Anders Scherstén

The Storo greenstone belt in Godthabsfjord, 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 Storo gold deposit. A 2.71 +/- 0.05 Ga arsenopyrite isochron and 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite from a mineralisation along a rock contact, together with a 2.64 +/- 0.02 Ga arsenopyrite isochron along the axial plane of the fold core indicate a two-stage mineralisation process. While the 2.707 +/- 0.008 Ga highly radiogenic arsenopyrite provides firm support for an early mineralisation event, a mixing origin cannot yet be excluded for the 2.71 +/- 0.05 Ga isochron. The 2.64 +/- 0.02 Ga 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 Os-187/Os-188 value of 0.56 +/- 0.16 for the 2.64 +/- 0.02 Ga isochron indicates a crustal source for the metals, whereas the initial Os-187/Os-188 = -0.1 +/- 0.6 for the 2.71 +/- 0.05 Ga isochron remains unconstrained. Re-Os data are best explained by relatively short crustal residence times of less than 0.1 Ga, wherein the Os, and associated metals, were extracted from the mantle at a time younger than 2.8 Ga, and in which the 2.64 +/- 0.02 Ga stage formed by mobilisation of an earlier mineralisation. Detrital zircon constraints imply volcanism and sediment deposition for parts of the belt at <= 2.84 Ga and that these units were tectonically juxtaposed to similar to 3.05 Ga rocks within the belt. The units that were deposited <= 2.84 Ga record metamorphic zircon U-Pb ages of similar to 2.63 Ga, but evidence for 2.72 Ga metamorphism, which is ubiquitous in adjacent terranes, is lacking. (C) 2012 Elsevier B.V. All rights reserved.