Abstract

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 explain the relatively low tenor of the orebody and the tenor zonation (through limited mixing and entrainment of melted exhalative iron sulfide melt with the komatiite lava), the poorly incised nature of the lava pathway and the abrupt change in internal komatiite stratigraphy across this zone.

There is good evidence that the southern terminations of both the MKD5 and the Cliffs orebodies are marked by growth faults. At a belt scale these points correspond roughly with the intersection of NNE- and NW-trending lineaments. These lineaments are on the order of 5 to 10 km in strike length and are visible in regional geophysical datasets (both gravity and magnetics). They are likely to mark the position of accretionary structures that were active at the time of volcanism and that probably controlled the volcanic architecture of the komatiite flow field and the location of Ni sulfide accumulations within komatiite lava pathways. Repeated reactivation of these early crustal-scale structures during subsequent orogenesis resulted in their upward propagation as sets of late brittle structures. Of broader significance to the understanding of the generation of komatiite-associated nickel sulfide deposits is the conclusion that many such deposits may be developed proximal to vent rather than in a distal setting, as has been proposed for the Kambalda camp.

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