DIAGNOSTIC GEOLOGICAL TOOLS TO RECOGNIZE METAMORPHOSED VHMS, SEDEX AND IRON OXIDES-CU-AU HYDROTHERMAL SYSTEMS IN THE FRONTIER HIGH-GRADE METAMORPHIC TERRAINS OF THE GRENVILLE PROVINCE

Significant magmatic arcs developed along the Grenville Province between Geon 17 to 12. Volcano-plutonic extensional and transtensional arc settings are known to be prospective for ore deposits (e.g., IOCG, VHMS Cu-Au, epithermal Au deposits). However, the magmatic arcs of the Grenville Province have been largely metamorphosed and their volcanic belts and rift zones are now concealed among metamorphosed batholiths and plutons forming large undifferentiated ‘pinkstone belts’. The intense metamorphism, the paucity of geoscience data in these largely uncharted terrains, and the need to adapt the classic Achaean greenstone belt-oriented exploration models to Proterozoic ‘pinkstone belts’ force the development of innovative ways to identify, document and explore prospective area among these frontier terrains. Targeted field-based research within the previously uncharted 1.4 Ga Bondy and 1.5 Ga La Romaine volcano-plutonic complexes unveiled extensive Cu-Au, Fe oxide-bearing hydrothermal systems. The presence of oxide-rich, meta-exhalite- and pelite-like units among undifferentiated gneisses were key steps in initiating a detailed search for protolith information and indicators of pre-metamorphic hydrothermal activity during field work. Evidences for volcanic textures and pre-metamorphic hydrothermally-driven Fe-oxide precipitation in association with sericitization, chloritization, advanced argillic alteration, and B-enrichment were discovered providing means to circumscribe U-Pb dating and protolith studies. To assess if a high mode of Fe-Mg-Al phases resulted from partial melting of metapelites (restitic residue) or metamorphism of hydrothermal rocks, leucosome mineral assemblages, shape and abundance were compared with those expected of in situ partial melting reactions. No compelling evidence was found for a restitic origin. In contrast, the presence of lapillis in garnet-cordierite-sillimanite gneiss constituted conclusive evidence for pre-metamorphic high-sulphidation alteration of a volcanic protolith. Mass balance calculations and Cu and Au mineralisations testify to hydrothermal activity whereas structural studies are unveiling syn-volcanic fault controls on channelling of hydrothermal fluids and metal precipitation.