Suggestions for Prospecting

U.S. Geological Survey / Department of the Interior Suggestions for Prospecting ^r< Suggestions for Prospecting For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Prospectors have contributed have been prospected although much to the development of this perhaps only at a reconnaissance Nation's mineral resources. Since scale. the time of the earliest settlement, Nearly one billion tons of surface the need for iron for tools and guns, and underground material are lead for bullets, and copper for mined annually in the United States utensils has prompted a search for to recover about one-half billion sources of these metals. The lure of tons of metallic ores, principally iron gold and silver provided the im and copper(U.S. Bureau of Mines' petus for much of the development "Minerals Yearbook, 1986," 1988, in the West between 1850 and v. 1, p. 25). Even greater amounts 1910. Later, prospectors carried out of ore must be found in the future successful ventures to fulfill the to meet the Nation's increasing country's expanding industrial needs and to replace exhausted demands for other metals such as deposits. The easily found deposits zinc, molybdenum, tungsten, have already been discovered; suc chromium, vanadium, and many cess in finding new deposits will de others. Even America's uninhabited pend more and more on modern rugged mountains or barren deserts prospecting techniques. IPir©§ij3®<sttiiinici] T@ U ^J Mining, Mineral, and Related The modern prospector has ad Terms" (Thrush and others, 1968), vantages that to some extent make is "a mineral deposit that can be up for the increased difficulty of worked at a profit under existing finding ore deposits. One advantage economic conditions." is greatly increased knowledge Remote Sensing is a relatively about the geologic factors that new method of mineral exploration, localize ore deposition. The search utilizing imagery of the Earth's sur for new deposits has become a face collected by instruments on air complex undertaking, and the pros craft and Earth-orbiting satellites. pector should be as well-informed Imagery of radar, color infrared, as possible. The prospector should thermal infrared, and other reflected acquire the ability to identify not or radiated electromagnetic energy, only ore minerals but also common can show features such as struc rocks and their minerals, as well as ture, vegetation, and rock types that many kinds of geologic structures. may not be discernable by conven This knowledge is best acquired by tional ground-based prospecting academic training, but much can be methods. learned from studying reference Prospecting equipment can be books such as "Exploration and used in many ways, according to Mining Geology" (Peters, 1987), the minerals sought and the meth "Handbook for Prospectors" (Pearl, ods employed in the search. Its 1973), and others listed at the end effectiveness depends in large of this booklet. measure on the operator's aware The first technique used in a ness of its applications. For exam prospecting venture is geological in ple, radiation counters (the scin- ference. The prospector studies re tillometer, for example) not only ports, geologic maps, and cross detect radioactive minerals for the sections of a region to pinpoint uranium prospector, but can also be areas where there are structures, useful to placer prospectors. Some rocks, and minerals with which ores placer deposits contain both gold are usually associated. These areas and heavy radioactive minerals warrant further exploration. Topo such as monazite; the radiation graphic maps or aerial photographs counter points out gold concentra of these targeted areas are ob tions by detecting the associated tained and used in plotting informa monazite. A "black light" (ultraviolet tion, such as locations for sampling. lamp), commonly used in prospec This booklet briefly describes basic ting for fluorescent ore minerals prospecting techniques geochem- such as scheelite (a tungsten ore), ical, geophysical, and combination is also useful in detecting fluores methods that can help lead to the cent rock-forming minerals such as discovery of an orebody. An calcite, barite, or fluorite, which can orebody, as defined by the U.S. be indicators of associated metallic Bureau of Mines' "Dictionary of minerals. presence remains. Commonly, iron oxides and iron hydroxides form and remain as brown or yellow stains and encrustations on the sur face rocks. The solution of minerals can result in a sponge-like, iron- stained, porous rock called gossan evidence that primary minerals have been oxidized and at least partly removed. A search for these minerals at depth in such places may be fruitful. Mineral com pounds containing copper, nickel, cobalt, molybdenum, uranium, and other metals may oxidize to form brightly-colored secondary minerals on the surface rocks. These minerals are found mainly in dry regions because many of them are Scintillometer soluble in water and would be leached away in areas of heavy rainfall. The residues left in rocks after Mineral deposits have also been ore minerals have been removed by located by following float (pieces of weathering also may be clues to ore rock found on the ground surface) deposits. The rusting of iron is a uphill to its source. When rocks are familiar example of the change in being eroded, fragments are carried materials exposed to weathering. downhill by gravity and downstream Primary ore minerals (that is, by water. The source of the float minerals deposited in the rock for can be found by tracing the pieces mation by ascending ore-forming up slope. solutions) near the surface may be The systematic panning of stream oxidized and carried in solution sediment and residual soil for gold downward from their original posi or other resistant heavy minerals tion. They may be either redepos- has long been used to find bedrock ited as secondary minerals in the lodes. The gold pan, an indispen rocks below, thus enriching lower sable prospecting tool, is versatile, parts of the mineral deposit; or the efficient, inexpensive and portable. minerals may be dissolved and From a sample, an experienced completely removed, resulting in a panner can recover 80 percent of process of depletion rather than the minerals that are heavier than enrichment. quartz (specific gravity 2.65). Even Regardless of what happens to an inexperienced person can obtain the ore minerals during weathering, a concentrate of heavy minerals some evidence of their former from an ore-bearing sample that will contain virtually all of the sample's as barite, zircon, sphene, monazite, coarse gold and platinum, and a and scheelite, in addition to the high percentage of its magnetite darker minerals such as magnetite, (specific gravity 5.2) and heavier ilmenite, hematite, and sulfides. All minerals such as cassiterite, these minerals can be clues to cerussite, coulumbite-tantalite, valuable deposits in nearby or scheelite, and silver. Many minerals upstream areas. such as hornblende, biotite, mus- Other devices can be used in covite, epidote, garnet, pyrite, cin conjunction with a gold pan to test nabar, and galena are easily large samples; dozens of varieties recognized during the washing proc of sluiceboxes, rockers, suction ess, and the skillful panner can devices, and spiral concentrators recover most of these minerals are available from prospecting and present in a sample. A sample that mining equipment dealers. An inex is "panned down" to "black sands" pensive portable sluicebox for use generally also contains a number of in sampling placer deposits can be light-colored heavy minerals such made from three pieces of 1/8-inch sheet aluminum, each 3 feet long Geochemical Prospecting and about 1 1/2 feet wide. The Geochemical prospecting is based metal is shaped into flat-bottomed on systematic measurement of troughs about 1 foot wide with sides chemical properties of rock, soil, 3 inches high; the three sections glacial debris, stream sediment, are bolted end-to-end to form a water, or plants. The chemical prop 9-foot sluice. The bottom of the erty most commonly measured is sluice box is fitted with carpet, the content of a key trace element. burlap, wooden cross-slats or riffles, Zones in the soils or rocks of com or other material to trap fine heavy paratively high, or anomalous, con mineral particles. Covering the centrations of particular elements material with a coarse screen may guide the prospector to the prevents pebbles from clogging the elements in rocks or soils that con trapping material. Water is delivered stitute a geochemical anomaly (dif to the sluicebox by means of a ferent from normal). The actual pump driven by a small gasoline amount of the key element in a engine, or the sluicebox can be sample may be very small and yet placed in the stream to use the constitute an anomaly if the sam natural flow of water. The bottom ple's concentration is high relative end of the sluicebox should be part to the concentration of the surround ly closed to slow down the water ing area. For example, if most flow and to aid in trapping heavier samples of soil are found to contain particles. The carpet or burlap con about 0.00001 percent (0.1 parts taining the heavy mineral concen per million, or ppm) silver, but a few trate is removed from time to time, contain as much as 0.0001 percent placed in a tub, and thoroughly (1 ppm), the few "high" concentra washed. The washings are then fur tions are geochemical anomalies. ther concentrated by panning. Plots of analytical results on a map Working diligently, one can wash a may indicate zones to be explored ton of gravel in a day and expect to further. recover 50 percent or more of the Geochemical anomalies are black sands present.

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