An aerial radiometric and magnetic survey of the Sierra Ancha Wilderness, Salome study area, Gila County, Arizona By Joseph S. Duval and James. A. Pit kin 1980 The U.S. Geological Survey has flown an Variations in total intensity can be related to aerial radiometric and magnetic survey of the the distribution of magnetic minerals at and Sierra Ancha Wilderness, Salome study area, and near the Earth's surface* Magnetic mineral vicinity* Gila County, Arizona to aid an evalua­ concentration varies widely with rock type and tion of the mineral potential of the area. can be a significant characteristic of a rock Radiometric data were obtained because uranium type* Measurement of the variation in total deposits are known in the area and uranium magnetic intensity by an aerial survey provides mineralization is thought to be widely distrib­ data that aids in geologic mapping and the uted in the upper member of the Dripping Spring understanding of geologic processes* The mag­ Quartzite and possibly in the upper member of netic field decreases as the distance from the the Meseal Limestone (Granger and Raup, source increases, but is not significantly 1969a,b). The magnetic data were obtained attenuated by intervening materials* For this because iron deposits are known in the area and reason, effects due to magnetic bodies as deep because iron mineralization occurs in associa­ as 40 km (25 mi) beneath the ground surface can tion with known asbestos deposits (Shride, 1967, be detected where the bodies are very large* 1969)* The radiometric and magnetic data are presented as contour maps overlaid on geologic EQUIPMENT and topographic maps* METHOD The equipment used for this survey consists of a four-channel gamma-ray spectrometer with its associated electronics, a large-volume An aerial radiometric survey measures gamma (113.3 liter) plastic detector (Duval and Pit- radiation from the decay products of the radio- kin, 1978), a proton-precession magnetometer, a elements K-40, U-238, and Th-232. The distribu­ radar altimeter, a digital magnetic-tape re­ tion of these elements and their decay products corder, an analog strip-chart recorder, and a are controlled by the complex interaction of 35-mm flight-track-recovery camera* Data from geological and geochemlcal processes such as the spectrometer, magnetometer, radar altimeter, hydrothermal fluid movement, groundwater flow, and a digital clock were measured during one- weathering, erosion, and transportation* Be­ second time intervals and recorded on magnetic cause of this relationship to geologic pro­ tape* The 35-mm camera photographed the flight cesses, measurements of the radioelement distri­ path of the aircraft at two-second intervals. butions can be used to aid geologic mapping and All equipment were mounted in a single-engine mineral exploration* Pilatus Porter STOL aircraft. 1 The measurement of the radioelement distri­ butions is made using scintillation detectors SURVEY PARAMETERS with associated electronic equipment* The survey aircraft generally follows the contour of the ground with a ground clearance of no more All data were measured at a nominal ground than 220 m (700 ft)* The low ground clearance clearance of 122 m (400 ft) and a speed of about Is -necessary for statistically valid measure­ 160 km/hr (100 ml/hr). The effective ground ments because the gamma rays are absorbed at an area measured by the radiometric equipment at exponentially increasing rate as the thickness this height above the ground is a strip along a of the air beneath the aircraft increases* flight line about 244 m (800 ft) wide. Fifty- Because the gamma-ray absorption by the surface five flight lines, 5 to 32 km (3 to 20 mi) long, materials also increases ' exponentially, the were flown along a northwest-southeast direction measurements reflect the radioelement concentra­ at intervals of 0.8 km (0.5 mi) (see Map A). tions only in the uppermost 0*5 m (1*5 ft) of This direction approximately parallels the rock and soil at the ground surface. geologic strike in parts of the area, and also An aerial magnetic survey measures the parallels the topographic grain* Flying paral­ total intensity of the Earth's magnetic field* lel to the topographic grain was necessary in The use of commercial trade names is for descriptive purposes only and does not consti­ tute endorsement of these products by the U.S. Geological Survey. order to be able to do contour flying at 122 m consist of Quaternary Tertiary alluvial and (400 ft) above the ground surface because topo­ colluvial deposits., small outcrops of Devonian- graphic relief in the area is greater than 1000 Cambrian sandstone, and rocks of Precambrlan m (3300 ft). age. The oldest outcrops are the Ruin Granite, which is a domlnantly coarse-grained quartz DATA REDUCTION monzonite. The Apache Group* which includes the Pioneer Formation, the Dripping Spring Quartz- ite, and the Me seal Limestone, overlie the Ruin The data were processed to obtain contour Granite. The Pioneer Formation is predominantly maps of the apparent surface distributions of maroon to purple tuff and pink to gray siltstone potassium (K), equivalent uranium (eU), equiva­ and sandstone, and is generally present only as lent thorium (eTh), the ratios eU/eTh, eU/K, narrow outcrops within the study area. The eTh/K, and the residual total magnetic field* Dripping Spring Quartzlte consists of siltstone These maps are shown on maps B through H. The and very fine to medium-grained feldspathlc- to radiometrlc data as presented are fully cor­ arkosic-sandstone and orthoquarzlte. Granger rected and are expressed in arbitrary units. and Raup (1969b) determined that siltstone in The "e" preceding the "U" and "Th" refers to the the upper part of the Dripping Spring Quartzlte fact that these measurements reflect equivalent is the most favorable host rock for uranium or apparent distributions of the elements. The mineralization, and that it also contains potas­ measurements are of gamma rays from the decay of sium oxide concentrations greater than 10 per­ Bl-214, a daughter product of U-238, and from cent. The Me seal Limestone Is massive dolomite the decay of Tl-208, a daughter product of Th- and limestone with siltstones and shales and 232. Because these elements and other predeces­ locally abundant chert and foldspar. In some sors In the decay chains can behave differently areas basalt remnants locally overlie or occur in the geochemical environment and become physi­ within the Me seal. Shride (1967) measured cally separated from the parent elements, U-238 potassium oxide concentrations greater than 10 and Th-232, the measurements may not accurately percent in the argillite member of the Me seal, reflect the parent-element distributions. For and Shride (1967) and Granger and Raup (1969b) this reason the measurements are designated as observed that asbestos mineralization in the being apparent or equivalent measurements. Me seal commonly has magnetite associated with Corrections applied to the radlometric data it. The Troy Quartzlte overlies the Apache include background subtraction to remove counts Group and is predominantly light-colored quartz- due to cosmic rays and aircraft contamination, ite and sandstone with conglomerate horizons. and altitude normalization to remove variations Precambrlan diabase sills and dikes intrude all caused by changes in the ground clearance. The of the older Precambrian rocks, but are found corrected radiometric data were reduced to mostly within the Meseal, Dripping Spring, and apparent surface distributions of the radioele­ Pioneer Formations of the Apache Group. ments using equations based upon calibration The structural features within the survey data obtained on the calibration pads estab­ area include numerous faults that are shown on lished by the U.S. Department of Energy at the maps and two generally north-south trending Walker Field airport, Grand Junction, Colorado monoclines that are not explicitly indicated on (Ward, 1978). the maps. One of these monoclines is parallel Corrections applied to the magnetic data to, and approximately along, the Globe-Young include removal of the diurnal variations in the highway. The other monocline follows the Cherry earth's magnetic field as measured by a base- Creek canyon. Shride (1967) briefly discusses station magnetometer and removal of the regional these monoclines. magnetic gradient. The regional gradient re­ moved was that of the International Geomagnetic DISCUSSION Reference Field (Barraclough and Fabiano, 1978) updated to the time of the survey. Map H shows the residual total magnetic intensity. eU map (Map B) GEOLOGY This la a contour map of the apparent surface distribution of equivalent uranium (eU) with values ranging from 1 to 40 (arbitrary The geologic report and map for the study units). Values of eU greater than 18 occur area are by Bergquist and others, (1980). The mostly In two generally north-south trending geologic map used as a gray-tone base for this zones. Most of the eU values greater than 18 report was modified from Bergquist and others appear to correlate spatially with the Mescal, (1980), and from unpublished maps and notes of Dripping Spring, and Pioneer Formations of the A* F. Shrlde. For this report the geology was Apache Group and many of them are associated generalized and some of the smaller outcrops with the Dripping Spring Quartzlte and the were dropped from the map. The locations of diabase intrusive. known mineralizations were taken from Granger East of the Globe-Young highway most of the and Raup (1969a, b), and Shride (1969). eU values less than 16 are associated with Troy The lithologlc units in the study area Quartz!te and diabase. Generally lower eU values also occur west of Salome Creek In an eU/K map (Map F) area of extensive diabase that lacks rocks younger than the Dripping Spring Quartzite. In This is a contour map of the ratio eU/K this area, the Dripping Spring is mostly lover with values ranging from 2 to 34 (arbitrary and middle members which are less radioactive units).