Compositional trends in Fe, Co and Ni sulfarsenides and their crystal-chemical implications: Results from the Arroyo de la Cueva deposits, Ronda Peridotite, southern Spain

Fe-Co-Ni sulfarsenides and diarsenides in a pyrrhotite matrix were found as localized segregations in the Arroyo de la Cueva area, at the northwestern margin of the Ronda Peridotite. The pyrrhotite matrix encloses alloclasite , lollingite and pentlandite, and is cross-cut by veins of chalcopyrite-cub anite intergrowths hosting arsenopyrite, cobaltite and sphalerite, as well as chlorite-magnetite and calcite-talc with minor arsenopyrite and lollingi te. Alloclasite, arsenopyrite, cobaltite, lollingite and pentlandite show s trong variation in their contents of Fe, Co, Ni, As and S. Textural and che mical evidence suggests that the ore initially formed at high temperatures and low fugacities of sulfur. The ore was subsequently metasomatized at con ditions of gradually increasing sulfur fugacity and decreasing temperatures . The variations in Fe, Cc and Ni in alloclasite, cobaltite and pentlandite describe linear trends, These trends were compared with similar trends fro m other, especially mafic-ultramafic associations. They depend on both mine ral association and the requirement that a structure-specific number of non bonding metal d electrons be maintained. Three types of trends are shown by the sulfarsenides. 1) Trends describing Co replacement by a mixture of Fe and Ni are shown by sulfarsenides occurring in metal-rich environments. 2) Trends where Co is replaced by Ni occur in associations containing either p yrite, sulfarsenides or diarsenides indicative of higher anion fugacities. 3) Trends shown by sulfarsenides associated with skutterudite describe repl acement of Ni by a mixture of Co and Fe, The trends defined by pentlandite compositions indicate the substitution Fe + Ni --> 2 Co and support previou sly suggested bonding models.