Abstract

Samples of normal soils, and soils near metalliferous veins were tested to investigate the behavior of copper, lead, and zinc during the formation of residual soil. The copper, lead, and zinc content of unmineralized bed rock ranges from 20 to 200 parts per million of each metal, and the content of these metals is of the same general magnitude in normal soil. The content of these metals is only slightly higher in soils from hillsides just above most of the fourteen veins studied. The soil directly over or immediately downhill from a vein commonly contains much higher concentrations of metal and may contain as much as 10,000 parts per million (1 percent). The ratios of the highest to the lowest metal value determined near each vein range up to 106 to 1 for copper, 170 to 1 for lead, and 11 to 1 for zinc. The range in metal content in the soils is high enough to permit easy detection of most of the geochemical anomalies by quick or "field-type" analytical methods.The geochemical anomaly in soil near veins is broad and low in contrast to the sharp, high anomaly in the unweathered rock. The anomaly in soil is characteristically asymmetric, with normal or "background" values just uphill from the vein, an abrupt high near the vein, and progressively lower values extending as much as several hundred feet downhill. In general, the anomalies in soils have characteristics that can be explained by assuming a simple mechanical mixing of the vein and wall rock materials during weathering, and a downhill creep of the weathered products.At some veins, chemical analysis of soils revealed the presence of significant quantities of ore metals for which no corresponding ore minerals were identified in the vein outcrop. Chemical analysis of soils also revealed the location of other veins even though they were completely hidden by residual soil. Two examples are described where chemical methods were used to locate concealed extensions of known veins. Prospecting by sampling surface soil at 50- to 100-foot intervals, and analyzing by quick tests apparently can locate some veins concealed by residual soil; however, there are many problems that must be solved before the limitations of this prospecting method can be defined adequately.

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