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Mineral Activities in
Latin America

By Norman J Page,
Charles G. Cummingham,
Gregory E. McKelvey, and others

r | Whe USGS Center for Inter-American

Mineral Resource Investigations

(CIMRI) encourages and advances studies of nonfuel mineral resources in South America, Central America, and the Caribbean. Headquartered in Tucson, Ariz., CIMRI projects include mineral information exchange, cooperative investigations and research, and training aimed at assisting Latin American countries in developing their mineral economies.

Training and Technology Transfer

CIMRI personnel develop and collect information relevant to mineral resources including geology, geochemistry, geophysics, remote sensing, and mineral deposits and occurrences for countries in the Caribbean and the Americas. The center contributes information to several computer data bases including the Mineral Resources Data System (MRDS), which is available to the public through Minerals Information Offices (MIO) in Washington, D.C.; Tucson, Ariz.; Reno, Nev.; and Spokane, Wash.

Coverage of mineral deposits and occurrences in the states of Sonora, Baja Sur, Baja Norte, and Chihuahua in Mexico, the Guayana Shield of Venezuela, the Altiplano of Bolivia, northern Chile, and Honduras has been greatly expanded. The Tucson MIO has taken the lead in assisting CIMRI in collecting, translating, and entering Latin American data in MRDS. Publicly available information and resources include bibliographies of geosciences related to mineral resources for Bolivia and the Guayana Shield of Venezuela and a partial bibliography of geophysical information on the Caribbean and the Americas, as well as archives of maps, books, and papers on Latin American earth science.

In a symposium on the mineral resources of the Chihuahua Desert, CIMRI representatives presented data on mineral deposits and occurrences in northern Mexico. At the Exposición Latinoamericana de Mineria in Santiago, Chile, grade and tonnage models were shown to be applicable in the search for

ore deposits that are amenable to small-scale mining, which does not require large capital outlays. While small-scale mining can be conducted on large deposits, certain deposits are particularly amenable to small-scale operations. By grading deposit sizes from smallest to largest, an explorationists can select deposit types that have appropriate size ranges for small-scale mining.

Deep weathering in the tropical climates in parts of Latin America has produced thick and extensive lateritic soils. These soils are rich in secondary oxides of iron or aluminum or both. Recently, concentrations of gold that form mineable deposits have been identified in laterites. By developing tonnage and grade models for these lateritic-saprolite-gold deposits, Latin American scientists now have another tool to aid in mineral exploration.

The USGS began a 2-year training project in May 1990 for earth scientists from the national geological institutes of Peru, Bolivia, and Chile. Investigations are centered on volcanic-hosted, epithermal gold-silver deposits of the Central Andes. The project, funded by the Inter-American Development Bank, includes some of the latest technology and concepts in mineral deposits, volcanology, geochemistry, and remote sensing. The project field areas are within a volcanic field of the central Andes and include known mining districts, as well as unexplored areas of altered rocks. Training includes learning how to use thematic mapper images from the Landsat 5 satellite to map the distribution of volcanic centers and related altered rock in the high Andes.

CIMRI Program Highlights


Cooperative investigations by the USGS and Tecnica Minera, C.A. (TECMIN) on mineral resources of the Guayana Shield in Venezuela are funded by the Corporación Venezolana de Guayana. As part of the USGS-TECMIN program, the USGS is preparing a set of 1:500,000-scale geologic and tectonic framework maps of the Guayana Shield based on an integration of available coverage of aeromagnetic data, radiometric data, gravity data, side-scanning radar, aerial photographs, and reconnaissance field work.

The first of these maps is of the Puerto Ayacucho region and shows a large granitic intrusion, which is suitable for use as building stone, along with potentially mineralized rock and several gold deposits. Three, previously unknown, ringlike structures have been

identified during data interpretation and assembly of the map. One of the structures is associated with uranium and may be a large carbonatite intrusion. Numerous small to medium sized buried magnetic bodies are also depicted on the map; it is effectively a representation of the top 6 to 9 miles of the Earth's crust in this area. This map series is the foundation for a detailed mineral-resource appraisal (completion: 1991) of the Guayana Shield.


The Ministry of Mines and Metallurgy and the USGS have a cooperative project with the Geologic Survey of Bolivia (GEOBOL). The project is funded by the Trade and Development Program, established by the International Development Cooperation Agency, to undertake a mineral resource assessment, coordinated by CIMRI, of the Altiplano-Cordillera Occidental of Bolivia.

As part of this work, geologic maps of the Altiplano prepared by GEOBOL at 1:250,000 scale were scanned into a computer system. These maps are used to compile a digital Altiplano-Cordillera Occidental geologic map, which will be published at a scale of 1:500,000 in a USGS Bulletin. Remote sensing imagery for the entire Altiplano is being analyzed to map mineralized and altered rock. Existing aeromagnetic data for nearly half of the Altiplano are obtained by GEOBOL, and these data, Combined with new aeromagnetic data currently being contracted, will provide coverage for nearly 70 percent of the area.

Collaborative teams of USGS and GEOBOL scientists are visiting known and suspected mineral occurrences found during an extensive review and cataloging of the unpublished literature in Bolivia. Mineral

occurrence data on about 300 sites are already

in the MRDS. Field work is completed in areas of known copper mineralization and regions containing industrial mineral potential. The remainder of the field work, including investigations into precious metals, will be completed by the end of 1990.


Cooperative investigations are underway in Uruguay by the USGS and the Dirección Nacional de Geología y Minas. Geophysical, geological, geochemical, and drill data are evaluated with comparable information obtained from the Dirección Nacional de

Geología y Minas for intrusive rock in the Cerro Mahoma area near San Jose de Mayo. This information and further field examinations established that some of the rock in this Precambrian intrusion is suitable for use as an ornamental stone, called granito negro. This intrusive rock may also contain copper-nickelplatinum-group minerals.


Mineral resources of the Taitao ophiolite in southern Chile are being studied by the USGS in conjunction with the Colorado School of Mines and the Servicio Nacional de Geología y Minería de Chile. This research project is supported in part by the National Science Foundation.

Remote southern Chile remains a frontier for basic geologic study and mineralresource development. This region, which has many similarities to the mountainous region of western North America from the eastern face of the Rocky Mountains to the Pacific Ocean, holds promise for mineral wealth. The Taitao region offers a unique location for studying the plate tectonic process of collision between an Oceanic ridge and the continental boundary and any associated mineralization. Here, on the Chile rise, new ocean floor is forming by upwelling of the Earth's mantle and submarine volcanism, and the ocean ridge is being subducted below the South American continental plate.

Geologic mapping, geochemical evaluation of rock and stream sediment, and petrology from the Taitao Peninsula are being used to understand the origin of the rock and to determine mineral-resource potential. The rock of the Taitao Peninsula is a likely host for deposits of platinum-group metals, chromium, copper, and gold. Field investigations uncovered an occurrence of copper and zinc mineralization. The results of this study have implications for the mineral-resource potential of areas further south in Chile where ridgeContinent collision has occurred in the more distant geologic past.

Future Plans

CIMRI will continue to provide geoscience information relevant to the mineral resources of the Caribbean and the Americas with the goal of synthesizing information on large regions in Latin America, such as the Andes. Discussions for developing mineral-resource assessment programs in Honduras, Argentina, and Uruguay and for a study of gold placers of the eastern Andes are underway.

Computer-Assisted Map Revision—Bolivia By Richard D. Sanchez

ne of the most significant mapping

problems facing many Latin Ameri

can countries today is the revision of outdated base maps. Because of the increasing cost of acquiring aerial photography—a key source of information for map revision—the Inter-American Geodetic Survey of the U.S. Defense Mapping Agency joined the USGS, under the auspices of the Pan American Institute of Geography and History, in examining the potential use of computer-assisted techniques and SPOT (Satellite Pour l'Observation de la Terre) and Landsat thematic mapper (TM) data to detect changes at the scale of 1:50,000.

Computer-assisted techniques are distinct from the conventional cartographic techniques used for photointerpretation and map revision. These techniques were developed with the use of a personal computer and include the image processing of electro-optical sensor data and the Superpositioning of digitally scanned map overlays for visual interpretation of change. The 1:50,000 scale was chosen because it is the standard scale for the topographic map series in Bolivia. The test area selected is in the eastern

piedmont region of the Andean Cordillera Oriental in the province of Santa Cruz, Bolivia. This relatively flat area of woodlands, cultivated fields, and pastures lies within the Amazon basin. Its principal river is the Rio Piray. Santa Cruz de la Sierra, the departmental capital, is a rapidly growing center of

commerce and a gateway to eastern neighboring nations.

To detect changes, a digital map file for time A is overlaid on a raster image file for time B; the two files are compared, and new information is noted. In a comparison of the 1967 planimetric data (time A) of the Santa Cruz area with a 1987 SPOT panchromatic image (time B), the tremendous expansion of the urban area and the striking increase in roads and other cultural features are readily apparent. On the SPOT image, cultivated fields, pastures, and woodlands are distinct from the urbanized area.

Approximately 75 percent of the revised features on the Santa Cruz de la Sierra map were detected by using a combination of SPOT panchromatic and Landsat TM imagery and computer-assisted techniques. Problems were encountered in the identification of certain features, such as urban structures, that require greater spatial detail or resolution than the satellite imagery can provide. Even though satellite images can detect new buildings and other structures and define new urban or suburban areas, supplemental aerial photographs or ground surveys may be required for the revision of intensely developed areas.

The joint effort showed that map revision at 1:50,000 scale is possible when the appropriate satellite images and other source information are used in conjunction with field verification. As a result of the findings in the Bolivia project, Mexico has asked the USGS to provide technical assistance in developing the capacity to do computer-assisted change detection and map revision using satellite data. Similar interest has been expressed by Venezuela.

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and sampling may substantially increase gold reserve estimates.

Phosphate Exploration

The DGMR and USGS conducted reconnaissance exploration in Saudi Arabia in 1965 to determine if the Middle East-Egyptian phosphate province extended into the northern region of the Kingdom. This study reveals phosphate-rich desert pavement, derived from what was to become known as the Umm Wu al phosphate deposit, to be part of the Sirhan-Turayf basin, which is approximately 42,000 square miles in area. Later geologic mapping and stratigraphic studies delineate phosphate deposits at Thaniyat and Al Jalamid. The potentially economic phosphorites of Saudi Arabia occur at three horizons in rocks of the Sirhan-Turayf basin west of the northwest-trending, gently north-plunging Hail arch. Rocks in the basin are cut by northwest-trending normal faults.

The Al Jalamid deposit has the greatest economic potential of all the known phosphorite deposits in Saudi Arabia. The deposit contains about 15.7 percent phosphorite, has a thickness of about 10.5 miles, and occurs in the lowermost of the three horizons. The beds in the deposit dip gently to the west. The rock commonly contains small solution cavities, and approximately 25 percent of the phosphorite is friable (easily broken or crumbled) or semifriable. Data from the few deep boreholes that penetrated unoxidized rock outside the

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Locations of current investigations A. - in Saudi Arabia. 36° 42° 48° 54°

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