Niobium Minerals Research Articles

Relationship between oxosilicate niobium mineralization and organic matter in alkaline pegmatites of the Khibiny pluton, Kola Peninsula

In alkaline pegmatites of the Khibiny pluton niobium oxosilicates with (Nb + Ti)/Si ≥ 1 and Nb > Ti occur in intimate fine intergrowths with endogenic organic matter, which makes it possible to assume a genetic relationship between them. Such intergrowths from pegmatites of Mounts Kukisvumchorr and Koashva have been studied by electron microscopy, electron microprobe, IR spectroscopy, and exclusion chromatography. Unsaturated hydrocarbon groups, including aromatic ones, and carboxylate groups are prevalent in the organic matter intergrown with the niobium oxosilicates. The molecular weight distribution in the part of the bitumen soluble in tetrahydrofurane is polymodal, probably due to its formation as a result of parallel reactions on several catalytic centers of polymerization.

Stable (C, O) and radiogenic (Sr, Nd) isotopes of carbonates as indicators of magmatic and post-magmatic processes of phoscorite-series rocks and carbonatites from Catalão I, central Brazil

The Late-Cretaceous Catalao I contains stockworks of thin dykes of phoscorite-series rocks, which can be subdivided into P1 (olivine-bearing, phoscorites) and P2/P3 (olivine-lacking, nelsonites). Dolomite carbonatites (DC) are intimately associated with nelsonites, as pockets and dykes. The P2 apatite nelsonite, the P3 magnetite nelsonite, and, to a lesser extent DC, host the Catalao I niobium mineralization. C–O isotopes signatures in carbonates reveal several distinct magmatic and post-magmatic processes. Limpid carbonates with stable isotopic mantle-like composition show Rayleigh fractionation and are interpreted as primary, while those with brittle-turbid aspect, and higher oxygen isotope composition, probably underwent recrystallization by interaction with H2O-rich fluids. A group of samples shows higher oxygen compositions and lower carbon values, which could be explained by degassing of carbonatite magma during cooling. A degassing pattern, parallel to magmatic degassing but at higher oxygen and lower carbon compositions, observed in carbonate veins, may indicate degassing of fracture filling fluids. Furthermore, C–O isotopes of carbonate from monazite-bearing carbonatite have a positive correlation, indicating a distinct, late-stage carbo-hydrothermal event. Though the Catalao I nelsonites and phoscorites are of igneous origin, they underwent several post-magmatic events, which sometimes overprinted partially or entirely the magmatic isotope signature.

Dissolution Factors of Ta, Th, and U Oxides Present in Pyrochlore

Air pollution can be a problem in industrial processes, but monitoring and controling the aerosols in the work place is not enough to estimate the occupational risk due to dust particle inhalation. The solubility in lung fluid is considered to estimate this risk. The aim of this study is to determine in vitro specific dissolution parameters for thorium (Th), uranium (U), and tantalum (Ta) associated to crystal lattice of a niobium mineral (pyrochlore). Th, U, and Ta dissolution factors in vitro were obtained using the Gamble solution (simulant lung fluid, SLF), particle induced X-ray emission, and alpha spectrometry as analytical techniques. Ta, Th, and U are present in the pyrochlore crystal lattice as oxide; however, they have shown different dissolution parameters. The rapid dissolution fraction (f r), rapid dissolution rate (λ r), slow dissolution rate (f s), and slow dissolution fraction (λ s) measured for tantalum oxide were equal to 0.1 and 0.45 and 0.00007 day−1, respectively. For uranium oxide, f r was equal to 0.05, λ r was equal to 1.1 day−1, and λ s was equal to 0.000068 day−1. For thorium oxide, f r was 0.025, λ r was 1.5 day−1, and λ s was 0.000065 day−1. These results show that chemical behavior of these three compounds in the SLF could not be represented by the same parameter. The ratio of uranium concentration in urine and feces samples from workers exposed to pyrochlore dust particle was determined. These values agree with the theoretical values of estimated uranium concentration using specific parameters for uranium oxide present in pyrochlore.

水素透過膜用 Nb-W-Ta 合金の水素雰囲気中における機械的性質と水素透過能

Pressure-Composition-Isotherm (PCT) measurements, in-situ small punch (SP) tests in hydrogen gas atmosphere and hydrogen permeation tests are conducted for Nb-W-Ta ternary alloys in order to investigate the alloying effects of tantalum on the hydrogen solubility, the mechanical properties and hydrogen permeability of Nb-5 mol%W alloy. It is found that the hydrogen solubilities of Nb-5 mol%W-Xmol%Ta (X=5, 10) alloys are very similar with that of Nb-5 mol%W alloy. Also, the mechanical properties in hydrogen gas atmosphere and hydrogen permeability of the alloys are comparable with Nb-5 mol%W alloy. Therefore, the addition of tantalum scarcely modifies the properties of Nb-5 mol%W alloys. Thus, it is not necessary to take into account the amount of tantalum in Nb-W alloys for hydrogen permeable membrane. In other words, it is not needed to remove tantalum impurity from niobium mineral ores in the course of the refinery process of niobium metal. This means that niobium mineral ore containing high tantalum impurity can be used to reduce the material cost of Nb-W-based hydrogen permeable membranes.

Tantalum Dissolution Rate in Simulant Lung Fluid

The aim of this study is to determine the solubility rate of tantalum contained in pyrochlore, columbite–tantalite and columbite in simulant lung fluid. The International Commission on Radiological Protection (ICRP), in its recommendation for limiting intakes of radionuclide by workers, has consistently recognized that the biological behavior of any specific material after incorporation can significantly diverge from model prevision. Model parameters should be adjusted to adapt the model for each specific substance material in order to estimate the dose due to this element intake. The most recent ICRP publication 66, respiratory tract dosimetry model, point out that information as data like particle size, aerosols solubility and the material chemical compounds are important parameters in the dose coefficients calculation. This paper studies the solubility in Simulant Lung Fluid (SLF) of Ta present in mineral dust particles. For this study 3 minerals were selected: pyrochlore, columbite–tantalite and columbite. Tantalum dissolution in vitro samples were obtained using the Gamble solution and PIXE (Particle Induced X-ray Emission) as analytical technique. In order to characterize the worker exposure to Ta bearing particles, one Brazilian niobium mine was selected. The mineral dust particles were collected using a six-stage cascade impactor and the elemental mass concentrations and the MMAD (Mass Median Aerodynamic Diameter) were determined. Concentrations of radionuclides from natural series of thorium and uranium were also determined. The results show that the workers are exposed to Ta bearing particles in the respirable fraction of aerosols (aerodynamic diameter (d aer) <2.5 μm) during the mineral processing to obtain Fe–Nb alloy. The solubility in Simulant Lung Fluid (SLF) of Ta present in mineral dust particles depends on the mineral characteristics. The solubility half-time varies between 34 and 62 h depending on the associated mineral.

Exposure of workers in a mineral processing industry in Brazil

In Brazil there are many regions where the extraction mining and processing of ores containing elements of great economical importance as tin, niobium and tantalum. Some of these ores have uranium and thorium natural decay series associated. This study was carried out in a niobium mine, where is obtained concentrates of niobates-tantalates, cassiterite and zirconite. The aim of this study is the evaluation of the occupational exposure to uranium, thorium, niobium and tin through urine bioassay data. In order to have it, 105 urine samples were analysed: 17 samples of exposed workers collected after a working day, 49 samples of exposed workers collected before a working day and 39 samples of local non-exposed people, assigned as a control group. The samples were analysed by mass spectrometry. The obtained results showed that the average concentration of Nb, Sn and U in the exposed group is statistically higher than those found in the control group indicating an occupational exposure. For Th there were no statistically difference between the exposed and the control group.

CONSIDERATIONS ABOUT TENORM: A STUDY CASE ON NIOBIUM FACILITIES

Two niobium mining facilities located at two different geological settings were selected as case studies for assessing the radiological impacts associated with mining and milling activities. The impacts were assessed both for operational and post-operational (future unrestricted use of the area) scenarios. The work methodology encompassed sampling and analysis of different materials along the operational process, including wastes, effluents and by-products, mass balance calculations, and dose assessment. The exposure scenario considered during the operational phase included the land use by hypothetical groups of members of the general public. In the case of post-operational phase the possible occupation of contaminated areas was considered. The results pointed out that during the operational phase of the investigated industries no relevant impacts could be attributed to them. However, in the case of the potential occupation of the contaminated areas where the wastes are being deposited, the unrestricted use of these areas cannot be accepted in terms of the relevant radiation protection requirements.

Exposure of workers in mineral processing industries in Brazil

The mining, milling and processing of uranium and thorium bearing minerals may result in radiation doses to workers. A preliminary survey pilot program, that included six mines in Brazil (two coal mines, one niobium mine, one nickel mine, one gold mine and one phosphate mine), was launched in order to determine the need to control the radioactive exposure of the mine-workers. Our survey consisted of the collection and analysis of urine samples, complemented by feces and air samples. The concentrations of uranium, thorium and polonium were measured in these samples and compared to background data from family members of the workers living in the same dwelling and from residents from the general population of Rio de Janeiro. The results from the coal mines indicated that the inhalation of radon progeny may be a source of occupational exposure. The workers from the nickel, gold and phosphate mines that were visited do not require a program to control internal radiological doses. The niobium mine results showed that in some areas of the industry exposure to thorium and uranium might occur.

Kinetics of pyrochlore flotation from Araxá mineral deposits

The niobium mineral deposits in Brazil account for more than 70% of the world's known reserves. Currently flotation of pyrochlore is carried out in the Araxá plant using amines as collector at low pH, in the presence of NaF as an activator and with the addition of a modifier. In the present work bench scale flotation kinetic testwork was conducted, aimed at a better understanding of some aspects of the process used. A rate model based on equations for fast floating and slow floating particles was used for describing the cumulative recovery versus time. The results showed that the control of collector (amine acetate) at a minimum level was essential for effective action of the activator and modifiers studied. In the rougher stage, best results were obtained with the collector at 100g/t. The use of NaF at a dosage higher than 1000g/t significantly affected the cleaner concentrate grade.

Geology and Copper Sulphide Mineralization of the Salmagorskii Ring Igneous Complex, Kola Peninsula, NW Russia

The Salmagorskii igneous complex represents a rare example of the carbonatite body of the Phalaborwa complex in South peralkaline rocks containing significant copper sulphide minAfrica contains one of the world’s largest copper deposits, eralization presented by chalcopyrite, djerfisherite and cubanite. but contrary to many other similar intrusions, it is charData on the geological structure of the complex and the copper acterized by the lack of niobium mineralization (Versulphide–apatite mineralization related to its foidolitic peralkaline woerd, 1986; Eriksson, 1989). On the other hand, even rocks have been obtained from drilling in recent years. The sub-economic copper sulphide ores (except for Phalsequence of rocks (dunite–clinopyroxenite–melilitolite–ijolite–urtite– aborwa) are rather uncommon in alkaline rocks and melteigite–carbonatite) forms a perfect concentric zoning within the carbonatites. This paper presents the first data on copper pluton indicating that fractional crystallization was the predominant mineralization from mafic peralkaline rocks of the Salpetrogenetic process. Copper sulphide mineralization formed at the magorskii complex. post-magmatic stage, when recrystallization processes within earlier Alkaline–ultramafic ring igneous complexes in the Kola ijolite and melteigite took place under the influence of a fluid rich Alkaline Province (Kukharenko et al., 1965), as well as in CO2, H2O, alkalis, S, P, Cu, and Cl. other regions (Woolley, 1987; Kogarko et al., 1995; Vasiliev & Zolotukhin, 1995), are usually concentrically zoned, pipe-like intrusions in which ultramafic rocks (dunite or clinopyroxenite) occupy the central parts of the complexes and melilitolites with foidolites surround

Occupational Exposures to Thorium in Two Brazilian Niobium Plants

The worker exposure to thorium-bearing airborne particulate was estimated in two Brazilian plants that process niobium minerals, one in the Amazon Forest (Plant A) and the other in the State of Goias (Plant B). The aerosol particle size and the thorium concentrations in the respirable fractions of aerosol concentrations were determined. Results indicate that in Plant A, the MMAD (Mass Median Aerodynamic Diameter) of particles containing thorium were in the range 1.1 to 1.8 μm: in Plant B they were in the range 1.1 to 3.4 μm. The thorium faeces concentrations before the vacation were higher than thorium faeces concentrations in the control group. After the vacation the thorium faeces concentrations were similar to the thorium faeces concentrations in the control group. These results indicate that the thorium incorporation by the workers are mostly due to ingestion.

A Cascade Impactor for Mineral Particle Analysis

ABSTRACT A cascade impactor (NCI) made of stainless steel with six stages was developed to collect particles using 50% cutpoints in the range of 0.64 to 19.4 μm. The sixth stage of the NCI was calibrated using monodisperse aerosols. The experimental MMAD (mass median aerodynamic diameter) was 0.634 μm, which agreed with the calculated MMAD. The NCI, stacked filter unit (SFU) and personal air samplers with the cyclone were used to characterize the workers exposure in two Brazilian plants where niobium minerals with thorium associated to the crystal lattice are processed. The Th, U, Nb, Zr, Pb, Fe, Sr, and Y air concentrations were determined. The MMAD of the particles containing these elements have shown the availability of mineral particles in the fine particles of aerosols. The Th concentration in the samples collected using SFU were in the same order of magnitude of those collected with the cascade impactor and are correlated.

Uranium and thorium exposures associated with conventional mining

Internal contamination with radioactive materials of mining workers is a common problem in Brazil. This is caused by the presence of uranium, thorium, and their natural decay series associated with the mined ore. The clear examples are the workers at the niobium mine located in the state of Goias. The niobium is associated with considerable quantities of uranium and thorium, but the mine is not legally subject to radiation protection requirements. Twenty mine workers were evaluated using “in vitro” bioassay techniques (urine and feces). The fecal samples were analyzed by alpha spectrometry using the method developed in the Bioassay Laboratory of the Instituto de Radioprotecao e Dosimetria/CNEN which evaluates thorium and uranium isotopes simultaneously. Minor modifications were introduced to measure a higher level of activity, around 1 Bq of uranium per sample. The urine samples were analyzed by alpha spectrometry for thorium and by fluorimetry for uranium. The results obtained show that a control of the occupationally-exposed workers is necessary.

Titanium, vanadium, and niobium mineralization and alkali metasomatism from the Magnet Cove Complex, Arkansas

The Christy Ti-V-Nb deposit is hosted by the Arkansas Novaculite, adjacent to the carbonatite-alkaline Magnet Cove Complex. Mineralized rock contains up to 8 wt percent Ti, 2.68 wt percent V, and 0.124 wt percent Nb. Minor amounts of Li and Mo are present, and there is evidence for the preferential concentration of the middle to heavy rare earth elements (REE) in altered clay-rich dikes. Mineralized rock consists of (1) thermally recrystallized novaculite that is partly replaced by actinolite, sodic amphibole, taeniolite (KLiMg 2 Si 4 O 10 F 2 ), pyrite,and siderite, (2) brookite-bearing amphibole- and taeniolite-rich rock that contains siderite and pyrite, (3) quartz-brookite aggregates, and (4) goethite-rich rocks that form the oxidized zone of the deposit. Unaltered rock is scarce at the Christy deposit. Comparison of mineralized lithologies from the Christy deposit with those from two other areas of mineralization associated with the Magnet Cove Complex, the Magnet Cove Titanium Corporation deposit (Rutile deposit) and the Hardy-Walsh prospect, provides mineralogical and chemical evidence for the source of the mineralizing fluids at the Christy deposit.Massive clay-rich (kaolinite + or - smectite) dikes are in contact with the mineralized zones of the Christy deposit. The presence of relict K feldspar and pyrite in the least altered dikes and certain characteristic trace elements (e.g., Ba, Rb) suggest that the Christy dikes are related to less pervasively altered rutile-mineralized, pyrite-bearing carbonate-feldspar dikes fromthe Rutile deposit. The feldspar-carbonate dikes from the Rutile deposit may be the products of intense alkali metasomatism of alkaline igneous silicate rocks that were emplaced prior to the intrusion of the Magnet Cove carbonatite.The Christy deposit formed through a series of complex processes. The initial phase of mineralization is directly related to the infiltration of novaculite by alkali-rich fluids that were probably derived from carbonatite magma. Introduction of the metasomatic fluids preceded emplacement of massive feldspar- and pyrite-bearing dikes that were subsequently altered to the clay-rich masses Titanium, V, Nb, and Li were introduced by the alkali-rich fluids at temperatures that were, as indicated by fluid inclusion data (Willis et al., 1991), as high as 600 degrees C. During the initial stage of mineralization, V was concentrated in aegirine and sodic amphibole, Li was concentrated in taeniolite, minor amounts of Ti were concentrated in aegirine, and pyrite formed. The replacement of novaculite by the aforementioned minerals yielded excess silica, which precipitated as quartz in veins and as clear overgrowths on recrystallized, inclusion-rich (partly replaced) quartz. Niobium- and V-bearing brookite precipitated with the quartz. Minerals formed during the first stage reacted with a second fluid at temperatures of 100 degrees to 300 degrees C, as indicated by fluid inclusion data (Willis et al., 1991), and V was then concentrated in smectite and goethite. The isotopic composition of siderite [delta 18 O SMOW = 23.8-25.0ppm, delta 13 C PDB = -7.1 to -8.9ppm suggests that the second fluid was a mixture of low-temperature metasomatic fluid and ground water. Vanadium was further concentrated in clay minerals (Breit et al., 1992), in goethite, and in vug minerals as low-temperature alteration proceeded. Brookite is not totally resistant to weathering and it is at least partly replaced by other oxides in the oxidized zone.

Biological monitoring of particulate pollutants during exploration work at a niobium mineralization in Greenland

The purpose of this investigation was to monitor airborne dipersion of particulates in connection with exploration activities at a niobium-mineralization in the Sarfartoq area, SW Greenland, during the summer of 1989, and to provide a basis for monitoring strategies in later phases of the development of the mining site. During three periods moss bags were exposed at 9 stations close to the outcrop and at 3 background stations, and indigenous lichens and mosses were sampled before and after the exploration work.Analyses of element concentrations showed that the natural enrichment of elements in the indigenous vegetation is restricted to areas immediately at or on the outcrop. The elements found in elevated concentrations were Nb, La, Ce, Th and U.The moss bags and the indigenous monitor organisms showed rises in concentrations of the elements Nb, Ce, Cd, Pb and Zn after the exploration activities during the summer. This shows that some dispersion of dust has occurred in the area.

The cancrinite-syenite/carbonatite complex of Lueshe, Kivu/NE-Zaire: petrographic and geochemical studies and its economic significance

The Luesche cancrinite-syenite/carbonatite complex is located in NE-Zaire about 50 km SSW of Lake Edward. The alkaline rocks have intruded metamorphic rocks of Burundian age (1.100 m.y.). The country rocks are quartzites and mica schists bearing chlorite, garnet, Kyanite and staurolite, which indicates P-T conditions of some 4 kbar and 430–500°C, maximum 6 kbar and 620°C. The K/Ar age of the biotites from the carbonatite is found to be 516 m.y. The core of the alkaline complex is composed of cancrinite-syenites which vary from place to place with respect to the contents of garnet, amphibole, aegirine, sodalite and cancrinite. Significant variations of the pyrochlore content are also observed. The core is surrounded by fine grained calcitic carbonatite (alvikite) which often shows a banded texture and grades into coarse grained calcitic carbonatite (søvite). In the SE of the søvite ring there is a separate dolomitic carbonatite body (beforsite) while at the contacts between the alkaline rocks and the country rocks a wide range of low to high grade fenites enriched in Na or K occurs. Within the alkaline complex the highest niobium values are found associated with metasomatic rocks of the contact zones such as carbosyenites, pyroxentic rocks and fenites. The main niobium mineral is a Ca, Na-pyrochlore enriched in Ti and/or Sr. The geochemical distribution patterns of the major elements as well as of the trace elements, especially the REE, suggests that the carbonatites are of primary magmatic origin derived from mantle material which was affected by later remobilizations, most likely as a result of postmagmatic hydrothermal solutions. Regarding the cancrinite-syenites, there is no evidence for a comagmatic formation with the carbonatites. They are rather of subcrustal origin; formed in the course of various metasomatic processes where the alkalic carbonate melts or volatile carbonate phases interacted with the metamorphic country rocks to form rheomorphic syenites which subsequently intruded. From an economic point of view only the lateritic zone derived from weathering of the carbo-syenites, pyroxenitic rocks and fenites is considered as ore due to the concentration of pyrochlore in the course of weathering. Ore grades vary from 1 % to 7 % Nb 2O 5. The ore is composed of clay minerals, iron oxides and phosphates. The weathering profile may be up to 150 m deep, whereby a gradual change with depth from crandallitic ore to crandallitic-apatitic ore and to apatitic ore in the bottom section occurs. Ore reserves amount to some 8 million tons with an average of 2.5 % Nb 2O 5. By these figures Lueshe is one of the few high grade niobium deposits in the world.

The mineral occurrences of central East Greenland

The present monograph on the mineral occurrences of central East Greenland is the result of an agreement between Nordisk Mineselskab A/S and the Mineral Resources Administration of Greenland (Råstofforvaltningen for Grønland) on compilation of the mineral-exploration activity carried out in central East Greenland by Nordisk Mineselskab A/S during the period 1952- 1984. The main aim is to present geological descriptions of all known mineral occurrences and to compile most of the existing geochemical data in a manner applicable to future exploration in the area. More than 200 individual mineral occurrences are described and many in great detail. Geochemical data are presented for scheelite and for 16 elements relevant to the evaluation of the mineral potential of the area. The data are presented as single-element anomaly maps for both rock samples and panned heavy-mineral concentrates and include data from a file consisting of more than 10 000 rock samples and close to 4000 panned heavy-mineral samples.&#x0D; Mineralization in central East Greenland occurs in rocks of Archaean to Oligocene age and has tentatively been grouped into several periods when mineralization took place.&#x0D; Mineralization hosted in Archaean-Lower Proterozoic rocks includes magnetite and chromite accumulations and iron-sulphide segregations associated with ultramafic and mafic igneous assemblages, volcanogenic massive-sulphide occurrences, titaniferous magnetite and ilmenite occurrences associated with original plutonic anorthositic massifs, banded iron-formations, gold-bearing quartz veins, a complex copper-skarn occurrence, and uranium and gold-uranium occurrences of unknown origin.&#x0D; Mineralization hosted in Middle and Upper Proterozoic rocks is restricted to lead-zinc and tungsten skarns and stratiform copper occurrences.&#x0D; Mineralization hosted in Lower Palaeozoic rocks is in general believed to be associated with Caledonian orogenic activity. It comprises tungsten skarn occurrences mainly associated with granodiorite intrusions, base-metal and tungsten mineralization associated with late-kinematic probably calc-alkaline granite, tin-tungsten-arsenic quartz veins associated with late-kinematic probably mildly alkaline granites, gold-bearing quartz veins associated with late-kinematic granite, uraniferous veins in probably Devonian alkaline granite, tungsten-antimony-gold and silver-bearing base-metal veins in low-grade metamorphic sediments, uranium-fluorite veins in Devonian felsic volcanic rocks and strata-bound uranium occurrences hosted in Devonian elastic rocks.&#x0D; Mineralization hosted in Upper Palaeozoic sediments (Carboniferous - Permian) includes base-metal quartz-baryte veins locally enriched in precious metals, goldbearing quartz veins. uraniferous veins, carbonate-hosted strata-bound baryte base-metal occurrences, carbonate-hosted strata-bound celestite occurrences, stratiform base-metal showings of Kupferschiefer type and strata-bound red-bed copper occurrences.&#x0D; Mineralization hosted in Mesozoic sediments is represented by Triassic stratiform and strata-bound base-metal occurrences of red-bed type and Jurassic placers rich in zirconium and rare-earth elements.&#x0D; Tertiary mineralization includes a major porphyry-molybdenum occurrence, granite roof-zone molybdenum mineralization, niobium mineralization associated with alkaline intrusive rocks, lead-zinc skarn occurrences, a minor magnetite-skarn showing, lead-zinc-bearing quartz veins and precious-metal, base-metal, molybdenum and fluorite mineralization associated with fumarolic volcanic activity.&#x0D; The location of all mineral occurrences is shown on a separate map: Mineral Occurrences in Central East Greenland.

Cancer incidence among workers exposed to radon and thoron daughters at a niobium mine

Cancer incidence among workers exposed to radon and thoron daughters at a niobium mine

Cancer incidence among workers exposed to radon and thoron daughters at a niobium mine.

The aim of this study was to investigate the incidence of cancer among 318 male employees of a niobium mining company which was only operated between 1951 and 1965. Many of the workers, especially underground miners, were exposed to the daughters of radon and thoron and also to thorium. The accumulated doses to the workers from short-lived radon and thoron daughters in the mine atmosphere were assessed to be relatively low; up to 300 working-level months. During the follow-up period 1953-1981, 24 new cases of cancer were observed compared to an expected number of 22.8. Twelve cases of lung cancer had occurred versus 3.0 expected. Among the 77 miners, 9 cases of lung cancer were observed against 0.8 expected. Associations between the occurrence of lung cancer and exposure to alpha radiation and smoking were found. For the radon and thoron daughter exposure, about 50 excess cases per million person-years at risk per working-level month were observed.

Hydrothermal alteration and mineralization of the alkaline anorogenic ring complexes of Nigeria

The Jurassic alkaline anorogenic granitic ring complexes in central Nigeria are discordant high level intrusions emplaced by piecemeal stoping through a collapsed central block. Biotite granites predominate over fayalite or amphibole bearing variants and are important for their ore deposits. A series of alteration processes, with related mineralization, can be recognized. Early high temperature sodic metasomatism may introduce niobium mineralization, can be recognized. Early high temperature sodic metasomatism may introduce beginning with potash metasomatism affect only the biotite granites. Accessory monazite, zircon, cassiterite, rutile, molybdenite and occasionally wolframite are associated with mica distribution. Subsequent acid metasomatism results in greisenization. The ore assemblage deposited at this stage is dominantly of oxides with early monazite, zircon and ilmenite followed by cassiterite—the major ore, wolframine, sometimes columbite or siderite and always rutile. During silica metasomatism early cassiterite is followed by abundant sphalerite, chalcopyrite, galena and occasionally arsenopyrite or pyrite. Chloritization and argillization are important but more restricted alteration processes. Fluid inclusion studies indicate that there was an initial separation of dense saline brine from the residual melt followed by vapour phase separation, to evolve a low density fluid. Fluids responsible for early metasomatic alteration and pegmatide development are highly saline and may contain glassy fluorosilicate phases. Homogenization temperatures are between 380 and 550°C. Later fissure-filling veins form in the range 300–380°C at salinites up to 15 eq. wt% NaCl, with the latest mineralized quartz veins in the range 200–300°C and salinities <10 eq. wt% NaCl. In addition, low temperature, monophase inclusions occur which are probably associated with late stage argillic alteration. Superimposed on this general pattern of decreasing temperature and salinity are important stages of immiscible phase separation related to CO 2 loss and local increase of salinity due to boiling as pressure is released. Mineralization may take the form of: (i) late magmatic pegmatites, (ii) pervasive metasomatic disseminations, (iii) pre- and postjoint pegmatitic lenses, (iv) quartz rafts, stockworks, sheeted veins and altered wall rock, (v) fissure-filling veins, (vi) irregularly shaped replacement bodies, (vii) quart veins, (viii) ring-dykes, (ix) alluvial and eluvial deposits. The different processes of alteration and associated mineralization characterize different parts of a granite pluton: (i) the roof zone, (ii) marginal zone, (iii) contact zone, (iv) the country rock, (v) the ring-dyke zone.