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The ruby and pink sapphire deposits of SW Greenland : geological setting, genesis, and exploration techniques Fagan, Andrew Jeffrey

Abstract

The Aappaluttoq, Siggartartulik, and Kigutilik gem corundum deposits are hosted by the Fiskenæsset Anorthosite Complex in SW Greenland. These deposits are some of the largest ruby and sapphire deposits in the world yet have received little scientific attention. During regional granulite metamorphism (2817 Ma; U-Pb EMPA monazite), leucogabbroic rocks alter towards more Na-enriched compositions causing Ca-feldspar to recrystallize. This liberates the Al required for metamorphic opaque corundum formation in an orthogneissic host. Subsequent retrogression through the amphibolite facies (2747-2555 Ma, U-Pb EMPA monazite) and local scale hydrous metasomatism (2660 Ma, U-Pb EMPA monazite) causes recrystallization of the mineral assemblage and the formation of three distinct gem corundum bearing lithologies: 1) phlogopitite, 2) altered leucogabbro, 3) altered ultramafic rock containing sapphirine and gedrite-anthophyllite. Intrusion of local granitoids (2747-2714 Ma, U-Pb TIMS zircon) provided a hydrous fluid which infiltrated and metasomatised the regional assemblage and was channeled along preexisting lithological boundaries (leucogabbro-ultramafic, leucogabbro-melanogabbro). The primary host for gem corundum is the 2-8 meter wide phlogopitite unit formed at 700 to 815°C and 5 to 8 kbar. This unit contains up to 20% corundum, of which at least 5% is gem quality. The phlogopite represents the quenched granitic-metasomatic fluid; EMPA and LA-ICP-MS analysis confirm a fluid composition depleted in S, Cu, Ca, Pr, Nd, Sm, Te, Cs, Ba, Se and transition metals, and enriched in Rb, Sr, Cr, and the HREEs. This is consistent with a granitic source rich in Si-K-Mg-Rb-Ba-Cs-Nb-Ta-Ga-H₂O±Cr±U±CO₂. The other two corundum-bearing units show similar variations except are typically more depleted in Cu, Te, S, Rb, and Sr. Corundum δ18O analyses define a tight range between 1.49‰ and 5.3‰, with a mean of 3.2 ± 0.9‰. This suggests the interaction or mixing of meteoric water with the metasomatic fluid at some point during gem recrystallization. Worldwide ultramafic-amphibolitic gem corundum deposits show similar low δ18O, mineralogy, mineral chemistry, and local geology. This study proposes the use of amphibolite-type as an all-encompassing gem deposit model to explain several enigmatic deposits elsewhere in the world.

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