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.

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° × 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

M

ore than 12 years after its 1977
launch, the Voyager 2 spacecraft

completed its fourth and final planetary 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

low number of impact craters attests to the geologic youth of the surface. The western hemisphere is dominated by a dense concentration of pits crisscrossed by ridges, dubbed the cantaloupe terrain. The eastern hemisphere consists of a series of much smoother units, including calderalike structures. These structures appear to be frozen lakes and are surrounded by successive terraces indicative of multiple episodes of flooding and collapse. The numerous, dark northeast-trending streaks seen on Triton's south polar cap are similar to wind streaks on Mars. However, some Voyager scientists doubt that Triton's tenuous atmosphere, exerting only 1/100,000 the atmospheric pressure at sea level as that on Earth, is sufficiently dense to entrain particles from the surface. These scientists propose that the streaks are the result of geyserlike venting of gas particles (fig. 2). Triton's nitrogen frost migrates from pole-to-pole every 80 years as the subsolar latitude varies +50°; therefore, the dark streaks are probably less than 80 years old. The presence of more than

NASA/USGS

Figure 3. Composite view showing Neptune on the horizon of Triton. The Neptune disk shows a great dark spot (the south pole is to the left). The foreground is a computer-generated view of Triton's icy volcanic plains as they would appear from a point about 28 miles above the surface. The terraces indicate multiple episodes of flooding, freezing, and collapse. This view was computed from a Voyager image and a photoclinometric topographic model. Topographic relief has been exaggerated about thirty fold; the actual difference in elevations is about 0.6 mile.

Figure 4. Color mosaic of Triton in polar stereographic projection, centered on the south pole. Grid indicates 30° intervals of longitude

and latitude. The entire south polar cap and bright fringe are visible. Diffuse bright rays extend northnortheast for hundreds of miles and emanate preferentially from the points of the scalloped cap margin. These rays probably consist of finegrained frost or snow from the bright fringe that was redistributed by prevailing northerly winds.

100 such streaks suggests ongoing venting activity.

Other conclusive evidence exists for active venting. By reprojecting and coregistering images acquired at different viewing angles, a match was found for all of the features except two sets of long westwardtrending dark streaks. The offsets between the paired images of the streaks indicate that these materials are located about 5 miles above the surface. Closer inspection of the images reveals vertical eruption columns extending from the surface to the eastern ends of the streaks. Apparently, these are active geyserlike eruptions in which plume material rises vertically for about 5 miles before being carried downwind above the transition zone between the troposphere and the stratosphere.

Images of Triton's complex surface became the highlight of the Voyager and Neptune flyby (figs. 3 and 4). About 50 highresolution images of Triton were acquired during a complex sequence that commenced 8 hours prior to the close flyby of the moon on August 25, 1989. These images were taken through various color filters, at different resolutions, and from rapidly changing spacecraft positions.

The computerized processing of such a data set is complex, and, in the past, months. or years have been required to produce highquality digital cartographic mosaics. For the Neptune flyby, however, USGS personnel were able to assemble a suite of highresolution, multispectral, geometrically controlled digital mosaics in less than 3 days, which gave the public their first glimpse at this new world. In addition, the USGS, working in collaboration with the JPL Digital Animation Lab, generated a three-dimensional time-lapse simulation of Triton's bizarre surface as it would be viewed by a spacecraft descending over the surface.

Final maps of Triton are being prepared, and the USGS is working with the International Astronomical Union to assign names to the many new features. The USGS also plans to publish a geologic map of Triton. Planetary geologic maps are used to gain an understanding of the processes now active on other planets that may have been active on Earth during its formation. Geologic maps also are essential for future planetary exploration that includes manned or unmanned spacecraft landings.

Flooding in the Arkansas,
Red, and Trinity Rivers
By Kenneth L. Wahl

I

n spring 1990, unusual amounts of rain produced record or near-record flooding during April and May in northeastern Texas, southeastern Oklahoma, western Arkansas, and along the Red River in Louisiana. The flooding was the culmination of an extremely wet winter and early spring. In Oklahoma, the statewide average precipitation for the first 4 months of 1990 was the largest January to April total reported since record keeping began in 1892; the 4-month total exceeded the previous high for the period by about 15 percent. The Dallas-Fort Worth Airport reported total precipitation for January to March of 22.05 inches, 129 percent above normal.

These extremely wet conditions were conducive to extensive flooding: by mid-April, soils were saturated, flows in the principal river systems were already near flood stage, and reservoirs and lakes were at or near capacity. Because of these conditions, two major storm sequences in late April and early