The Aguablanca Ni–Cu–PGE deposit, Southwestern Iberia: Magmatic ore-forming processes and retrograde evolution

Abstract

The Aguablanca Ni–Cu–PGE ore deposit, in southwestern Iberia, has a magmatic origin, and has been reworked by later skarn-related hydrothermal activity. Base-metal sulfides are associated with igneous cumulates displaying subvertical layering consisting of gabbro, norite and dolerite, with fragments of pyroxenite and peridotite. These rocks show a pervasive retrograde alteration with an early overprint of actinolite ± chlorite ± epidote ± albite ± serpentine, followed by later growth of talc ± chlorite ± carbonates. The base-metal sulfides are concentrated in two subvertical bodies and consist of pyrrhotite, pentlandite and chalcopyrite, overprinted by fluid-deposited pyrite. The following types of mineralization have been recognized: 1) disseminated ore (1 < Ni/Cu < 1.5), with sulfides occurring interstitially to the igneous silicates, 2) semimassive to massive ore (2 < Ni/Cu < 5), with net-textured to poikilitic sulfides, 3) breccia ore, consisting of fragments of unmineralized ultramafic rocks in a massive sulfide matrix, 4) nodules of sulfides, 5) pyrrhotite veinlets, and 6) chalcopyrite veinlets. The PGE content and (Pt + Pd):(Ru + Ir + Os) ratio have their highest values in the Cu-rich disseminated ore and associated veinlets of chalcopyrite. The platinum-group minerals (PGM) associated with the base-metal sulfides at Aguablanca include michenerite, merenskyite, moncheite, palladian melonite and sperrylite. These mostly occur enclosed within sulfide minerals or close to contacts between them. The PGE tellurides show an extensive substitution of Bi for Te, indicating temperatures of formation below 500°C. A PGE oxide assemblage has also been discovered in samples from the surface gossan developed above the sulfides. The composition of the PGM and their association with magmatic base-metal sulfides suggest that they exsolved from the sulfides during subsolidus recrystallization, although some remobilization may have occurred in areas of pervasive circulation of fluid.