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Figure 3. Digitally produced, derivative map of eastern Loudoum County showing the percent difference in construction costs as geologic units change (yellow, 0 percent difference; orange, 5 percent increase; and red, 10 percent increase). Superimposed are the proposed western bypass corridors and existing major transportation routes.

cost estimates of road development. The geologic map information used for this case study was acquired and digitized for $1.2 million. With the geologic information as a fact in the cost estimates, the change in total costs for the Loudoun County segment of the Washington Bypass is a savings of $10.5 million for W1-a, $5.9 million for W2, and $2.3 million for W3.

The difference between the total costs of each route with and without geologic map information illustrates the benefit of using geology in planning and development. Geologic maps provide decisionmakers with a tool, in this instance, for determining which route is the best for the county and, in general, for solving many land use, natural hazard-management, and mineral and energy resources issues.

World Energy Resources Assessment

By Donald L. Gautier and Charles D. Masters

uring the summer of 1990, political D developments in the Middle East and

rapidly rising gasoline prices bluntly reminded the people of the United States of

their precarious petroleum-supply situation. While domestic production continues to decline, increasing imports, added to by growth in demand for crude oil, now amount to about 50 percent of supply. The United States, having one-twentieth of the world population, consumes more than one-fourth of total world crude oil production. Oilproducing countries of the Middle East increasingly will dominate world petroleum supply.

The Organization of Petroleum Exporting Countries (OPEC) produced about onethird of the world's oil supply in 1990, but their share is increasing and is expected to exceed 50 percent by the year 2010. Within OPEC, oil production from proven reserves and supporting undiscovered resources will be dominated by countries of the Persian Gulf well into the 21st Century.

Crucial decisions will have to be made in the United States concerning energy conservation, new technologies, use of strategic supplies, exploratory drilling, and the use of coal, nuclear, and nonconventional energy sources. Despite the indisputable dominance of Middle East oil reserves, additional sources of oil exist, and it is in the interest of the world's consumers that oil supplies remain geographically dispersed to as great a degree as possible. Countries such as Venezuela, the U.S.S.R., China, Indonesia, and Nigeria are large oil producers, and other countries may also become contributors to world oil supply. Energy strategies require a clear understanding of the potential for additional sources of petroleum to the U.S. energy supply. This understanding depends upon reasoned and credible estimates of the undiscovered oil and gas resources of the United States and the world.

The World Energy Resources Program (WERP) of the USGS was established to provide an unbiased assessment of the occurrence of world oil and gas resources. The WERP is the only Federal program that makes such assessments, and the assessments are made available to all government agencies and to the public. The program was developed to coordinate with the Federal Energy Supply Assessment Program of the Energy Information Administration, an Energy Department program that focuses on determining world reserves of oil and gas. In recent years the WERP has conducted several assessments of the world's undiscovered oil and gas resources, and the results have been presented to the World Petroleum Congress.

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The World Petroleum Congress, a quadrennial technical meeting, was established in 1933 to give the international exploration and production industry a perspective on world petroleum activities, capabilities, and resource potential. Congress membership is held by countries, petroleum companies, and interested professional groups. The 13th Congress will be held in Buenos Aires, Argentina, October 1991, under the theme of New Horizons for the Petroleum Industry—Technical and Economic Challenges and Opportunities.

The WERP continuously conducts assessments of oil and gas resources on a regionby-region basis. Regions are selected according to their significance to world petroleum supply. The assessments are supported by background information from various types of topical and regional studies. Recent accomplishments include a study of the petroleum geology and resources of Antarctica, a report on the petroleum resources of West Siberia, a petroleum basin map of China, and a USGS bulletin on the distribution of major petroleum source rock and its relation to discovered occurrences of oil and gas and to plate tectonics.

The world petroleum source rock study concluded that the bulk of the world's discovered petroleum has come from several comparatively thin sequences of source rock deposited during a relatively small fraction of geologic time. For example, two intervals during the Cretaceous Period (138 to 63 million years ago) are responsible for more than 54 percent of all the world's petroleum. Future work may identify additional oil source rock that has generated significant amounts of petroleum.

During 1990, the WERP has become increasingly integrated with the USGS Domestic Resource Appraisal Program. This integration is necessary to meaningfully place domestic resources in the context of world petroleum supply. The WERP initiated cooperative projects in several areas of the world to investigate oil and gas resources of critical regions where resources are large and geologic uncertainties exist. In particular, cooperative projects with scientists of the U.S.S.R. Ministry of Geology will redefine categories and methods of resource assessment and improve our estimates of the resources and reserves of the Soviet Union. Similarly, cooperative projects with Saudi Aramco will develop a program of mutually beneficial topical research that will update assessments of the undiscovered resources of the Arabian Peninsula and Persian Gulf. Discussions of program cooperation with other countries, including Norway and Venezuela, will further enhance our global petroleum resource understanding.

Tapping the Potential Mineral Resources of Alaska

By Donald J. Grybeck

or more than a century, the favorable Fo of Alaska's mineral resources has been cited in the popular and pro

fessional press, by politicians, by geologists and mining engineers, by industry, and by many in the public and private sectors. Since systematic work began in Alaska in 1895, the USGS has devoted a major share of its work to defining those mineral resources.

The several large mines that have come into production in Alaska within recent years may signal a promising future for the Alaskan mining industry. Future mining activity depends on identifying undiscovered deposits and delineating other deposits that, while known, have not yet been measured with any assurance. Undiscovered deposits are those that cannot now be identified and are thus by definition individually unpredictable in their size and location. While many mineral deposits are already known in Alaska, only one in several hundred will ultimately prove to be mineable.

How can one predict the number of undiscovered deposits in Alaska? One technique is by geologic analogy; that is, study the geology of a given area and predict the number of undiscovered deposits by considering the type of deposits that are likely to occur based on known deposits, worldwide, that have comparable geology.

Other predictions can be based on the number and type of mineral deposits found during exploration. Some examples are the discovery of the Red Dog zinc-lead-silver deposit in the western Brooks Range in 1968 (now the largest zinc mine in the world), the Greens Creek lead mine on Admiralty Island in 1973, and the U.S. Borax molybdenum deposit in southeastern Alaska in 1974—all in areas not previously known to be mineralized.

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Red Dog mining waste disposal area. Close scrutiny shows a lone caribou at center.

These discoveries indicate that more large mineral deposits are yet to be found in Alaska. Another predictive method is to compare the mineral production of Alaska with that of other producing regions. For instance, most Western States have produced many times more metals per square mile than Alaska has produced or even contains in known deposits. The unavoidable conclusions are that Alaska almost certainly contains numerous undiscovered metal deposits and that some of them probably will be of world-class dimensions. The process of assessing Alaska's mineral resources and the subsequent mineral exploration and development of those resources follows a series of steps: 1. Regional geologic analysis of the State or large areas, 2. Definition of areas of mineral potential, 3. Systematic studies, usually for specific types of deposits, in selected areas, 4. Detailed studies of deposits or mineralized areas, usually by drilling, 5. Detailed economic, logistical, and metallurgical studies of selected deposits that have the potential of becoming a mine, and 6. The actual development of the property into an operating mine. The USGS is primarily concerned with the first four steps of the process, and, in Alaska, has an active role in the first two steps, in particular, geologic mapping and mineral-resource assessment at the regional scale. Detailed studies of selected areas and deposits establish the scientific framework for studying mineral deposits. Development of mineral deposits into actual mines is carried

out by private industry aided by the U.S. Bureau of Mines. At this time, about 30 to 40 percent of Alaska has potential for undiscovered mineral deposits; however, less than 1 percent of Alaska will contain mineable mineral deposits. A mineral deposit, even one that can be developed into a world-class mine, is usually difficult to locate. Many deposits are concealed by tundra and soil or are buried beneath glacial drift or other transported materials. Sophisticated exploration techniques and considerable work are necessary to locate them. Currently, most USGS Alaskan regional mineral-resource work leading to the identification of deposits is being carried out under the Alaska Mineral Resource Assessment Project (AMRAP). AMRAP, a component of the National Mineral Resource Assessment Program, includes systematic multidisciplinary studies of 1:250,000-scale quadrangles. Geologic mapping, geochemical techniques using sophisticated analyses of stream sediments, soils, and heavy minerals concentrated in streams, and geophysical techniques that measure the magnetic and electrical properties of rocks and mineral deposits are the primary methods used to generate data for a mineral assessment. In most quadrangles, 3 to 4 years are necessary to collect and synthesize field data into a regional mineral-resource assessment. The goal is to define those areas that have mineral potential, to assemble all the geologic and mineral deposit data for the quadrangle in a usable form, and to predict the number and size of undiscovered deposits. To date, AMRAP studies have been published or are now in press for 38 of the approximately 100 1:250,000-scale Alaska quadrangles that have mineral potential. An additional 16 quadrangles are under study. In 1981 Congress passed the Alaska National Interest Lands Conservation Act (ANILCA). Among the many provisions of the act, Section 1010 charged the Secretary of the Interior with assessing the mineral potential of all Federal lands in Alaska. That responsibility has been delegated to the USGS and the U.S. Bureau of Mines. ANILCA placed land management responsibilities for most of Alaska's Federal lands with the National Park Service, the U.S. Fish and Wildlife Service, the Bureau of Land Management and, in the Department of Agriculture, the U.S. Forest Service. These agencies are engaged in extensive land use planning, and the estimated value of mineral resources is usually factored into these plans. Thus, in addition to the

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traditional clients for minerals information (industry and the public), Alaskan land managing agencies increasingly need up-to-date, Comprehensive mineral-resource data for their lands. For example, the USGS currently is working with the Bureau of Land Management to assess the geology and mineral potential of the southern part of the National Petroleum Reserve in northwestern Alaska and with the U.S. Forest Service in assessing the mineral potential of the Tongass National Forest in southeastern Alaska.

Common metallic minerals, such as gold, silver, copper, molybdenum, lead, and zinc, are known or are likely to be present in major quantities in Alaska. However, another class of minerals in Alaska is especially important to the Nation. This class, the strategic and Critical minerals, includes mineral commodities that play a critical role in modern technology or are largely imported from foreign sources. Among these minerals are the platinum-group elements, tin, chromium, manganese, and certain rare-earth elements, Commodities that are essential in the production of steel alloys, as chemical catalysts, and in the semiconductor industry.

Many of Alaska's known deposits have potential for these strategic and critical minerals. Strategic minerals in Alaska have been mined intermittently in the past, usually during periods of war or international unrest. The USGS and the U.S. Bureau of Mines currently have plans for an expanded program of research on Alaska's strategic and critical minerals, not only to better understand and inventory known deposits but also to search for new deposits. The improved information will serve the Nation during periods of national emergency and identify new sources of strategic and critical minerals to make the United States less dependent on foreign sources in the future.

Idaho National Forest Roadless Areas

By Ronald G. Worl

r | Whe USGS, U.S. Bureau of Mines (USBM), and Idaho Geological Survey (IGS) are conducting joint mineral resource studies of roadless public lands in Idaho. These public lands may be included in the National Wilderness System. The studies will provide geological, geophysical, geochemical, mining, and mineral deposit information on roadless public lands that will benefit the

domestic mining industry and other users of earth-science data responsible for making informed land use decisions. The roadless areas under study are located mainly within central Idaho in areas underlain by the Idaho batholith, a large composite granitic intrusive body, which is 70 to 95 million years old, and by younger granitic rock, which is about 50 million years old. Defining the geologic setting of the mineral deposits and determining the extent of the important mineral-bearing terranes are the focus of the geologic studies. This 3-year (fiscal years 1989–91) program of geologic mapping by the USGS and IGS will complete 1:100,000-scale geologic maps for most of the previously unmapped roadless areas. The IGS, with funding from the USGS, is mapping more than 3,000 square miles of ground in the Elk City and Hamilton 1° x 2° quadrangles. Studies in the Edwardsburg and Profile mining districts indicate that known mineral deposits are aligned along a major shear zone that extends through one of the proposed wilderness areas. The proximity of many of the known gold deposits to 75-million-year-old two-mica granite has been shown in several areas. Gold in large placer deposits in the Warren and Florence mining districts, for example, was derived by weathering and erosion of networks of precious-metal-bearing quartz vein and veinlets emplaced near the upper boundary of two-mica granite bodies and overlying rock. Field interpretation of these regional geophysical data indicates that magnetic highs correspond to the younger granitic intrusive rock, some of which is mineralized. Several regional magnetic and radiometric highs indicate the presence of granitic intrusions that are not exposed. The regional gravity data indicate that younger granitic rock underlies more of central Idaho than was previously recognized. Site-specific studies include geoelectrical surveys of possible deep-seated fracture systems and gravity surveys in areas that may contain unrecognized intrusive bodies. Interpreting existing geochemical data from the Challis area led to the delineation of 10 areas that contain significant amounts of cobalt in stream sediment samples. Four of the areas are associated with exposures of the more than 1-billion-year-old Yellowjacket Formation, a known host for cobalt deposits elsewhere. Six are in the Bayhorse area and are underlain by 45-million-year-old volcanic rock not known to host cobalt deposits. Site-specific geochemical surveys show that black shales and mafic dikes in the Borah Peak area may host unrecognized mineral deposits. Samples of black shale contain anomalous amounts of molybdenum, silver, and zinc, and heavy-mineral concentrates of stream sediment collected throughout the area contain significant amounts of barite. Metals associated with the mafic dikes include chromium, nickel, cobalt, and copper. In the Smokey Mountains area, anomalous amounts of gold were found in heavy-mineral concentrates from a cluster of sample sites in an area of about 10 square miles. Studies of stream sediment and mechanically panned concentrate samples in the northern Lemhi Range led to the discovery of a large area containing anomalous amounts of gold.

Detailed mining area studies by the USBM have delineated several locations where known mineral resources occur within roadless areas that are being considered for inclusion in the National Wilderness System. The areas are within or next to the Profile, Edwardsburg, Warren, Relict, Big Smoky, and Skeleton Creek mining districts. Known resources in these areas include gold, silver, lead, and zinc.

Several mineral deposit models are being developed or revised for the assessment of undiscovered deposits in Idaho roadless areas. Two types of productive gold vein deposits have been recognized—massive quartz veins and complex quartz veins. The massive quartz veins are characterized by several generations of quartz deposition, simple mineralogy, and close proximity to two-mica granites. The structural setting of this deposit type is along major fracture systems formed by compression in the Earth's crust. These deposits formed 78 to 57 million years ago at depths of 5 to 10,000 feet. The complex quartz veins are characterized by open-space quartz filling, the presence of numerous complex minerals, and close proximity to felsic dikes. The structural setting of this deposit type is along major fracture systems formed by tension in the Earth's crust. These deposits formed 50 to 25 million years ago at depths of less than 4,000 feet.

Summary reports on the accomplishments of all studies under this plan will be prepared jointly by the three agencies at the end of the project in fiscal year 1991. The summary reports will include maps showing areas of known mineral resources, maps showing areas that have potential for undiscovered resources, descriptions of mineral deposit types, and an estimate of the number of undiscovered deposits in terranes delineated as having potential for selected mineral deposit types.

Voyage of the Century— Neptune and Triton

By Laurence A. Soderblom, Randolph L. Kirk, and Alfred S. McEwen

ore than 12 years after its 1977 M launch, the Voyager 2 spacecraft completed its fourth and final plan

etary encounter—the flyby of Neptune and its large companion satellite Triton—in August of 1989 (fig. 1). The Voyager spacecraft have provided the reconnaissance exploration of the major part of the Solar System and have revealed a diversity of planets, moons, and rings that is almost beyond comprehension. Voyager has been the premier extraterrestrial exploration of the twentieth century.

The Voyager project, conducted by the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration, has been a coordinated effort of Federal agencies, industries, research institutions, and universities. USGS scientists participated with JPL scientists in planning and executing data gathering efforts and analyzing the resulting information from the Voyager encounters of Jupiter, Saturn, Uranus, and Neptune. USGS scientists also mapped and analyzed the surfaces of the 57 known outer-planet moons, 17 of which were discovered by Voyager. The largest of the outer-planet moons—the four Galilean satellites (at Jupiter), Titan (at Saturn), and Triton (at Neptune)—are about the size of Earth's moon. In addition, Voyager recorded the surfaces of 12 medium-sized moons orbiting Saturn and Uranus. Each of these moons were revealed to be unique and varied worlds, and they have increased the number of bodies available for comparative planetary geologic study from 5 (Mercury, Venus, Earth, Moon, and Mars) to 23.

Triton provides some of the major surprises of the Neptune flyby in that it has an unusual and geologically young surface and at least two active geyserlike plumes (fig. 2). A huge polar cap, probably composed of nitrogen and methane ice and frost, covers almost the entire southern hemisphere of Triton. The cap has a slight reddish tint, possibly due to the presence of organic compounds produced from methane and nitrogen by the actions of photochemistry and energetic particle bombardment. A very bright and slightly bluish fringe occurs around the margin of the cap and probably consists of fresh nitrogen frost or snow.

Northward of the polar cap, the surface has a variety of exotic terrains. The relatively

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