Simple deposition versus replacement and re-equilibration at the Crescencia Ni-(Co-U) deposit (Central Pyrenees, Spain)

The mineral assemblage of the Crescencia mine contains Ni sulfarsenides, arsenides and diarsenides, uraninite and pyrite. Three stages of mineral deposition can be distinguished: Stage I: nickeline, gersdorffite I, and rammelsbergite; Stage II: gersdorffite II; Stage III: uraninite. Gersdorffite II has been subdivided into six groups on the basis of textural and compositional criteria. Some of these gersdorffite types are clearly pertained to disequilibrium processes. Gersdorffite B, C, E and F represent intermediate steps during the nickeline replacement and progressive re-equilibration with residual fluids, representing by gersdorffite D. Thus, compositional variations found in gersdorffite are the result of nickeline replacement and reequilibration processes rather than a direct precipitation under different conditions of the ore-forming fluids.