Prospecting and Exploration

The mining enterprise needs to secure satisfactory reserves, to guarantee production for the economic life of its existing mines. It also needs to locate new deposits to replace production from exhausted mines. Searching for new prospects, and investigating extensions of existing orebodies, are routines for the mining company.

The prospector looks at the surface lithology, observing irregularities. The geology may be established by pieces of coloured rock, mineralized blocks appearing in glacial drift areas, or sand in streams and waterways.

Such ocular inspection only identifies surface mineralization, and cannot be used in areas covered by alluvium or water. Where minerals show on surface, the downward extension is a matter of guesswork. Ancient miners may already have taken the most valu­able mineral.

Different geophysical methods are used to explore the bedrock, based on physical properties of rock and metal-bearing minerals. Magnetism, gravity, electrical conductivity, radioactivity, and sound velocity are used. Two or more geophysical methods are often combined in one survey, to acquire a more reliable database. Results from the surveys are compiled, and matched with geological information and records from core drilling, to decide if exploration activities are worth continuing.

Magnetic surveys measure variations in the Earth's magnetic field, caused by magnetic properties of subsurface rock formations. In prospecting for metallic minerals, these techniques are particularly useful for locating magnetite, pyhrrotite and ilmenite. Magnetic logging inside drill holes is also used, to obtain information for directing holes in core drilling programmes.

Electromagnetic surveys are based on variations of electrical conductivity in the rock mass. An electric conductor is used to create a primary alternating electromagnetic field. Induced currents produce a secondary field in the rock mass. The resultant field can be traced and measured, indicating the presence of a conductor. Electromagnetic surveys are mainly used to discover mineral deposits and to map geological structures. Sulphides containing copper and lead, magnetite, pyrite, graphite and certain manganese minerals, are examples of discoveries by electromagnetic surveys.

Electric surveys measure either the natural flow of electricity in the ground, or galvanic currents are directed into the ground, and accurately controlled. Electrical surveys are used to locate mineral deposits at shallow depth, map geological structures, determine the depth of overburden to bedrock, or identify the groundwater table. When prospecting for ore, the electric survey is often used to differentiate conducting zones located by electromagnetic surveys.

Gravimetric surveys detect small variations in the gravitational field caused by the pull of rock masses, which may be located several kilometres below the surface. The variation in gravity may be caused by faults, anticlines, and salt domes that are often associated with oil-bearing formations. Gravimetric surveys are also used to detect high-density minerals, like iron ore, pyrites and lead-zinc mineralization.

Radioactivity surveys measure the intensity of radiation of rock formations containing radioactive minerals, which will be considerably higher than the normal background level. Measuring radiation levels helps locate deposits containing uranium and thorium, and other minerals associated with radioactive substances.

Seismic surveys are based on variations of sound velocity experienced in different geological strata. The time is measured for sound to travel from a shock point to one or more detectors placed on the ground. The source of sound might be the blow of a sledgehammer, a heavy falling weight, a mechanical vibrator, or an explosive charge. Seismic surveys determine the quality of bedrock, and locate the upper surface of compact mineral deposits deep in the ground, and are also used to locate oil-bearing strata.

Geochemical survey is a comparatively recent technology, featuring several specialities. One is to investigate the presence of various metals in the topsoil cover. By taking numerous samples over a large surface area, and analyzing for minute traces of each metal, regions of interest are identified. Areas can then be delineated for more detailed studies.

Date: 2015-12-24; view: 633