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AN INTRUSIVE ORIGIN FOR THE KOMATIITIC DUNITE-HOSTED MOUNT KEITH DISSEMINATED NICKEL SULFIDE DEPOSIT, WESTERN AUSTRALIA

N. M. Rosengren^,* and S. W. Beresford

School of Geosciences, P.O. Box 28E, Monash University, Clayton, Australia 3800

B. A. Grguric

WMC Exploration, P.O. Box 91, Belmont, Western Australia 6984

R.A.F. Cas

School of Geosciences, P.O. Box 28E, Monash University, Clayton, Australia 3800

Corresponding author: e-mail, Nic.Rosengren{at}wmc.com

The MKD5 nickel deposit at Mount Keith is hosted within the Mount Keith Ultramafic Complex, a thick komatiitic dunite body previously interpreted as either a large volume lava flow or as a dikelike intrusion. The upper contact relationships of the dunite body are critical for the evaluation of an extrusive versus intrusive origin. New drill core examined during this study has revealed preserved upper contact relationships between the Mount Keith Ultramafic Complex and the enclosing dacitic volcanic rocks. These contacts have lobate geometries with apophyses of the ultramafic material intruding the overlying dacite and dacitic xenoliths within the ultramafic rock along all observed margins. These contact features indicate an intrusive relationship between the Mount Keith Ultramafic Complex and the enclosing stratigraphy, which is consistent with the lack of definitive extrusive features. Our new interpretation of the Mount Keith Ultramafic Complex suggests that thick komatiitic dunite bodies may be regarded as subvolcanic sills emplaced within and below an extrusive komatiite pile. Importantly this model implies that komatiitic dunite bodies are not an integral or even necessary feature of a komatiite flow field.

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