Reconnaissance for Uranium in the Tocopilla Area, Province of Antofagasta, Chile

722/ RME-4534( Rev.) OBE Go\\ec 04

RECONNAISSANCE FOR URANIUM IN THE TOCOPILLA AREA, PROVINCE OF ANTOFAGASTA, CHILE

By William A. Bowes Paul H. Knowles Mario Serrano C. Rudolfo Grnienwald S.

February 1959

Division of Raw Materials, AEC Washington, D. C. .and Instituto de Inve stigaciones Geologicas Santiago, Chile

UNITED STATES ATOMIC ENERGY COMMISSION * OFFICE Of TECHNICAL. INFORMATION metadc 783694 LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any employee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor.

This report has been reproduced directly from the best available copy.

Printed in USA. Price $0.50. Available from the Office of Technical Services, Department of Commerce, Washington 25, D. C.

USAEC Office of Technical Information Extension Oak Ridge, Tennessee RME-h53h (rev.) GEOLOGY AND MINERALOGY

U. S. ATOMIC ENERGY COIISSION DIVISION OF RAW MATERIALS IN COOPERATION WITH INSTITUTO DE INVESTIGACIONES GEOLOGICAS DE CHILE

RECONNAISSANCE FOR URANIUM IN THE

TOCOPILLA AREA, PROVINCE OF ANTOFAGASTA, CHILE

William A. Bowes, Paul H. Knowles, Mario Serrano C. and Rudolfo Grenwald S.

February 1959 Santiago, Chile

RECONNAISSANCE FOR URANIUM IN THE

TOCOPILLA AREA, PROVINCE OF ANTOFACASTA, CHILE

CONTENTS

Pag

ABSTRACT - * . * * . * . * * . . * * * * * . * ...... 5

INTRODUCTION . - ...... 5 Geography ...... 7 Location and accessibility ...... 7 Topography, vegetation and climate ...... 7 Exploration and mining history ...... 7 Acknowledgements ...... 9

GENERAL GEOLOGY ...... * . . . . . ** , 9 Stratified rocks ...... 9 Intrusive igneous rocks ...... 9 Structure ...... 10

RECONNAISSANCE FOR URANIUM ...... 10 Tocopilla district ...... 11 Minita mine ...... * . . * ...... 11 San Jose'mine ...... lb Gatico district...... 15 Guanillos district ...... 5 ...... 5 Tres Puntas - San Juan district . 1 Cerrillos district ...... 15 Other localities ...... 16

CONCLUSIONS ...... 16

REFERENCES ...... 17

ILLUSTRATIONS

Figure 1. Index map, Tocopilla area, Province of Antofagasta, Chile 6

2. Geologic plan of portions of the Minita mine, h30 and 610 levels, showing sample localities ...... 13

Table 1. Radioactivity in milliroentgens per hour ...... 11

Plate 1. Geologic sketch, Tocopilla area, Antofagasta Province, Chile ...... 19 -3-

RECONNAISSANCE FOR URANIUM IN THE

TOCOPILLA AREA, PROVINCE OF ANTOFAGASTA, CHILE

ABSTRACT

In September-October 1958 a six-day reconnaissance in the Tocopilia area, Antofagasta Province, Chile, was made by members of the U. S. Atomic Energy Commission and the Instituto de Investigaciones Geologicas de Chile for the purpose of evaluating reported uranium occurrences. Bed rocks in the western part of the area are a Jurassic volcanic sequence. The eastern part is underlain by Paleozoic sediments, the two sectors being separated by a regional north-trending fault, called the Salar del Carmen fault. All subsidiary faults and fractures appear related to movement on this, or parallel, faults in the same major fault system. Both the older sediments and the volcanics are intruded by facies of the Andean diorite complex. Two principal plutons of this diorite are located, respectively, near Tocopilla and Ggtico.

Uranium occurs .near Tocopilla in east- to northeast-trending tensional fractures associated with quartz, actinolite, and copper and iron minerals in veins paralleling andesite dikes. The dioritic to syenitic host rock is part of the Tocopilla pluton. The highest radiometric assays from veins were 0.09 percent equivalent uranium. In view of the persistence and character of the northeast-trending tensional structures, a limited pro- gram of more detailed geologic mapping seems warranted, as the area may possibly contain significant uranium concentrations.

INTRODUCTION

Reconnaissance for uranium in the Tocopilla area was performed during the winter of 1958 by geologists of the U. S. Atomic Energy Commission (USAEC) and the Instituto de Investigaciones Geol6gicas de Chile (IIG) under a cooperative agreement between the American and Chilean governments.

The Tocopilla area (fig. 1) was selected for reconnaissance on the basis-of its favorable geology indicated by literature research (Hague, 1958), as well as by the presence of uranium in certain areas.

Field work was accomplished in a six-day period in September and October by two field parties, each equipped with four-wheel drive vehicles. The work center was Tocopilla. Liberal use was made of aerial photographs of the U. S. Army Tri-Metrogron type (1:4O,000) and also U. S. Army Map Service (1:60,000) vertical photographs in stereoscopic pairs. The base map for plate 1 was copied from topographic maps compiled by the Instituto Geografico Mi.litar de Chile (scale 1:250,000). Both scintillation and Geiger counters were used in the investigation. R ME 4534 (rev,)

To 690 680 PERU(

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0 0 23 z Son Pedro de Atacomo. E w a Solar . de - ANTOFAGASTA Atocame / 0

:z 24 z w I 700 (9 6 90 .. f 680

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Figure I- Index Map of Tocopilla area, Province of Antofogasta Chile. The scope of the work was limited to reconnaissance of the area for favorable geologic indications of uranium; this included visits to selected localities and mining districts. Geologic information thus compiled may be used in the assembly of a new geologic map of Chile, an Instituto de rnvestigaciones Geologicas project scheduled for completion two years hence.

Geography

Location and accessibility

The Tocopilla area falls'within latitudes 21 0l5' to 220301 S. and longitudes 690h5' to 70015' W. The coastal port of Tocopilla (population, 20,000) is the capital of the Department of Tocopilla, Province of Anto- fagasta, Chile. It lies near the center of the western edge of the rectangular, north-elongated, 50- by 80-kilometer area, and is accessible by boat, rail, or auto.

Access to most of the high ground east of Tocopilla is by jeep roads branching from the main east-west Chuquicamata-Tocopilla highway. Access along the coast is limited to a surfaced road south of Tocopilla, as the northern coastal road is blocked by stream washes and mud flows that occurred during freak storms in 1911. The coastal barrier is cut only rarely by valleys deep or continuous enough to make road construction possible in an east-west direction; in the area at present the only inland- coastal access is by the Tocopilla-Chuquicamata road. Much of the northern cart of the area is relatively inaccessible due to lack of access roads.

Topography, vegetation and climate

Elevations in the Tocopilla area range from sea level to 2,300 meters in the coast range mountains. The western slope of the coast range mountains is exceptionally steep; peaks as high as 1,900 meters elevation are found only 3 kilometers inland from the coastline. The coastal mountains, in an early mature stage of erosion, slope'gradually to the east and merge into the wide alluvial plains of the central.valley. The entire Tocopilla area is dry and nearly devoid of natural vegetation. Precipitation is negligible and has annually averaged 4.5 millimeters in rainfall over a period of 15 years. Drinking water is supplied by pipeline from the Andes Mountains. The climate is hot in summer and cool in winter; the coastal area is subject to frequent fogs. Rock exposures are generally good throughout the mountainous parts of the area.

Exploration and mining history

The city of Tocopilla has been an important copper-producing center for over a century.

-'7- The rich Rosario veins in the Minita-Portezuelo Despreciada mine group, 3 kilometers northeast of Tocopilla, were worked 'until damaged by a great earthquake in 1877. Later this group of mines was consolidated into one company in the year 1920 and has since been worked more or less continuously as the Compa1 i'a Minera de Tocopilla. The Company has a sea-water flotation plant in Tocopilla which can treat up to 300 tons of the lower grade 4- to 5- percent copper ores per day. An aerial tram with a capacity of 30 tons per hour has been replaced by trucks to haul ore to the plant. This plant also treats ore from the company-owned Buena Esperanza mine located about 7 kilometers south of Tocopilla.

Adjoining the Despreciada group are the Lincoln group on the north and the Sloman group on the east. The latter includes the San Jose'vein dis- cussed in another part of this report.

The famous old copper mines of Gatico are situated 2 kilometers inland from the sea coast. The locality is shown in the southwest corner of plate 1, approximately 40 kilometers south of Tocopilla. The mines have been worked discontinuously from the time of the Spanish occupation. A smelter located there handled up to 200 tons of ore daily. The properties have been idle for many years, and the plant, mine buildings, and equipment are in ruins.

Other important copper districts of past years include the Tres Puntas- San Juan, 10 kilometers southeast of Tocopilla, and the Guanillos, 10 kilometers north of Gatico. The latter district includes the Bandurrias, Gitana, San Ram'n, and Mantos de la Luna mines. A recently constructed aerial tramway serves the Mantos de la Luna mine, a 5-percent copper deposit of large tonnage, and it is reported locally that this mine will resume operation. At present the only mining activity in the Guanillos area is a shaft-sinking operation within 500 meters of the sea. Nearby, a small pilot flotation mill is testing the low-grade (2-percent) copper ores in this district. Access to copper deposits in the mountainous area here and to the north is difficult and costly; only a few such deposits have been developed.

At least two occurrences of lead minerals are known in the Tocopilla area. One displaying wulfenite and galena, the Anita mine, is shown south of Cerrillos, about 40 kilometers east of Tocopilla; the other, the "Galenosa,"' is shown due north of the Anita, approximately 10 kilometers northeast of Ojeda.

One of the largest salitre operations in Chile, the Maria Elena, is 20 kilometers east of Cerrillos, just beyond the eastern map limit.

Andes Copper Company's steam power plant is located in Tocopilla and supplies both the electrical needs of the Chuquicamata mine, located 125 kilometers to the east, and the city of Tocopilla.

Before a pipeline was installed, drinking water was supplied to the city by this plant through a sea-water distillation process.

-8- Acknowledgements

The writers acknowledge the kind assistance of Sr. Walter Homann W., engineer of the Compani'a Minera de Tocopilla, who provided guidance and valuable information on the Minita and other mines in the Tocopilla area; and Sr. Heriberto Ramfrez, concentration-plant superintendent of the same company, who guided the authors in a tour of the plant operation.

Sr. Isaias Encina of the Corporacioni de Fomento de la Produccidn, ,and Sr. Juan Mu2ioz of the CAja de Crddito Minero, both of Antofagasta, were helpful to the writers in supplying base maps and general information of the area.

GENERAL GEOLOGY

In the course of this study a fault of regional proportions traversing the entire map area in a northerly direction was discovered. This fault brings Jurassic volcanic rocks on the west against a Paleozoic sedimentary sequence on the east.

Stratified rocks

The oldest rocks in the area, a sedimentary sequence, are of Late Paleozoic (Permian) age, according to' plant studies by Wetzel (1927) in the Toco area several kilometers to the east of the area mapped. These sediments occupy the entire eastern margin of plate 1. Included are argillitic quartzites, quarzitic schists, mica schists, and medium- to coarse-grained grey clastic sediments of a greywacke type.

Volcanic rocks of probable Jurassic age occupy the largest part of the Tocopi-lla area (pl. 1). No sedimentary intervals in which fossils could be collected were observed, dating being based on relation of Cre- taceous sediments near Antofagasta (Briiggen, 1950) and by regional correlation. The volcanic sequence cropping out throughout the western portion of the Tocopilla area is andesitic in composition and includes grey, brown, and reddish andesite porphyries, porphyritic tuff breccias, and volcanic breccias and conglomerate. Low-grade metamorphism is exhibited in the volcanic sequence immediately south of the Tocopilla intrusion, resulting in local epidote masses, dark flinty hornfels, chloritization, and discoloration. These effects decrease to the south of the intrusion, suggesting igneous metamorphism.

Recent alluvium is widely exposed in fans, intermontane valleys, and in the interior desert.

Intrusive igneous rcclcs

The plutons shown on plate 1 intrude the Jurassic and Paleozoic rocks, and are part of the widespread Andean diorite complex of Late Cretaceous

-9- to early Tertiary age. The intrusions range in composition from a diorite to a granite; are medium to coarse grained, and are grey to reddish grey in color.

Two main, intrusive masses of dioritic to syenitic composition are shown near Tocopilla and Gatico. A portion of another intrusion of a granite composition is shown near the central eastern margin of plate 1.

The intrusions and older rocks are traversed by aplite and basic dikes. In places andesite dikes are associated with veins in both the Tocopilla and Gatico intrusions. Aplite dikes are prominent in the Gatico intrusions.

Structure

An alignment of intrusive masses, Mesozoic and Paleozoic rock strata, and major north-trending faults (previously described) characterize the structure in the area. A persistent set of en-echelon faults of east to northeast trend apparently are associated with movement on the north- trending major fault zone. Secondary fractures of northwest trend are displayed adjacent to the northeast set in the coastal area.

The faults shown on plate 1 were mapped with the aid of aerial photographs. The large fault traversing the area from north to south was traced northward on aerial photographs beyond the map limit for 120 kilometers to and beyond Salar Grande; to the south it was traced for 320 kilometers to where it crossed the sea coast 20 kilometers north of Taltal, a total distance of over 500 kilometers. This fault has been mentioned in the literature by Briiggen (1950), who describes it as a "recent fault" and includes a sketch showing a well-known scarp in alluvium in the Salar del Carmen 10 kilometers east of Antofagasta. In a USAEC uranium reconnaissance of the Antofagasta area the fault was named the "Salar del Carmen" fault (Knowles, Bowes, Serrano and GriienwalJd, 1959). Apparent movement on the fault is normal, with the west side down; this is indicated by the Mesozoic-Paleozoic relationship in this area and by the proximity of an ocean deep off the coastline extending over 200 kilometers from above Antofagasta southward to beyond Taltal.

RECONNAISSANCE FOR URANIUM

The Tocopilla area was selected for reconnaissance on the basis of the presence of uranium in a specimen from the Despreciada mine and in Wilfley- table concentrates from the Compania Minera de Tocopilla plant, as well as favorable vein structures at the contact of intrusions at Tocopilla and Gati co.

Passable roads and jeep trails were traversed in this reconnaissance, with the objective of determining the radioactivity and character of rock units, contacts, structures, and metallization. Routes traversed are shown on plate 1. The radioactivity levels for individual rock units and mineralized structures within them are listed in table 1.

-10- Table 1. Radioactivity in imilliroentgens per hour

Mineralized Rock type Background structures

Alluvium 0.008 - 0.01

Intrusions, Andean diorite complex

Tocopilla 0.015 - 0.03 0.03 - 3.0

Gatico 0.015 - 0.02 0,015 - 0.03

Eastern t 0.010 not observed

Mesozoic volcanic rock 0.010 - 0.03 0.025 - 0.10

Paleozoic sedimentary rocks 0.01 - 0,03 0.01 - 0.03

There is little difference in the level of radioactivity for individual rock units, but radioactivity in structures at the south end of the Toco- pilla intrusion is distinctly higher. The average background level of radioactivity in the area is 0.015 mr/hr.

Tocopilla district

Included in this district, located a few kilometers northeast of Toco- pilla, are the Rosario, Minita, Portezuelo, Despreciada and San Jos. mines. With the exception of the San Jose, all are now properties of the Campani'a Minera de Tocopilla, in whose beneficiation plant uranium was first reported. The concentrates were produced from radioactive ores mined at the Minita' mine. An underground tour, of the mine workings was made in an effort to localize and determine the spatial position and .character of the radioactive minerals. Radioactivity in the nearby San Jose veins was revealed in the course of this investigation. District-wide host rocks are facies of the Andean diorite complex.

Minita mine

The mine is reached by road 3 kilometers northeast of Tocopilla. The workings are a short distance north of the main Tocopilla highway. The rock host is diorite.

The mine is serviced by a 670-meter-deep inclined shaft. The lowest level, the 670, is 180 meters below sea level. The old Despreciada mine shaft, on the same vein system, is approximately 600 meters to the east. Between the two, at a higher elevation, is the old Portezuelo shaft. The old Rosario shaft is west of the Minita headframe. Most of the

-11- upper levels in the Minita shaft are inaccessible.. Due to limited time available with the mine engineer, only the deep levels, 430 and 610, were visited in addition to the 190 level, the present entrance level for the mine.

Oxidized material extends less than 100 meters below the surface due to rapid erosion along the steep coastal front. Oxidized minerals are mainly sulfates and carbonates of copper. The copper mineral in the primary zone is chalcopyrite, along with a very small amount of g old and silver. The thin enriched zone.contains native copper, covellite and chalcocite. Most of the mine production has been from primary ores.

The major set of veins strike N. 650 to 800 E. and dip 800 S. (fig. 2). Three principal parallel veins, the Rosario, San Antonio, and Portezuelo, are mined from the Minita shaft. The veins are spaced from 10 to 30 meters apart and are from 1 to 2 meters in thickness. The principal vein structures are in places intruded by andesite dikes from 1 to 5 meters thick and 60 to 120 meters long. The dikes parallel the veins and, in places, ore is found on both sides of a dike. They appear to be later than the ore, as a 1-meter-thick dike crosses the Rosario vein on the 430 level north of the shaft station. A persistent set of minor vein structures strikes N. 450 E. and dips 800 SE. These structures intersect the major veins and link one major vein to the other. No dikes have been observed crossing the northeast-trending veins.

The east-northeast-trending veins and the dikes with northeast-trending minor veins comprise the Rosario--Minita-Portezuelo-Despreciada vein system, which extends, with intermittent ore shoots, up to 1 kilometer in length with a maximum depth of 650 meters. Chalcopyrite is present in these veins in disturbed massive crystals within a grey gouge, the result of continuing post-mineral movement.

Radioactivity ranges from 0.05 mr/hr on a portion of the 190 level to a maximum of 3.0 mr/hr in a select portion of the Portezuelo vein on the 610 level (fig. 2). The average reading in the vein zones visited in the mine was around 0.20 mr/hr. The most persistent radioactivity was associated with the thin, northeast-striking minor veins as observed on the 610 level, with readings ranging from 1.0 to 2.0 mr/hr (fig. 2).

Radiometric assays of samples indicate that the uranium is extremely erratic in distribution within the vein. An 8-centimeter sample across the zone of highest radioactivity observed in the mine (Portezuelo vein, 3.0 mr/hr) ran only 0.08 percent equivalent uranium (eU). Obviously, the radioactive mineral portion of the vein was very small and was only partially taken in the sample. A sample taken at the same locality across the full vein width of 1 meter assayed only 0.012 percent eU. The radioactive mineral has been identified by the Compa'a Minera de Tocopilla as uraninite. This identification is supported by the presence of uranium in the heavy mineral concentrate at the plant site. Further mineralogic study is now being made.

-12- RME -4534(rev.)

430 LEVEL MINITA MINE

300 METERS.BELOW SHAFT COLLAR

A kir en ,.c C A nIte.I e A I V AN b UV METlER ABOV Et EA LEVEL

R OSAR IO VEIN - -0.08 mr/hr

Basic dike 1-11/2 meters A- 1886 ( select -0.001 percent eU)

0.08 mr/hr. .-- N.M DIORITE 0.20 mr/hr 0. 10 m r /hr

SAN ANTONIO VEIN.0/

A - 1887 ( s elect - 0.001 percent e U.)

610 LEVEL MINITA MINE

APPROXIMATELY 500 METERS BELOW YV SHAFT COLLAR This vein reported to be 0.13mr/hr stronger on 640 level. A-1888 0.20mr/hr (10 cm 0,072 percent eU) -18 89 .12 mr/h".A 1.0 mr/hr .f'(3cm - 0.026 percent eU)

0.2 mr/hr 2.0 mr/hr 2.5mr/ hr

1.0 /h0. mr/hr 2.5 mr /hr 3.0 mr /hr 80*2 /rDDI OR I T E 0.3 mr/hr A-1890 SAN ANTO NIO VEINPORTEZUELO VEIN 800 (8cm-0.081 percent eU) A-1891 (Imeter-0.Ol2percent eU) 0 10 20 30 Feet

0 5 10 15 Meters

Figure 2 - Geologic plan of portions of the Minita mine 430 and 610 levels showing sample localities. The flotation mill located at Tocopilla has treated an average of 240tons per day of h.10-percent copper ore, producing a concentrate with 27 percent Cu and 3 grams Ag per ton. The recovery is over 90 percent with sea-water flotation cells. The float concentrate contains, in addition, 0.11 percent Mo;. 30percent Fe; 0.05'percent Ni and Co; 34.5 percent S; 0.94 percent Al; 0.95 percent Ca; 0.36 percent Mg; 3 percent- Si; and 0.75 percent C0 2. After crushing, grinding, and classification, the mineral passes into two primary groups of flotation cells. The concentrate then passes to refining cells. Low-grade float concentrate from a third primary cell returns to the classifier, along with middlings from both refining cells. Sink products from the third primary cell pass directly to a Wilfley table, from which tailings go to the sea shore and the heavy concentrate returns to the classifier. This is a closed circuit, and it is apparent that an accumulation of heavy radioactive minerals could take place over extended periods of time. The highest radioactivity was observed in this plant at the time when uranium was first discovered in the table concentrates about 10 years ago. A 90.7-kilogram (200-pound) sample of the'radioactive concentrate registers over 5 mr/hr and is reported to assay 20 percent eU308. Chemical assays will also be made. At the time the plant was visited, the radioactivity of the table concentrate was 0.5 to 1.0 mr/hr. A radiometric assay of the concentrate (sample A-1876) ran 0.024 percent eU.

A sample of the mill feed from the Minita mine below the 400-meter level (sample A-1877) ran 0.001 percent eU.

San Jos' mine

This mine is 3 kilometers east of Tocopilla and is accessible from the highway to Chuquicamata. Radioactivity was detected here by instru- ment from the auto. The host rock has been identified locally as syenite, and the geology of the mine is much the same as the Minita, 1 kilometer to the north. The principal vein is parallel to the major veins in the Minita mine, striking N. 65 E. and dipping 800 S. The main vein of the San Josemine has a mineralized zone up to 1 meter in thickness with secondary copper minerals, clay, and limonite. Actinolite occurs in irregular masses along with magnetite, and andesite dikes, up to 2 meters wide, parallel the vein on the hanging wall (southeast) side. Mine workings include four adits on the north side of the canyon and one or more on the south. The vein trend is repeated by numerous parallel fractures in the syenite. There are minor prospects on many of these.

Radioactivity on the San Jose vein averages 0.3 mr/hr. A 20-centi- .meter sample (A-1885) taken from the vein on the south side of the road assays 0.023 percent eU. A select sample from the vein, 15 meters in from the portal at the third adit above the road on the north side, assays 0.092 percent eU and registers 1.5 mr/hr. The average radioactivity of parallel veins in the vicinity is approximately 0.1 to 0.2 mr/hr. Gatico district

The Gatico mines, within sight of the ruins of the port city of Gatico, include the old Toldo copper mine, the principal producer in past years. Mine workings are down to 500 meters on veins up to 1.5 meters wide. Veins strike easterly and northwesterly and have been followed irregularly for 1.5 kilometers. The depth of the oxidized zone varies but, in general, extends to 80-meters. Minerals in this zone are carbonates of copper, native copper, cuprite-limonite, and small amounts of molybdenum, arseno- pyrite and niccolite. Below, chalcopyrite and pyrite are found with some magnetite. Gangue minerals are quartz, calcite, siderite and some tourma- line. The radioactivity of the host diorite is low--0.015 to 0.02 mr/hr0 Exposed veins and dumps have approximately the same degree of radioactivity.

Guanillos district

This area, 10 kilometers north of Gatico, was selected for reconnaissance because of its similarities to the Tocopilla district. Veins exposed at the coastline in volcanic rocks on the west side of the intrusion have an easterly to northeasterly trend. Radioactivity is low in both the host rock and the vein structures, averaging only 0.02 mr/hr. Farther east, exposed veins and dumps in the Gitana mine area were tested for radioactivity and found to average 0.02 to 0.025 mr/hr. The diorite host rock averages a little less than 0.02 mr/hr. The veins trend northwesterly and dip 700 S. Crushed calcite gangue encloses secondary copper minerals, and some silver has been reported.

The radioactivity of Mantos de la Luna ore registers 0.02 mr/hr0 This copper deposit is located high on the mountains in volcanic rocks near the diorite.contact.

Tres Puntais-San Juan district

This district is 10 kilometers southeast of Tocopilla. The host rock is the Mesozoic volcanic sequence. The most important vein structures trend northeasterly; others trend northwesterly. Secondary copper minerals in a calcite-quartz gangue display weak radioactivity-up to 0,03 mr/hr. The background of the volcanics is approximately 0.03 mr/hr.

Cerrillos district

The outcrop areas south-southeast of the Cerrillos railway station (latitude 22 17' S., longitude 69 52' W.) were examined. Bedrock is metamorphosed Paleozoic argillite showing radioactivity up to 0.03 mr/hr. The area has been prospected for lead minerals, and one active mine, the Anita, was visited. The vein structure dips 800 W. and trends northerly, parallel to the Salar del Carmen fault zone 1 kilometer west. A breccia zone, up to 1 meter wide, contains wulfenite, which changes to galen below a depth of 17 meters. Radioactivity of the vein ranges from 0.01 to 0.03 mr/hr.

The vein occurs at the crest of a small anticline, which may be related to movement on the main fault zone.

Other localities

A traverse was begun along the road extending north of Tocopilla in order to explore the set of northeast tension faults for radioactivity. Since the road was cut by stream wash was 5.5 kilometers north of the city, the traverse was not completed. The bacjkgrourid radioactivity in the diorite is 0.02 mr/hr.

An attempt was also made to traverse the total distance from the Tocopilla-Chuquicamata highway southward from the vicinity of Barriles to Gatico. The road was cut near Gatico. The traverse crossed a number of northeast-trending tension fractures cutting the volcanic sequence, but radioactivity never ranged above 0.025 mr/hr.

The coastal area between the Gatico and the Tocopilla intrusions includes east-dipping volcanic rocks altered in the northern portion and mineralized in mantos and veins. The Buena Esperanza mine, located 7 kilometers south of Tocopilla, is a manto-type deposit with disseminated copper sulfides. The area displays a maximum of 0.10 mr/hr, the highest radioactivity noted in volcanic host rocks in the area.

CONCLUSIONS

On the basis of the reconnaissance in the Tocopilla area, the following conclusions have been made:

(1) The north-trending Salar del Carmen fault is a major structure. All other structures shown on plate 1 appear subsidiary to it although, as yet, insufficient mapping has been done to make a complete structural analysis.

(2) The east- to northeast-trending structures along the coastline are the ore hosts. From the presence of numerous andesite dikes and actinolite fissure fillings, these structures appear to have been open tensional fractures, the result of movement on the Salar del Carmen, or parallel, seaward faults of the same system. The east-northeast structures have hosted the important Tocopilla and Gatico deposits and numerous minor copper bodies. Most of the copper production has come from veins in crystalline rocks, especially near the margins of the intrusions. Uranium is

-16-e present in tensional structures within the diorite-syenite of the Tocopilla intrusion. This is an important and widespread geologic environment that could contain significant uranium deposits.

(3) Additional geologic mapping is desirable for the east- northeast trending host structures in and adjacent to the southern part of the Tocopilla intrusion. Also, the intrusion-wide area extending from the Tres Puntas-San Juan district northward approximately to the Cabo Paquica-Cerro. del Rumbo (about 30 kilometers in length.) should be mapped in greater detail (preferably 1:50,000), noting on the map all of the structures and mineral occurrences detected by ground reconnaissance and aerial photograph study. The structures should then be radiometrically surveyed, and all available underground workings should be -briefly reviewed to determine the mineral suites for use in conducting zoning studies. These investigations could then serve as a basis for evaluating possible physical exploration for uranium in this vein system.

REFERENCES

Bruggen M., Juan, 1950, Fundamentos de la geologi'a de Chile: Santiago, Instituto Geogrifico Militar, p. 97, 311.

Flores W.,, Hector, 1956, Descripcion geologica del mineral de Gatico, in Geologia economica de yacimientos minerales: Santiago, Edugal, pt. 2, p. 31-36.

Hague, "R. S., 1958, U. S. Atomic Energy Comm. unpublished report.

Knowles, Paul H., Bowes, William A., Serrano C., Mario and Grenwald, Rudolfo, 1959, U. S. Atomic Energy Comm. unpublished report.

Kuntz, Julio, 1928, Monografi'a minera de la Provincia de Antofagasta: Santiago, Soc. Nac. de Mineria Bol. Minero, v. 40, no. 38, p. 190-200.

Leiding, Benjamin, 1936, Monografia de La Compania minera de Tocopilla: Santiago, Bol, de Minas y Petr'leo, v. 6, no. 56, p. 177-190.

Wetzel, W., 1927, Beitrage zur Erdegeschichte der mittleren Atacama: Neues *Jahrb. Mineralogie, Geologie u. Palaontologie, v. 58, p. 505-578.

70015 700 69*45' R.E-4534 (Rev.) + + + + + 21045' 1++ + 111111

+ ,+ + + 'I- -I LEGEND C111 r1 n11111111111 Pozo Perdi'do + +t1 1 1 1 11 I

++ +< / ernory All u vi urm + + + + l 111111111, 111 11111 J Quotes

+9 9-4 Ctaceous OPeC + +I 2o e to +Cerrde Ru I Tertiary + + Andean diorite complex + + Co + + +\ 8III

+asp+r1861 1rt-"t J r ssi c Andesite extrusive sequence i~~O~i~5@: :I\II IIII1111 Ii T C PILL -+""t-r+ + +f(I I'l11 111 I'll'111I('+

+ + +v + + I~lkil O1+ +l erro 0aCerro I PluItiI I [ nian Metamorphosed sedimentary sequence

+ + Cerro R

IIIq T PunPLLnc +Cero a17+B+Li " J r o + 0 111 Contacts 49oI III -II\ I{I II I1 1

i c'o Fault with dip where shown PLATE .- GEOLOGIC SKETCH OF TOCOPILLA AREA , ANTOFAGASTA PROVINCE, CHILE .

Strike and dip of strata, high angle.

0 15 Kilometers

erolnCorr ie Strike and dip of strata,low angle. SantiagoChile. 0 15 Miles January, 1959 +\rden+ +

Fold axis

Chindos

+ Tp Roads

trrlUS \ 4 11 111111111 22101 7OCP CL + + 71l-04 4. 111911145'1111

Cu Mine with name,showing mineral produced Diana

" Elevations in meters 1861 C9leta GuaniSpe

ogy by U.S. Atomic Energy Commission William A. Bowes and Paul H. Knowles. C_____+_____ oC'0Irr ./i11!r11 institute de Investigociones Geolcgicas de Chile. +Ceroeate d -er~ uo -Go Mario Serrano C y Rudolfo Gruenwald S. 0 graphy Instituto Geogrofico Militar de Chile. Maps N 62170 and 2270

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