Abstract

Considerable exploration interest has been generated by the platinum-group element (PGE) and Ni-Cu potential of the Archean layered mafic-ultramafic intrusions in the West Pilbara Craton, Western Australia. The Munni Munni intrusion contains the largest resource of PGE associated with a layered intrusion in Australia, and the Radio Hill and Mount Sholl intrusions host significant resources of Ni-Cu-Co sulfides. Titaniferous magnetite layers, remobilized sulfides, and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years. The ca. 2.9 Ga Munni Munni, Andover, Radio Hill, Mount Sholl, and Sherlock layered intrusions are a cogenetic suite of high-level (<5 kbars) bodies that represent some of the oldest mineralizing systems of their type in the world. Although they display similar field relationships and mineralogical, geochemical, and isotopic features, their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the S saturation event were critical for the development of PGE-enriched sulfide-bearing layers and basal segregations of base metal sulfides. The intrusions form thick (>5.5 km) dikelike bodies or relatively thin (0.5-2 km) sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust. Rhythmically layered ultramafic components are generally thinner than, and occur along the northern sides of, more massive overlying mafic components. The ultramafic zones consist of dunite, lherzolite, wehrlite, olivine websterite, clinopyroxenite, and websterite. Inverted pigeonite gabbronorite, magnetite gabbro, olivine gabbro, anorthositic gabbro, and anorthosite comprise the mafic sequences. Olivine and clinopyroxene were generally the first minerals to crystallize, except in the Andover intrusion, where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material. The crystallization of chromite was inhibited in the ultramafic zones by the partitioning of Cr into early crystallizing clinopyroxene, thus downgrading the potential for PGE-chromite associations. The PGE mineralization in the Munni Munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic-gabbroic zone contact. Mineral compositional trends and Nd isotope data indicate that a Pd-Pt-Au-enriched S-undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone. A major influx of more fractionated, S-saturated tholeiitic gabbroic magma related to the resident magnesian magma, rapidly inflated the chamber, and induced turbulent magma mixing that resulted in the formation of the PGE-bearing porphyritic websterite layer. In contrast, the parent magmas that formed the Mount Sholl, Radio Hill, Andover, and Maitland intrusions were saturated in S before they were emplaced into the magma chambers. In these intrusions gravitational and structural controls were important for the concentration of PGE-poor (5-400 ppb Pt + Pd + Au) massive Ni-Cu-Co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic-ultramafic cumulates.
Google map showing geographic bounding box with values North bound -20.0 East bound 121.1 West bound 116.0 South bound -23.0
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Product Type/Sub Type

dataset - External Publication - Scientific Journal Paper

Constraints

license
Creative Commons Attribution 3.0 Australia Licence

IP Owner

Commonwealth of Australia (Geoscience Australia)

Author(s)

Date (publication)

2002

Product Type

dataset

Topic Category

geoscientificInformation

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External Publication
Scientific Journal Paper
Earth Sciences

Resource Language

English

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utf8

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unclassified

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North bound
-20.0
East bound
121.1
West bound
116.0
South bound
-23.0

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Unknown

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Organisation Name
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Australia
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a05f7892-fa9e-7506-e044-00144fdd4fa6

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ANZLIC Metadata Profile: An Australian/New Zealand Profile of AS/NZS ISO 19115:2005, Geographic information - Metadata

Metadata Standard Version

1.1

Metadata Date Stamp

2010-11-16

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unclassified

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Geoscience Australia
City
Canberra
Administrative Area
ACT
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2601
Country
Australia
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Related Links
For information on acquiring this product,
please contact the Geoscience Australia Sales Centre via:

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Please note that support hours are 9 am to 5 pm weekdays