Mineralogy and chemistry of Cu-Fe-Ni sulfides in orogenic-type spinel peridotite bodies from Ariege (Northeastern Pyrenees,France)

Mantle-derived peridotite bodies of Ariège are composed of spinel lherzolites and harzburgites ranging from remarkably fresh (less than 5% serpentinized) samples with protogranular texture to secondary foliated samples, which are generally 10%–20% serpentinized. The foliated samples have passed through two cycles of deformation and re-crystallization, the earlier ones occurring at temperatures above 950° C for 15 kbar pressure, the later ones at temperatures between 950° and 750° C for 8–15 kbar. Microscopic investigation of 140 samples reveals an accessoy sulfide component which is more abundant in lherzolie than in harzburgite. This component occurs in two differet textural locations, either as inclusions trapped within silicates during the first stage of re-crystallization or as interstitial grains among silicates. Mineralogy and chemistry of both sulfide occurrences are quite similar, at least in samples less than 5% serpentinized. In these fresh samples, sulfides are composed of complex intergrowths between nickel-rich pentlandite and pyrite, coexisting with minor primary pyrrhotite (Fe₇S₈) and chalcopyrite. Pentlandite and pyrite are interpreted as low-temperature breakdown products of upper mantle monosulfide solid solutions. The mineralogy and chemistry of interstitial sulfides in serpentinized rocks vary in parallel with the degree of serpentinization. In samples less than 10% serpentinized, primary pyrrhotite grades into FeS. In samples more than 10% serpentinized, pyrite is replaced by secondary pyrrhotite, and then disappears totally, whereas the coexisting pentlandite is Fe-enriched and replaced by mackinawite. This sequence of alteration indicates a decrease of sulfur fugacity, resulting from serpentinization of olivine at temperatures below 300° C. This is also the case for the inclusions which have been fractured during the tectonic emplacement of the host peridotites within the crust. The presence of non-equilibrium sulfide assemblages in both cases reflects the sluggishness of solid state reactions at near-surface temperatures. It is inferred from these results that sulfides disseminated within orogenic peridotite massifs are so sensitive to serpentinization that most sulfur fugacity estimates based on fractured inclusions and intergranular sulfides are unreliable.