Corundum formation by metasomatic reactions in Archean metapelite, SW Greenland: Exploration vectors for ruby deposits within high-grade greenstone belts

摘要

Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Stor?), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO 2 in combination with addition of Al 2 O 3 , MgO, K 2 O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO 2 . The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO 2 allowing corundum to stabilize in the former rocks. Furthermore, addition of Al 2 O 3 via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface. Graphical abstract Display Omitted Highlights ? Corundum (ruby) formation in metapelite juxtaposed to ultramafic rocks. ? Two case-studies are presented from Archean greenstone belts in SW Greenland. ? Amphibolite-facies metamorphism of low-Si high-Al rocks favor corundum stability. ? Spatial proximity to ultramafic rocks is critical for maintaining low-silica conditions. ? Prospecting for ruby in Archean terranes should target high-grade ultramafic rocks.