SURFICIAL URANIUM DEPOSITS IN ARGENTINA

F. RODRIGO. H. OLSEN, A.E. BELLUCO

Comisiôn National de Energfa Atômica Buenos Aires, Argentina

ABSTRACT

SURFICIAL URANIUM DEPOSITS IN ARGENTINA

In Argentina, several surficial Cenozoic uranium occurrences have been found. They are all of low grade, wide area! extent, occur near the surface, have varied thicknesses, are highly irregular in distribution, and have significant potential. Two deposits only have been fully evaluated, while the rest have only been found recently and remain unexplored.

Three main types have been identified:

1. Uraniferous calcite- and gypsum-cemented clastic sediments of Cenozoic age.

2. Autunite and meta-autunite in intensely fractured granites.

3. Uraniferous calcareous sediments associated with hot springs in Quaternary volcanic fields.

1. INTRODUCTION

Several Cenozoic uranium deposits have been found in Argentina (Figure 1 ). Some deposits are merely showings and are not economically viable, while others have considerable uranium resource potential.

Three main types of surficial uranium deposits have been identified:

1. Those associated with calcite and gypsum, cemented Cenozoic clastic sediments, which contain small amounts of manganese and iron oxides. They are generally of a low grade but have a significant uranium resource potential.

2. Occurrences associated with areas of intense fracturing in Carboniferous granites.

3. Occurrences in travertine, aragonite, crystalline limestone, and tufa deposited by hot springs in the Late Pleistocene volcanic cycle.

2. TYPE 1 — IN CENOZOIC CLASTIC SEDIMENTS 2.1 Valle de Punilla-Rodolfo Deposit

The Rodolfo deposit occurs in Cordoba Province at latitude 31°15' south and longitude 64°30' west.

General Geology

It is one of a number of uranium deposits that are hosted by Tertiary continental sediments situated on the eastern flank of the north-trending Punilla valley. They are found in a 50 km belt between the towns of Bialet Masse in the south and Capilla del Monte in the north. The most important occurrence is Rodolfo, which has a mineralized zone of 6 km in length, 50 to 300 m wide, and a thickness of 1 to 2 m that locally reaches a maximum of 8 m.

The uranium occurs principally in the middle section of the Eocene Cosquin Formation, which is overlain by the non-uraniferous Casa Grande Formation, both of which unconformably overlie Palaeozoic granites similar to those of the Sierra Grande. Covering these rocks are Quaternary sediments such as fanglomerates and loess (Figure 2).

The Cosquin Formation is about 115m thick and can be divided into three members:

1. A lower member consisting of 90 m of calcite-cemented arkosic sandstones with siltstone lenses;

2. a middle member consisting of 5 to 12 m of calcite-cemented red silts and montmorillonitic clays; and 3. an upper member consisting of calcite-rich sediments, similarto the lower members in composition, but with

a greater abundance of calcite cement.

Clastic sediments of all three members were derived from the granitic rocks of the Sierra Grande to the west. The lower two members are red with green-grey bleached zones, mostly along joints and in sandy layers. The upper member contains abundant limestone nodules, and a few sandstone lenses and siliceous beds are present. These sediments were deposited in a lacustrine or low-energy stream environment.

Clastic sediments in the three members are composed of clear to white, sometimes smoky, quartz, potassic feldspars, pistacite, minor light-blue fluorite, white mica, red garnets, magnetite, ilmentite, and trace monazite.

The Casa Grande Formation is more than 150 m thick and consists of a monotonous sequence of calcite-cemented silty sandstones and gravelly conglomerates. A basal well-calcite-cemented conglomerate is similarto the

REPUBLIC OF ARGENTINA C.N.E.A.

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Figure 1

The distribution of surficial uranium deposits in Argentina.

Figure 2

Geological cross-section of Valle de Punilla-Rodolfo uranium deposit

upper Cosquin Formation, but with some compositional differences. The conglomerates are polymictic and contain clasts derived from the metamorphic terrane of the Sierra Chica to the east.

These clastic sediments are composed principally of milky quartz, feldspar, pistacite, muscovite, biotite, minor magnetite, garnet, hematite, and monazite. Abundant plutonic and metamorphic rock fragments transported from the east and northeast are present.

Tectonics

The Cosquin and Casa Grande Formations dip homoclinally 40° to 50° to the east and are repeated by the north-to northwest-trending east-dipping thrust faults. To the east, a major fault at the foot of the Sierra Chica juxtaposes Precambrian and Lower Palaeozoic rocks against the Tertiary formations (Figure 2). To the west another fault juxtaposes Tertiary and Quaternary sediments. This north-south system of faulting is intersected by a later east-west system.

Mineralization

The uranium is hosted by red and silty clays of the middle member of the Cosquin Formation. It occurs as carnotite- and tyuyamunite-cemented nodules and lenses that occur over an average thickness of 3.3 m. The uranium is irregularly distributed and tends to follow different layers in the middle part of the formation. No uranium has been found in the Casa Grande Formation or in the Quaternary sediments. Reasonably assured resources are 3 700 t U308 with a grade of 0.04 % U30g.

2.2 Valley Conlara

In the Conlara River valley of San Luis Province, uranium resources occur in an area 40 km long and 10 km wide, against the flanks of the valley from Santa Rosa to the north to Concaran to the south between the Comechingones ridge to the east and the Santa Luis ridge to the west

Uranium occurs as carnotite and tyuyamunite in silts and sandy silts, and in the regoliths of metamorphic rocks.

The sediments are Tertiary in age and are found along the rims of the valley. Two occurrences have been studied:

Las Toscas

Las Toscas occurs near Santa Rosa at latitude 32°30' south and 65°14' west Spotty impregnations of carnotite and tyuyamunite occur in a silty sandstone in an area 300 by 300 m which has a thickness of 2 m. Some samples

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Figure 3

Surficial uranium deposits in Patagonia.

yield up toi 000 ppm U308 over a thickness of 0.8 m. The mineralized beds are high on the rim of the valley in the old river terraces lying on the Precambrian and Lower Palaeozoic schists and gneisses.

Humberto

Humberto is loctated at latitude 32°34' south and 65°15' west. Carnotite and tyuyamunite impregnate strongly fractured and altered metamorphic rocks with abundant kaolinite and calcium carbonate. The mineralization is controlled by a faulted zone that strikes 330°. Biotite schists with some muscovite and yellow uranium minerals are found to the west of this area. To the east, there is an increase of hematite and a decrease of carbonates and uranium mineralization. Drilling in the faulted area gave no indication of uranium mineralization below the water table and is considered to be epigenetic and surficial. The resources of the deposit have been estimated at 50 t U308 at a grade of 220 ppm.

2.3 Patagonia

Many surficial uranium deposits occur in Chubut Province in the southern region of Argentina known as Patagonia (Figure 3). The region is desertic, receiving less than 200 mm rain per year, a mean temperature of about 12 °C, and a high evaporation rate, due to strong winds.

The uranium occurrences are associated with closed lacustrine basins which are highly calcareous and usually contain gypsum. Since they have not been explored, little is known about their potential uranium resources or detailed geology.

The source of uranium is the Manuel Arce Formation, a fluvial conglomeratic sandstone which forms the upper part of the middle Upper Cretaceous Chubut Group. The Chubut Group is a continental volcano-sedimentary sequence that contains uranium deposits and occurrences in many different stratigraphie horizons. The Manuel Arce Formation contains autunite, schroeckingerite, carnotite, and tyuyamunite associated with plant debris, mostly partially silicified fossil logs. It occurs within the drainage basin of the Laguna Salada (Salt Lake), is about 200 km long and has an associated playa with an area of about 1 500 km2 (Figure 3).

The eastern playa is composed of 1 m of thick brown sandy silt covered by the unconsolidated conglomerates of the Rodados Patagonicos Formation of Pleistocene age. Both the silts and the conglomerates are locally impregnated with tyuyamunite and calcium carbonte in an area of about 70 x 20 km.

Four groups of occurrences have been located in the Laguna Salada and adjacent areas in the Chubut Province:

Laguna Salada

Laguna Salada occurs between latitudes 44°18' and 44°26' south and between longitudes 67°11 ' and 67°28' west. Tyuyamunite occurs as coatings on pebbles and in fractures, joints, and cracks in the underlying sandy silts, all of which contain abundant calcareous cement. Theuraniferous layer is 1 m thick, but only 0.4 m has a grade as high as 300 ppm. There is a direct correlation between the calcium carbonate and the uranium concentration.

La Madresetva

La Madreselva occurs between latitudes 44°02' and 44°14' south and between longitudes 67°11 ' and 67°22' west, and constitutes another group of anomalies in the same environment with calcareous and gypsiferous cement. Tyuyamunite occurs in layers 0.2 to 0.5 m thick in sandy silts with uranium grades of up to 650 ppm.

Cerro La Virgen

Cerro La Virgen is the third group of anomalies in the Laguna Salada area which occur at latitude 44°1 9' south and 68°03' west.

El Mirasol

El M irasol, occurring at latitude 43°07' south and longitude 67° 13' west, constitutes another group of radiometric anomalies in Chubut Province.

3. TYPE 2 — IN FRACTURED GRANITIC TERRANES 3.1 Los Gigantes

In the Los Gigantes area, surficial uranium deposits are formed in zones within a granitic terrane where the intense fracturing created paths through which solutions could move. These fractures, which contain finely divided material, acted as sponges, thereby retaining the uranium. Exploration has failed to show uranium mineralization at depths of more than 40 to 50 m, in particular at Schlagintweit, the most important of this type of uranium deposit.

Schlagintweit

Schlagintweit is located on the eastern slope of the Los Gigantes ridge at latitude 31°20' south and longitude 67°47' west in Cordoba Province and has been extensively explored. It is situated within the granites of the Achala batholith which intruded into Upper Precambrian-Lower Palaeozoic high-grade metamorphic rocks that form the Sierras Pampeanas. The granite (329+1 5 Ma) is coarse-grained and locally porphyritic with pegmatitic textures and contains potassic feldspar (perthitic microcline), lesser oligoclase, andxenomorphic quartz. During the upper Tertiary it was exposed and extensively fractured.

The granite is cut by NNW-trending thrust faults which control the orientation of the Los Gigantes ridge forming part of the Sierra Chica. Another thrust fault system strikes WNW-ESE. The intersection of the two systems has created structural traps which contain concentrations of uranium. At Schlagintweit, there is a long compressive NNW-SSE fault, dipping WSW, that is intersected by a WNW-ESE fault to form a dihedral angle where the granite is intensely fractured and uranium occurs as a massive body. There are several sets of joints, the horizontal set being the most important in the distribution of uranium. The mineralized body has a elliptical shape with a log axis of about 600 m and a maximum depth of 45 to 50 m. The uranium minerals are of secondary origin and consist of autunite and meta-autunite with lesser beta-uranophane and phosphuranylite.

Schlagintweit was formed by leaching of the granitic rocks of the Achala batholith, which is slightly anomalous, having a uranium content of 6 to 9 ppm U308, mainly as disseminated uraninite and uranium in biotite grains. The intense fracturing in the granite created favourable conditons for leaching of uranium from the biotites and was

transported and deposited in the topographic depressions in the granite which occur in areas of more intense faulting and fracturing. The main faults acted as paths that controlled the movement of the mineralizing solutions.

Reserves of the deposit are approximately 2 000 t of U308 with an ore grade of 250 ppm. The average thickness of the deposit is 33 m. Other lesser important occurrences located along the NNW-SSE fault include Los Europeos;

La Morenita; and Cerro Aspero. On the western flank of the Achala batholith, there are other similar uranium deposits such as the Los Riojanos, Don Vicente, and Don Alberto, but these have not been completely explored.

The uranium minerals are autunite, meta-autunite, and phosphuranylite, with uraninite in the biotite.

4. TYPE 3 — CALCAREOUS SEDIMENTS ASSOCIATED WITH QUATERNARY VOLCANIC ROCKS

4.1 Las Penas

The Las Penas uranium occurrence is found at latitude 34°14' south and longitude 68°49' west in the Sierra Pintada area of the Mendoza Province, where subeconomic uranium occurs in Pleistocene calcareous sediments.

These rocks are more than 20 m thick and are composed of travertines, aragonite, crystalline calcite, and tufas.

The mineralized unit is a banded travertine 1 m thick and 400 m long which contains 100 ppm U3 08-4.2 Lahuen-Co

Lahuen-Co is situated at latitude 35°10' south and longitude 69°50' west in Mendoza Province where limonitic travertine, deposited by hot springs, contains up to 200 ppm U3O8. Radioactivity is low and the mineralization appears to be out of equilibrium.

4.3 Arizaro

Arizaro is situated at latitude 25°01' south and longitude 67°53'west in Salta Province. There are several occurrences of uranium located around the closed basin of the Arizaro Salt Lake, the most important of which are: