Extrusive Origin and Structural Modification of the Komatiitic Mount Keith Ultramafic Unit

Gole, Martin J.; Robertson, J.; Barnes, Stephen J.

doi:10.2113/econgeo.108.7.1731

Cited by 24 publications

(14 citation statements)

References 49 publications

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“…Forsterite contents, estimated from fresh, unmineralized olivine adcumulate rocks from all slices, typically range from 92 to 94%, in keeping with the estimates of Dowling and Hill (1990) and Barnes et al (2011Barnes et al ( , 2012a and similar to those reported from Fred's Flow (Arndt et al, 1977). The chilled upper margin of the Mount Keith ultramafic unit varies in composition in a predictable way, depending upon the internal stratigraphy of the unit; fractionated sequences are overlain by basaltic to basaltic-andesite chilled margins, whereas olivine orthocumulate rocks are overlain by ultramafic chilled margins, confirming observations made by Gole et al (2013) in the immediate Mount Keith MKD5 mine area. Furthermore, pyroxene and olivine spinifex-textured rocks are reported from the upper margin of the Mount Keith slice near the northwest wall of the Mount Keith pit (Gole et al, 2013).…”

Section: Mount Keith Ultramafic Unitsupporting

confidence: 86%

“…The chilled upper margin of the Mount Keith ultramafic unit varies in composition in a predictable way, depending upon the internal stratigraphy of the unit; fractionated sequences are overlain by basaltic to basaltic-andesite chilled margins, whereas olivine orthocumulate rocks are overlain by ultramafic chilled margins, confirming observations made by Gole et al (2013) in the immediate Mount Keith MKD5 mine area. Furthermore, pyroxene and olivine spinifex-textured rocks are reported from the upper margin of the Mount Keith slice near the northwest wall of the Mount Keith pit (Gole et al, 2013). Spinifex textures are also rarely preserved at the upper contact of the Spinifex Park slice, above the fractionated zone (Duggan et al, 2008).…”

Section: Mount Keith Ultramafic Unitsupporting

confidence: 78%

“…The "Early Flows" ultramafic unit was first described by Dowling and Hill (1990) and consists of blocks of spinifex-textured komatiite and olivine orthocumulate rock with oikocrystic pyroxene caught up in a major shear zone in the immediate footwall of the Mount Keith ultramafic unit (Gole et al, 2013). Drill intercepts of komatiite are found in this structural setting over a strike length of approximately 3 km, from the center of the MKD5 deposit south to the vicinity of Shed Well.…”

Section: Other Ultramafic Unitsmentioning

confidence: 90%

“…Forrestania: Perring et al, 1995;Kambalda: Swager, 1997), (3) the presence of Ni sulfides (massive ore overlain sequentially by matrix and disseminated ore) at or close to what is interpreted as the basal contact, and (4) the presence of fractionated pyroxenitegabbro sequences close to what is interpreted as the upper contact (cf. Gole et al, 2013). Most komatiite country-rock contacts are sheared and the metasomatized nature of the country rock and the paucity of outcrop make it almost impossible to determine whether or not the contacts of cumulate units are intrusive or extrusive (e.g., Trofimovs, 2003).…”

Section: Komatiite Volcanology: Interpretation and Terminologymentioning

confidence: 99%

“…Intercepts are up to 10 m thick. Hayward (2004) and Rosengren et al (2007) interpreted this unit as a footwall apophysis off the main Mount Keith ultramafic unit but this was challenged by Gole et al (2013).…”

Section: Other Ultramafic Unitsmentioning

confidence: 99%

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Volcanological and Structural Controls on Mineralization at the Mount Keith and Cliffs Komatiite-Associated Nickel Sulfide Deposits, Agnew-Wiluna Belt, Western Australia—Implications for Ore Genesis and Targeting

Perring

2015

Economic Geology

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The Mount Keith region of the Agnew-Wiluna belt in Western Australia is perhaps best known for hosting the giant komatiite-associated disseminated Ni sulfide deposit known as MKD5. It also hosts the sizeable Cliffs massive Ni sulfide deposit, which is the eighth largest in the Archean Yilgarn block. A concerted program of regional-scale three-dimensional model construction has integrated a plethora of geological, geochemical, and geophysical data and led to a much better understanding of the tectonic and volcanic architecture of this ancient greenstone belt.The disseminated MKD5 orebody occupies a curvilinear thickening in the olivine adcumulate core of the host Mount Keith ultramafic unit. The orebody plunges shallowly to the south from surface, over a distance of 3.5 km, before bending sharply to plunge steeply to the north-northwest at depth. The thickest part of the orebody and the best Ni grades are associated with the bend in the overall ore trend. This also marks the approximate intersection point of two growth faults that appear to control the thickness variations in the adcumulate core and the plunge variations in the ore trend. The olivine adcumulate core is mantled by olivine orthocumulate rocks, which are best developed above and below the flanks of the orebody, and a fractionated sequence of pyroxenitic and gabbroic rocks occurs at the top of the Mount Keith ultramafic unit above the inside of the bend in the ore trend. These observations suggest a strong volcanological control on the position of the MKD5 orebody, and the local curvilinear thickening in the adcumulate core is interpreted as a fluid pathway, the locus of which was in turn controlled by the intersection of two broadly orthogonal growth faults.The Cliffs ultramafic unit consists of a lower olivine ortho-to mesocumulate unit, which varies in thickness along strike, overlain by a sequence of thin, spinifex-textured komatiite flow units intercalated with variably sulfidic tuffaceous shales. The Cliffs massive Ni sulfide deposit occurs at the base of a relative thickening (to ~100 m) in the lower olivine ortho-to mesocumulate unit. The orebody is typically less than 6 m thick, 500 m wide and plunges gently south from surface over a distance of 1.5 km, terminating in a structurally complex zone across which there are marked differences in the internal stratigraphy and thickness of the Cliffs ultramafic unit. The orebody has relatively low tenor (maximum ~8% Ni, typically ~5% Ni) and the tenor is strongly zoned with extremely low tenors (1-3% Ni) developed on the flanks. A barren exhalative massive Fe sulfide unit is typically present at the base of the Cliffs ultramafic unit on the flanks of the orebody and probably acted as a source of sulfur. The tenor zonation pattern suggests that the melting and entrainment of the underlying exhalative sulfide horizon occurred more vigorously in the center of the lava pathway. The structurally complex zone at the southern end of the Cliffs orebody is interpreted as a possible vent location; this would e...

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