Summary
The Sompujärvi (SJ) PGE Reef is located at the border between the third and fourth megacyclic units, 400–1000 metres above the base of the Penikat layered intrusion. It usually occurs in the basal bronzititic portion of megacyclic unit IV, but can occur in the overlying peridotitic cumulates or the gabbroic cumulates at the top of megacyclic unit III in places. PGE concentrations have been found to occur mainly in association with either base metal sulphide (mainly pyrite-chalcopyrite-pentlandite) or chromite disseminations, the latter type usually representing a distinctly higher grade of mineralisation.
When the magma which formed megacyclic unit IV penetrated into the Penikat layered intrusion, its lower part intermixed with the older residual magma. It may be assumed that this mixing of magmas was responsible for the precipitation and PGE enrichment of sulphides in the lower part of the olivine cumulates, accounting for the sulphide-disseminated mineralization type. The entry of the new magma pulse was probably quite powerful, and when this magma spread out over the uppermost, partly consolidated gabbroic cumulate of megacyclic unit III it caused pronounced local erosion in the uppermost crystal layer and excavated elongated channels and/or depressions, as associated with faulting. These channels and depressions then trapped older residual liquid enriched in platinum-group elements and the chromite-disseminated type within the SJ Reef was formed.
Zusammenfassung
Das Sompujärvi (SJ) PGE Reef liegt im Grenzbereich von Megazyklus III und IV, 400 1000 m oberhalb der Basis der Penikat Intrusion.
Normalerweise tritt es in den liegenden Bronzititen des Megazyklus IV auf; es kann aber sehr wohl auch in den peridotitischen und gabbroiden Kumulaten in den obersten Teilen von Megazyklus 111 entwickelt sein. PGE Anreicherungen sind entweder an Sulfide (hauptsächlich Pyrit-Kupferkies-Pentlandit) oder an disseminierten Chromit geknüpft. Der letztere Vererzungstyp ist durch höhere Gehalte gekennzeichnet.
Während der Intrusion der Magmen des Megazyklus IV kam es zu einer Magmenmischung zwischen diesen und älteren Magmen. Es kann angenommen werden, daß dieses “magma-mixing” für die Ausfällung und Anreicherung der PGE's in den Sulfiden, die in den tieferen Anteilen der Olivin-Kumulate auftreten, verantwortlich zu machen ist und der Grund für diese disseminierte an Sulfide gebundene Vererzung ist. Das Eindirngen mächtiger neuer Magmenschübe und die Ausbreitung dieses Magmas über die obersten, bereits konsolidierten Gabbro-Kumulate des Megazyklus III, verursachten eine lokal intensive Erosion in diesen obersten bereits kristallisierten Lagen. Längliche Spaltensysteme und/oder Einsenkungen, ähnlich wie bei Störungen, wurden angelegt. In diesen Spaltensystemen und Einsenkungen sammelte sich ältere Schmelze, die an Elementen der Platingruppe angereichert war. Sie bildete die an disseminierten Chromit gebundene Vererzung im SJ Reef.
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Halkoaho, T.A.A., Alapieti, T.T. & Lahtinen, J.J. The Sompujärvi PGE reef in the Penikat layered intrusion, northern Finland. Mineralogy and Petrology 42, 39–55 (1990). https://doi.org/10.1007/BF01162683
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DOI: https://doi.org/10.1007/BF01162683