CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS GEOCHEMICAL EXPLORATION DISTRIBUTION OF SELECTED ELEMENTS IN ROCKS, SOILS, AND PLANTS, MINERAL BUTTE COPPER DEPOSIT, PINAL COUNTY, ARIZONA By MAURICE A. CHAFFEE ABSTRACT A geochemical study was made of rock, soil, and plant samples collected in the vicinity of the Mineral Butte copper deposit in Pinal County, Ariz. This deposit is in a Precambrian granite host rock near its contact with a small Upper Cretaceous biotite quartz monzonite stock. Of the potentially economic minerals only secondary copper minerals are present in bedrock exposures. Bedrock and residual-soil samples were collected on a 150-metre (500-foot) grid. Two soil fractions were separated for analysis: a 12 millimetre coarse fraction and a <0.063 millimetre fine fraction. Leaf and stem samples from four plant species (mesquite, catclaw acacia, blue paloverde, and ironwood) were collected from plants growing in stream channels in the area of the deposit. The rock and soil samples were analyzed for 39 elements. The soil samples were also analyzed for soil pH. The analyses of 31 elements did not provide sufficient information, and these were not evaluated further. Eight elements provided sufficient useful analyses; cobalt, fluorine, gold, lead, molybdenum, silver, and zinc are thought to be geochemically associated with copper and are discussed in this report. Anomalously low pH values were found in soil samples collected near the Mineral Butte deposit. The areal extent of these low values seems to be related to areas of silicified Precambrian granite. This distribution of low values might also represent a halo around the copper-rich area, thereby providing a larger exploration target than that of the copper geochemical anomaly. The distributions of concentrations of cobalt, copper, gold, molybdenum, silver, and zinc in bedrock samples yield positive anomalies. The areal extent of the cobalt anomaly most closely matches that of copper. The distributions of concentrations of fluorine and lead in bedrock samples yield what appear to be negative anomalies that also coincide closely with the copper anomaly. Another interpretation of the fluorine and lead data suggests that these two elements show a positive aureole around the copper anomaly. A more regional geochemical sampling program is needed in order to interpret better the anomalies of these two elements. In contrast to many other copper districts, there does not seem to be a close association between the distributions of copper and molybdenum in the Mineral Butte area. The distributions of anomalies of cobalt and gold in soil samples most closely match the known copper anomaly. For copper, cobalt, gold, and molybdenum, samples of the fine-soil fraction provide more widespread and (or) more easily interpreted anomalies than do samples of either rock or the coarse-soil fraction. For these elements, at least, a regional reconnaissance sampling program using a finesoil fraction should be considered. Physical and chemical data from this study indicate that eolian contamination of soil samples is not a serious problem to consider in anomaly interpretation in the Mineral Butte area. Ash from the leaves and stems of the four plant species was analyzed for 38 elements. The analyses of most of these elements did not provide sufficient information. This report describes the results for copper, zinc, and molybdenum. Examination of the geochemical data for these three elements indicates that the concentrations of these elements in plant ash vary according to the particular species and plant part sampled. Those plant-element populations that produced the greatest spread of analytical values also produced the most useful vegetation maps. Of the three elements studied in samples of plant ash, copper clearly provided the most consistent and meaningful information for locating the known deposit. Molybdenum was not as useful. No spatial correlation was found between zinc anomalies in rock and soil samples and zinc anomalies in the ash of any of the plant species studied. The copper deposit was best located using analyses of mesquite samples. Those of blue paloverde and catclaw acacia were almost as effective. In general, those samples that contain anomalous metal concentrations in both plant parts, rather than in just one part, are likely to represent the most significant anomalies. Deep-rooted plants can be an effective regional reconnaissance sample medium for geochemical surveys in arid lands, especially in the search for metal deposits buried under a thin layer of overburden. INTRODUCTION AND ACKNOWLEDGMENTS A geochemical study has been made of rock, soil, and plant samples collected in the vicinity of the Mineral Butte copper deposit, Pinal County, Ariz. This study has a threefold purpose. The first is to provide basic information on the abundances of many elements in a variety of sample types collected in the vicinity of a known copper deposit. The second is to determine, by evaluation and comparison of the different sample types, the optimum type(s) to use. The third is to identify those elements in the different sample types that would be most useful in the search for copper deposits in similar environments. Emphasis in this investigation has been to determine in different sample media well as, or better than, would copper itself. No attempt has been made to study the trends in concentrations of the selected elements on a truly regional scale. The Mineral Butte copper deposit is located in the Blackwater mining district, Pinal County, Ariz., about 13 km (8 mi) northwest of the town of Coolidge (fig. 1). Elevations in the study area range from 581 m (1,907 ft) to 411 m (1,350 ft). FIGURE 1. Location of the report area. The climate and flora in the vicinity of the study area are typical of the semiarid lower Sonoran Desert of Arizona. The weather station nearest Mineral Butte is at Casa Grande National Monument, just to the north of Coolidge, about 13 km (8 mi) southeast of the study area. The mean annual precipitation at that station for the years 1908 through 1957 was 22.0 cm (8.74 in.). The mean daily maximum and minimum temperatures were approximately 31°C (about 87°F) and 10°C (about 51°F), respectively (Green and Sellers, 1964). The Mineral Butte area supports a large variety of herbs, shrubs, and trees. Many trees growing in or near the active stream channels reach heights of 10 m (about 33 ft) or more. The plant species selected for this investigation are among those that commonly grow in the stream channels of the region. The area of the Mineral Butte deposit was selected for this geochemical study for several reasons. First, the surface has not been greatly disturbed by man's activities. Wilson (1969) noted that the area was originally prospected for copper, silver, and gold, although mining activity to date has been restricted to a few small shafts and pits. Second, the area is geologically simple. Only three geologic units of major importance crop out in the area, and one of these comprises from 80 to 90 percent of the total surface exposures. Premineralization-age bedrock is exposed throughout much of the study area and overall relief is low, allowing a comparison of bedrock geochemical data with corresponding residual-soil data. Finally, although all streams in the area have only intermittent flow, the chemistry of water crossing this deposit and continuing downstream toward the Gila River can be studied indirectly by analyzing the plants growing in the one main channel that crosses directly over the deposit. These analyses can then be compared to those from samples of plants growing in other nearby stream channels that do not cross the deposit to determine how well and how far from any outcrop the analyses from the plant samples would indicate the known deposit. The Mineral Butte copper deposit is on land leased from the Gila River Indians by the Duval Corporation. The assistance and cooperation of the staff of this corporation, especially Mr. R. A. Metz, are gratefully acknowledged. The following members of the U.S. Geological Survey assisted the author in collecting, preparing, and analyzing the samples for this report: R. N. Babcock, R. D. Coolidge, C. L. Forn, C. W. Gale, III, J. D. Hoffman, J. M. Mitchell, R. M. O'Leary, J. H. Reynolds, D. F. Siems, C. D. Smith, Jr., J. H. Turner, R. L. Turner, and E. P. Welsch. In addition, appreciation is extended to Mr. A. A. M. A. Gayayel of the Geological Survey of Egypt, who assisted the author in the field while working for the U.S. Geological Survey as a United Nations Special Fund Fellow. GEOLOGIC SETTING Six different geologic units have been mapped in the Mineral Butte area (fig. 2). Exposures in this area of the oldest unit, the Pinal Schist of Precambrian X age1 (Drewes, 1972, 1975), are gneiss rather than schist. The Pinal Schist was intruded by a coarsegrained, equigranular Precambrian granite, which is thought to be equivalent to the Oracle Granite north of Tucson (Balla, 1972). The Oracle Granite has been dated as 1,420 m.y. (million years) (Damon, 1968, p. 38). This granite is the most extensive of the six 'An interim scheme for subdivision of Precambrian time recently adopted by the U.S. Geological Survey assigns Precambrian X time to the interval 1,600-2,500 m.y. ago and Precambrian Y time to the interval 800 |
