Abstract

The alpine lherzolite massifs of southern Spain (Serrania de Ronda) and northern Morocco (Beni Bousera) are mantle intrusions emplaced at high temperature in the continental crust. Three types of mineralization occur in the lherzolites: (1) chromite-Ni arsenide ores (Cr-Ni ores) with orthopyroxene and/or cordieritc, (2) chromite ores (Cr ores) with clinopyroxene and/or orthopyroxene, (3) Ni-Fe-(Cu) sulfide-graphite ores (S-G ores) with chromite, plagio-clase, and phlogopite. The chromite ores are located in the plagioclase lherzolite core of lherzolite massifs, whereas the two other ore types are distributed in the outer spinel and garnet lherzolite zones.Chromite and Cr-Ni ores show primary magmatic textures, which are mostly overprinted by secondary high-temperature plastic deformation, and a compositional trend of chromite and orthopyroxene that corresponds fairly well to a magmatic Mg-Fe fractionation. Chromite also displays an unusual Zn and V enrichment. High gold contents (3-35 ppm) and up to chondritic platinum-group element values are found in the Ni arsenide-rich ores. S-G ores comprise a pyrrhotite-pentlandite-(chalcopyrite-cubanite)-graphite association which also includes Fe-rich chromites with high Zn and V contents; their platinum-group element and Au contents are much lower than those of Cr and Cr-Ni ores (<0.5 ppm platinum-group elements and <4 ppm Au).The proposed metallogenic model comprises (1) crystallization of chromite and pyroxene between 1,500 degrees and 950 degrees C; (2) segregation of an immiscible As-S-rich liquid (following the hypothesis of Oen, 1973) that accumulated downward and started crystallization between 800 degrees and 400 degrees C, with the formation of an arsenide matrix among chromite grains; and (3) later crystallization of an S-rich residual liquid in graphite-rich assemblages. During this magmatic evolution Os-Ir-Ru minerals were included in chromite, whereas Au and Rh-Pt-Pd were concentrated in the As-S-rich liquid and precipitated with the Ni arsenides. The residual S-rich liquid was therefore depleted in platinum-group elements and Au. This model implies a lherzolite source for the ore-forming elements and the fractionation of a magmatic liquid migrating at moderate- to low-pressure conditions from the core to the borders of the rising lherzolite body.

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