Summary
The PGE mineralized zones referred to as the Ala-Penikka PGE Reefs (AP I and AP II) are located about 250 m and 340 m above the base of megacyclic unit IV in the Penikat layered intrusion, both mineralizations being hosted by plagioclase-augitebronzite and narrow poikilitic plagioclase cumulates. A depression structure (pothole) about 300 m long and 100 m deep is encountered in the area of the AP I Reef, and it is in this structure that the AP I Reef, which is normally 30 cm thick, attains its maximum thickness of 20 m.
The dominant sulphide paragenesis in AP I is pyrrhotite-chalcopyrite-pentlanditepyrite and that in AP II chalcopyrite-pentlandite-pyrite. The platinum-group minerals identified comprise almost thirty species, the most common being Pd-Te-(Bi) and Pd-As-Sb minerals and sperrylite (PtAs2).
The AP Reefs are interpreted as having been formed from an upward-migrating fluid-enriched intercumulus melt in which PGE, S, Ni, Cu and related elements occurred in the fluid phase. The poikilitic plagioclase cumulate in both of the AP Reefs acted as a layer which trapped the upward-migrating intercumulus melt at its lower contact. The depression structure developed when a disturbance of some kind in the magma chamber caused the unconsolidated cumulate layers to collapse.
Zusammenfassung
Die als Ala-Penikka Platin Reefs (AP I und AP 11) bezeichneten vererzten Zonen liegen ungefähr 250 m und 340 m oberhalb der Basis der vierten megazyklischen Einheit in der Penikat-Intrusion. Beide Vererzungen kommen in Plagioklas-Augit-Bronzit und dünnen Bändern poikilitischer Plagioklas-Kumulate vor. Eine Depressions-Struktur (Pothole) von etwa 300 m Länge und 100 m Tiefe kommt im Bereich des AP 1 Reefes vor. Hier erreicht das AP I Reef, das durchschnittlich nur 30 cm mächtig ist, seine maximale Mächtigkeit von 20 m.
Die wichtigste Sulfidparagenese in AP I ist Magnetkies-Kupferkies-Pentlandit-Pyrit, und die in AP II Kupferkies-Pentlandit-Pyrit. An die dreißig verschiedene Platin-Minerale konnten identifiziert werden; die verbreitetsten sind Pd-Te-(Bi) und Pd-As-Sb Minerale, sowie Sperrylit (PtAs2).
Die AP-Reefs haben sich aus einer aufwärts migrierenden Fluid-angereicherten Interkumulusschmelze gebildet, in der PGE, S, Ni, Cu und assoziierte Elemente in der fluiden Phase vorkommen. Das poikilitische Plagioklas-Kumulat in beiden AP Reefs fungierte als eine Barriere die für die aufwärts migrierende Interkumulusschmelze undurchlässig war. Eine Störung in der Magmenkammer, die die noch nicht konsolidierten Kumulat-Lagen betraf, wird für die Entstehung der Depressions-Struktur verantwortlich gemacht.
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Halkoaho, T.A.A., Alapieti, T.T., Lahtinen, J.J. et al. The Ala-Penikka PGE reefs in the Penikat layered intrusion, Northern Finland. Mineralogy and Petrology 42, 23–38 (1990). https://doi.org/10.1007/BF01162682
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DOI: https://doi.org/10.1007/BF01162682