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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

View larger version (51K): [in a new window]   FIG. 1. Local geology and stratigraphy of the Mount Keith region, outlining the three ultramafic belts and intercalated mafic and/or felsic volcanic rocks. The MKD5 nickel deposit is hosted within the Mount Keith Ultramafic Complex (modified from Dowling and Hill, 1990 unpub. report for CSIRO).

View larger version (173K): [in a new window]   FIG. 2. A. Schematic architecture and geology of the MKD5 deposit, showing drill hole locations. Boxed area indicates northwest corner, highlighting large-scale ultramafic apophyses in overlying dacite. Dashed lines indicate tectonized contacts around the thickened Mount Keith Ultramafic Complex. B. and C. Photographs of pit exposures of ultramafic apophysis in the dacite hanging wall. Note the lobate contacts and partial enclosure of a dacite xenolith. D. Ultramafic apophyses in dacite footwall to the MKD5 deposit. Their location is indicated by the rectangle on the lower contact of the MKD5 deposit in A.

View larger version (53K): [in a new window]   FIG. 3. Details of drill holes described in the text. A. Schematic log for drill hole MKG47. B. Schematic log for drill hole MKD288. C. Geochemical profile and schematic log and photos for drill hole MKG59. Note that the peaks in Al2O3 correspond with bastite (after oikocrystic pyroxene-rich orthocumulates). OAD= olivine adcumulate, Omc = olivine mesocumulate, Ooc = olivine orthocumulate.

View larger version (173K): [in a new window]   FIG. 4. A. Ultramafic-dacite contact from drill hole MKG47. Note the planar contact between the ultramafic and dacite units and the inclusion of a dacitic pendant within the ultramafic rock. B. Contact between ultramafic and dacite in drill hole MKD288. Note the angular sawtooth texture of the contact. C. Dacitic xenolith below the contact in drill hole MKD288. Note the resorbed edges and crosscutting ultramafic material. D. Entire contact between ultramafic and dacite in drill hole MKG59. Note the lobate apophysis of ultramafic material to the left of the photograph and the apparent zone of mixing between dacite and ultramafic material.

View larger version (84K): [in a new window]   FIG. 5. A. Photomicrograph of the ultramafic-dacite contact in drill hole MKD288, showing small-scale lobate interfingering between dacite and ultramafic material. Note the feldspar phenocryst in the dacite unit. B. Photomicrograph of stellate actinolite within the ultramafic apophysis, which is part of the drill hole MKG59 contact.