Platinum group element,mineralisation in the Munni Munni Complex,western Australia

Summary The late Archaean Munni Munni Complex occupies an elliptical area of 9 by 25 km, the southern half of which is covered unconformably by a 2.7 Ga volcanic sequence. The Complex consists of a lower 1850 m thick Ultramafic Series (UMS) and an upper Gabbroic Series (GS) at least 3600 m thick, and is in the form of an elongate funnel. The UMS is made up of macrorhythmic cycles of dunite, wehrlite and clinopyroxenite, while the GS shows an uninterrupted fractionation trend from pigeonite gabbros through pigeonite-magnetite gabbros to granophyres. The base of the GS is very sharp, and marked by simultaneous appearance of cumulus plagioclase and pigeonite. GS cumulates show a monotonous upward increase in Fe/Mg and an absence of cyclic layering, indicating crystallization in a closed chamber.The top of the UMS is a distinctive 30 m thick layer of bronzite-porphyritic orthocumulate websterite, which continues up the side walls as a marginal zone in contact with progressively more fractionated gabbros. A pyroxenite dyke intersects the sloping floor of the intrusion at a level close to the top of the UMS, and appears to have fed the uppermost layers of the UMS.Cu-rich magmatic sulphides are weakly disseminated throughout the porphyritic websterite layer, increasing in abundance to 1–3% in a semi-continuous augite orthocumulate layer a few metres below the gabbro. This layer extends over 8.2 km, averages 2.5 m in thickness, and has an average grade of 2.9 g/t Pt + Pd + Au, 0.2% Ni and 0.3% Cu with local higher grade zones. In about 40% of intersections, peak PGE, Au, Cu and Ni grades are coincident, while in the remainder peak PGE grades are offset about 1–2 m below the peak Cu and Ni grades.Coincident intersections are probably derived by homogenization of original offset intersections. Peak PGE grades become lower and more widely dispersed farther away from the intrusion walls.PGE-enriched sulphides also occur close to the websterite-gabbro contact where the websterite occupies a marginal position on the side wall. The marginal websterite zone and the porphyritic websterite layer are physically contiguous and petrographically similar, and are probably correlative.Microprobe data on cumulus pyroxenes indicate that the porphyritic websterite layer crystallised from a mixture of a relatively Mg- and Cr-rich M magma, parental to the Ultramafic Series, and an Fe-rich, strongly Cr-depleted gabbroic G magma. Pyroxenes from the PGE horizon are very low in Cr, suggesting that they crystallised from a G-rich hybrid.The websterite formed as a result of an influx of dense G magma which mixed with hotter resident M magma. The upper few metres of the websterite, including the PGE-rich sulphides, accumulated during a period of quiescence at the end of the influx phase. The PGE-rich sulphides formed by fractional segregation of sulphide liquid from a 500 to 1000 m thick layer of silicate magma.Munni Munni PGE mineralisation shows some striking similarities to that of the Great Dyke, particularly in the stratigraphic position of the mineralisation, the vertical distribution of PGE through the sulphide layer, and the lateral distribution of grades.

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Platin-Mineralisation im Munni Munni-Komplex, West-Australien

Zusammenfassung Der spät-archaische Munni Munni-Komplex bedeckt eine elliptische Fläche von 9 × 25 km, deren südliche Hälfte diskordant von einer 2.7 Ga alten vulkanischen Abfolge überlagert wird. Der Komplex besteht aus einer unteren, 1850 m mächtigen ultramafischen Serie (UMS) und einer oberen gabbroischen Serie, die mindestens 3600 m mächtig ist und die Form eines länglichen Trichters hat. Die UMS besteht aus makrorhytmischen Zyklen von Dunit, Wehrlit und Klinopyroxenit, während die GS einen ununterbrochenen Fraktionierungs-Trend von Pigeonit-Gabbros über Pigeonit Magnetit-Gabbros zu Granophyren zeigt. Die Basis der GS ist scharf und wird durch das gleichzeitige Erscheinen von Cumulus-Plagioklas und Pigeonit definiert. GS Cumulate zeigen gegen das Hangende zu eine monotone Zunahme von Fe/Mg und ein Fehlen zyklischen Lagenbaues, was auf Kristallisation in einer geschlossenen Kammer hinweist.Der oberste Teil der UMS ist eine deutlich ausgebildete, 30 m mächtige Lage von Bronzit-porphyritischem Orthokumulat-Websterit, welche sich an den Seitenwänden als randliche Zone fortsetzt, die in Kontakt mit zunehmend mehr fraktionierten Gabbros ist. Ein Pyroxenet-Gang durchschlägt den geneigten Boden der Intrusion im Bereich der obersten UMS, und dürfte als Zufuhrkanal für die obersten Lagen der UMS gedient haben.Eine schwache Dissemination von Cu-reichen magmatischen Sulfiden ist im Gesamtbereich der porphyritischen Websterit-Lage zu beobachten; in einer Augit-Orthocumulat-Lage wenige Meter unterhalb des Gabbros steigt diese auf 1–3% Cu-Sulfide an. Diese Lage erstreckt sich über 8.2 km, ist im Durchschnitt 2.5 m mächtig, und hat einen Durchschnittsgehalt von 2.9 g/t Pt + Pd + Au, 0.2% Ni und 0.3% Cu, mit lokal reicheren Zonen. In etwa 40% der untersuchten Bohrkerne fallen maximale Gehalte an PGE, Au, Ni und Cu zusammen, während sonst maximale PGE-Gehalte etwa 1–2 m unterhalb der Cu- und Ni-Maxima auftreten.Zusammenfallende Maxima dürften durch Homogenisation ursprünglich separater Maxima entstanden sein. Mit zunehmender Entfernung von den Rändern der Intrusion nehmen PGE Gehalte ab und werden unregelmäsiger.PGE-reiche Sulfide kommen auch nahe am Websterit-Gabbro-Kontakt vor, wo der Websterit eine randliche Position einnimmt. Die randliche Websterit-Zone und die porphyritische Websterit-Lage hängen zusammen, sind petrographisch ähnlich, und sind wahrscheinlich zu korrellieren.Mikrosonden-Analysen von Kumulus-Pyroxenen zeigen dass die porphyritische Websterit-Lage aus einer Mischung von relativ Mg- und Cr-reichem M-Magma dem die ultramafische Serie zuzuordnen ist, und einem Fe-reichen, Cr-armen gabbroischen G-Magma entstanden ist. Pyroxene aus der PGE-Lage führen sehr niedrige Cr-Gehalte; dies dürfte auf Kristallisation aus einem G-reichen Hybrid-Magma zurück gehen.Der Websterit wurde als das Resultat der Zufuhr von dichtem G-Magma das sich mit höher temperiertem M-Magma mischte, gebildet. Die obersten Meter der Websterit Abfolge, mit den PGE-reichen Sulfiden, bildeten sich während einer ruhigen Periode am Ende der Influx-Phase. Die PGE-reichen Sulfide sind das Produkt fraktionierter Segregation von sulfidischer Schmelze aus einer 500 bis 1000 m mächtigen Lage silikatischen Magmas.Die PGE-Mineralisation des Munni Munni-Komplexes ist der des Great Dyke von Zimbabwe in vieler Hinsicht ähnlich, besonders was die stratigraphische Position, die vertikale Verteilung der PGE in der Sulfid-Lage, und die laterale Verteilung der Gehalte betrifft.

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With 7 Figures and 1 Plate     

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