complicated. Magmas consist of partly molten rock, and they transport a variety of elements, including tin, molybdenum, iron, nickel, copper, the rare earths, and the platinum group. Such metals may then either precipitate directly from the magmas into magmatic deposits or be incorporated into aqueous fluids as the magmas solidify.

Whether aqueous fluid or igneous melt acts as the carrier in transport, the mineral must reach a trap and accumulate in significant concentrations before a deposit can be considered economical. These traps are the targets of mineral exploration. A great deal of progress has been made in quantitatively modeling individual accumulation processes, such as boiling, cooling, mixing with dilute waters, reactions with wallrock, adiabatic expansion, and sulfurization of metal-bearing magmas. These individual process models require considerable additional data before they can be integrated into a comprehensive form that can handle all the known variables together.

Computers, using data bases on both fluid and mineral characteristics, have enabled modeling the complex geochemical and hydrologic processes related to ore formation. For example, the flow paths, flow rates, and residence times for fluids can be predicted, given a geological starting point such as a sedimentation interval within a basin or the intrusion of a granite magma into a sequence of rocks. The anticipated temperatures, pressures, and minerals can be calculated for every point within a flow system. This exercise is analogous to watching a mineral deposit form. It is also an experiment ideally suited to predicting the behavior of chemical wastes injected into rocks and of fluids used for in situ mining.

Mineral Exploration and Exploitation

Mineral deposits display great diversity in material; grade; size; and style of localization, accessibility, and minability. These largely independent variables complicate the search for profitable mineral concentrations. The ideas developed by early geoscientists about the origin of mineral deposits were based on surmise because only a few cases of ore formation could be observed directly. These few included placers forming in streams, volcanic fumaroles yielding sulfur and metallic sublimates, and dried playa lakes and marine lagoons depositing salt. Additional examples of direct observation of ore formation have been recognized in recent years as exploration of our planet has reached into even the most hostile environments (see Figure 4.10).

Since 1979 spectacular discoveries of huge thermal springs have been made at spreading centers along the mid-oceanic ridges in the Pacific and Atlantic oceans. Heat from subseafloor basalt creates a massive circulation of hot seawater, and reactions between the water and the basalt form acidic solutions. Leached metals and sulfur from the rock are deposited from vents on the seafloor above the spreading center to form cones containing iron, copper, zinc, gold, and other metals. Active vents, which range in temperature up to 350°C, are characterized by rising plumes of rapidly precipitating sulfides, graphically called black smokers. These precipitates resemble the deposits hosting the copper ores that gave Cyprus its name as well as other minerals valued since ancient times. These long-familiar mineral deposits are now recognized as ocean-floor rocks formed at extinct spreading centers and then incorporated into the continents at arc and continental collision sites. Variants occur in Mexico's Gulf of California and off the continental shelf west of Washington and Oregon. Close to continents, active sedimentation buries the spreading center, resulting in mineralization that is disseminated throughout the sediments as well as around the black smokers on the seafloor.

Of even greater economic interest are the vents associated with rifts along the crests of island arcs, such as that in the China Sea west of Okinawa; they are associated with magmas that are richer in silica than ocean basalts. These magmatic heat engines are apparently larger and longer lived than those immediately above oceanic crust and could produce deposits similar to the Kuroko-type massive sulfides, rich in copper, zinc, lead, silver, and gold. The Kuroko deposits in northern Honshu, Japan, formed about 13-million-years ago when a rifting episode split the axis of the island arc. Oceanic waters flooded the rift to a depth of more than a kilometer, and the massive sulfides were deposited where seawater and magma interacted.

Another mineral deposition environment occurs in deep depressions along the axis of the Red Sea. The water descending to the heat source along that axis dissolves large amounts of rock salt from deposits buried along the margins of the sea. The resulting brine forms dense pools at the bottom of the Red Sea. Large amounts of zinc and iron are precipitating in the brine pools, demonstrating that a deep stratified brine environment may have been a progenitor of the ancient strata-bound ore deposits exposed on the continents.

Active thermal springs and solutions encountered in the course of drilling for geothermal power



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement