NOBLE-METALS SEGREGATION AND FRACTIONATION IN MAGMATIC ORES FROM RONDA AND BENI-BOUSERA LHERZOLITE MASSIFS (SPAIN, MOROCCO)

Three types of mineralization are found in high-temperature lherzolite massifs of Southern Spain and Northern Morocco: (Cr) chromite, (Cr-Ni) chromite-nickel arsenide, (S-G) sulphide-graphite. The ore veins are distributed in this order from the plagioclase-lherzolite core to the garnet-lherzolite border of the massifs. These hightemperature ore assemblages (1200-600°C) have cumulate textures including orthopyroxene and/or cordierite as main silicate minerals. High average PGE concentrations are present in the Cr-Ni ores (2000 ppb) in relation to the Ni-arsenide abundance. The Cr ores have only 900 ppb PGE, and the S-G ores are PGE-poor (350 ppb). Gold roughly follows the PGE distribution: 13,000 ppb in Cr-Ni ores, 570 ppb in Cr ores, and only 88 ppb in S-G ores. The chondrite normalized PGE patterns of the Cr-Ni ores are chondritic, whereas those of the Cr and S-G ores have respectively negative and positive slopes. The Pd/Ir ratio strongly increases from the Cr ores (0.39) to the Cr-Ni and the S-G ores (2.7 and 3.4)). There are some (Os, Ru)S2 inclusions in the chromite of the Cr ores. In the Cr-Ni ores, some minute Au, Au-Cu, and Au-Bi-Te grains are observed. No PGM have been found, except in a weathered Cr-Ni ore sample where abundant PGM (PtAs2, IrAsS) are present., suggesting that PGE may be hidden as solid solution in the Ni-arsenide. The ore-forming magma probably has a mantle source-rock. The earliest chromites (Cr ores) contain Os-Ir-Ru mineral inclusions, whereas most of the gold and the remaining PGE with higher Pd/Ir ratio were partitioned into an immiscible As-S-liquid, which fractionated later into an earliest PGE-Au-rich NiAs-phase (Cr-Ni ores) and then a PGE-Au-poor MSS-phase (S-G ores). © 1990 Springer-Verlag.