three holes revealed no significant mineralization, but the fourth hole hit highgrade ore, containing as much as 60 grams of gold per ton of ore. After additional drilling, USGS geologists estimated a possible resource of 1.1 million tons, with an average grade of 27 grams of gold and 73 grams of silver per ton of ore. When subsequent drilling confirmed the USGS results, the orebody was put into production.

After the success at Mahd adh Dhahab, during the late 1970's, emphasis shifted toward complete geological mapping of the Arabian Shield, as a basis for future minerals exploration and development. In addition to mapping, the USGS explored for copper, zinc, gold, silver, tin, tungsten, and nickel and carried out topical studies on the geochronology and

petrology of the felsic plutonic rocks of Saudi Arabia. These previous studies provide the background for current USGS efforts in gold exploration.

Gold prospects now being explored and evaluated are in the Jabal IshmasWadi Tathlith gold belt, a regional concentration of gold occurrences that lies within a suture zone in the southeastern part of the Arabian Shield. Current projects are at the Farah Garan ancient mine and in the Farah Garan-Kutam and Halahila mineral belts, in the southeastern corner of the Arabian Shield, and at Ishmas Kabir and B'ir Jarbuah about 200 miles north of Halahila.

At Farah Garan, the host rocks contain finely laminated to massive sulfides of iron, copper, and zinc, along with minor amounts of arsenic, gold, and sil

ver. The gold-bearing deposits of Farah Garan are similar to those of Al Masane to the north, where mineable reserves of copper-zinc-gold-silver ore are awaiting production. The current work at Farah Garan follows the discovery of significant amounts of gold (8.7 grams per ton over 8.6 meters) encountered during previous USGS drilling. Detailed geological, geochemical, and electromagnetotelluric geophysical surveys have been done as the basis for a five-hole core drilling program that is in progress. In addition, stream-sediment geochemical sampling and geological mapping are being done to explore a large area around Farah Garan for the presence of other deposits like Farah Garan and Al Masane.

The Ishmas Kabir ancient gold mine is located at the intersection of a suture zone with a younger strike-slip fault zone. The ancient mine is marked by sand-filled trenches surrounded by a row of mine dumps. Dump samples from the ancient workings show moderate grades (3 to 7 grams per ton). Four core holes, drilled to intersect the steeply dipping orebody at depth, showed that it pinches out and gold grades decrease with depth.

Although drilling results at Ishmas Kabir were negative, geological work in the area has revealed another, perhaps more promising, prospect at B'ir Jarbuah. The ancient mines of B'ir Jarbuah are in and along quartz veins and veinlets. Mapping, trenching, and sampling have revealed spotty, high-grade concentrations of visible native gold in some of the quartz veins and veinlets. The highest assay reported was 163 grams per ton of gold over 7.1 meters. Further geological and geochemical work is planned to test the continuity and extent of the gold deposits of B'ir Jarbuah and the vicinity.

Reconnaissance to identify new prospects is being initiated with a review of known mineral belts and gold occurrences. The USGS library in Jeddah and the computerized Mineral Occurrence Documentation System (MODS), established during geologic mapping of the Arabian Shield, are the starting points for this work. In addition, satellite imagery will be used to help identify hydrothermally altered areas, and wadisediment geochemical surveys will be used to identify new prospects in promising areas. Emphasis will be placed on the

search for near-surface, bulk-mineable, and heap-leachable gold deposits, which can now be economically mined to very low grades and which may have been missed by the ancients. One deposit of that type has recently been discovered in Saudi Arabia, and the chances are good that others remain to be found.

Mineral Resource Investigations in Venezuela

By Norman Page, Jeff Wynn, and Gary Sidder

In late 1987, a technical assistance project, supported financially by Corporacion Venezolana de Guayana, was initiated between the U.S. Geological Survey (USGS) and Tecnica Minera (TECMIN), which is part of the Corporacion, to investigate the mineral resources of the Guayana shield, in Venezuela.

The shield consists of largely unexplored areas of Precambrian (more than 570 million years old) volcanic, granitic, and sedimentary rocks that are possible hosts of undiscovered mineral deposits containing one or more of the following commodities: gold, diamonds, bauxite, iron, manganese, chromium, titanium, copper, lead, zinc, tin, rare-earth elements, nickel, and platinum-group elements. In general, the shield in eastern Venezuela is composed of thick sequences of low-grade metavolcanic and metasedimentary rocks, basalt, tuff, and other pyroclastic rocks, which are intruded locally by ultramafic and gabbroic rocks. To the west and south, flows, tuffs, breccias, and subvolcanic and plutonic granite are notably unmetamorphosed but do contain local areas of contact metamorphism and hydrothermal alteration.

TECMIN is charged with exploring the shield area and identifying undiscovered and undeveloped ore deposits; two groups of personnel, each using different techniques, are employed. The Inventory Group examines the entire area of the

bly also for massive sulfide deposits. One area contains gold-quartz-carbonate veins and possible chromite deposits. Other areas were evaluated for potential mineralization associated with intrusive dikes, felsic volcanic rocks including ash-flow tuffs, and placer diamond-gold deposits. At least two new prospective areas containing gold-quartz-carbonate veins were identified. Recommendations also were made to discontinue exploration efforts in two areas.

To improve the technology for mineral resource assessment and exploration in Venezuela, the facilities for preparing and analyzing samples are being upgraded. At the same time, training in handling, preparing, and analyzing samples has begun. The Exploration Group of TECMIN also assesses the available geophysical information, both airborne and ground data. Aeromagnetic data for several concession areas were examined, and preliminary interpretations were made, to be verified during field work.

The Inventory Group examined areas drained by the Caura and Paragua Rivers in the south-central part of the shield. Besides various types of sandstones, much of the area appears to be underlain by felsic volcanic rocks including welded ash-flow tuffs. Gabbroic, diabasic, and granitic intrusive rocks also appear to be major components. Evidence for mineralization, including copper and tin minerals, alteration products, and visible gold, was observed at several localities. The geologic field observations are integrated with the interpretations of the geology made from photographic and side-looking airborne radar (SLAR) images in an attempt to locate areas where mineral deposits may be present.

Training activities by the USGS, both in the field and laboratory and through teaching of short courses, form an important part of the project. Short courses on mineral deposit modeling, mineral resource assessment techniques, and digital geologic map preparation systems were presented to counterpart geoscientists. The courses have contributed to increasing the number of Venezuelan geologists who can recognize the types of rocks and geologic environments that may host a broad range of different types of mineralization.

Antarctic Mapping—

Thirty Years Since the International Geophysical Year

By Frederick S. Brownworth

The U.S.Geological Survey first participated in Antarctic surveying and mapping activities in 1957-58, when William H. Chapman, a USGS cartographer, traveled to Antarctica to take part in the International Geophysical Year program effort in the Ronne Ice Shelf-Filchner Ice Shelf area. The following year, in 1958-59, Chapman wintered over at Byrd Station and subsequently established geodetic control for the Survey's first three Antarctic mapping projects: the Horlick Mountains, the Whitmore Mountains, and the Executive Committee Range. Since then, 30 employees have wintered over, principally at the South Pole Station, and 91 employees have participated in 136 summer assignments, either conducting geodetic field survey operations or acquiring aerial photography for future mapping programs.

Early Reconnaissance Mapping

In 1959-60, the USGS began a major program to prepare topographic reconnaissance map coverage at

1:250,000 scale for the unmapped and largely unexplored coastal regions and rock outcrop areas of western Antarctica. The plan was to provide base maps to support scientific research programs, aerial reconnaissance activities, geologic field investigations, and thematic mapping. Today, 30 years later, 91 topographic maps have been published that provide coverage for the Transantarctic Mountains from Cape Adare to the Shackleton Range, Marie Byrd Land, the Ellsworth Mountains, the Pensacola Mountains, and the Executive Committee Range. The USGS has recently resumed the compilation of nineteen 1:250,000scale maps in the southern Antarctic

Peninsula areas and along the coast westward to Thurston Island. Twenty-three published sheets are being revised to show recently established geodetic control stations, U.S. Board on Geographic Names place-name additions, new Antarctic Treaty nations' research stations, and the boundaries of Specially Protected Areas and Sites of Special Scientific Interest.

Large-Scale Topographic Mapping

In December 1970, the USGS began 15-minute topographic quadrangle mapping at 1:50,000 scale. The first project consisted of eight quadrangles covering the Taylor and Wright Valleys in the Dry Valleys area of northern Victoria Land. More recently, in March 1988, a mapping requirements meeting was held to identify future needs. These new requirements include 1:50,000-scale topographic mapping for selected areas within the Freyberg Mountains, Queen Alexandra Range, Ellsworth Mountains, and Pensacola Mountains, to support geologic studies, and portions of the Dry Valleys area, to support detailed glaciological investigations.

Geodesy

USGS cartographers first wintered over at the old South Pole Station in February 1973 in support of two projects, which are still active today. The first project, Doppler Satellite Tracking, was to collect data from U.S. Navy navigational satellites for geodetic and mapping purposes. The data are also used annually to reset the true geographic South Pole marker on the basis of Doppler icemovement data. The second project was the Antarctic Seismic Project, a part of the Worldwide Standardized Seismograph Network. The data from this project are transmitted daily to the USGS National Earthquake Information Service to help locate earthquake hypocenters and pinpoint origin times.

The Survey's first practical field experience with Doppler satellite surveying technology was during the austral summer of 1971-72. This technology provided a means to upgrade and to add to the existing geodetic control network and to recompute the data on the 1972 earth-centered coordinate system. Plans for the future are to use the more advanced Global Positioning System and the 1984 coordinate system.