Exploration

Exploration for mineral deposits. The result of prospecting is the discovery of potentially economic mineralization, that is, the prospect. Mineral exploration continues beyond prospecting to include the delineation and evaluation of the prospect to determine its minability as an orebody or economic mineral deposit. A successful prospect is developed into a mine.

Prospecting generally pertains to the search for deposits of metallic ore minerals, but it also includes the search for nonmetallic or industrial minerals and rocks such as sulfur, potash, and limestone, and mineral fuels such as petroleum, coal, and oil shale.

With much of the Earth's readily accessible surface having been investigated for minerals, prospecting is increasingly directed toward the discovery of deeper mineralization in recognized mining districts; mineralization hidden beneath overlying rocks, sediments, and soils; and mineralization in the less-known jungle, arctic, and offshore parts of the world. See also Marine mining; Oil and gas, offshore. 

Prospecting is done on the basis of the guides to ore associated with a conceptual image of the anticipated orebody. The image is referred to as an exploration model, and it is drawn from the characteristics of known orebodies in similar terrain. The exploration model and its guides to ore are expressed in terms of the regional and local geologic pattern; it has a certain diagnostic mineralogical character, it will commonly have a halo or envelope of associated guide minerals, and it will be expected to have a recognizable geochemical and geophysical expression.

The topography itself may give evidence of abrupt depressions related to the leaching and collapse of sulfide ore minerals, or it may show boldly exposed silicified zones associated with ore. Some of the latter expressions of ore mineralization represent outcrops of siliceous iron formation host rocks, quartz-filled breccia pipes, and the prospector's classic quartz reefs that indicate vein deposits. Aerial photography and satellite imagery are valuable in searching for the topographic expression of potential ore mineralization.

Outcrops of gossan (the residue of red, brown, and yellow iron oxides and silica that remains from the weathering and near-surface leaching of sulfide ore minerals) are examined in the field for evidence of underlying ore mineralization, and trains of float (fragments of ore and gossan) are traced toward their apparent topographic origin. In glaciated terrain, trains of ore boulders are mapped and traced systematically toward their apparent sources.

Placer gold and placer accumulations of other minerals such as platinum, cassiterite, rutile, and diamonds are sought as economic deposits in themselves and are used as guides to upstream deposits of associated minerals. In addition, resistant and relatively dense minerals in stream gravels and residual heavy minerals in soil are collected by the long-established prospector's method of panning the loose material, and these are traced to a source area.

Geochemical prospecting is based on two characteristics of orebodies: an association with anomalous concentrations of chemical elements within primary halos in the surrounding rock, and an association with secondary dispersal patterns of chemical elements in the surficial products of their weathering and erosion. Geochemical methods involve the field and laboratory analysis of sampled rock, soil, vegetation, and other natural materials for trace amounts (in parts per million or billion) of the principal indicator elements of an orebody and of the related pathfinder elements that provide more recognizable or farther-reaching anomalies.

Imagery provided by remote sensing from aircraft and orbiting satellites is of fundamental importance in prospecting and in the patterns of exploration data associated with geographical information systems. Aerial photography in spectral bands of the near-infrared and near-ultraviolet frequencies is also used in photogeology for discriminating between types of exposed rock and soil and for emphasizing the appearance of bleached and stained areas as well as geobotanical anomalies. Airborne remote-sensing systems have provided radar imagery of terrain in the prospecting of cloud-covered jungle regions, and they have furnished thermal-band infrared surveys for recognizing anomalously warm areas that may be associated with mineralization. Airborne multispectral sensors with the capability of identifying some of the specific kinds of minerals in altered zones have been tested for use in prospecting.

Geophysical exploration is based on the measurement of physical properties associated with geologic features. As a means of both airborne and ground prospecting for mineral deposits, it involves the recognition of contrasts in properties between the deposit and the adjacent rock, generally to depths on the order of 330–660 ft (100–200 m), and the definition of deeper structural and lithologic features to be used as guides to ore mineralization. Magnetic, electrical, electromagnetic, and radioactive methods are the most widely used in prospecting for ore and industrial minerals deposits. Geophysical surveys are often made by several methods, so that more than one physical property can be taken into account.

Drilling is the principal method of subsurface prospecting where evidence of ore mineralization and geophysical or geochemical anomalies indicates a target for prospecting at a depth of more than a few feet. Geophysical information is obtained by the probing or logging of drill holes. Electrical and electromagnetic logging is done in holes drilled in search of metallic orebodies; with these methods, the radius of search is extended considerably beyond that of the small-diameter cylinder of sampled rock. Gamma-ray methods of geophysical drill-hole logging have become standard practice in prospecting for uranium ore.