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Principles of Geochemical Prospecting GEOLOGICAL SURVEY BULLETIN 1000-F CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS PRINCIPLES OF GEOCHEMICAL PROSPECTING By H. E. HAWKES ABSTRACT Geochemical prospecting for minerals includes any method of mineral exploration based on systematic measurement of the chemical properties of a naturally occurring material. The purpose of the measurements is the location of geochemical anomalies or of areas where the chemical pattern indicates the presence of ore in the vicinity. Anomalies may be formed either at depth by igneous and metamorphic processes or at the earth's surface by agents of weathering, erosion, and surficial transportation. Geochemical anomalies of deep-seated origin primary anomalies may result from (1) apparent local variation in the original composition of the earth's crust, defining a distinctive "geochemical province" especially favor­ able for the occurrence of ore, (2) impregnation of rocks by mineralizing fluids related to ore formation, and (3) dispersion of volatile elements transported in gaseous form. Anomalies of surficial origin-^secondary anomalies take the form either of residual materials from weathering of rocks and ores in place or of material dispersed from the ore deposit by gravity, moving water, or glacial ice. The mobility of an element, or tendency for it to migrate in the.surficial environment, determines the characteristics of the geochemical anomalies it can form. Water is the principal transporting agency for the products of weathering. Mobility is, therefore, closely related to the tendency of an element to be stable in water-soluble form. The chemical factors affecting the mobility of elements include hydrogen-ion concentration, solubility of salts, coprecipitation, sorption, oxidation potential, and the formation of complexes and colloidal solutions. The mobility of the elements may be further modified by biological factors. Secondary anomalies may occur in residual materials or in materials transported by ice, frost, underground water, animals, soil-forming processes, plant activity, and surface water. Each one of these transporting agencies gives a characteristic distribution pattern to the weathering products of ore deposits. Geochemical methods have been applied most extensively in the Soviet Union, Scandinavia, the United States, Canada, Africa, and Japan. The most uniformly successful geochemical prospecting work has been based on sampling and analysis of residual soil and vegetation; anomalies caused by 225 226 movement of metals in ground and surface water show promise as an ' effective means of locating buried ore deposits. Some suggestions for the execution of geochemical surveys and the interpretation of geochemical dataV«"* in terms of possible ore are presented. h INTRODUCTION Geochemical prospecting for minerals, as denned by common V usage, includes any method of mineral exploration based on sys­ tematic measurement of one or more chemical properties of a *" naturally occurring material. The chemical property measured is most commonly the trace content of some element or group of elements; the naturally occurring material may be rock, soil, gos­ san, glacial debris, vegetation, stream sediment, or water. The purpose of the measurements is the discovery of a geochemical "anomaly" or area where the chemical pattern indicates the pres­ ence of ore in the vicinity. * The broader science of geochemistry, as originally defined by f Goldschmidt and summarized by Mason (1952, p. 2), is concerned with: 1. The determination of the relative and absolute abundance of the ele­ ments * * * in the earth. 2. The study of the distribution and migration of the individual elements «. in the various parts of the earth * * * with the object of discovering the principles governing this distribution and migration. * The application of the principles of .geochemistry to practical > problems in mineral exploration requires the same basic approach. It is built upon the investigation of the average abundance of certain elements in earth materials and of the laws that govern A the distribution of elements and the formation of geochemical anomalies. A thorough understanding of geochemical anomalies, * how they form, how to find them, and how to appraise them when they are found that is, an understanding of some of the funda- ( mental laws of geochemistry is essential in effective geochemical prospecting. This report is intended primarily as a review of the most im­ portant geochemical principles involved in the evolution of geo- * chemical anomalies. A section has also been included on the his­ torical development and application of .geocnemical methods Of prOSpeCting. The data Of geochemical prospecting surveys will be reviewed only insofar as they illustrate the principles of geo- (_ chemical dispersion; detailed information on the results of geo­ chemical prospecting surveys may be found in the reviews cited ^ in Harbaugh (1953) or the literature reviewed. The methods of chemical analysis used in geochemical prospecting are covered in PRINCIPLES OF GEOCHEMICAL PROSPECTING 227 other publications. In this report only incidental reference is made to geochemical methods of exploration for petroleum. A glossary of terms and conversion tables for units of measurement are in­ cluded at the end of the report. This report summarizes the results and conclusions of many workers over a period of 20 years. The intent has been to give full acknowledgment by reference to the literature or other nota­ tion for all ideas and data that have gone into the preparation of this summary. Many of the concepts, however, have grown out of personal conversations and contacts where the writer may have adopted the ideas of others without realizing the source. In this regard, special mention should be made of the intangible contri­ butions of W. H. Newhouse, T. S. Lovering, L. C. Huff, Helen Cannon, H. W. Lakin, V. P. Sokoloff, F. C. Oanney, V. C. Kennedy, J. S. Webb, and J. E. Riddell. The literature used in assembling the data for this report includes only that published before March 1954. ABUNDANCE OF ELEMENTS IN EARTH MATERIALS Geochemical anomalies, indicative of commercially important concentrations of elements, can be recognized only by their con­ trast with unmineralized areas. The normal abundance of an ele­ ment in any material where the equilibrium has not been upset by the presence of a mineral deposit is commonly referred to as "background." Background values can vary widely owing to natural chemical and physical processes whereby certain elements are enriched and others correspondingly impoverished. For a general guide to the range of background values that may be ex­ pected, it is helpful to refer to data that have been compiled on the composition of average igneous rocks, fresh water, and terrestrial plants (table 1). The data on igneous rocks and terrestrial plants have been obtained from published estimates. The data in table 1 on the major element composition of fresh river water have been taken from estimates by Con way (1942) and Polynov (1937). Estimates for the minor constituents of fresh water have been newly computed from individual published analy­ ses of representative river waters, as summarized in table 2. So far as the writer is competent to judge, only the most reliable data were selected. Because of the wide range in the reported content of many elements in surface waters, the median value as well as the average was computed. As Ahrens (1954) has pointed out, the median of a series of widely scattered values may give a more sig­ nificant indication of the normal value than the arithmetic mean, .228 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS -Abundance of some dements in igneous rocks, fresh water, and terrestrial plants Fresh water 2 Terrestrial plants (ppm) 3 Igneous Element rocks ' (ppm) Water (micro- Residue Dry grains per liter) (ppm) weight Ash Aluminum __ . 81,300 700 4,800 20 800 Antimony. _ _ _ 1 5 1 7 250 160 1,100 30 1,200 Beryllium. _ ____ 6 Bismuth. ________ .2 Boron. _______ . 3 110 750 5 200 Cadmium. __ _ . .15 Calcium _ _ ... 36,300 29,600 202,700 5,000 200,000 200 5.3 36 Cobalt........... 23 4.9 33 Copper _ _. ... 70 20-70 140-480 4 5 4 200 Fluorine 600-900 250 1,700 Gallium _ ...... 15 Germanium ..... 7 Gold.. __._...... .005 .0033 .023 .1 Iron.. ______ ... 50,000 510 3, £00 Lead. _______ . 16 3.4 23 65 .1 4 Magnesium _ .... 20,900 4,400 30,300 700 28,000 Manganese. 1,000 52 360 0.077-0.5 .05 .34 2.5-15 .5 20 Nickel.... _______ 80 15 ioo Niobium __ 24 Platinum. .005 25,900 2,900 20,200 3,000 120,000 120 .4 16 Rhenium _ ______ .001 310 2 80 .09 8 55 277,200 8,000 54,500 1,500 60,000 ftjlvpT .1 .0071 .048 28,300 3,800 26,300 200 8,000 300 20 800 Sulfur.. ......... 520 5,500 37,900 500 20,000 Tantalum __ ... 2.1 Tellurium........ .0018 Thallium _ ..... 5 1.3 Tin..... ....... ._ 40 Titanium ... 4,400 25 170 ~2 80 Tungsten.. ______ 1.5-69 150 .2 8 Zinc. ---._ 132 ~I6 ~70 4 32 4 1,300 220 1 Rankama and Sahama (1950, p. 39-40), except as noted. a For sources, see table 2. 'Hutchinson (1943) except as noted; ash computed on basis of 2.5 percent average ash content ot errestrial plants. « Shaw (1952). TABLE 2. Source data for abundance of elements in fresh water Fresh water Number Element of Water (micrograms per liter) Residue (ppm) ' Reference samples Average Median 2 Range * Average Median (1) (2) (3) (4) (5) Aluminum 700 4,800 Polynov (1937). Arsenic - 37 1 <3 7 Kehoe and others (1944). Barium. _ 27 160 120 50-300 1,100 820 Braidech and Emery (1935); Wilska (1952). Boron. 51 110 80 20-lcO 750 550 Braidech and Emery (1935); Huberty and others (1945).

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