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Munni Munni Complex, Karratha, Roebourne Shire, Western Australia, Australia

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Latitude & Longitude (WGS84): 21° 7' 11'' South , 116° 51' 0'' East
Latitude & Longitude (decimal): -21.11972,116.85000
GeoHash:G#: qs5pfgr8d
Locality type:Complex
Köppen climate type:BWh : Hot deserts climate

A 250 sq km complex of in-situ Palladium, Platinum, Gold and significant Rhodium (up to 0.1 g/t) about 55 kilometres south of Karratha. For the purposes of this article, a smaller well studied area 4 x 9 kilometres was used in the reference for the geology.

The complex is found in the West Pilbara Block of east-west trending granite-greenstone terranes consisting of domal granitic batholiths, separated by belts of greenschist to lower amphibolite facies meta-sedimentary and meta-volcanic rocks of the Mount Bruce Supergroup.

A significant mineralised PGE-Au horizon was discovered here by John Ferguson in the 1980's. Hunter Resources Limited explored the area in the mid 1980's and Helix Resources in late 2000. In 2013 three companies held exploration tenements over the Munni Munni Complex.

It is one of the best preserved mafic-ultramafic intrusives in Australia, consisting of a 1.8 kilometre ultramafic zone overlain by a 3.6 kilometre gabbroic zone.

The ultramafic zone is rythmically layered in nine zones of minor dunite, with iherzolite, olivine websterite, clinopyroxene, norite, with chromitic and platiniferous websterite near the top of the zone. Some zones are olivine dominant. The dominant three cumulus mineral phases are augite, bronzite and olivine. Inter-cumulus minerals are plagioclase, biotite, magnetite, ilmenite, pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor phases traped in inter-cumulus liquid. Uppermost in the ultramafic zone is a mineralised horizon of mediium to coarse grained porphyritic websterite containing PGE. Olivine serpentinization and talc alteration is pervasive in the ultramafic zone. Plagioclase shows in the basal area of this zone.

The gabbroic zone is composed of gabbronorite, anorthositic gabbro, and minor anorthosite displaying a pronounce tholeiitic fractionation trend. The upper part is a sequence of poorly layered gabbro to gabbronorite, fairly homogenous with only local variations of pigeonite in its lower sections. There are also minor anorthosite and anorthosite bands. Magnetite is found near the base of this zone. Cumulus minerals in the gabbro are plagioclase, augite, inverted pigeonite and magnetite. Inter-cumulus minerals are inverted pigeonite, clinopyroxene, orthopyroxene, and minor quartz and biotite.

Evolution of S largely governed the behaviour of the PGE during fractionation of the Munni Munni magmas. PGE occurs immediately below the ultramafic-gabbroic contact, and has resulted from combined magmatic processes of crystal fractionation and magma mixing.

Hoatson and Keays (1989) suggest hot boyant sulfide-saturated tholeiitic magma rose through the density stratified PGE enriched sulfide un-saturated resident ultramafic magma until reaching its own density near the top of the chamber. Here it spread laterally for 12 kilometres. Due to the crystallisation of plagioclase and Fe enrichment of the melt, the density increased, overturned, and the magma mixed.

The platinum group minerals in the deposit show arsenides, sulfarsenides, tellurides, native metals, and mercury bearing phases forming small grains, enclosed by silicate minerals, or along silicate grain boundaries, or are associated with chalcopyrite-pyrrhotite-pentlandite blebs. All are in a mineralised websterite layer. The mineralised zone shows (Pt and Pd)/Se, (Pt and Pd)/S, (Pt and Pd)/Cs, (Pt and Pd)/Zr and Cu/Zr ratios, and Cu, Au, Cs, Zr, Sr, S, Se, Rb, and St concentrations.

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Hoatson, D.M., Keays, R.R.(1989): Formation of Platiniferous Sulfide Horizons by Crystal Fractionation and Magma Mixing in the Munni Munni Layered Intrusion West Pilbara Block Western Australia, Economic Geology (Nov 1989):84(7): 1775-1804
Albertson, G.A.(2000): Western Australia Platinum Complexes (2000)
Barnes, S. J., 1993 Partitioning of the Platinum Group Elements and Gold Between Silicate and Sulphide Magmas in the Munni Munni Complex, Western Australia Geochimica et Cosmochimica Acta, Volume 57, p. 1277 - 1290
HOATSON,DM, WALLACE,DA, SUN,S-S, MACIAS,LF, SIMPSON,CJ and KEAYS,RR (1992) Petrology and platinum-group-element geochemistry of Archaean layered mafic-ultramafic intrusions, west Pilbara block, Western Australia. AGSO Bull. 242, 319pp.

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