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GEOREF RECORD

Deformed chromitite layers in the Coobina Intrusion, Pilbara Craton, Western Australia

Steve Barnes and Sarah Jones
Deformed chromitite layers in the Coobina Intrusion, Pilbara Craton, Western Australia
Economic Geology and the Bulletin of the Society of Economic Geologists (March 2013) 108 (2): 337-354

Abstract

The Archean Coobina intrusion contains a number of tectonically dislocated and folded chromitite seams within a strongly deformed ultramafic-mafic layered intrusion. The intrusion consists of predominantly serpentinized olivine cumulates, overlain by gabbroic rocks. The entire sequence, along with the overlying greenstone belt sediments and volcanic rocks, shows evidence for multiple episodes of ductile deformation and a later phase of shearing associated with the Capricorn orogen. This history has resulted in isoclinal folding and boudinage of the chromite layers and disruption of original lateral continuity. This deformation appears to have been accommodated by micrometer-scale microshears within the massive chromitites rather than by penetrative fabric development. The Coobina chromitites have compositions at the more Cr-rich end of the range observed in layered intrusions worldwide. Chrome contents and Cr/Al ratios are indicative of a parent magma of komatiitic affinity. Mg/Fe ratios vary unsystematically and have probably been modified by varying degrees of subsolidus exchange of Mg and Fe with surrounding olivine-rich cumulates. Chromitites are consistently Pt and Pd poor, have typical 100-ppb levels of Ru and Ir, and show no systematic relationship to stratigraphy or major element chromite chemistry. The lack of Pt and Pd enrichment is attributed to formation from uniformly S-undersaturated magmas, consistent with evidence for high temperatures of formation. Crystal sizes in the Coobina chromites show linear trends on crystal-size distribution plots. Such trends are considered to be characteristic of homogeneous populations of growing crystals. There is no evidence of grain-size sorting that might be expected from crystal settling processes, and a mechanism of in situ nucleation and growth is inferred. Chromite seams are interpreted as the result of magma-mixing processes during magma chamber recharge, but with mixing taking place entirely between olivine-saturated magmas, accounting for the lack of pyroxene-bearing cumulates in the sequence, and the higher than usual Cr/Al ratios.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 108
Serial Issue: 2
Title: Deformed chromitite layers in the Coobina Intrusion, Pilbara Craton, Western Australia
Affiliation: CSIRO, Earth Science and Resource Engineering, Perth, West. Aust., Australia
Pages: 337-354
Published: 201303
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 29
Accession Number: 2013-030578
Categories: Economic geology, geology of ore deposits
Document Type: Serial
Bibliographic Level: Analytic
Annotation: A digital suppl. to this paper is avail. at http://economicgeology.org/ and at http://econgeol.geoscienceworld.org/
Illustration Description: illus. incl. sects., sketch maps
S24°00'00" - S23°00'00", E120°00'00" - E120°30'00"
Secondary Affiliation: Consolidated Minerals, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201319
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