Aluminous gneiss derived by weathering of basaltic source rocks in the Neoarchean Storø Supracrustal Belt, southern West Greenland

Abstract The origin of amphibolite-facies aluminous gneiss from the gold-hosting Neoarchean Storo Supracrustal Belt in the Nuuk region of southern West Greenland is investigated in this study. An improved understanding of the formation of such aluminous gneiss has implications for genetic models (epithermal vs. orogenic style) for a local gold occurrence, which is hosted by sheeted quartz-veins within amphibolite in the hanging wall adjacent to the aluminous gneiss on the island of Storo. The aluminous gneiss mainly consists of garnet, plagioclase, sillimanite, quartz and biotite, which suggest a pelitic protolith. However, it has previously been postulated that the aluminous gneiss represents a hydrothermal alteration product, formed by leaching of a mafic precursor that was subsequently transformed to the current mineral assemblage during later regional metamorphism. In support of this hypothesis are decimeter-scale relicts of amphibolite, found within the decameter-wide aluminous gneiss, that share similar ratios of commonly fluid immobile elements, such as Al, Ti, Zr, Hf, Nb and Lu. Metasedimentary rocks sensu stricto are also present within the Storo Supracrustal Belt. These mostly comprise biotite schist with a similar mineralogy as the aluminous gneiss, and are located adjacent to the latter in the footwall. The aluminous gneiss has high bulk-rock Al 2 O 3 contents and low SiO 2 relative to the biotite schist, which is less aluminous, has higher SiO 2 , and is more potassic than the aluminous gneiss. The immobile element ratios of the biotite schist are significantly different from those of the amphibolite and the aluminous gneiss. Additionally, the biotite schist yields distinct detrital zircon age populations, whereas only metamorphic zircon has been found in the aluminous gneiss and amphibolite. In the present study, the isocon method was applied to provide mass balance constraints on the alteration of basalt to the protolith of the aluminous gneiss prior to regional metamorphism of the entire Storo supracrustal sequence. Accepting minor fractionation among otherwise immobile elements, an error of at least ± 20% is estimated for the isocon mass-balance model. The results indicate that all major elements, except for K 2 O were leached from the basaltic precursor, resulting in a net mass loss ranging from − 20 to − 40 wt.%. However, despite the similar trace element patterns and ratios of the aluminous gneiss and the adjacent amphibolite, in situ alteration or weathering of the basaltic precursor rock is not possible given the small, but persistent, fractionation among fluid immobile elements, due to the unusual accumulation of Cr, U, Ni and Th, as well as the observed modal layering within the aluminous gneiss. Instead, this points to physical transport during sedimentary reworking of a mafic protolith and potentially the addition of redox-sensitive elements such as U and Cr from the water column, and therefore suggest that this aluminous gneiss simply represents a metasediment with a mafic provenance. The implication of this study for gold exploration within the Archean supracrustal belts of the SW Greenland is that aluminous gneiss is unlikely to represent an indicator of acidic hydrothermal alteration as previously postulated. Instead, aluminous gneisses within these supracrustal belts are likely of sedimentary origin and may provide a venue to further understand the exogenous environments of the Archean Earth, and thus further geochemical studies of such rocks are recommended in order to place constraints on the composition of the hydrosphere at that time.