Platinum group element behaviour and thermochemical constraints in the ultrabasic–basic complex of the Vizcaino Peninsula, Baja California Sur, Mexico

Abstract

The Vizcaino complex is characterized by tectonised harzburgites overlain by serpentinized dunite bodies and wherlites layers containing several cumulate chromitite deposits and by fractional crystallization sequences: cyclic repetititons of dunites, pyroxenites with pegmatoidal textures, and gabbros overlain by a succession of gabbronorites to diorites in which olivine is followed by pyroxene and by plagioclase. Enriched sulfide horizons generally associated with pegmatoidal rocks are lacking. The fractionation of platinum-group element (PGE) displays evidence of erratic dispersion throughout the intrusion. PGE patterns of chromitites show high Ir and Ru contents, relatively high Rh content and an unusual Pt–Pd depletion lower than in ophiolitic chromitites. These chromitites do not contain laurite-type sulfides, which are commonly observed in association with Os–Ir–Ru alloys. Dunites show relatively flat patterns and gabbros present typical trends with depletions in the Iridium-group (IPGE) relative to PPGE. Ru, Rh, and especially Ir are very low in these silicate rocks, indicating the precipitation of these elements during the earliest stages of fractional crystallization and their concentration in chromitites. Spilitized dolerite dykes from the metamorphic basement show significant PGE enrichments similar to those of gabbros. Based on our previous thermochemical data, an extremely low f S 2 (<10 −2 atm) was in equilibrium with the silicate melt as indicated by the absence of laurite included in chromitites. In the absence of complexation of Pt (+Pd) by sulfur in the early stage of magmatic differentiation, a complexation of these elements by Cl as chloride complexes in a Cl-rich fluid phase may be expected. This hypothesis is reinforced by the notable absence of any sulfide horizon throughout the intrusion. Part of precious metals (Pt+Pd+Au) may have been remobilized at low temperature and transported as chloride or hydroxide complexes by fluid phases introduced during low-grade metamorphic processes or later during deformation events. In this interpretation, the erratic behaviour of PGE at Vizcaino was partially controlled by solubilities of Pt and Pd as chloride complexes at magmatic temperatures as early as chromite crystallization and the control of local saturation of the magma by variation of f S 2 was overshadowed.