Meso- and Neoarchaean geological history of the Bjørnesund and Ravns Storø Supracrustal Belts, southern West Greenland: Settings for gold enrichment and corundum formation

• Geological history of the Bjørnesund region, southern West Greenland (c. 62–63° N).
• Locus of gold enrichment defined by competence contrast during shearing folding.
• Gem-corundum result of reaction anorthite + ultramafic rock triggered by pegmatite.
• Connection between Fiskenæsset complex and Ravns Storø supracrustal belt.
• New metamorphic petrology and zircon U–Pb LA-ICP-MS geochronology data.

To understand the Meso- and Neoarchaean geological history of the Bjørnesund region, southern West Greenland (ca. 62–63° N), we have undertaken an integrated study combining field observations with U–Pb zircon geochronology by LA-ICP-MS, metamorphic petrology, and microstructural analyses. The Bjørnesund region links the southern part of the Fiskenæsset anorthosite complex to the well-preserved metavolcanic rocks of the Ravn Storø Supracrustal Belt and the Bjørnesund Supracrustal Belt, so the development of this region is of key interest to unravelling the Archaean geology of the entire region. In this study, we couple the observed phases of gold-enrichment and corundum formation to the geological history.The leucogabbroic rocks of the Bjørnesund region, which are part of the Fiskenæsset complex, intruded at ca. 2.95 Ga into the tholeiitic and calc-alkaline metavolcanic rocks of the two studied belts. Following isoclinal folding, the rocks were intruded by diorite (2.92 Ga) and granodiorite (2.91 and 2.88 Ga), which are now texturally orthogneisses. The area was affected by three major phases of folding; F1 (undated), F2 at 2.85–2.83 Ga, and F3 at 2.71–2.70 Ga. The former deformation event is associated with the gold-enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss. Intrusion of granitic sheets during the F3 deformation triggered the formation of corundum, which is locally of gem quality, at the contact between leucogabbroic and ultramafic rocks. Peak metamorphic conditions were calculated as 580–630 °C and 6–4 kbar at 2.85–2.83 Ga. Later retrogression in the southern part of the area at ca. 500–450 °C and 4.5–3 kbar, and extensional fault reactivation overprinted some of the older structures.