Amphibolite to granulite-facies Cu-Fe-oxydes hydrothermal system in the Wakeham Group, Eastern Grenville Province, Québec: a metamorphic study

CIM Montreal 2003
Louise Corriveau, Otto Van Breemen, Alain Tremblay,
Abstract The 1.5 Ga Wakeham Group of the eastern Grenville Province in Québec host in its eastern, Lake Musquaro-La Romaine, extension a series of Cu mineralizations among granulite-facies volcanic-hosted alteration zones typical of a VHMS-type hydrothermal environment. In the area, deposition of arenaceous sediments and volcanic and pyroclastic rocks (1.52-1.49 Ga) preceded by a few million years emplacement of a sub-volcanic 1.5 Ga granitic batholith (SHRIMP U-Pb age data). In supracrustal rocks, metamorphism increases southward from amphibolite facies at Lake Musquaro (garnet-biotite-kyanite or fibrolite paragenesis with only local incipient partial melting) to granulite facies at La Romaine (e.g., orthopyroxene-bearing anatectic leucosomes in amphibolite). This contrasts with the uniform late Grenvillian (1.01 Ga) amphibolite-facies metamorphism recorded by the granitoids and suggests early, batholith-related, high-temperature metamorphism in the supracrustal rocks. The regional architecture recorded by fabrics within the granitoids and the supracrustal rocks, consists of km-wide, dome-like structures related to polyphased folding.
Hydrothermally-derived rock types consist of a variety of aluminous gneiss, aluminous veins and nodules, and meta-exhalite. The latter form an atypical package of finely-laminated garnetite, garnetiferous hornblendite, Mn or Ba-rich coticule, garnet or clinopyroxene amphibolite to which are associated some epidosite, biotitite, albitite, calc-silicate, marble and anorthite-rich units. Metamorphic assemblages, mode and mineral colour vary according to whole-rock composition. Garnet varies in colour from red to dark pink, to orange, to yellowish in the field, a testimony of their more aluminous, manganiferous or calcic composition respectively, while the mode of zircon and monazite significantly increases in hydrothermally-altered units. A metapelite-looking sillimanite-garnet-cordierite gneiss still preserves lapilli textures at La Romaine, providing evidence for severe pre-metamorphic alkali leaching of a volcanic protolith. This unit, which could have been mistaken for a metapelite (aluminous metasediment), extends sporadically along tens of km structurally below a composite amphibolite unit hosting Cu mineralization. Though field assessment could not resolve the origin of some aluminous gneiss, mineral chemistry indicates a very high Fe and low Ca content of their garnet (almandine 79-83, grossularite 0-0.2), a composition that contrasts with those from normal metapelite. Biotite commonly presents unusually high concentration of Ba, F and Cr compatible with an hydrothermal origin for its host. The use of mineral chemistry to characterize major and trace element anomalies in metamorphic minerals refines protolith identification in high-grade metamorphic terrains and the search for concealed mineral deposits.
Keywords: VHMS hydrothermal system, mineral chemistry, high-grade metamorphism
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