Sulfur saturation and second-stage melts; application to the Bushveld platinum metal deposits

Abstract

A model is presented for the formation of platinum-group element ores by precipitation from sulfur-deficient, platinum-group element-rich, second-stage magmas. Numerous investigations indicate that basaltic magmas originating from undepleted or mildly depleted source regions are sulfur saturated at the time of eruption. Mantle-derived peridotites residual from midocean ridge basalt magma genesis have Pd abundances that are uniformly too high for it to have originated solely from silicate melt retained in the source region at the time of segregation; they imply the presence of an accessory immiscible sulfide melt to host the platinumgroup elements. The very high sulfide melt-silicate melt partition coefficients of the platinumgroup elements would result in strong fractionation of these metals into the accessory sulfide component during the melting event. The extreme platinum-group element enrichment of this accessory sulfide component is evidenced by the discovery ofplatinoid minerals in residual mantle nodules containing Pd (and Pt) abundances at the low ppb level (Keays et al., 1981).Magmas generated by low to moderate degrees of partial melting of undepleted mantle are therefore sulfur saturated at the time of segregation. These (first-stage) magmas become impoverished in platinum-group elements during the early stages of fractional crystallization because of coprecipitation of an immiscible sulfide phase (e.g., midocean ridge basalt glasses average <0. ppb Pd). Further melting of this depleted source leads to complete dissolution of the residual platinum-group element-rich sulfide component and production of secondstage magmas enriched in them but poor in sulfur.Boninitic magmas, generally regarded as second-stage melts generated from strongly depleted mantle sources, have enhanced platinum-group element abundances (mean Pd = 15 ppb) in agreement with the above model. During fractionation of these S-deficient magmas the concentration of platinum-group elements will build up until the onset of sulfur saturation and segregation of sulfides. These sulfides will generally have a higher platinum-group element tenor than those segregating from first-stage melts.A marked similarity in the composition of boninitic magmas and early parental magmas proposed for the Bushveld Complex (Davies et al., 1980;Sharpe, 1981), and possibly other intrusions hosting platinum-group element ores, suggests that their source regions have evolved in a similar manner. This accounts for the timing of deposition of the UG-2 and Merensky cyclic units in the upper partition of the critical zone and the unusual platinum-group elementrich nature of their ores. If this model is relevant to the Bushveld platinum-group element mineralization, then it provides a means of assessing the potential of other stratiform intrusions to host economic platinum-group element deposits.

Chromite and PGE in the Logar Ophiolite Complex, Afghanistan

“…TiO 2 is also sensitive to melt-rock reaction and can fingerprint melt composition 34 . Data for the LOC in Table 3 Magmas producing chromitites similar to those seen at C1 tend to be Sundersaturated, Ti-poor, PGE-enriched, have a high Cr# and are boninitic in composition 14,22,10,37 . Magma of this type is formed from widespread fluid-induced melting of previously melted lithospheric mantle beneath an arc 22 .…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…By contrast, other workers Taylor and McLennan (1985) and Sun and McDonough (1989) Phanerozoic and Archean boninites are from the database on the website: http://georoc.mpch-mainz.gwdg.de/georoc/Entry.html. The field of SHMB is from , Seitz and Keays (1997) and Sensarma et al (2002); the island arc basalts and MORB fields are from Polat et al (2002); the Taishan komatiites and pillow basalts are from Polat et al (2006b). disagreed with this interpretation and believed that the parental magma responsible for the PGE mineralization of the Merensky Reef in the Bushveld Complex was boninitic (Hamlyn and Keays, 1986;Hatton and Sharpe, 1989).…”

“…Some workers believe that most Archean SHMBs were derived from komatiites through assimilation-fractional crystallization (AFC) processes (Barley, 1986;Arndt and Jenner, 1986;Seitz and Keays, 1997;Sensarma et al, 2002;Zhou et al, 2002), since SHMBs are commonly associated with komatiites. However, other workers propose a similar petrogenesis to Phanerozoic boninites based on the fact they are not always closely associated with komatiites, they show no geochemical evidence of significant crustal contamination, and their major element compositions are similar to boninites (Cameron et al, 1979;Redman and Keays, 1985;Hamlyn and Keays, 1986;Hatton and Sharpe, 1989;Srivastava, 2006Srivastava, , 2008. Hence, identification of SHMBs and probing their origin by using geochemical data, particularly radiogenic isotopes, has important implications for understanding their origin.…”

utilisation du rapport S/SE dans les gisements magmatiques à sulfures de Ni-Cu-EGP et ses implications dans la caractérisation des processus pétrogénétiques :

“…The ultramafic cumulates of the Heazlewood River Complex (Tasmania) contains low S concentrations (< 100 ppm) formed by a S-undersaturated magma (Peck and Keays, 1990) derived from a depleted mantle (Hamlyn and Keays, 1986). These rocks were strongly serpentinized and most of the S/Se ratios are located outside the field of igneous rocks (Fig.…”