SCHEELITE-POWELLITE AND PARANIITE-(Y) FROM THE FE-MN DEPOSIT AT FIANEL, EASTERN SWISS ALPS

Small syngenetic exhalative Fe-Mn deposits embedded in Triassic marble s of the Suretta, Starlera, and Schams nappes (Eastern Swiss Alps) wer e subjected to a Tertiary regional metamorphism under blueschist- to g reenschist-facies conditions. In one of the deposits (at Fianel, Val F errera), this polyphase metamorphism led to the formation of quartz+do lomite veinlets containing beryl, scheelite-powellite, paraniite-(Y), monazite(Ce), fluorapatite, bergslagite, fluor-romeite, and antimonian betafite; these veinlets crosscut dolomite breccia lenses that are em bedded in hematite-quartz-carbonate ores. Scheelite-powellite displays a continuous range of compositions between 28 and 70 mol% CaMoO4; its As2O5 contents range from 0.73 to 3.96 wt%, and are positively correl ated with the Y2O3 contents that vary between 0.33 and 2.37 wt%. The s cheelite-powellite grains display a two-stage chemical zoning: stage A generally produced a core and a rim that, relative to the core, is ri cher in W, As, and Y. During the second stage (stage B), W-rich scheel ite-powellite replaces stage A grains along fractures and rims. Crysta ls of paraniite-(Y), ideally (CaWO4)(2). YAsO4, occur as small inclusi ons (=1 mu m) in stage-B scheelite-powellite. The Fianel deposit is on ly the second locality where paraniite-(Y) has been reported. The para niite-(Y) from Fianel displays, like the type material, no polysomatic stacking fault in the scheelite-YAsO4 layering. At Fianel, paraniite- (Y) is characterized by elevated Mo contents, and seems to have crysta llized under influence of W- and LREE-rich fluids during stage B, i.e. , during the metasomatic replacement of Y- and As-rich scheelite-powel lite produced in stage A.