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Uranium was discovered in this area by the Onuba mining company
in the 1950s. This exploration phase continued until the demise
of the first uranium boom in the early 1960s. Investigations were
continued by the JEN from 1974 to 1978. A series of uranium occurrences
were discovered either by Onuba or the JEN. The mineralisation is
hosted by members of the late Precambrian Schist-Greywacke Complex
along the northern contact of the Hercynian, Cabeza de From 1982 to 1988 exploration was continued by ENUSA, and later, from 1989 to 1994, by CISA, the Spanish subsidiary of COGEMA. There appears to have been no further investigation since that time. The area with most potential is covered by a single application for a ‘Permiso de Investigacion’. The permit covers an area of some 85 km² and traces the northern contact of the Cabeza de Araya granite for some 22 km in a WNW-ESE direction.
Geology The stratigraphic sequence consists of a thick succession of late Precambrian sediments that correspond to the Schist-Greywacke Complex. This sequence is an alternation of pelitic and arenaceous sediments with intercalations of coarser-grained materials. These sediments experienced deformation, metamorphism and igneous intrusion during the Hercynian Orogeny. They have been folded into a series of NW-SE trending, steeply-dipping isoclinal folds. Only one phase of penetrative deformation has been definitely recognized, but there are a series of late Hercynian structures including a crenulation cleavage, and various episodes of fracturing. Metamorphic grade corresponds to the lower greenschist facies. The intrusion of the porphyritic two-mica Cabeza de Araya granite that forms the southern limit of the prospective area produced a pronounced thermal aureole in the metasediments. Following the Hercynian Orogeny, there was an extensive period of erosion and peneplanation that extended through the Mesozoic. During the Tertiary, a relatively thin cover of continental clastic sediments was laid down. This cover has been partially removed by erosion during the Pliocene-Quaternary, exposing the various uranium deposits. Uranium Occurrences Mineralisation is epithermal consisting of pitchblende and coffinite associated with pyrite/marcasite and carbonate gangue. There is a strong similarity with the deposits in Salamanca Province. Valdemarco, La Espigadera, El Castillo, Hocino Norte, Hocino Sur, El Piojo and Valdescabon are considered to be the most interesting occurrences. The Valdemarco deposit consists of two main zones of mineralisation, ‘Viesgo Oeste’ and ‘Viesgo Este’. ‘Viesgo Oeste’ is situated close to the northeastern limit of Ceclavin village. Drilling on this property has intersected strong radiometric intervals in fractured pyritic and carbonaceous black slates with visible autunite and torbenite (CISA, 1989). The zone extends for some 400m, but the continuity of the mineralisation is not clear. Drilling was also carried out on the ‘Viesgo Este’ property. The La Espigadera deposit has been investigated several times by at least three different entities -JEN, ENUSA and CISA (Cogema). An initial drilling program was carried out by ENUSA and subsequently by CISA, providing significant data for future targets. The mineralised zone appears to be related to a WNW-ESE fracture zone that dips at a low angle to the northeast. The primary mineralisation is hosted by fine fractures in pyritic and carbonaceous black slates. El Castillo is a further occurrence related to fractures in black carbonaceous slates. Targets An estimation by ENUSA of the in-situ uranium resource in the Ceclavin-Acehuche Area at the end of 1995 provides a target of 5-5.5m lb of contained U3O8 at a grade from 0.09-0.1%. However the breakdown of this resource figure by deposit is not available from data reviewed to date. |
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