Rare earth element geochemistry of the metamorphosed volcanogenic massive sulfide deposits of the Manitouwadge mining camp, Superior Province, Canada; a potential exploration tool?

Abstract

Trace and rare earth element (BEE) geochemistry indicates that rocks in the Manitouwadge mining camp were derived from three major lithological units: mafic volcanic rocks, acid volcanic tuffs, and dacites. The Geco, Willroy, Willecho, and Big Nama Creek Cu-Zn-Ag deposits are contained within the 2720 Ma acid volcanic tuffs. Our study suggests that the thickness of the felsic volcanic unit (acid tuffs and volcaniclastics) may be significantly underestimated in the area. Medium- to high-grade metamorphism (upper amphibolite, lower granulite) gave rise to specific alteration assemblages which crystallized during several episodes of deformation and plutonism. Because major element geochemistry of the rocks was modified during ore- forming processes and later by several episodes of metamorphism and tectonism, the present alteration minerals merely reflect the cumulative effect(s) of these episodes and are often misleading in the identification of the protoliths. Thus, due to extensive alteration, the protoliths of various lithological units can be positively identified only from trace and rare earth element geochemistry. Extensive (up to 97%) light REE depletion was observed in the felsic volcanic rocks around the Geco, Willroy, and Willecho deposits. The positive correlation between BEE mobility and mineralization suggests that REE geochemistry can be successfully used in the exploration for volcanogenic massive sulfide deposits around the Manitouwadge mining camp. The absence of a well-defined discordant alteration pipe at Geco and localized enrichment in Fe, Al, + or - K at the Willroy, Willecho, Big Nama Creek, and Geco deposits implies that they may be highly metamorphosed equivalents of Mattabi-type deposits where the original iron-rich carbonates have recrystallized to calc-silicate or amphibole-rich assemblages during regional metamorphism. Localized enrichment in Fe, Al, and K in the felsic tuffs could be used to identify possible alteration pipes or semicomformable alteration zones associated with mineralization.