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,
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.
VHMS hydrothermal system, mineral chemistry, high-grade metamorphism