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MoG Geoscience vol. 28 1992              Order this book on-line

Hans Pauly
Topaz, prosopite and closing stages of formation of the Ivigtut cryolite deposit, South Greenland.
22 pp, 20 figs. 66 DKK

The evolution of the cryolite deposit at Ivigtut is divided into three stages. This work deals with the last two.
Siderite-cryolite (2.500.000 tons) followed by the body of pure, white cryolite accompanied by chiolite (500.000 tons) crystallized in Stage 1. An abrupt change in conditions led to Stage 2 in which the fluorite zone and the fluorite-cryolite (1.000.000 tons) formed. Finally, in Stage 3, fluids left from Stage 2 in fissures and cavities, dissolving cryolite gave rise to thomsenolite and other secondary minerals, (80.000 tons).
The cruptocrystalline topaz, emplaced in breccias and vein breccias with fluidized emplacement and penetrating cryolite along, unusual, cleavage planes, indicates that the transition from Stage 1 to Stage 2 was marked by extensive gas-driven explosions related to a Ca-F-rich residue. During these events colloform fluorite precipitated and formed with topaz the fluorite zone in the deeper part of the deposit; above, mechanically admixed cryolite gave rise to the fluorite-cryolite.
Concentrations of the minor elements Mg, K and Sr-Ba resulted in formation of weberite, spherulitic K-mica, jarlite and other Sr-Ba resulted in the last part of Stage 2. Chiolite broke down but cryolite remained stable during most of Stage 2.
Fluids left after Stage 2 dissolved cryolite and thomsenolite, ralstonite and other secondary Ca-fluorides formed. In an early phase of Stage 3 limited explosions secondary Ca-fluorides formed. In an early phase of Stage 3 limited explosions precipitated prosopite in fluidized emplaced vein breccias similar to those carrying topaz. These events also led to the formation of the peculiar stalactitic or agate-like fluorite where the explosions hit cavities with solutions rich in Ca.