Geochemical exploration techniques based on distribution of selected elements in rocks, soils, and plants, Mineral Butte copper deposit, Pinal County, Arizona

A geochemical study was made of rock, soil, and plant samples collected in the vicinity of the Mineral Butte copper deposit in Final 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 1-2 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 Dl D2 CONTRIBUTIONS TO GEOCHEMICAL PROSPECTING FOR MINERALS 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, Final 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 what elements, if any, would characterize the copper deposit as EXPLORATION TECHNIQUES, FINAL COUNTY, ARIZONA D3 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, Final 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).