Steve Beresford

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

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.

The behaviour of the fronts of komatiite lavas in medial to distal settings

Abstract Field characteristics of komatiites and observations of flow behaviour and propagation processes for subaerial basalt lavas are used to reconsider the flow behaviour of Archean komatiite lavas. Planar, conformable bases at contacts with pelitic sedimentary strata, the existence of coherent to quench fragmented tops, but the absence of autobreccias, indicate that many komatiite lavas were emplaced passively under laminar flow conditions. To determine how widely this may apply to active komatiite lavas, we model flow front thicknesses. Noting that the preserved flow thickness is thus significantly greater than the thickness of the propagating flow front, we apply the concept of inflation of basalt lava flows to komatiites. Application of the Jeffreys equation allows lava thickness to be determined as a function of flow velocity, viscosity, density and slope of the terrain. Using the results together with estimates of flow front velocities and viscosity in the Reynolds number equation indicates that at expected low flow front velocities in medial to distal settings, most komatiites would have propagated in a laminar flow state. We therefore envisage that komatiites were turbulent in near vent settings and capable of physical erosion, and at times, as previously proposed, channel-forming thermal erosion. As flow area increased downstream, and magma supply rate to the flow front and the flow front velocity decreased significantly, the flow state would have transformed to laminar flow. Preserved flow thicknesses are often considerably greater than the calculated flow front thicknesses for such low viscosity lavas indicating that the final preserved thicknesses are either due to flow inflation, ponding in topography, or that some komatiites were intrusions.

Vesicles in thick komatiite lava flows, Kambalda, Western Australia

Traditionally, vesicles have been considered to be rare in komatiites, however, field studies at Kambalda, Western Australia, and critical literature examination suggest that vesicle presence and content in komatiites are grossly underestimated. At Kambalda, vesicles are observed in significant amounts (up to 30%) in internal horizons within komatiite lavas. The vesicular horizons are inferred to represent bubbles frozen in place below the upper crust, during multiple episodes of flow through of bubble‐rich lava. We conclude that vesicle and crystal distribution are compatible with many komatiites being emplaced under laminar flow conditions, analogous to models for basalt lava emplacement.

Facies architecture of an Archean komatiite-hosted Ni-sulphide ore deposit, Victor, Kambalda, Western Australia: implications for komatiite lava emplacement

Nickel sulphide mineralisation is located in structural embayments or troughs at the base of thick basal komatiite lava flows at the Victor ore body, Kambalda, Western Australia. Primary contact relations, and textural and vesicle distribution of the host komatiite lavas, particularly the development of coherent or quench fragmented margins is consistent with emplacement of komatiites under laminar flow conditions. However, the absence of sedimentary units beneath komatiites with coherent flow tops in the ore environment seems contradictory and suggests both turbulent and passive laminar emplacement, respectively. The presence of erosive basal contacts in the ore environment suggests the komatiite lava was initially turbulent and probably open channel fed. It is during this initial stage that both erosion and deposition of the NiS deposits occurred. As the flow evolved, widened and thickened, laminar flow conditions prevailed. The komatiites are inferred to have flowed in a laminar state, through the development of interior magma tubes beneath surface crusts, and to have grown endogenously.

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the worlds largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

Contrasting komatiite belts, associated Ni-Cu-(PGE) deposit styles and assimilation histories

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew–Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) MgO contents of 25–30% in inferred parental magmas; and (ii) Al/Ti ratios of ∼20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e.g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni–Cu–(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.

Geology of an Archaean metakomatiite succession, Tramways, Kambalda Ni province, Western Australia: Assessing the extent to which volcanic facies architecture and flow emplacement mechanisms can be reconstructed

The Kambalda Ni province, located in the Archaean Norseman‐Wiluna greenstone belt of Western Australia, boasts the largest known concentration of komatiite‐associated magmatic Fe–Ni–Cu sulfide deposits. These are found as long, linear massive to disseminated bodies at the base of a thick komatiite sequence. The sulfide bodies are closely associated with, or contained within, trough structures at the contact with the underlying basaltic unit. In this study, the McComish Prospect, located 40 km south of Kambalda at Tramways, was studied to assess the relationships between volcanic facies, mineralisation and trough structures. The rocks in this region have variably experienced four phases of deformation, upper greenschist ‐ lower amphibolite facies metamorphism, granitoid intrusion, and subsequent alteration. Relict igneous textures are locally preserved at McComish, however, enabling the evaluation of existing geological models and interpretations. The McComish trough is considered to be entirely structural in origin and unrelated to primary volcanic processes (e.g. thermal erosion). The association of volcanic textural facies in individual flow units, and the distribution of flow units across the trough is more complex than predicted by prevailing models, suggesting an alternative komatiite lava emplacement mechanism. Results are consistent with the proposal that komatiites did not flow turbulently as widely accepted, nor did they cool by vigorous convection. Alternatively, the lavas were emplaced as inflated, lobate basalt pahoehoe‐like flows. Although Fe–Ni–Cu sulfide mineralisation at McComish is most likely volcanic in origin, its present distribution appears to be structurally controlled or modified. The zone of weakly to strongly disseminated sulfides at the base of the komatiite sequence is thickened adjacent to a major north‐northwest‐trending fault on the western margin of the trough. This fault is interpreted to have been a fluid conduit, remobilising the ore during metamorphism and deformation.

Hydrothermal remobilisation around a deformed and remobilised komatiite-hosted Ni-Cu-(PGE) deposit, Sarah’s Find, Agnew Wiluna greenstone belt, Yilgarn Craton, Western Australia

The Sarah’s Find nickel deposit, located 4.5xa0km north of the Mount Keith nickel mine, Western Australia, was chosen as a case study to investigate the nature and three-dimensional geometry of a geochemical halo created by the hydrothermal remobilisation of base metals and platinum group elements into the country rock surrounding a small massive Ni sulphide orebody. Portable and laboratory-based XRF analyses were carried out on samples from a shear zone localised along the basal komatiite-dacite contact that hosts the orebody. A geochemical halo was identified that extends along the shear zone up to 1780xa0m away from the massive sulphides, parallel to a prominent stretching lineation. Elevated Ni and Pd are associated with high As, Co, Cu and S. Palladium and Pt concentrations increase with proximity to massive sulphides (from 6 to 1190xa0ppb Pd). These anomalous concentrations reflect the presence of sulfarsenides and sulphides, either physically remobilised and forming veinlets close to the massive sulphides, or hydrothermally transported and redeposited within the foliation. In situ laser ablation ICP-MS indicates that Pd and Pt are hosted within these nickel sulfarsenides. This Ni-Co-As-Pd geochemical halo, observed around the Sarah’s Find ore body, is interpreted as forming syn deformation, by the circulation of As-rich hydrothermal fluids dissolving base metals, Pd and Pt from the orebody and redepositing them along the sheared footwall contact. Similar Ni-Co-Pd-Pt-As geochemical haloes could potentially exist around any magmatic nickel sulphide mineral system that has undergone a phase of arsenic metasomatism and may be a generally applicable proximity indicator for nickel sulphides in hydrothermally altered terranes.

Evidence of water degassing during emplacement and crystallization of 2.7 Ga komatiites from the Agnew-Wiluna greenstone belt, Western Australia

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old, which formed through >30% partial melting of the mantle. This study addresses the crucial relationship between volcanology and physical manifestation of primary magmatic water content in komatiites of the Agnew-Wiluna greenstone belt, Western Australia, and documents the degassing processes that occurred during the emplacement and crystallization of these magmas. The Agnew-Wiluna greenstone belt of Western Australia contains three co-genetic komatiite units that (1) display laterally variable volcanological features, including thick cumulates and spinifex-textured units, and (2) were emplaced as both lava flows and intrusions at various locations. Komatiite sills up to 500xa0m thick contain widespread occurrence of hydromagmatic amphibole in orthocumulate- and mesocumulate-textured rocks, which contain ca. 40–50 wt% MgO and <3 wt% TiO2. Conversely, komatiite flows do not contain any volatile-bearing mineral phases: ~150-m-thick flows only contain vesicles, amygdales and segregation structures, whereas <5–10-m-thick flows lack any textural and petrographic evidence of primary volatile contents. The main results of this study demonstrate that komatiites from the Agnew-Wiluna greenstone belt, irrespective of their initial water content, have degassed upon emplacement, flow and crystallization. More importantly, data show that komatiite flows most likely degassed more water than komatiite intrusions. Komatiite degassing may have indirectly influenced numerous physical and chemical parameters of the water from the primordial oceans and hence indirectly contributed to the creation of a complex zonation at the interface between water and seafloor.