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1.
The Igarapé Bahia, situated in the Carajás Mineral Province, is a world-class example of a lateritic gold deposit. It has developed under tropical weathering conditions since at least the Eocene and resulted in a regolith cover of at least 100 m thickness. The regolith is dominated by ~ 80 m thick ferruginous saprolite containing gossan bodies that constitute the main Au ore. Above saprolite the regolith stratigraphy has been established considering two distinct domains. One composed of residual materials and the other transported materials deposited over palaeochannels. In the residual domain the ferruginous saprolite grades upwards into a fragmental duricrust, interpreted as a collapsed zone, and then into different types of ferruginous duricrusts. Over palaeochannel the ferruginous saprolite is truncated by poorly sorted ferruginous sediment of variable composition that grades upwards into the ferruginous duricrusts formed over transported materials. Lateritization took place during a marked period that transformed the colluvium of the residual domain, and the transported materials accumulated in the channel depressions, into the ferruginous duricrust units. A later bauxitization event has overprinted all duricrust types but has mostly affected the duricrusts over the palaeochannel forming gibbsitic nodules. All duricrusts were finally covered by a transported layer of latosol which flattened the whole landscape in the Carajás region. Gold shows a depletion trend across the regolith but is enriched in the fragmental duricrust below the ferruginous duricrust from which gold is leached. Gold is also chemically dispersed laterally into the fragmental duricrust, but lateral Au dispersion in the ferruginous duricrusts of the residual domain is probably also influenced by colluvial transport. Metals associated with Au mineralization (Cu, U, Mo, Pb, Ag, LREE, Sn, W, Bi, Sb and P) are generally depleted in the saprolite but most of them are still anomalous. The fragmental and ferruginous duricrusts are more leached but the tests performed to estimate the dispersion potential of metals contained in the ferruginous duricrust show that some metals are still significantly anomalous especially Au, Ag and Cu. However, if ferruginous duricrusts are used as an exploration sample media their environment of formation must be considered. Metal depletion is generally more advanced in the ferruginous duricrusts developed in the vicinities of palaeochannels as oppose to those developed in residual domain. On the contrary, Au over palaeochannel areas is enriched in the upper bauxitized ferruginous duricrusts and in their gibbsitic nodules as a result of lateral chemical transport that is more widespread than in the colluvium over residual domain. The latosol is highly depleted in most metals due to its transported nature. However, the nodular fractions of the latosol show the greatest dispersion potential especially for Au, Ag, W, U, Bi and Sn. It can incorporate magnetic nodules that bring a rich suit of metals associated to the magnetic gossans, and non-magnetic nodules, classified as concretion and pisolites, which bring metals enriched or dispersed in the ferruginous duricrusts. This suggests that Lag constitutes a promising sample medium for geochemical exploration in the lateritic terrains of the Carajás region. 相似文献
2.
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. 相似文献
3.
Raymond E. Smith 《Journal of Geochemical Exploration》1987,28(1-3)
In some glaciated regions, weathered mantles, formed under previous climatic regimes, were not always eroded bare by glacial activity, but instead lie buried beneath the glacial overburden. It is obviously important that geochemical exploration programs in such terrain should take into account some of the known regolith features seen in deeply weathered regions. The effects of glacial action upon deeply weathered terrain are considered in three conceptual models. The critical factor in each is the depth of truncation of the preglacial weathering profile. All three proposed models are dynamic systems and a wide range of intermediate situations must be expected.In Model 1 a complete, largely undisturbed, weathering profile is preserved beneath till. The key characteristic is preservation of a lateritic duricrust which may contain areally large (up to 200 km2) geochemical anomalies. It would be critical, firstly, that the duricrust be sufficiently continuous to allow a reasonable success rate in its being sampled, and secondly that the duricrust be recognizable in drill spoil. The most efficient geochemical exploration could be based upon wide-spaced overburden drilling directed at sampling the duricrust. For reconnaissance, holes could be based upon a 1-km grid where stratabound massive polymetallic sulphide or stratabound gold deposits are sought. Even wider spacing could be tried where larger-sized ore deposits are expected.Model 2 is characterized by a weathered profile that was partly stripped prior to glaciation. The essential feature is a vertically zoned weathering profile in the basement rock in which the upper levels of the weathering profile have undergone leaching of certain elements and lower levels contain enrichments of these elements. Any gossans present would show strong vertical zonation. Relatively strong sources for dispersion in till are likely to be enriched gossans where erosion has cut deep into the profile, zones of supergene enrichment of ore deposits, and supergene ore deposits themselves. In many situations, saprolite under glacial drift would be too soft to provide boulders so important in conventional till prospecting.The essential ingredient in hypothetical Model 3 involves progressive planing away of the weathering profile by glacial action. Thus any halo in duricrust is initially dispersed, followed by dispersion of progressively deeper levels of saprolite. If taken deep enough the supergene enriched zone of a mineral deposit could add to the glacial dispersion. A broadly zoned anomaly would be expected; a till anomaly would be characterized distally by elements from the former duricrust anomaly, with elements more characteristic of supergene zones closer to the source. 相似文献
4.
This essay traces the development of geochemical exploration from its early beginnings in the modern era during the 1930s, concentrating especially in its application to deeply weathered terrain in the tropics and sub-tropics. Following promising results obtained in temperate regions in North America and Europe, test orientation surveys were conducted to see whether similar procedures were applicable in the tropics, where conventional geological prospecting was largely precluded due to the extensive cover of a deep lateritic regolith and consequent lack of outcrop. After initial work in Sierra Leone and Nigeria, the emphasis transferred to East Africa in the 1950s and 1960s, aimed principally at Cu exploration. Many of the basic principles for exploration in dominantly residual, free-draining terrain were quickly established in this period. Exploration in terrains with more complex weathering histories, however, raised a number of difficulties due to leaching and secondary concentrations of elements, problems in selecting and identifying appropriate sample media, and extensive transported overburden. These were encountered especially in more arid regions in Australia and Africa during exploration for Ni and Au during the 1970s and 1980s. This led to a change in approach, placing weathering and geochemical dispersion in the context of regolith and landscape evolution –a return to the early concept of landscape geochemistry. The 3D expression of mineralization in the landscape is depicted as empirical conceptual models, that account for both relict features and active processes, and portray element associations, dispersion mechanisms and host materials. They also indicate suitable sample media, sampling intervals and procedures for analysis and interpretation. 相似文献
5.
Weathering intensity changes due to climatic variability across tectonically stable portions of continental crust can generate a thick and extensive weathered cover, resulting in regolith-dominated terrains (RDTs). Mineral exploration in RDTs is challenging because of the lack of bedrock outcrop, and the difficulty of linking surface regolith geochemistry to the geology at depth. Complex weathering obscures the expression of the basement geochemistry in the regolith, and therefore the footprints of mineral systems are difficult to detect. The southeast of the Yilgarn Craton and the Albany–Fraser Orogen (AFO) in the south of Western Australia is an RDT that extends along the coastline and the Eucla basin.This study proposes a landscape evolution model of the AFO, driven by transgression–regression sea-level changes that resulted in the formation of numerous islands and development of estuarine zones. This model contrasts with the river system-dominated landscape evolution present in the Yilgarn Craton. This difference has significant implications for mineral exploration and geochemical interpretation of the regolith in this region.Weathering profiles developed “on inland” and “on island” are thicker and more mature than those developed in sea-inundated areas. Even if in the Yilgarn Craton local areas display reworking of weathering profiles and other complexities from Permian, non-marine Tertiary sediments and Quaternary fluvial and aeolian sediments, at a regional scale, if vertical geochemical mobility of elements has occurred, “on inland” and “on island” are more reliable for understanding geochemical anomaly-basement relationships, whereas the “marine inundated” areas require a more detailed investigation, because of the role of marine reworking of weathering profiles and possible mixture of sediments from different provenances.Landscape changes from the topographically high, dissected Yilgarn environment with thick saprolite development and uneven basement topography, to the nearly flat regions dominated by sand dunes and thin saprolite development at the coastline. These regions are the result of the erosional and depositional effects of successive sea-level transgression–regression cycles. Within this framework, the following four different regolith settings have been identified in a progressive change from Yilgarn Craton environments to the modern coastline: (1) Albany; (2) Kalgoorlie–Norseman; (3) Esperance; and (4) Neale.Mapping the palaeocoastlines, islands and estuarine zones, as well as the region of influence of marine limestones and sediments, can significantly improve the understanding of how surface geochemistry relates to the landscape, and how it links with the geology at depth, and therefore, how it may reflect the presence of mineral systems. Understanding the difference in the landscape evolution between the AFO and Yilgarn Craton is essential to properly calibrate mineral exploration protocols in both regions. 相似文献
6.
《Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy》1999,24(10):855-860
In regolith-dominated terrains, the nature of contemporary processes and the surface distribution of regolith materials may be a poor guide to the character and history of regolith materials at depth. The nature of regolith materials at depth is often critical to unravelling the development of a landscape. Conventional mapping aids such as air photos, multispectral remote sensing and airborne radiometrics are not wholly adequate in this context, as they penetrate limited depths (<0.4 m). Airborne electromagnetics (AEM) on the other hand, has the potential to map regolith materials to considerable depths (>100m).The application of AEM to regolith mapping and its potential as a tool in geomorphology are illustrated by reference to an AEM survey flown at Lawlers in the Yilgarn Craton of Western Australia. At Lawlers, AEM identifies a palaeochannel that has no surface expression. It cannot be seen in images of the Landsat, airborne radiometric or airborne magnetic data. The disposition of this channel in the landscape, and in particular its association with ferruginous materials forming breakaways, suggest that inversion of relief has been a significant factor in the evolution of the Lawlers landscape.The AEM data at Lawlers have also been used to map the weathering front. The topography of the weathering front not only reflects the movement of water through the landscape in a general sense, but also reflects the influence of lithology and structure. Different lithologies are clearly weathering to different depths. Information on the nature of the weathering front is potentially an important constraint on models of groundwater flow, and by association, models of solute dispersion. 相似文献
7.
8.
Detailed geochemical mapping of superficial lateritic formations is compared with recent high-resolution SPOT satellite images in the Dagadamou prospect in southern Mali.The two main landscape features standing out in SPOT images are expressed by thematic indices derived from multispectral data. They reflect the distribution of vegetation cover and the distinction between silty clay soils in the valleys and ferruginous duricrust exposed on the plateaus. Geochemical differentiation factors are closely related to the nature of sampled materials and reflect the relative amounts of major constituent minerals estimated by normative calculation.Duricrust samples with high Fe and Al content are also enriched in trace elements, P, V, Cr, As, Mo, Nb and Cu, immobilized in the weathering profile with secondary oxihydroxide minerals.Soils of flats are composed mainly of quartz and kaolinite; they also concentrate heavy minerals characterized by high contents of Zr, Ti, Ce and Y.Accumulation of detrital material at the periphery of duricrust plateaus is marked by a geochemical halo of higher contents in Zr and quartz at their periphery. This feature is clearly visible on SPOT imagery as a zone of high reflectance, devoid of vegetation.The density of vegetation on duricrust is related to its kaolinite content. Its distribution is characterized by a striped pattern, which is probably controlled by bedrock lithological structures preserved in the lateritic cover.This study shows the relationships, both direct and indirect, existing between spectral reflectance and geochemical composition of superficial lateritic formations. Many other landscape features identified with high resolution on SPOT images provide complementary information, which could be very helpful at various stages of geological and mineral exploration in lateritic terrain. 相似文献
9.
T. P. Hopwood 《Australian Journal of Earth Sciences》2013,60(4):345-360
A technique for obtaining age estimates for regolith profiles in Australia, based on the oxygen‐isotope composition of the clay mineral assemblage in a profile, is applied to a variety of regolith profiles and kaolinitic sediments from across Australia. Excluding monsoonal regions in the north of the continent, it is possible to distinguish profiles formed in the Late Mesozoic‐Early Tertiary (δ18O values between +15 and +17.5%δō) from profiles formed in post‐mid‐Tertiary times (>+17.5%ō). In addition it is concluded that there remain widespread remnants of a deep‐weathered regolith which developed in pre‐Late Mesozoic (Early Cretaceous or Jurassic?) times when Australia was at high latitude. The low δ18O values associated with clays formed in pre‐Late Mesozoic times (+10 to +15%o) suggest that deep weathering took place in a cool to cold and presumably humid climate, contrary to the traditional belief that deep weathering requires tropical to subtropical temperatures. The formation of deep‐weathered profiles at high latitude in a comparatively cold climate may be linked in part to higher past atmospheric CO2 levels. 相似文献
10.
11.
Geochemical responses in weathered and oxidized surface metasedimentary rocks associated with stratiform lead-zinc mineralization at Stirling Hill (6 km west of Broken Hill) are compared with the geochemical responses in fresh drill core from an equivalent lithostratigraphic section with stratiform lead-zinc mineralization at the Pinnacles Mine (8 km south of Stirling Hill). Mineralization is interpreted as being volcanic exhalative and it lies within highly metamorphosed (sillimanite grade) rocks of the Willyama Supergroup.Surface rocks were classified into groups by discriminant analysis using drill core data from the Pinnacles Mine as the initial training set. The behaviour of elements in surface rocks varies with the rock group but Zn, Pb, Mn, Fe, and Co are leached from all surface rocks relative to fresh drill core.Nothwithstanding the leaching effects of weathering, common geochemical responses to mineralization have been identified in drill core and surface rocks. Coincident positive anomalies for Zn/Ba and Fe/(Na × Ba) ratios and negative anomalies for Na/(Mn × Ca) ratios uniquely define mineralization in both weathered surface rocks and in fresh drill core.The results demonstrate that the pattern of geochemical responses to Pinnacles-type stratiform volcanic-exhalative mineralization in surface rocks has survived the intensive weathering regime in the Broken Hill region. 相似文献
12.
C.D. Ollier 《地学学报》1992,4(3):312-319
Landscape evolution is on the same time-scale as global tectonics, biological evolution, and major climatic change. Some features of global change reflected in landscape evolution result from the breakup of Pangaea. Others relate to major climatic changes, and yet others to a major change from a dominantly plains landscape of the Mesozoic to an increasingly mountainous landscape in the Tertiary. Worldwide deep weathering profiles of Cretaceous and early Tertiary age suggest widespread planation and warm, moist climates. Erosion of deeply weathered regolith following the formation of new continental margins and tectonic uplift led to the deposition of unusual mature sediments. Many river patterns can be traced to early Tertiary or older antecedents.
Some climatic changes are due to orbital forcing, but others may be related to changing positions of continents, creation of new seaways, or to mountain building. The timing of mountain uplift, climatic changes and biological changes must be resolved to test models of their mutual interaction. Geomorphic input on tectonics, drainage evolution, and deep weathering will be an essential component in dealing with these problems. 相似文献
Some climatic changes are due to orbital forcing, but others may be related to changing positions of continents, creation of new seaways, or to mountain building. The timing of mountain uplift, climatic changes and biological changes must be resolved to test models of their mutual interaction. Geomorphic input on tectonics, drainage evolution, and deep weathering will be an essential component in dealing with these problems. 相似文献
13.
A comparison of selective extraction soil geochemistry and biogeochemistry in the Cobar area, New South Wales 总被引：1，自引：0，他引：1
D. R. Cohen X. C. Shen A. C. Dunlop N. F. Rutherford 《Journal of Geochemical Exploration》1998,61(1-3)
In parts of the deeply weathered and semi-arid environments of the Cobar area (NSW, Australia), detection of mineralisation using conventional soil sampling and total metal analysis is impeded. This is due to the intense leaching of trace elements within the weathered profile, discontinuous coverage of transported materials and the existence of diffuse regional geochemical anomalies of ill-defined source. Selective chemical extractions, applied to various regolith components, and biogeochemistry offer a means of isolating localised geochemical patterns related to recent dispersion of trace elements through the overburden. Lag geochemical patterns across the McKinnons deposit (Au) and Mrangelli prospect (Pb–Zn–As) reflect mechanical dispersion processes and minor hydromorphic effects. Concentrations of more mobile elements tend to be higher in the non-magnetic fraction, due to higher proportions of goethite and poorly crystalline hematite than in the magnetic fraction. The subdued soil geochemical responses for metals extractable by cold 40% hydrochloric acid (CHX) and for total element concentration reflect the leached nature of the residual profile, low grade of mineralisation, dilution by aeolian components and disequilibrium of fine fractions with coarser, relict Fe-oxides. The stronger contrast for CHX for most metals, compared with total extraction, indicates surface accumulation of trace elements derived from underlying mineralisation. Enzyme leach element anomalies are intense but generally located directly over bedrock sources or major structural breaks, irrespective of the nature of the overburden. Though mechanisms for the dispersion of trace elements extracted by enzyme leaching are not well established, the lack of lateral transport suggests vertical migration of volatile metal species (atmimorphic dispersion). The strong, multi-element response to mineralisation in cypress pine needles indicates significant metal recycling during the present erosional cycle. However, a comparison of the trace element concentrations in vegetation (cypress pine needles) and selective extractions of soils indicates that recycling by the plants is not the dominant mechanism for transportation of metals through the overburden. The vegetation may be responding to hydromorphic dispersion patterns at depth. The use of selective extractions may be useful in detecting mineralisation through deeply leached profiles, but offers even greater potential when integrated with biogeochemistry to detect targets buried by significant thickness of transported cover. 相似文献
14.
张家口北部是河北省重要的铅锌银多金属矿床富集区.区内多金属矿床成矿作用和控矿因素相似,矿化形成均与燕山期酸性小岩体侵入变质结晶基底有关.区域多金属矿床化探异常具可对比性.化探异常图显示区内多金属矿床与化探异常在地表总体呈现无偏性空间分布特征,化探数据因子分析结果表明成矿元素相关性在水系沉积物中未发生改变.据此可将原生晕中依据不同元素组合识别矿体剥蚀程度的思路运用于1:20万化探数据,以此识别区域多金属矿床剥蚀程度.本文以张家口研究程度较高的蔡家营多金属矿床为例,选用可有效识别矿体剥蚀程度的(Bi+V+Ba)/(B+Sb+U)、(U/Th)元素比值,将区域化探数据用于多金属矿床剥蚀程度分析,结果显示区内多金属矿床均出露于剥蚀强度较高区域,受到不同程度的剥蚀.累加NAP值方法显示兰闫多金属矿床剥蚀程度最高,蔡家营、青羊沟矿床剥蚀程度明显降低.区内多金属矿床多分布于岩石风化程度较高的变质结晶基底出露区域及其周边.火山岩盖层下伏变质岩中多金属矿床多呈隐伏-半隐伏状态分布,应是区内下步多金属矿床找矿方向. 相似文献
15.
At Avoca, Eire, Kuroko-like sulfide mineralization comprising massive stratiform cupriferous pyrite, accompanied by hanging-wall galena-sphalerite mineralization and footwall stringer pyrite, is hosted by Ordovician sediments, calc-alkaline lavas and pyroclastics. The sequence has been subjected to low-grade regional metamorphism, isoclinal folding and thrusting. The surficial cover comprises nonexotic glacial drift some 2 m thick. Previous work has demonstrated that major-element lithogeochemistry reflects the wall-rock alteration associated with the mineralization, but these signatures are absent from the overlying till. There is a poor heavy metal-expression at surface of the concealed mineralization.Analysis of wall rock, basal till and surface till samples for the chalcophile pathfinder elements As, Sb, Bi and Se (by rapid techniques which involve the introduction of their volatile hydrides into an inductively coupled plasma emission spectrometer) shows that a primary zonation of these elements around the mineralization can, in part, be traced to surface. In wall rocks, Bi enrichment is associated with the massive pyrite and footwall mineralization, Sb and Se anomalies occur in the hanging wall, and an As halo extends across all three types of mineralization. At surface, Bi and As anomalies are found over the massive pyrite and footwall zones, and an Sb anomaly occurs above the hanging-wall mineralization. The development of these patterns is attributed to mainly hydromorphic dispersion.The primary and surficial dispersion patterns of the chalcophile pathfinders should prove useful in exploration for other examples of Kuroko-type mineralization. The determination of these elements in geochemical exploration can be carried out quickly and cost-effectively. 相似文献
16.
Stephen J. Barnes L. A. Fisher R. Anand T. Uemoto 《Australian Journal of Earth Sciences》2013,60(2):269-285
Large, high-quality multi-element geochemical datasets are becoming widely available in the exploration industry, and afford excellent opportunities to investigate geochemical processes. A dataset of over 2500 analyses of unweathered and variably weathered mafic and ultramafic rocks for over 50 elements has been collected by Gold Fields Ltd. in the auriferous Agnew-Lawlers area of the eastern Yilgarn Craton of Western Australia. This dataset is used to investigate changes in element abundances and inter-element ratios through varying degrees and styles of weathering in an area of thick regolith characterised by deep in situ weathering. Systematic interrogation of the data, using lithostratigraphic controls derived from regional mapping and geophysics, reveals that a suite of elements, including Ti, Al, Zr, Th, La, Sc and Nb, and to a lesser extent Cr and Ni, behave as essentially immobile components during saprolite formation. In some cases diagnostic element ratios persist into siliceous duricrust. Ratios of these elements are used as reliable discriminants of bedrock type, and delineate features such as cryptic layering within fractionated sills and subtle geochemical variants in a sequence of tholeiitic and komatiitic basalts. Mapping on the basis of discriminant element ratios greatly extends previous trace-element ratio-based schemes for rock type discrimination. The potential to determine several of these elements with adequate precision and accuracy using portable XRF technology opens a potentially useful technique for rapid geochemical bedrock mapping in residual terrains. 相似文献
17.
Desert terrains in northern China are covered by widespread regolith sediments which mask geochemical signals from ore bodies and are major obstacles to mineral exploration. There is a critical need to study the vertical distribution of elements in this regolith and to establish optimum sampling and analytical methods. The aim of this study is to understand the dispersion and variation of elements throughout the cover in a vertical profile. The results demonstrate that the main elements show a distribution pattern of enrichment in clay layers in the vertical profile, i.e., most elements tend to be enriched in the most upper part of the profile above the orebodies except for some elements like Cu, Ca and Ag. Meanwhile, both fine (-160 mesh) and coarse (+20 mesh) fraction samples from clay-rich horizons are favorable samples and selective leaching of the elements absorbed on clays or oxide coatings is effective for localizing buried deposits. The distribution of active Cu and Mo is quite different because chalcopyrite is weathered into Cu sulfate and CuCO3, and is precipitated in alkaline environments in a layer below close to the surface. Mo is soluble in the alkaline environment so it can penetrate the caliche layer and be enriched on the surface of soil enriched with weakly cemented clay. 相似文献
18.
Weathering and erosion during the Tertiary period in Ireland gave rise to widespread secondary dispersion in the vicinity of exposed mineralization. Resulting patterns are still partly traceable under the glacial cover and constitute geochemical anomalies that are found to be more revealing than corresponding anomalies in the glacial till.A large near-surface lead and zinc anomaly in northeast County Offaly, Eire, was found to overlie an iron hydroxide-filled Tertiary(?) sinkhole. Tracing the dispersion patterns from this preglacial hydromorphic anomaly led to the discovery, to the north of the sinkhole and under the glacial drift, of two residual anomalies characterized by highly concentrated secondary zinc mineralization (up to 10% Zn). One of these follows a regional structural trend and may be of economic significance.Assessment of the depth of overburden was made using shallow refraction seismic equipment and sampling was carried out with two types of portable drills. 相似文献
19.
Thick, commonly lateritic, regoliths are widespread in inter-tropical regions of the world and present particular challenges in exploration. These are best tackled through a sound understanding of the evolution of the landscapes in which they occur. The regoliths formed under humid, warm to tropical conditions and, although they may have been modified by later climatic changes, i.e., to more humid or more arid conditions, many chemical and mineralogical characteristics are retained. These include the geochemical expressions of concealed mineralization. Erosional and depositional processes control the preservation and occurrence of specific regolith units that may be used as sample media and, in turn, target size, element associations and contrast, thereby influencing sampling procedures, analysis and data interpretation. These parameters are best summarized in terms geochemical dispersion models based on the degree of preservation of the pre-existing lateritic regolith. Regolith–landform mapping permits an assessment of the terrain in terms of such models. In relict regolith–landform regimes, in which the lateritic regolith is largely preserved, broad multi-element anomalies in the upper ferruginous horizons (lateritic residuum) can be detected using sample intervals of 1 km or more. In contrast, in erosional regimes, where this material is absent, anomalies in upper saprolite, and the soil and lag derived from it, are more restricted in area and closer sampling intervals, (200×40 m or less) may be necessary. Lag and soil are, generally, ineffective in depositional areas, except where the sediments are very thin (e.g.,<2 m) or overburden provenance can be established. Stratigraphic drilling is necessary to establish whether the overburden overlies a buried lateritic horizon or an erosion surface cut in saprolite. Lateritic residuum remains an excellent sample medium if present, again with widespread haloes, but where it is absent, leaching and the restricted haloes in upper saprolite present formidable problems. Ferruginous saprolite or composites across the unconformity may be effective, but otherwise carefully targeted drilling and sampling through saprolite and saprock may be necessary. Partial extraction analyses have yet to demonstrate significant results except in very specific environments. In arid regions, pedogenic carbonate (calcrete, caliche) may be a valuable sample medium for Au exploration, principally in erosional regimes, and in depositional areas where the overburden is shallow. Sample intervals range from 1 km for regional surveys, through to 100×20 m in prospect evaluation. Saprolite is an essential sample medium in all landform environments, but the restricted halos and possibility of leaching requires that drilling and sampling should be at close intervals. 相似文献
20.
Lithogeochemical-mineralogical haloes around unconformity-type uranium deposits in northern Saskatchewan can expand the size of drill targets up to fifteen times. The deposits are located at or near the unconformity between Aphebian metamorphosed basement rocks in close proximity to Archean granitoids and overlying unmetamorphosed sandstones of the Helikian Athabasca Group. Deposits studied include Key Lake (Deilmann), Midwest Lake and Eagle Point.Unconformity-type deposits are associated with broad alteration haloes in the overlying sandstones and more restricted haloes in the basement rocks. The haloes in the sandstone are localized around steeply dipping fault structures and are characterized by zones of intense leaching of specular hematite and detrital heavy mineral layers and by changes in chemistry related to clay mineral alteration and tourmalinization.Clay mineral alteration haloes are common in the sandstone and basement host rocks. Interstitial clays consist mainly of kaolinite and illite with lesser amounts of chlorite. The ratio K2O/AI2O3 proved useful in delineating illite-kaolinite patterns in the sandstone and in the uppermost portions of the paleoweathering profile in basement rocks. The Midwest deposit is characterized by a broad bell-shaped zone (500 m across strike) of high K2O/AI2O3 ratios (>0.18) in which illite is the dominant clay mineral. The sandstone above the Deilmann deposit, on the other hand, is characterized by silicification and kaolinitization with low K2O/AI2O3 ratios (<0.04). This kaolinite cap overprints a preexisting illite zone.Anomalously high boron values are characteristic of the three deposits considered in this study. Boron anomalies are similar in extent to the anomalous clay mineral alteration haloes. Altered sandstones commonly contain aggregates of radiating magnesium-rich dravite needles within the clay matrix. The interpretation of boron patterns is problema tical however, mainly because of the detrital tourmaline component in sandstones and metamorphic tourmalines present in the Aphebian metasediments.Trace elements such as U, Ni, As and Co are generally of limited use in expanding targets in sandstone because their haloes are restricted to a few tens of metres horizon-anomalies along the steeply dipping zones of mineralization related to fault structures. Uranium (>3 ppm) does form an anomaly more than 200 metres laterally across the Midwest deposit. At the Deilmano deposit anomalous uranium dispersion is restricted to within a few metres of high grade ore.In the basement rocks, the various layers of the paleoweathering profile are geochemically overprinted up to 250 m from mineralization. Bleaching related to illitization and chloritization is associated with enrichment in K2O, MgO, B, S, U, As, Ni and P2O3. The ratio Fe2O3/MgO is useful in delineating chloritization in the upper portion of the paleo weathering profile.Deposits with large root extensions in the Aphebian metasediments such as Eagle Point, have intense dravite-chlorite-illite alteration zones which are restricted to within a few metres of mineralization across strike. ‘Quartzite” units are alteration related. The complexity of the basement lithology inhibits the use of individual elements as alteration guides other than in the intensely altered zone. The application of multivariant techniques, element ratios and clay mineralogical work prove useful in identifying the mineralogical changes at Eagle Point. 相似文献