Summary
Fourteen different platinum-group minerals (PGM) have been found in massive chromitite from the Osthammeren ultramafic tectonite (serpentinite) body, Norway. The PGM-inclusions occur in two distinctly different textural groups.
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1.
Os, Ir, Ru and minor Pt occur in euhedral-subhedral, small (< 5–20 , µm), mainly single-phase inclusions of Os-free laurite, Os-laurite, osmiridium and Pt2 (Ir, Os)Fe0,65. They are totally enclosed in fresh chromite. A few PGM-inclusions are however associated with small blebs of Na-bearing hornblende and phlogopite, included in the chromite, indicating that the presence of volatiles at an early stage (i.e. preceding chromite crystallization) may have influenced the formation of these PGM. The occurrence of PGM within unaltered chromite suggests a primary magmatic origin of these.
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2.
Purely secondary PGM occur as anhedral-(subhedral), texturally often very complex grains or grain-aggregates of varying size (5–70 µm) and consisting of from one to eight PGM-phases plus Ni-sulphide and Ni-arsenide. These PGM always occur within, or in contact with, late formed cataclastic cracks or very fine fissures, in the primary magmatic chromite grains. The finer fissures are generally tightened by ferrit-chromite, a hydrothermal alteration product of chromite. The wider cracks are usually serpentineand chlorite-filled and are rimmed by ferrit-chromite against the chromite. Os, Ir, Ru, Rh, Pt and minor Pd are represented in grains of this group, and the secondary PGM—association found consists of Os-free laurite, Os-laurite, erlichmanite, Ir-rich erlichmanite, native Os, iridosmine, osarsite, irarsite, hollingworthite, Rh-rich platarsite (?), Ru-rich platarsite, sperrylite, (Ir, Rh)SbS, IrSbS, (Ir, Pt, Pb)S2 (new?), Pd-antimonide (probably stibiopalladinite) plus the associated phases pentlandite, heazlewoodite and niccolite.
Zusammenfassung
14 verschiedene Platingruppen-Minerale (PGM) wurden in einem massiven Chromitit aus dem ultramafischen Osthammeren Tektonitkörper (Serpentinit), Norwegen, gefunden. Zwei texturell verschiedene Gruppen von PGM-Einschlüssen treten auf.
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1.
Os, Ir, Ru und untergeordnet Pt kommen als eu-bis subhedrale, kleine ( < 5–20 , µm) hauptsächlich einphasige Einschlüsse in Form von Os-freiem Laurit, Os-Laurit, Osmiridium und Pt2 (Ir, Os) Fe0.65, vor. Sie sind alle in frischem Chromit eingeschlossen. Einige PGM-Einschlüsse sind jedoch mit kleinen Körnern von Na-führender Hornblende und Phlogopit, die ebenfalls als Einschlüsse im Chromit vorliegen, vergesellschaftet. Dies weist darauf hin, daß die Anwesenheit von Volatilen in einer Frühphase (d.h. vor der Chromitkristallisation) die Bildung dieser PGM's beeinflußt haben könnte. Das Auftreten von PGM in nicht alteriertem Chromit legt einen primär magmatischen Ursprung nahe.
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2.
Selten treten sekundäre PGM's in Form von anhedral (-subhedralen), texturell oft komplexen Körnern oder Kornaggregaten verschiedenster Größe (5–70 ,µm), auf. Sie bestehen aus bis zu 8 PGM-Phasen, sowie Ni-Sulfiden und Ni-Arseniden. Diese PGM's kommen immer in oder in Kontakt mit spät gebildeten kataklastischen Rissen und schmalen Gängchen in primären magmatischen Chromitkörnern vor. Die schmalen Gängchen werden generell von Ferritchromit, einem hydrothermalen Alterationsprodukt des Chromits, augefüllt. Die breiten Risse sind gewöhnlich mit Serpentin und Chlorit gefüllt; Os, Ir, Ru, Rh, Pt und untergeordnet Pd sind die in dieser Gruppe vertretenen Elemente. Os-freier Laurit, Os-Laurit, Erlichmannit, gediegen Os, Iridosmium, Osarsit, Irarsit, Hollingworthit, Rh-reicher Platarsit (?), Ru-reicher Platarsit, Sperrylith, (Ir, Rh)SbS, IrSbS, (Ir, Pb, Pb)SZ (neu?), Pd-Antimonit (wahrscheinlich Stibiopalladinit), sowie die Phasen Pentlandit, Heazlewoodit und Niccolit sind die Minerale dieser sekundären PGM-Assoziation.
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Nilsson, L.P. Platinum-group mineral inclusions in chromitite from the Osthammeren ultramafic tectonite body, South Central Norway. Mineralogy and Petrology 42, 249–263 (1990). https://doi.org/10.1007/BF01162694
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DOI: https://doi.org/10.1007/BF01162694