Mesoarchean Epithermal Gold Mineralization Preserved at Upper Amphibolite-Facies Grade, Qussuk, Southern West Greenland

摘要

The Qussuk gold occurrence, located in southern West Greenland (North Atlantic Craton), is a flattened, transposed, and metamorphosed, 20 km long Au ± Cu system interpreted as an epithermal mineralization system of Mesoarchean age. The Qussuk mineralization is hosted in the 3.075 Ga volcanosedimentary rocks of a relict andesitic arc. The volcanic arc component is embedded in 3.06 to 3.0 Ga plutonic, predominantly tonalitic rocks and was gradually deformed and metamorphosed as the original arc matured into a microcontinent. Metamorphism culminated with a sillimanite-grade amphibolites-facies thermal event and mobilization of granite at around 2.995 to 2.975 Ga. The premetamorphic hydrothermal alteration associated with the mineralization includes acid leaching revealed by quartz-alumina rocks with very high rare earth element (REE) contents and Ga-Al fractionation, and subsequent reintroduction of LIL elements including K, Th, and U together with the mineralizing agents. Premetamorphic carbonate alteration is widespread but unrelated to the mineralized system. Complexly zoned zircon with igneous-hydrothermal-metamorphic histories and metamorphic monazite bracket the mineralizing event to 3.075 to 3.02 Ga (most likely >3.06 Ga) in age. The zoned zircons have volcanic/igneous, 3.075 Ga oscillatory zoned cores with igneous-type REE distributions, <3.07 Ga hydrothermal spongy growth phases, diverse REE distributions and abundant inclusions of quartz and biotite, and homogeneous metamorphic, 2.995 Ga rims. Geochronological, petrographical, and geochemical evidence demonstrates that the gold mineralization at Qussuk was formed prior to peak deformation and metamorphism. The Qussuk gold occurrence illustrates that epithermal gold deposits existed at the Archean, and that they can be preserved up to upper amphibolite metamorphic conditions, further highlighting the prospectivity of ancient high-grade cratons and volcanic belts worldwide for primary gold deposits.