Wiluna Greenstone Belt Research Articles

The komatiite-hosted Perseverance Ni-sulphide deposit, Agnew-Wiluna greenstone belt, Western Australia; new insights into the Perseverance komatiite channel and footwall lithostratigraphy

The paper presents new geological and geochemical data on the Perseverance komatiite complex (PKC) that hosts the Perseverance Ni-sulphide deposit and the volcanic and sedimentary rocks forming the footwall of the PKC. Using the 3D reconstruction of the PKC and its’ footwall volcano-sedimentary succession the heterogeneity of the komatiite channel has been characterised on the basis of spatial distribution of the geochemical petrogenetic indicators.The study shows that the komatiite facies proximal to the mineralised bodies and the main pathway are expressed in the 3Dmodels as the segments with anomalously high MgO concentrations coupled with low contents of Al2O3, TiO2and CaO. The geochemical indicators of komatiite fertility, Ni/Cr and Ni/Ti ratios, commonly are in the range of 1 to 3 for Ni/Cr and 2 to 5 for Ni/Ti within the mainpathway, The Ni/Crexceeds 5and Ni/Ti20 in the mineralised parts of the channel. In the peripheral zone komatiites thesevalues commonly drop to less than 1, corresponding to barren komatiites. Similarly large variations are exhibited by the ratios of mantle normalized Pt and Pd to Al and Ti over short distances from 1 to 25. Closely spaced high and low values of these ratios hinder their use for discriminating fertile from barren komatiites. However, within the komatiite main lava pathway they can be used as geochemical pathfinders for magmatic sulphides.Reconstruction of the structurally complex komatiite sequences can be facilitated by using lithogeochemical correlation of the host volcano-sedimentary successions and particularly tracing geochemically distinctive marker units. For these purposes a special reference in this study was made to the geochemical systematics of rocks that make up footwall succession of the PKC complex. It was shown that amphibolites of the host sequence can be geochemically subdivided into different types, including mafic volcaniclastic rocks (mafic-1) and intrusive mafic rocks, apparently metagabbro sills (mafic-2). The mafic volcaniclastic rocks are strongly enriched in LREE, possibly due to presence of sedimentary epiclastic material or reflecting magma contamination by the crustal material during ascent through the crust. Additionally, they have notably high-Th contents, allowing them to be classified as having high-Th geochemical affinities. The second group (mafic-2), interpreted as coherent in origin, are characterised by flat REE patterns and lower overall abundances of the rare earths and Zr corresponding to a tholeiitic composition. Contents of Th and TiO2of these rocks are clustered in the two narrow intervals of compositions, corresponding to low-Th and intermediate-Th basalts, which form different amphibolite bodies. Mafic volcanics intercalate with the felsic volcanics, and quartz-feldspar-biotite-amphibole (QFBA) schists. It is shown that QFBA schist composition corresponds to metamorphosed clastic sedimentary and volcaniclastic rocks of mixed composition, which contain both clastic materials derived from an Archaean provenance and volcaniclastics material derived from mafic and felsic tuffs. The different geochemical affinities of the mafic rocks distributed in the footwall succession of the PKC apparently suggests different magma sources and, possibly, can also reflect the position of the rocks within the mantle plume structure. In conclusion, abundances, and ratios of the trace elements in composition of the mafic rocks matches that of the different parts of the Agnew Wiluna greenstone belt and can be used for overall stratigraphic correlations.

Integrated interpretation of geology and geophysics, using inversions, to predict geology under cover

Predictive 3D geological models of the subsurface can be developed using a range of available tools. Each tool is suited to slightly different problems and datasets. The method described here, using the UBC-GIF inversions algorithms, allows rapid development of models using an objective, automated procedure. It has flexibility to include as little or as much geological information as is available, making it ideal for greenfields exploration or mapping programs. The steps involved are: 1) develop a solid understanding of the expected physical properties; 2) convert geological observations into physical property constraints; 3) perform geologically-constrained inversions; 4) apply geological classifier to recovered 3D physical property models. The procedure is demonstrated for the southern Agnew- Wiluna greenstone belt in WA, a highly mineralised region with a high proportion of surface cover. The predictive 3D lithology models developed for the area are particular effective at mapping the extent of dense mafic and magnetic ultramafic rocks, and provide new insights about their distribution at depth.

The West Jordan deposit, a newly-discovered type 2 dunite-hosted nickel sulphide system in the northern Agnew–Wiluna belt, Western Australia

The West Jordan nickel deposit, in the northern Agnew–Wiluna greenstone belt of Western Australia, is a newly-discovered Type 2 dunite-hosted, low-grade, large tonnage, disseminated sulphide system. Located in the core of a large dunite body, mineralisation is dominated by intercumulus sulphide blebs (20μm to 6mm across) in assemblages containing pentlandite, pyrrhotite, heazlewoodite and locally, native nickel, sphalerite and chalcocite. Mineralisation grades between 0.2 and 2wt.% Ni, with the majority of samples in the 0.35–0.7% Ni range, were consistent with most komatiitic Type 2 systems. Hypogene alteration of the ultramafic host rock is interpreted to have been effected by retrograde metamorphic fluids, and has resulted in extensive serpentinisation and localised, structurally-controlled, talc-magnesite alteration. This gangue alteration has resulted in modification of original magmatic sulphide assemblages, and localised remobilisation of the minor Cu and Zn components of the magmatic sulphides. The deposit is deeply weathered, and all samples utilised in this study were obtained from a series of 12 diamond drill holes which were comprehensively assayed. An igneous stratigraphy is presented which is interpreted to be west-younging, consistent with along-strike deposits to the south, such as the Mount Keith and Yakabindie Type 2 nickel deposits.

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 world's 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.

Towards a volcanic–structural balance: relative importance of volcanism, folding, and remobilisation of nickel sulphides at the Perseverance Ni–Cu–(PGE) deposit, Western Australia

Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional DI deformation event (corresponding to the local DP1 to DP3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F3 folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.

Mass and magnetic properties for 3D geological and geophysical modelling of the southern Agnew–Wiluna Greenstone Belt and Leinster nickel deposits, Western Australia

Physical property measurements provide a critical link between geological observations and geophysical measurements and modelling. To enhance the reliability of gravity and magnetic modelling in the Yilgarn Craton's Agnew–Wiluna Greenstone Belt, mass and magnetic properties were analysed on 157 new rock samples and combined with an existing corporate database of field measurements. The new samples include sulfide ore, serpentinised and olivine-bearing ultramafic host-rocks, granitoid, and felsic and mafic volcanic and volcaniclastic country rock. Synthesis of the data provides a useful resource for future geophysical modelling in the region. Several rock types in the region have sufficiently distinct physical properties that a discriminant diagram is proposed to facilitate a basic classification of rock types based on physical properties. However, the accumulation of emplacement, metamorphic, hydrothermal and structural processes has complicated the physical properties of the rocks by imposing duplicate and sometimes opposing physical property trends. The data confirm that massive sulfide and ultramafic rocks have the most distinctive mass and magnetic properties but with variability imposed by their complex history. Sulfide content imposes the strongest control on densities, but can only be identified when comprising >10 vol% of the rock. The pyrrhotite-rich Ni-sulfide assemblages generally have similar magnetic properties to the host ultramafic rocks, but can have much lower susceptibilities where the thermal history of the rocks has favoured development of hexagonal pyrrhotite over monoclinic pyrrhotite. In ultramafic rocks that contain <10 vol% sulfides, density and susceptibility are primarily controlled by serpentinisation, with olivine breaking down to serpentine and magnetite in the presence of water. Serpentinisation dramatically lowered densities and increased susceptibilities, but had limited influence on the intensity of remanent magnetisation. All ultramafic rocks contain multidomain magnetite, and most contain low coercivity grains prone to overprinting by in situ viscous remanent magnetisation or drilling-induced isothermal remanent magnetisation during extraction. Despite the low coercivities, Koenigsberger ratios of 1–20 are observed indicating that viscous remanent magnetisation aligned parallel to the present Earth field must be considered in any magnetic modelling. It is also noted that coarser-grained intrusive varieties of all rock types (e.g. granite, gabbro) show remanent magnetisation intensities 1–2 orders of magnitude greater than their extrusive equivalents (felsic and basaltic volcanics).

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites, Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ∼200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.

Controls on the emplacement and genesis of the MKD5 and Sarah’s Find Ni–Cu–PGE deposits, Mount Keith, Agnew–Wiluna Greenstone Belt, Western Australia

The Mount Keith (MKD5) nickel sulfide deposit is one of the largest komatiite-hosted nickel sulfide deposits in the world; it is hosted by a distinctive spinifex-free, cumulate-rich, ultramafic horizon/unit termed the Mount Keith Ultramafic (MKU). The Mount Keith Ultramafic shows significant variation along its lateral extent. The internal architecture is made up of adcumulate-textured pods and lenses, which are flanked by thinner meso- and orthocumulate-textured units, overlain by pyroxenitic and gabbroic horizons. The lateral and vertical changes in the geometry and internal architecture reflect variations in the lithological association and emplacement conditions along the strike extent of the belt. The chilled margins of the Mount Keith Ultramafic unit contain ∼1,200 ppm Ni. Olivine cumulates average ∼2,500–3,500 ppm Ni, with few exceptions (Ni > 4,500 ppm) reflecting occurrence of minor nickel sulfides, whereas pyroxenites and gabbros generally contain, respectively, ∼1,500–2,000 and ∼100–1,000 ppm Ni. Olivine cumulates generally contain low Cr concentrations ( 5,000 ppm Cr). The internal stratigraphy of the Mount Keith Ultramafic unit may be subdivided into two groups based on rare earth element distribution. The chilled margins and the internal units of the Main Adcumulate domain display LREE-enriched patterns [(La/Sm)n > 1–3] and negative Eu, Hf, Zr, Nb, and Ti anomalies. The internal units in the Western Mineralized Zone generally display flat chondrite-normalized REE patterns and only minor negative Nb anomalies. The pattern of platinum-group element (PGE) distribution varies greatly along the strike extent of the Mount Keith Ultramafic unit. The chilled margins display relatively low absolute concentrations [PGE (excl. Os) ∼16 ppb] and relatively fractionated patterns, with subchondritic Pt/Pd ratios (∼1.5), and superchondritic Pd/Ir ratios (∼3). The PGE trends in the thick adcumulate-textured pods containing widespread nickel sulfide mineralization display positive correlation with sulfide abundance, whereas fractionated pyroxenites and gabbros in the thinner domains display highly depleted PGE concentrations and generally show compatible PGE trends. The nickel sulfide ore typology and style vary greatly along the strike extension of the Mount Keith Ultramafic unit. Basal massive nickel sulfide mineralization (e.g., Sarah’s Find) occurs in the thinner meso- and orthocumulate-textured units, whereas stratabound disseminated nickel sulfide mineralization (e.g., MKD5 Ni Deposit) is hosted in the adcumulate-textured pods. We hypothesize that the very low PGE content of the initial liquid of the Mount Keith Ultramafic unit indicates that the initial magma pulse that penetrated through the dacite host-rock had already equilibrated with sulfides at depth and/or carried entrained immiscible sulfide blebs. We argue that upon emplacement, the intruding magma experienced a significant thermal shock at the contact with water-saturated volcaniclastic breccias. The sudden chilling would have increased the viscosity of the magma, possibly to the point where it was no longer able to sustain the suspension of the immiscible sulfide liquid. As a result, the sulfide blebs coalesced and formed the basal massive sulfide nickel sulfide mineralization at the base of the sill (i.e., Sarah’s Find). Prolonged focused high volume magma flow within the sill resulted in the emplacement of a thick, lens-shaped accumulation of olivine adcumulate. Local variations in intensive parameters other than crustal assimilation (e.g., T, fO2, fS2) may be principally responsible for sulfide supersaturation and controlled the local distribution of stratabound disseminated nickel sulfide mineralization (e.g., MKD5 Ni Deposit), generally localized within the core of the thicker dunite lenses.

P-T-X conditions of hydrothermal fluids and precipitation mechanism of stibnite-gold mineralization at the Wiluna lode-gold deposits, Western Australia: conventional and infrared microthermometric constraints

The Wiluna lode-gold deposits are located in the Archean Wiluna greenstone belt, in the northern sector of the Norseman-Wiluna belt in the Yilgarn Craton of Western Australia. They are hosted in subgreenschist facies meta-basalts, and controlled by the Wiluna strike-slip fault system and associated shear veins and breccias. The 13 individual lode-gold deposits have produced around 115 t Au from 1901 to 1946 and 1986 to today. Historically, they also produced 38.3 t As and 3.5 t Sb. Gold formed in two stages: stage 1 gold-pyrite-arsenopyrite is finely disseminated in the wallrock and breccia fragments, whereas stage 2 gold-stibnite is located in massive shear veins and breccia matrix, as fracture-fill and in banded-colloform textured veins. Stibnite-gold orebodies only occur in some of the deposits (e.g., Moonlight and northern part of the West Lode) and also display a restricted vertical extent, being preserved only in the uppermost 200 m of stibnite-bearing lodes. Petrographic, conventional, and infrared microthermometric and laser-Raman analysis on stibnite-bearing quartz veins and breccias reveal that the antimony- and gold-rich hydrothermal fluid was of mixed H2O-NaCl-CO2±CH4 type. Microthermometric measurements reveal maximum homogenization temperatures of 340 °C (average 290±25 °C), and a wide range of salinities between 0.2 and 23 eq. wt% NaCl. Aqueous-carbonic fluid inclusions contain variable XCO2+CH4 (0.03 to 0.82), with the carbonic phase containing a maximum XCH4 of 0.21. Combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low-salinity, medium XCO2+CH4, homogeneous parent fluid at about 290 °C and pressures between 700 and 1,700 bar. Fluid immiscibility was triggered by cyclic pressure release during fault-zone movement. The decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to lower temperatures, significantly reduced the degree of stibnite undersaturation, and caused stibnite to precipitate. The deposition of stibnite reduced the ore-fluid H2S concentration, thereby destabilized gold bisulfide complexes in solution, and caused gold precipitation locally. This mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite-gold orebodies at Wiluna.

The Archean Chalice gold deposit: a record of complex, multistage, high-temperature hydrothermal activity and gold mineralisation associated with granitic rocks in the Yilgarn Craton, Western Australia

The Chalice gold deposit (∼20 t Au produced), in the southwestern portion of the Late-Archean Norseman–Wiluna greenstone belt of the Yilgarn Craton, Western Australia, is hosted both in a sequence of intercalated mafic and ultramafic amphibolites and a post-peak metamorphism monzogranite dike. The deposit is flanked on the western side by calc-alkaline plutonic rocks, and to the east by a predominantly monzogranitic pluton, with at least four generations of monzogranitic dikes that intrude the local mine stratigraphy. Locally, four deformation events (D 1–D 4) have affected this sequence of amphibolites, and two stages of gold mineralisation are recognised, controlled by the progressive D 2 event. Main Stage gold mineralisation (95% of the resource), which is demonstrably later than all granitic bodies except second generation and subsequent monzogranite dikes, is controlled by localised D 2 asymmetric folds developed in the mafic amphibolite. It is expressed as veins and wall-rock replacement, characterised by a prograde assemblage of quartz–diopside–plagioclase–K-feldspar–titanite–pyrrhotite–pyrite–magnetite±garnet, hornblende, scheelite and biotite. Second Stage gold mineralisation (5% of the resource) is associated with the intrusion of a monzogranite dike and is expressed as disseminated gold in the dike, as well as foliation-discordant veins of quartz–gold, quartz–diopside–gold, actinolite–gold and molybdenite–tellurobismuthite–gold. Detailed field mapping and petrography suggest that Main Stage gold mineralisation at Chalice is equivalent to hypozonal orogenic lode–gold mineralisation in other Archean greenstone belts of the Yilgarn Craton, whereas monzogranite-associated Second Stage gold mineralisation is atypical of such deposits. The Main Stage mineralisation cannot be correlated to any particular granitoid in the local environment, although temporally, it is bracketed by granitic magmatism. A deep magmatic or metamorphic source cannot be resolved from the field data. It is also uncertain as to whether Second Stage mineralisation represents a true magmatic-gold event, or is a product of the assimilation and remobilisation of Main Stage gold, during the intrusion of the monzogranite dike. However, its distinctive metal association of Au–Bi–Mo–Te–W suggests the former. Irrespective of this uncertainty, Chalice remains as a deposit which is more intimately related both spatially and temporally to granitic magmatism than other Yilgarn deposits.

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.

Chromite in Komatiites, 1. Magmatic Controls on Crystallization and Composition

Chromite is a widespread accessory mineral in komatiites, ranging Zhou & Kerrich, 1992). This is the first in a series of papers which addresses the occurrence of chromite in from skeletal through euhedral and equant to lobate and rarely poikilitic habits depending on cooling regime and the composition komatiites and its chemical variability. This paper looks at the factors controlling the crystallization of chromite of the coexisting olivine. Chromite is least abundant in highly magnesian chrome-undersaturated lavas, and most abundant in from komatiites, and at the textural and compositional variation within chromites which have apparently restrongly differentiated layered cumulate bodies. Abundances are typically lower than the expected cotectic proportions. Chromite tained some or all of their igneous characteristics. Textural and compositional features of chromites are compositions reflect variations in lava composition and oxygen fugacity; reaction with trapped intercumulus liquid; and sub-solidus interrelated in complex and interesting ways, and show systematic relationships to host rock chemistry and crysFe–Mg exchange with olivine. Primary chromites in thick dunitic channels and sheet flows have very low Fe contents, whereas tallization regime. Future papers in the series will deal with superimposed metamorphic effects. comparable thin flow chromites contain higher Fe. This is attributed to an initial reduced state of the magma, thin flow lavas A large body of new data on chromite compositions, including high-precision wavelength-dispersive analyses being subsequently oxidized because of post-eruption processes. Extensive reaction of chromite with trapped liquid causes decreasing of minor and trace elements Ti, V, Mn, Zn, Ni and Co, is presented from the Norseman–Wiluna Greenstone Belt Mg/(Mg+ Fe), and enrichment in Fe, TiO2, V2O5, MnO, CoO and ZnO. Nickel trends depend on sulphide content. These in Western Australia, and synthesized with a compilation of literature data taken mainly from Roeder (1994) and trends are best developed in rocks combining presence of trapped intercumulus liquid with prolonged cooling histories. The deficiency various unpublished sources. Whole-rock geochemical data are similarly compiled from the literature and some of chromite in channelized environments is partly a consequence of more primitive lavas being found in these environments, but is unpublished sources, and include a large body of data collected over the last 15 years at CSIRO. Details of mainly due to the difficulty in nucleating chromite crystals under low degrees of supercooling. data sources are listed in Appendix A, and details of electron microprobe analytical techniques in Appendix B.

Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

The Bulletin lode-gold deposit is within the northernmost part of the Norseman–Wiluna greenstone belt in the Archaean Yilgarn Block, Western Australia. It is located within a brittle–ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite–calcite, intermediate calcite–dolomite, outer proximal sericite and inner proximal dolomite–sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, CO 2, K, Rb, S, Sb, Te and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for >150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Te (>10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1–35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, though stepwise, vector towards ore.

Constraints on the timing of lode‐gold mineralisation in the Wiluna greenstone belt, Yilgarn Craton, Western Australia

40Ar‐39Ar ages of hydrothermal muscovites constrain the age of gold mineralisation in the Wiluna greenstone belt, Western Australia. A single muscovite sample from the Matilda M1 deposit, hosted in the greenschist‐amphibolite facies Matilda domain, has a 40Ar‐39Ar plateau‐like segment corresponding to an age of 2623 ± 12 Ma (2 σ). This is interpreted as the probable age of mineralisation and is similar to radiometric ages of mineralisation from other parts of the craton. Two muscovite samples from an altered microdiorite dyke from the East Lode deposit, located within the prehnite‐pumpellyite facies Wiluna domain have 40Ar‐39Ar plateau‐like segment ages of 2565 ± 12 Ma and 2563 ± 12 Ma. A third sample, consisting of muscovite from an altered high‐MgO basalt, has an enigmatic spectrum with no obvious plateau‐like segment and the highest temperature steps have apparent ages of ca 2600 Ma. Two alternative interpretations are considered for the data from the East Lode deposit: (i) mineralisation in the Wiluna...

Two stages of CO2 metasomatism at the Munro mine, Munro Township, Ontario: evidence from fluid-inclusion, stable-isotope, and mineralogical studies

The Munro asbestos mine is hosted by a differentiated ultramafic sill of Archean age. Localized carbonate alteration at the mine has resulted from two separate episodes of CO2 metasomatism, and the fluids were unrelated. The first episode affected only the serpentinized peridotite and occurred at 250 °C. The fluid was a saline brine (up to 24 wt.% NaCl–CaCl2), and had an oxygen isotopic composition of −3‰, and δ13C was equal to −7.8‰. Calcite veins were emplaced into the overlying, fractured pyroxenite at approximately 300–400 °C during the second episode. The salinity of this fluid was only 1–5 equiv. wt.% NaCl, the oxygen isotopic composition was +7.5‰, and δ13C equaled −3 to −5‰. The first episode was probably associated with burial metamorphism (diagenesis) and the second episode with regional metamorphism. The widespread occurrence of two separate stages of CO2 metasomatism in the Abitibi belt and in other well-documented Archean terranes, such as the Norseman–Wiluna greenstone belt in Western Australia, suggests that this may be an important factor in the tectonic evolution and metamorphic history of Archean greensone belts.

A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States

A comparison of high-alumina mineral assemblages spatially associated with the Haile and Brewer gold deposits in the Paleozoic Carolina slate belt (CSB) of the southeast United States and the Sons of Gwalia and Mount Celia gold deposits in the Archean Norseman–Wiluna greenstone belt (NWGB) in Western Australia suggests a similar hydrothermal origin and subsequent metamorphic and deformational history. A common hydrothermal origin is supported by the striking similarity in whole-rock chemistry, even though there were probably significant variations in original unaltered protoliths. An analysis of rocks from each of the deposits that contain high-alumina minerals demonstrates that the protoliths were leached of alkalis with respect to aluminum and have a ratio of Al2O3/Na2O + CaO + K2O greater than three. Although the rocks contain abundant high-alumina minerals, the aluminum content in these rocks is not unusually high, and it does not appear that there has been any significant transport of aluminum either into or out of the rocks that were altered.The most common high-alumina minerals found in rocks affected by the feldspar destructive alteration event include andalusite, kyanite, pyrophyllite, kaolinite, and sericite. Other minerals present in variable amounts include diaspore, chloritoid, alunite, natroalunite, paragonite, and topaz. It is possible that some of these minerals formed during the early hydrothermal alteration event and were recrystallized during metamorphism. Regardless of when the high-alumina minerals formed, textural relations suggest that the rocks had been hydrothermally altered by the time the metamorphic minerals formed. A comparison of alteration from the deposits studied in the CSB and the NWGB suggests there are many similarities to acid-sulfate alteration associated with geothermal areas, such as Yellowstone Park, Wyoming, and with acid-sulfate gold deposits, such as Goldfield, Nevada. Thus, it is possible that the protolith of the metamorphosed rocks in the CSB and NWGB contained an alteration assemblage that included alunite, pyrophyllite, and kaolinite.A generalized paragenetic sequence determined from petrographic and field observations, beginning with regional metamorphism, follows: (i) formation of andalusite, kyanite, chloritoid, and topaz during prograde metamorphism, depending on whole rock chemistry, (ii) formation of pyrophyllite and quartz-rich pods during silicification of aluminosilicate-bearing rocks, (iii) bedding parallel schistosity and fracturing produced by a deformational event, (iv) fractures filled by quartz, sericite, pyrophyllite, or calcite, (v) folding of early layering in the rocks to form a crenulation cleavage accompanied by introduction of quartz veins at high angles to foliation; and (vi) retrograde metamorphism of andalusite–kyanite-pyrophyllite to produce an assemblage of kaolinite ± diaspore.The spatial association of the acid-sulfate alteration with gold mineralization, together with comparison of analogous alteration associated with younger unmetamorphosed acid-sulfate gold deposits, suggests that at least some of the gold was introduced during the early premetamorphic alteration event. The present location of gold in each deposit is a result of local changes brought about by later metamorphic and deformational events.

Archaean carbon reservoirs and their relevance to the fluid source for gold deposits

It is commonly assumed1,2 that seawater-derived carbon in altered volcanic rocks is the only major pre-metamorphic carbon-in-carbonate reservoir in Archaean greenstone belts. Thus carbonates from Archaean gold deposits with stable isotope ratios too negative (median δ13C ≈ −3‰) to have been derived from such carbon (δ13C usually assumed to be near 0‰) have been interpreted as being derived from a local felsic magmatic source1. Barley and Groves3, however, recognize two carbonate-alteration styles that predate regional metamorphism and gold mineralization in the Norseman–Wiluna greenstone belt of Western Australia: evidence is summarized below. These are seafloor alteration and fault-controlled regional alteration, which, as shown below, have completely different carbon isotope compositions. The latter (median δ13C = −4.8‰) implies a major juvenile carbon (CO2) flux from the mantle during greenstone-belt evolution, and is thus of fundamental importance in itself. However, here we show why the mere existence of two carbon-in-carbonate reservoirs and variation in carbon isotope ratios of carbonates between individual gold deposits in one area, negate some of the fundamental assumptions used to support magmatic-fluid models1.

Revised Sm–Nd systematics of Kambalda greenstones, Western Australia

Sm–Nd isotope systematics in mafic–ultramafic lavas from the Norseman–Wiluna greenstone belt, Western Australia, have been reinvestigated. A previous age based on mixing lavas and granites is not confirmed: instead, a Sm–Nd age of 3,262 ± 44 Myr is indicated for the lavas, 400 Myr older than intruding granites, and predating gneisses that have been proposed as basement rocks.