Gold in meteorites and in the earth's crust

By Robert S. Jones

GEOLOGICAL SURVEY CIRCULAR United States Department of the Interior

TABLES

GOLD IN METEORITES AND IN THE EARTH'S CRUST

This report is one of several that summarize available data on the occurrence of gold. They have been prepared as background material for the Heavy Metals program of the U.S. Geological Survey, an intensified program of search for new sources of heavy metals, including gold.

Data on the occurrence of gold in meteorites and tektites are summarized, and recent estimates of the abundance of gold in the earth's crust are compiled.

Table 1 shows reported gold contents of tektites, aerolites, siderolites, and siderites. The table is arranged so that the data on tektites, which have the lowest iron contents, are at the top of the table and the data on siderites, which have the highest iron contents, are at the bottom. The other meteorite groups are intermediate in iron contents except for the siderolites. Gold is most abundant in the siderites and least abundant in the tektites; therefore, meteorites supply good evidence of the siderophilic character of gold. The tektites and the achondrites are relatively low in gold contents and are distinct from the other groups of meteorites in this respect. The gold contents of tektites and achondrites are of the same order of magnitude as those of terrestrial rocks. The other meteorites, on the average, contain appreciably more gold.

Although the iron contents of meteorites are similar in many respects to those of mafic and ultramafic rocks, the meteorites tend to contain much more gold. In chondrites, the golc, seems to be almost entirely in the dispersed netallic phase (Vincent and Crocket, 1960), and this is probably true of the other meteorite? .. The gold content of the metallic phase of the chondrites is about 1.4 ppm (parts per million), which is similar to the gold contents of siderolites, octahedrites, and ataxites (Vincent and Crocket, 1960; Goldberg and others, 19£~1).

The carbonaceous chondrites are primitive, relatively undifferentiated matter from which the other meteoritic types have evolved (Mason, 1962; Baedecker and Ehmann, 1965). The occurrence of gold in such primitive types may be of special interest. The average gold content for 13 carbonaceous chondrites is 0.16 ppm, an amount greater than that in the average terrestrial rock by a ratio of about 40 to 1.

It has been suggested (Aller, 1961) tl'at the best approximation to the average comp')sition of the earth's mantle or even the entire earth is provided by the composition of the chordrites. They are similar in chemical composition to the ultramafic rocks, and their isotopic constitution for several elements is basically the sr.me as that for the rocks of the mantle.

Fouche and Smales. (1966) analyzed 70 siderites and found gold contents that ranged from 0.055 to 3.61 ppm. The correlation coefficients between gold and rhenium and between gold and chromium were low and negative, giving values of -0.41 and - 0.31, respectively, but the correlation between gold and arsenic was +0.82 and between gold and palladium +0.68.

Goldschmidt and Peters (1932) analyzed the Coahuila, Mexico, meteorite and reported that it contained 1 to 5 ppm gold, whereas analysis by the neutron-activation method by Goldberg, Uchiyama, and Brown (1951) gave 0.743 ppm gold; by Fouche and Smales (1966), 0.70 ppm gold; and by Cobb (1967), 0.43 ppm gold. Goldschmidt and Peters (1932), analyzed the Mount Joy, Pa., meteorite and reported that it contained 5 to 10 ppm gold, whereas analysis by the neutron-activation method by Goldberg, Uchiyama, and Brown (1951) gave 0.994 ppm gold. These comparative values along with others in this report seem to indicate that lower values are obtained for gold when neutron-activation methods are used.

Parker (1967) has pointed out the difficulty in estimating the composition of the earth's crust, which forms less than 1 percent of the earth's mass (Aller, 1961). Differences among the estimates given by various authors since Clarke and Washington (1924) are due partly to different concepts of what constitutes the earth's crust, the depth to the Mohorovicic discontinuity, the composition of the oceanic crust compared with the continental cru~t, and the changes in crustal composition with depth. Also, with respect to gold specificall:", the newer method of analysis, that of neutrc n activation, has resulted in a general downward revision of gold values.

The estimates of gold and silver in the ~arth's crust have varied little since those of Clarke and Washington in 1924, although the estimates for platinum have varied substantially. The Ag :Pt :Au ratios, based on Tung and Chi-Lung's (1966) figures, are 21 :13:1.

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