ers continue to discover and develop new deposits. An ore body is defined as a new class of deposit if, in the absence of knowledge of its mineral content, an experienced exploration geologist would not recognize its economic potential. Even with current supposedly sophisticated knowledge of the Earth, new classes of deposits continue to be discovered. Classes recognized for the first time during the past 30 years include unconformity-related uranium, Olympic Dam type of ore, Carlin-type gold, salt-dome sulfides, and sediment-hosted tungsten.

Unconformity-related uranium deposits contain the largest known concentrations of uranium. The world's richest and largest known deposit is in Canada at Cigar Lake, Saskatchewan. It contains 150,000 metric tons of uranium metal in the form of high-grade ore—8 percent U3O8. This type of deposit also occurs in Africa and Australia but was not recognized until 1968.

The Olympic Dam deposit, an iron-rich breccia in South Australia, contains more than 1 billion tons of ore rich in copper, gold, uranium, and rare earths. No portion of the deposit was exposed, and it was discovered by drilling located on subsurface regional lineaments and magnetic signatures.

The Carlin-type gold deposits of Nevada are low-grade, large-volume deposits containing very fine-grained gold. Now the source of most of the current U.S. gold production, they were recognized only recently, although a few high-grade zones had been mined in the region during the past century. A major contributor to the successful production from such low-grade resources has been the advent of heap or pad leaching, which extracts the gold from the ore at minimal cost.

The cap rock of salt domes in the Texas Gulf Coast has been found to contain lead, zinc, and silver in minerals whose quantities and grades approach the profitable. The existence of similar sulfide concentrations in other salt-dome cap rocks is anticipated.

The largest known tungsten deposit in Europe, at Felbertal, Austria, was discovered relatively recently in metamorphosed sedimentary and volcanic rocks. This deposit of sediment-hosted tungsten is remote from the igneous rocks with which other tungsten mineralization is closely associated.

Thus, nonconventional ore deposits continue to be found, and prospects continue for discovering additional unique mineral concentrations. Such ore bodies provide a stimulus for continually revising the conceptual models of mineral deposits. Revisions either validate hypotheses embodied in existing models or generate completely new models and hypotheses to be tested.

Studies that build models for mineral exploration have several similarities with petroleum studies—questions about source, migration, and traps. However, mineral deposits are generally more complex because there is not a single source material, as there is in petroleum formation. Minerals may originate in the mantle or in the crust and may represent concentrations in a particular environment, such as dissolved salts in the ocean, which are concentrated by evaporation until they form salt deposits called evaporites. Transport processes are equally varied, as are controls on the deposition of mineral accumulations.

The variety of possible formation environments for copper deposits can illustrate the complexity of mineralization processes in contrast to the relative simplicity of petroleum development. Major copper deposits occur in primary or altered sedimentary environments, in veins within rocks of all types, in products of seafloor hot-spring vents, in segregates from mafic lavas as native copper, and in other distinct geological settings. This multiplicity of host environments suggests a variety of deposition and transport processes, which complicates the task of inventorying copper resources. Deposit types for other metals are just as diverse, requiring the design of various models and consideration of many alternatives. The source, transport, and accumulation processes that concentrate minerals in valuable deposits are seldom fully understood. New ideas that seem to answer outstanding questions form the bases for new models.

Transport, or migration, involves two basic steps. The first is mobilization of materials by means of a carrier, such as the solution of metals from source rocks into aqueous fluids or igneous melts. The second step involves movement of the carrier to the site of deposition. Research into transport by aqueous fluid is based on the recognition that the common ore minerals are extremely insoluble in pure water and that complex ions are required to increase solubility. Most of the ore minerals in vein deposits are sulfides, so sulfur chemistry is particularly important. Aqueous sulfur chemistry is strongly influenced by an environment's oxidation-reduction status and by its acidity. Reactions with wallrock minerals, water from sources including groundwaters, and loss of boiling gases are important factors in metal transport and eventual deposition. At temperatures above 400°C, ionic links tend to be replaced by molecular links, and solution chemistry becomes simpler in concept but experimentally very challenging.

Transport by means of igneous melt is even more



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