Behavior of highly-siderophile elements during magma degassing: A case study at the Kudryavy volcano

The capacity of natural vapor phase to transport metallic elements is not unambiguously established relative to that of a liquid hydrothermal phase. We measured highly-siderophile element (HSE) and Au abundances in gas condensates and mineralized rocks in order to examine the geochemical behavior of these elements during magma degassing at the Kudryavy volcano, Kurile Arc. Gas condensates of the Kudryavy volcano are enriched with Re, Os and Au (to 210 ppb Re, 0.907 ppb Os, 2.4 ppb Au, 0.49 ppb Pt, 0.4 ppb Pd, 0.04 ppb Ir, 0.07 ppb Rh, 0.009 ppb Ru). The measured enrichment factors demonstrate that Os is the element that is most strongly compatible with fluid. Fluid compatibility decreases in the sequence: OsNReNAuNPtNPd over the temperature range from 480 to 850 °C. The mobility of HSE and Au in fluid is confirmed by the sublimation of their compounds, amongst which rheniite ReS2 and K perrhenate KReO4, native Pt, Pt–Pd selenide and various Au alloys have been identified with a scanning electron microscope [Nature 369 (1994) 51; Miner. Deposita 40 (2006) 828]. In addition, new HSE compounds, including ReO2, ReO3, Pt(OH)2 and metal-chloro-organic complexes, were detected in the sublimates using X-ray photoelectron spectroscopy. In contrast to the chalcophile behavior of Pb, Re and Os exhibit a dual behavior in the gaseous phase, since both sulfide and oxide phases containing these metals precipitate throughout the entire temperature range. However, available mineralogical, experimental and thermodynamic modeling data indicate that Re and Os are preferentially transported as oxygen-bearing species. Data on metal contents in fumarolic crusts of the volcano confirm that a high-temperature low-density fluid can concentrate these metals to economic grade.Newly obtained data on the Pb and Sm–Nd isotopic composition of volcanic gas condensates and host rocks were correlated with available data on Re and Os abundances and with the Re–Os isotopic composition of the same sample set in order to identify the possible sources of the magmatic melts. The homogeneity of the Pb and Nd isotopic composition of volcanic rocks (206Pb/204Pb: 18.33–18.41, 207Pb/204Pb: 15.52–15.54, 206Pb/204Pb: 38.19–38.24; n=6; 143Nd/144Nd: 0.513067–0.513118; n=5)indicates that the main source of the melts was metasomatised depleted MORB mantle. This is consistent with the relatively low radiogenic 187Os/188Os isotope ratios of younger basaltic andesites and fumarolic gas condensates, but is inconsistent with theradiogenic Os isotope characteristics of the acid volcanic rocks and the high Re abundance in rocks and fluids [Geochem. Cosmochem. Acta 72 (2008) 889].The results of this study suggest that similar elemental and isotope HSE signature can be characteristic of HSE fractionation in other environments of low-density oxidizing fluid stability

Facies Variation in PGE Mineralization in the Central Platreef of the Bushveld Complex, South Africa

AfricaAabsStractThe lateral variation in platinum-group mineral (PGM) assemblages in the magmatic constituents of the Platreef, Bushveld Complex, have been investigated using whole-rock PGE data, scanning electron microscope observations and electron-microprobe data on the PGM. The Platreef comprises a series of mafic–ultramafic sills together with interlayers and xenoliths of metamorphosed sedimentary rocks variably mineralized in PGE, Ni and Cu. In the Central Platreef, the zone with the highest exploitable PGE grade (referred to as PGE reef) occurs at the top of the sequence and is hosted by feldspathic pyroxenite partially replaced by feldspathic harzburgite down-dip. Elevated PGE contents are also related to zones of massive and disseminated chromitites that occur as discontinuous layers within the top reef and at the bottom of the Platreef close to the footwall contact. The harzburgite-hosted reef and chromitites contain PGE mineralization with a high PGE tenor, enriched in Rh and Ru that is taken to indicate proximity to a feeder zone for the Platreef. Textural relationships show that an early succession of laurite ± PGE sulfarsenides ± sperrylite crystallized at a high temperature from a hot magma that was unsaturated with respect to sulfide. Once sulfide saturation was achieved, cooperite and eutectic mixtures of Pt3Fe, Pt sulfide and Fe–Ni–Cu sulfides exsolved first from the sulfide liquid. With decreasing temperature, sperrylite and hollingworthite continued crystallizing across a wide range of conditions from early magmatic to hydrothermal and formed a series of solid solutions and exsolution products. The composition of the PGM assemblages changes along strike within a discrete magmatic layer, and the variation is a function of both the footwall and magmatic rock lithologies. The proportions of Pt–Pd sulfide, Pt–Fe alloy and Rh–Ir–Ru minerals are the most useful in discriminating between different magmatic facies and associated mineralization, whereas Pt–Pd bismuthotellurides are more widespread, and their abundance is dependent on the PGE grade. Early magmatic crystallization of Rh–Ir–Ru-rich PGM distinguishes the PGE profile characteristic of harzburgite and chromitite from the PGE profile typical of the pyroxenite-hosted PGE mineralization that has exsolved from the PGE-rich sulfide liquid