Low-temperature, platinum-group elements-bearing Ni arsenide assemblage from the Atrevida mine (Catalonian Coastal Ranges, NE Spain)

Abstract

The Atrevida vein is located at the south-western edge of the Catalonian Coastal Ranges, Spain. This vein extends for more than 3 km and is hosted by Silurian and Carboniferous metasediments, late Hercynian granites and continental conglomerates of Lower Triassic age. The Silurian metasediments contain massive sulphide layers with disseminations of platinum-group element (PGE)-bearing, Fe-Ni arsenides. Vein fi lling largely consists of banded and brecciated baryte rimmed by a complex polymetallic assemblage of Co-Ni-As-Bi-Ag(-U) ores. The Ni arsenide and sulpharsenide assemblage occurs only where the vein cuts the sulphide-bearing Silurian metasediments, and consists of nickeline, maucherite, rammelsbergite, gersdorffi te and Ni-skutterudite. These Ni arsenides and sulpharsenides display rosettes, spherules, colloform and botryoidal textures, as well as subhedral crystals. The chemical compositions of these arsenide-sulpharsenide minerals exhibit limited cation substitution but substantial anion substitution of As by S and Sb, exceeding the expected theoretical values at low temperatures. This suggests that most of them formed under disequilibrium conditions. Ni arsenides and sulpharsenides also contain trace PGE. Palladium (up to 157 ppm) is the most abundant PGE, and occurs in solid-solution in all the phases studied, but shows the highest values in rammelsbergite and Niskutterudite. Pt only occurs in massive nickeline (54–1338 ppm Pt) and in zoned gersdorffi te (29–360 ppm Pt) formed at the end of the depositional sequence of arsenides. The Ni arsenide and sulpharsenide ores at the Atrevida mine formed by reduction of PGE-bearing, oxidizing fl uids at temperatures close to, or slightly above 100 °C. These oxidizing fl uids most probably collected their PGE budget from the nearby mineralized black shales. During the crystallization of the veined, Ni arsenide and sulpharsenide ores, Pd partly concentrated in the early nickeline and maucherite, but mainly fractionated to the late solutions from which rammelsbergite and Ni-skutterudite precipitated. Pt concentrated only in the latest nickeline and gersdorffi te.