GEOLOGICAL SURVEY CIRCULAR 746

and Other Apatite-Bearing 

Sedimentary Rocks in the Precambrian 

of Northern Michigan

Report prepared in cooperation with

Geological Survey Division,

Michigan Department of Natural Resources
Phosphorite and Other Apatite-Bearing 

Sedimentary Rocks in the Precambrian 

of Northern Michigan

By W. F. Cannon and J. S. Klasner

GEOLOGICAL SURVEY CIRCULAR 746

Report prepared in cooperation with

Geological Survey Division,

Michigan Department of Natural Resources

1976
United States Department of the Interior

THOMAS S. KLEPPE, Secretary

Geological Survey

V. E. McKelvey, Director

library of Congress catalog-card No. (IS 76 600065

Free on application to Branch of Distribution, U.S. Geofogicaf Survey 

7200 South fads Street, Arlington, Va. 22202
CONTENTS

Page

Abstract ______ ___________________________________-___ 1

Introduction _______________________________ ____________________________ 1

Geologic nature and setting of the deposits ___________________ ____ 1

Description of outcrops ___________________________-_ - 3

Section 15 locality ________________________ _ 3

Silver Lake locality ____________________________ ___ 5

Michigamme locality ___________________________________ ______ 5

Huron River and Slate River localities ____________________-_ 5

Possibility of additional deposits ___________________________ _ 5

References cited ________________________________________ 6

ILLUSTRATIONS

Page

FIGURE 1. Geologic sketch map showing location of five occurrences of phosphate-bearing rocks _________-____________-___ 2 

2. Sketch map of Section 15 locality ______---______________ 4

III
Phosphorite and other apatite-bearing sedimentary 

rocks in the Precambrian of northern Michigan

By W. F. Cannon and J. S. Klasner

ABSTRACT

Phosphatic pebbles are widespread in basal units of 

the Marquette Range Supergroup (Precambrian X) in 

the central part of the Upper Peninsula of Michigan. At 

one locality, a conglomeratic bed about 15 m thick averages about 15 percent P*O5, and many thinner beds are 

of comparable grade. This occurrence is believed to be 

the richest sedimentary phosphate deposit known in the 

Precambrian of the United States, as well as one of the 

oldest. Although outcrops are generally scarce, similar 

material has been found at four other localities in the 

Upper Peninsula of Michigan.

INTRODUCTION

Although phosphate-rich sedimentary rocks 

of commercial grade are known at many places 

in the United States and throughout the world, 

phosphate-bearing sedimentary rocks in the 

Precambrian of the United States are rare; 

none that approach economic grade have been 

known previously. We have recently discovered 

that pebbles of apatite are relatively widespread in basal units of the middle Precambrian (Precambrian X) Marquette Range 

Supergroup in the central part of the Upper 

Peninsula of Michigan (fig. 1) and have found 

one locality (Sec 15 locality) where a bed about 

15 m thick averages about 15 percent P2 05 

and where many other thinner beds are of comparable grade. This occurrence is believed to 

be the richest sedimentary phosphate deposit 

known in the Precambrian of the United States, 

as well as one of the oldest. Mancuso, Lougheed, 

and Shaw (1975) reported beds of carbonateapatite in carbonate iron-formation at about 

the same stratigraphic horizon in nearby areas.

The purpose of this paper is to call these occurrences to the attention of those interested in

the geology and mineral exploration of the area 

and to provide information to assist them in the 

further field examination of the phosphatebearing rocks. Our research on these deposits is 

continuing.

GEOLOGIC NATURE AND SETTING 

OF THE DEPOSITS

All known occurrences of phosphate-bearing 

rocks reported here are in metasedimentary 

rocks of the Marquette Range Supergroup, 

about 2 billion years old, and stratigraphically 

near the basal unconformity with lower Precambrian (Precambrian W) gneiss and greenstone. The two occurrences reported previously (Mancuso and others, 1975) are beds of carbonate-apatite in lean carbonate iron-formation 

near the base of the Baraga Group in the 

Baraga basin (Huron River and Slate River 

localities, fig. 1). The three new localities reported here differ from these earlier known 

beds in that most phosphate is in well-rounded 

pebbles commonly 1 to 3 cm long in beds of conglomerate. At the Silver Lake and Michigamme 

localities, the pebbles are in a basal conglomerate within a meter or two of the unconformity with Precambrian W gneiss and are only a 

minor component of the rock. At the Section 15 

locality, the pebbles of phosphate are the major 

component of conglomeratic zones throughout 

the lowermost 100 m of the sedimentary section; one bed is as much as 15 m thick. The 

pebbles are black and very fine grained. On 

fresh surfaces, they are difficult to distinguish 

from fragments of black slate. On weathered 

surfaces, the phosphatic pebbles weather into 

negative relief and have a characteristic light-
MARQUETTE TROUGH

0t10

10 

1

1 

20

I 

30

20 

1

KILOMETERS

30 

1

MILES

EXPLANATION

LOWER PALEOZOIC SEDIMENTARY ROCKS Mostly flat-lying sandstone

JACOBSVILLE SANDSTONE Rat-lying sandstone and 

shale, probably of Keweenawan age (Precambrian Y)

MARQUETTE RANGE SUPERGROUP (PRECAMBRIAN X) Metasedimentary and metavolcanic 

rocks. Phosphate deposits are near base

-GRANITIC GNEISS AND GREENSTONE (PRECAMBRIAN W)

Contact

FIGURE 1. Geologic sketch map of the central part of the northern peninsula of Michigan showing the location of

five occurrences of phosphate-bearing rocks.
gray to white chalky coating. In this respect, 

they resemble limestone or dolomite and were 

originally mistaken for carbonate pebbles by us. 

The Michigamme locality is in the Ajibik 

Quartzite, the basal unit of the Menominee 

Group. All other occurrences are near the base 

of the Baraga Group, which unconformably 

overlies the Menominee Group, although the 

Menominee Group is not present in the areas of 

the Baraga Group phosphate localities.

DESCRIPTION OF OUTCROPS

SECTION 15 LOCALITY

The Section 15 locality is in the NE^i, sec. 

15, T. 49 N., R. 28 W. There, on a low topographic knob, outcrops of rock in the lowest 

200 m of the Baraga Group are relatively abundant, and phosphate-bearing beds of some economic potential are abundant in a stratigraphic 

unit 90 to 100 m thick. The unit is underlain by 

massive to thick-bedded dolomitic quartzite (fig. 

2), which is the basal unit of the Baraga 

Group; the unit is overlain by greenish-black 

thinly laminated argillite. Bedding strikes 

about N. 70° W. and dips 70°-80° SW. Phosphate beds contain well-rounded flat pebbles of 

black aphanitic apatite. Diameter: height ratios of pebbles are mostly 3:1 to 5:1, and the 

largest pebbles are about 10 cm long. The pebbles are in a matrix of fine-grained quartz sand, 

which in the richest beds also includes abundant 

apatite. Muscovite and pyrite are common in 

small amounts, generally a percent or two. The 

apatite pebbles weather into negative relief and 

are coated with a grayish-white-weathering 

residue. Interbedded with the phosphate-rich 

beds is black to light-gray chert and less commonly argillite and arkosic quartzite.

The richest beds contain 40 to 60 percent 

apatite and about 15 percent P2 05 . In spite of 

relatively abundant outcrops (fig. 2), thin soil 

cover obscures much of the bedrock, and our 

data do not completely characterize the P2 05 

content of the entire unit. What follows is a 

general appraisal of the unit based on field observations and limited assays.

The lowest 45 m of the unit (lower part of 

unit 3a in fig. 2) contains many rich beds of 

phosphatic rock about 0.1 to 2 m thick, which, 

by comparison with other assayed beds, contain

as much as 15 percent P205 ; however, they are 

widely interspersed in cherty beds. Two composite chip samples, representing about 20 of 

the 45 m, contain 4 to 5 percent P205 .

Stratigraphically above this unit is a unit 25 

to 30 m thick (upper part of unit 3a) that in 

outcrops is very poor in apatite pebbles and was 

not assayed. It is overlain by a rusty-weathering unit about 5 m thick which contains abundant carbonate rhombs and pyrite and for 

which a composite chip sample indicates about 

9 percent P205 .

The richest unit (lower part of unit 3b) is 

immediately above the rusty zone. A composite 

chip sample representing about the lowest 15 m 

of unit 3b contains slightly more than 15 percent P2 05 . The uppermost part of the unit (upper part of 3b), about 30 m thick, is known 

from field examination to contain many phosphate-rich beds, but it has not been adequately 

sampled. Two composites of two and three grab 

samples contain only about 0.5 percent P205 , 

which, from field observations, we believe is not 

representative of this part of the section; the 

P2 05 content is probably considerably higher. 

High scintillometer readings, typically produced by apatite-rich beds, were measured over 

parts of the uppermost 30 m of section, and a 

scintillometer profile measured across the outcrop indicates that this uppermost part of the 

section also contains considerable apatite.

The limited sampling indicates that the P2 05 

content of the entire 100-m-thick unit is 5 to 10 

percent but that individual beds, the thickest 

being at least 15 m, contain about 15 percent. 

The outcrops are somewhat weathered, and 

apatite has been preferentially dissolved out of 

the siliceous matrix. Although we attempted to 

avoid the most deeply weathered material in 

our sampling, this preferential solution may 

have resulted in lower assays than would be 

characteristic of unweathered rock.

Samples were also analyzed for 65 elements 

by a semiquantitative spectrographic technique. 

The richest apatite unit contains about 120 ppm 

cerium and about 200 ppm yttrium, but no 

other anomalous values were detected. The apatite-rich beds are radioactive, generally producing readings of two to three times background, 

measured on the outcrop, and show a slight 

trend toward being more radioactive in strati-
DC 

CQ

I

UJ 

DC CX

X

Approximately 180 m south and 

240 m west to center of sec. 15, 

T. 49 N., R. 28 W.

EXPLANATION

Thinly laminated gray-green argillite

Phosphate-bearing unit a, gray to black banded chert 

containing beds 0.1-2 m thick of pebbly phosphate rock, b, mostly pebbly phosphate rock containing some cherty beds

Rusty-weathering dolomitic quartzite 

Greenstone and granitic gneiss

 Contact 

.CT^-C^Cr Sheared zone 

f|l||; Area having abundant outcrops

FIGURE 2. Sketch map of the Section 15 locality showing the distribution of outcrop areas and contacts between

lithologic units.
graphically higher units. No uranium was detected in the semiquantitative analyses, but the 

method is very insensitive to uranium, the limit of detection being 320 ppm.

SILVER LAKE LOCALITY

Along the north shore of Silver Lake in the 

NW}4, sec. 8, T. 49 N., R. 28 W., are many exposures of the basal units of the Baraga Group 

and of the Precambrian W gneisses, on which 

the Baraga Group rests unconformably. The 

roughly planar unconformity dips southeastward toward the lake, so that only the lowermost meter or two of the Baraga Group is exposed above lake level. The sedimentary rocks 

are mostly poorly sorted conglomerate, mostly 

of local derivation, containing clasts of gneiss 

and vein quartz, but shaly beds are also present. 

Phosphate occurs in rare, well-rounded pebbles 

composed of very fine grained apatite. The pebbles are as much as 2 cm long and etch away 

selectively, leaving a characteristic chalkywhite-weathering coating. No concentrations 

approaching commercial grade have been found, 

but only the lowermost beds of the section are 

exposed; richer beds might be found higher in 

the section beneath the lake.

MICHIGAMME LOCALITY

At the Michigamme locality in the NWVi, 

sec. 24, T. 48 N., R. 31 W., phosphate is mostly 

in well-rounded apatite pebbles, but thin (~1 

cm) beds of apatite have also been recognized. 

Outcrops can be reached from a gravel road 

leading from U.S. 41 to Keewaydin Lake. About 

1.1 km north of U.S. 41, steep south-facing 

bluffs of Precambrian W granitic gneiss rise 

abruptly from nearly featureless lowlands of 

glacial outwash. The lowlands are-underlain by 

Ajibik Quartzite and Siamo Slate of the Menominee Group, but outcrops are extremely 

sparse (Klasner and Cannon, 1974). Within 

about 200 m east and west of the road, the basal 

conglomerate of the Ajibik Quartzite can be 

seen along the base of the bluffs. This conglomerate lies on gray-green sericitic rock, probably 

a regolith, which grades downward into granitic gneiss. The conglomerate is very crudely 

bedded, dipping about 60° S., and contains 

clasts of vein quartz, gneiss, older quartz-pebble conglomerate, and apatite. The matrix is 

quartzose and contains disseminated pyrite and 

magnetite grains. Apatite in both pebbles and 

beds is extremely fine grained and difficult to 

identify, even with a microscope, but nowhere 

does it appear to compose more than about 1 

percent of the rock. Only the basal 1 to 2 m of 

the sedimentary section is exposed here, and 

because of the lack of outcrops, neither the 

lateral extent of the conglomerate along strike 

nor the possibility that it becomes more phosphate-rich up section can be determined.

HURON RIVER AND SLATE RIVER LOCALITIES

The Huron River locality is in the bed of the 

Huron River in the NWi/4, sec. 35, T. 52 N., R. 

30 W. The Slate River locality is west of the 

Slate River near the center of sec. 28, T. 51 N., 

R. 31 W. These two occurrences were described 

by Mancuso, Lougheed, and Shaw (1975). Carbonate apatite occurs as laminated brownishblack beds in a cherty carbonate iron-formation 

about 20 m thick near the base of the Baraga 

Group. The apatite beds contain traces of calcite and variable amounts of pyrite (0-25 

percent).

POSSIBILITY OF ADDITIONAL DEPOSITS

All presently known phosphate-bearing localities, which are roughly within a circle 40 

km in diameter, were found "accidentally" during reexamination of field notes and hand specimens in the laboratory or office. None were 

identified initially in the field. The fact that 

five localities have been found through such a 

cursory examination and that outcrops are very 

sparse suggests that phosphate-bearing rocks 

may be fairly common and widespread in the 

area. On the basis of the exposed rocks, only 

one locality seems to have economic potential; 

however, so little is known of the rocks beneath 

the glacial cover that at present the potential 

cannot be fully determined. At both the Silver 

Lake and Michigamme localities, outcrops are 

restricted to the lowest 1-2 m of the sedimentary section, and the nature of the overlying 

sediments is unknown. Because at the Section 

15 locality, the richest beds are well above the 

basal unconformity, similar rich beds might be 

present at these other localities.
The stratigraphic horizons of interest, the 

lower 100-200 m of the Precambrian X section, 

are so poorly exposed throughout the region 

(only a very small fraction of 1 percent of these 

rocks crops out) that the Section 15 locality is 

unlikely to be the only rich deposit or the best 

deposit in the region, especially because the 

phosphate-rich rocks tend to weather rapidly. 

We suggest, therefore, that other phosphaterich units may well be present beneath the 

glacial cover in this area.

REFERENCES CITED

Klasner, J. S., and Cannon, W. P., 1975, Preliminary 

geologic map of the southern part of the Three 

Lakes 7 }&-minute quadrangle, Baraga County, 

Michigan: U.S. Geol. Survey open-file rept. 75-8, 

scale 1:24,000.

Mancuso, J. J., Lougheed, M. S., and Shaw, R., 1975, 

Carbonate-apatite in Precambrian cherty iron-formation, Baraga County, Michigan: Econ. Geology, 

v. 70, no. 3, p. 583-586.