Composition and origin of lateritic nickel ore at Tagaung Taung,Burma

The recently discovered lateritic nickel ore deposit on the summit area of the Tagaung Taung contains about 40 million DMT nickel-saprolite ore with approximately 2% Ni. The serpentinite massif is covered with a thick weathering mantle consisting of saprolite, limonite, and an allochthonous lateritic surface layer. The thickness, nickel content, and physical properties of the ore are very variable. The main minerals in the nickel saprolite ore are nickeliferous serpentine and smectite. Microprobe analyses show a strong depletion of Mg with increasing nickel content in the serpentine minerals; smectite generally contains less nickel than serpentine. Chemical analyses of samples from more than 100 drill holes are used to calculate the mean chemical compositions, mean accumulation factors, and gain and loss of major and trace elements for the various layers. This permits a quantitative approach to be made to the genetic history of the weathering mantle. Gain and loss determinations on the basis of constant chromium content and of constant volume give nearly identical results for the saprolite layer. Calculations on the basis of Ni/Cr ratios indicate that three-quarters of a former 20-m-thick limonite layer has been eroded away. Only the lower part of the original limonite layer with a relatively high SiO₂ content is preserved. The kaolinite- and illite-rich surface layer is a mixture of limonite and weathered clayey material, which was probably derived from adjacent rocks. Part of the autochthonous weathering section is contaminated with infiltrated allochthonous surface material. It is not possible that colluvial-alluvial deposition of clayey material from nonultramafic rocks could have taken place under the present geomorphic regime. Thus, lateritization of the serpentinite and deposition of siallitic material must have occurred before exposure of the Tagaung Taung in its present form.     

红土型镍矿床成因的多样性：基于全球尺度的对比研究

红土型镍矿床是全球镍矿成因体系中资源储量最大的一类.为探讨红土型镍矿床成因机制的多样性,基于全球广域尺度,选择以载镍矿物为分类视角,开展铁氧化物型、水镁硅酸盐型和粘土型等3类矿床的对比研究.铁氧化物型矿床在矿化剖面上以铁质红土层为优势岩(土)相层,以针铁矿为主要载镍矿物,矿石中镍品位偏低但伴生钴矿化.矿床的成矿条件依赖于热带气候环境、稳定大地构造背景、纯橄榄岩母岩和剪切构造带等多种因素的集成耦合.强氧化和水解作用是载镍铁氧化物和氢氧化物发育的主要机制,镍在红土化过程中主要以类质同象进入低结晶度针铁矿晶格或被吸附的方式富集成矿;水镁硅酸盐型矿床以风化壳中出现厚大腐岩层为标志,以镍蛇纹石、镍滑石等水镁硅酸盐矿物为主要载镍矿物相,矿石多为高品位硅镁镍矿.矿床的发育严格受控于热带雨林或热带草原气候环境,与活跃大地构造背景及富镁橄榄岩母岩条件密切相关,属强淋滤表生作用的产物.离子交换作用和次生沉淀作用共同构成了水镁硅酸盐矿床发育的成矿机制;粘土型矿床以红土剖面中出现稳定的粘土矿物层为标志,局限于偏干旱的热带气候背景,与低缓的地形地貌、排水受阻的水文环境及蛇纹岩母岩条件等具有专属性联系,属未遭受充分表生淋滤作用的产物.镍与次生蒙脱石类矿物发生离子交换作用是粘土型矿床发育的主要机制.     

Lateritization processes of ultramafic rocks in Cretaceous times: The fossil weathering crusts of mainland Greece

The lateritic weathering crusts exposed in mainland Greece were developed on ophiolitic ultramafic lithologies during lower Cretaceous times. The lateritic profile consists of four zones: bedrock, saprolite clay (nontronite) and goethitic. The profiles show large variations in thickness, continuity, mineralogy and chemical characteristics. They are broadly similar to clay nickel laterite deposits. The uppermost gravelly ferruginous sector was eroded and the material reworked and redeposited partly on the lateritic crust. Silcrete layers, characteristic of groundwater silcretes, were formed into the clay and goethitic zones. Significant supergene nickel enrichments occur in the clay and saprolite zones, indicating that water moved downward to a very low water table. The structure and mineralogy of the weathering crusts indicates that environmental conditions were likely to have been dominated by alternating wet and dry periods.     

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.     

Post-Gondwana pedogenic ferromanganese deposits, ancient soil profiles, African land surfaces and palaeoclimatic change on the Highveld of South Africa

Several ferromanganese wad deposits are developed on the Archæan Malmani dolomite succession of the Transvaal Supergroup along the plateau forming the watershed between rivers draining to the Indian and Atlantic Oceans from the Highveld area between Johannesburg and Lichtenburg. The deposits were studied at the Wes Wits Gold Mine and the Ryedale, Houtkoppies and Klipkuil ferromanganese mines. The ferromanganese wad deposits are located in the Waterval saprolite, which formed by deep chemical weathering along the post-Gondwana African surface. The parent rocks for the wad are the Fe- and Mn-rich dolomites of the Malmani Subgroup of the Transvaal Supergroup at Wes Wits Mine and at Klipkuil, and Fe- and Mn-rich blackband Fe ores of the Ecca Group of the Karoo Supergroup at Ryedale and Houtkoppies. This saprolite is unconformably overlain by a ferruginous alluvial succession, informally defined as the Weswits formation. Diamondiferous gravel bars occur in the lower part of the fluvial succession. The unconformity at the base of the succession with incised valleys is thought to correspond to the post-African I event of uplift and erosion. Manganiferous soil nodules, derived from a lateritic weathering profile that originally covered the Waterval saprolite, are concentrated in the lower part of the Weswits formation. Climatic conditions became drier during the deposition of the Weswits formation, and plants with deep taproots vegetated the surface of the alluvium, giving rise to the formation of a ferric poclzol. Further aridification took place and eventually the plants with deep taproots died and a pediment developed along which a stone lag was concentrated. This pediment is thought to represent the post-African II surface of erosion. The pediment is overlain by the Hutton soil, representing Kalahari sand and dust that have been reworked by fluvial and peclogenic processes. Massive ferruginous soil nodules grew in situ in the Hutton soil, indicating more pluvial climatic conditions at times, most probably corresponding to the Quaternary ice age events in the Northern Hemisphere. A thin, modern orthic soil developed on top of the Hutton soil in the present day mild, subhumid climatic conditions.     

Mineral deposit formation in Phanerozoic sedimentary basins of north-east Africa: the contribution of weathering

The intra- and epicontinental basins in north-east Africa (Egypt, Sudan) bear ample evidence of weathering processes repeatedly having contributed to the formation of mineral deposits throughout the Phanerozoic.The relict primary weathering mantle of Pan-African basement rocks consists of kaolinitic saprolite, laterite (in places bauxitic) and iron oxide crust. On the continent, the reaccumulation of eroded weathering-derived clay minerals (mainly kaolinite) occurred predominantly in fluvio-lacustrine environments, and floodplain and coastal plain deposits. Iron oxides, delivered from ferricretes, accumulated as oolitic ironstones in continental and marine sediments. Elements leached from weathering profiles accumulated in continental basins forming silcrete and alunite or in the marine environment contributing to the formation of attapulgite/saprolite and phosphorites.The Early Paleozoic Tawiga bauxitic laterite of northern Sudan gives a unique testimony of high latitude lateritic weathering under global greenhouse conditions. It formed in close spatial and temporal vicinity to the Late Ordovician glaciation in north Africa. The record of weathering products is essentially complete for the Late Cretaceous/Early Tertiary. From the continental sources in the south to the marine sinks in the north, an almost complete line of lateritic and laterite-derived deposits of bauxitic kaolin, kaolin, iron oxides and phosphates is well documented.     

世界红土型镍矿的资源分布及勘查、开发利用现状

目前世界上最具开发利用价值的镍矿床类型为岩浆硫化镍矿床和红土型镍矿床。本文在全面收集、整理和综合世界红土型镍矿相关资料的基础上, 对世界红土型镍矿的资源概况、地理分布、地质特征以及勘查和开发利用现状进行了系统的分析与阐述, 文章对我国地勘单位和矿业企业走出去勘查和开发利用国外红土型镍矿资源具有现实的借鉴和指导意义。     

全球红土型镍矿床的基本成矿规律

全球红土型镍矿床具有明显的时空分布规律, 红土型镍矿体则具有明显的垂向分带规律。红土型镍矿化强度明显受成矿母岩成分(包括镍含量、岩石地球化学成分、矿物成分)、气候、地形地貌、水文地质、构造地质、成矿时间长短及风化壳成熟度的影响和控制。红土型镍矿的矿床类型主要受气候、大地构造背景、构造隆升状态控制, 并受地形地貌、排水条件、成矿母岩岩性及其蛇纹石化程度影响。     

Red Clay Type Gold Deposits in China

<正>Red clay type gold deposits,located in the south of China,are situated not only in orogenic belts,but also in inner cratons,where climate is tropical-subtropical with clear arid and humid.The lateritic weathering crust often can be divided into five zones,including topsoil,siliceous duricrust zone,multi-color zone(or red clay zone in some deposits),pallid zone and saprolite zone from surface to the base rock,several of which are absent in some deposits.The base rocks are composed mainly of carbonate rocks with minor clastic rocks,intermediate-basic volcanic rocks and intermediate-acid and alkalic intrusions.The orebodies are mainly located in the multi-color zone with part of them in the pallid and saprolite zones.The ore sources include orebodies of Carlin-type gold deposits and porphyry gold deposits,as well as gold-rich base rocks.The red clay type gold deposits experienced early-stage endogenic gold mineralization and laterization during the Tertiary and Quaternary.The areas with endogenic gold deposits,especially Carlin-type gold deposits and porphyry gold deposits in karst depressions on the plateau,structual erosional platforms in the middle-lower mountains,and intermountain basins in southern China are well worth studying to trace red clay type gold deposits.     

Evolution of regoliths and landscapes in deeply weathered terrain — implications for geochemical exploration

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.     

Geochemical exploration challenges in the regolith dominated Igarapé Bahia gold deposit,Carajás,Brazil

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.     

Biogeochemistry of a regolith: The New Caledonian Koniambo ultramafic massif

This work aims to elucidate the factors governing the mobility of metals in soils of a lateritic nickel ore deposit in New Caledonia. The transfer of nickel and associated metals is determined along a topographic sequence ranging from a plateau to a thalweg. Mining exploration borehole data and soil pit data enabled us to define the general geochemical trends of the lateritic weathering. The homogeneous topsoil, which consists mainly of iron oxy-hydroxides, exhibits various amounts of plant available metals. Two interrelated factors control the differences in metal availability: (1) the presence of nickel-rich soil horizons underlying the topsoil and formed of silicate minerals, and (2) the biological cycling of metals from these horizons to the surface. These results have major implications for topsoil management during the mining activity where it is important to minimise metal inputs to water systems and to restore endemic vegetation after mining.     

Karst bauxite deposits in the Zagros Mountain Belt,Iran

More than 190 occurrences of bauxitic–lateritic deposits were investigated in seven areas within the Zagros Simply Folded Mountain Belt in southwestern of Iran. The bauxitic horizons are situated in eroded major NW–SE trending anticlines and occur in karst cavities near or at the boundary between the Sarvak and Ilam Formations. Uplift in the Cenomanian–Turanian period had exposed the Sarvak limestone to karst weathering and, during a period of unconformity, layers of ferruginous–argillaceous limestone debris developed and accumulated on its surface. The ferruginous–argillaceous debris was partly converted to bauxite. Folding and faulting in Oligocene–Miocene time, with ensuing erosion, exposed the bauxitic horizons on the limbs and cores of anticlines. The karst bauxite deposits are probably of authigenic origin, as evidenced by their lithologic associations, textural and mineralogy.     

Genesis of nickel laterites and bauxites in greece during the jurassic and cretaceous, and their relation to ultrabasic parent rocks

Nickel laterites and bauxites, including their proposed parent rocks from the Mesozoic of Greece, have been investigated by means of mineralogical and geochemical methods. The results are discussed in order to recognize the genetic sequence which comprises: pre-lateritic alteration and reworking of ophiolites and associated rocks, lateritic in-situ weathering, reworking and redeposition of the alteration products in an epicontinental transition environment, and post-depositional events affecting the mineralogical and geochemical properties.The ultramafic massifs of the Euboea and Locris area, i.e. the parent rocks of the Ni---Fe deposits, are primarily harzburgites which represent the erosional outliers of a probable “complete” ophiolitic nappe that were transformed to a monomineralic lizarditite. Xenoliths of basic and sedimentary rocks are included in the serpentine matrix of the basal tectonic melange. Lateritic Ni---Fe deposits resting as in-situ alterites on ophiolites or as mechanically reworked laterite detritus, either on serpentinite or karstified limestone, are mainly derived from serpentinites. The ore deposits in the Locris area have been affected by a strong supergene epigenetic overprint, mainly resulting in a downward Ni redistribution and enrichment. A continuous transition from karstic Ni---Fe deposits towards bauxitic material in a southern direction is interpreted as sedimentary admixture of weathering products of different origin.The three bauxite horizons B₁, B₂ and B₃ and their satellite horizons are intercalated in epicontinental shallow-water limestones within an Upper Jurassic to Middle Cretaceous sequence. A karstic surface (unconformity) forms the substratum of these bauxite horizons.Similar to nickel laterites on karst, the detrital parent material was transported from a terrestrial hinterland by widely ramified river systems into a brackish lagoonal or marine environment from a northeastern to southwestern direction. Colloidal matter, fine muds and coarse material were deposited on a karst topography in mechanical traps by successive debris flows during cycles of emersion and marine regressions.Diagenesis resulted in (a) leaching of silica and iron under partly reducing conditions, and (b) recrystallization of iron minerals and neoformation of Al minerals, i.e. boehmite and/or diaspore. During tectonic subsidence and early marine transgression a strong supergene-epigenetic downward mobilization of Fe, Mn and associated elements took place. They were reprecipitated near the footwall in chemical traps.Bauxites of all horizons originate from serpentinites as well as from metamorphic and magmatic rocks. This is indicated by a high content of siderophile elements and lithic components. The sequences of regression and transgression and their erosional, sedimentary and geochemical processes are interpreted as cyclic events.     

菲律宾迪纳加特岛红土型镍矿床地质特征及找矿勘查方法

通过对菲律宾迪纳加特岛风化壳氧化镍-硅酸镍矿床的成矿地质条件、矿床地质特征、成矿机制的分析和研究,认为矿床是由于超镁铁岩-橄榄岩在中生代、第三纪、第四纪的热带、亚热带气候条件下,经过风化作用,镍从含镍的硅酸盐矿物中淋滤出来,随地表水向下渗透到风化壳的下部,形成富含镍的次生矿物,具有典型的风化壳型硅酸镍矿床特点。找矿勘探工作要在找矿区域进行地质填图及工程取样扫面来确定矿化区。     

论铝土矿床成因及矿床类型

铝土矿全部是风化作用形成的，无一例外。地球上含铝（含少量铝质也可以）岩石在适宜的气候和地形条件下，风化成红土矿物，包括铝土矿物及少量粘土矿物、含铁矿物及少量含钛矿物等风化壳铝土物质（红土铝土物质）。第四纪以前的古风化作用形成的是古风化壳铝土物质（古红土铝土物质），迁移就位以后便形成风化壳铝土矿床（红土铝土矿床）。深埋地下经过成岩变化，再随地壳抬升进入地壳浅部，或地表的叫古风化壳铝土矿床。我国９８％是古风化壳铝土矿床，即国外所称的喀斯特铝土矿床。由于其迁移就位方式不同，便形成了不同的风化壳和古风化壳（亚型）矿床     

Geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in Central Africa

A simple geochemical balance of lateritization processes governing the development of several tens of meters of weathering profiles overlain by ferricretes is estimated on the basis of detailed mineralogical and geochemical data. The lateritic weathering mantle of the “Haut–Mbomou” area in Central Africa is composed of different weathering layers described from the base to the top of vertical profiles as a saprolite, a mottled clay layer, a soft nodular layer, a soft ferricrete, and a ferricrete in which kaolinite, gibbsite, goethite, and hematite occur in various quantities. Incongruent dissolution of kaolinite leads to the formation of gibbsite in the upper saprolite, whereas the hematite does not clearly replace the kaolinite according to an epigene process in the upper ferruginous layers of the profiles. Instead, that kaolinite is also transformed into gibbsite according to an incongruent dissolution under hydrated and reducing conditions induced by a relatively humid climatic pattern. The respective relations of the silica, iron, and aluminum balances and the Al substitution rate of the hematite on the one hand, and of RHG [RHG = 100 (hematite/hematite + goethite)] and the kaolinite on the other hand, to the consumption or the release of protons H⁺ permit differentiation of aggrading ferruginization and degradation processes operating in the different lateritic weathering profiles. The Al substitution rate of the Fe–oxyhydroxides varies according to the nature of lateritization processes, e.g., saprolitic weathering and aggrading ferruginization vs. degradation. The observations and results indicate that the ferruginization process of the weathering materials of parent rocks is not a simple ongoing process as often thought. This suggests that the actual lateritic weathering mantle of the Haut–Mbomou area may result from different stages of weathering and erosion during climatic changes.     

Banded iron formation to high-grade iron ore: a critical review of supergene enrichment models

All the major worldwide direct-shipping iron ore deposits associated with banded iron formations (BIF) are characteristically deeply weathered. They extend to considerable depths below the water table and show well-preserved primary structures and textures, but characteristically most deposits contain no evidence of chert bands being present prior to weathering. Recent studies have found evidence of hydrothermal and/ or metamorphic influences in the development of certain ore deposits and new genesis models such as the supergene-modified hypogene model have been postulated for major high-grade iron ore deposits. Nevertheless, there are many high-grade deposits that show no evidence of hypogene alteration and for which a hypogene or metamorphic genesis is unreasonable that are automatically ascribed to supergene enrichment, commonly erroneously attributed to lateritic weathering in tropical environments. Laterite (sensu lato) is a soil formation in which primary textures are destroyed and is underlain by a pallid zone showing the preservation of chert and the depletion, not enrichment, of iron oxides and thus is totally incompatible with the formation of the high-grade ore deposits. Various theories and models that purported to explain the conditions under which such a uniquely BIF-related dissolution of quartz and residual accumulation of hematite could occur by supergene processes typically conflict with current understanding of groundwater hydrology, chemistry, weathering processes and soil formation.Supergene enrichment of ore is universal in the leaching of gangue minerals such as iron silicates, carbonates and apatite and supergene enrichment of BIF to low-grade ore is common in near surface environments above the water table such as ferrugenised BIF outcrops, detrital ore deposits, and some shallow ore deposits that have been subjected to prolonged exposure to fresh meteoric water. In all cases of supergene enrichment traces of the chert bands are visible and the dissolution or replacement processes for the removal of quartz are clear, in direct contrast to the most important deep saprolite ore deposits that show no trace of chert bands.The widespread acceptance of an inappropriate and untenable supergene enrichment model inhibits search for the true origin of the ore and our ability to predict and find concealed high-grade ore deposits.     

晋北硅铁建造型铁矿床铁矿石相、沉积环境判别及铁矿找矿方向的研究

硅铁建造型铁矿床是太古代——早元古代火山作用、构造作用,沉积作用、变质作用的结果。硅铁建造型铁矿石担的研究对确定铁矿床成因类型、判别沉积环境、评价利用铁矿石都有着现实意义。詹姆斯(James H·L)金伯利(Kimberley M·M)古德温(Goodwin A·M)格罗斯(Gross G·A)等从铁矿物组合、岩石组合上划分了铁矿石相类型,提出形成铁矿石相的沉积环境,但并未对铁矿石相划分、相环境判别建立定量数值指标和相环     

沉积型铝土矿的陆表淋滤成矿作用: 兼论铝土矿床的成因分类*

铝土矿是化学风化作用的细粒终极产物,与强烈的化学风化作用密切相关。根据母岩的类别及作用过程,风化作用进一步分为铝硅酸盐岩强化学风化形成的红土化作用和碳酸盐岩强化学风化形成的钙红土化作用。在强烈的化学风化过程中,地表的原始沉积物(母岩)的原生矿物发生溶蚀、水解、水化、碳酸化、氧化,破坏原始的矿物结构,形成新的细粒矿物(主要是黏土质矿物)。在适合的地质条件下,持续的强烈化学风化作用会造成大部分活动的元素(如K、Na、Ca、Mg、Si)的流失与Al的残留富集从而形成铝土矿。现在观察到的沉积型铝土矿,虽然与古风化壳具有密切联系,但沉积型铝土矿多数是由沉积过程搬运到沉积盆地中所形成的强化学风化产物的沉积层,与古风化壳的残坡积层具有显著差别,只有少数工业价值不大(品位低、品质差)的残坡积相铝土矿。铝土矿含矿岩系的沉积环境与铝土矿(尤其是高品位、高品质的铝土矿)的成矿环境不尽相同。铝土矿主要形成于暴露于大气中的陆表环境(而非水下环境),由地下水淋滤作用形成(在渗流带由活动元素流失、Al等稳定元素残留富集而成)。本研究在铝土矿成矿作用分析等基础上,提出了以铝土矿沉积物等物源和沉积、成矿作用为依据的中国铝土矿床分类方案,包括原地或准原地残坡积物成因的红土型和喀斯特型,和异地物源沉积成因的沉积型。