Geologic Interpretation of an 

Aeromagnetic Survey of the 

Iron Springs District, Utah 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 516-B 

Geologic Interpretation of an 

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 

Abstract __________________________________________ _ 

Introduction--------------------------------------- General geology ___________________________________ _ Itock units ____________________________________ _ 

Ore deposits ___________________________________ _ 

Magnetic properties of the rocks _____________________ _ 

The aeromagnetic survey __ ------------- _______ ------ Interpretation of the aeromagnetic map ______________ _ 

Three Peaks area ______________ ---------- ______ _ 

Outline of the intrusion _____________________ _ 

Thickness of the intrusive body----- _________ _ 

East fault __________ ------------------------

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Interpretation of the aeromagnetic map-Continued 

Three Peaks area-Continued 

Local anomalies ___________________________ _ 

Granite Mountain area ______ -------------- __ ----

Geometry of the intrusion ______ -----_------- Local anomalies ___________________________ _ 

Southwest termination ______ ----- _____ ------ Iron Mountain area ____________________________ _ 

Geometry of the intrusion ___________________ _ 

Local anomalies ___________________________ _ 

Summary and Conclusions_--------------- __________ _ Iteferences ________________________________________ _ 

ILLUSTRATIONS 

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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 

A total-intensity aeromagnetic map is presented as an overprint on a generalized geologic map of the Iron Springs district, 

southwestern Utah. The isomagnetic contours roughly delineate 

the three bodies of quartz monzonite porphyry exposed in the 

district-the Three Peaks, Granite Mountain, and Iron Mountain 

intrusions. The Three Peaks intrusion extends at shallow depth 

beneath alluvium to the east and southeast of the area of outcrop. Granite Mountain is an upbulge on a largely concealed 

intrusive body extending to the north, west, and southwest; additional relief on the concealed surface of this body is inferred 

from the aeromagnetic data. There is some magnetic evidence 

for a considerable western extension of the Iron Mountain 

intrusion. 

The zone of "selvaged" joints beneath peripheral rock of the 

Three Peaks intrusion has a higher effective SIUsceptibility than 

normal interior rock and produces a local magnetic high superimposed on the general pattern of the anomaly. The amplitude 

of the high appears to be proportional to the size and abundance 

of magnetite veins and joint coatings characteristic of this zone. 

Some contact-replacement ore bodies also produce strong local 

anomalies ; others were not detected by the survey. 

INTRODUCTION 

The Iron Springs district is in Iron County, southwestern Utah, at the eastern margin of the Great Basin. 

Cedar City is the nearest sizable community. For many 

years the important steel industry at Provo and Geneva, 

225 miles north of Cedar City, was almost wholly dependent on the open-pit mines of the district as a source 

of iron ore. As late as 1965 the district still ranked as 

one of the largest iron -ore producers in the western 

conterminous United States, although in 1966 production was drastically curtailed. 

In 1947 the U.S. Geological Survey made an aeromagnetic survey of the district (Dempsey, 1951) as part of 

a systematic investigation of the geology and ore deposits. Detailed ground-magnetic surveys were subsequently carried out by the U.S. Bureau of Mines 

(Cook, 1950), as well as by private parties. A geologic 

map of the district at a scale of 1 : 24,000 was prepared 

by Mackin. Mackin's geologic map in generalized form 

is presented in this report together with the results of 

the aeromagnetic survey. Interpretation of the aeromagnetic data was undertaken jointly by the two authors, but responsibility for geophysical conclusions 

rests on Blank alone. 

The paper has benefited from the critical comments of 

D. M. Lemmon and F. C. Frischknecht, to whom the 

authors are grateful. Frischknecht independently investigated the magnetic properties of the intrusive rocks, 

oomputed theoretical magnetic-response curves, and 

arrived at a model of the Three Peaks intrusion which 

agrees closely with the results given here. 

GENERAL GEOLOGY 

Because of its economic importance, the Iron Springs 

district has been the subject of numerous geologic investigations. The most pertinent published reports are 

those of Leith and Harder ( 1908) ; Butler, Loughlin, 

Heikes, and others (1920, p. 568-580); Wells (1938); 

and Mackin ( 194 7, 1954, 1960). Relevant structural 

and stratigraphic informS~tion is contained in reports 

of peripheral areas by Thomas and Taylor (1946), 

Gregory ( 1950) , Cook ( 1957), and Threet ( 1963). A 

comprehensive geologic report on the district by Mackin 

is currently in preparation. Only a brief outline of the 

geologic relations is given in this report. 

ROCK UNITS 

The oldest rocks exposed in the district belong to the 

Jurassic Carmel Formation of the Colorado Plateau 

(Gregory, 1950). In Iron Springs the formation is 

about 250 to 300 feet thick and consists of a thin basal 

siltstone member overlain by the Homestake Limestone 

Member (Mackin, 1954), which contains the main replacement ores of the district. Overlying the Homestake is a shale-sandstone sequence 0 to 220 feet thick 

tentatively correlated with the Jurassic Entrada SandBl 
B2 GEOPHYSICAL FIELD INVESTIGATIONS 

stone of Gregory ( 1950). The two Jurassic formations 

are shown on the geologic map by a single pattern. 

A succession up to 4,000 feet thick of clastic bedspredominantly mustard-colored sandstones-rests disconformably upon the Entrada(~) Sandstone. These 

beds compose the Iron Springs Formation of Late( n 

Cretaceous age (Mackin, 1947, 1954). 

Conglomerate, siltstone, and lacustrine limestone of 

the Claron Formation, of probable Eocene age, overlie 

the Iron Springs Formation disconformably or locally 

with marked angular unconformity. The basal part 

of the Claron w~s deposited in an environment of considerable topographic relief following the Laramide 

deformation which involved the Iron Springs and older 

formations. The formation becomes increasingly tuffaceous toward the top, a fact reflecting the onset of 

volcanic activity in the region. Its maximum thickness 

is about 1,000 feet. 

Volcanic rocks of the Needles Range, Isom, and 

Quicha pa Formations were extruded onto a flattish 

Claron surface in early ( ~) and middle Tertiary time. 

These deposits are composed of welded ash flows 

(ignimbrites), lava flows, and flow breccias, chiefly of 

andesitic to quartz-latitic composition. The maximum 

aggregate thickness is about 1,500 feet. The welded 

tuffs are parts of vast sheetlike deposits that are distinguishable throughout much of the region. These 

formations were named by Mackin ( 1960). The type 

locality for the Needles Range Formation was designated as the east side of the Needles Range at and 

south of the Garrison-Milford highway, T. 28 S., R. 

11 W., Beaver County, Utah (p. 99). There the formation consists of at least two ignimbrites and has a 

total thickness of 500 to 1000 feet. Regionally it rests 

on an erosion surface cut in Paleozoic rocks; locally on 

older Tertiary deposits filling depressions on that surface. It is overlain by rocks of the Isom Formation. 

Type localities for the Isom and Quichapa Formations are in the Iron Springs district. For the Isom, 

the type locality was designated as the southern slope of 

an east-west ridge just north of Isom Creek in The 

Three Peaks quadrangle, sec. 5, T. 35 S., R. 12 W. 

(Mackin, 1960, p. 98). The Isom consists of three ignimbrite members with a total thickness of about 200 

feet. The contact with the underlying Needles Range 

Formation is exposed at the foot of the ridge; capping 

the ridge is the basal ignimbrite member of the overlying Quichapa Formation. 

The type locality of the Quichapa Formation is Right 

Quichapa Canyon, northeast quarter of T. 37 S., R. 13 

W., in the Harmony Hills southeast of Iron Mountain. 

Except for a few tens of feet at the base, where rocks of 

the underlying Isom Formation (and in some places, the 

Claron Formation) are exposed, the canyon is cut entirely in ignimbrites of the Quichapa. Four ignimbrite members with a total thickness of about 900 feet 

compose the type section. 

The Needles Range and Isom are believed to be 

Eocene or early Oligocene in age; the Quichapa is Oligocene or early Miocene (Mackin, 1960, p. 98, 103; 1963, 

p. 76). For convenience and because their distinction 

serves no purpose here, the Needles Range and Isom 

Formations are represented on the generalized map as 

one unit. 

Three bodies of quartz monzonite porphyry-the 

Three Peaks, Granite Mountain, and Iron Mountain intrusions-crop out in the district. The emplacement 

of these bodies, at or near the end of the Quichapa 

episode of volcanism, arched up the sedimentary and 

volcanic roof rocks and produced steep-sided topographic features. All three are roughly alined along 

a northeast-trending Laramide anticlinal flexure which 

is locally overthrust; a number of similar intrusions, ineluding those of the Bull Valley district, 40 miles to the 

southwest, lie along the same structural feature. The 

intrusions are believed to be laccoliths or bysmaliths 

floored by the Jurassic and Triassic( n Navajo Sandstone. This formation, however, has neither been seen 

in outcrop nor penetrated by drilling. The observed 

marginal contacts either are concordant with the Carmel 

Formation or are ruptures in the roof rocks, here called 

intrusive faults, formed during emplacement. (See 

Mackin, 1947 and 1954, for a discussion of structural 

features of intrusive origin.) Along the southeast side 

of Iron Mountain the contact is a breccia zone on the 

Laramide thrust. 

The quartz monzonite porphyry consists of phenocrysts of intermediate plagioclase feldspar, hornblende, 

biotite, and augite set in a fine-grained groundmass of 

orthoclase and quartz. Magnetite is the principal accessory mineral, occurring both as pyrogenic crystals 

and as blebs produced by deuteric (endomorphic) 

alteration. The magnetite content is generally between 

1 and 3 percent of the rock volume. 

Gravity sliding from the roof and from the southern 

to eastern flanks of the Iron Mountain intrusion formed 

extensive zones of tectonic breccia composed of finely 

shattered rock of the Claron, Isom, and Quichapa Formations. The breccias are identified on the geologic 

map by their predominant material-either Claron or 

Quichapa. 

Volcanic rocks of the Rencher and Page Ranch Formations of Cook (1957), also lumped as a single unit, lie 

stratigraphically above the Quichapa Formation. 

They have a maximum thickness of 2,000 feet and are 

composed predominantly of rhyodacite to quartz latite 
AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH B3 

welded tuff and tuff breccias of middle and late ( ~) 

Tertiary age. The basal member of the Rencher Formation in the Swett Hills is a wedge of volcanic debris 

evidently derived from erosion of the Iron Mountain 

intrusion and later deformed by emplacement of a nowburied extension of the Granite Mountain intrusion. 

The Page Ranch Formation, on the other hand, is 

younger than the Granite Mountain intrusion. 

Surficial Quaternary deposits blanket the rmnainder 

of the district. In many places a thin veneer of pediment gravels rests on truncated older formations. 

ORE DEPOSITS 

Two distinct types of iron ore occur in the district : 

replacement bodies in limestone of the roof rock, and 

vein or fissure-filling ores in the porphyry. According 

to Young (1947, p. 6), the replacement ores are composed mainly of hematite in association with magnetite 

(the magnetite ranging from 10 to 47 percent within 

each ore body), whereas the vein ores are predominantly 

magnetite. The principal commercial deposits, nearly 

all of the replacement type, are indicated diagrammatically on the geologic rna p. 

The iron mineralization has been shown by Mackin 

(1947, 1954) to represent concentrations of iron released 

from the intrusive magma by deuteric processes during 

the final stages of consolidation. Fresh porphyry is 

limited to the rapidly chilled peripheral shell of each 

intrusion-a zone 100 to 300 feet thick adjacent to the 

original upper margins. Elsewhere the rock is deuterically altered, and the primary minerals have been transformed into pseudomorphic aggregates of orthoclase 

and iron oxides. Iron was concentrated in joints and 

open fissures produced by late distension of the partially 

solidified intrusive mass. Magnetite-coated joints and 

magnetite veins have borders, or "selvages," leached of 

iron. The zone of selvaged joints forms a resistant 

layer of irregular thickness between the peripheral shell 

and crumbly interior. Replacement bodies occur in 

places where the ore-bearing fluids gained access, 

through fractures in the peripheral shell, to the capping 

limestone. 

Peripheral shell rock, interior rock with selvaged 

joints, and normal interior rock are given separate map 

designations. Normal interior rock is exposed only in 

the Three Peaks intrusion; peripheral shell and selvaged 

interior rock are not separated on Iron Mountain. 

MAGNETIC PROPERTIES OF THE ROCKS 

The response of the airborne magnetometer depends 

upon the intensity of magnetization of the rocks and the 

distance between the rocks and the detector. The intensity is the resultant of both induced and permanent 

components of polarization. 

To evaluate the effect of induced polarization, a description of the inducing field and an estimate of rock 

susceptibility are required. The following parameters 

for the earth's normal field in the Iron Springs district 

are taken from the charts of Vestine and others (1947): 

Total intensity=54,100 gammas 

Declination=16.5° (east) 

Inclination=64.3° 

No direct measurements of magnetic properties of the 

Iron Springs rocks were made. However, other workers-for example, Slichter (1929), Werner (1945), and 

Mooney and Bleifuss (1953)-have shown empirically 

that the susceptibility of rocks containing up to several 

percent magnetite is approximately linearly proportional to magnetite content. Satisfactory results in the 

interpretation of ground-magnetometer surveys for iron 

deposits in the Iron Springs district are obtained if the 

magnetite component of the ore is assumed to have a 

susceptibility of 0.25 cgs (centimeter-gram-second) unit 

(Cook, 1950). The susceptibility of magnetite in the 

ore is not necessarily the same as that in the porphyry, 

but if the value 0.25 cgs unit applies, the susceptibility 

of quartz monzonite porphyry containing 1 to 3 percent 

magnetite should range from 2.5 to 7.5 X 10-s cgs unit. 

Cook measured the susceptibility of 116 specimens of 

porphyry on the western margin of the Three Peaks 

intrusion and concluded that specimens from the 

border z.one (peripheral shell) have a higher susceptibility than "normal" porphyry, but data were not listed. 

The actual susceptibility range of the porphyry is no 

doubt greater than the range given above because of 

atypical values of magnetite content and departures 

from the susceptibility assumed for pure magnetite. 

(For a list of measured susceptibilities of magnetite 

from various sources, see, for example, Birch and others, 

1942, p. 295.) Volcanic rocks of the district probably 

have susceptibilities comparable to or less than those of 

the porphyry; susceptibilities of the sedimentary rocks 

are assumed to be negligible. 

A study of permanent polarization, or natural remanent magnetization (NRM), of the Granite Mountain 

and Three Peaks intrusions has been made by R. L. 

DuBois of the University of Arizona. He (oral commun., 1964) reported that the NRM for these bodies 

ranges from 158 to 5000 X 10 -a cgs unit, with azimuth 

averaging 6° W. of true north and inclination 40° to 50° 

N. Thus the orientation of the resultant total magnetization vector for the Iron Springs bodies is probably 

not greatly different from the orientation of the earth's 

present field in the region, but the magnitude of the vector may be significantly larger than that of the induced 

component alone. 

To rapidly obtain an approximate value for the average effective susceptibility of the porphyry (that is, a 
B4 GEOPHYSICAL FIELD INVESTIGATIONS 

proportionality factor relating the total intensity of 

magnetization to the ambient field intensity) , magnetic 

north-south traverses were made with an Arvela vertical 

magnetometer across two topographic features in the 

Three Peaks area. Magnetic and topographic profiles 

are given in figure 1. 

Traverse A-A' was made across the Irene monocline, a structural feature whose topographic expression 

is a ridge composed of interior porphyry of the selvaged 

zone. The magnetic profile is erratic, but the smoothed 

curve shows an amplitude increase of 600 to 900 gammas 

associated with the ridge. A graphical two-dimensional 

analysis of the topographic effect was made with a 

Pirson polar chart (Pirson, 1940) on the assumptions 

that the selvaged-zone rock responsible for the topography is uniformly magnetized in the direction of the 

earth's field and rests on a subhorizontal surface of normal interior rock. Effective susceptibilities of 5 and 

7.5 X 10-3 cgs unit, corresponding to the lower and upper 

limits of the observed amplitude, were obtained. These 

values span the upper half of the range of susceptibilities computed on the basis of magnetite content. 

Traverse B-B' was made over Big Peak, a hill 

having a total relief similar to the Irene monocline 

ridge (about 300 feet) but composed almost entirely 

of normal interior porphyry. The thin capping of 

peripheral shell rock on the summit makes a negligible 

contribution to the average anomaly; the zone of selvaged joints is absent. This anomaly is obviously difficult to evaluate, but the topographic effect appears to 

be about 200 to 400 gammas. On the basis of a comparison with traverse A-A', the effective susceptibility 

of normal interior porphyry is probably lower than 

that of selvaged zone porphyry by a factor of 2 or 3, 

and it lies in the lower half of the range of susceptibilities calculated on the basis of magnetite content. 

Both profiles demonstrate the highly irregular magnetization of the porphyry from outcrop to outcrop. 

The difference in average effective susceptibilities of 

the selvaged zone and normal interior rock is in part 

attributed to differences in magnetite content, but 

permanent polarization probably plays a more important role. The average magnetite content of the 

zone of selvaged joints, including magnetic veins, is 

probably close to that of fresh peripheral shell rock and 

is only slightly higher than that of normal interior 

rock because deuteric alteration chiefly affected the 

hydroxyl-bearing mafic minerals rather than magnetite. 

However, the effect of a slight increase in magnetite 

content could be greatly magnified by permanent 

polarization. The NRM of shell rock is locally strong 

enough to have a conspicuous effect on a hand-held 

Brunton compass; much of the vein magnetite is 

lodestone. 

The ratio of the magnitudes of permanent to induced 

polarization ( Q ratio) is very high in some volcanic 

rocks of the Iron Springs district. This is illustrated 

by the basal vitrophyre of the Bauers Tuft' Member 

(Mackin, 1960) of the Quichapa Formation, a highly 

welded ash-flow deposit (ignimbrite). Although this 

vitrophyre is only 5 to 10 feet thick, edges of the sheet 

produce a strong response in the airborne magnetometer. Fortunately, the volcanic rocks are everywhere 

separated from the porphyry by weakly magnetic sedimentary rocks so that their anomalies are easily distinguished from those of the intrusive bodies. 

THE AEROMAGNETIC SURVEY 

The aeromagnetic survey was made under the direction of W. J. Dempsey of the U.S. Geological Survey. 

The detector was a total-intensity flux-gate magnetometer model AN/ASQ 3A towed by a twin-engine 

AT-11 Beechcra:ft at an elevation of 1,000 feet above 

the terrain. Similar equipment and procedures have 

been described by Balsley ( 1952) and others. Where 

the topography is rugged, as over exposed parts of the 

intrusive bodies, terrain clearance along an individual 

traverse may have differed as much as several hundred 

feet from the prescribed value. Profiles were flown in 

an east-west direction and spaced approximately a quarter of a mile apart; control lines were flown at right 

angles to the main family of traverses to correct :for instrument drift so that all profiles could be reduced to a 

common (arbitrary) datum. The instantaneous position of the aircraft during each flight traverse was ascertained by referring fiducial marks on the recorder 

output chart to corresponding marks on continuous strip 

photographs. 

INTERPRETATION OF THE AEROMAGNETIC MAP 

Each of the three areas of porphyry exposed in the 

Iron Springs district-Three Peaks, Granite Mountain, 

and Iron Mountain-is encompassed by a complex magnetic high (see map). The extent of the general anomalies is a rough indication of the lateral extent of 

concealed porphyry. Thus the Three Peaks and Granite Mountain intrusions represent larger buried bodies 

that are probably continuous at depth; the Iron Mountain intrusion appears to be a discrete body whose 

southern and western extremities are concealed. However, deeply buried intrusive rock may not everywhere 

have a distinct magnetic expression, particularly if its 

polarization is less than that of surface samples. Cook 

and Hardman ( 1966) concluded on the basis of a gravity 

survey of the region that intrusive rock underlies a 
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AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH 

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FIGURE 1.-Vertical intensity ground magnetometer profiles across ridges of Three Peaks porphyry. Traverses by H. R. Blank~ 

Jr., assisted by G. W. Green, 1964. 

greater area than is inferred from the aeromagnetic 

data and that all three mapped intrusions have a subsurface continuity. 

Anomalies with short spatial wavelengths, here called 

local anomalies, are superimposed on the broader anomalies associated with each intrusion. They bear no 

consistent relation either to topographic relief or to the 

mapped intrusive margins. The relation of known 

ore deposits to these anomalies is also erratic, although 

228-959-66-2 

niost deposits are associated in a general way with positive magnetic features. The principal source of the 

local anomalies appears to be the magnetic inhomogeneities discussed previously. 

Other groups of local anomalies characterize the areas 

of outcrop of volcanic rocks northwest of the Three 

Peaks intrusion and east of the Granite Mountain and 

Iron Mountain intrusions. The traverse spacing of· the 

aeromagnetic survey is too wide to permit detailed inter-
B6 GEOPHYSICAL FIELD INVESTIGATIONS 

pretation of these anomalies. They reflect magnetic 

inhomogeneities in the volcanic rocks and the edges of 

individual thin sheets, and their true configuration is 

poorly known. 

Even a well-delineated anomaly cannot, of oourse, be 

interpreted uniquely. The following discussion of the 

aeromagnetic map presents interpretations which are 

believed to be geologically reasonable. 

THREE PEAKS AREA 

OU'l"'l~INE OF THE INTRU'SION 

The northern part of the Three Peaks intrusion is 

largely obscured by pediment gravels. However, it is 

clear that the northern half of the western boundary of 

the body is an intrusive fault, the Northwest fault, 

which places porphyry against Cretaceous and Jurassic 

rocks; the fault passes southward into a monoclinal flexure. Porphyry is generally concordant against the 

Homestake Limestone Member of the Carmel Formation along the entire southwestern and southern margins 

in a series of monoclines and monoclinal flexures. No 

marginal contacts are exposed on the east, but there the 

outcrops of intrusive rock terminate along a line believed to represent the trace of a fault, called the East 

fault, which postdates the intrusion (pl. 1). Geologic 

evidence favoring the existence of the fault includes the 

following: (1) The line is parallel to the main set of 

north-trending faults that postdate the intrusion and 

are associated with the Cedar City graben (Thomas and 

Taylor, 1946, pl. 3), and (2) pediment gravels east of 

the line are derived from both Colorado Plateau and 

Three Peaks sources whereas those west of the line are 

derived from the Three Peaks area only. These provenances suggest that the western· extent of Cedar City 

graben fill that is older than the pediment was limited 

here by some topographic obstruction, such as a fault 

scarp or faultline scarp. The discontinuous joint pattern of the intrusive body at the line also supports the 

hypothesis of faulting after intrusion rather than faulting at the time of intrusion. 

The aeromagnetic data may be used to investigate 

the configuration of the concealed parts of the intrusive 

body. The northern edge of the intrusion coincides 

approximately with the northern boundary of the survey,· but the subsurface eastern margin is apparently 

about a mile east of the East fault, in a region of steep 

magnetic gradient. The problem of the eastern extension will be considered later. Concealed intrusive 

rock also extends to the south of the exposed southern 

contact, as was predicted from the dip of the concordant 

contact in this vicinity (about 45 ° S.) . Similarly the 

Bouthwest nose (Mackin, 1947, fig. 2), a smaller feature 

reflected in surface dips of the sedimentary rocks, is 

shown by the isomagnetic contours (pl. 1). The nose 

produces an anomaly extending due west from the 

2,063-gamma anomaly at the southwestern margin of 

the intrusion. 

THICKNESS OF THE INTRUSIVE BODY 

Various attempts to determine the depth extent of 

an intrusive body from aeromagnetic data have been 

reported; for examples, see Zietz ( 1960) and Allingham 

and Zietz (1962). The magnetic method, even when 

used in conjunction with other methods, is subject to 

severe limitations. The amplitude of an anomaly is 

relatively insensitive to depth extent of a thick causative body. Also, it is difficult to isolate near-surface 

effects from the general anomaly, and any calculation 

based on the assumption of uniform intensity of magnetization throughout the body must be treated with 

caution. Nevertheless, it is instructive to compare the 

observed Three Peaks anomaly with the theoretical 

magnetic field produced by geologically plausible 

models. 

The gross pattern of the Three Peaks anomaly 

corresponds to that of a thick tabular body magnetized 

in a direction approximately parallel to the earth's 

present field. A. rather sharp magnetic minimum (only 

partly delineated by this survey) is present at the northern edge; minimums on the sides are produced by a 

combination of lateral flaring of the southern half of 

the body and locally steep margins. The position and 

amplitude of the minimums provide a preliminary 

indication of the depth extent of the intrusive body. 

Consider, for example, the 1,185-gamma minimum associated with the western edge of the Three Peaks positive anomaly. This minimum is about 1,200 to 2,500 

feet west of the intrusion in an area where the intrusive 

contact is known to be nearly vertical. A precise flight 

elevation cannot be given, but it may be assumed to be 

between 500 and 1,000 feet above the upper surface of 

the intrusion at the western edge. Therefore the minimum is 2.4 to 5.0 times the flight elevation from the 

western edge for the lower limit and 1.2 to 2.5 times 

the flight elevation for the upper limit. For a given 

flight elevation, the westward displacement of the 

minimum is a function of the thickness of the body. 

Examination of the edge effect of an ideal vertical-sided 

two-dimensional body (fig. 2) shows that the thickness 

of the intrusive body in this vicinity does not exceed 

about 1 mile, and probably is considerably less. This 

conclusion is independent of the susceptibility assumed 

for the body. Departures from the ideal case, such as 

end effects and outward-sloping sides, tend to displace 

the minimum farther west, so that the thickness 

estimates are maximum values, 
AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH B7 

Magnetic West 

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FIGURE 2.-Edge effect of two-dimensional slabs. Position of west minimum is shown for magnetic east-west traverse 1 depthunit (DU) above slabs of different thickness. Vertical and horizontal SC'ale for models are in depth-units; scale for 

response curves is in percentage of total half space subtended by lower surface of model minus percentage of total 

half space subtended by upper surface. Dashed curves and crosses refer to models 8 depth-units wide; solid lines and 

circles refer to models 16 depth-units wide. Minimum is displaced westward by increasing thickness .of slab. 

A cross section passing through the intrusive body 

and intersecting the two lateral minimums approximately along a magnetic meridian was selected for twodimensional analysis (section 0-0', pl. 1). The intrusive body was represented by hypothetical models 

of several thicknesses, and the magnetic response was 

computed over a range of susceptibilities. Calculations 

were made with a Burroughs 220 Datatron computer 

using a program, based on the theoretical response due 

to bodies of trapezoidal cross section, which was written 

and compiled by Gerald Evenden of the Geological 

Survey. A simulated flight elevation of 6,600 feet 

above sea level was used; this corresponds to a terrain 

clearance of about 1,000 feet at the ends of the profile 

but somewhat less over the intrusive body, as was suggested by examination of the horizontal extent of the 

steepest aeromagnetic gradients. The response curves 

were taken directly from the output of an X-Yplotter. 

The models and theoretical curves for a susceptibility 

of 4 X 10 -a cgs units are shown in figure 3. For comparison, the observed aeromagnetic profile (as obtained 

from the contour map) is reproduced in the same figure, 

with 1,400 gammas as the zero datum. The best overall 

match to the observed curve is probably produced by the 

model about 4,000 feet in thickness. Observed field intensities over the exposed part of the intrusive body no 

doubt reflect near-surface magnetic inhomogeneities as 

well as a contribution from the Irene monocline just 

north of the traverse, and will not be matched by the 

theoretical response of any simple two-dimensional 

model of uniform susceptibility. A model having no 

vertical step on the East fault, but only a change of slope 

of the porphyry surface, produces an improved fit in this 

area. This model, however, seems to contradict the independent geological and geophysical evidence presented in the next section (p. B8-B10). Moreover, any 
B8 GEOPHYSICAL FIELD INVESTIGATIONS 

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<( 

600 

500 

~ 400 

< 

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>= 300 

t-

(/i 

z 

LIJ 

~ 200 

() 

i= 

LIJ 

z 100 <.9 

<( 

:::2: 

-100 5000' 

f----- T=2000 ft 4000' 

f----- T=3000 ft 3000' 

f----- T=4000 ft 2000' 

L---- T=5000 ft 1000' 

SEA LEVEL 

~-1000' 

Magnetic West L___j____l _ _L___L _ _J__---L-L--L _ __JL__L_____L _ _L___L_..L_ _ __L_!_____L____l _ _L____t_ _ _L____J.__.J._____t__L.____ Magnetic East 

0 5000' 10,000' 15,000' 20,000' 25,000' 

FIGURE 3.-Total intensity aeromagnetic anomalies generated by hypothetical two-dimensional models of the Three Peaks 

intrusion, k=4X10-a cgs units. Observed aeromagnetic and topographic profiles 0-0' are indicated by dashed lines. 

increase in the displacement of the porphyry surface at 

the East fault would produce a worse fit. A modification of the shape of the east side of the model is evidently necessary to reconcile the observed and computed 

curves near the eastern minimum; another step fault 

may be present, the intrusive body may extend farther 

east, and the side is probably not vertical. 

The susceptibility used for the curves in figure 3, k= 

4 X 10 -a cgs units, is less than that computed for selvaged-zone rock from the traverse of Irene monocline 

but is rather high for normal interior rock as obtained 

from the traverse of Big Peak. Other combinations of 

susceptibility and thickness could of course be used to 

produce a satisfactory amplitude match of theoretical 

·and observed profiles. However, for thicknesses much 

less than 3,000 feet the required susceptibility is unreasonably large and the discontinuity at the East fault is 

exaggerated; for thicknesses much greater than 5,000 

:feet the required susceptibility for the amplitude match 

might be satisfactory, but the shape of the western minimum is unacceptable. A more precise knowledge of 

susceptibility and natural remanent-magnetization 

values and their distribution is required before a more 

refined quantitative interpretation would be meaningful. 

EAST FAULT 

As a test of the validity and significance of the East 

fault, the trend of this inferred structural :feature was 

traversed at right angles with a Worden gravimeter. 

Observations were made along approximately east-west 

roads, and the reduced values were projected onto true 

east-west profiles. The results are reproduced in figure 

4, together with magnetic profiles taken from the isomagnetic contour map. Profiles D-D' show a distinct 

gravity discontinuity at the indicated fault trace 

amounting to about 3 milligals and a steep magnetic 

gradient having a point of inflection somewhat :farther 

west. The magnetic profile may be influenced by magnetic inhomogeneities in the porphyry. Profiles E-E' 

also show a gravity step across the :fault trace and an 

associated steep magnetic gradient. 

The gravity data reflect the density contrast between 

alluvium and bedrock but do not distinguish between 

the porphyry and the Homestake Limestone Member; 
:::: r 1700 

i 1600 ~ 

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1300 

1200 

w 

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1900 

1800 

1700 

en 

<( 1600 ~ 

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1400 

1300 [ 

1200 

AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH 

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ffil 

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------------------------~jr~~~~/.~r/.7.~~~/.~~7-w.~r/,~~/,~~~r/,~~?.~~r/.~~?.rh~r/.7.~~~/,~r/.7.~~~~~=--------------------------------- E 

5000 0 5000 FEET 

B9 

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>- 10 0::: 

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7000' 

6000' 

5000' 

4000' 

3000' 

2000' 

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5000' 

4000' 

3000' 

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FIGUBE 4.-Gravity. and aeromagnetic profiles across the East fault, Three Peaks intrusion. Gravity Ua.verses by B. R. Blank, 

Jr., assisted by G. W. Greene, 1964. 
BlO GEOPHYSICAL FIELD INVESTIGATIONS 

the magnetics are sensitive to the surface configuration 

of the intrusive body only. In combination, the data 

suggest that a small displacement of the porphyry surface, downward to the east, coincides with the trace of 

the East fault. The density of the porphyry is about 

2.65 grams per cubic centimeter (Leith and Harder, 

1908, p. 47). If the surficial material is assumed to be 

dry unconsolidated sediments with a density of 1.85 

grams per cubic centimeter, a displacement of about 300 

feet satisfies the gravity discontinuity. The :fault apparently dies out to the south where the magnetic break 

disappears. Local disturbances of the magnetic field in 

this vicinity are most likely caused either by irregularities in the bedrock topography or by changes in the 

effective susceptibility in the porphyry, such as would 

be produced by concentrations of magnetite veinlets. 

The presence of porphyry at a shallow depth east of 

the East fault suggests that this area has potential economic significance. The contact of intrusive porphyry 

with Homestake limestone--a :favorable environment 

for ore deposition-could be concealed beneath a thin 

cover of alluvium, and therefore local magnetic anomalies could also represent replacement ore bodies amenable to exploitation. The 1,940-gamma high, apparently superimposed on the linear anomaly associated 

with the fault, may be produced by a replacement body 

just east of the fault zone and should probably be 

investigated further with a ground-magnetometer 

survey. 

LOCAL ANOMALIES 

Most of the local magnetic anomalies within the exposed margins of the Three Peaks intrusion are due 

to magnetic inhomogeneity of the porphyry, magnetite 

veins, steep intrusive contacts, and topographic relief. 

In general the zone of selvaged joints indicated on the 

geologic base map is represented by a magnetic high. 

The high cannot be only an edge effect because it 

diverges from the margin of the intrusive body and 

follows the Irene monocline. It is most pronounced 

where magnetite veins and iron-coated joints are most 

numerous. Some maximums (the 2,060- and 2,110-

gamma anomalies) coincide with vein ore. The Great 

Western vein deposits, perhaps the largest in the area, 

occur in what is contoured as a magnetic sa-ddle on the 

selvaged-zone high (immediately south of the 2,063-

gamma anomaly), but they were not crossed by a flight 

line. Even in the area of rather homogeneous interior 

rock, a weak magnetic ridge (containing the 1,885- and 

1,870-gamma maximums, near the East fault) follows 

a zone of abundant mineralized joints extending south 

of Big Peak. Two of the three negative closures in 

interior rock occur where there are fewest joints (1,735-

gamma minimum) ; the third ( 1,675-gamma minimum) 

is in part a polarization low associated with the positive 

anomaly of Irene monocline. 

Because selvaged-zone rock is generally more resistant to erosion than normal rock of the interior, and has 

a higher effective susceptibility, there is a partial correlation between local magnetic highs and topography; 

however, topographic relief in interior rock is not reflected by the magnetic pattern. The effect of peripheral shell porphyry on the western and southern 

margins is merged with that of the selvaged zon~. 

Isolated outcrops of peripheral shell on Big Peak or 

elsewhere do not disturb the magnetic field appreciably 

at the elevation flown. 

GRANITE MOUNTAIN AREA 

GEOMETRY OF THE INTRUSION 

Previous studies of the Granite Mountain intrusion 

(Mackin, 1954) indicate that the area of outcrop 

represents the easternmost, and structurally highest, 

part of a largely concealed mass. The isomagnetic contours show that porphyry extends beneath the sedimentary and alluvial cover for about 4 miles southwest of 

the southwesternmost exposures (near Desert Mound) 

along the major Laramide trend and northwest of 

Desert Mound approximately to the limits of the survey. The maximum amplitude of the anomaly over 

exposed intrusive rock is comparable to that of the 

Three Peaks maximum; a slightly lower maximum is 

present at the buried southwest end. Like the Three 

Peaks intrusion, the Granite Mountain intrusion probably has the overall form of an elongated slab several 

thousand feet thick, whose upper surface is locally 

up bulged. 

The exposed southeastern intrusive contact is a highangle reverse fault (Gory-Armstrong fault) placing 

porphyry against the Cretaceous and Tertiary sedimentary sections. The magnetics suggest that this fault, 

or at least a steep intrusive contact, continues to the 

southwest beneath alluvium. In the alluvium-covered 

area intrusive rock is probably also present on the southeast side. Similarly the intrusive fault on the northwest side of Granite Mountain (Clive fault) can be 

extrapolated through a zone of steep magnetic gradient 

past Desert Mound toward the southwestern end of the 

intrusion. These two structural features define an upbulged area, which can be regarded as an intrusive 

horst on the surface of the intrusive body. They give 

way at the northeastern end to an intrusive nose, where 

the porphyry contact dips to the north beneath the 

sedimentary cover. The presence of the nose is reflected by a partly exposed plunging anticlinal flexure 

i:ri the sedimentary rocks, designated the Iron Springs 

arch. According to the aeromagnetics, the axis of the 
AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH Bll 

nose swings sharply to the northwest. The axial trace 

as inferred here was not entirely included in the Bureau 

of Mines ground-magnetometer surveys previously 

cited. Because such a structural feature may contain 

selvaged feeder joints produced by late distension of 

the outer part of the intrusion, the supra-adjacent limestone beds constitute a favorable environment for replacement ores and should be thoroughly tested. 

This nose is only about 1 mile from the southwest 

nose of the Three Peaks intrusion. The intervening 

terrane is probably also floored by intrusive rock, so 

that the Three Peaks and Granite Mountain intrusions 

are connected. Depth to porphyry in the gap probably 

exceeds 2,000 feet. 

A broad Inagnetic anomaly, the 1,660-gamma high, 

occurs immediately northwest of the community of Iron 

Springs. The anomaly is attributed to porphyry constituting a second upraised part of the largely unexposed Granite Mountain intrusive slab. Although 

the body is too deeply buried to produce strong magnetic detail, it seems to be bounded by rather steep contacts (intrusive faults?) on three sides, and its surface 

probably dips gently to the southwest. It does not 

affect the dip of Cretaceous rocks on the nearest exposures and consequently could not be detected by surface mapping. The peak of the anomaly was outlined 

by private ground-magnetometer surveys; the presence 

of subsurface porphyry (as well as ore in the Homestake) is reportedly confirmed by commercial drilling. 

The aeromagnetic survey does not encompass the 

entire Granite Mountain anomaly, but it is evident that 

the north west perimeter of the intrusion is deeply 

buried. The broad poorly defined 1,615-gamma 

anomaly is roughly at the center of the body. 

At the southwestern end of the Granite Mountain intrusion, another major upbulge is indicated by the 

2,035-gamma anomaly. This feature is partly isolated 

from the main intrusive horst, although it is alined with 

it and is likewise bounded by steep contacts on the 

northwest and southeast sides. The upbulge is probably responsible for the arcuate pattern traced by the 

northwest-facing scarp of the volcanic sequence in the 

Swett Hills. Drilling near the magnetic maximum reportedly located high-grade replacement iron deposits 

in limestone of the roof rock and bottomed in quartz 

monzonite porphyry. The magnetic expression of the 

ore would be merged with that of the porphyry. 

LOCAL ANOMALIES 

The Granite Mountain anomaly also shows local irregularities over exposures of porphyry. All the exposed rock is either the selvaged or peripheral-shell type 

and should have a relatively high effective susceptibility. Topography must therefore make some contribution to the anomalies. Comparison with a detailed 

geologic map of the intrusion (Mackin, 1954) shows 

that the 2,195-gamma maximum coincides with a zone 

in which vein magnetite is most abundant; the remainder of the pattern is probably caused by a combination of edge effect, mineralized joints, and topography. 

Exceptions are the 1,860-gamma maximum on the north 

nose, which is produced by the so-called Pioche replacement ore body, and the 2,005-gamma anomaly in the 

vicinity of Desert Mound, which probably is largely due 

to a group of replacement deposits. 

Flight traverses passed directly over some replacement ore bodies that do not disturb the isomagnetic contour pattern. Glicken (1955) presented an interesting 

illustration of the effect of the Short Line replacement 

body (due west of the Desert Mound pit) on an airborne 

magnetometer at different flight levels. At 1,000 feet 

or more above the terrane the anomaly of this deposit 

is not distinguishable from the anomaly due to the 

porphyry intrusion. 

SOUTHWEST TERMINAT'ION 

One of the most interesting features of the Granite 

Mountain anomaly is the pattern of isomagnetic contours produced by the concealed southwestern end of the 

intrusion. The pattern implies th3Jt the termination of 

the intrusion is alined with an extension of the Woolsey 

Ranch fault which forms the southwestern border of the 

Swett Hills.\ 

This fault is, however, genetically associated with the 

Iron Mountain intrusion rather than with the Granite 

Mountain intrusion; it marks the northern boundary of 

a zone of chaotic structure formed by gravity sliding 

from Iron Mountain (Mackin, 1960, p. 119-122, and fig. 

8). Within this zone (south of the fault) the structural 

relation of the Iron Springs Formation to the Claron 

Formation, the multiple repetition of the Claron section, and the presence of tectonic breccias, considered as 

a whole, indicate that the bulk of the post-Cretaceous 

section moved generally east-southeastward, accompanied by shearing off and rotation of the Claron. 

Vertical displacement on the Woolsey Ranch fault is 

greatest near Iron Mountain and decreases rapidly to 

the east; the movement becomes largely translational 

before dying out altogether. The fault is not found 

northwest of Iron Mountain at the foot of the Antelope 

Range. The geologic evidence strongly implies that 

the fault was formed in conjunction with the gravity 

· movements. 

Combined lithologic and structural evidence suggesting that the Iron Mountain intrusion was emplaced 

prior to the Granite Mountain intrusion was adduced in 

a previous paragraph. The aeromagnetic data tend to 

rule against a shallow subsurface continuity of the two 
B12 GEOPHYSICAL FIELD INVESTIGATIONS 

bodies or a vertical displacement of the Granite Mountain intrusion by the fault. On the premise, then, th31t 

the Granite Mountain intrusion is a discrete body that 

postdates the fault movement, it seems reasonable to 

suppose that the Woolsey Ranch fault served as a structural barrier to southwestward extension of the Granite 

Mountain magma. Examples of other early faults 

which influenced the shape of the spreading intrusions 

are the Cory-Armstrong fault on the north side of 

Granite Mountain and the Northwest fault of Three 

Peaks. 

The origin of the east-southeast trend of the Woolsey 

Ranch fault is purely speculative, but the trend may 

represent a Laramide line of weakness approximately 

transverse to the axis of the Iron Springs Gap anticline. 

IRON MOUNTAIN AREA 

GEOMETRY OF THE INTRUSION 

The position of the Iron Mountain aeromagnetic 

anomaly corresponds rather closely to the area of porphyry outcrop. The anomaly has the general form 

which would be produced by a thick ovate slab elongated 

slightly in the direction of the main Laramide trend 

and dipping west-southwestward. The magnetic margin and outcrop margin of the intrusive body in the 

northwest; north, and northeast, although obscured by 

the pattern of local anomalies, are approximately coincident. Only in the south and west is there a significant 

discrepancy. 

The curving southeastern boundary of the intrusion is 

a near-vertical intrusive monocline having a throw of 

several thousand feet, which corresponds in position to 

a steep magnetic scarp. This scarp is not completely 

delineated by the survey, but probably continues along 

the southern border of the intrusion; the effect of the 

Mount Stoddard intrusion to the south of the area flown 

distorts the southernmost contours. On the west, the 

porphyry surface slopes downward beneath a cover of 

sedimentary rock and alluvium to form what appears 

to be a nose analogous to the southwest nose of Three 

Peaks. The base of the Homestake (the horizon of intrusion) has been calculated from the westerly dip of 

Claron and Iron Springs rocks to be about 4,500 feet 

below the nearest outcrops of the Claron (more than 2 

miles west of the exposed margin of the intrusion). The 

steep magnetic gradients associated with the Iron 

Mountain anomaly disappear within about half that 

distance. 

Surface geologic evidence, on the other hand, suggests 

that intrusive rock occurs west of the region of steep 

gradient. The westward continuity of south-dipping 

rocks of the Claron Formation tangent to the southeastern and southern margin of Iron Mountain, and the 

arcuate pattern (convex to the north) of the Claron 

northwest of Iron Mountain, are strong structural indications of an intrusive body beneath the alluvium. 

Moreover, porphyry has reportedly been penetrated by 

deep test drilling in this region. Although the critical 

area is not thoroughly covered by the aeromagnetic survey, the data suggest that the weak magnetic ridge with 

a 1,330-gamma maximum and the high of plus 1,400 

gammas at the western edge of the map area are expressions of deeply buried porphyry, probably continuous 

with the Iron Mountain body proper. Thus Iron Mountain, like Granite Mountain, is apparently the highest 

part of a more extensive intrusive body. 

LOCAL ANOMALIES 

Although phases of intrusive rock at Iron Mountain 

are not distinguished on the geologic map, all exposed 

porphyry represents either peripheral or selvaged 

interior phases. The relation of local anomalies to 

magnetic inhomogeneities in the porphyry cannot be 

demonstrated. It is apparent, however, that both topographic relief, which is as great as 1,700 feet, and 

major ore deposits make significant contributions to the 

local pattern. 

The 2,705- and 2,260-gamma highs at the southwest 

end of the intrusion are produced by one of the most 

commercially important ore deposits in the district, a 

group of replacement bodies in the Homestake Limestone Member of the Carmel Formation. Ore and 

Homestake limestone are exposed only at the southern 

end of the positive anomaly. The well-defined polarization low (1,000-gamma minimum) shows that the strike 

of the total magnetization vector is approximately true 

north. No aeromagnetic flight path crosses the rather 

sizable workings immediately to the southeast of the 

deposit; the 1,800-gamma minimum closure shown 

there may have little meaning. The plus 2,200-gamma 

high on the southern periphery and the 2,540-gamma 

high near the eastern margin also represent major replacement ore bodies. Mineralization in the area of the 

2,540-gamma high has occurred at the base of an infolded septum of the roof rock. 

Two local highs in the central part of the intrusion 

probably reflect the topography to some extent but require an additional explanation. A. 2,640-gamma magnetic peak is associated with a magnetite vein system 

that crops out just north of the traverse; this anomaly 

probably is largely caused by the veins. A. 2,765-

gamma high-much broader than the 2,640-gamma 

high-is crossed by the trend of a high-angle reverse 

fault which is believed to extend from the infolded 

septum of roof rocks southwestward across the intrusion. (For a schematic section of this structural. 

feature see Mackin, 1960, fig. 8, sections B and C~) 
AEROMAGNETIC SURVEY, IRON SPRINGS DISTRICT, UTAH B13 

Mineralized Homestake limestone crops out in several 

places along this trend and near the anomaly. The 

anomaly is conceivably produced (at least in part) by 

mineralization of a large infolded or infaulted roof 

pendant almost wholly concealed by quartz monzonite 

porphyry. Alternatively, the anomaly could be caused 

by an increase of effective susceptibility due to abundant mineralized joints and veinlets. 

The magnetic trough just east of Iron Mountain is 

interpreted as an edge anomaly associated with polarization of the intrusive body. Local irregularities in the 

western part of the trough, including three distinct 

lows (the 1,030-, 1,000-, and 1,035-gamma minimums) 

should probably be attributed to the irregular margin 

of the porphyry and to variations in effective susceptibility. Where the porphyry is concealed, the local 

anomalies may have economic significance. However, 

the precise configuration of these anomalies is not 

represented by the isomagnetic contours. 

SUMMARY AND CONCLUSIONS 

1. The aeromagnetic survey roughly delineates the 

lateral subsurface extent of the Three Peaks, Granite 

Mountain, and Iron Mountain intrusions. Granite 

Mountain is an upbulged part of a largely concealed 

body; two additional upbulges on this body are indicated by the isomagnetic contours but do not crop 

out at the surface. 

2. Analysis of the Three Peaks anomaly suggests that 

the intrusive body is floored at relatively shallow 

depth, probably less than 1 mile. 

3. The zone of selvaged and mineralized joints in the 

exposed Three Peaks intrusion has a higher effective 

susceptibility than normal interior rock and is associated with a belt of local magnetic highs; maximums 

occur where magnetite fissure veins are most abundant. Such anomalies in the Three Peaks and Granite Mountain bodies are larger than those produced 

by exploitable replacement deposits, some of which 

are not detected by the survey. 

4. The East fault of Three Peaks probably produces a 

maximum displacement of several hundred feet on 

the surface of the porphyry. The area east of the 

fault is underlain by intrusive rock at relatively shallow depth and could be a :favorable target for further 

prospecting. 

5. The axis of the concealed northern nose of Granite 

Mountain (Iron Springs arch) lies partly to the west 

of the area covered by published ground-magnetometer surveys and may also be favorable for further 

prospecting. 

6. Quartz monzonite porphyry is continuous at depth 

:from Granite Mountain to a northwest-trending line 

possibly representing an extension o:f the Woolsey 

Ranch :fault which forms the southern margin o:f the 

Swett Hills. The fault may have determined the limit 

of southwestward expansion of this intrusion. 

7. Weak but incompletely delineated anomalies west 

of Iron Mountain probably represent deeply buried 

extensions of the main body of porphyry. 

8. A local anomaly associated with a structural trend 

crossing the interior of Iron Mountain may be caused 

by mineralization of a concealed infolded septum of 

the Homestake Limestone Member of the Carmel 

Formation. 

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Birch, A. F., Schairer, J. F., and Spicer, H. C., eds., 1942, Handbook of physical constants : Geol. Soc. America Spec. Paper 

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Cook, E. F., 1957, Geology of the Pine Valley Mountains, Utah: 

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Minnesota : Geophysics, v. 18, no. 2, p. 383-393. 
B14 GEOPHYSICAL FIELD INVESTIGATIONS 

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Slichter, L. B., 1929, Certain aspects of magnetic surveying, in 

Geophysical Prospecting : Am. Inst. Mining Metall. Engineers Trans., v. 81, p. 238-260. 

Thomas, H. E., and Taylor, G. H., 1946, Geology and groundwater resources of Cedar City and Parowan Valleys, Iron 

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210 p. 

Threet, R. L., 1963, Geology of the Parowan Gap area, Iron 

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Utah: Intermountain Assoc. Petroleum Geologists 12th 

Ann. Ji"'ield Conf., Salt Lake City, Utah, 1963, p. 136--146. 

0 

Vestine, E. H., and others, 1947, Description of the earth's main 

magnetic field and its secular change, 1905-1945: Carnegie 

Inst. Washington Pub. 578,532 p. 

Wells, F. G., 1938, The origin of the iron ore deposits in the 

Bull Valley and Iron Springs districts, Utah: Econ. Geology,v.33,no.5,p.477-507. 

Werner, Sture, 1945, Determinations of the magnetic susceptibility of ores and rocks from Swedish iron ore deposits: 

Sveriges Geol. Undersokning, Arsb. 39, no. 5, 79 p. 

Young, W. E., 1947, Iron deposits, Iron County, Utah: U.S. Bur. 

Mines Rept. Inv. 4076, 102 p. 

Zietz, Isidore, 1960, Configuration of the lON pluton, Three 

Forks, Montana: U.S. Geol. Survey Prof. Paper 400-B, 

art.l02, p. B229-B230.