Airborne and Ground 

Reconnaissance of Part of 

the Syenite Complex 

Near Wausau, Wisconsin

GEOLOGICAL SURVEY BULLETIN 1042-B

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of 

the Commission

CONTRIBUTIONS TO ECONOMIC GEOLOGY

AIRBORNE AND GROUND RECONNAISSANCE OF PART

OF THE SYENITE COMPLEX NEAR WAUSAU,

WISCONSIN

By R. C. VICKERS

ABSTRACT

Twelve radioactive mineral localities were found as a result of airborne and 

ground reconnaissance for radioactive minerals in part of the syenite complex 

near Wausau, Marathon County, Wis. The rocks in the area are of Precambrian 

age and consist of syenite and nepheline syenite, which have intruded older 

granite, greenstone, quartzite, and argillite. Outcrops are few, and much of 

the rock is deeply weathered and covered by residual soil.

Thorium-bearing zircon pegmatite float was found within the area of syenite 

and nepheline syenite at 4 localities. Reddish-brown euhedral to subhedral 

crystals of well-zoned zircon (variety cyrtolite) comprise more than 40 percent 

of some specimens. The radioactive mineral at 4 localities outside the area of 

syenites was identified as thorogummite, which occurs as nodular masses in 

residual soil. Alinement of the thorogummite float and associated radioactivity 

suggests that the thorogummite has resulted from weathering of narrow veins or 

pegmatites containing thorium-bearing minerals. Unidentified thorium-bearing 

minerals were found at 3 localities, and a specimen of allanite weighing about 

2 pounds was found at 1 locality.

Shallow trenches at two of the largest radioactivity anomalies showed that 

the radioactive material extends down into weathered bedrock. The occurrences 

might warrant additional exploration should there be a sufficient demand for 

thorium.

INTRODUCTION

As zirconium, thorium, and certain rare earths are among the 

elements that are notably enriched in many alkalic and nepheline 

syenites (Rankama and Sahama, 1950, p. 179-180, 518-528, 564- 

567, 570-572), the literature concerning the syenites near Wausau, 

Marathon County, Wis., was reviewed. Radioactive zircon and 

several minerals that contain rare earths had been reported. The 

area of syenites, therefore, was examined to study the reported 

occurrences, to test methods of prospecting for radioactive minerals 

where outcrops are few, and to use these methods in the search for 

additional occurrences. This work was done by the U. S. Geological. 

Survey on behalf of the Division of Raw Materials of the U. S. 

Atomic Energy Commission.

 . - 25 '
26 CONTRIBUTIONS TO ECONOMIC GEOLOGY

ACKNOWLEDGMENTS

X-ray equipment at the University of Wisconsin was used to 

identify some of the minerals, and the use of these facilities is greatly 

appreciated. The author also wishes to express his appreciation to 

Ernest F. Bean, formerly State geologist of Wisconsin, for providing 

specimens of zircon-bearing rock from the Wausau area. George F. 

Hanson, State geologist of Wisconsin, was especially helpful in allowing 

the writer access to unpublished information. Appreciation is also 

expressed to Elaine Geisse and Daniel S. Turner, who permitted 

publication of thesis material, and to Loren C. Hurd, of Rohm & 

Haas Co., for permission to use unpublished geologic data.

LOCATION, ACCESSIBILITY, AND SURFACE FEATURES

Figure 2 shows the location of the area investigated and the boundary of the syenite complex. The investigation was conducted in 

sees. 9, 10, 11, 14, 15, 16, 21, 22, 23, 26, 27, and 28, T. 29 N., R. 6 E., 

Marathon County, Wis. The area is about 7 miles west of Wausau, 

Wis., and is accessible by many secondary roads.

The area consists of low, rolling hills having a maximum relief of 

about 200 feet. It is part of the Driftless Area of Wisconsin, from 

which weathering products have not been extensively removed; consequently, residual soil covers most of the area, and outcrops are very 

scarce.

Because of the fertility of the residual soil, more than 75 percent of 

the area is under cultivation; the remainder is wooded. The relatively high value of the land for agriculture has caused a lack of interest in exploration for valuable minerals.

PREVIOUS GEOLOGIC WORK

Weidman (1907) did the first comprehensive geologic work in the 

Wausau area and first reported the occurrence of nepheline syenite. 

Emmons and Snyder 1 studied the geologic structure of the Wausau

area to determine the probable conditions under which the nepheline

syenite was formed and to appraise ike nepheline' reWVGS. A 

possible origin for the syenite and nepheline syenite has been proposed by Emmons (1953).

Several workers nave described unusual mineral occurrences in the

area. Weidman (1907, p. 312-315) noted abundant zircon in quartzfeldspar pegmatites associated with nepheline syenite in the NWK

sec. 22, T. 29 N., R. 6 E. Marignacite, a variety of pyroclore, containing about 18 percent rare earths and 0.20 percent thoria, was 

Originally described from the same area (Weidman, 1907, p. 308-312).

i Emmons, R. O., and Snyder, F. O., 1944, A structural study of the Wausau area: Wis. Qeol. Survey file 

report, Madison, Wis.
.R.6 E. R. 7 E.

Geology modified from Emmons, 1953

FIGURE 2. Index maps showing the location of area of investigation near Wausau, Marathon

County, Wis.

In addition, Weidman (1907, p. 252-253, 217-218) suggested that 

lavenite (silicate containing zirconium) and mosandrite (silicate containing zirconium and rare earths) possibly occur in the NE% sec. 27, 

T. 29 N., R. 6 E., and that allanite or piedmontite may occur at 

the rapids in the Wisconsin River at Wausau; Geisse 2 described 

eucolite, a variety of eudialyte containing rare earths, from near the 

NE# sec. 27, T. 29 N., R. 6 E.

The radioactivity of the igneous rocks in the Wausau area was first 

investigated by Turner.3 He found that the gamma activity of the 

various rocks was directly proportional to the frequency of occurrence

* Oeisse, Elaine, 1951, The petrography of the syenites, nepheline syenites, and related rocks west of 

Wausau, Wis. [Unpublished Master of Science thesis in files of Smith College Library, Northampton, 

Mass.] . '. . ....

8 Turner, D. S., 1948, Heavy accessory mineral and radioactivity studies of the igneous rocks in the Wansau 

area, 107 p. [Unpublished Doctor of Philosophy thesis in flies of Univ. Wis. Library, Madison, Wis.]
28 CONTRIBUTIONS TO ECONOMIC GEOLOGY

of zircon, the zircon-bearing pegmatites being the most radioactive, 

then the aplites and syenite pegmatites. His gamma-ray measurements were done with a laboratory Geiger-Miiller counter, and no 

attempt was made to measure the radioactivity of the rocks in the field.

PREVIOUS EXPLORATION

Many attempts have been made to exploit the zircon-bearing 

pegmatites reported in the NW% sec. 22, T. 29 N., R. 6 E. About 

3,100 pounds of float containing about 25 percent zircon was obtained 

from rock piles during the 1920's. During World War II interest 

was revived somewhat when some of the favorable ground was leased 

to Mr. H. L. Geisse who mined about 7 tons of zircon-bearing rock 

from shallow open pits and shipped it to the Rohm & Haas Co. of 

Philadelphia, Pa, for beneficiation tests and chemical analyses.

During 1946 a magnetic and resistivity study, together with 

diamond drilling, trenching, and soil sampling, was conducted by the 

(J. S. Bureau of Mines in the area of zircon-bearing pegmatites in the 

NW% sec. 22. No zircon-bearing rock of a grade comparable to that 

present as float in the area was found.

GENERAL GEOLOGY

Because outcrops are scarce, only the general features of the bedrock 

geology are known. Syenites of various kinds have intruded an 

older igneous and metamorphic series consisting of granite, greenstone, 

quartzite, and argillite. Within the area of younger igneous rocks 

shown in figure 3, the bedrock is mainly medium- to coarse-grained 

syenite, which contains numerous dikes of syenite pegmatite, and 

locally nepheline syenite and quartz-feldspar pegmatite. Most of 

the dikes are restricted to the area of syenite. The size and shape 

of the intrusive rocks are not well known, but there is general agreement among the several workers in the area 4 (Weidman, 1907) regarding the time relations of the various rock types.

A generalized geologic column is given below.

Younger intrusive rocks Syenite and nepheline syenite pegmatite

fl Syenite

.S Aplite

.o 

a 

as Older igneous and Granite

£ metamorphic rocks Greenstone

^ Quartzite

Argillite

A single age determination by the Larsen method of zircon from a 

syenite pegmatite has recently been completed by H. W. Jaffe, of the 

U. S. Geological Survey. The results indicate an age of about 560

.« Emmons, R. 0., and Snyder, F. O., 1944, op. cit.
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WATJSATT, WIS. 29

Base from uncorreeted girpnotos Geology modified from Emmons and Snydef, 1944

AREA COVERED BY AIRBORNE 

RADIOACTIVITY RECONNAISSANCE

Approximate contact

 

Radioactivity anomaly found by

Radioactive mineral locality 

Ao*3 ridiofctivg mirartl prtser. 

wfaro /y«/)///7«rf; Zr, ifnoni 

Tn, tfiorogummltt

FIGUBE 3. Geologic map of part of the syenite complex near Wausau, Marathon County, Wis.

878760 66
30

million years which would correspond to Upper Precambrian (Keweenawan). This age, although preliminary, is in agreement with the 

geology.

RADIOACTIVITY RECONNAISSANCE METHODS

Preliminary work in the Wausau area during May 1953 showed that 

zircon-bearing rock could be easily detected with a Geiger counter. 

Several specimens of float containing abundant zircon were found near 

the center of the NW^ sec. 22 (reported occurrence of zircon-bearing 

pegmatite), and a general reconnaissance was made to find additional 

zircon occurrences or other radioactive mineral localities. To obtain 

as much coverage as possible in the shortest time, all roads shown in 

figure 3 were traversed with carborne equipment consisting of a 

survey meter and two 2- by 48-inch Geiger-Miiller tubes mounted 

on the roof of a half-ton truck. Reconnaissance of the interroad 

areas was then conducted with a scintillation survey meter. Because 

of the small size of the known anomalies and reported zircon occurrences, traverses were made about 200 feet apart. However, as each 

square mile required about 25 miles of walking, only the areas considered as the most favorable were covered, that is, almost all of sec. 

22 and the W% sec. 23.

The anomalies gave relatively high readings (from 0.5 to more than 

5,0 nxr/hr; background 0.03 mr/hr), SO it was proposed to learn whether 

tJiey could be detected With the scintillation survey meter in a light

airplane WHiU flying at a Jow ievel- To test this method of prospecting a scintillation survey meter in a. ngLi ai^flr-ftf t W8iS flown over two 

known anomalies which were found during the ground reconrLa.i3Sance 

One of the anomalies (locality 5) was easily detected from the air at 

an elevation of about 30 feet above ground. Another radioactivity 

anomaly (locality 8) could not be detected because of transmission 

lines and trees that prevented flying less than about 50 feet above 

ground. From this work it was evident that relatively Small areas 

of abnormal radioactivity could be detected from the air if the elevation of the airplane could be kept within 20 or 30 feet above ground. 

Reconnaissance of about 12 sections (shown in fig. 3) was then 

completed with the equipment mentioned above. The flight lines 

were spaced about 200 feet apart and were flown at elevations ranging 

from 10 tO 80 feet, depending upon obstructions such as transmission 

lines and trees. TKo ground speed Varied between 40 and 65 miles

per hour. With a background count 01 a\>w 0.02 mr/kr all readings

greater than 0,05 mr/hr were recorded. The pOSitiOn Of tKe anomauea 

was indicated on airphotos. The scintillation counter was watched 

continuously, as most of the anomalies represented only a very 

momentary full-scale deflection of 0.05 mr/hr. Some of the most
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSAU, WIS. 31

significant anomalies were reflown to check them and mark their 

position more accurately on the airphoto.

Because of the abnormal radioactivity of the syenites and related 

rocks, many of the radioactivity anomalies recorded over these rock 

types were due to the mass effect of large areas of slightly radioactive 

rocks. However, in the areas flown outside the syenite complex, 

ground checking indicated that all but one of the anomalies were 

related to concentrations of radioactive minerals in float and residual 

soil.

All the anomalies found outside the area mapped as syenite and 

related rocks were detected while flying less than about 20 feet above 

the ground. Coverage was not at all complete because many obstructions prevented flying lower than 40 to 80 feet above ground over 

perhaps 50 percent of the area, and because the flight lines were 

spaced farther apart (about 200 feet) than the width of the known 

anomalies.

RADIO ACTIVE MINERAL OCCURRENCES

Twelve radioactive mineral occurrences were found as a result of 

the ground and airborne reconnaissance. Thorogummite (a variant 

of thorite containing hydroxyl) was identified from localities 8, 9, 11, 

and 12 (fig. 3); thorium-bearing zircon was found at localities 2, 3, 5, 

and 6; allanite was found at locality 1; and unidentified thoriumbearing minerals occurred at localities 4, 7,' and 10.

Except at locality 4, there are no outcrops in the immediate vicinity 

of the radioactivity anomalies, and the radioactivity is due mainly to 

residual accumulations of radioactive minerals in the soil or radioactive minerals in float. Shallow trenching at two of the largest 

anomalies showed that the radioactive material extends down into 

weathered bedrock. The source of the radioactive float at three 

localities was not determined.

MINERALOGY

Three minerals that contain thorium and rare earths have been 

identified from the radioactive localities. These are thorogummite, 

zircon (variety cyrtolite), and allanite. Detailed mineralogic work 

would probably show the presence of other minerals containing 

thorium and rare earths.

THOROGUMMITE

Reddish thorogummite (Th(Si04)i-x (OH) 4x ) occurs as nodules 

commonly from one-half to 1 inch in diameter in residual soil and 

'weathered bedrock. X-ray powder photographs of the material 

show that the mineral is crystalline (not metamict) and is associated
32 CONTRIBUTIONS TO ECONOMIC GEOLOGY

with various amounts of hematite. Although thorogummite and 

thorite give almost identical X-ray powder photographs, thorogummite is believed to be formed by the alteration of primary thorium 

minerals, including thorite, to occur as fine-grained aggregates that 

are not metamict but crystalline, and to contain essential water 

(Frondel, 1953, p. 1007-1017). Because crystalline aggregates of 

thoritelike material occur in residual soil and appear altered, the 

material is probably thorogummite. A semiquantitative spectrographic analysis of an impure sample of the thorogummite by G. W, 

Boyes, of the U. S. Geological Survey, follows.

Element Percent ' 

Th XX. 

Fe X.+ 

Si X. 

Al, Ca, Mg, Y X.- 

Ti .X+ 

Mn, Na, Ce, Dy, Er, Gd, La, Sm .X 

Nd, Yb .XCu, Nb, Pb, V .OX+ 

Ba, Co, Sc .OXBe .OOX

1 Speetrographic notations, in percent, are: XX=greater than 10; X+=5-10; X=2-5; X~=l-2.

The sample contained 2.5 percent equivalent uranium and 0.001 

percent chemical uranium.

ZIRCON (VARIETY CYRTOUTE)

Zircon occurs in quartz-feldspar pegmatite float as reddish-brown

euhedral to subhedral crystals that range in length from 1 mm to 

at»outj 1O mm. Xnin sections ol tlie zircon-bearing rooK show that

the zircons are well zoned, altered, iron stained, and in part metamict, 

A chemical analysis of altered zircon concentrates from sec. 22

done by the Rohm & Haas Co. as reported by Turner 5 is given 

below.

Constituent Percent

Zr02 and Hf02 49. 6 

Si02 30.1

Rare earths as Y2 03 2. 6

Cb2O6 and Ta2O6 0. 9

U2O3 < 05

Th02 <. 05

A12 03 5.1 

Fe2 03 3. 5

TiO2 2. 6

Mn02 1.8

CaO <-05

»Turner, D. S., 1948, Heavy accessory mineral and radioactivity studies of the igneous rocks in the Wsusau 

area, 107 p. [Unpublished Doctor of Philosophy thesis in flies of Univ. Wis. Library, Madison, Wis.]
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WATJSATJ, WIS. 33

Constituent Percent

MgO <. 05

Pb <.05

Zn <.05

S <. 05

Ignition loss at 1000° C. 3. 5

100.25

Spectrographic analysis of impure zircon concentrates from sec. 

22 by G. W. Boyes, of the Survey, follows.

Element Percent

Si XX.

Zr X.+

Fe X.

Al, Ce, Th .X +

Ti, Hf, La, Y .X

Nd .XMn, Ca, Dy, Er, Gd, Nb, Yb .OX+

Mg, Ba, Pb .OXCr, Cu, Sc .OOX+

Be .OOXAJLLANITE

The allanite reported as float from locality 1 occurs as a single 

massive specimen weighing about 2 pounds. The specimen has been 

altered to a yellowish to brownish color on the exterior but appears 

black and fresh in the interior. The specific gravity of fresh-appearing 

material from the center of the specimen is 3.92. The specimen contains 0.57 percent equivalent uranium and 0.001 percent chemical 

uranium. A spectrographic analysis of the material by R. C. Havens, 

of the Survey follows.

Element Percent

Ce, La XX. 

Si, Fe, Ca, K, Nd, Th X.+

Al X.

Gd, Sm, Y X.-

Dy .X +

Ti, Mg, Er .X

Mn, Sc .XPb, Yb .OX +

Ba, Sr, V .OX

B .OXZr, Sn .OOX +

Be, Ga .OOX

Co, Cu, Ni .OOXX-ray powder photographs of the material show a very diffuse 

pattern, indicating that the material is probably metamict. The 

mineral gelatinizes with HC1.

Based on the above data, the material is tentatively identified as 

allanite.
34 - CONTRIBUTIONS TO ECONOMIC GEOLOGY 

DISTRIBUTION OF BABE EABTHS

Rare-earth elements (lanthanides) are associated with all the radioactive mineral occurrences, but the relative proportions vary. The 

thorium-bearing zircon occurrences contain cerium as the dominant 

rare-earth element whereas the thorogummite specimens are characterized by greater abundance of gadolinium and erbium. The relative 

abundances of rare-earth elements in specimens of thorogummite 

and zircon (cyrtolite) are shown in figure 4.

DISTRIBUTION OF THORIUM AND URANIUM

The results of analyses of samples collected in the area show that 

thorium is the principal radioactive element present, is generally associated with small quantities of uranium, but the ratio of these two 

elements is not constant. The highest uranium contents were found 

in samples of zircon concentrates that contained as much as 0.054 

percent uranium and about 0.8 percent thorium. The highest thorium 

Contents were found in specimens of thorogummite which contained 

as much as an estimated 15 to 20 percent thorium but only about 0.001 

percent uranium.

Hematite is closely associated with thorogummite, a secondary 

mineral, and thus the hematite may also be of secondary origin. The 

low uranium content of hematite-bearing samples may have resulted 

from the leaching of uranium during weathering. Analyses of samples 

of radioactive hematite-bearing TOCkS and Samples of zircon by S. P. 

Furman, R. F. Dufour, J. S. Wahlberg, G. W. Boyes, J. W. T. Meadows, Wayne Mount]oy, and P. J. Dunton, of the Survey, are listed 

below in order to compare their uranium and thorium contents.

Local- Equivalent ura- Chemical uranium Thorium

Uy Sample nium (percent) (percent) (percent) '

8 Thorogummite and hematite. ___ 4. 2 . 001 XX.

8 ____do__-_----_.------------- 2.5 .001 XX.

5 Hematite (radioactive)--_______ 3.0 .001 X. +

6 Zircon and quartz..----.- ._ .12 .008 .X+

3 ..._do-.__------------------ .10 .034 .X

6 Zircon concentrates-.-.-.------ .21 .054 .X+

1 Spectrographic determination.

Inasmuch as most of the samples in this report have been taken 

near the surface, differential leaching has probably decreased the

original uranium/thorium ratios because Of tin greater mobility of

uranium during weathering. Larger amounts of uranium may be 

expected in unweathered samples taken at depth, but the grade of 

material that may be present is believed to be too low to be of commercial value.
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSAU, WIS. 35

40-

30-

£ 20-

10-

20-

O

cc in u, 1-

La I PrI PmI EuI Tb I Ho I Tm I Lu 

Ce Nd Sm Gd Dy Er Yb'

Total rare earths about 2 percent

La I Pr I Pm ' Eu I ' 

Ce Nd Sm Gd

. I Ho ' T 

Dy Er

I Lu 

Yb

THOROGUMMITE 

Total rare earths about 8 percent

FIGURE 4. Graphs showing relative abundances of the rare-earth elements In specimens of thorogummite and zircon from the Wausau area, Marathon County, WIs. 

Compiled from spectrographic analyses by O. W. Boyes, U. S. Geological Survey.
36 CONTRIBUTIONS TO ECONOMIC GEOLOGY

STRUCTURE

The structural patterns of the various syenites in the area are very 

incompletely known because of the lack of outcrops. Emmons and 

Snyder 6 have interpreted the syenites as being narrow dikes that trend 

northwestward. They further suggest that the zircon-bearing pegmatites may well occur along northwestward-trending faults. The 

zircon-bearing pegmatite float from sec. 22 (locality 3) originally was 

reported 7 to have a northwesterly trend, but later exploration of the 

pegmatite revealed a northerly trend. Detailed reconnaissance with 

a scintillation counter at locality 5 showed a northerly trend of the 

radioactivity anomaly and presumably the zircon-bearing rock. The 

radioactive fine-grained phase of syenite at locality 4 has a strike of 

N. 45° E. Outside the area mapped as syenite and nepheline syenite, 

all the radioactive anomalies in the vicinity of the thorogummite 

occurrences have a westerly trend. The radioactive anomaly at 

locality 8 trends N. 70° W.; anomalies at localities 9, 10, and 12 trend 

almost due west. This alinement of the anomalies and then* narrow 

width (2 to 20 feet) as compared to their length (120 to 200 feet) 

suggest that the primary radioactive mineral (possibly thorite) has 

been localized along westward-trending faults or fractures in the roof 

rocks. It would thus appear that structures of various trends have 

localized the radioactive minerals.

The shape and attitude of the intrusive syenites in the area canfbe 

inferred from the results of ground reconnaissance with a scintillation 

counter. Some of the rock types, namely coarse syenite and nepheline 

syenite and their residual soils, were found to be abnormally radioactive (as much as 0.2 mr/hr, background 0.03 mr/hr), and thus the 

type of bedrock below residual soil could be determined by traversing. 

Although HO attempt was made to determine the exact shape and

extent of such anomalies, the radioactivity of the residual soil indicated 

that much of the coarse syenite was not restricted to narrow dikes but 

OCCUrred in large irregular shaped bodies underlying several tens of 

acres. Northwesterly trends of radioactivity were H0t apparent in the 

area of syenites examined, except in the SWK sec. 22 at the contact

between coarse syenite and the older rocks. Although the geologic

map of part of the Wausau area (Emmons, 1053) SuOWS ft nOrthwtttely 

trend for all the syenite and nepheline syenite dikes, the author was 

unable to find evidence which would support this interpretation.

r»Escnn>TiONS OF RADIOACTIVE LOCALITIES 

LOCALITY 1

GrOUDd reconnaissance in the S#NWH sec. is m tue ^ic^it^ O f tKe

anomalies detected from the air snowed slightly abnormal rftdlO-

«Emmons, R. C., and Snyder, F. G., 1944, A structural study of the Wausau area: Wis. Oeol. Survey 

file report, Madison, Wis. 

7 Wisconsin Geological Survey Township report, T. 29 N., R. 6 E. [Unpublished.]
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSATJ, WIS. 37

activity (0.1 mr/hr, background 0.03 mr/hr) over a large area. Moreover, two abnormally radioactive specimens were found among rocks 

piled along a fence row. The first specimen, weighing about 2 pounds, 

is composed mainly of allanite with minor amounts of fluorite and 

quartz. A description and an analysis of this specimen are given on 

page 33. The specimen contains 0.57 percent equivalent uranium 

and 0.001 percent uranium. A second specimen, weighing about 10 

pounds, is composed mainly of pyroxene (50 percent), quartz (5 

percent), and feldspar (40 percent), with small amounts of allanite (?) 

and fluorite. It contains 0.065 percent equivalent uranium.

LOCALITY 2

Specimens of zircon-bearing pegmatite were found along a fence 

row in the SW^SW^ sec. 15, T. 29 N., R. 6 E. The zircon, estimated 

to make up about 20 percent of the rock, occurs as yellowish to reddishbrown euhedral to subhedral grains that are commonly about 

4 mm in diameter. The source of the zircon-bearing rock was not 

found during a scintillation counter survey of the adjacent fields.

LOCALITY 3

The reported site where several tons of zircon-bearing pegmatite 

were removed during the 1940's is about 100 feet southeast of the 

center of the NW% sec. 22, T. 29 N., R. 6 E. The former open pits 

have been filled, and the area now is under cultivation. Abnormal 

radioactivity was detected in a few small areas, and several specimens 

of zircon-bearing pegmatite were found on or very near the surface. 

A description of the zircon-bearing rock is given on page 32-33.

According to Fiedler 8 the zircon occurred as thin but continuous 

bands in highly sheared feldspar and quartz and also in rich lenses in 

quartz. He reported that three zircon bands, each about 6 inches 

wide, were followed continuously for about 40 feet along the strike 

of the shear zone in the main pit, and a rich lens of zircon surrounded 

by barren and fractured quartz-rich pegmatite was found about 

100 feet south of the main pit along the strike of the zircon-rich bands.

LOCALITY 4

Abnormal radioactivity is associated with a fine-grained phase of 

gray syenite exposed in an abandoned quarry in the NE^NWK sec. 

23, T. 29 N., R. 6 E. The radioactive syenite is about 8 feet long and 

from 6 inches to 1 foot thick. Thin sections show that the rock is mainly 

orthoclase with abundant fluorite and some quartz. No radioactive 

minerals were identified in thin section. Analyses show that the

8 Personal communication from W. M. Fiedler to Rohm & Haas Co., Philadelphia, Pa., 1942.
38 CONTRIBUTIONS TO ECONOMIC GEOLOGY

rock contains 0.23 percent equivalent thorium and 0.004 percent 

uranium. Spectrographic analyses show that thorium is the most 

abundant rare element present.

LOCALITY 5

Abnormal radioactivity was first detected in the SW%NW% sec. 23, 

T.29 N., R. 6 E., in the specimens found along a fence row. These 

specimens, composed of acmite, microcline, and zircon contain fracture 

coatings of highly radioactive hematite. A scintillation counter 

traverse of adjacent cultivated fields shows anomalous radioactivity 

in an oval-shaped area that measures about 90 by 40 feet. An 

isorad map of the anomaly, made by tracing out each of the contours 

of equal radioactivity on a gridded surface, is shown in figure 5.

The thorium content of channel samples taken in several shallow 

trenches, which were dug in the area of anomalous radioactivity, is 

also shown in figure 5. Bedrock was penetrated in all the trenches 

except in the east end of trench no. 5. In general the highest thorium 

contents in the bedrock were found in the area of highest radioactivity 

on the surface. However, the isorad map shows mainly the radioactivity of the residual soil before trenching and therefore may show 

little or no relationship to the presence of radioactive minerals at 

depth.

The results of analyses by S. P. Furman, H. E. Bivens, J. N. Rosholt, 

Jr., and G. W. Boyes, of the Survey, of channel samples taken in the 

trenches are tabulated below.

Trench no.

1. ..................

2 _ ___ _

3-.--.-~ -_. ._

4 ....

5..- 

7

Length 

(feet)

1

2

2

3

3

1

2

0

4

2

2

1

Equivalent 

uranium

0.004

nee

. O41 

.021

.057

07^

.073

.064

. 026

.021

. 014 

. 006

Chemical

uranium

0. 018 

. 006

. 012

.003

.001

Th>

nil

0 15

3. 12 

3. 06

. 17

.22

.22

.20

3.08

». 06

3.04

nil

Ce»

O.X 

.X 

.X

.X-

.bx+

"V" _

Zr»

O.X +

"V I

.X+

.x+

,x

,x1 Radiochemical analysis. Thorium content is computed by determining the amount of a particular 

daughter product present (in this case Bi^) by alpha particle count after chemical separation. If the 

sample is in equilibrium, the amount of Bi2" is proportional to the amount of thorium in the sample.

»Spectrographic analysis.

' Based on eTJ/Th ratios of other samples. ' 
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSAU, WIS. 39 

' li

Trench 

2-3 ft deep 10 Feet

FIGUBE 6. Isorad map of radioactive anomaly at locality 5 (SWHNWH sec. 23, T. 29 N., R. 6 E.) 

showing results of channel sampling of trenches.
40 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Specimens taken from the trenches and pieces of float show that 

the wall rock is pegmatite consisting mainly of perthite, hornblende, 

and quartz. Some hornblende crystals are as much as 1 foot long. 

Abnormal radioactivity is confined mainly to areas in the pegmatite 

where the hornblende has been altered to pyroxene (acmite). In thin 

sections and in hand specimens all stages of alteration of hornblende 

to pyroxene can be seen. The pyroxene is much finer grained than the 

hornblende, the slender pyroxene crystals being 3 to 5 mm in length. 

Alteration of hornblende was accompanied by decrease in grain size of 

microcline, probably caused by brecciation, and introduction of 

zircon (?), which occurs ias reddish altered grains about 0.6 mm in 

diameter. Earthy hematite that is highly radioactive coats many 

fracture surfaces. Spectrographic analysis of selected hematite 

coatings is given below.

Element Percent

Th, Fe XX.

Ca X.

Zr X.- 

Si, Ti, Ce, Nb, Y .X+

Nd .X

Er, La .XX-ray powder photographs of hematite coatings show a good hematite 

pattern with only a few, very faint additional lines. This suggests 

that the thorium and related elements are in either an amorphous 

material or a metamict mineral.

The hematite is probably a product of weathering. The primary 

source of the tWium in the hematite may be zircon.

LOCALITY 6

Several gpecimeilS Of ZirCOn-bearing pegmatite, similar to those at

locality 3, were found in the wxsWH sec. 23, T. 29 N., R. 6 E. Two

specimens weighing about 4 pounds each COUSlSt Of quartz and about

40 percent zircon. A heavy liquid concentrate of the zircon contains 

0.054 percent uranium, 0.8 percent thorium, and about 2 percent total

rare earths. The specimens were found on rock piles, but no areas of abnormal radi0activity covvld be found in fae adjacent fields,

LOCALITY 7

A radioactivity anomaly, dfiteCtCd. during the airborne recomialSsance, in the NW}jNE^ sec. 27, T. 29 N., R. 6 E., was checked on the

ground and found to be due to a small area Of nepheline syenite float. 

A sample of the nepheline syenite contains 0.009 percent equivalent 

uranium, which is about the same order of magnitude as other 

nepheline-bearing rocks in the area.
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSAU, WIS. 41

LOCALITY 8

Abnormal radioactivity in the N%NW% sec. 27, T. 29 N., R. 6 E., 

was found by road traverse with car-mounted Geiger counter. A 

maximum count of about 5 times background was detected. A 

scintillation counter survey of the area adjacent to the road showed 

anomalous radioactivity at several places. The location of the radioactivity anomalies is shown in figure 6, which also includes an isorad 

map of the largest anomaly and the results of channel sampling. Because part of the area is within the gravel roadbed and the adjacent 

ditches, some of the radioactive material probably has been moved 

about during road building.

At the most radioactive anomaly (plus 2.0 mr/hr) thorogummite 

was found in the soil within a foot from the surface. A shallow trench, 

2 feet deep, was then dug. Bedrock was not penetrated in the trench, 

but many fragments of greenstone and decomposed syenite (?) were 

found. Thorogummite occurs as nodules as much as 1 inch in diameter in the decomposed rock. Results of analyses by S. P. Furman, 

H. E. Bivens, and R. G. Havens of the channel samples taken from 

the bottom of the trench (12.5 feet long and 2 feet deep) are as follows:

Field no.

WM-101-53--------_----_--..-

WM-102-53- __ ______ ___

WM-103-53_- _--_.___ ..__-...

WM-104-53-------__--__---_

WM-105-53.--- ......_...... .

WM-10&-53.. ----- __._.. _ .

WM-107-53-. _----.._-.-..-._

WM-108-53_. ...... ..-.__-.._

Length 

(feet)

1

1

3

2

1.5

1

1.5

1.5

Equivalent 

uranium 

(percent)

0. 007

.022

.29

.055

.009

.28

.021

.008

Chemical 

uranium 

(percent)

<0. 001

<. 001

Thorium ' 

(percent)

O.OX+

X.-

.ox

.OX+

Cerium 

O.X-f

.X

.X

.X

' Spectrographic determination.

Small quantities of hematite are associated with the thorogummite, 

and both were identified by X-ray powder photographs. Because of 

the lack of outcrops in the area and the weathered condition of the 

bedrock, little information is known about this occurrence. The 

alinement of the anomalies away from the road suggests that a pegmatite or vein trending about N. 70° W. may be the source of the 

primary thorium minerals.

The small anomaly about 450 feet east of the largest anomaly may 

be due to a similar thorium-bearing body.
42 CONTRIBUTIONS TO ECONOMIC GEOLOGY

vo

r

§i

= UJ

<m 

ujo.
RECONNAISSANCE OF PART OF SYENITE COMPLEX, WAUSAU, WIS. 43

LOCALITY 9

Anomalous radioactivity in the NEJiNWJi sec. 28, T. 29 N., R. 6 E., 

was found during the airborne reconnaissance. A ground scintillation 

counter survey showed slight abnormal radioactivity (0.1 mr/hr, 

background 0.03 mr/hr) in the area and two local areas of high radioactivity caused by nodules about 1 inch in diameter composed of 

hematite and thorogummite. Spectrographic analyses of the nodules 

show XX. percent each of iron and thorium. The nodules contain 

4.5 percent equivalent uranium and 0.002 percent uranium.

LOCALITY 10

An anomaly in the SW%NW# sec. 28, T. 29 N., R. 6 E., which was 

substantiated by ground surveys, was due to abnormally radioactive 

fragments of syenite(?) in the soil over an area about 100 feet in 

diameter. Specimens of weathered syenite(?) contain 0.015 percent 

equivalent uranium and 0.001 percent uranium. No radioactive 

minerals were identified from this locality, but most of the radioactivity appears to be concentrated along fracture surfaces in the 

weathered syenite, A sample of the syenite contains O.X percent 

zirconium and O.OX+ percent thorium, suggesting that the radioactive mineral might be thorium-bearing zircon.

LOCALITY 11

Abnormal radioactivity at locality 11 (SW%SE# sec. 28, T. 29 N., 

R. 6 E.) was found during the airborne reconnaissance. Ground 

checking indicated a westward-trending anomaly 140 feet long and 5 

to 20 feet wide with a count of more than 0.2 mr/hr (background 

0.03 mr/hr). Local areas of high radioactivity within the 0.2 mr/hr 

isorad give gamma counts of more than 5.0 mr/hr. Samples of the 

soil from one of the local areas of high radioactivity contain disintegrated granite(?) and a reddish radioactive mineral that was identified 

as a mixture of hematite and thorogummite.

LOCALITY 12

A ground scintillation counter survey of an anomaly in the SWKSE/4 

sec. 27, T. 29 N., R. 6 E., showed high counts at several places, which 

were caused by float composed mainly of thorogummite in residual 

soil. The local high counts are alined in a westerly direction for a 

distance of about 160 feet.

OTHER AREAS

Except for the radioactive localities described above, all the radioactivity anomalies in sees. 15 and 22 (fig. 3) recorded in the airborne 

reconnaissance survey are apparently related to large areas of slightly
44 CONTRIBUTIONS TO ECONOMIC GEOLOGY . . . . ,

radioactive syenite and nepheline syenite. The radioactivity of the 

large areas of syenite and nepheline syenite would tend to mask the 

radioactivity from small concentrations of radioactive minerals.

CONCLUSIONS

Reconnaissance for radioactive minerals in part of the Wausau 

area has resulted in the discovery of six radioactive mineral localities 

that were found by airborne reconnaissance and six radioactive 

localities found by ground reconnaissance. Although there are few 

outcrops, the methods employed have been effective in finding bedrock 

occurrences of radioactive minerals by detecting abnormal radioactivity in the overlying residual soil. Zircon-bearing rocks have 

been found mainly within the area mapped as syenite and nepheline 

syenite; thorogummite has been found only outside the syenite complex. Further reconnaissance in the area of discussion and hi other 

parts of the syenite complex would doubtless lead to discovery of 

additional occurrences.

The commercial value of the occurrences is not known, because 

several factors, such as ease of recovery of the valuable minerals from 

the gangue minerals, depth and lateral extent of the radioactive 

minerals, grade of the rock, and salability of the ore or concentrates, 

are not known.

LITERATURE CITED

Emmons, R. C., 1953, Petrogeny of the syenites and nepheline syenites of central

Wisconsin, in Emmons, R. C., ed., Selected petrogenic relationships of

plagioclase: GeoL.Soc. America Memoir 52, p. 71-87. 

Frondel, Clifford, 1953, Hydroxyl substitution in thorite and zircon: Am.

Mineralogist, v. 38, p. 1007-1018. 

Rankama, Kalervo, and Bahama, T. G., 1950, Geochemistry: University of

Chicago Press, 912 p. 

Weidman, Samuel, 1907, The geology of north central Wisconsin: Wis. Geol. and

Nat. History Survey Bull. 16, 697 p.

U. S. GOVERNMENT PRINTING OFFICE: 1986