Argillite debris converted to bauxite during karst weathering: evidence from immobile element geochemistry at the Olmedo Deposit, Sardinia

The Olmedo bauxite deposit occurs in the Nurra district of northwest Sardinia. It forms a stratiform horizon in Cretaceous limestone and marl. Uplift in mid-Cretaceous had exposed recently deposited limestone to karst weathering, and a layer of argillaceous debris accumulated on its surface and was partly converted to bauxite. Intermediate products were desiccated marl, bauxitic argillite and argillaceous bauxite. Subsidence followed, and the bauxite was preserved by the deposition of late Cretaceous limestone and other sediments. Uplift in Oligocene-Miocene time, with ensuing erosion, exposed the bauxite horizon to its present configuration. Concentrations of normative minerals illustrate chemical processes and the build-up of Al in the bauxite horizon. Plots of chemical data and correlation coefficients show that Al, Ti, Zr, Nb, Th, Cr and V were immobile during the bauxitization process. Mass changes point to large net removal of Si, Mg and K from the system, although some of this material and slightly mobile Al were reprecipitated in the underlying argillite and altered marl. Immobile element ratios trace the source of the bauxite to the underlying argillaceous limestone. Al in the bauxite was accumulated from the degradation of 25 to 50 m of the argillaceous limestone. Received: 10 June 1996 / Accepted: 15 April 1997     

Mineralogical and geochemical evolution of the Bidgol bauxite deposit,Zagros Mountain Belt,Iran: Implications for ore genesis,rare earth elements fractionation and parental affinity

The present study focuses on the Late Cretaceous Bidgol bauxite deposit in the Zagros Simply Fold Belt, SW Iran. The orebody is located in the eroded major NW–SE trending Koh-e-Hosseyn anticline and hosted as discontinuous stratified layers and lenses within the upper member of the Cenomanian–Turonian Sarvak Formation. Detailed mineralogical analysis reveals that diaspore, hematite, goethite, anatase, clinochlore, chamosite, and calcite are the major mineral components accompanied by minor amounts of detrital and REE-bearing minerals such as rutile, zircon and parisite. The ore texture suggest that the bauxite material has an authigenic origin but in some parts it has been transported short distances from a primary in situ environment and redeposited in karstic depressions. The spheroidal pisolites of the Bidgol bauxite formed under conditions of low water activity, favouring the formation of large diaspore cores and a single dry-to-wet climatic fluctuation. The mass change calculations relative to the immobile element Ti show that elements such as Si, Fe, Mg, K, Na and Sr are leached out of the weathered system; Al, Ni, Zr, Ga, Cr and Ba are concentrated in the residual system; and Hf, Ta, Co, Rb, Cs, Be, and U are relatively immobile during the bauxitisation processes. The Nb, Th, Y, V, Sc, Sn and ΣREE are relatively immobile in the initial stage of bauxitisation processes in the bauxite ores, but were slightly mobile at the later stage of bauxitisation. Geochemical data reveal progressive enrichment of the REE and intense LREE/HREE fractionation toward the lower parts of the bauxite profile. Cerium behaves differently from the other REEs (especially LREE) and show positive anomalies in the upper horizons that gradually become negative in the deeper parts of the profile. The distribution and fractionation of trace elements and REEs during the bauxitisation process in the Bidgol deposit are mainly controlled by the presence of REE-bearing minerals, fluctuations in soil solution pH, REE ionization potential and the presence of bicarbonates or organic matter. Geochemical analyses confirm a protolith contribution from the bedrock argillaceous limestone and suggest that the source material for the Bidgol bauxite was provided from a siliciclastic material derived from a continental margin. The mid-Turonian uplift led to the formation of karstic topography, rubbly breccia and a layer of ferruginous–argillaceous debris that was affected by lateritic weathering under humid tropical climate. Subsequently, mobile elements are removed from the profiles, while Al, Fe and Ti are enriched, resulting in the formation of the pristine bauxite materials. When the platform subsided into the water again, the pristine bauxitic materials were partly converted to bauxite. During the exposure of bauxite orebodies on the limbs and crests of anticlines and subsequent eroding and accumulation in the karstic depressions during folding and faulting in Oligocene–Miocene, important factors such as intensity of the weathering, drainage and floating flow may have improved the qualities of the bauxite ores.     

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     

Immobile elements and mass changes geochemistry at Sar-Faryab bauxite deposit,Zagros Mountains,Iran

The Sar-Faryab bauxite deposit is located in 250 km east of Ahvaz city in southwestern Iran. Structurally the deposit located in the Zagros Simply Folded Mountain Belt. Outcrops of the bauxite horizons in the area are distributed irregularly over an area of about 20 km² but have fairly uniform thickness averaging 1 to 1.5 m. The Sar-Faryab bauxite is situated in NW–SE trending Mandan anticline and occurs in karst horizons near or at the boundary between the Sarvak and Ilam Formations. Based on field observation, mineralogy and stratigraphy an unconformity during Cenomanian–Turonian times has exposed the Sarvak Limestone to karst weathering and the layers of Marly Limestone, Argillite, Oolitic–Pisolitic, Yellow, Red and White Bauxite were formed and accumulated in the karstic areas.     

Mineralogical and geochemical investigations of the Mombi bauxite deposit,Zagros Mountains,Iran

The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al₂O₃, SiO₂, Fe₂O₃ and TiO₂ are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce³⁺ to Ce⁴⁺), ionic potential, and complexation of Ce⁴⁺ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.     

Geological,geochemical, and mineralogical characteristics of the Mandan and Deh-now bauxite deposits,Zagros Fold Belt,Iran

The Mandan and Deh-now bauxite deposits are located 40 km northeast of the Dehdasht city in the Zagros simply fold belt. These deposits occur in eroded major NW–SE trending anticlines and occupy karst cavities near or at the boundary between the Sarvak and Ilam Formations. Local uplifts at the end of the Cenomanian and the mid-Turonian caused erosion and karstification of the Sarvak Formation. These unconformities in the Upper Cretaceous favoured the formation and enrichment of bauxite deposits in the Zagros fold belt. The bauxite sequence in the Mandan deposit consists of white, gray, black, pisolitic, red, and yellow bauxites. This sequence was repeated in the Deh-Now area, but without gray and black bauxites. The present mineralogical studies of the Sarvak Formation and the Mandan and Deh-now bauxite deposits indicate oxidizing to reducing conditions during the Upper Cretaceous in the Zagros fold belt, which had a significant effect on the compositions of the bauxites. At least two phases of bauxitization can be distinguished in the study area: (i) an oxidizing phase represented by boehmite, diaspore, hematite and kaolinite; and (ii) a reducing phase represented by pyrite and chlorite. Geochemical data show that trace elements, like Zr, Hf, Nb, Ta, Th, and U, were enriched during bauxitization. The bauxite deposits and carbonate rocks show similar REE patterns, namely they are enriched in REEs although the LREEs are more enriched than the HREEs. Mass change calculations demonstrate that Mg, Mn, Ca, K, and P₂O₅ were leached out of the weathered system whereas Al, Fe, and Si become concentrated in the residual system. This study indicates that the Mandan and Deh-now deposits are karst-type bauxites formed by karstification and weathering of the Sarvak Formation.     

桂西二叠纪喀斯特型铝土矿地质成矿过程

桂西二叠纪喀斯特型铝土矿是第四纪萨伦托型铝土矿的矿源层,但是其具体成矿地质过程并不清楚.在矿带东部平果矿田1∶5万区域地质调查基础上,针对性地对二叠纪铝土矿床进行了合山组含铝岩系基本层序、铝土矿成矿物质来源、古喀斯特地貌对铝土矿的控制作用及含铝矿物生成顺序的研究,并阐述了该类型铝土矿从源岩风化到搬运沉积的具体成矿过程.综合前人研究成果,提出桂西二叠纪喀斯特型铝土矿的四阶段成矿模式,分别为孤立台地隆升接受火山喷发沉积物阶段、原地深度风化阶段、积水潜育化阶段和埋藏成矿阶段.      

华北克拉通南缘石炭系本溪组铁-铝黏土矿物质来源: 以河南三门峡大安铝黏土矿床为例*

华北克拉通在中奥陶世至晚石炭世期间一直出露地表,经历了长期的风化作用,形成大规模的铁-铝黏土矿,其成矿物源一直是研究的热点,尤其是本溪组底部铁矿和铁质黏土矿与上部铝黏土矿是否为同一来源尚未查清。本研究选取克拉通南缘大安铝黏土矿床作为研究对象,展开微区矿物及元素地球化学组成分析,进一步探讨铁-铝黏土矿物质来源。大安矿床内含矿岩系自下而上包括铁质黏土岩、铝土矿、铝质黏土矿;局部喀斯特高地缺失铝土矿,铝质黏土矿直接覆盖于铁质黏土岩之上。铁质黏土岩在洼地以菱铁矿、黄铁矿和伊利石为主,在隆起区以赤铁矿、伊利石和高岭石为主。铝土矿以硬水铝石、伊利石和锐钛矿为主;铝质黏土矿主要矿物为伊利石。矿物微区分析在黏土矿底部发现大量的碳化硅和少量自然硅、硅铁矿、铬铁矿;区域对比揭示北秦岭造山带内商丹缝合带和二郎坪群中的蛇绿岩为铝黏土矿形成提供了成矿物质。本溪组底部铁质黏土与上部铝黏土矿稳定元素比率(例如Zr/TiO₂、Hf/TiO₂、Nb/TiO₂、Ta/TiO₂)存在明显差异,揭示二者为不同来源: 底部铁质黏土岩和铁矿层为底板碳酸盐岩原地风化的产物;而上部铝黏土矿是异地搬运物,北秦岭造山带在晚石炭世的整体抬升为华北铝黏土矿形成提供了重要的成矿物质。     

Provenance of iron,bauxite and clay deposits of the Carboniferous Benxi Formation in southern margin of North China Craton: An example from Da'an bauxite and clay deposit of Sanmenxia area,Henan Province

During the Middle Ordovician to Late Carboniferous period,the North China Craton(NCC)was exposed and experienced prolonged weathering that resulted in the formation of large-scale iron,clay and bauxite deposits. The source of ore-forming material has always been a research focus,in particular,whether the sources of the iron ore and the Fe-bearing clay at the bottom of Benxi Formation are the same as the upper bauxite and clay deposit is still unclear. In this study,the Da'an bauxite and clay deposit at the southern margin of the NCC was chosen to carry out a detailed analysis of the micro-region mineral composition and elemental geochemical characteristics for further exploring the sources of iron,bauxite and clay deposits. The composition of the ore-bearing rocks in the Da'an bauxite deposit from the bottom to top includes Fe-bearing clay(locally iron ore),bauxite,and bauxitic clay;locally,in karstic uplift,bauxitic clay layer is directly overlying on the Fe-bearing clay. The Fe-bearing clay is dominated by siderite,pyrite,and illite in the karstic depression,and hematite,illite,and kaolinite in the uplift. Bauxite is mainly composed of diaspore,illite,and anatase,while bauxitic clay is mainly composed of illite. Mineral microanalysis revealed the development of large amounts of moissanite and small amounts of natural silica,silicalite,and chromite at the bottom of bauxitic clay layer. Regional comparison and correlation reveal that the ophiolite in the Shangdan suture zone and Erlangping Group in the North Qinling orogenic belt(NQOB)likely provides source materials for bauxite and clay deposits. The obvious differences in immobile element ratios (e.g., Zr/TiO₂,Hf/TiO₂,Nb/TiO₂,Ta/TiO₂)between the bottom Fe-bearing clay layer and the upper bauxite and clay layer in Da'an deposit,revealing that they are from different sources. The bottom Fe-bearing clay and iron ore layers are the products of in-situ weathering of underlying carbonates,while the top bauxite and clay are allochthonous. The regional uplift of the NQOB during the Late Carboniferous period provided important ore-forming materials for the formation of the NCC bauxite and clay deposits.     

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.     

The “coal–bauxite–iron” structure in the ore-bearing rock series as a prospecting indicator for southeastern Guizhou bauxite mines

Bauxite deposits in southeastern Guizhou occur in the lower Permian Liangshan Formation. The rock series bearing the sedimentary bauxite exhibit a typical “coal–bauxite–iron” structure, in which the lower part consists of bauxitic shale intercalated with siderite concretions, the middle part consists of bauxitic rock intercalated with multilayer lenticular or earthy bauxite, and the upper part consists of carbonaceous shales and sandstones intercalated with coal seams. The paralic, coastal and paludal depositional environments at one time had a stable tectonic setting. By studying the elemental geochemistry of the ore-bearing rock series, it can be seen that the common presence of the “coal–bauxite–iron” structure has resulted from (1) the extensive desilication and iron depletion during the formation of the sedimentary bauxite due to the varying physical and chemical environment, and (2) the sufficient supply of organic matter by the external environment. Such geological anomalies that resulted from the physical and chemical changes during the formation of the layered structure in the ore-bearing rock series can serve as a mineralization indicator in prospecting for new deposits.     

Importance of hydrogeological conditions during formation of the karstic bauxite deposits,Central Guizhou Province,Southwest China: A case study at Lindai deposit

Karstic bauxite deposits are widespread in Central Guizhou Province, SW China, and high-grade ores are frequently sandwiched with overlying coal and underlying iron-rich layers and form a special “coal–bauxite–iron” structure. The Lindai deposit, which is one of the most representative karstic bauxite deposits in Central Guizhou Province, was selected as a case study. Based on textural features and iron abundances, bauxite ores in the Lindai deposit are divided into three types of ores, i.e., clastic, compact, and high-iron. The bauxite ores primarily comprise diaspore, boehmite, kaolinite, illite, and hematite with minor quartz, smectite, pyrite, zircon, rutile, anatase, and feldspar. The Al₂O₃ (53–76.8 wt.%) is the main chemical contents of the bauxite ore samples in the Lindai district, followed by SiO₂, Fe₂O₃, TiO₂, CaO, MgO, S, and P etc. Our geological data on the Lindai deposit indicated that the ore-bearing rock series and its underlying stratum have similar rare earth elements distribution pattern and similar Y/Ho, Zr/Hf, and Eu/Eu¹ values; additionally, all ore-bearing rock samples are rich in MgO (range from 0.16 wt.% to 0.68 wt.%), and the plots of the dolomites and laterites lie almost on or close to the weathering line fit by the Al-bearing rocks in Zr vs. Hf and Nb vs. Ta diagrams; suggesting that the underlying Middle Cambrian Shilengshui Formation dolomite is the parent rock of bauxite resources in the Lindai district.Simulated weathering experiments on the modern laterite from the Shilengshui Formation dolomite in the Lindai bauxite deposit show that hydrogeological conditions are important for karstic bauxite formation: Si is most likely to migrate, its migration rate is several magnitudes higher than those of Al and Fe under natural conditions; the reducing inorganic acid condition is the most conducive to Al enrichment and Si removal; Fe does not migrate easily in groundwater, Al enrichment and Fe removal can occur only in acidic and reducing conditions with the presence of organic matter.The geological and experimental studies show that “coal–bauxite–iron” structure in Lindai deposit is formed under certain hydrogeological conditions, i.e., since lateritic bauxite or Al-rich laterite deposited upon the semi-closed karst depressions, Si can be continuously removed out under neutral/acidic groundwater conditions; the coal/carbonaceous rock overlying the bauxitic materials were easily oxidized to produce acidic (H₂S, H₂SO₄, etc.) and reductant groundwater with organic materials that percolated downward, resulting in enrichment of Al in underlying bauxite; it also reduced Fe³⁺ to its easily migrating form Fe²⁺, moving downward to near the basal carbonate culminated in precipitating of ferruginous (FeS₂, FeCO₃, etc.) strata of the “coal–bauxite–iron” structure. Thus, the bauxitic materials experienced Al enrichment and Si and Fe removal under above certain hydrogeological conditions forming the high-quality bauxite.     

Characterization of bauxite deposits from Kachchh Area,Gujarat

The bauxites deposits of Kachchh area in Gujarat are investigated to characterize them based on mineralogical and petrographic studies. The major bauxitic mineral in these occurrences is gibbsite, with minor concentration of boehmite and diaspore. Apart from the bauxitic minerals, the other associate minerals are kaolin, calcite, alunite and the iron ore minerals such as hematite and goethite and titanium rich anatase. The iron ore minerals (hematite and goethite) are 10-50microns in size and are disseminated throughout the oolitic and pisolitic bauxitic minerals. At places the goethite exhibits colloform texture. The preservation of basaltic texture in some of the samples indicate that the insitu nature of these bauxites, which are formed by the alteration of calcic plagioclase from the parent basalt. Although, the basalt occurs as the main parent rock for these bauxites, the presence of calcite in some of the samples represent the possibility of having a limestone parent rock at least in some of the bauxite occurrences.     

A Study on Palaeogeography and Metallogenic Conditions of Early Carboniferous Bauxite Deposits of Old Weathered Crust in Guizhou,Sichuan, Hunan and Hubei Provinces1

Abstract There are four subtypes, namely, la, lb, 1c and Id of the Early Carboniferous bauxite deposits of the old weathered crust type in Guizhou, Sichuan, Hunan and Hubei provinces. They are all distributed in the Upper Yangtze old land. As a result of the Hunan-Guizhou palaeo-faulting, the crust on the northern side of the fault was uplifted and became land, thus suffering denudation. The bauxitic substances left in the weathered crust evolved and were reworked into bauxite. On the other hand, the crust on the southern side of the fault sank and remained to be part of an ocean. The bauxite deposits of la and lb subtypes were formed by in-situ enrichment of residual bauxitic substances in the weathered crust of the old land, but bauxite deposits of 1c and Id subtypes resulted from transportation, accumulation and sedimentation of allothogenous bauxitic substances on the weathered crust of the old land. The processes of transportation may be distinguished as dry transportation and wet transportation, both of which proceeded under atmospheric conditions.     

重庆南川-武隆铝土矿矿物学、地球化学特征

重庆南川-武隆铝土矿属于渝南-黔北铝土矿成矿带,为喀斯特型铝土矿床。经显微镜、X射线粉晶衍射、矿物自动分析仪（MLA）、扫面电子显微镜等方法对该矿床矿物学的研究,发现组成铝土矿的主要矿物为一水硬铝石、高岭石、绿泥石,次要矿物为伊利石、一水软铝石、三水铝石、鲕绿泥石、菱铁矿、赤铁矿、针铁矿、黄铁矿、锐钛矿、金红石、磷灰石、石英、锆石、方解石、长石及稀土矿物等。矿石组构及矿物组合表明形成铝土矿的沉积/成岩环境为接近于潜流的环境。矿石结构和锆石形态指示成矿物质经过了短距离的搬运。地球化学研究结果显示,组成铝土矿的主要化学成分为Al2O3、TFeO、SiO2和TiO2,铝土矿化过程中REE、Zr、Hf、Nb、Ta、Th、Sc、Li和Ga发生富集。形成铝土矿的母岩物质主要来自下伏页岩的风化作用,灰岩和酸性火山岩对铝土矿的形成也有一定的贡献。结合稳定同位素资料,认为铝土矿的形成可能与生物作用有关。     

Mineralogical and geochemical studies of boron-rich bauxite ore deposits in the Songqi region,SW Henan,China

The Songqi region, SW Henan, is an important bauxite province in China. The bauxite ore deposits occur unconformably on the top of Middle Ordovician argillaceous carbonates. The bauxite ores from the Songqi region are characterized by unusually high boron contents. In this paper, we report the mineralogical and chemical compositions of selected bauxite ores from the region. These new data, together with existing geochemical data of soils in the region, are used to evaluate the origin of boron enrichment in the bauxite ores. The compositional characteristics of tourmaline in the bauxite ores suggests that this mineral was likely derived from a meta-sedimentary protolith. Mass balance calculation reveals that tourmaline only accounts for a small portion of boron in the bauxite ores; the majority of boron in the ores must occur in other forms, possibly by ion absorption on the surfaces of diaspore and clay minerals. In the Songqi region, the Precambrian meta-sedimentary rocks are known to contain abundant tourmalines whereas the Cambrian–Ordovician argillaceous carbonates and the Upper Proterozoic shales are all enriched in boron in free ion state. We suggest that these rocks provided tourmaline and boron ion to the bauxite deposits in the region. Most known bauxite deposits in the Songqi region occur at the margins of boron-rich soil domains, suggesting that boron in soils is a potential exploration indicator for unknown bauxite deposits in this region.     

孝义-霍州一带铝土矿形成的古地理环境及找矿前景

孝义—霍州一带铝土矿是指山西省孝义市—霍州市包括交口县等区域间石炭系本溪组下段的Ｇ层铝土矿。其本溪期古地理环境为一狭长的低凹盆地,其中心部位为泻湖相,由泥质岩—铁质岩或铁质岩—泥质岩区组成,泻湖边缘相由铝质岩区、泥质岩—铝质岩区、铝质岩—泥质岩区构成,铝土矿主要形成分布于该相区内。该区已成为我国最大的铝土矿资源地之一,预计该区南部（汾西西南部）西侧仍具较大找矿前景,可望今后实现地质找矿的重大突破。     

豫西济源西部铝土矿成矿地质环境

文章在对含矿岩系岩石组合、地球化学分析的基础上,认为矿区铝土矿形成于华北地块从早古生代准平原状态向晚古生代陆表浅海过渡时期,早期海侵之后的陆地湖泊环境中.被碳酸盐岩覆盖、较小地形差异的陆地准平原状态,湿热的气候,繁茂的植物使得岩溶湖泊成为铝土矿富集和保存的有利场所.     

山西交口县庞家庄铝土矿矿物学与地球化学研究

山西交口县庞家庄铝土矿属于典型的喀斯特型铝土矿,矿体赋存于上石炭统本溪组,其下部为中奥陶统灰岩。矿石以隐晶质结构为主,含少量碎屑结构与鲕粒结构;矿石的结构特征显示一些风化物质经过了一段距离的搬运,并且矿体在晚期遭受了构造变形作用。x射线衍射分析显示铝土矿石中的主要组成矿物为硬水铝石、高岭石、赤铁矿、针铁矿、方解石与少量...     

Lateritic nickel deposits of Brazil

Many nickel deposits are known in Brazil, accounting for about 350 · 10⁶ tons of ore with an average of 1.5% Ni. All are of the lateritic type. These deposits are scattered throughout the country, being rarer in the Northeastern Region and in the South, below 25 °S latitude. They are mainly associated with mafic-ultramafic massifs of large dimensions and ultramafic alkaline complexes, and occur in climatic regions of contrasting seasons. The weathering profile developed over the fresh rock consists, from bottom to top, of the following horizons: altered rock, coarse saprolite, argillaceous saprolite, ferruginous saprolite and lateritic overburden. The thickness of each horizon varies from one deposit to another, the whole profile generally exceeding 20 m. The saprolitic horizons with inherited minerals (serpentine, chlorite) or neoformed minerals (smectites) constitute the silicated nickel ore and are thicker were climatic conditions are drier; the ferruginous upper horizons made up of iron oxide-hydroxides are more developed in more humid regions. In Brazil, the silicated ore generally prevails over the oxidized ore. The main Ni-bearing minerals are serpentine, smectite, garnierite and goethite. The lateritic nickel deposits of Brazil may be correlated with two erosion surfaces, corresponding to the Sul Americano (Lower Tertiary) and Velhas (Upper Tertiary) levelling cycles. The degree of dismantling of the higher and more ancient surface and the consequent development of the Velhas Surface control the position of the nickel accumulation in the landscape. Thus, the deposits may be found either in the lowlands or in the highlands, where they are always covered by a silcrete layer. The alteration profiles in the Brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world. However, they present two characteristic features: the silicated ore prevails over the oxidized ore, and a silicified layer covers the profies developed on the highlands.