chalcopyrite-pyrite rich lenses in the garnetiferous biotite schists and as disseminated to massive cobaltite-rich orebodies. Geochemical data support field relationships that the garnetiferous biotite schists originated from the protoliths of the amphibolites. Mass balance calculations revealed significant gains in Fe, Mn, and K, but losses in Ca, Na, and Mg, similar to those found for the formation of chloritic zones from seafloor hydrothermal alteration of basalts. The garnetiferous biotite schists are characterized by depletion in whole rock δ¹⁸O values (+4 to +6.5‰ V-SMOW) relative to the associated amphibolites (+6.8 to 8.3‰) and ultramafic rocks (+5.6 to 6.9‰). This depletion is interpreted as resulting from high-temperature interactions with seawater and thus supports a seafloor hydrothermal origin for the Werner Lake Co-Cu-Au deposit. The δ³⁴S values of sulphides and sulpharsenides in the ores of the Werner Lake deposit are uniformly close to 0‰, similar to those sulphides in the alteration assemblages (-0.5 to +2.3‰) and amphibolites (+0.3 to +1.8‰), suggesting a juvenile source for the S. The Werner Lake Co-Cu-Au deposit differs in ore-element chemistry, host lithology, and alteration assemblages from the Cu-Zn and Zn-Pb-Cu VMS deposits and therefore is interpreted to represent a separate type of seafloor hydrothermal deposit.

The granulite facies metamorphism (640-750°C and 4.5 to 5.5 kbar) at the Werner Lake deposit was accompanied by penetrative deformation followed by retrogression and late hydrothermal alteration. However, this high-grade metamorphism did not obliterate the possibly syngenetic mineralization and alteration zonation. This preservation of primary features was attributed to an absence of pervasive fluids during and after the granulite facies metamorphism, as indicated by the oxygen isotope heterogeneity of the ores and associated lithologies and by the preservation of distinct oxygen isotope signatures in the garnetiferous biotite schists. Localized solid-state remobilization of cobaltite-rich ores during the granulite facies metamorphism and deformation is indicated by thickening of the orebodies close to the F₂ fold hinges and alignment of cobaltite porphyroblasts parallel to the S₂ foliation. The presence of chalcopyrite-rich veins associated with biotite-rich selvages suggests a local, fluid facilitated remobilization of chalcopyrite during retrogression. Both granulite facies metamorphism and retrogression may have resulted in the preferential removal of Fe-Cu-rich sulphides from the ores owing to their higher ductilities than cobaltite and silicates, thereby increasing the proportion of cobalt-rich ores. However, late hydrothermal alteration does not appear to have significantly disturbed the Co mineralization in the Werner Lake Deposit.