Diamond exploration in Western Australia

Lamproitic rocks of Western Australia have produced significant amounts of diamond and have given rise to the world's largest diamond mine. This has led to a reappraisal of the relationship between diamond and its host rock as well as new exploration techniques in the search for diamond. Procedures for success follow a modelling system using the most modern methods available in the Earth-related sciences.     

Fuzzy inference systems for prospectivity modeling of mineral systems and a case-study for prospectivity mapping of surficial Uranium in Yeelirrie Area,Western Australia

A Mamdani-type fuzzy inference system for prospectivity modeling of mineral systems is described. The system is a type of knowledge-driven symbolic artificial intelligence that is transparent, intuitive and is easy to construct by geologists because they are built in natural language and use linguistic values. No examples are used for training the system and expert-opinions are incorporated indirectly in terms of objective mathematical functions, which reduce the possibility of over-emphasizing the known deposits usually used as training data. The cognitive reasoning of the exploration geologist is captured in explicit if–then type of statements written in natural language using linguistic values. Conditional dependencies in the exploration data sets are managed through the use of fuzzy operators. A case study for surficial uranium prospectivity modeling in the Yeelirrie area, Western Australia, is used to demonstrate the approach. In the output prospectivity map, the SE-NW trending Yeelirrie and E-W trending Hinkler's Well palaeochannels show high prospectivity, while other channels show very low prospectivity ranges. The known surficial uranium deposits fall in high prospectivity areas, although minor showings and anomalies in the southern part of the study area fall in low prospectivity areas. A comparison of the prospectivity model with the radiometric image shows that several channels showing high surface uranium concentrations in the NW and NE quadrants may not be prospective.     

Weights-of-evidence and logistic regression modeling of magmatic nickel sulfide prospectivity in the Yilgarn Craton,Western Australia

Bayesian weight-of-evidence and logistic regression models are implemented in a GIS environment for regional-scale prospectivity modeling of greenstone belts in the Yilgarn Craton, Western Australia, for magmatic nickel sulfide deposits. The input variables for the models consisted of derivative GIS layers that were used as proxies for mappable exploration criteria for magmatic nickel sulfide deposits in the Yilgarn. About 70% of the 165 known deposits of the craton were used to train the models; the remaining 30% was used to validate the models and, therefore, had to be treated as if they had not been discovered. The weights-of-evidence and logistic regression models, respectively, classify 71.4% and 81.6% validation deposits in prospective zones that occupy about 9% of the total area occupied by the greenstone belts in the craton. The superior performance of the logistic regression model is attributed to its capability to accommodate conditional dependencies amongst the input predictor maps, and provide less biased estimates of prospectivity.     

澳大利亚西部油气勘探投资前景分析

阐述了澳大利亚良好的石油投资外部环境，西澳地区尤其是坎宁盆地具有极大的油气勘探开发潜力，是近两年来招标活动频繁、推出区块最多的地区之一。坎宁盆地分为南、北两个拗陷和中部隆起，其古生界发育了有利的生储盖组合，且以隆起、断阶和斜坡为成藏有利区带。初步评价认为，位于Fitzroy凹陷及其南侧断阶上的招标区块勘探风险较低，凹陷另侧(北侧)斜坡已发现6个油田。     

澳大利亚西澳州2004-2005年度矿产勘探综述

介绍了澳大利亚西澳州2004—2005年度金、镍、铁、钛、锆、金刚石矿床的勘探情况和取得的找矿成果。简述了2004—2005年度西澳州矿产勘探(包括石油勘探)消费支出、不同矿产品所占勘探支出的份额、钻探、矿山土地复垦及环境保护的情况。     

Regional exploration for channel and playa uranium deposits in Western Australia using groundwater

Shallow calcrete aquifers in the central north of the Yilgarn Craton in Western Australia are the host to numerous secondary carnotite U deposits. Sampling and analysis of approximately 1400 shallow aquifer groundwaters were conducted to test if U mineralisation of this type may be found using a >5 km sample spacing. Results show this can be achieved. All the economic deposits and most of the minor deposits and occurrences are associated with groundwater that has carnotite (KUO₂VO₄) approaching or exceeding saturated conditions. Soluble U concentrations alone identified the largest deposit (Yeelirrie) and several smaller deposits, but this parameter was not as successful as the mineral saturation indices. Palaeodrainage distribution and thickness of cover combined with surface drainage and catchment boundaries provided background information of U primary sources and for areas with the highest exploration potential for channel and playa U deposits. Granites in the SE of the study area are less prospective with regard to secondary U deposits. Groundwater geochemistry in conjunction with palaeodrainage mapping may greatly improve exploration through cover where radiometric geophysics is not effective. The study of regional, shallow groundwater for U shows multiple benefits for mineral exploration, the economy and potable water quality.     

Regional prospectivity analysis for hydrothermal-remobilised nickel mineral systems in western Victoria,Australia

Fuzzy logic mineral prospectivity modelling was performed to identify camp-scale areas in western Victoria with an elevated potential for hydrothermal-remobilised nickel mineralisation. This prospectivity analysis was based on a conceptual mineral system model defined for a group of hydrothermal nickel deposits geologically similar to the Avebury deposit in Tasmania. The critical components of the conceptual model were translated into regional spatial predictor maps combined using a fuzzy inference system. Applying additional criteria of land use restrictions and depth of post-mineralisation cover, downgrading the exploration potential of the areas within national parks or with thick barren cover, allowed the identification of just a few potentially viable exploration targets, in the south of the Grampians-Stavely and Glenelg zones. Uncertainties of geological interpretations and parameters of the conceptual mineral system model were explicitly defined and propagated to the final prospectivity model by applying Monte Carlo simulations to the fuzzy inference system. Modelling uncertainty provides additional information which can assist in a further risk analysis for exploration decision making.     

Geochemical exploration for base and precious metal sulphides associated with the Jimberlana Dyke, Western Australia

A geochemical exploration case history is described, dealing with the search for Ni and Cu sulphides, platinoids and chromite associated with the Jimberlana Dyke in Western Australia. The Jimberlana Dyke is a strongly differentiated mafic-ultramafic intrusive exhibiting similarities with the Great Dyke of Rhodesia.Soil sampling was used as the main search technique, the minus 80-mesh fraction of the near-surface soils being analysed for Ni, Cu, Co and Cr by atomic absorption spectrophotometry. Evaluation methods relied initially on visual inspection of element profile plots. Follow-up soil sampling and auger drilling were carried out in anomalous areas to define targets for deeper drill-testing. Subsequent statistical evaluation of the geochemical data has largely confirmed the initial interpretation but has in addition helped to elucidate the excellent correlation existing between the near-surface soil sampling and bedrock geochemistry.The programme was successful in locating a number of occurrences of sulphide mineralization, many of which contained copper and nickel sulphides. The most significant anomaly was at Bronzite Ridge, being related to a small pipe-like body of sulphides with grades up to 2% Ni and 2% Cu. All sulphide intersections tested recorded low values for the platinoid metals. No significant chromite segregations were located and high Cr values in soils (> 1%) can be attributed to a combination of lateritic and eluvial concentration processes.Although no economic mineralization was located the programme demonstrated the extreme sensitivity of Cu as an indicator of sulphides even through moderately thick soil cover.     

双频激电法在西澳矿产勘查中的应用

结合双频激电法在西澳大利亚典型覆盖区(热带稀树草原覆盖区)的应用实践,讨论了在特殊地质景观条件下,双频激电法在工作设计中装置选择、工作频率选择、接地电阻改善、电磁耦合克服等方面遇到的问题以及解决这些问题所提出的技术方法,总结整理了一些具有代表性的工作准则和野外工作经验。结论表明:双频激电法在澳洲某金矿的特殊地理环境下的应用效果良好,圈定的激电异常符合地质预期,有进行工程验证的必要;使用双频激电法,在设计及施工过程中应综合分析各方面的影响因素,科学决策,并对可能影响测量数据准确性的因素采取有效的措施,这样才能充分发挥其技术优势,取得令人满意的成果。     

Reducing subjectivity in multi-commodity mineral prospectivity analyses: Modelling the west Kimberley,Australia

Predicting realistic targets in underexplored regions proves a challenge for mineral explorers. Knowledge-driven prospectivity techniques assist in target prediction, and can significantly reduce the geographic search space to a few locations. The mineral prospectivity of the underexplored west Kimberley region was investigated following interpretation of regional gravity and magnetic data. Emphasis was placed on identifying geological structures that may have importance for the mineral prospectivity of the region. Subsurface structure was constrained through combined gravity and magnetic modelling along three transects. Crustal-scale structures were interpreted and investigated to determine their depth extent. These interpretations and models were linked to tectonic events and mineralization episodes in order to map the distribution of minerally prospective regions using a knowledge-driven mineral systems approach. A suite of evidence layers was created to represent geological components that led to mineralization, and then applied to each mineral system where appropriate. This approach was taken to provide a more objective basis for prospectivity modelling. The mineral systems considered were 1) magmatic Ni-sulphide, 2) carbonate-hosted base metals, 3) orogenic Au, 4) stratiform-hosted base metals and 5) intrusion-related base metals (including Sn–W, Fe-oxide–Cu–Au and Cu–Au porphyry deposits). These analyses suggest that a geologically complex belt in the Kimberley Basin at the boundary to the King Leopold Orogen is prospective for magmatic-related hydrothermal mineral systems (including Ni, Au and Cu). The Lennard Shelf is prospective for carbonate-hosted base metals around a feature known as the 67-mile high, and parts of the King Leopold Orogen are prospective for stratiform-hosted base metals. These results show that knowledge-driven mineral system modelling is effective in identifying prospectivity in regional-scale studies of underexplored areas, as well as drastically reducing the search space for explorers working in the west Kimberley.     

Tertiary stratigraphy of Western Australia

This paper is a summary of the present knowledge of the Tertiary stratigraphy of Western Australia. Also included is new information on the Cainozoic of the Carnarvon Basin, a result of petroleum exploration in the area.

Tertiary rocks formed during more than one cycle of deposition in three basins (Eucla, Perth, and Carnarvon), and also as thin units deposited in a single transgression along the south coast. The Tertiary stratigraphy of the Bonaparte Gulf Basin is not well known.

Drilling in the Eucla Basin has encountered up to 400 m of Tertiary in the south central part, with uniform thinning towards the margins. The section begins with a middle‐upper Eocene carbonate unit which represents the dominant event in the Tertiary sedimentation in this basin. More carbonates were deposited in the late Oligocene‐early Miocene and middle Miocene.

Along the south coast, the so‐called Bremer Basin, the Plantagenet Group (up to 100 m) of siltstone, sandstone, spongolite, and minor limestone, was deposited during the late Eocene.

The Perth Basin contains up to 700 m of Tertiary sediment, formed during at least two phases of sedimentation. The upper Paleocene‐lower Eocene Kings Park Formation consists of marine shale, sandstone, and minor limestone, with a thickness of up to 450 m. The Stark Bay Formation (200 m) includes limestone, dolomite, and chert formed during the early and middle Miocene. Events after deposition of the Stark Bay Formation are not well known.

The northern Carnarvon Basin and Northwest Shelf contain by far the most voluminous Tertiary sediment known from Western Australia: 3500 m is known from BOCAL's Scott Reef No. 1. A more usual maximum thickness is 2500 m. Most sediments were laid down in four episodes, separated by unconformities: late Paleocene‐early Eocene; middle‐late Eocene; late Oligocene‐middle Miocene; and late Miocene to Recent.

The Paleocene‐early Eocene cycle consists of about 100–200 m (up to 450 m in the north) of carbonate, shale, and marl of the Cardabia Group containing rich faunas of planktonic foraminifera.

The middle‐late Eocene sediments include diverse rock types. Marine and nonmarine sandstone formed in the Merlinleigh Trough. At the same time, the Giralia Calcarenite (fauna dominated by the large foraminifer Discocyclina) and unnamed, deeper water shale, marl, and carbonate (with rich planktonic foraminiferal faunas) formed in the ocean outside the embayment. Thickness is usually of the order of 100–200 m.

The main cycle of sedimentation is the late Oligocene‐middle Miocene, during which time the Cape Range Group of carbonates formed. This contains dominantly large foraminiferal faunas, of a wide variety of shallow‐water microfacies, but recent oil exploration farther offshore has recovered outer continental shelf facies with abundant planktonic foraminifera. A minor disconformity representing N7 and perhaps parts of N6 and N8 is now thought to be widespread within the Cape Range Group. The last part of this cycle resulted in sedimentation mainly of coarse calcareous marine sandstone (unnamed), and, in the Cape Range area, of the sandstone and calcareous conglomerate of the Pilgramunna Formation. Maximum thickness encountered in WAPET wells is 900 m.

After an unconformity representing almost all the late Miocene, sedimentation began again, forming an upper Miocene‐Recent carbonate unit which includes some excellent planktonic faunas. Thickness is up to 1100 m.

Thin marine sediments of the White Mountain Formation outcrop in the Bonaparte Gulf Basin. They contain some foraminifera and a Miocene age has been suggested.     

Significance of weathering and regolith/landscape evolution for mineral exploration in the NE Albany-Fraser Orogen,Western Australia

The Albany-Fraser Orogen (AFO), southeast Western Australia, is an underexplored, deeply weathered regolith-dominated terrain that has undergone complex weathering associated with various superimposed climatic events. For effective geochemical exploration in the AFO, integrating landscape evolution with mineralogical and geochemical variations of regolith and bedrock provides fundamental understanding of mechanical and hydromorphic dispersion of ore and pathfinder elements associated with the different weathering processes.In the Neale tenement, northeast of the AFO, a residual weathering profile that is 20-55 m thick was developed under warm and humid climatic conditions over undulating Proterozoic sheared granitoids, gneisses, schists and Au-bearing mafic rocks. From the base, the typical weathering profile consists of saprock, lower ferruginous saprolite, upper kaolinitic saprolite and discontinuous silcrete duricrust or its laterally coeval lateritic residuum. These types of duricrusts change laterally into areas of poorly-cemented kaolinitic grits or loose lateritic pisoliths and nodules.Lateritic residuum probably formed on remnant plateaus and was transported mechanically under arid climatic conditions over short distances, filling valleys to the southeast. Erosion of lateritic residuum exposes the underlying saprolite and, together with dilution by aeolian sands, constitutes the transported overburden (2-25 m thick). The reworked lateritic materials cover the preserved silcrete duricrusts in valleys. The lower ferruginous saprolite and lateritic residuum are well developed over mafic and sulphide-bearing bedrocks, where weathering of ferromagnesian minerals and sulphides led to enrichment of Fe, Cu, Ni, Cr, Co, V and Zn in these units. Kaolinitic saprolite and the overlying pedogenic silcrete are best developed over alkali granites and quartzofeldspathic gneisses, which are barren in Au and transition elements, and enriched in silica, alumina, rare earth and high field strength elements.A residual Au anomaly is formed in the lower ferruginous saprolite above a Au -bearing mafic intrusion at the Hercules prospect, south of the Neale tenement, without any expression in the overlying soil (< 20 cm). Conversely, a Au anomaly is recorded in the transported cover, particularly in the uppermost 3 m at the Atlantis prospect, 5 km southwest of the Hercules prospect. No anomalies have been detected in soils using five different size fractions (> 2,000 μm, 2,000-250 μm, 250-53 μm, 53-2 μm and < 2 μm). Therefore, soil cannot be efficiently applied as a reliable sampling medium to target mineralization at the Neale tenement. This is because mechanical weathering was interrupted by seasonal periods of intensive leaching under the present-day surface conditions and/or dilution by recently deposited aeolian sediments which obscure any signature of a potential Au anomaly in soils. Therefore, surface soil sampling should extend deeper than 20 cm to avoid dilution by aeolian sands and seasonal leaching processes. Regolith mapping and the distinction between the residual and transported weathering products are extremely significant to follow the distal or proximal mineralization.     

Vectorial fuzzy logic: A novel technique for enhanced mineral prospectivity mapping,with reference to the orogenic gold mineralisation potential of the Kalgoorlie Terrane,Western Australia

Modern exploration is a multidisciplinary task requiring the simultaneous consideration of multiple disparate geological, geochemical and geophysical datasets. Over the past decade, several research groups have investigated the role of Geographic Information Systems as a tool to analyse these data. From this research, a number of techniques has been developed that allow the extraction of exploration‐relevant spatial factors from the datasets. The spatial factors are ultimately condensed into a single prospectivity map. Most techniques used to construct prospectivity maps tend to agree, in general, as to which areas have the lowest and highest prospectivities, but disagree for regions of intermediate prospectivity. In such areas, the prospectivity map requires detailed interpretation, and the end‐user must normally resort to analysis of the original datasets to determine which conjunction of factors results in each intermediate prospectivity value. To reduce this burden, a new technique, based on fuzzy logic principles, has been developed for the integration of spatial data. Called vectorial fuzzy logic, it differs from existing methods in that it displays prospectivity as a continuous surface and allows a measure of confidence to be incorporated. With this technique, two maps are produced: one displays the calculated prospectivity and the other shows the similarity of input values (or confidence). The two datasets can be viewed simultaneously as a three‐dimensional perspective image in which colour represents prospectivity and topography represents confidence. With the vectorial fuzzy logic method, factors such as null data and incomplete knowledge can also be incorporated into the prospectivity analysis.     

Tectonic setting and regional implications of ca 2.2 Ga mafic magmatism in the southern Hamersley Province,Western Australia

Although the presence of post-Hamersley Group mafic intrusive and extrusive rocks in the southern Hamersley province of Western Australia has been known since the area was first mapped in the early 1960s, details of the age, tectonic setting and significance of these rocks have only recently been determined, and are still controversial. These rocks are most commonly interpreted as the products of two temporally distinct periods of continental extension, separated by a hiatus of ～370 m.y. represented by the unconformity at the base of the Wyloo Group. However, the integration of published geochronological and geochemical data with detailed field observations documented in this study shows that ca 2.2 Ga dolerite sills pre- and post-date this unconformity, and were intruded during Ophthalmian orogenesis in a retroarc foreland basin. Furthermore, ca 2.2 Ga mafic magmatism is interpreted to include the Cheela Springs Basalt and is related to subduction beneath the southern Hamersley province, most likely resulting in accretion of part of the Gascoyne Province to the Pilbara Craton.     

Soil and landscape studies in Western Australia

The main features of climate and geology relevant to a discussion of the soils and landscapes of Western Australia are briefly reviewed. The pattern of drainage is described in terms of six drainage divisions, each with characteristic physiography and soils. In the interior, this involves a reconstruction of an ancient drainage system, based on the maps of the Atlas of Australian Soils. It is shown, on a continental scale, that the major divides are characterised not only by deep, chemically weathered, often lateritic profiles, but also by extensive sandy deposits derived from these preweathered materials.     

Mesoproterozoic rift sedimentation,fluid events and uranium prospectivity in the Cariewerloo Basin,Gawler Craton,South Australia

The Cariewerloo Basin formed in the Mesoproterozoic following assembly of the Gawler Craton, South Australia, and was filled by arenaceous redbeds of the Pandurra Formation. While previous regional-scale work reveals a basin with similar size and sedimentary fill to the Proterozoic Athabasca and Kombolgie basins that host unconformity-related uranium deposits, few details of the Cariewerloo Basin are known. In this study, stratigraphy, petrography, lithogeochemistry, stable isotope geochemistry and ⁴⁰Ar/³⁹Ar geochronology are integrated to clarify the depositional history of the Pandurra Formation, and to assess fluid events in the basin that could be linked to the formation of uranium deposits. In the study area, the Pandurra Formation was deposited in two eastward-thickening packages that terminate at faulted basement uplifts, interpreted as half-grabens that formed in a continental rift system as the eastern Gawler Craton underwent extension. Deposition occurred between 1575 Ma (latest Hiltaba Suite age) and ca 1490 Ma, the ⁴⁰Ar/³⁹Ar age of diagenetic illite in the basal Pandurra. Diagenesis involving fluids having δ¹⁸O and δ²H values between –2.1 and 3.6‰, and between –66 and –8‰, respectively, occurred at around 150°C. Protracted diagenesis preferentially occurred in the upper Pandurra Formation based on petrography and Pearce Element Ratios that show complete replacement of detrital lithic and feldspathic grains by diagenetic phyllosilicates, and younger ⁴⁰Ar/³⁹Ar ages between ca 1330 and 1200 Ma that record fluid events later into basin history. Conversely, the basal Pandurra Formation shows better preservation of detrital grains, and older ⁴⁰Ar/³⁹Ar ages around 1450 Ma that suggest these strata became closed to fluid flow earlier in basin history. Although, based on O-isotope ratios, fluid–rock interaction did not occur in the Cariewerloo Basin to the same extent as that in the Athabasca or Kombolgie basins, it is possible that a uranium deposit formed where the upper Pandurra Formation was in contact with metasedimentary basement units outside the present basin margins.     

Groundwater calcrete deposits in Australia some observations from Western Australia

Groundwater calcretes are a non‐pedogenic form of calcrete occurring in broad fossil drainage systems. In Australia, they only occur north of about latitude 30°S and their formation requires arid conditions with very high potential evaporation as well as the periodic recharge of groundwater systems. They are linear, tabular limestone bodies, averaging about 10 m thick, occurring at or close to the surface and forming gentle mounds. The most extensive are located in the centre of the zone of distribution, where the climatic conditions are optimal, and are of the order of 100 km by 10 km. Some groundwater calcretes are fossil and strongly dissected, others are modern and still in process of formation.

A model of carbonate precepitation below the water table, i.e. in the phreatic zone, resulting in continuous calcrete growth, accounts for their morphology, their textures and the apparently inverted stratigraphy suggested by carbon‐14 dating. In areas of maximum growth, small anticlines develop which have structures engendered by both shear and concentric folding due to upward growth pressure.