Re–Os and U–Pb constraints on gold mineralisation events in the Meso- to Neoarchaean Storø greenstone belt, Storø, southern West Greenland

Abstract

The Storø greenstone belt in Godthåbsfjord, southern West Greenland consists of juxtaposed rock units of different age and origin, and hosts gold mineralisation that is associated with arsenopyrite along a contact between lithological units and along the axial plane of a large fold core. The age and origin of the gold is debatable, but in this paper, we present new arsenopyrite Re–Os and zircon U–Pb data to constrain the age of the Storø gold deposit. A 2.71±0.05Ga arsenopyrite isochron and 2.707±0.008Ga highly radiogenic arsenopyrite from a mineralisation along a rock contact, together with a 2.64±0.02Ga arsenopyrite isochron along the axial plane of the fold core indicate a two-stage mineralisation process. While the 2.707±0.008Ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event, a mixing origin cannot yet be excluded for the 2.71±0.05Ga isochron. The 2.64±0.02Ga isochron is in perfect agreement with recent U–Pb zircon data (Nutman et al., 2007. Precambrian Research 159, 19–32) and these data are best explained by orogenic mineralisation during amphibolite facies metamorphism along structural weak planes. The initial 187Os/188Os value of 0.56±0.16 for the 2.64±0.02Ga isochron indicates a crustal source for the metals, whereas the initial 187Os/188Os=−0.1±0.6 for the 2.71±0.05Ga isochron remains unconstrained. Re–Os data are best explained by relatively short crustal residence times of less than 0.1Ga, wherein the Os, and associated metals, were extracted from the mantle at a time younger than 2.8Ga, and in which the 2.64±0.02Ga stage formed by mobilisation of an earlier mineralisation. Detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at ≤2.84Ga and that these units were tectonically juxtaposed to ∼3.05Ga rocks within the belt. The units that were deposited ≤2.84Ga record metamorphic zircon U–Pb ages of ∼2.63Ga, but evidence for 2.72Ga metamorphism, which is ubiquitous in adjacent terranes, is lacking.