minerals precipitated as the hydrothermal fluid cools during mixing with seawater; the mounds result from direct precipitation from the accumulation of particulate matter expelled through the chimney and from the erosion and collapse of chimney debris. Some vents are currently active and vigorously discharging large volumes of cloudy black (sulfide- rich) or white (sulfate- or silica-rich) fluid. Other vents are inactive and rapidly disintegrating. Many active vent systems are inhabited by exotic communities of worms, crabs, and other organisms evidently supported by abundant sulfur-reducing bacteria in the hotspring environment.

The mounds and chimneys are composed largely of sulfide minerals, especially zinc, copper, and iron. Parts of the vent deposits contain considerable amounts of anhydrite, barite, amorphous silica, and talc, and some mounds are largely composed of these minerals. Native elements commonly present include gold, silver, and sulfur. Sediment and basalt surfaces near the mounds are coated with deposits of iron and manganese oxide deep-sea sediments on the flanks of oceanic ridges and in nearby basins also are enriched hydrothermally in metals such as iron, manganese, nickel, copper, lead, zinc, arsenic, and mercury.

Analogs of the massive sulfide deposits of the East Pacific Rise are found in ophiolite sequences on land. Onshore ophiolites are layered sequences of volcanic rocks up to 6 miles thick that have lithological, structural, and geophysical characteristics similar to the crust beneath present-day ocean basins. Thus, ophiolites are thought to be fragments of ancient oceanic crust that formed at an oceanic spreading center and subsequently were transported away from the axis of spreading, uplifted, and placed onto the margins of continental land masses.

Sulfide deposits in ophiolite belts of the eastern Mediterranean region, particularly in Cyprus and Oman, have been important production centers for copper, iron, gold, and silver since 2500 B.C. Other large deposits of this type occur in Turkey, the Phillippines, Canada, the United States, and in several other countries. Ophiolitic terranes in the United States with massive sulfide deposits are found in California, Nevada southwestern Oregon, and Alaska. The massive sulfide deposits in ophiolites typically comprise 50,000 to 20 million tons of ore containing 0.5 to 10 percent copper, 0.5 to 3 percent zinc, and a few ounces of gold and silver per ton. In addition, several deposits may have important reserves of cobalt.

Like their counterparts on the East Pacific Rise and other modern oceanic spreading centers, the

massive sulfide deposits in ophiolites are associated originally with pillow lavas (so called because of their pillowlike shape) erupted on the sea floor. The deposits contain two types of mineralization: massive (discrete orebodies) and stockwork (vein-type) (fig. 3). Compact massive sulfide occurs in lenses or dish-shaped bodies interlayered with pillow lava or pillow lava fragments. The massive sulfide body is typically underlain by hydrothermally altered pillow lava and a network of interconnected veins or stockwork that extend downward in rootlike fashion into the ophiolite. The stockwork veins represent the channelways followed by orebearing hydrothermal fluids as they migrated upward toward the sea floor. The mineralogy of the massive sulfide in ophiolites is dominated by iron sulfide minerals with lesser amounts of copper and zinc minerals. Quartz is the principal accessory mineral in the massive ore.

FORMATION PROCESSES OF SUBMARINE MASSIVE SULFIDE DEPOSITS

The massive sulfides of the midocean ridges are hydrothermal deposits, that is, the heavy metals and sulfur were carried in dissolved form in deep-seated hot-water fluids and deposited when these fluids discharged as geysers on the sea floor. Chemical measurements of the hot water sampled along the East Pacific Rise site indicate the fluid was originally seawater that had undergone various chemical changes as a result of heating and chemical interaction with volcanic rocks beneath the sea floor.

The great oceanic spreading centers are one of the largest geologic features on Earth and account for the bulk of the Earth's volcanic activity. During the process of sea floor spreading, great amounts of heat are brought up from the mantle of the earth to the sea floor along these zones. Although of great linear extent, the actual zone of active lava production is narrow, usually only 1 mile or less in width. Such a geometry and the fact that the spreading center crests are generally more than 6000 feet below sea level gives rise to cooling of the spreading centers by convecting seawater. Such convection transports about 60 percent of the total heat introduced by sea-floor volcanism. The volcanic rocks on the sea floor are highly permeable to seawater, which is taken into the sea floor and absorbs heat from the top of the magma chamber along the spreading ridge axis. The intense pressures at these depths permits seawater to remain in the liquid state at approximately 800°F. Although boiling cannot occur

FIGURE 3.-Example of massive and stockwork sulfide mineralization found in ophiolites. The slab of massive sulfide shown above exhibits a crude layering and consists mainly of iron compounds (light-gray areas) and five minerals (dark-gray spots and layers). The sample shown below exhibits basalt cut by stockwork veins. The veins are composed of iron compounds (bright edges), quartz (white areas), and copper minerals (gray areas). The massive sulfide and veins represent sea-floor and subsea-floor deposition of metals respectively.

at these great pressures, the sea water with its dissolved minerals, is considerably expanded at the elevated temperatures. Its buoyancy becomes so high that it rises and is discharged onto the sea floor in a manner analogous to coffee in a percolator. Discharge of the ascending fluid is con

fined to a few localized and narrow channels that give rise to the isolated and rapidly discharging geysers observed on the sea floor.

The concentrations of heavy metals in normal seawater are extremely low, so low, in fact, that simply measuring them accurately has been one of the most difficult problems in marine chemistry during the past 30 years. Seawater at room temperature is slightly alkaline and, thus, has little or no tendency to react with and to dissolve metallic elements in the sea-floor rock.

Experiments conducted by Geological Survey scientists, however, have revealed that seawater changes from alkaline to acidic at temperatures above 575°F., when in contact with volcanic rocks. The acidity, in turn, acts on the rock to leach it of its heavy metals. The metals will remain dissolved as long as the acidity is maintained, and concentrations of the metals from the experiments are of the same magnitude as those found in the sea floor discharges.

Experiments further revealed that if flow rates are sufficiently slow, then acidity would be neutralized via reaction with successive amounts of unaltered volcanic rocks, and the metals would be then lost from solution. Thus, for a seafloor deposit to form by such a process, the flow rates must be high and must involve large volumes of water compared to the amount of rock altered.

On the sea floor, a dramatic reaction takes place where the hot acidic metal bearing fluid discharges into cold (35°F.) oxygenated alkaline seawater at the sea floor. Mixing one with the other produces a drastic and nearly instantaneous change in the chemical and physical conditions of the fluid, and a black plume is produced (fig. 4). The plume consists of iron, copper, and zinc sulfide minerals that have precipitated on mixing.

JUAN DE FUCA RIDGE

The Survey is initiating a program to better understand the mineral-forming processes at these spreading-center hydrothermal systems. The program will emphasize comparison of modern submarine hydrothermal deposits with ore deposits in ophiolite sequences exposed in the Western United States and Alaska. The study is designed to improve the ability to locate and define oneland sulfide mineral deposits in ophiolite sequences. The Juan de Fuca Ridge, 250 nautical miles west of the Oregon coast, will be the focal point of the Survey study of modern spreadingcenter mineralization. The Juan de Fuca Ridge has a separation rate of 2.5 inches per year, and

evidence of hydrothermal activity is already known. A series of cruises designed to locate, photograph, and sample fluids and mineral deposits along the Ridge crest was started in 1980.

The most recent of these cruises to the Juan de Fuca Ridge recovered samples from a hydrothermal vent area. The samples had typical values of about 55 percent zinc, 3,200 parts per million copper, 2,500 parts per million lead, and 300 parts per million silver. Deep-sea fauna observed in bottom photographs at the vent areas appear to differ in composition from the communities observed at the Galapagas and East Pacific Rise sites. Continued investigations of the Juan de Fuca deposits will enable the Survey to investigate the connection between presently forming and ancient ore deposits.

Offshore Hard Mineral Resources within U.S. Jurisdiction

The sea floor surrounding the United States, including Alaska, Hawaii, and the island territories, is the depository for vast quantities of hard mineral resources. These resources attracted growing interest from industry in the 1960's until moratoria on their leasing and exploitation on the Federal Outer Continental Shelf were emplaced, which has limited exploration and evaluation by government and industry. In early 1981, however, policy statements by the incoming Administration indicated the intention to include offshore hard

FERROMANAGANESE NODULES AND
METALLIFEROUS DEPOSITS

Manganese nodules in the Blake Plateau, off South Carolina and Florida were considered subeconomic up to 1978, despite the fact that they make up the largest potential U.S. resource of managanese. In the late 1960's, the nodules were the site of pilot studies by the Deep Sea Ventures Company, which recovered nodules from water approximately 3,000 feet deep using airlift techniques (fig. 2).

Renewed interest in the nodules located within the presumed U.S. exclusive economic zone has been spurred by industry requests for leases. Recent analytical studies by the Geological Survey have also shown that the Blake nodules have the highest platinum concentrations (nearly 0.5 part per million) of any oceanic nodules studies to date.

In the Pacific area, Geological Survey scientists edited a cooperative monograph on the prime Pacific nodules sites and have pointed out the economic potential of hydrothermal polymetallic ore deposits located along the Juan de Fuca Ridge. Additional interest centers on cobalt-rich nodules and crusts in seamount areas within the

FIGURE 1.-Percentage of imported (1979) mineral requirements of the United States. Arrows indicate minerals potentially available for sea floor and subsea-floor deposits within U.S. jurisdiction.

Percentage of U.S.imports, 1979 75

0

Fluoride
Nickel

? Potassium

probable U.S. exclusive economic zone around

the Hawaiian Islands and other island and coastal areas.

SAND AND GRAVEL

Aggregate materials for building and other construction are in short supply, or in potential short supply, in several coastal urban areas. In the New York Bight area, onshore reserves of sand and gravel will be depleted within the next 5 to 10 years. A critical situation may develop once existing deposits are depleted because net costs of construction aggregate are closely tied to transportation distance. However, economic models showed that offshore supply could

replace onshore sources and recover capital costs in less than 5 years. Sand resources on the Atlan

tic Continental Shelf of the United States were estimated to be adequate to supply coastal Atlantic States for several thousand years at current annual consumption rates of 150 million to 200 million tons per year (fig. 3).

Other areas where offshore sands and gravels could fill near-term needs include areas off Portland, Oregon, southern California, southern Florida, Hawaii, and the Virgin Islands. Off northern Alaska, material is needed in the construction of artificial islands to serve as oil exploration and development platforms 1 to 3 years after conclusion of lease sales in the Beaufort Sea.

PHOSPHORITE

New economic studies indicate that phosphorite deposits offshore South Carolina to Georgia may be economically competitive with