Geologic and Airborne 

Radioactivity Studies 

in the Rock Corral Area 

San Bernardino County 

California

GEOLOGICAL SURVEY BULLETIN 1021-C

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

A CONTRIBUTION TO GENERAL GEOLOGY

GEOLOGIC AND AIRBORNE RADIOACTIVITY STUDIES IN 

THE ROCK CORRAL AREA, SAN BERNARDINO COUNTY,

CALIFORNIA

By R. M. MOXHAM, G. W. WALKER, and L. H. BATJMGARDNER

ABSTRACT

The investigation in the Rock Corral area was undertaken to determine the 

relation between the anomalously high radioactivity recorded during an airborne 

survey and the distribution and mode of occurrence of radioactive material. 

Thorium-bearing minerals occur in relatively small, highly radioactive biotiterich inclusions in a porphyritic quartz monzonite. Radioactive accessory minerals are also disseminated in the porphyritic quartz monzonite and in detritus 

derived from the porphyritic quartz monzonite. The configuration and amplitude 

of the major radioactivity anomalies detected from the air indicate that they 

have resulted chiefly from the large masses of porphyritic quartz monzonite 

rather than the biotite-rich inclusions. An analysis of the recorded radioactivity 

anomalies in the Rock Corral area and the equivalent-uranium content of the 

source rocks indicates that the lower limit of sensitivity of the airborne equipment, with respect to gross geologic features, is probably 0.001 percent.

INTRODUCTION

FIELDWORK

The investigation was undertaken to determine the relation between 

the anomalously high radioactivity detected during an airborne survey 

and the distribution and mode of occurrence of radioactive material in 

an area of about 1% square miles near Rock Corral, San Bernardino 

County, Calif, (fig. 29). A secondary objective was to determine 

whether the concentrations of radioactive minerals might constitute 

potential sources of thorium or uranium.

In 1949, prospectors discovered a number of small masses of rock 

containing thorium-bearing minerals in the vicinity of Rock Corral. 

In early 1952, preliminary reconnaissance examinations of the known 

occurrences and adjacent area by D. F. Hewett, G. W. Walker, and 

L. H. Baumgardner indicated that some biotite-rich inclusions in 

plutonic rocks exposed in the Rock Corral area contained concentra109
110 CONTRIBUTIONS TO GENERAL GEOLOGY

ROCK CORRAL 

AREA

es 15 and 16

ROCK CORRAL AREA

Twentynine Palms

il

15 Miles

F.GUKB 29. Index map of the Rock Corral area, San Bernardino County, Calif.

tions of radioactive minerals. An airborne radioactivity survey" 

seemed desirable to prospect for additional biotite-rich inclusions or 

other concentrations of the radioactive materials. In March 1952, 

an airborne radioactivity survey of about 80 square miles was made- 

(Moxham, 1952) in the vicinity of Eock Corral (pi. 15). The airborne survey determined that anomalous radiation intensity, which.

was recorded in many parts of the area, was concentrated principally

in four localities to the east and southeast of Eock Corral, and alsothat some of the anomalies were of the greatest amplitude recorded 

by the airborne equipment over natural sources. One of the four

a,ms mentioned includes me anomaly o! greatest peak intensity recorded in the Mojave Desert region, as well as several lesser anomalies,, 

and for that reason was selected for the detailed study on the ground..
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 111

The investigation, made in June 1952, consisted of geologic reconnaissance mapping of an area of approximately 1% square miles, referred to as the Rock Corral area. A radioactivity survey was made 

on a 300-foot grid in a selected portion of the area (pi. 16), and the 

various rock types exposed in the area were sampled for petrographic 

study and for analyses of their equivalent-uranium content. This 

work was done in behalf of the Division of Raw Materials of the 

Atomic Energy Commission.

LOCATION OF THE AREA

The area is in San Bernardino County, Calif., 50 miles east-northeast 

of San Bernardino. The mapped portion is on the northeast flanks 

of the San Bernardino Mountains, on the southern edge of the Mojave 

Desert. Most of the area is in sees. 4 and 9, T. 2 N., R. 5 E. San 

Bernardino meridian, though a part extends into sec. 5 and part may 

extend into sec. 8. Rock Corral, which is located at one of the few 

permanent springs in the region, lies 5 miles northwest of the center 

of the mapped portion.

STRATIGRAPHY

No previous geologic mapping has been done in the area described, 

although reconnaissance mapping by Vaughan (1922) in an extensive 

area a few miles to the west and southwest has established a background adequate for a discussion of the geologic environment of the 

radioactive minerals. The Rock Corral area is geologically similar 

to the area mapped by Vaughan and to the other areas in the western 

Mojave Desert. In these adjacent areas the plutonic rocks, which 

intrude pre-Cambrian metamorphic rocks, are divided by Vaughan 

into "heterogeneous plutonic rocks" of undetermined age and a younger 

intrusive called the Cactus granite of probable Late Jurassic age. 

According to D. F. Hewett, U. S. Geological Survey, recent studies 

indicate the intrusive rocks named Cactus granite by Vaughan are 

probably Cretaceous rather than Late Jurassic in age (oral communication, December 1951).

PRE-CAMBRIAN ROCKS

Metamorphic rocks of pre-Cambrian age are exposed predominantly 

in the northeast and east part of the mapped area (pis. 15 and 16) 

and consist of biotite gneiss, siliceous metasedimentary rocks, and 

dark intercalated recrystallized metavolcanic( ?) rocks. The metamorphic rocks are contorted and intricately intruded by porphyritic 

quartz monzonite of pre-Cretaceous age and are commonly recrystallized adjacent to contacts with the quartz monzonite. Many apophyses 

and small separate masses of quartz monzonite containing substantial
CONTRIBUTIONS TO GENERAL GEOLOGY

quantities of wall rock occur in these recrystallized zones. Some of the 

recrystallized metamorphic rocks and the quartz monzonite containing 

wall rock are megascopically similar, and in many localities the 

various petrographic types are so intricately related as to preclude 

mapping as individual units. They have been grouped, therefore, into 

:a single unit labelled "igneous and metamorphic complex" (pis. 15 

.-and 16).

PBE-CRETACEOUS BOCKS

The pre-Cambrian rocks have been intruded by buff to gray pre-

 Cretaceous quartz monzonite, which in detail has a considerable range 

of textures and compositions; in most places the rock is porphyritic,

 containing alined, but erratically distributed, orthoclase phenocrysts 

in a medium-grained groundmass. An average sample of the quartz 

'monzonite contains orthoclase (about 40 percent), plagioclase that 

Tanges in composition from oligoclase to calcic andesine (about 30 

percent), quartz (about 20 percent), biotite (3 to 5 percent), small

 amounts of magnetite, allanite, apatite, zircon, muscovite, fluorite, and 

.minor amounts of many other accessory minerals. Chlorite, derived 

from biotite, is present in small amounts, and most of the feldspar 

.has either been sericitized or saussuritized. Locally, as in the southwest part of the area, the orthoclase, phenocrysts predominate over 

the groundmass, and the quartz monzonite contains only sparse mafic

- and accessory minerals. In other outcrops, the mafic and accessory 

minerals are abundant.

Near contacts with the pre-Cambrian metamorphic rocks, the quartz 

monzonite contains many metasomatically altered biotite-rich inclusions of wall rock, These inclusions range in shape and size from 

nearly equidimensional masses as much as 20 feet in diameter to 50- 

foot tabular plates, which may represent reconstituted sections of

-sedimentary beds. Some inclusions, or small roof pendants, are gray 

mica Schist and Others are nonfoliated black masses composed of 80 to 

^90 percent biotite, minor amounts of andesine, magnetite, alknite, 

.sphene, apatite, and zircon.

Parts of the Rock Corral area are underlain by a fine-grained in- 

.truSive ( ?) Crystalline rOCk With a texture and composition like the

.groundmass of the porphyritic quartz monzonite. The fine-grained 

TOCk contains sparse accessory minerals except biotite, no orthoclase

phenocrysts, and is commonly more strongly foliated than the quartz

imonzonite. Locally, Contacts between the quartz monzonite and the 

fine-gained, rock are sKar-p, wkereas in other areas, contacts appear 

tO D6 grSiUcltlOIial j probably the fine-grained rock represents an eari7

:pliase Of quartz monzonite intrusion^ perhaps correlative with ti\Q, 

porphyritic quartz monzonite. Small intrusive bodies of quartz-

-feldspar pegmatite, probably genetically related to the quartz mon-
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 113

zonite, occur in areas underlain by the igneous and metamorphic 

complex and the pre-Cambrian metamorphic rocks.

CRETACEOUS BOCKS

The Cactus granite of Vaughan (1922), which is younger than the 

porphyritic quartz monzonite, intrudes both the pre-Cretaceous quartz 

monzonite and the pre-Cambrian metamorphic rocks. On the west 

edge of the area, in a typical exposure, the granite is uniformly coarse 

grained, light gray, and composed of orthoclase, andesine, quartz, 

and minor amounts of biotite, magnetite, chlorite, muscovite, zircon, 

apatite, sphene, and allanite. The plagioclase and orthoclase may be 

present in equal amounts, though commonly the plagioclase predominates; according to some rock classifications the rock would be 

a quartz monzonite rather than a granite.

Many fine-grained felsic sills and dikes extend from the main 

granite body into the older rocks. The texture and composition of 

the sills and dikes near the main intrusive mass are identical with the 

Cactus granite, but at localities more remote from the main mass 

these apophyses are commonly very fine grained, locality porphyritic, 

and resemble dacite. A few of the fine-grained dikes, not definitely 

correlated with the Cactus granite, may possibly be Tertiary volcanic 

intrusive rocks, but in this report all of them are grouped with the 

Cactus granite.

TERTIARY AND QUATERNARY ROCKS

Small remnants of a Tertiary oli vine basalt flow are exposed in the 

southwest part of the mapped area. Areas adjacent to the dry washes 

are covered by Quarteriiary bench gravels or slope wash. The washes 

and valleys of the mapped area are floored with Recent alluvium.

STRUCTURE

The dominant faults trend northwest and dip at steep angles both 

east and west. Although the regional structure is poorly known, it is 

believed that some of the northwest-trending faults in the Rock Corral 

area are extensions of normal faults which other workers have mapped 

along the northeast edge of the San Bernardino Mountains. The 

movement on these faults has dropped the valley, or northeast block, 

in relation to the mountain, or southwest block. Some of the northwest-trending faults may have existed before the intrusion -of the 

porphyritic quartz monzonite, though recurring movement along these, 

or later parallel faults, has displaced masses of Cactus granite of 

Vaughan and possibly also displaces the oli vine basalt of Tertiary age. 

A second set of faults, which trends northeast, locally offsets the northwest-trending faults. Some of the faults, which trend northeast, are

340908 55 2
114 CONTRIBUTIONS TO GENERAL GEOLOGY

displaced by northwest-trending faults. Many of those zones of 

rupture are filled with intrusive Cactus granite or with a fine-grained, 

dacitelike rock comparable to the Cactus granite.

OCCURRENCE OF RADIOACTIVE MINERALS

Petrographic studies of 18 samples of rocks of the area indicate 

that several radioactive accessory minerals are present. Significant 

concentrations of these accessory minerals occur in metasomatically 

altered, biotite-rich inclusions in the quartz monzonite, in porphyritic 

quartz monzonite, and in detritus deposited in washes that drain outcrop areas of the porphyritic quartz monzonite. Not all outcrops of 

the rock types mentioned above, however, contain significant concentrations of the radioactive accessory minerals.

The minerals responsible for the radioactivity of the various rock 

types were for the most part identified either by means of radioactivity 

measurements of relatively pure fractions of a particular mineral, or 

by the presence of pleochroic halos around a particular mineral species. 

Some accessory minerals such as monazite (?) and xenotime (?) were 

present in such small quantities that it was not possible to separate a 

pure fraction for radioactivity tests nor were they found in association 

with biotite. Probably such minerals do contain radioactive elements 

and have contributed to the total radioactivity in the Eock Corral 

area.

The radioactive minerals that have been identified include allanite, 

zircon, apatite, sphene, monazite(?), and xenotime(?). Allanite is 

the principal source of the anomalous radioactivity in the Bock Corral

area.

Eight samples representative of each of the four major lithologlC 

types in the area were collected for determination of equivalenturanium content. Each bedrock sample consisted of fresh chips collected at about 3-foot intervals along lines indicated on plates 15 and 

16; the samples of alluvium represent surficial material collected at 

about 3-foot intervals along the lines indicated on the same plates.

Equivalent-uranium content determined, by beta-gamma radioactivity measurements 

[Analyst, B. A. McCall, tr. s. Geological Survey]

Sample

l.................................._....

2.... .......... ................... 

5................................... ... 

6.......-.............. ~ T....... 

O-_ ---------......_ 10..........................:..:.;;;;;;".: 11............. ................... .......

Description

Porphyritic quartz monzonite.. ------------------ 

__ do ------ - ------

 ..do --- --- .......-....--..-.. __ ..

... ..uo........__..__..._.___._._...__._.__.

Alluvium........ _ .. . __ . __ .............

AA £tR.msri!nnont'Qrv rn/>to

Equivalent

uranium

(percent)

O nio

.007

.008 

.008

.007

.0023 

.0026

.0026
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 115

The heavy minerals (>3.3 specific gravity) were separated from the 

rest of sample 6 and further separated .into magnetic-susceptibility 

fractions by means of an alnico magnetic and a Frantz isodynamic 

separator. Each magnetic fraction was analyzed for equivalenturanium content.

Analysis of heavy-mineral portion of sample 6 

[Analyst, B. A. McCall, U. S. Geological Survey]

Magnetic-susceptibility Equivalent uranium

fraction (percent) 

Alnico magnetic (few grains) 

Frantz, 0.3 amp___________________________ 0.09 

Frantz, 0.6 amp___________________________ .17 

Frantz, 0.9 amp____________________________ 1.00

Nonmagnetic________________________ . 24

PORPHYRITIC QUARTZ MONZONITE

Samples of various phases of porphyritic quartz monzonite collected 

from different localities in the area contain an unusual number of accessory minerals, and the relative abundance of the accessory minerals 

differs in each phase. The porphyritic quartz monzonite, in which 

the groundmass predominates over the orthoclase phenocrysts, invariably contains a greater abundance and probably a greater assortment of accessory minerals than the quartz monzonite in which the 

phenocrysts predominate. The most abundant accessory mineral 

identified is biotite, but the rocks also contain magnetite, ilmenite, 

fluorite, apatite, allanite, many varieties of zircon, two varieties of 

sphene, and lesser amounts of zoisite, rutile, xenotime(?), corroded 

hornblende, tourmaline, garnet, monazite(?), and topaz(?). Some 

outcrops of quartz monzonite, in which the groundmass predominates, 

contain appreciably more biotite and fluorite than similar-textured 

quartz monzonite elsewhere. An increase in the biotite and fluorite 

content generally indicates an increase in the content of allanite, zircon, 

and apatite and may indicate an increase in the monazite (?) and 

xenotime(?). A point-count petrographic analysis was made of a 

thin section of porphyritic quartz monzonite which contains fluorite, 

moderate amounts of biotite, and other accessory minerals. The analysis shows that locally this rock contains as much as 4.5 percent allanite 

and nearly 1.5 percent zircon.

In all types of porphyritic quartz monzonite, the allanite and some 

types of zircon are definitely radioactive; both are invariably surrounded by pleochroic halos where found in contact with biotite. 

Nearly pure fractions of the allanite and zircon are appreciably radioactive. Monazite (?) and xenotime (?) occur only in minute quantities 

and are not found in association with biotite; they have been classed, 

therefore, as probably radioactive. Apatite, locally abundant and
116 CONTRIBUTIONS TO GENERAL GEOLOGY

commonly associated with biotite, is essentially nonradioactive; however, a few apatite grains, containing minute unidentified inclusions 

are surrounded by faint pleochroic halos in the enclosing biotite. 

Sphene, like apatite, is considered essentially nonradioactive, though 

one grain of sphene observed in a thin section is apparently surrounded by a faint pleochroic halo. Tests of the other accessory 

minerals indicate that they are not radioactive.

Porphyritic quartz monzonite, which, contains concentrations of 

both biotite and fluorite and appreciable amounts of radioactive minerals, crops out extensively along the ridge northwest of check point 76, 

flight line ±A (pis. 15 and 16) and north to the fault contact between 

the quartz monzonite and the igneous and metamorphic complex. 

South of the ridge, to midway between flight lines -Ld and 5, the quartz 

monzonite contains appreciably less fluorite; the biotite, zircon, and 

allanite content south of the ridge varies locally, but is commonly less 

than in the quartz monzonite along the crest and north of the ridge. 

Allanite, zircon, and probably other radioactive minerals are present 

in the masses of the porphyritic quartz monzonite exposed in the north 

and northeast part of the area. However, no attempt has been made 

to determine the quantity or distribution of the radioactive minerals 

in these masses.

Spectrographic analysis of sample 6 

[Analyst, H. W, Worthing, U. S. Geological Survey]

Element Percent 

Al, Si ______ over 10 

Ca, K, Na, Fe_____________________________ 1.O-10.0 

Mg, Ba____________ ___ _ - 0.1-1.0

Ce, Sr, Ti, Nd, B, Or, La, Mn, Pr, Co______________ 0.01-0.1

Pb, Ga, V, Cu, Sc_ 0.001-0.01

Yb,Be______-______-_________________________-__ 0.0001-0.001

INCLUSIONS HIGH IN BIOTITE

The mineralOgiC Composition of the biotite-rich inclusions varies 

only slightly from one inclusion to another but the. inclusions illVariably contain not less than 60 percent of olive-green to brown biotite. 

A thin section of a sample from an inclusion about 15 feet in diameter 

was analyzed petrographically by the point-count method. The 

analysis ShOWS about 74 percent biotite, about 8 percent feldspar,

slightly more than 7 percent allanite, 5 to 6 percent magnetite, about

3 percent apatite, and 1 percent of each sphene and zircon. The radioactivity of the inclusion is due largely to the high content of allanite; 

distinct pleochroic halos surround crystals of both allanite and zircon. 

Other biotite-rich inclusions exposed in the area are also appreciably 

radioactive, probably due to the content of allanite.

Biotite-rich inclusions in the porphyritic quartz monzonite are 

exposed at only a few localities in the Kock Corral area. All of the
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 117

appreciably radioactive inclusions occur in the south-central part of 

the area. The total outcrop area of all the biotite-rich inclusions is not 

more than a few hundred square feet in an area of nearly 1% square 

miles.

ALLUVIUM

Some of the alluvium, which fills the washes that drain areas underlain by porphyritic quartz monzonite containing significant quantities of wall rock, is nearly identical in mineralogic composition to 

the source rocks. The alluvium in these washes is composed of abundant quartz and feldspar, and lesser amounts of biotite and accessory 

minerals. Some placer concentrations of dark-colored minerals occur 

as thin discontinuous layers in the alluvium and, locally, on the surface 

of the alluvium. The principal constituents of the dark layers are 

quartz, feldspar, biotite, and magnetite; the content of radioactive 

minerals is only slightly greater than that found in typical specimens 

of porphyritic quartz monzonite.

RADIOACTIVITY SURVEYS

AIRBORNE SURVEY

The airborne radioactivity survey in the Rock Corral area was made 

with gamma-radiation detection equipment mounted in a multiengine aircraft. A nominal flight level of 500 feet was maintained 

during all traverses, and the distance of the aircraft from the ground 

was measured with a radar altimeter and continuously recorded by a 

graphic milliammeter. Aerial photographs were used for pilot guidance, and the flight path of the aircraft was recorded by a 35 millimeter 

gyrostabilized camera using continuous-strip film (Jensen and 

Balsley, 1946).

The observer integrated the data by means of a synchronous circuit 

that simultaneously marked the radiation and altimeter records and 

the strip film, thereby relating the flight records to the ground.

All measurements of gamma radiation intensity were made with 

scintillation equipment developed by the Oak Ridge National Laboratory, Division of Health Physics, in cooperation with the Geological 

Survey. The components of the equipment are shown in figure 30. 

The equipment consists of two independent detecting and recording 

units operated from a common power supply. The detectors of each 

unit consist of three thallium-activated sodium iodide crystals 2 inches 

thick and 4 inches in diameter. A photomultiplier tube, mounted in 

direct contact with each crystal, detects the scintillations and feeds 

the resultant pulses into the amplification stages. The amplified pulses 

are fed into a count-rate recorder which is essentially similar to a 

scaling circuit. The incoming signal is measured over a 1-second 

period and is recorded in the following 1-second interval. A con-
118 CONTRIBUTIONS TO GENERAL GEOLOGY

3 Nal 

crystals

High 

voltage

3 Nal 

crystals

Pre-amplifler Linear 

amplifier

Linear 

amplifier

Timer

Radar 

altimeter

1

E-A 

recorder

Count-rate

recorder

Altitude 

compensator

Count -rate

recorder

E-A 

recorder

E-A

recorder

FIGURE 30. Diagram of the scintillation detection equipment.

tinuous record is provided through the incorporation of two identical 

channels in each count-rate recorder which alternately measure and 

record the radiation intensity. A mechanical timer driven by a synchronized motor provides the necessary switching. The development 

of this recording technique has been described elsewhere.1

A d-c bucking voltage is utilized to eliminate, in effect, the cosmic 

component from the total counting rate so that the recorded intensity 

represents the net counting rate. To compensate automatically for 

variation in radiation intensity with variation in distance from the 

ground, the output of the radar altimeter is fed into the recording 

circuit in such a manner as to modify the sensitivity of the count-rate 

recorder proportional to e~> (where /*=linear absorption coefficient 

of the air, and 3*= height of the aircraft above the ground). This

function is an approximation of the relation between the distance and 

radiation intensity from a semi-infinite plane source. 

The flight lines of the airborne survey (fig. 29) were oriented in

a northwestward direction and, in the greater part of the area, were

spaced at quarter-mile intervals. In the eastern part of the area,

intermediate flight lines were spaced at eighth-mile intervals to provide greater detail near known occurrences of radioactive materials,

Seven flight lines (24,3,34,4,44,5, and 54), spaced at approximate 

eighth-mile intervals, cross the area (pis. 15 and 16).

TKG. x^iati^. of ti*o ^one on tiie ground from wiiicn anomalous radioactivity is measured at the nominal 500-foot flight altitude varies

with the areal extent and tenor of the SOUTC6 rOCkS. HOW6Ver, at 

quarter-mile flight-line spacing the radiation from smaller areas of

considerable intensity, located midway between the flight lines, probably would not be resolved. By employing an eiglltll-mlle

1 Stead, F. W., 1951, Airborne radioactivity survey In the vicinity of Grants, McKinley, 

and Valencia Counties, New Mexico: U. S. Geol. Survey, Trace Elements Memo. Rept. 161.
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 119

interval at a nominal 500-foot altitude, smaller variations of the 

source distribution are better delineated.

The data of the airborne radioactivity survey were compiled as 

an isoradioactivity contour map (pi. 15) in order to determine the 

relation between the measurements of radiation intensity at the 500- 

foot flight level and the surface distribution of source materials. 

The intensities shown are net values (total minus cosmic).

The maximum radiation intensity recorded at the 500-foot flight 

level over the middle of the mapped area is related to the main mass 

of quartz monzonite. On flight line 4J. the radiation intensity 

exceeded the limit of the graphic recorder so that the configuration 

of the peak of the anomaly is not fully known. The shape of the 

profile of radiation intensity along this line suggests that at least 

one, and possibly two, additional contours would be needed to delineate 

the true radiation magnitude. One additional contour has been 

sketched on plate 15, but its location may be considerably in error. 

The areal extent of the principal anomaly in the central part of the 

area conforms in most places to the principal outcrop area of quartz 

monzonite. A northeastward extension of the principal anomaly conforms in general to the alluvium-filled wash that drains the northern 

part of the main mass of quartz monzonite. Another extension in 

the east-central part of the area is situated over outcrops of the igneous 

and metamorphic complex, which contains a considerable amount of 

quartz monzonite. In most localities a rather sharp decrease in the 

radiation intensity was recorded over the fine-grained intrusive(?) 

rock that borders the porphyritic quartz monzonite on the north 

and east.

The gneiss of pre-Cambrian age and metasedimentary and metavolcanic( ?) rocks are generally low in radioactivity, as is the Cactus 

granite, which occupies the western part of the area. The areas of 

basalt of Tertiary age in the southwest and south-central part of the 

area are rather small and appear to have had little influence on the 

configuration of the radiation flux.

The typical relation between the profile of radiation intensity at 

the 500-foot flight level along flight line 5, and the various geologic 

units in the mapped area is shown in figure 31.

GROUND SURVEY

The radioactivity survey on the ground was undertaken to determine the surface distribution of radioactive materials in the Rock 

Corral area and to relate the surface distribution to the data obtained 

by measurements at the 500-foot flight level.

The survey was made by occupying stations on a 300-foot grid, with 

one coordinate of the grid coinciding with projections of the flight
ELEVATION

SECOND PER COUNTS INTENSITY, RADIATION 

OZl 01- IVHSNaO 
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 121

lines. The data obtained were compiled as an isoradioactivity contour 

map (pi. 16).

All radioactivity measurements were made with a portable survey 

meter, equipped with a brass-walled Geiger counter 2 inches in diameter and 24 inches long. The net radioactivity was determined at 

each station and the results were contoured at an interval of 6 /ir/hr 

(microroentgens per hour). The resulting isoradioactivity map presents only the gross configuration of the radioactivity field, owing 

to the rather wide spacing of the survey grid necessitated by the 

relatively large area to be covered. For instance, the felsic dikes, 

related to the Cactus granite, and the biotite-rich inclusions are1 relatively small geologic features which are quite negatively anomalous 

insofar as their radioactivity is concerned, but few of these small 

features could be resolved on the 300-foot grid.

The occurrences of radioactive materials are confined chiefly to 

the biotite-rich inclusions and the quartz monzonite host rock. The 

minerals responsible for the radioactivity have been described.

The biotite-rich inclusions in the quartz monzonite contain the 

most significant concentrations of radioactive minerals in the Rock 

Corral area. A chip sample, collected across the biotite-rich mass at 

sample location 1, contains 0.032 percent ell (equivalent uranium), 

which is the maximum radioactivity found in the area. None of the 

biotite-rich masses were so situated geographically or were of sufficient size or tenor to influence materially the configuration of the radiation flux.

The average equivalent-uranium content of the principal mass of 

quartz monzonite in the central part of the area is about 0.006 percent. 

The radioactivity is highest in the northeastern part of the outcrop 

area where the quartz monzonite is in fault contact with the igneous 

and metamorphic complex. In the areas of maximum radiation intensity the quartz monzonite, there moderately porphyritic, probably 

contains as much as 0.009 percent ell; to the southwest, the rock 

becomes increasingly fine grained and the equivalent-uranium content 

decreases to an average of 0.003 percent.

In most areas there is a sharp decrease in radioactivity from the 

porphyritic quartz monzonite to the foliated fine-grained intrusive (?) 

rock that bounds the quartz monzonite on the northeast (geologic 

contact shown by dashed line in pis. 15 and 16). The radioactivity 

of the igneous and metamorphic complex varies directly with the 

amount of quartz monzonite included but is generally considerably 

lower than that of the main mass of the quartz monzonite.

The pre-Cambrian rocks are generally low in radioactivity. Sample 10, collected from an outcrop of metasedimentary-rocks in the 

southeastern part of the area, contains 0.0026 percent eU. Likewise, 

felsic dikes related to the Cretaceous Cactus granite are relatively
CONTRIBUTIONS TO GENERAL GEOLOGY

low in radioactivity, probably containing less than 0.002 percent eTL 

The composition of the slope wash is essentially the same as the quartz 

monzonite from which it was derived and in general is comparable in 

radioactivity to the parent material. Two samples of alluvium (9,11) 

contain 0.0023 and 0.0026 percent eU respectively. The main outcrop 

areas of the Tertiary olivine basalt flows are a short distance beyond 

the limits of the radioactivity survey on the ground. The radiation 

intensity measured in the airborne survey indicates that the basalt 

contains negligible quantities of radioactive material.

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eU CONTENT OF THE SURFICIAL MATERIALS

FIGURE 32. Relation between surface radiation intensity and equivalent-uranium content

of the source rocks.

The relation of the surface radiation intensity of the various rock

iypes in the surveyed, area to trie equivalent-uranium, content of the

rocks is shown in figure 32.

or n^rno^cxivixY or THE ROCKS TO 

MEASUREMENTS FROM THE AIR

This study was undertaken to determine the relation between the 

distribution of radioactive materials in the rocks and measurements of

the radioactivity from the air. More specifically it was desired to

fenOAv -wKetlxer or- j^ot the laigli meii.siiT-eni.e:n.ts ax-e cliae to biotite-riola

thorium-bearing inclusions or from some other material.

The principal anomalies in the mapped area, as shown on plate 15 r

are symmetrically disposed in relation to the outcrop areas of the 

quartz monzonite of preLCretaceous age, the igneous and metamorphic
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 123

complex containing quartz monzonite, and the alluvium containing a 

high percentage of quartz monzonite detritus. Assuming that the 

major anomalies have resulted solely from the quartz monzonite masses 

described above, with no contribution from the biotite-rich bodies, then 

the areal extent of the source materials would for the most part constitute a semi-infinite plane, with respect to the solid angle of response 

of the airborne detector. The behavior of the airborne equipment 

with respect to a semi-infinite plane source has been determined 2 as 

follows :

h=(kS/A0S0 ] (1-0.342^) e^-^r (\\ 

where /&= response of the detector in cps (counts per second)

S= tenor of the semi-infinite source in percent eU (equivalent

uranium)

S0 tenor of the calibration source (0.35 percent eU) 

.A 0 =area of the calibration source (1,600 square feet) 

k = instrumental constant (3.19 X107 ) 

ju= linear absorption coefficient (1.4X1Q-3 ft-1 ) 

x= altitude (500ft)

At flight line 4, which crosses the northern part of the principal 

quartz monzonite outcrop, the peak intensity recorded (/&) is about 

1,000 cps. Solving equation (1) for $, an equivalent-uranium content of 0.0075 percent is obtained. Sample 6, collected a short distance 

south of the vertical projection of flight line 4, contains 0.008 percent eU.

In the eastern part of the area, flight line 4 crosses an area (between 

check points 34 and 35) underlain by alluvium. The average /&. 

over this material as shown on plate 16 is about 350 cps. From equation (1), S= 0.0026 percent eU. Kadiometric assay of sample 11 

shows an equivalent-uranium content of 0.0026 percent.

It would appear that the gross source materials in the Kock Corral 

area closely follow behavior typified by semi-infinite plane sources. 

This fact becomes more apparent if the opposite assumption is made, 

that is, that the large anomalies have resulted from a point source, 

such as a biotite-rich body. The response of the airborne detector 

with respect to a point source has been determined 3 as follows :

/ = (k ASe-^x/AoS^) ( I + p-r - 0.342 1>] 2 ) (2) 

where 7P = net response of the detector in cps above background (seebelow)

S= tenor of the point source (percent equivalent uranium) 

/S0 = tenor of the calibration source (0.35 percent equivalent 

uranium)

3 Sakakura, A. Y., 1954, U. S. Geol. Survey unpublished report. 

* Sakakura, A. Y., Idem.
124 CONTRIBUTIONS TO GENERAL GEOLOGY

A = area of the point source (square feet) 

A0 = area of the calibration source (1,600 square feet)

k=instrumental constant (3.19 X107 )

/*= linear absorption coefficient (1.4X1Q-3 ft"1 )

o?= altitude (500ft)

r= air distance from source to detector (The value of x/r will 

, be assumed to be unity, that is, the center of the source is 

vertical with respect to the detector, and r=500 feet.)

In the point source equation, Ip is the net amplitude of the anomaly, 

that is, the peak intensity minus the intensity recorded over the adjacent "normal" rocks. The peak intensity recorded in the vicinity of 

the biotite-rich masses was in excess of the full-scale deflection of 

1,000 cps. However, assuming that the peak value is 1,000 cps and 

the background intensity 350 cps (see plate 15), Ip then becomes 650 

cps. Solving equation (2) for SA, the grade X area of the assumed 

point source must have a value of 3,770. Now, there is a total of five 

biotite-rich masses along line 4A, west of check point 76, so close together that it is conceivable that they would not be resolved individually, but would be recorded as a single anomaly. Most of the biotiterich masses are very small, rarely exceeding a few tens of square feet 

in extent, so that a value of 1,000 square feet for a total A for the five 

biotite-rich masses would be a liberal figure. Thus, SA having a value 

of 3,770, S would be approximately 3.77 percent eU. A sample of one 

of the more radioactive of the biotite-rich masses contains 0.032 percent eU, so it seems certain that the anomalies in the Rock Corral area 

did not result from the small thorium-bearing masses.

. Some speculation on the lower limit of sensitivity of the airborne 

equipment may be warranted from the flight data obtained during the

Rock Corral survey. The sensitivity scale on which the equipment was 

operated had a full-scale deflection of 1,000 cps. Full-scale deflection

.was obtained over relatively broad source rocks containing about

0.007 percent eU so that a change of 0.001 percent in the equivalenturanium content of the source rocks would result in a change of about

14 percent in the recorded radiation intensity, if a linear relationship

may be assumed. The range of statistical deviation of the measurements during the Rock Corral survey was about 5 percent, so that

changes on the order of 0.001 percent in the equivalent-uranium content of broad source materials should be within the detectable limit of

the airborne equipment, assuming conditions of geometry and energy 

spectrum comparable to those existing in the Rock Corral area.

For example, consider the area of anomalous radioactivity measurements outlined by the isoradioactivity contours (pi, 16), which extends 

northeastward from flight line 3J. across lines 3 and 2J.. The peak 

intensity along the crest of the anomaly is somewhat in excess of
GEOLOGIC AND RADIOACTIVITY STUDIES, ROCK CORRAL AREA, CALIF. 125

30/ir/hr, and the average equivalent-uranium content of the source 

rocks (as determined from fig. 32) is approximately 0.004 percent. 

In a northwestward direction from the crest, the radiation intensity 

at ground level decreases to about IS/^r/, indicating that the equivalent-uranium content has decreased to between 0.001 and 0.002 percent. 

The radioactivity anomaly detected from the air, outlined by the 

isoradioactivity contours (pi. 15), has therefore resulted from an 

overall difference in equivalent-uranium content of about 0.003 percent in source rocks occupying about 10,000 square feet. The anomaly 

in question is well defined by the airborne data and it seems likely 

that a lower sensitivity limit of 0.001 percent eU is possible for 

geologic units of similar extent.

CONCLUSIONS

Radioactive materials in the Eock Corral area are confined chiefly 

to: Quartz monzonite intrusive masses, containing as much as 0.009 

percent eU the radioactivity is attributed to disseminated thoriumbearing accessory minerals, chiefly allanite and zircon; biotite-rich 

inclusions in the quartz monzonite, usually less than a hundred square 

feet in extent, containing as much as 0.03 percent eU the radioactivity is attributed chiefly to allanite; and alluvium composed largely of 

quartz monzonite detritus.

The radioactivity anomalies detected from the air resulted from 

the outcrops of quartz monzonite, rather than from the small biotiterich inclusions.

As the inclusions are associated with the most radioactive geologic 

unit in the area, it may be possible to determine other areas of the 

Mojave region most favorable for the occurrence of biotite-rich inclusions by delineating the outcrops of the abnormally radioactive host 

rock. «,

Well-defined anomalies were recorded over source rocks of about 

10,000 square feet in areal extent, which are anomalous only to the 

extent of 0.003 percent eU, with respect to contiguous rocks. It is 

concluded that a lower sensitivity limit of the airborne equipment is 

at least 0.001 percent eU for geologic units of comparable extent.

LITERATURE CITED

Jensen, Homer, and Balsley, J. R., 1946, Controlling plane position in aerial

magnetic surveying: Eng. and Min. Jour., v. 147, no. 8, p. 94-95, 153-154. 

Moxham, R. M., 1952, Airborne radioactivity surveys in the Mojave Desert region,

Kern, Riverside, and San Bernardino counties, California: U. S. Atomic

Energy Commission, Tech. Inf. Service, TEM-360. 

Vaughan, F. E., 1922, Geology of the San Bernardino Mountains north of San

Gorgonio Pass: Calif. Univ., Dept. Geol. Sci., Bull., v. 13, no. 9, p. 319-411.

o