Abstract

At San Manuel, near Tucson, Arizona, recent churn drilling has blocked out large reserves of low-grade "porphyry copper" ore. This virgin deposit has a small outcrop and seems ideally suited for a geochemical study of the dispersion pattern produced by weathering in a desert climate. Samples of soils, alluvium, ground water, and vegetation were analyzed for copper. To avoid grinding, to accentuate differences in copper concentration, and to decrease sampling error, sampling was confined to the silt and clay fraction of the soil and alluvium. A sensitive field test for copper, using hydrochloric acid for a digestant and dithizone for copper determination, proved both quick and reliable.The results of the study show that at present little copper from the ore body dissolves in the ground water or runoff, and also that very little is taken up by plants growing on the ore outcrop. For this reason, prospecting for similar deposits by the sampling and analysis of ground water or vegetation is unlikely to be fruitful. A study of plant ecology, however, shows that certain plant species grow preferentially on outcrops of copper ore and may be useful as indicators of ore.Chrysocolla along joints carries most of the copper in the oxidized zone. The chrysocolla is slowly disintegrated mechanically as the rock weathers and enters the fine fraction of the soil. Slope wash and soil creep carry the copper along with the other soil materials into the nearby washes, where they are incorporated in the alluvium and swept downstream during floods. Abrasion probably continues to concentrate the copper in the fines, but the net change downstream is a decrease in copper content caused by dilution.As copper can now be readily determined by chemical analysis in the field, analysis of soil and alluvium seems to be one of the best geochemical methods of prospecting for copper in a desert environment. Wherever appreciable copper is found in alluvium or soils, upstream or upslope sampling can be used to trace the copper back to its source.

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