Lessons from a joint interpretation of vibroseis wide-angle and near-vertical reflection data in the northeastern Yilgarn, Western Australia

A wide-angle reflection seismic experiment was carried out in the Eastern Goldfields granite–greenstone terrane of the Archaean Yilgarn Craton during 2001. This was the first time in Australia that wide-angle data were collected using a vibrator source and with a high density of observations. Unlike other wide-angle surveys carried out in other parts of the world, our survey used both a smaller number of sweeps, and shorter sweeps. We recorded three sweeps (each with its own frequency range) at each vibration point. The experiment demonstrated that the sum of three 12 s sweeps using 3 large vibrators provides enough energy to record signal at offsets up to up to 60–70 km. A comparison of individual shot gathers from near-vertical data and receiver gathers from wide-angle data demonstrated higher reflectivity in near-vertical data. This may be due to differences in the frequency bands of the recording equipment. The after stack section obtained from dense wide-angle data is different from that obtained from conventional near-vertical reflection data. The conventional reflection section provides higher quality image of the crust compared to the wide-angle section. This could be explained by the low-fold in wide-angle data and differences in the acquisition and processing methodology. The wide-angle survey, which was coincident with a regional vibroseis seismic reflection transect, was focused on the Leonora–Laverton region. The survey was designed to supplement the deep seismic reflection studies with velocity information. This also created an opportunity to compare velocity model derived from wide-angle reflection seismic data with a structural image obtained from the deep common mid-point seismic reflection data, and thus refine our geological understanding of the area. A high velocity body reaching a maximum thickness of 2 km was identified exclusively from the seismic velocity model derived from wide-angle study. This body is interpreted as mafic rocks within the Archaean Granite–Greenstone Belt. The joint interpretation also shows that structural boundaries do not always follow lithological boundaries in our study area. The combination of wide-angle reflection and near-vertical reflection data has facilitated a more complete geological interpretation of the seismic data.     

4D architecture and tectonic evolution of the Laverton region,eastern Yilgarn Craton,Western Australia

The Laverton region, located in the eastern Yilgarn Craton (EYC) Western Australia, is second only to the Kalgoorlie region for gold endowment. The integration of high-density, potential-field data, regional- and camp-scale seismic reflection data, regional- and mine-scale structural analysis, and geochronologically-constrained stratigraphy, provided new insights into the 4D architecture and tectonic evolution of Laverton region.     

澳大利亚伊尔岗地块诺斯曼-维卢纳绿岩带金矿成矿作用

诺斯曼-维卢纳金成矿带位于澳大利亚西澳克拉通伊尔岗地块东部黄金省次级地体内,是澳大利亚最重要的绿岩带型金矿成矿区。区内矿床可分为3类：剪切带中的蚀变晕±石英脉型,是区内最为重要的矿化类型,典型矿床为卡尔古利金哩矿床,矿床赋存在金哩粗玄岩中,矿化发生在绢云母-黄铁矿化蚀变带内;石英网脉型,典型矿床为卡尔古利夏洛特山金矿床。矿床也产于金哩粗玄岩中,矿化主要发生在硅化外围的碳酸盐-黄铁矿化蚀变带内;纹层状石英脉型,矿化表现出层控的特点,并多发育绿泥石化。通过对区域和典型矿床地质特征和区域成矿特征的总结,认为诺斯曼-维卢纳金成矿带的金主要赋存于铁硫化物或毒砂中,矿化与大规模的蚀变作用联系密切,太古宙绿岩带和条带状铁建造、深大断裂、褶皱等因素控制了矿床的分布。在此基础上,总结了绿岩带型金矿的找矿标志,并为在区内找矿提出若干建议。     

澳大利亚伊尔岗地块诺斯曼—维卢纳绿岩带金矿成矿作用

诺斯曼—维卢纳金成矿带位于澳大利亚西澳克拉通伊尔岗地块东部黄金省次级地体内,是澳大利亚最重要的绿岩带型金矿成矿区。区内矿床可分为3类：剪切带中的蚀变晕±石英脉型,是区内最为重要的矿化类型,典型矿床为卡尔古利金哩矿床,矿床赋存在金哩粗玄岩中,矿化发生在绢云母—黄铁矿化蚀变带内;石英网脉型,典型矿床为卡尔古利夏洛特山金矿床,矿床也产于金哩粗玄岩中,矿化主要发生在硅化外围的碳酸盐—黄铁矿化蚀变带内;纹层状石英脉型,矿化表现出层控的特点,并多发育绿泥石化。通过对区域和典型矿床地质特征和区域成矿特征的总结,认为诺斯曼—维卢纳金成矿带的金主要赋存于铁硫化物或毒砂中,矿化与大规模的蚀变作用联系密切,太古宙绿岩带和条带状铁建造、深大断裂、褶皱等因素控制了矿床的分布。在此基础上,总结了绿岩带型金矿的找矿标志,并为在区内找矿提出若干建议。     

Auriferous sediments of the Yilgarn Block, Western Australia

Gold-bearing sediments of mixed sedimentary-igneous composition, and associated largely with various basic volcanic rocks (greenstones), abound in the Archaean of the Yilgarn Block. In the Coolgardie district, where perhaps they are best developed, many important gold mines and several series of minor gold deposits occur in a few such horizons. Practically identical occurrences, even when highly metamorphosed, are also found elsewhere in greenstone belts-e.g. in the Southern Cross-Westonia-Yellowdine metamorphic zone.     

Low temperature Phanerozoic history of the Northern Yilgarn Craton, Western Australia

The Phanerozoic cooling history of the Western Australian Shield has been investigated using apatite fission track (AFT) thermochronology. AFT ages from the northern part of the Archaean Yilgarn Craton, Western Australia, primarily range between 200 and 280 Ma, with mean confined horizontal track lengths varying between 11.5 and 14.3 μm. Time–temperature modelling of the AFT data together with geological information suggest the onset of a regional cooling episode in the Late Carboniferous/Early Permian, which continued into Late Jurassic/Early Cretaceous time. Present-day heat flow measurements on the Western Australian Shield fall in the range of 40–50 mW m⁻². If the present day geothermal gradient of  18 ± 2 °C km⁻¹ is representative of average Phanerozoic gradients, then this implies a minimum of  50 °C of Late Palaeozoic to Mesozoic cooling. Assuming that cooling resulted from denudation, the data suggest the removal of at least 3 km of rock section from the northern Yilgarn Craton over this interval. The Perth Basin, located west of the Yilgarn Craton, contains up to 15 km of mostly Permian to Lower Cretaceous clastic sediment. However, published U–Pb data of detrital zircons from Permian and Lower Triassic basin strata show relatively few or no grains of Archaean age. This suggests that the recorded cooling can probably be attributed to the removal of a sedimentary cover rather than by denudation of material from the underlying craton itself. The onset of cooling is linked to tectonism related to either the waning stages of the Alice Springs Orogeny or to the early stages of Gondwana breakup.     

Numerical modelling of orogenic processes and gold mineralisation in the southeastern part of the Yilgarn Craton,Western Australia

We use numerical modelling codes to simulate aspects of some current hypotheses for the origin of gold deposits and hydrothermal systems in the Yilgarn Craton of Western Australia. In particular, we investigate conceptual models advocating vertically continuous hydrothermal systems as well as those invoking extensive lateral flow and possible links with advection of heat by late orogenic granitic magmatism. Numerical models of part of the Eastern Goldfields Province and Southern Cross Province have been built with FLAC3D, to simulate crustal‐scale coupled interaction between deformation and fluid flow. These illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upwards flow of fluids derived from melting and metamorphism in the deep crust. In some cases, downflow also occurs within the middle crust, at depths where fluid influx might trigger melting if the geothermal gradient were appropriate. The models indicate that tectonic wedging within a layered crust and diverging thrust systems that generate ‘pop‐up’ wedges may be important in facilitating efficient fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. However, the effect of topography appears more important than the precise depth or location of the site of fluid production in the deep crust. The effects of thermal convection and fluid‐fluid interaction have also been numerically modelled for a simplified section across the Kalgoorlie Terrane. Modelling under both hydrostatic and lithostatically overpressured pore‐pressure gradients has effectively delineated domains of convective fluid flow within the middle and upper crust, and has identified two generic sites that are favourable for fluid mixing, notably hangingwall and footwall environments in major shear zones, such as the Bardoc Shear, and in broad antiforms, such as the Goongarrie ‐ Mt Pleasant Antiform. The thermal effect of small plutons embedded in a regional metamorphic regime can cause significant lateral displacement of fluid convection patterns, over distances greater than pluton diameter, as well as more proximal effects on precipitation and dissolution of mineral species. However, these results are highly dependent on the pore‐pressure gradient and the permeability structure of the crust, and require magmatic and metamorphic fluid generation to be precisely timed with respect to deformation, thus reinforcing the dynamic feedback between deformation, magmatism and fluid production and migration.     

Chloritoid and staurolite stability: implications for metamorphism in the Archaean Yilgarn Block, Western Australia

Abstract The stability of quartz-chloritoid-staurolite-almandine-cordierite and aluminium silicates is used to constrain both metamorphic conditions and pressure-temperature trajectories for two localities within the 2700 Ma Archaean Yilgarn Block in Western Australia. Available experimental data are used to calculate thermodynamic data for a self-consistent set of equilibria between these minerals. A lower amphibolite facies locality from the margin of a lower strain area contains assemblages including quartz-chloritoid-staurolite-garnet-biotite with altered cordierite replacing chloritoid, quartz-staurolite-andalusite, and quartz-cordierite-andalusite-biotite. This locality was heated to 530–560°C in the andalusite field, at 4.2 kbar. A sample from a mid- to upper-amphibolite facies, highly strained locality contains relict staurolite enclosed by andalusite, in turn replaced by cordierite and muscovite with biotite and sillimanite in the matrix. The assemblage was heated isobarically from conditions near the maximum experienced by the lower grade locality of 560°C at 4.2 kbar to temperatures in excess of the andalusite-sillimanite transition but within the quartz plus muscovite stability field (600–650°C). The higher grade locality is close to a granitoid dome and sections based on gravity profiles reveal that this locality is underlain by granitoid at shallow depths. The higher grade metamorphism apparently reflects superposition of the thermal aureole on regional metamorphic conditions similar to those in the lower grade areas.     

Metamorphic evolution of the central Southern Cross Province,Yilgarn Craton,Western Australia

Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D₁) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造(BIF)有关,可分为2种类型:1深成—表生矿床;2表生—富集矿床,主要分布在尤恩米(Youanmi)地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

西澳大利亚伊尔岗克拉通铁矿床研究进展

伊尔岗克拉通位于澳大利亚西南部,是地球上最古老的克拉通之一。该克拉通内产出的铁矿床均与条带状含铁建造（BIF）有关,可分为2种类型：①深成—表生矿床;②表生—富集矿床,主要分布在尤恩米（Youanmi）地体中。深成—表生型铁矿床具有相似的变形历史、镁铁质火成岩围岩、深成热液蚀变事件和高品位的铁矿石类型。深成热液蚀变包括早期碳酸盐-磁铁矿蚀变、中期形成磁铁矿矿石、晚期碳酸盐-赤铁矿蚀变,但是这些矿床在岩相、变质程度、矿物学和地球化学方面都存在差异,目前还没有统一的成因模型。表生—富集型铁矿床可能是通过表生淋滤BIF中的硅质条带形成的,但不含硅质条带的BIF的出现,说明没有对硅质条带的选择性表生溶解也可以形成高品位矿体。     

An indirect lead isotope age determination of gold mineralization at the Corinthia mine,Yilgarn Block,Western Australia

The Corinthia lode‐gold deposit in amphibolite‐facies greenstone belt rocks in the Southern Cross Province of the Archaean Yilgarn Block contains a largely undeformed pegmatite dyke emplaced during the last phases of movement along the Fraser's‐Corinthia shear zone. Gold mineralization and shear zone development were synchronous, and a Pb‐Pb isochron age of 2620 ±6 Ma for pegmatite emplacement either indirectly dates mineralization, or places a minimum age constraint on the timing of mineralization. This age is in accord with a broadly synchronous dominant episode of Archaean lode‐gold mineralization throughout the Yilgarn Block.     

Archaean basin development and crustal extension in the northeastern Yilgarn Block,western Australia

Supracrustal sequences in the northeastern Yilgarn Block, Western Australia, form two contrasting lithotectonic associations. Most sequences were deposited in subaqueous to subaerial environments in a basin 120 km in width. The preserved succession reaches a maximum of e. 10 km in thickness in the central part of the basin, although there is no record of the basement on which it was deposited. In the later stages of its development, the basin was intruded by calc-alkaline granitoids, that are in part comagmatic with calc-alkaline volcanic rocks in the upper part of the stratigraphic succession. Brittle fracture (normal strike-faults and conjugate cross-faults), open, upright folds and peak low-strain metamorphism were coincident with granitoid emplacement.The eastern and western margins of the basin were greatly modified during a period of increased crustal extension, marked initially by the generation of a mildly peralkaline volcanic-plutonic suite, the volcanic members of which were deposited in linear, faultbounded depressions. A major growth fault indicated by thin persistent units of oligomictic and polymictic conglomerate and localized intense metasomatism was probably contemporaneous with felsic volcanic activity. Subsequent sediments, derived largely from granitic source areas, were deposited in narrow grabens. The youngest felsic plutonic rocks in the area include small bodies of alkali feldspar syenite many of which are found near the margins of the late, sediment-filled grabens. Tectonism at the eastern basin margin extended into the Proterozoic. In contrast to sequences at the centre of the basin, those at its margin have undergone isoclinal folding, ductile deformation and high-strain metamorphism.The geological framework of the northeastern Yilgarn Block emphasizes the importance of crustal extension in the development of younger Archaean supracrustal sequences and provides valuable support for extension-dependent greenstone belt evolutionary models (e.g., Groves and Batt).     

Chemical fingerprinting of multiple large-scale magmatic events in the Mesoproterozoic Bangemall Supergroup,Western Australia

Three major igneous events, dated at ～1465, ～1070 and ～500 Ma, are represented in the Proterozoic of central Western Australia, yet their extent is poorly understood. The compositions of dated mafic rocks from the western Bangemall Supergroup of Western Australia have been used to establish a chemical fingerprint for the ～1465 Ma and ～1070 Ma intrusive events, and to assign sills of an unknown age to one of the two events. A similar approach has been used to identify the extent of ～500 Ma magmatism. Low-pressure fractionation or accumulation has exerted a strong influence on the chemistry of rocks from all magmatic events, but distinctive trace-element ratios (e.g. Th/Nb, Nb/Zr) and rare-earth element (REE) chemistry (e.g. Eu/Eu?, (Gd/Yb)_CN) can discriminate different events. These ratios remain constant regardless of the degree of fractionation or accumulation, and reflect the chemistry of the respective mantle sources. Based on chemistry, ～500 Ma igneous rocks are not found in the Bangemall Supergroup. Neodymium model ages for ～1465 Ma sills overlap with crystallisation ages of subduction-related felsic intrusive rocks from the adjacent Gascoyne Complex, and this, combined with trace-element and REE chemistry, suggests that the mantle source for these sills underwent ～5% crustal contamination approximately 450 Ma prior to melting in a subduction zone environment. Unrealistically large amounts of contaminant are required to explain the chemistry of most ～1070 Ma sills, and their chemistry is better explained by melting of a heterogeneous mantle source, consistent with the overlap of Nd model ages and crystallisation ages. However, the relatively low εNd(T) and high ⁸⁷Sr/⁸⁶Sr(T), elevated light REE, Rb and Zr concentrations, and high Th/Nb of some ～1070 Ma eastern Bangemall Supergroup sills are indicative of crustal contamination. These sills are relatively depleted in chalcophile elements and platinum-group elements, consistent with coeval precipitation of sulfides and crustal contamination. The overlap in the maximum depositional age of host-rocks with the crystallisation age of some sills, the confining of most ～1465 Ma sills to older parts of the stratigraphy, and field evidence showing that some sills belonging to both the ～1465 and ～1070 Ma events were intruded into wet sediments, indicate a close temporal relationship between sedimentation and sill intrusion.     

Stratigraphy and timing of eolianite deposition on Rottnest Island, Western Australia

Over 100 whole-rock amino acid racemization (AAR) ratios from outcrops around Rottnest Island (32.0° S Latitude near Perth) indicate distinct pulses of eolian deposition during the late Quaternary. Whole-rock d-alloisoleucine/l-isoleucine (A/I) ratios from bioclastic carbonate deposits fall into three distinct modal classes or “aminozones.” The oldest, Aminozone E, averages 0.33 ± 0.04 (n = 21). Red palaeosol and thick calcrete generally cap the Aminozone E deposits. A younger Aminozone C averages 0.22 ± 0.03 (n = 63); comprising two submodes at 0.26 ± 0.01 (n = 14) and 0.21 ± 0.02 (n = 49). Multiple dune sets of this interval are interrupted by relatively weak, brown to tan “protosols.” A dense, dark brown rendzina palaeosol caps the Aminozone C succession. Ratios from Holocene dune and marine deposits (“Aminozone A”) center on 0.11 ± 0.02 (n = 15), comprising submodes of 0.13 ± 0.01 (9) and 0.09 ± 0.01 (6). Calibration of A/I averages from Aminozones E and A are provided by U/Th and ¹⁴C radiometric ages of 125,000 yr (marine oxygen isotope stage (MIS) 5e and 2000-6000 ¹⁴C yr B.P. (MIS 1), respectively. The whole-rock A/I results support periodic deposition initiated during MIS 5e, continuing through MIS 5c, and then peaking at the end of MIS 5a, about 70,000-80,000 yr ago. Oceanographic evidence indicates the area was subjected to much colder conditions during MIS 2-4 (10,000 to 70,000 yr ago), greatly slowing the epimerization rate. Eolianite deposition resumed in the mid Holocene (∼6000 yr ago) up to the present. The A/I epimerization pathway constructed from Rottnest Island shows remarkable similarity to that of Bermuda in the North Atlantic (32° N Latitude). These findings suggest that, like Bermuda, the eolian activity on Rottnest occurred primarily during or shortly after interglacial highstands when the shoreline was near the present datum, rather than during glacial lowstands when the coastline was positioned 10-20 km to the west.     

Granite weathering and silcrete formation on the Yilgarn Block,Western Australia

Weathering profiles developed on granitic rocks, exposed in the breakaways of the Barr‐Smith Range in the N of the Yilgarn Block of Western Australia, consist of kaolinitic saprolites merging upwards into silcrete, sandstone and grit. The sandstones and silcretes may also form columns or dykes, penetrating downwards into the saprolite. The silcretes are cemented by quartz and anatase, with zircon (QAZ‐cement), and‐the sandstones are cemented by aluminosilicates, either apparently amorphous (as siliceous allophane) or partly crystalline, as kaolinite and opaline silica. Transitional zones between silcretes and sandstones have all cement types. The profiles are characterized by low concentrations of alkalis and alkaline earths and most metals. The QAZ‐silcrete horizons may contain over 3% TiO₂ and 1000 p.p.m. Zr. The profiles evolved through at least four stages: (i) Formation of the deep saprolite‐sand weathering profile by kaolinization of feldspar and mica at depth, and the solution of kaolinite near the top of the profile, causing settling of resistant quartz grains, (ii) Precipitation of QAZ‐cement, the TiO₂ and SiO₂ being derived partly by lateral migration from upslope. (iii) Precipitation of aluminosilicates, in the sandstone and the saprolite. (iv) Erosion and exposure of the profiles by pedimentation. A similar profile occurs further S, at Gabbin, but no QAZ‐silcrete is present and the only exposures are in exploration pits. The kaolinitic saprolite‐quartz sand profiles probably formed under humid conditions, as the equivalents of ferruginous laterite developed on more basic rocks nearby and of lateritic bauxite in the Darling Range. However,’ the sand was a surface horizon and there is no evidence that there was ever a ferruginous zone at these sites. The sequential precipitation of QAZ‐ and aluminosilicate‐cements was probably, a response to increasing aridity and reduced groundwater flow. Aluminosilicate‐cemented materials tend to disaggregrate on exposure but they are probably more abundant than the more prominent QAZ‐silcretes.     

Structural-event framework for the eastern Yilgarn Craton,Western Australia,and its implications for orogenic gold

The NNW-trending tectonic grain of the eastern Yilgarn Craton (EYC) was established as a result of predominantly ENE–WSW directed extension (D1 and D3) and E(ENE)–W(WSW) (D2, D4) to NE–SW directed (D5) contraction. The result has been a succession of NNW-striking temporally discrete fabric elements, which can be difficult to interpret reliably at any single location. Despite this, many past workers interpreted the NNW-striking fabric as the result of only one regional contractional event, and used it as a marker for correlating structural events across the region. In order to unravel the complexity, this paper presents a new sixfold (D1–D6) deformation nomenclature based on >10,000 new mesoscale structural observations, including their kinematic analysis and cross-cutting relationships. These mesoscale data were referenced with regional 3D map patterns, stratigraphic-magmatic-metallogenic considerations, and deep seismic reflection images. This integrated geodynamic-architectural approach is applicable to solving structural-event histories in other polydeformed terrains. Gold mineralisation occurred during the first five events, but was particularly concentrated from D3 onwards. The D3 event marked the most profound change in the tectonic evolution of the EYC, with changes in greenstones, granites and tectonic mode (lithospheric extension and core complexes), with the first significant gold deposited within extensional shear zones that dissect the crust. Later contraction (D4) was imposed at a high angle to the previously established anisotropic architecture. The outcome was the creation of a new dynamic permeability framework, which resulted in gold mineralisation during NNW-striking sinistral strike-slip faulting and associated thrusting. A further stress switch (D5) further modified the architecture resulting in N- to NNE-striking dextral strike-slip faulting, and the final period of gold mineralisation, before late-stage extension (D6).     

Archaean basic volcanism in the Eastern Goldfields Province,Yilgarn Block,Western Australia

Petrographic, petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the Eastern Goldfields Province, Yilgarn Block, Western Australia. These are termed the high magnesian series basalts (HMSB), the low magnesian series basalts (LMSB) and the siliceous high magnesian series basalts (SHMSB).The HMSB and SHMSB constitute differentiation series which contain both high MgO (9.5–14 wt.%) and low MgO (<9.5 wt.%) members. These suites are commonly characterized by igneous textures indicative of very rapid crystallization suggesting high eruption temperatures. This feature clearly distinguishes those low MgO members of HMSB which contain amphibole pseudomorphs after spherulitic-textured pyroxene from compositionally similar LMSB. The LMSB are generally characterized by an intergranular texture consisting of plagioclase laths and interstitial amphibole pseudomorphs after pyroxene grains. Variolitic-textured basalts are common and appear to be restricted to the SHMSB suite of basic volcanics.The HMSB and LMSB were derived from source mantle regions which were variably depleted in the incompatible elements. Archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived. The depleted nature of the source regions may have been induced by earlier small degree (< 5%) partial melting events with subsequent extraction of a proportion of that melt. Variations in both the degree of such melting, and the proportions of melt removal, could induce considerable heterogeneity of incompatible elements in the Archaean upper mantle.Source mantle regions of the SHMSB were variably enriched in the incompatible elements and water and parental magmas of the SHMSB were derived from moderately hydrous conditions of partial melting.The relative proportions of each basalt suite varies considerably between the layered successions examined. For example, the basic volcanics overlying the komatiites at Kambalda are SHMSB, while the footwall volcanics consist predominantly of HMSB with subordinate LMSB. However, the Norseman succession, where no ultramafic volcanics are known to occur, is comprised mainly of LMSB with a smaller proportion of HMSB.     

Early extension in the Late Archaean northeastern Eastern Goldfields Province,Yilgarn Craton,Western Australia

A series of linked extensional detachments, transfer faults, and sediment- and volcanic-filled half-grabens that pre-date regional folding are described in the Late Archaean Margaret anticline, Eastern Goldfields Province, Yilgarn Craton, Western Australia. Coeval structures and rock units include layer-parallel extensional detachments, transfer faults (high-angle rotational faults rooted in the detachments and linking layer-parallel shear zones with varying amounts of extension); felsic intrusions, either as granitoids emplaced in or below the detachments, or as fine-grained intrusive bodies emplaced above the detachments and controlled by the high-angle faults; and half-grabens controlled by the high-angle faults and filled with clastic sedimentary and volcanic rocks. At least 1500 m of section is excised across the detachments. The detachments and high-angle faults are folded by the east-northeast regional compression that formed the Margaret anticline. Extensional deformation in the Margaret anticline is correlated with the regionally recognised felsic magmatism and associated volcanic and volcaniclastic basin fill dated at approximately 2685–2670 Ma across the Eastern Goldfields Province. This suggests the extensional event was province-wide and post-dated initial greenstone deposition (at around 2705 Ma) but pre-dated regional compressive deformation. We suggest the extension is the result of a thermal anomaly in the crust, generated by the insulating effect of a thick pile (of the order of 10 km or greater) of mafic and ultramafic volcanic rocks on precursor Archaean felsic crust. The thermal anomaly has generated renewed production of felsic and mafic volcanic rocks, coeval with uplift and extension in the upper crust.     

The crustal continuum model for late-Archaean lode-gold deposits of the Yilgarn Block,Western Australia

Most Archaean gold ores belong to a coherent genetic group of structurally controlled lode-deposits that are characteristically enriched in Au with variable enrichments in Ag, As, W, Sb, Bi, Te, B and Pb, but rarely Cu or Zn, and are surrounded by wallrock alteration haloes enriched in K, LILE and CO₂, with variable Na and/or Ca addition. Evidence from the Yilgarn Block of Western Australia, combined with similar evidence from Canada and elsewhere, indicates that such deposits represent a crustal continuum that formed under a variety of crustal régimes over at least a 15 km crustal profile at PT conditions ranging from 180°C at <1 kb to 700°C at 5 kb. Individual deposits, separated by tens to hundreds of kilometres, collectively show transitional variations in structural style of mineralisation, vein textures, and mineralogy of wallrock alteration that relate to the PT conditions of their formation at varying crustal depths. Specific transitions within the total spectrum may be shown also by deposits within gold camps, although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt. Recognition of the crustal continuum of deposits implicates the existence of giant late-Archaean hydrothermal systems with a deep source for the primary ore fluid. A number of deep fluid and solute reservoirs are possible, including the basal segments of greenstone belts, deep-level intrusive granitoids, mid-to lower-crustal granitoidgneisses, mantle lithosphere, or even subducted oceanic lithosphere, given the probable convergent-margin setting of the host greenstone terranes. Individual stable and radiogenic isotope ratios of fluid and solute components implicate fluid evolution from, or equilibrium with, a number of these reservoirs, stressing the potential complexity of pathways for fluid advection to depositional sites. Lead and strontium isotope ratios of ore-associated minerals provide the most persuasive evidence for fluid advection through deep-level intrusive granitoids or granitoid-gneiss crust, whereas preliminary oxygen isotope data show that mixing of deeply sourced fluid and surface waters only occurred at the highest crustal levels recorded by the lode gold deposits.