Two metamorphic gold mineralization events confirmed by Lu-Hf isotope dating of garnet in the Late Archean Storø Au deposit, Nuuk region of SW Greenland

Abstract

We report Lu-Hf isotope ages for garnet from the Late Archean Storø Supracrustal Belt (SSB) ranging from 2697 ± 8 Ma to 2646 ± 13 Ma. These ages essentially overlap with two previously reported Re-Os isotope ages for arsenopyrite associated with Au mineralization within the SSB, which implies that both Au-forming events took place while garnet was stable and thus occurred during amphibolite-facies metamorphic conditions.Furthermore, we report bulk-rock Lu-Hf isotope data for amphibolite and aluminous gneiss from the SSB. The latter rock type has previously been suggested to represent the pre-metamorphic hydrothermal alteration product associated with low temperature epithermal Au formation. However, the initial Hf isotope compositions of these two rock types are significantly different, and therefore demonstrate that they do not share a common origin, because the Lu-Hf isotope system is essentially fluid-immobile. Instead, the aluminous gneiss represents a pelitic rock with mixed mafic and felsic sources, which is in agreement with the formation of both Au mineralization events during orogenic amphibolite-facies metamorphic processes, which were therefore structurally controlled.A too young bulk-rock errorchron age of 2553 ± 72 Ma for the amphibolite unit (>2720 Ma) is interpreted to reflect resetting of the system during the intrusion and melt infiltration of the ca. 2560 Ma Qôrqut Granite Complex.