The San Juan de Plan deposit, in the central Pyrenees of Spain, is situated in an ankeritic horizon in Silurian black shales. The main association consists of pyrite, sulfarsenides, diarsenides, and Co triarsenides (skutterudite), as well as a late stage consisting of cobaltite, Fe, Cu and Bi sulfides and native Bi. The sulfarsenides are arsenopyrite, alloclasite, members of the gersdorffite–cobaltite solid-solution series, arsenic-rich gersdorffite and cobaltite, whereas the diarsenides range from rammelsbergite to safflorite. Rammelsbergite is characterized by a composition within the range (Ni0.71–0.97Co0.02–0.29Fe0.00–0.02)As1.75–2.01S0.00–0.26, exceeding the theoretical value (As1.93S0.07) given by Yund (1962). Two generations of the gersdorffite–cobaltite solid-solution series have been found on the basis of textural features and chemical composition; the first, (Co0.10–0.77Ni0.07–0.80Fe0.03–0.58)As0.95–1.27S0.75–1.06, shows a positive correlation between S and Fe contents, and between Co and As contents, whereas the second one, (Co0.02–0.91Ni0.02–0.96Fe0.00–0.28)As0.99–1.38 S0.67–1.01, exhibits a considerable compositional field in the Fe-poor region of the system CoAsS–NiAsS–FeAsS, providing evidence of extensive substitution of Co by Ni, with a positive correlation between As and Ni contents. Arsenic-rich gersdorffite displays a broad range of As and S mutual substitution, (Ni0.65–0.98Co0.02–0.27Fe0.00–0.06)As1.37–1.83S0.22–0.64, exceeding the experimental value of NiAs1.77S0.23 reported by Yund (1962). The presence of alloclasite and the extent of the solid-solution fields of the cobaltite–gersdorffite series suggest that the main assemblage of minerals at the San Juan de Plan deposit formed at temperatures as high as 600°C.C. Preliminary geothermometric data obtained on graphite support such a high temperature.

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