[en] The association of pitchblende-fluorite veinlets with pink-coloured episyenites is here reported as a new type of uranium deposit in Portugal. It is located at Aljao, 9km northwest of Gouveia (central Portugal). The feldspatic episyenitic lenticular body that crops out along 250m and goes down to 80m, grades abruptly to the encasing Hercynian planar-foliated biotite-muscovite medium-fine-grained monzonitic granite. This granite has sharp contacts against the surrounding Hercynian post-tectonic coarse-grained porphyritic biotite granite; NE-SW, E-W and ENE-WSW bearing quartz veins and basic dyke rocks, some of them with uranium mineralization, cut across these latter granites. Pegmatitic and aplitic pods and stringers affiliated with the coarse-grained porphyritic granite are common. Arkoses and conglomerates make up the Ceno-Antropozoic unit which has been mapped at the northern margin of this ore deposit. The albite + K feldspar + chlorite assemblage and the conspicuous cataclastic granular fabric of the episyenite replace the quartz + K feldspar + oligoclase + biotite + muscovite assemblage and the faintly strained planar fabric of the granite. As far as the major elements are concerned a remarkable loss of SiO₂ is reported and an overall balance of the other elements. The episyenite has lower density and higher porosity and permeability than the parental granite. This deposit was discovered by ground radiometric survey and further evaluated by drilling. The uranium mineralization occurs as disseminated phosphates, mostly autunite within the episyenite, as well as under the form of veinlets of pitchblende + fluorite that tend to be more frequent on the transition zone between the episyenites and the granites. The genetic aspects and the guides for prospection which arise with this new type of uranium deposit are considered. (author)

[en] Mineralogical and isotopic studies of vein-type uranium deposits revealed the presence of several generations of radioactive mineral assemblages that formed and survived under specific environmental conditions. Aqueous solutions, percolating along the rejuvenated fractures, dissolved radioactive and radiogenic minerals in one part of the deposit and redeposited them in the other part of the deposit in a different crystalchemical form during a time span of several hundred million years. The following secondary mineral assemblages and their spatial and temporal evolution have been studied: (1) Secondary pitchblende rims formed on sulphides, arsenides, carbonates and quartz between 870 and 200 million years ago; (2) Uraniferous mixed-layer phyllosilicates along fractures in altered rocks where uranium was trapped in the interlayer of montmorillonite component about 400 million years ago; and (3) Reactions between mobilized uranium and other ions, such as silica, sulphur, carbon and water to form coffinite, uranyl-bearing silicates, carbonates, sulphates and hydrocarbons continue at the present time. It is concluded that, by changing chemical-mineralogical compositions and adapting suitable crystal structures, uranium can survive under diverse environmental conditions. (author)

[en] Co-ordinated mineralogical techniques, supported by partial bulk-rock chemical analysis, have been applied in an investigation of drill-core samples of granite from both unaltered and sericitized zones in the Urgeirica uranium deposit, Viseu-Guarda region, Portugal. Significant proportions of the whole-rock uranium values are accounted for by uraninite, although titanium-uranium phases, formed from lateration of ilmenite, are important in the sericitized rocks. The distribution of uranium in the latter suggests several stages of development during alteration. Comparisons are made between the mineralogical and chemical characteristics of uraninite in these samples and vein-uraninite (pitchblende). The results are discussed in relation to the genesis of the vein-type mineralization. (author)

[en] The Madadzang uranium occurrence is situated about 250km SSW of N'djamena and at only 5km from the Cameroon border. Tectonically the occurrence is located close to the eastern arm of the Benue Trough and it lies within a well-defined intrusion of alkaline granite. Two uranium occurrences at a distance of 7km from each other have been recognized but only the southern one, Madadzang, has been investigated so far. The uranium mineralization consists of autunite at surface, and uraninite, Y-rich-coffinite and kasolite at depth. Some sulphides are present. Thorium is very low in all mineral phases but yttrium is unusually high. Two inclined reconnaissance drill holes have intersected 20 and 29m of mineralization, assaying more than 700ppm U, the maximum value being 0.95% U over 1.3m. The Madadzang prospect consists of an essentially subvertical uranium mineralization of primary uraninite and uranium silicates, probably of hydrothermal origin, in a homogeneous but fractured alkaline intrusion. The mineralization is occasionally expressed in highly mineralized veinlets probably structurally controlled. A possible metallogenic relationship between the mineralization and the Benue aulacogene is suggested. Both the vertical and the lateral extension of the mineralization are still untested. (author)

[en] Geomorphology, river drainage patterns and land form may be related to and indicative of, the presence of metalliferous vein structures, particularly if these are composed of either harder or softer gangue material than the enclosing host rock, and where the structures are relatively recent or uncomplicated by later tectonic movements. The Beira region of Portugal is an excellent example of the relationship between uranium-bearing vein and fault structures and the local geomorphology. The Serra da Estrela mountain range dominates the region and is a horst block whose long axis trends N48⁰E and is bounded by well-defined fracture systems N60⁰E on the north-west side and N35⁰E on the south-east side. The uranium veins, the main river courses and the diabase dyke structures to the north-west and south-east of the mountains are parallel to these two bounding fault systems. The close relationship between the uranium vein structures and the geomorphology, particularly the river courses, must indicate that the veins are young in age and closely related to, but immediately following, the uplift of the Serra da Estrela mountains. The uplift of the Serra da Estrela mountains is caused by the effects of the Alpine movements on the Iberian Peninsula and dated at a probable mid- to late Oligocene age. The uranium vein structures must therefore be close to and immediately following that age. (author)

[en] Using the data base developed during Phase I of IUREP (International Uranium Resources Evaluation Project, 1978, NEA/IAEA), the authors have estimated that between 0.5 million and 1.3 million tonnes of the world's speculative resources (excluding countries with centrally planned economies) could occur in vein deposits and have identified those countries in which these deposits are likely to occur. The contribution of 'small veins' to the total speculative resources is probably less than that made by other major deposit types and is significantly less than that made by deposits of the Proterozoic unconformity-related type. However, several factors favour the exploration for, and exploitation of, these deposits compared with the larger deposits, and it is concluded that small veins will play an appreciable role in meeting future uranium demands. (author)

[en] Some vein-type and similar uranium deposits and occurrences are briefly described to show different models identified in Argentina. Practically all of them were formerly thought to be related to hydrothermal-magmatic processes, but at present few are considered to be so; some are classified as typically exogenous and opinions differ about the genesis of the remaining ones, especially because of a lack of sufficient research on the matter since this group of accumulations only contributes less than 10% to the entire uranium resources of Argentina. The typical vein-type ore bodies are small (including less than 200t U) with grades varying from 0.1 to near 1%U. Other deposits, resolved as stockworks, could be from small to medium size (more than 200t U to 2000t U) with a uranium content from 0.7 to 0.03%, respectively. The mineralogical associations are variable, from complex ones in veins considered as magmatic-endogenous (with U, Ni, Co, Pb, Cu, Zn, etc.) to very simple ones in the exogenetic accumulations, which only comprise uranium minerals. The paragenetic studies available are not complete enough to define the possible relation of uranium with the other metals in the complex ores. The age of the mineralization has been defined in some cases, but not in others. There are examples of mineralizing processes occurring from Palaeozoic to very recent times. Some of the uranium deposits mentioned here have been exploited in the past; one of them will be re-opened very shortly; and a new one will be put into operation in 1981. The geological composition of Argentina is not favourable for uranium deposits related to the Proterozoic unconformity, and the best possibilities for finding interesting accumulations of vein and similar type are in the large Hercynian granitic environments which have outcrops that cover more than 150,000km² (Pampean Hills and North Patagonian Massif). (author)

[en] In the Bertholene deposit three fracture lines, N20, N70 and N120, meet to form a tectonic intersection where the fracturing density is greater, giving rise to a morphological anomaly. The N20 and N70 lines converge towards the north-east and determine the extent of the deposit; the deformation at that point takes the form of a heterogene strain producing mineralized bodies of decametric size, the configuration and the attitude of which depend on the characteristics of the fractured network. The tectonic analysis provides a better understanding of the geometry of the mineralized fracture networks forming the deposit. (author)

[en] The granites surrounding the three Portuguese uranium deposits studied here (Urgeirica, Bica and Cunha Baixa) are characterized by high and variable uranium tenors, ranging from 4 to 17ppm at the surface, compared with thorium tenors of 20 to 37ppm. The minor minerals containing the uranium and thorium are as follows: uraninite with low ThO₂ tenors, xenotime, monazite, zircon and, to a lesser extent, apatite. On the sides of the uranium-bearing veins, intense sericitic alteration is caused by low-salinity aqueous fluids (1.1. to 5.8% eq.wt. NaCl) at variable temperatures rising to 200-250⁰C. Several generations can be identified from detailed chemical data on the white micas: magmatic muscovite, deuteric muscovete and phengite occurring along the borders of the mineralized veins. There is evidence of a complex evolution of the fluids within the country rock enclosing the uranium-bearing veins, suggesting that the primary uranium mineralizations are of hydrothermal origin. (author)

[en] As has been pointed out by a number of workers, the development of quartz-pebble conglomerate uranium deposits was confined to the period of 2800-2200 million years (Ma). The first appearance of these deposits coincides with major igneous, sedimentary, biogenic and tectonic changes, and it is suggested that all these factors jointly contribute to the uniqueness of quartz-pebble conglomerate uranium deposits. On the other hand, vein-type uranium deposits made their appearance throughout time post-2200 Ma. However, over 90% of vein-type uranium deposits are found in rocks dated between 2200 and 1700 Ma. It is suggested that this skewness is real. As continental crustal development in post-2200 Ma times appears largely to follow uniformatarian lines, the only variable which could explain the concentration of vein-type uranium in the 2200-1700 Ma period appears to be a steadily evolving atmosphere. It is suggested that during these times the hydrosphere was sufficiently oxidizing for uranyl transport, but rapidly reducing conditions were met short distances into the lithosphere. Reduction resulted in precipitation of uranium of UO₂ from meteoric water into suitable structural traps which were largely developed during periods of prolonged erosion. Rapid development of impermeable cover rocks preserved the uranium deposits. (author)