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Geologic Interpretation of an Aeromagnetic Survey of the Iron Springs District, Utah

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Geologic Interpretation of an Aeromagnetic Survey of the Iron Springs District, Utah Geologic Interpretation ofan Aeromagnetic Survey of the Iron Springs District, Utah GEOLOGICAL SURVEY PROFESSIONAL PAPER 516-B Geologic Interpretation ofan Aeromagnetic Survey of the Iron Springs District, Utah By H. RICHARD BLANK, Jr., and J. HOOVER MACKIN G E 0 P H Y S I C A L F I E L D I NiVjEfS)T I G AT I 0 N S GEOLOGICAL SURVEY PRO.FE!SSIONAL PAPER 516-B UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY William T. Pecora, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Page Abstract__________________________________________ _ B1 Interpretation of the aeromagnetic map-Continued Introduction--------------------------------------- 1 Three Peaks area-Continued General geology ___________________________________ _ 1 Local anomalies ___________________________ _ B10 Itock units ____________________________________ _ 1 Granite Mountain area ______ -------------- __ ---- 10 Ore deposits ___________________________________ _ 3 Geometry of the intrusion ______ -----_------- 10 Magnetic properties of the rocks _____________________ _ 3 Local anomalies ___________________________ _ 11 The aeromagnetic survey __ ------------- _______ ------ 4 Southwest termination ______ ----- _____ ------ 11 Interpretation of the aeromagnetic map ______________ _ 4 Iron Mountain area____________________________ _ 12 Three Peaks area ______________ ---------- ______ _ 6 Geometry of the intrusion___________________ _ 12 Outline of the intrusion_____________________ _ 6 Local anomalies ___________________________ _ 12 Thickness of the intrusive body-----_________ _ 6 Summary and Conclusions_--------------- __________ _ 13 Iteferences ________________________________________ _ East fault __________ ------------------------ 8 13 ILLUSTRATIONS PLATE 1. Aeromagnetic and generalized geologic map of the Iron Springs district, Utah---------------------------~ In pocket Page FIGURE 1. Vertical-intensity profiles _______________________________________________________ ----__ -_-------------- B5 2. Computed magnetic profiles showing edge effect of two-dimensional slabs _____________ ---------------------- 7 3. Diagram showing total-intensity aeromagnetic anomalies---------------------------------------·---------- 8 4. Gravity and aeromagnetic profiles across the East fault--------------------------------------------------- 9 III GEOPHYSICAL FIELD INVESTIGATIONS GEOLOGIC INTERPRETATION OF AN AEROMAGNETIC SURVEY OF THE IRON SPRINGS DISTRICT, UTAH By H. RicHARD BLANK, JR., and J. HooVER MAcKIN ABSTRACT by Mackin. Mackin's geologic map in generalized form A total-intensity aeromagnetic map is presented as an over­ is presented in this report together with the results of print on a generalized geologic map of the Iron Springs district, the aeromagnetic survey. Interpretation of the aero­ southwestern Utah. The isomagnetic contours roughly delineate magnetic data was undertaken jointly by the two au­ the three bodies of quartz monzonite porphyry exposed in the thors, but responsibility for geophysical conclusions district-the Three Peaks, Granite Mountain, and Iron Mountain intrusions. The Three Peaks intrusion extends at shallow depth rests on Blank alone. beneath alluvium to the east and southeast of the area of out­ The paper has benefited from the critical comments of crop. Granite Mountain is an upbulge on a largely concealed D. M. Lemmon and F. C. Frischknecht, to whom the intrusive body extending to the north, west, and southwest; addi­ authors are grateful. Frischknecht independently in­ tional relief on the concealed surface of this body is inferred vestigated the magnetic properties of the intrusive rocks, from the aeromagnetic data. There is some magnetic evidence oomputed theoretical magnetic-response curves, and for a considerable western extension of the Iron Mountain intrusion. arrived at a model of the Three Peaks intrusion which The zone of "selvaged" joints beneath peripheral rock of the agrees closely with the results given here. Three Peaks intrusion has a higher effective SIUsceptibility than normal interior rock and produces a local magnetic high super­ GENERAL GEOLOGY imposed on the general pattern of the anomaly. The amplitude Because of its economic importance, the Iron Springs of the high appears to be proportional to the size and abundance of magnetite veins and joint coatings characteristic of this zone. district has been the subject of numerous geologic in­ Some contact-replacement ore bodies also produce strong local vestigations. The most pertinent published reports are anomalies ; others were not detected by the survey. those of Leith and Harder ( 1908) ; Butler, Loughlin, Heikes, and others (1920, p. 568-580); Wells (1938); INTRODUCTION and Mackin ( 1947, 1954, 1960). Relevant structural The Iron Springs district is in Iron County, south­ and stratigraphic informS~tion is contained in reports western Utah, at the eastern margin of the Great Basin. of peripheral areas by Thomas and Taylor (1946), Cedar City is the nearest sizable community. For many Gregory ( 1950) , Cook ( 1957), and Threet ( 1963). A years the important steel industry at Provo and Geneva, comprehensive geologic report on the district by Mackin 225 miles north of Cedar City, was almost wholly de­ is currently in preparation. Only a brief outline of the pendent on the open-pit mines of the district as a source geologic relations is given in this report. of iron ore. As late as 1965 the district still ranked as one of the largest iron-ore producers in the western ROCK UNITS conterminous United States, although in 1966 produc­ The oldest rocks exposed in the district belong to the tion was drastically curtailed. Jurassic Carmel Formation of the Colorado Plateau In 1947 the U.S. Geological Survey made an aeromag­ (Gregory, 1950). In Iron Springs the formation is netic survey of the district (Dempsey, 1951) as part of about 250 to 300 feet thick and consists of a thin basal a systematic investigation of the geology and ore de­ siltstone member overlain by the Homestake Limestone posits. Detailed ground-magnetic surveys were sub­ Member (Mackin, 1954), which contains the main re­ sequently carried out by the U.S. Bureau of Mines placement ores of the district. Overlying the Home­ (Cook, 1950), as well as by private parties. A geologic stake is a shale-sandstone sequence 0 to 220 feet thick map of the district at a scale of 1 : 24,000 was prepared tentatively correlated with the Jurassic Entrada Sand- Bl B2 GEOPHYSICAL FIELD INVESTIGATIONS stone of Gregory ( 1950). The two Jurassic formations Claron Formation) are exposed, the canyon is cut en­ are shown on the geologic map by a single pattern. tirely in ignimbrites of the Quichapa. Four ignim­ A succession up to 4,000 feet thick of clastic beds­ brite members with a total thickness of about 900 feet predominantly mustard-colored sandstones-rests dis­ compose the type section. conformably upon the Entrada(~) Sandstone. These The Needles Range and Isom are believed to be beds compose the Iron Springs Formation of Late( n Eocene or early Oligocene in age; the Quichapa is Oli­ Cretaceous age (Mackin, 1947, 1954). gocene or early Miocene (Mackin, 1960, p. 98, 103; 1963, Conglomerate, siltstone, and lacustrine limestone of p. 76). For convenience and because their distinction the Claron Formation, of probable Eocene age, overlie serves no purpose here, the Needles Range and Isom the Iron Springs Formation disconformably or locally Formations are represented on the generalized map as with marked angular unconformity. The basal part one unit. of the Claron w~s deposited in an environment of con­ Three bodies of quartz monzonite porphyry-the siderable topographic relief following the Laramide Three Peaks, Granite Mountain, and Iron Mountain in­ deformation which involved the Iron Springs and older trusions-crop out in the district. The emplacement formations. The formation becomes increasingly tuf­ of these bodies, at or near the end of the Quichapa faceous toward the top, a fact reflecting the onset of episode of volcanism, arched up the sedimentary and volcanic activity in the region. Its maximum thickness volcanic roof rocks and produced steep-sided topo­ is about 1,000 feet. graphic features. All three are roughly alined along Volcanic rocks of the Needles Range, Isom, and a northeast-trending Laramide anticlinal flexure which Quicha pa Formations were extruded onto a flattish is locally overthrust; a number of similar intrusions, in­ Claron surface in early ( ~) and middle Tertiary time. eluding those of the Bull Valley district, 40 miles to the These deposits are composed of welded ash flows southwest, lie along the same structural feature. The (ignimbrites), lava flows, and flow breccias, chiefly of intrusions are believed to be laccoliths or bysmaliths andesitic to quartz-latitic composition. The maximum floored by the Jurassic and Triassic( n Navajo Sand­ aggregate thickness is about 1,500 feet. The welded stone. This formation, however, has neither been seen tuffs are parts of vast sheetlike deposits that are dis­ in outcrop nor penetrated by drilling. The observed tinguishable throughout much of the region. These marginal contacts either are concordant with the Carmel formations were named by Mackin ( 1960). The type Formation or are ruptures in the roof rocks, here called locality for the Needles Range Formation was desig­ intrusive faults, formed
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