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The Lower Banded series of the Stillwater Complex, Montana: whole-rock lithophile, chalcophile, and platinum-group element distributions

Barnes Sarah-Jane, Pagé Philippe et Zientek Michael L.. (2020). The Lower Banded series of the Stillwater Complex, Montana: whole-rock lithophile, chalcophile, and platinum-group element distributions. Mineralium Deposita, 55, p. 163-186.

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URL officielle: http://dx.doi.org/doi:10.1007/s00126-019-00887-3

Résumé

The principal rock types of the Lower Banded series of the Stillwater Complex are gabbronorite and norite with minor troctolite and anorthosite. The whole-rock composition is largely controlled by the cumulate minerals plagioclase and pyroxene (±olivine). The rocks are very poor in incompatible elements, at the 0.1 to 2 times mantle levels, indicating < 10% liquid fraction. The Pd and Pt are enriched in a layer known as the J-M Reef which occurs in a zone containing olivine-bearing rocks (OB I). This zone contains the same rock types as the rest of the Lower Banded series, but troctolite, olivine gabbronorite, and anorthosite are more abundant in OB I. The concentrations of lithophile elements in OB I are similar to the rocks above and below OB I. In particular, the reef rocks show no additional signs of continental crust contamination such as additional Th or LREE enrichment, and the reef rocks are not enriched in incompatible elements. The strongly chalcophile elements Cu, Ni, Se, Bi, and Au all correlate with S, indicating that these elements are controlled by sulfides. The PGE show two trends. The rocks below the J-M Reef and the reef rocks have high PGE+Au to S ratios and fall on a single trend, whereas those above the reef have low metal to S ratios. These low ratios for the rocks above the reef could arise because the magma was depleted in PGE+Au as it had already segregated some PGE+Au-rich sulfides. The relatively high PGE, Au, Cu, and Se concentrations of the rocks below the reef imply that the rocks contain ~ 0.1 wt% cumulate sulfides. However, they have very low S contents (< 300 ppm). The low S/Se ratios of these rocks (< 2000) and most of the rocks outside of the reef suggest that they have lost more than half of their S. The Pd/Pt and Pd/Ir ratios of the reef (~ 3.4 and 800, respectively) are much higher than those of the rocks below and above the reef. The high Pd/Pt and Pd/Ir ratios of the reef imply that the sulfides formed from a fractionated magma; however, there is no correlation between the Pd/Ir and Pd/Pt ratios and the host rock type. These ratios are dissimilar to the Bushveld reefs and the Main Sulfide Zone of the Great Dyke. They more closely resemble the reefs in more fractionated rocks such as the AP Reef of the Penikat Intrusion, the Roby Zone of the Lac des Iles Complex, or the disseminated sulfides of the Noril’sk I intrusion. The very high Pd/S and low Cu/Pd ratios of the J-M Reef require that the sulfides collected Pd from a very large volume of magma. None of the current models is entirely satisfactory; however, we favor a model where at depth the OB1 magma partially melted a komatiitic massive sulfide and then transported the sulfide droplets into the magma chamber at the level of the J-M reef.

Type de document:Article publié dans une revue avec comité d'évaluation
Volume:55
Pages:p. 163-186
Version évaluée par les pairs:Oui
Date:Janvier 2020
Sujets:Sciences naturelles et génie > Génie
Sciences naturelles et génie > Sciences appliquées
Sciences naturelles et génie > Sciences naturelles > Sciences de la terre (géologie, géographie)
Département, module, service et unité de recherche:Départements et modules > Département des sciences appliquées > Unité d'enseignement en sciences de la Terre
Mots-clés:Stillwater Complex, Lower Banded series, whole-rock geochemistry, platinum-group elements, chalcophile elements, lithophile elements, J-M Reef, PGE reefs, Complexe Stillwater, série à bandes inférieures, géochimie de la roche entière, éléments du groupe platine, éléments chalcophiles, éléments lithophiles, récifs J-M, récifs PGE
Déposé le:09 juin 2021 16:24
Dernière modification:09 juin 2021 16:25
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