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Platinum-group minerals in chromitites from the Eastern Rhodope ultramafic complex, Bulgaria

Minerale der Platinggruppe in Chromititen des Ultramafit-Komplexes des Ost-Rhodopen Massivs, Bulgarien

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Summary

The podiform chromitites investigated in the course of this study occur in intensely serpentinized dunites and peridotites of unknown age (paleozoic or older) within a metamorphic complex consisting of gneisses, amphibolites and marbles. Concentrations of platinum group elements (PGE) and the distribution of platinum group minerals (PGM) have been investigated in the chromitite occurrences of Dobromirci and Pletene.

PGE concentrations in chromitites vary from 787 to 891 ppb (Dobromirci). The highest value was recorded in chromite ore from Pletene (1274 ppb). The enrichment is due to high contents of Os, Ir and Ru, whereas the contents of Rh, Pt and Pd are relatively low. The Ru-contents (480-600 ppb) are remarkable and correspond to the average content in chondrite Cl. Chondrite-normalized PGE distribution patterns of chromitites of both localities reveal a distinctly negative trend from Ru to Pd, which is typical for chromites from ophiolites.

Irrespective of their chemical composition, most chromites carry numerous PGM inclusions which have formed during the magmatic stage at high sulphur fugacity (fs2). In addition to laurite, the main mineral, there are sulpharsenides of Ru-Ir-Os (ruarsite, irarsite, osarsite).

Textural aspects and the results of chemical analyses show that the concentration of PGE is not caused by substitution in the lattice of chromite, but by magmatic formation of discrete PGM before or contemporaneously with chromite. All PGM apparently remained unaltered. No evidence for remobilization or redistribution of PGE by serpentinization has been found.

Zusammenfassung

Die untersuchten podiformen Chromite tretey in stark serpentinisierten Duniten und Peridotiten unbekannten Alters (paläozoisch oder älter) innerhalb eines hochmetamorphen Komplexes auf, der aus Gneisen, Amphiboliten und Marmoren besteht. In den Chromitit-Vorkommen von Dobromirci und Pletene wurden Konzentrationen der Elemente der Platingruppe (PGE) und die Verteilung der Minerale der Platingruppe (PGM) untersucht.

Die PGE-Konzentration der Chromitite variiert zwischen 787 und 891 ppb (Dobromirci). Die höchste Konzentration wurde im Chromiterz aus Pletene (1274 ppb) gefunden. Die Anreicherung geht auf hohe Beteiligung von Os, Ir und Ru zurück, da die Gehalte an Rh, Pt und Pd relativ niedrig sind. Auffallend hoch sind die Ru-Gehalte (480-600 ppb), die dem mittleren Gehalt im Chondrit Cl entsprechen. Chondritnormalisierte PGE-Verteilungsmuster von Chromititen beider Lokalitäten zeigen einen stark negativen Trend von Ru zu Pd, der für Ophiolith-Chromite typisch ist.

Unabhängig von ihrem Chemismus führen die meisten Chromite zahlreiche PGME-Einschlüsse, die sich magmatisch bei hoher Schwefelfugazität (fS2) gebildet haben. Neben dem Hauptmineral Laurit, wurden Sulfarsenide von Ru-Ir-Os (Ruarsit, Irarsit, Osarsit) festgestellt.

Texturelle Merkmale der PGM und Ergebnisse der chemischen Analysen führen zu der Schlußfolgerung, daß die Konzentration der PGE nicht auf eine Substitution in Chromit, sondern auf die Frühbildung der selbständigen PGM vor oder gleichzeitig mit den Chromiten zurückzuführen ist. Die PGM zeigen keine Alterationserscheinungen. Es wurden keine Hinweise für eine Remobilisation oder Umsetzung der PGE durch Serpentinisierung gefunden.

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Authors and Affiliations

  1. Institute of Mineralogy and Petrology, University of Hamburg, Grindelallee 48, DW-2000, Hamburg 13, Germany

    M. Tarkian

  2. Department of Ore Deposits and Geochemistry, Sofia University, 15 Ruski Blvd., 1000, Sofia, Bulgaria

    E. Naidenova &  M. Zhelyaskova-Panayotova

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  1. M. Tarkian
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  2. E. Naidenova
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  3. M. Zhelyaskova-Panayotova
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Tarkian, M., Naidenova, E. & Zhelyaskova-Panayotova, M. Platinum-group minerals in chromitites from the Eastern Rhodope ultramafic complex, Bulgaria. Mineralogy and Petrology 44, 73–87 (1991). https://doi.org/10.1007/BF01167101

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  • DOI: https://doi.org/10.1007/BF01167101

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