Occurrence of LREE- and Y-arsenates from a Fe-Mn deposit, Ligurian Brianconnais Domain, Maritime Alps, Italy

We report the occurrence of some rare and unknown LREE- and Y-arsenates. Th ey are present within the Ponte dei Gorrazzi Fe-Mn deposit, associated with quartz arenites and subordinately with marbles of the Corsaglia Valley seq uence, in the Ligurian Brianconnais Domain, Maritime Alps, Italy. These min erals mainly occur within microveins and microcavities as microcrystalline aggregates or small isolated prismatic crystals (up to 10 mu m in length). Three different groups have been distinguished on the basis of electron-mic roprobe data. 1) Group A is characterized by Y-rich grains with low LREE co ntents whose composition is consistent with the formula of chernovite (YAsO 4). 2) Group B is characterized by LREE-rich grains (mainly La), with very low Y contents; their chemical composition does not correspond to any known natural phase, but it is consistent with that of the synthetic compound La AsO4. 3) Group-C grains are characterized by intermediate contents of Y and the LREE. Although they could be related to micrometer-scale intergrowths of Group A and B minerals, they are characterized by significant enrichment s in Gd, Th and Ca, thus suggesting the possibility of a distinct mineral s pecies.