REPRODUCED FROM BEST AVAILABLE COPY

RME-4533

UNITED STATES ATOMIC ENERGY COMMISSION DIVISION OF RAW MATERIALS

WASHINGTON, D.C.

URANIUM EXPLORATION AND DEVELOPMENT IN ARGENTINA

By

H. E. Stocking H. B. Wood W. W. Keyes

REPRODUCED FROM BEST AVAILABLE COPY

Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, productaor process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise,bdoe n necessarilyonttt or imply its endorsement,

recommendation,

or favoring by the

United rnStstes Government or

any agency thereof. The

views

andopinosof authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Washington, D. C.

April 1959

RMe-4533

URANIUM

EXPLORATION AND DEVELOPED

NT

IN ARGENTINA

CONTENTS

Page

INTRODUCTION . . . . . . . . . . .o . . . . . .

DISTRICT UIPLORATION AND DEVELOPMENT * . . , . ." .

Malargle area . . . . . . . . . . . . . . .

Huemul-Agua Botada locality . . . . Agua Botada mine, South Agua Botada, and

area north

to

Huemiil . . . . . . . . . .

Pampa Amarilla anticline prospects .. . . Guandacol . . . . . . . . . . . . . . . . . . Sierra Cuadrada . . . . . . . . .*. . . . . . Canadon Krueger . . . . . . . . . . . ...

La Estela mine and Sierra de Comechingones .

Chihuido del Medio . . . .

San Sebastian mine, Sanogasta area . . . . .

Rahueco . . . . . . . . . . . . . . . .

Cosquin . . . . . . . . . . . . . . . .

San Roque mine. . . . . . . . . . . . . . .

Canadon

Gato ^{. . . . . . . . . . . . .} . . .

Miscellaneous districts . . . . . . . . . Plateau of Huincul . . . . . . . . . . Charahuilla area . . . . . . . . . . .0 .

Cordoba Carboniferous-Permian sediments

Capilla del Monte . . . . . , . . . .

APPENDICES

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1. Physical exploration at North Agua Botada-Arroyo Seco, Agua Botada mine, and South Agua

Botado localities . ^. . . . . . . . . . . . . .

2. Physical exploration at the Uryco prospect . . 3. Physical exploration at La Estela mine . . . . ^.

1.

Physical exploration at the San Sebastian mine .

5.

Physical exploration at Cosquin

.

. . . . . . . .

4

5

9 9

10

14

16

19 22 25 29

31 34 36 1l 42 45 48 48 49 so 50

51 54

58

62

65

. .0.0.0

APPENDICES (continued)

6.

7.

8.

Exploration methods and techniques ^. .

General outline for reports ^{. . . . .} Hydrogeochemical references

Part I: Procedures and equipment for field

extraction of uranium from waters by-ion exchange resin

. . . .

Part II:' Mobile geochemical units for uranium exploration

IIUSTR&ATIONS.

1. Location map of Argentine uranium districts.

2. Hueml mine, uranium grade thickness . . . 3. Uryco, copper and uranium grade thickness .

4.

Plan and cross section B-B' -B",

Quilico

mine

^{. * * . . .. . . . .} . * . . .

5.

Plan and cross section A-At,

Labor de los Troncos . . . . . . . . 6. Geologic section of Labor L.1

south wall, Cosquin ^{. .} . . . . . . .

7. Uryco prospect, exploration plan . . . . . 8. La Estela mine plan . . . . . . . . . .

9. Cross section A-A', La Estela mine ^. . ^. . 10. Rodolfo uranium deposits, Cosquin . . . . .

FIGURE

Page

68 74

83

6 12 18 38 39

43

55 59 60 66

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RIME-4533

URANIUM EXPLORATION AND DEVELOPMENT IN ARGENTINA INTRODUCTION

Geologists of the United States Atomic Energy Comaission (AEC)

H. E. Stocking, H. B. Wood, and W. S. Keys, visited uranium prospects and mines in Argentina during the period September 17 to December 6, 1958, at

the invitation of the Comision Nacional de la Energia Atomica of Argentina (CNEA). In this invitation it was requested that the AEC geologists

"examine the existing uranium deposits and cooperate with their Argentine colleagues in formulating new prospecting and exploration programs".

In response to this request, plans for the further investigation of each of the localities visited were developed and are presented herein.

Because most Argentine deposits are in an early stage of development, the plans involve considerable geologic study and physical exploration to evaluate more completely uranium ore reserves and potential. The order

of presentation represents the authors' evaluation of the relative importance of the areas studied. However, the authors have not presumed to recommend the overall level of exploration activity which might be appropriate in Argentina and therefore the extent to which the plans presented should be

carried out.

Since details of the geology of each district are available in other reports, only sufficient background geologic information to provide the

setting for the plans is included in this report.

The authors are deeply appreciative of the many courtesies extended during their stay in Argentina and sincerely hope, when the results of the exploration proposals are available, that they merited the generous hospitality of their hosts.

SUWAART

Argentina is considered a uranium province because of the broad geographic distribution of numerous deposits and the many types of geologic environments containing uranium. In general, we believe that areas underlain by continental sedimentary rocks offer the best potenti- ality for development of uranium ore reserves and warrant the greatest exploration effort.

A brief summary of the uranium districts visited (fig. 1) follows in the order of our evaluation of their relative importance; detailed

discussions of these districts appear later in the report. In our view, deposits in sedimentary rocks in the Malargue, Guanadacol, Sierra Cuadrada, and Canadon Krueger districts deserve highest priority in future exploration programs. However, in accordance with our instructions, exploration plans are presented for all of the other districts as well. The deposits listed as items 8 through

14

are not considered of sufficient importance to

warrant relative evaluation and may be considered of generally- equal priority.

RME - 4533

"13

17

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LOCATION INDEX

3 I MALARGUE 10 PAPAGAYOS, SOBERANIA,

12 INDEPENDENCIA

2 GUANDACOL

4- II SAN ROQUE

3 SIERRA CUADRADA ~

12 CANADON GATO / 4 CANADON KRUEGER v

13 TINOGASTA

5 LA ESTELA 14 PLATEAU OF HUINCUL

6 CHIHUIDO IS CHARAHUILLA

7 SAN SEBASTIAN 16 CORDOBA CARBONIFEROUS-

9 RAHUECO PERMIAN SEDIMENTS

9 COSOUIN IT CAPILLA DEL MONTE

O 0 4w aw aw

Sesle in Kilometers

FIGURE I. LOCATION MAP OF ARGENTINE URANIUM DISTRICTS

1. The Malargue area₇ which includes the Huemal and Agia Botada. mines and the region of the Pampa Amarilla anticline and areas with

similar rocks to the south, appears to- have the best possibilities

for uranium at present. It contains the largest Argentine uranium deposits and, more importantly, host rocks and structural features with favorable characteristics,

2. The Panacan formation, particularly in the northern part of the

uraniferous belt in the Guandacol area, is favorable because of the amount of ore exposed at La Marthita mine and the widespread occurrence of uranium minerals in the formation.

3o The Chubutiano formation in the Sierra Cuadrada area should be

thoroughly explored because .it presents a favorable environment for

uranium deposits such as the one recently found on the Ferrin ranch.

1. Asphaltic sandstones in the. Riochiquense formation in the Canadon Krueger area present a very favorable environment for large uranium deposits in. spite of the erratic distribution of the uranium found to date,

5. La Estela mine and similar veins in the Sierra Comechingones are considered to have the greatest potential of any of the hydrothermal vein deposits seen in Argentina. This opinion is based on the

geologic similarity between this deposit and some vein deposits in

the Western U. S. A. and Western Europe.

6. Uranium in the Rayoso formation on the southern part of the Chihuido del Medio anticline occurs in very small ore bodies.

It is not likely that large deposits will be found in this

environment. However, larger deposits may be found in other

host rocks or different facies.of the same host rocks.

7. We do not consider the potentiality of the San Sebastian mine and adjacent similar deposits to be great because of the small size of the known ore bodies. Both development and extraction costs will be greater in this type of deposit than in the deposits in sediments.

8. The Kimmeridgian formation in the Rahueco area is characterized by small uranium deposits. The discovery of much larger ore bodies in such consistently fine-grained rocks does not appear to be likely.

9. Known deposits in tertiary sediments in the Cosqpin area contain only low-grade material which cannot be considered economically exploitable under present conditions. Some exploration is recom- mended here to test the possibility of higher-grade ore at depth.

10. The uranium deposits at the Papagayos, Soberania, and Independencia mines have, we believe, a very small potentiality because of the erratic distribution of low-grade uranium ore and the lack of

evidence of strong hydrothermal activity. No exploration proposals are made for these mines.

11. The San Roqie mine is a unique geolog Lc type, and although it

does not appear to be a potentially important uranium source, the area should be considered for additional prospecting and study.

12. Some of the uranium at Canadon Gato is in a structural environment similar to vein deposits but there is considerable doubt that it

is of hydrothermal origin. In any event, the main uranium poten- tiality of this area is, we believe, in disseminated deposits in continental sediments.

13. Continental sedimentary rocks observed in the Tinogasta area are not considered favorable for the discovery of deposits of a size

and grade to permit profitable extraction, therefore, no explora- tion proposals are made for this area.

1U. Other miscellaneous areas, visited only briefly, such as the Plateau of Euincul, Charahilla, Cordoba Carboniferous-Permian sediments, and Capilla del Monte, can be assigned a potentiality only after additional geologic exploration.

DISTRICT EXPLORATION AND DEVELOPMENT lalargue area

The Malargue area, about 30 kilometers south of the village of Malarguie, Mendoza Province, includes the Huemul and Agua Botada mines as well as the

outcrops of the Diamantiano formation around the Pampa Amarilla anticline and adjacent regions.

This uranium district, of considerable potential magnitude, is under- lain by a series of Mesozoic and Cenozoic sedimentary rocks which have been deformed into a series of gentle anticlines, synclines, and domes.

Tensional fault systems transect the sediments, and volcanic features and intrusive igneous masses are prominent. The lack of obvious carbonaceous

"trash" and the localized and erratic distribution of the asphaltic material were especially noted. Petroleum accumulations, however, are known through-

out the area.

Huemnl-Agua Botada locality

General geology-Uranium ore deposits at the Huemil and Agra Botada mines occur along the southwest flank of the Pampa Amarilla anticline in

continental

mdstone,

sandstone, conglomeratic sandstone, and conglomerate of the upper one-third of the Diamantiano formation. Both uranium and asphalt are concentrated in the more-porous units. Clearly defined paleostream channels are not readily recognizable, but lensing of the

various facies of the formation, and abrupt. facies changes are character- istic and apparently influence uranium ore deposition. Color changes

accompany the lithologic changes, and the sandstones and conglomerates generally have a yellowish to buff color; the mdstones are variegated, mostly shades of red and gray. Unconformably overlying the Diamantiano

formation and in a nearly horizontal attitude are Tertiary volcanic ash

beds which have anomalous radioactivity.

The dissemination of uraninite in hard hydrocarbon, referred to as

asphaltite, and the association of uranium in vanadates in the oxidized

zone permit a comparison of these deposits with those located at Temple Mountain, Utah, U. S. A., although copper is not present in significant

quantities in the latter locality.

Geologic and physical exploration--Huemul mine

1. Trends in mineralization at the Huemul mine have been deline- ated by compiling a grade-thickness map (fig. 2) from assqrs recorded on CEA Plan

484

as multiplied by the thickness of the rock from which the sample was cut. Although complete in-

formation between levels "0" and "-24" is lacking, the following tentative conclusions are suggested:

a. Grade thickness increases nearly threefold downward from the "*10" level to the "-24" level.

b. Grade-thickness contours, reflect a primary sedimentation feature trending S. 700 W.

c. Drilling objectives may be expected to approximate this orientation.

d. Extension of closely spaced grade-thickness contours from the north end of level "0" into the ground north of level

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Grade - thickness contour

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Outline ^of levels and inclines

<1234 Product of grade x thickness

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Data from CNEA Pion 484

0 10 ^{20 30 40} 50

Scale ^{in Meters}

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"f10" seems possible.

From the above, it is proposed that level X10" be extended northwest to test the possibility of ore in the general area enclosed in the dashed

1,000 grade-thickness contour.

2. Some southwesterly trends along level"-24" may extend to level "-4o"

which is currently being driven. To test this idea as well as the possibility of extending such trends beyond level '40" into ground

drilled with negative results (holes III and III, Huemnl Plan

S72),

channel samples should be cut at intervals similar to the

spacing on level "t10" and assayed. If results are favorable so that high grade-thickness contours can be extended at depth, drill- ing or deeper workings might be indicated.

3. Uranium and copper vary directly at Huemul, though not in the same proportion, and the action of supergene solutions, as revealed by the presence of malachite, azurite, and chrysocolla, has undoubtedly affected the distribution of both elements. Enrichment at the

water table by such solutions has not been demonstrated. To resolve this important exploration problem and to provide a basis for any later drilling programs, channel samples should be taken from level "0" down the incline to level"'-2lL", across that level to the deeper level of "-40" and to the base of the short incline

below that bottom level. This sampling would, unless overly

complicated by intersecting ore trends, allow an interpretation of the presence and degree of supergene enrichment.

Agua Botada mine, South Ag a Botada, and area north to Hemnl Geologic exploration

1. In-mapping the Agta Botada mine and the South Agua Botada area give particular emphasis to lithology, sedimentary features, and structure, and their relation to radioactivity. A representative suite of samples for mineral identification and paragenetic

studies would be useful in evaluating alteration and leaching phenomena.

2. Channel samples taken at 1- to 2-meter intervals from outcrops of uraniferous beds at Agua Botada and within the mine itself when analyzed for uranium, vanadium, copper, and calcium carbonate will provide information for grade-thickness contour maps similar

to those for Hueml. This information will contribute to an evaluation of anomalous radioactivity in the outcrops not yet subjected to underground exploration on the Pampa Amarilla anticline. The channel-sample assay information will also-be useful in estimating the size, shape, and orientation of drilling

targets and may yield clues on possible chemical or mineralogic halos surrounding or associated with the uranium ores.

3. Stratigraphic sections in the Agua Botada mine area and vicinity

would serve as a basis for correlation of ore-bearing units and preparation of sandstone/mudstone ratio maps.

U.

Stratigraphic information developed in the previous step could also be used in the preparation of structural contour maps and cross sections from the Hueml-Agua Botada area to beyond the

outcrops of Beds B-1, B-2, and B-3.

Physical exploration-Physical exploration programs totaling 6,000 meters of drilling in 65 to 70 holes are considered (Appendix 1) separately for the North Agua Botada-Arroyo Seco, Agra Botada, and South Agra Botada localities. Because of the presence of sandstone and conglomerate beds up to 4 meters thick and andesite sills up to 10 meters thick, diamond drill- ing with a rotary rig using water as a circulating medium is, needed. If cuttings are logged and the holes are probed radiometrically, core drilling is not required unless an ore body is intersected. A geologist should be assigned to exploratory drilling and should be authorized to "bottom" the holes and, if necessary, change the location of holes.

From the drilling information, detailed subsurface geologic maps of the potential host-rock beds can be prepared showing structure contours on the oldest hosts-rock bed, isopachs of each ore-bearing bed, calcium carbonate and asphalt concentrations and lithofacies.

Pampa Amarilla anticline prospects

General geology-In addition to the Huemal and Agua Botada deposits, other uranium occurrences on the Pampa Amarilla anticline include the Uryco, Molla, and Cerro Mirano prospects. The host rocks at the Cerro Mirano prospect, which has been adequately explored, are too siliceous and impermeable for a large uranium deposit. However, the Uryco and Molla prospects warrant additional exploration because of the folloing favorable geologic characteristics:

1. Friable, gray, permeable sandstone and conglomerate interbedded with claystone lenses containing asphalt either disseminated or

in pockets.

2. Structures favorable for deposition of large-size ore bodies.

3. Visible secondary uranium and copper minerals. The presence of secondary minerals, as elsewhere on the Pampa Amarilla anticline, suggests the possibility of surface leaching and higher-grade ores at depth.

Geologic exploration--Ulryco and Molla prospects

1. Continued mapping of the Diamantiano formation around the south and southeast side of the Pampa Amarilla anticline may permit correlation of the ore-bearing beds from Agua Botada to Uryco.

Preparation of a geologic cross section from the Uryco prospect south across the cuesta into the Valle de los Leones will also

be helpful.

2. Determination of sedimentary trends, lithofacies changes, and

thickening or thinning of the ore beds in Bed D in the vicinity

of Uryco may aid in predicting flow

directions

of uraniferous

solutions through the sediments and possible directions of ore- body elongation.

3. As a further aid to defining mineralization trends prior to drilling, channel samples should be cut and assayed from the outcrop of the uraniferous bed at Hemnl (at intervals similar to those of Plan

484)

and from the outcrop of Bed D in the

vicinity of the Uryco incline. Bed D should be sampled as far

down the dip as the topography will allow. Information derived from this sample-assay program will permit comparison of the betteri-known Huemal locality with Uryco and possibly suggest mineralization trends in Bed D at Uryco. Also note WIW-WS3

mineralization trends indicated by grade-thickness contour maps for copper and uranium (fig. 3).

P1ygsical exploration-Following the surface studies outlined above, .air rotary drilling is necessary to learn the distribution of the uranium mineral concentration exposed at the surface, the extent of asphalt-

saturated porous sandstones and conglomerates, and the thickness and

character of the uraniferous beds in both the oxidized and unoxidized zones.

RME - 4533

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Data from CNEA Plan 218 Grade-thickness contour

1234 Product of grade X thickness

0 _{I _} 2 _lI 4 s " 10

Scale in Meters

January 1959

FIGURE 3. URYCO, COPPER AND URANIUM GRADE THICKNESS

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Details of the drilling program are presented in Appendix 2.

Guandacol

General geology-The Guandacol area in La Rioja and San Juan

Provinces includes numerous uranium occurrences in a north-south outcrop belt of the Panacan formation of Pennsylvanian or Permian age. Uranium occurrences at Los Rastrojos, La Marthita, La Helenita, Sonia, El Pedregal, and Cerro Aspero extend for approximately 15 kilometers along a belt west of the town of

Quandacol.

The Panacan formation averages

800

^{meters in}

thickness thinning from l,500 meters in the north to 700 meters in the south at El Pedregal, and consists of interbedded sandstones, siltstones, and mtdstones. The dip of the mineralized rocks ranges from 15O-400W . Uranium and copper minerals are found associated with carbon trash in a medium-grained, gray to yellow-brown sandstone. The most promising de-

posits are at La Marthita, one of the northernmost occurrences, and at Sonia about

5

kilometers to the south. At La Marthita and Sonia, uranium is in sparsely distributed pitchblende nodules, 1 to 3 centimeters in dia- meter. At other localities, uranium minerals and anomalous radioactivity

are associated with carbonaeegus debris and silicified logs. La Marthita mine has shipped about 70 tons of ore reported to assay 0.70 percent U₃0₈. With the exception of La Marthita mine, the ore is very discontinuous and

only a few centimeters thick.

Geologic exploration-Because of the great thickness of the Panacan

formation and the repetition of apparently favorable sandstones, geologLc information can be a great aid in restricting the area to be prospected.

A regional geologic mapping program of the area from Cerro Aspero to La Marthita should suggest the most favorable host rocks for prospecting and exploration. The following features should be stressed in a geologic study:

1. Stratigraphic position of the uranium deposits-Although most of the uranium appears to be in the upper part of the Panacan formation, the real stratigraphic relationship of the various deposits is not known. However, the more-promising deposits appear to be in the

thicker part to the north. The mapping of the marker beds and the measuring of sections should suggest how this thinning takes place

and stratigraphically locate the uraniferous beds.

2. Structural control-There appears to be a close spatial relation- ship between a

number

of the deposits and fractures and faults.

Detailed geologic mapping of the area around La Marthita, Sonia, and several other prospects should give information on this relationship and suggest other ore controls or guides useful for exloration.

3. Igneous rocks-The apparent close spatial relationship between the various dikes and sills cutting the Panacan formation and the uranium deposits should be studied by geologic mapping. It has been suggested that the uranium deposits in the Sonia-La Marthita

area are a constant distance above a dacite sill. This relation- ship, if proven, would be a useful prospecting guide.

!t. Sedimentary features-.In addition to determining the sandstone/mud- stone ratio and other lithofacies characteristics during both

detailed and regional geologic mapping, it would be helpful to outline the bleached zones and record all sedimentary trends.

Bleaching of the normal reddish sandstones appears to be a reliable visual prospecting guide in the Panacan formation, but the value of sedimentary trends in exploration here will not be known until more information is developed.

5.

Zirconium--The extremely high zirconium content of the pitch- blende at Sonia and La Marthita may provide information on the genesis of the deposits and a guide to favorable areas in the Panacan formation. Chemical analyses for zirconium should be made of the host sandstone near the deposits and unmineralized

sandstones. Similar analyses should be made of the dikes and sills at varying distances from the uraniferous areas.

Physical exploration-Because part of the Guandacol area is still under- going preliminary prospecting, physical exploration can best be limited to a few of the more-promising occurrences. Completion of preminary

prospecting and geologic evaluation of the area should precede extensive underground work so that expenditures are not made on undeserving deposits.

It is suggested that several inclines in the Cerro Asperoy El Pedregal, Sonia and La Marthita areas respectively would aid in the proper evaluation

of thee deposits. The inclines should follow the bedding until completely barren sandstone is reached in order to test the theory that many of these ore lenses pinch out down dip. If sedimentary trends can be delineated, several of the inclines should follow these trends. At La Marthita, the present incline should be driven to follow the underlying

andstone,

^and

long holes in the roof of the incline should be probed for an overlying ore horizon. Crosscuts should be driven every 10 meters from this incline to the limits of the ore. Long holes from the workings will probably be the best method of thorough exploration, because the steep slope and dip would' make drilling from the surface expensive.

Sierra Cuadrada

General geology-Sierra Cuadrada is a large mesa of nearly horizontal sedimentary rocks of Upper Cretaceous and Paleocene age, located about 150 kilometers northwest of Comodoro Rivadavia, Chubut Province. Minor uranium occurrences, some associated with silicified logs in the Chubatiano formation of Upper Cretaceous age, led to prospecting of the area by the CNEA.

Disseminated secondary uranium minerals in sandstones of the upper Chubutiano formation on the northeast side of the Mesa have been explored by means of five trenches. Where the sandstone and conglomerate are friable and contain claystone galls and fragments of silicified logs, radioactivity is higher

and there is a greater abundance of visible carnotite. Where the sand- stone and conglomerate are cemented with calcium carbonate and hard nodules are common, very little or no radioactivity is present. A 70

centimeter channel sample of this sandstone assayed

0.15

percent U

³

⁰

8

and a selected sample assayed

2.3

percent 1305. These samples were

taken from a poorly cemented and sorted sandstone bed that is continuously mineralized for about 60 meters along the strike. Scattered uranium occurrences associated with silicified logs are reported for

4

^kilometers

along the outcrop at this locality and on the southern side of Sierra

Cuadrada.

Geologic exploration

1. Areal mapping--All uranium occurrences should be located on a general geologic and topographic map of the most-promising area with particular emphasis on stratigraphic position of the

occurrences. Other geologic features rhich can be plotted include facies changes, orientation and concentration of logs, sedimentary trends, fractures and attitude of bedding. The gathering and in- terpretation of this type of geologic information should provide guidance for both a prospecting and physical exploration program.

2. Prospect mapping-Detailed long-wall sections should be made of all of the trenches, and assays of channel samples located on these sections. These cross sections and accompanying assay results are

the most important results of trenching and will aid in the interpretation of drilling esults. Supplementing these long- wall sections, radiometric surveys along the outcrop of the

uraniferous rock are advised.

3. Aerial radiometric surveying-The Sierra Cuadrada area is suitable for an aerial prospecting program. The area lends itself very well to this type of prospecting during the less-windy months. A geologic observer who is familiar with the area should be aboard the plane. Considerable time

and

money can be saved by preflight selection of the favorable stratigraphic units to be followed by flight lines. Marking with paint bombs will help to assure location of anomalies.

Physical exploration

1. The Sierra Cuadrada type of deposit is ideal for exploration by noncore air-percussion drilling except for the excessive thickness of overlying rocks encountered away from the outcrop. The first line of drill holes should be located from 20 to 30 meters from the outcrop and the holes spaced no more than 10 to 12 meters apart. However, the location of drill holes should depend more on geologic interpretations than general rules, and the drilling program should be flexible to allow location of holes based on information derived from previous holes. The holes should be

gamma logged and the cuttings logged by a geologist and saved

for assay, pending results of radiometric probing. Information on color, grain size, carbon content, and type of cement. can be derived from these cuttings. and contribute to the geologic

knowledge on the area. Subsurface maps and cross sections of the

mineralized bed will help to correlate the lithology and radio-

activity. These maps should include lithofacies, isopachs, and isorads of the host rocks and structure contours on marker beds.

2. Trenching is recommended only for those areas where the host rock is shallow and close to the rim. Tunneling may be necessary where the overlying rocks are too thick for economical drilling. All underground workings should be mapped for geologic details.

Canadon Krueger

General geology-The uranium deposits of Canadon Krueger are located on the southeast side of Rio Chico about 70 kilometers northwest of Comodoro Rivadavia, Chubut Province. The area is about 8 kilometers southeast of Canadon Gato along the same vertical dike that bears uranium at Canadon Gato. Canadon Krueger appears the more favorable because of the dissemi- nation of uranium wnth widespread impregnations of asphalt in the nearly horizontal Riochiquense formation of probable Eocene age. The highest- grade ore has been found in a coarse-grained, poorly cemented asphaltic sandstone about

5

meters to the north of the dike. A vertical channel

sample 2 meters long cut in one of the trenches assayed 0.21 percent

U₃0₈. Four dikes similar to the one at Canadon Gato crop out in the area.

They're filled with clay, asphalt pods, and veinlets and, in some places,

basalt.

The Canadon Krueger area is thought to have a potentiality for significant uranium deposits for two reasons. First, the small uranium occurrences in the vicinity of the dike suggest it may have either acted as a vertical feeder bearing uraniferous solutions or, more likely as a structural trap restricting the lateral flow of uraniferous solutions through the permeable sandstones. Second, the asphaltic, poorly sorted and poorly cemented sandstones of the Riochiquense formation provide an extremely favorable host rock for uranium deposits. The lithology of this sandstone is very similar to sandstones that are host to same of the biggest uranium deposits in the Western U. S. A. The exploration program is

predicated on the belief that the potentiality of this area is in bedded- type rather than vein-type deposits.

Geologic

exploration

1. Geologic mapping-The collection of geologic date in the Canadon Krueger will be difficult because of the lack of good outcrops, however, an attempt should be made to construct geologic cross sections of the area to correlate the uranium-bearing beds with the asphaltic beds. Information necessary for the construction of

cross sections can probably be derived from trenches and by logging the cuttings from drill holes. To provide information on characteristics of the sediments that might have provided a favorable environment for the deposition of uranium away from the dike, maps showing asphalt distribution, facies and color changes, and sedimentary trends will be useful.

2. Radiometric surveys-The area around Canadon Krueger is well suited to airborne exploration during the less-windy months.

Since outcrops are not continuous and the favorable host rocks are not well defined, it is suggested that the area be flown on a grid with flight lines oriented perpendicular to the dike in order to give a clear picture of the distribution of anomalous radioactivity along that structure. The flat topography of much of the area is al]o ideal for a carborne scintillation-counter survey. A detailed radiometric survey of the surface and con- struction of an isorad map is suggested for any areas of anomalous radioactivity where information from outcrops does not reveal the source of the radioactivity. Such a map may be the only guide to the location of drill holes.

Physical exploration-Air rotary, truck-mounted drilling with a 300 cfm air compressor should be a rapid and economic method of evaluating the

uranium potentiality of this area. The holes should be logged radiometrically

and about

75

percent of the cuttings should be saved for logging so that

lithologic characteristics can be determined at least every 30 centimeters.

It is recommended that the holes be drilled in lines nearly perpendicular to the dikes and

placed to intersect anomalies on the isorad maps. For

example,

the first

row

of holes could be drilled along a

line oriented about N. 100 E. located to pass through the center of the radiometric high at the west workings and through the center of the high 380 meters to the north. Along the line of drill holes, the spacing should not be more than 10 meters in the vicinity of the dike and the anomalies and possibly 50 meters apart in the intervening areas. The spacing of the holes should be designed to test the continuity of the mineralized sandstone and to allow stratigraphic correlation along the line of section. Because of the lack of outcrops in this area, drilling will probably be the only way of locating the mineralized beds stratigraphically to guide future prospecting.

Several of the holes in the vicinity of the dikes should be drilled to the top of the Chunbtiano formation in order to test the unconformity at the base of the Tertiary sediments for uranium deposits. Uranium deposits in continental sedimentary rocks are commonly located just above erosional nnconformities. Any other radiometric anomalies along these dikes should be drilled in a like manner with the length and spacing of the line of drill holes depending to a large degree on the results of the previous drilling.

The first holes should always be drilled in the most favorable area, and the

location and depth of subsequent holes should depend on interpretations of the drilling results by the geologist in charge of the program.

La Estela mine

and

Sierra de Comechingones

General geology-At La Estela mine, San Luis Province, uranium occurs in a breccia zone in a coarse-grained porphyritic granite which has under- gone moderate kaolinization and sericitization. Hematite staining is common

but no sulfides or carbonates have been observed. Irregular fracture

surfaces, some with slickensides, are common, and both quartz and feldspar

grains are fractured. The ore minerals are predominantly uranophane uL th a

little autunite occurring as a coating on the fracture surfaces and as a film around the phenocrysts. Uraninite (pitchblende) has been identified from this deposit, and purple fluorite occurs as stringers and in small

pockets throughout the uranium ore and as fluorite ore veins. Both fluorite and uranium mineral concentrations in this locality occur at irregular intervals along the outcrop of a 10-meter wide east-west shear zone which dips 600-800 S. for a distance of about 250 meters. This shear zone with associated minor faults appears to have been the major channel for rising solutions. Prominent cross fractures or joints exposed in the open pit have a north-south strike atd an undcilating dip of 300-600 .

In general, La Estela deposit is very similar in geologic and mineralogie

nature to some of the uranium deposits in the Sierra Morena in south-

central Spain and to certain deposits of this same type in the Vendee and La Crouzille districts of France.

The highest-grade ore seems to occur in a highly brecciated zone at the intersection of the east-west shear zone with north-south

^Pointe.

These joints have undulating planes, occur at intervals of 30 or more

centimeters, and are very evident on the south wall of the open pit. They appear to have been displaced slightly by more-recent fractures of a nearly flat-lying system. On the west slope of the open pit, the ore appears to be cut off by one of these east-dipping joint planes.

It is not known whether the cross joints continue to the east or west

of the open pit because the surface is covered with talus. The cross joints

were not observed during the quick inspection of level "-9".

The direction of inclination of the intersection of the east-west shear zone and the north-south joints ranges from S.

450-85o

^{E. and the}

plunge ranges from 150-60 E. In the open pit, this intersection may control the ore shoot, and similar favorable conditions may be found else- where along the east-west shear zone.

Geologic exploration-Because fracturing and brecciation are important ore controls, it will be helpful to up date the geologic maps of all under- ground workings and the open pit, giving special emphasis to mapping

fractures, degree of brecciation, kaolinization and sericitization and to

make a radiometric survey.

.Brecciation

may be confined to' the width of the shear zone or it may extend into the wall rock, thereby increasing the

size of a potential ore body. If the geologic features observed indicate that the intersection of the east-west shear zone

and north-south joint

systems controls the ore distribution, then a physical exploration program, as outlined in Appendix 3, would be warranted.

Comments on the Sierra de Comechingones region

It is understood that other uranium prospects in the Sierra de

Comechingones, similar geologically to La Estela, have been found by

CNEA geologists to the south. Because of the favorable geologic setting of La Estela mine, surface prospecting may be contained along east-west shear zones in this area, and in some places, surface mapping may be warranted.

The numerous intermittent and perennial streams draining the west .slope of the mountains afford a good opportunity to carry out a hydrogeochemical prospecting program.

The Cerro Negro and San .Guillermo fluorite mines and others of this type occurring along the east slope of the Sierra de Comechingones do not appear to have a favorable potential for immediate uranium exploitation although they contain minor uranium mineralization.

Chihuido del lfedio

General geology-The Chihuido del Medio anticline is located 160

kilometers northwest of the city of Neuquen, Neuquen Province. The uranium

deposits at Chihuido del Medio occur at several stratigraphic levels in

gently dipping fluvial sandstones of the Rayoso or Diamantiano formation of

Upper Cretaceous age. Uranium, vanadium, and copper are associated with

carbonaceous material in hundreds of pods, mostly less than 1 ton in size..

The highest grade and greatest number of these pods are found on the Maria

Teresa and Palo Quemado claims, several kilometers apart, near the southern end of the anticline. Similar deposits are found at Mariquena, 8 kilometers to the west, and Ramblon de la Vidriera,

5

kilometers to the southeast.

CNEA work to date appears to have established the size character of the ore pods in the Chihuido area, and indicates that significantly larger deposits will not be found in this environment. Nevertheless, there is the possibility that the development of the area could be analogous to Pumpkin Buttes, Wyoming, U. S. A., where discovery of similar ore pods led to the subsequent discovery of larger disseminated deposits in adjacent areas.

The principal objective of a geologic exploration program in this area should be to establish ore controls; fully investigate the known mineralized areas; locate new areas for exploration on the anticline; and to suggest other environments where larger deposits might be found.

Geologic exploration -A base map of the southern part of the Chihuido del Medio anticline should be prepared on a scale large enough to permit the the superposition of the following transparent overlay maps containing the data indicated:

1. Structure map--Where a marker horizon is available, structural contours should be drawn. Faults and

joints

should also be shown.

2. Isopach map-From measurement of sections, it may be possible to construct an isopach map of the sandstone unit containing the ore.

3. Lithofacies map-It is apparent that the ore at Mariquena lies in a zone of facies change. This zone of facies change may include the deposits at Palo Qiemado and Maria Teresa. If this

observation is supported by a lithofacies map based on sandstone/

mudstone ratio, a belt of ground favorable for exploration

would

^be

indicated. Sedimentary trends, as determined from the direction of crossbedding, current lineation, ripple marks, log orientations, scours, and sandstone and ore lenses should also be plotted.

lt. Isorad maps-In areas where the surface has not been contaminated by physical exploration, a radiometric grid survey may be useful in suggesting ore trends that are not immediately apparent.

5. Miscellaneous maps-Maps showing bleached areas, particularly where the normal red color has been bleached to yellowish-grey

or grey, can provide guides to favorable host rocks. Maps showing the distribution of other elements and their relationship to

uranium are useful for comparison and as indicating possible halos around ore bodies.

Physical exploration

1. Because the known "ore" pods are shallow and sporadically dis- tributed, it is believed that trenching would be the most

economical method of blocking out ore. Where the ore horizon is deeper than several meters, air-percussion drilling may be the

best method of exploration. The pattern and spacing of trenches and drill holes should be based on a study of ore-body size and distribution. Aside from providing better sampling of the ground, shallow trenching may be paid for by the ore removed. During trenching, the position of each ore lens should be plotted and long-wall sections made of the completed trenches. This infor- mation will be of value in interpreting the cuttings and

radiometric logs of the drill holes.

2. Drilling to block out ore should only be carried to an economic mining depth, but if any purely exploratory holes are drilled along trends delineated by the geologic program, they should go through the ore-bearing unit. Such exploration holes would only be drilled for the purpose of finding larger ore bodies that would pay for deeper exploration and mining.

San Sebastian mine Sanogasta area

General geology-The San Sebastian mine is in the Sanogasta area,

which is located on the east side of the Sierra de Famatina, about 20 kilometers south of Chilecito, La Rioja Province. The San Sebastian and

the nearby San Victorio mines are fissure veins in schists, slates, and

quartzites of Ordovician age. At the Santa Brigida mine,

6

kilometers to the south, the vein is along the contact between granite

aid

^{schist. Ore}

production of approximately 1,700 tons of 0.7 to 0.8 percent U₃0₈ has been realized by handsorting ore from the San Sebastian and San Victorio mines.

At these mines, the ore occurs in discontinuous bodies along northwest- to north-striking vertical- to west-dipping veins. These veins are apparently offset by northeast- to,east-striking post mineral faults. The ore bodies are as much as 20 meters long and 1.5 meters thick, but are usually smaller in size. Uranophane and autunite are found near the surface and pitch- blende in the deepest workings. Associated minerals are copper carbonates and sulfides, purple fluorite, calcite and barite.

Geologic exploration-In many uranium veins such as those at San Sebastian and San Victorio, the ore is localized in cymoid curves and intersections of structures; this type of control can only be interpreted by detailed geologic mapping. The preliminary underground mapping to date shows that the ore-bearing veins are offset by transverse faults in a number of places. A study of the paragenesis of the vein minerals, which vary in nature between sets of fractures, may aid in determining the age relations of the various fractures and assist in locating possible displaced ore.