TEXTURES AND COMPOSITIONAL VARIABILITY IN GERSDORFFITE FROM THE CRESCENCIA Ni (Co U) SHOWING, CENTRAL PYRENEES, SPAIN: PRIMARY DEPOSITION OR RE-EQUILIBRATION?

Abstract

At the Crescencia showing in the Pyrenees, Spain, three stages of mineral deposition can be distinguished: stage I: nickeline and pararammelsbergite, stage II: gersdorffite, and stage III: uraninite. Gersdorffite has been subdivided into seven groups on the basis of textural and compositional criteria. Some of these groups of gersdorffite clearly show disequilibrium processes that may have been induced by secondary reactions associated with a re-equilibration of the system during cooling. Formation and subsequent growth of gersdorffite nuclei on the nickeline (0001) surface, self-organization, or a coupled dissolution-and-reprecipitation moving interface through pararammelsbergite or nickeline are some of the processes invoked during the re-equilibration. Thus, the compositional variations found in the seven groups of gersdorffite are likely due to intermediate steps during the bulk of the replacement and re-equilibration processes at low temperature, rather than a direct precipitation from the ore-forming fluids under different conditions.