# 26 

UNITED STATES DEPARTMENT OF THE 

INTERIOR J. A. Krug, Secretary 

GEOLOGICAL SURVEY W. E. Wrather, Direct~r 

CIRCULAR 26 July 1948 

MAGNETIC SURVEY AND GEOLOGY OF THE 

EASTERN AND SOUTHEASTERN PARTS OF 

THE IRON RIVER DISTRICT 

IRON COUNTY, MICHIGAN 

By 

Harold L. James and Kenneth L. Wier 

Prepared with the cooperation of the Geological Survey Division, 

Michigan Department of Conservation 

WASHINGTON, D. C. 
MAGNETIC SURVEY AND GEOLOGY OF THE -EASTERN 

AND SOUTHEASTERN PARTS OF THE IRON RIVER DISTRICT. 

ffiON ·COUNTY, MICHIGAN 

By 

Harold L. James and Kenneth L Wier 

Page 

Introduction ••••• :...................... 2 

The magnetic survey..................... 2 

General geology......................... 3 

Stratigraphy....... . • • • • • • • • • • • • • • • • • • • • 3 

Greens tone • • • • • • • • • • • • • • • • • • • • • • • • • • 5 

Footwall strata..................... 5 

Iron-f'orma tion. • • • • • • • • • • • • • • • • • • • • •. 6 

Hanging-wall strata................. 6 

Magnetic slate...................... 7 

Structure.............. • • • • • • • • • • • • • • • • • 10 

Ore deposits ••••• : •••••••••••••••••••••• 10 

Interpretation of' the magnetic survey ••• 11 

Summary des cription·s. • • • • • • • • • • • • • • • • • • • 13 

Section 19, T. 43 N., R. 34 W ••••••• 13 

Section 30, T. 43 N., R. 34 w ••••••• 13 

Section 24, T. 43 N., R. 35 w ....... 13 

Spies mine area ( EisEt) •••••• ·• • • 13 .Swt· sec. 24.. • • • • • • • • • • • • • • • • • • • 14 

.Section 25, T. 43 N., R. 35 w ••••••• 14 

Section 36, T. 43 N., R. 35M ••••••• 14 

Cyr-Lennox properties (Ei-Swt)... 14 

CONTENTS 

Summary descriptions (continued) . Barras property (S~NB~) •••••••• . . Bengal-Tully area l SEi) •••••••• Section 1, T. 42 N., R. 35 W ••••••• Caspian mine area (NE~) •••••••• Morgan exploration (NWfSE~) •••• Cottrell exploration (NE~SEt) •• Fogarty mine (SBtSst) •••••••••• Section 12, T. 42 N., R. 35 W~ ••••• Section 5, T. 42 N., R. 34 W ••••••• Section 6, T. 42 N., R. 34 W ••••••• Borland exploration (NiNWf) •••• Corry exploration (NEiSwf} ••••• Berkshire mine (NWfswf) ...•.... · Buck mine ( Si;Swf) •••••••••••• • ~ 

Section 7, T. 42 !~I R. 34 W ••••••• Baltic mine (~~NWt) ••••.•••••• 

Zimmerman mine area ( EiNWf). • • • 

DeGrasse area (Silmfl·········• Section 8, T. 42 N., R. 34 W ••••••• Section 9, T. 42 N., R. 34 W ••• ~··• 

ILLUSTRATIONS 

Plate 1. 

2~ 

3. 

4. 

Figure 1. 

Hap showing magnetic data and inf'erred geology in the eastern part of' the 

Iron River district, Iron County; Michigan.* 

Hap of' magnetic data in the southeastern part of' the Iron River district, 

Iron County, Michigan* Hap showing inf'erred geology of' the southeastern part of' the Iron River 

district, Iron County, Michigan* Geologic cross sections in the eastern and southeastern parts of' the 

Iron River district (to accompany Plates 2 and 3) * 

Index map of' Iron County, Michigan, showing areas covered by preliminary 

Page 

14 

14 

15 

15 

15 

15 

15 

15 

16 

16 

16 

16 

16 

17 

17 

17 

17 

18 

18 

18 

Page 

reports ••••••••••••••••••••••••••••••••••••••••••••• ·• ••••••••• ·• • · ••••.••••••• • • 4 \.. 

TABLES 

Page 

Table 1. Stratigraphic chart summarizing characteristics of' the iron-f'ormation and 

associated rocks •••••••••••••••••••• ,............................. • • •.• • • • • • • • • • 7 

* Available f'or purchase, Directo~ Geological Survey (see page 18). 

1 

INTRODUCTION 

The Iron River-Crystal Falls district, 

which is in the northern peninsula of 

Michigan, has been under study by the U. s. 

Geological Survey, in cooperation with the 

Geological Survey Division of the Michigan 

Department of Conservation~ for several years. Preliminary reports have been issued on the 

Mineral Hills area,!/ the Ice Lake-Chicagon 

Cr.eek area ,gj and the Crystal Falls-Alpha area (two reports),~ and a report on the · 1central part of. the Iron River district is in 

preparation.!/ The present report is thus 

the fifth of a series of reports issued in 

advance .of a more formal publication in which 

the results of the entire study will be integrated. These .preliminary reports serve a dual purpose; they permit critical evaluation 

of geologic concepts by other geologists and 

by actual exploration prior to the final pub- lication, and they permit release of :factual 

inf'ormation, such as the magnetic data, soon- er than would otherwise be possible. The 

reports are entirely preliminary in nature and 

do, not include discussion of theoretical con- cepts, the results of microscopic study of the 

rocks and ores, nor an appraisal of pertinent 

literature. 

This report describes an area of about 

nine square miles e~st and southeast of the 

town of Iron River; it ipcludes the towns of 

Caspian, Gaastra, and Stambaugh. Within the area is one producing group of mines, the 

Buck group, comprising the Buck, Fogarty, Berkshire, Baltic, Zimmerman, and DeGrasse 

properties, from which 708~324 tons of iron ore were shipped in 1947 by Pickands, Mather 

and Co. Some of the workipgs of the Spies 

mine are in the northern· part of the map area, 

but the shaft is outside the area considered 

in this report. Other pr6-pert1-es that have 

been productive in the past are the Caspian, 

Bengal, and Tully. The ~engal-Tully mine is now being dewatered by theM. A. Hanna Co., 

and it is•expected that mining will be re- eumed in 1948. The ore throughout the. area is 

$ soft, red, non-Bessemer hematite. Some of 

that produced from the Bengal mine was highly JI!.S:ngani f'ero]J:~ ~ · · 

1/ Dutton, c. E., Park,. C. ·F., Jr., and Balsley, 

J. R.,-Jr., General character and succession of tentative divisions in th~ stratigraphy of Mineral Rills district, Iron River, Iron County, Michigans u. s. Geol. · 

Survey Preliminary Report, 1945. . 

2/ James, H. L., Clark, 1. D., and Smith, L. E •• 

Mar;netTc survey and geoloa of the Ice lake-Chicagon 

Creek area, Iron County, Michigan: U. S. Geol. Survey 

Preliminary Report 3-213, 1947. 

3/ Pettijohn, F. J., Geology of the Crystal 

Falh-Alpha iron-bearinc; district, Iron County, Michigans u. s. Geol. Survey Preliminary Map No. 3-181, 

1947. Pettijohn, F. J., Magnetic and geologic data of 

parts of the Crystal Falls-Alpha iron district, Iron 

County, Michic;an: Michir;an Dept. of Cons., Geol. 

Survey Div., 1948. 

4/ Dutton, C. E., Geology of the central part of 

the Iron River district, Iron County, Michigan: U. S~ 

Geol. Survey, report in preparation. 

2 

Most .of the field work, which consisted 

of mappingof all accessible underground openings and the making· of a detailed magnetic 

survey of the area,. was done in 1946 and 1947. 

Several hundred drill holes have been sunk in 

the area, and although the core from some has 

been discarded, that from a relatively large 

percentage ls still available. Core from 

approzimately· 125,000 feet of drilling was ex- amined in the course of thi~ study. 

Despite the volume of data available 

:from drilling, underground development, and 

detailedmagnetic surveys, the complex structure of the area makes it quite impossible to 

determine the position of bedrock boundaries 

with any real degree of accuracy. It is believed that the major :features are outlined on the accompanying maps, but it is certain 

that almost every new drill hole or development will reveal details that will cause some modification in the map pattern. 

The work was possible only with the 

wholehearted cooperation of the mining com- panies. Complete access was given to the 

mines and the core storehouses, and to i'i1e 

information. Prints of hundreds of· maps and 

of drill-hole data .were made ,uponrequest. 

This assistance and the many other courtesies 

extended by officials of Plekands; Mather and 

Co. , the 1'1. A. Hanna .. Co. , the ClevelandCliffs Iron Co., the Mineral Mining Co., and 

the Jones and Laughlin Ore Co. , are gratefully 

a9knowledged. 

Other members of. the U. S. Geological 

Survey who contributed substantially to this 

study .are Carl E. Dutton, Charles F. Park, 

Jr., John J. Hill, and Robert B. Hall, and 

the writers have had the benefit of numerous discussions with F. G. Pardee and G. E. Eddy 

of the Geological Survey Division of the 

Michigan Department of Conservation. 

THE .HAGNETIC SURVEY 

The areawas surveyed with vertical magnetometers of the Schmidt type. Host of the 

survey was made with a temperature-compensated Wolfson ins trumen.t (model I'l-l), with a sensitivity that varied from 28.8 to 29.9 sa-as EJ per scale d1 vision during the course of the survey. A smaller part was surveyed with a temperature-compensated Askanla magnetometer with a sensitivity of 34.5 gammas 

perscale division. No base-station instrument was used; the diurnal correction curve was obtained by three or .fouv observations at a base-station·during the day. The average ~rror is believed to be about 10 gammas, most ~f which probably is due to nonlinear diurnal 

variations and mild magnetic s~orms that 

occurred b~~een base-stat1on checks. 

5/ A ganuna is 1/100~000 of a gauss, which is the 

fundamental unit of magnetic intensity. The total magnetic field of .the earth is about 65,000 gammas in this 

latitude. ·The common dip needle has a sensitivity of 

300 to 500 gar:unas per scale division. 
For.most of the areas, determinations were made at. paced intervals of 100 feet 

along compass lines 300 feet apart. Eacn 

half-mile traverse was tied in to adjacent 

land boundaries. Approximately 7,000 

stations were occupied. The primary base 

station (zero value) is the sal'l:.e as that used on the maps of the Ice Lake-Chicagon Creek area (Preliminary map 3-213); it is located on a narrow side road in the NEiNE~ sec. 28, 

T. 43 N., R. 34 W., about 30 feet west of the 

road along the east side of the section. 

Cultural objects, such as pipe lines, that 

might affect the determinations were avoided so far as possible, and determinations sus- pected of being affected were not used in 

drawing the isomagnetic contours. The survey was hampered in the southern part of sec. 24 

because of magnet~c disturbances set up by the direct current used for tramming in the 

adjacent mine workings. These disturbances 

have been noticed more than half a mile away 

from the nearest workings. 

GENERAL GEOLOGY 

The Iron River-Crystal Falls district is a tightly folded synclinorium which roughly 

forms a triangular-shaped area, with apices 

at Iron River, Michigan, Crystal Falls, 

Michigan, and Florence, Wisconsin (see index 

map, fig. 1). The bedrock geology of this 

basin consists of a complexly folded series 

of slates, graywackes, and iron-formation, 

which res't on a "·greenstone" basement complex 

comprising pillow lavas, agglomerates, tuffs,. some slate, and some iron-formation. The 

iron-formation and associated rocks within 

the basin are generally considered to be 

Upper Huronian, younger than the major ironformation of t · .• e Gogebic, Marquette, and old 

Menominee ranges. The age of the greenstone 

and associated rocks is in doubt. 

The map area covers part or· the western 

apex of the basin. .The geology of the 

southeastern part of the map area (pl. 3} consists, essentially, of a fold belt of ironformation bounded successively by older slate 

and greenstone o~n the south and southwest, 

and by younger strata on the north and northeast. However, at of this area of younger 

rocks contains folds that expose iron-formation at ledge surface; the central part of 

the synclinorium--that is, the part that con- tains only the younger rock at ledge sur- face--lies farther to the east and northeast. 

The major feature of the northwestern part of 

the area (pl. 1) is the deep synclinal basin 

in which younger beds that include a magnetic 

slate are preserved. 

The iron-formation and ore in this area are not magnetic, but some of the beds 

stratigraphically above the iron-formation are strongly so. Much of the geologic interpretation of the area hinges on the pattern 

of anomalies produced by these magnetic rocks. 

Outcrops are very scarce, and in most · places the bedrock is mantled by glacial 

deposits which average about 150 feet thick 

and locally attain a thickness of more than 

300 feet. Both the present surface and the 

ledge surface are of low to moderate relief. 

Except for a few altered diabase dikes, 

intrusive rocks are unknown in this area. 

STRATIGRAPHY· 

The stratigraphic succession in adjacent 

parts of the Iron River-Crystal Falls di·trict 

has been described in the preliminary reports 

mentioned above. The stratigraphy of this 

map area agrees closely with that described 

in general terms for the Ice Lake-Chicagon 

Creek area and, with one important difference, 

with that described in the,report on Mineral 

Hills. Continued investigation by the senior 

author and C. E. Dutton subsequent to the · release of the latter report has shown that 

the concept of a nlower iron-format onn, in 

which the ore of the Bates and east Spies 

-mines was presumed to occur, is not valid, 

and that the ore on those properties is in 

the same stratigraphic unit as that elsewhere 

in the district. 

Detailed work in all parts of the 

district has demonstrated that the stratigr ·phic succession is reasonably uniform 

throughout. The almost bewildering repeti- tion of rock types encountered in most places, 

which led earlier workers to the concept of 

deltaic conditions of sedimentation with 

lenticular beds, actually is due to repeti- tion of the same beds on exceedingly tight 

folds and by faults that are sometimes quite 

inconspicuous. 

In no part of the area can an entirely 

normal stratigraphic succession be examined. 

In all places, overt rned folds and cross- folds (which also are locally overturned), 

coupled with s me adjaoent faulting and 

changes in thickness as a result of faulting, 

shearing, or squeezing, make determipation of 

the succession extremely difficult. Determination of tops of eds is always a problems no fossils, of course. are available; cross- bedding and gradational bedding are exceedingly rare; ripple marks are absent. Drag folding is the most useful criterion for the spot 

determination of tops, but it must also be 

used with caution. Use of inadequately ex- posed drag folds may yield apparently anoma- lous results, and in some places the axes or the folds are overturned on cross folds so that an anticline may have the structural 

form of a steeply plunging syncline, Determination of the stratigraphic succession has 

thus required much detailed mapping and con- tinual reexamina· .ion of concepts. However, 

the stratigraphic succession that has evolved 

is presented with c n~idence, and it is 

believed that although fUture work may add to 

the detail, the framework will be found 

correct. · 

Inasmuch as the great bulk or exploration is naturally limited so far as possibl&, 

to the general zone of iron-formation, the 

stratigraphic succession of that zone is 

known in much greater detail than that of 

parts farther removed from the iron-formation. 

The stratigraphy of the ro·n-forma tion and 

associated rocka is presented in summary form on table 1, as well as in the following paragraphs. 
llt.ST W. R . SSW. 

R 0 N 

• Report in preparation 

llt.SZW R.~IW. 

COUNTY 

N 

INDEX MAP OF IRON COUNTY 

MICHIGAN SHOWING AREAS 

COVERED BY PRELIMINA-RY 

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REPORTS 

Arto covered ·by this report. 

Mtural Hilts Arto. 

Crystal Falla- Alptta Area .. 

Ice Lokt-Chicoton Crttk Arto. 

Stater AIM.* 

Central Iron River Mea.• 

Cryatal Falla-:- Paint Rlv.tr Area.* 

Work ilt J&'Of ...... 

Figure 1. 

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1-' 
Greenstone.--The oldest rock in the area is greenstone, which .makea up the basement on 'Which the yoimger sedimentary 'rocks· were deposited. 'l'b.e rock is not exposed in the 

area, buti was reportedly cut by two drill 

holes in the southwestern part of sec. 8, 

T. 42 N., R. 34 w. It is expos&d several 

places south ot . the map area, as in the · 

sec. 21, .T. 42 N., R. 34 w. Here tihe green• ·stone is massive and shows coarse agglomera tic 

&true ture. Some layers of . the greens tone., 

apparently originally tuffaceous, are strongly 

magnetic. In sec. 18, about li miles south of' 

the map area, the greenstone. contains interbedded slate with some magnetic iron-formation. Green11tone is exposed in the northern 

part of sec. · 13, rr. 4:2 N., R. 35 w •. ;. here the 

rock is diabasic. in appearance and is prob- ably intrusive.•!' P'-llow structures are .not 

present in the:'cfew outcrops of the basement ·greenstone near this area, though common .enough elsewhere. 

'l'b.e age of the greenstone is, as yet, indeterminate. Whether it is equivalent to 

volcanics such as the Middle Huronian Hemlock 

greenstone, or the greenstone of the Keewatin 

series, is notas yet known. 

Footwall strata.-Tb.e footwall. . strata 

comprise those· rocks between the greenstone 

and the iron-formation. Little or n,othing is 

known withiri this area .or · the rock immediatel7 

above the greenstone. In the Ohicagon Oreek 

area,y in sec. 25, T. 43 N •. , :R· 34 ~., it .is a greenish graywacke, and in the Oryetal 

Falls-Alpha area, J./ it is a gray-black slate. 

The oldest footwall rocks in this area for 

which any description can be given are slates 

exposed in the NWfNE~ sec. 12. These rocks are fissile, sericitic to talcose slates in 

which the cleavage is the dominant structure. 

·The color of the outcropping rock varies !'rom 

gray to limonitic yellow. The rock is common- ly contorted~ and bedding is determinable only 

with difficulty, it at all. Drill holes in 

the SE-fSEi sec. 8 cmt similar rock. 

Above the contorted sericitic slate is a series consisting largely of' massive fine- to 

medium-grained gray graywacke and gray sericitic slate. This rock is encountered in 

many of the deeper drill holes and in some · of' 

the mine workings. ·The shaft stations of the 

lower levels of the Berkshire mine are in this 

rock. In the fine-grained facies, cleavage is 

generally more or less well developed and the 

rock possesses a distinct nongraphitic sheen 

not :found in any of the younger slates with 

which it might be confused. Fine banding is 

'common in the silty layers and these almost 

invariably show numerous ·vecy small faults 

that are particularly evident in the drill core. The lower part of this series is largely fine-grained, banded graywac'ke. In many 

places the rock exhibits an ill-defined, 

pseudocong1omeratic structure that is due to 

minor faulting of some layers accompanied by 

y James, H. L., Clark, L. D., and Smith, L. E., 

op. cit. 

]/ Pettijohn, F. J.:. op. cit. 

. 5: . 

shearing and rotation or. the raul ted :fragments • The upper part of the series,which is common- ly entered in mine workings;. is largely a dark 

gray, sericitic; silty slate readily distingu1abable from the overlying graphitic slate 

b1'· 1ts sheen ·and by its gray, nongraph1t1c 

streak. Tne contact~ith the graphitic slate 

is gradational,; with \seric1tic slate alternatil)g with graphitic slate for several :feet. ·As shown by drill core analyses, the seri,c1 tio 

slate charact&ris.tioally contains 6 to 8 .per .. cent iron, and the graywacke 8 to 12 percent 

iron, most of vhich is in the form of interstitial siderite.· .Oxidation o:r· these rocks, 

although not common, has been observed 'Where 

structural conditions werefavorable. 

Overlying the sericitic slates 1s a graphitic, highly pyritic slate which :forma 

the stratigraphic footwall for the iron-to~ .. tion throughout the Iron River-Oeystal Falla 

district. ·The bulk of the rook in this area is, where unsheared, a thin-bedded to laminated, dark,..gray to black rock with a graphitic . streak. .Pyrite forms 35 to 50 percent of the · rock, but is only rarely visible; for the most 

part it is indistinguishable from the other 

fine-grained cc;>ns ti tuents. ·Where sheared, the 

rock is glossy black and parts on innumerable 

slickensided planes. Near or at th.e base of 

the graphitic slate is a characteristic ' 

breccia .§/ (the "speckled gray• of :f~eld 

terminology), which is a marker hOrizon ·· throughout the Iron River district. In this area it is rarely more than 10 feet thick and 

commonly much less. Tb.e breccia is rather 

Jllassive, in contrast to the laminated character of the adjacent slate, and .contains numerous small, angular fragments of slate in a dense, graphitic, pyritic matrix. The trag• 

ments, which rarely exceed half an inch in 

diameter, are most commonly of graphitic slate, 

though :fragments of gray slate are not rare and chert f~gments are found occasionally. 

In places,. many of the graphitic slate fragments show partial or complete replacement by coarse pyrite. 

The true thickness of the graphitic .slate 

is probably between 20 and 40 .feet, although 

because o:f :folding or squeezing' it may appear 

to be much thicker or thinner. The iron 

analyses, as shown in drilling records, are either high or low, depending upon the ~ethod 

of determination. Quite commonly the rock is . roasted to clear the dark color for ti tra tionJ as a result, the pyrite is·converted to · soluble oxide and the records will show an iron content of 15 to 25 percent. If the- ro~k 

is not roasted, most of' the iron remains aai 

insoluble pyrite and the records .will .show en , iron content of 2 to 6 percent. Oonsiderabl• 

contusion bas arisen because of this di.f't'e~~ 

ence in analytical procedure. '!'b.e Sftph1tt o 

slate, because.o:f its high pyrite content; f:s 

responsible :for the numerous mine :fires that 

have occurred in the district. In opecimena 

and drill core this "burning" of' the pyri·te · causes an e:f:t'lorescence or complete breakdown 

to whitish powder within a . :few yeara • 

y Dutton, c. E., Park, c., F., Jr., and 

Balsley, J. R., Jr., op. cit • 
Iron-formation-The iron-formation rests on the graphitic slate with an abrupt but con- formable contact. The iron-formation, where 

unoxidized, consists of interlayered dark gray 

chert and gray siderite with occasional layers 

of sideritic slate. Graphitic slate partings 

and seams as much as a few inches thick are not uncommon. The thickness of individual 

chert and siderite layers may be as much as 4 

inches, but more typically range from a quarter of an inch to 2 inches. The siderite 

and chert are very fine-grained, and layers of one generally contain a small percentage of 

the other mineral. No iron silicates, such as gruneri te, have been found in the uno·xidized 

iron-formation in this area. 

Oxidation has converted the siderite to 

hematite in many places to yield "oxidized 

iron-formation." The chert that remains is 

most commonly bleached to white or gray. 

Leaching of the chert or replacement by iron 

oxides converts the oxidized iron-formation to ore in which the original banding is generally 

preserved. The average iron content of the 

unoxidized iron-formation is approximately 24 

percent, that of the ore 50 to 60 percent. 

.Most of the iron-formation is oxidized in 

part , so that the iron content of the formation as a whole is about 35 percent. On the 

maps and sections accompanying this report, no distinction is made between oxidized ironformation and ore. 

TP.e iron-formation, in almost all places, 

is tightly folded, so. much so that this 

feature is practically a definitive criterion. 

Because of this folding, the true thickness of 

the formation is difficult to determine; in 

most places it is 100 to 140 feet, but locally 

it may be as little as 10 feet (see cross section E-E' plate 4) as a result of erosion 

at the end of iron-formation time. 

The top of the iron-formation is, in 

many places, marked by an unconformity (or 

disconformity) with the overlying rocks. This 

unconformity, although widespread in the Iron 

River-Crystal Falls district,y is rarely of 

sufficient intensity to significantly change 

the thickness of the iron-formation. Within 

the iron-formation it is commonly marked by the occurrence, generally in the upper part, 

of breccias consisting of angular chert fragments and rounded, ill-defined siderite fragments in a sideritic matrix, the whole retaining the same bulk composition as the bedded 

iron-formation. In most places, the chert 

fragments have obviously been transported, but 

locally the fragments line up as segmented portions of originally continuous layers. This rock is believed to be the product of 

slumping and submarine erosion of the partlyconsolidated, banded sediment as a result of a minor but widespread tectonic dis~rbance 

~hich caused changes of sea-bottom slope. 

Pettijohn, F. J., op. cit. 

James, H. L., Clark, L. D., and Smith, L. E., 

op. cit. 

6 

Hanging-wall strata.-Because of the 

unconformity mentioned above, the lower part 

of the hanging-wall strata is subject to rapid 

changes in lithology and thickness. The "normal" contact with the iron-formation, though 

not necessarily the most common,is gradational. 

At the typical gradational contact, layers of 

chert, siderite, and sideritic slate alternate 

with increasingly thicker layers of graywacke 

until the rock is mostly massive graywacke. 

Such relations are clearly shown on the lOth 

level of the Buck mine near .the west boundary 

of the NWfNEt sec. 7. In the area of the east 

Spies mine (sec. 24), the upper part of the 

iron-formation is interbedded with layers of · massive, nonpyritic black slate. Elsewhere, 

almost every combination of bedded or brecci~ 

ated iron-formation with a hanging wall of 

sideritic slate, silty slate, sideritic 

graphitic slate, graywacke, graywacke with 

chert fragments, or sideritic slate with chert 

fragments can be found. Locally, the rock is 

graphitic enough to be confused with the footwall graphitic slate, as in the Spies mine area (sec. 24). This rock, however, is aider- · itic rather than pyritic, as indicated by the 

soluble iron content. Furthermore, it is most 

commonly massive rather than laminated, and 

typically contains scattered chert nodules. 

As a group, these varieties of hanging-wall 

strata are characterized by a high iron con- tent (rarely less than 14 percent) and by 

association with sideritic graywacke that 

commonly contains chert fragments. The most common type of contact relation, as indicated 

by examination of the core of many drill holes 

and examination of exposed contacts, shows 

bedded or slightly .brecciated iron-formation 

overlain by thin-banded silty sideritic slate 

which grades within a short distance into 

graywacke carrying scattered chert fragments. 

In some places, notably in the Bengal 

mine area. iron-formation breccia grades into 

sideritic or hematitic graywacke with chert 

fragments·. The two rocks are much alike in · appearance, the graywacke being distinguished 

only by the presence of scattered glassy . quartz grains, a more granular . texture, and· .a slightly lower iron content. 

Some of the underground holes that ~ere 

drilled westward from the Caspian mine encoun- tered breccias consisting of chert fragments 

in a dark, graphitic, sideritic matrix, and 

there were indications that bedded iron-formation was thin or absent. It seems likely that 

this indicates erosion of a significant amount 

of the iron-formation; furtner, that in some part of the surrounding area the graphitic 

footwall beds were eroded in the period of 

disturbance following the deposition of the 

iron-formation and the material from this 

footwall rock contributed to the breccia. 

The types of the lower hanging-wall 

strata most commonly encountered in some parts 

of the map area are listed below: 

1. st sec. 24 (pl. 1). Massive to 

bedded, black, sideritic, silty, somewhat 

graphitic slate overlain by slate or graywacke 

with or without chert fragments. Commonly ~i th interbedded layers of ,,siderite and sideritic graywacke. Locally graywacke with chert 

fragments directly above iron-formation. 
KUOR UHIT 

MAGNETIC SLATE 

HANGING-WALL STRATA (GRAYWACKE-SLATE 

SERIES) 

Table 1. Stratigraphic chart summarizing characteristics of the iron-formation and associated rocks. 

NATURE OF CONTACT THICKNESS (feet) 

T 100 'l 

Unit 

Slaty f'aci s 

Banded facies 

SUBDIVISIONS 

Description 

Nearly black; composed of very fine grained micaceous mineral. Hassi ve, but comm6nly will break along parallel cleavage planes. 

Tt>.J.n-banded to finely laminated, w1 th lighter and darker layers. 

Hard, tough, subconcholdal fracture. Some layers of identical appear- ance are nonmagnetic. Color gray, but rock assumes characteristic 

IRON CONTENT1/ 

(percent) 

15-25 

DISTINGUISHING CHARACTERISTICS 

Generall'f strongly magnetic. In slaty facies, dark color and finely micaceous habit of predominant min- eral. In banded facies , hardness and toughness, fine banding. Mauve color highly characteristic of' .a slightly oxidized rock. Slump struc- tures ( tiny faul ta and folds) C01111!10n in thin banded types. 

Soft, massive, light-colored. 

Apparently gradational. .I 1· . I mauve tint where oxidized. I I I ~i~;t!~p~~;~~;~s b~r~:;;d. Gra-y slateY 

cal pr~cipitates. r-------~----------------r--------------------------_, Massive graywacke 

Graywacke with 

:::~!~rag- !l:l 

100'-400 ~ Slate with 

~ chert., 5rag- : mentsg,.. «> 

g Massive_ qlack 

~· slate.Y 

f !:? 

Massive, thick-bedded for most part. Loosely packed glassy 

quartz grains in dense .matrix. 

Dark gray where unoxidized, red and greenish where oxidized. Some interbedded silty gray slate; 

locally layers of chert and siderite. In a few places, gray- wacke with slate fragments occurs in lower part, and the graywacke may be somewhat graph! t1 c. 

Hassi ve, gray, coarse graywacke with angular chert fragments as much as 2 inches long . 

Sideritic slate, locally graphitic, with angular chert frag- ments. 

Massive, slightly graphitic. Individual layers commonly as much as 6 inches thick. Commonly with interbedded layers of gray s1deri te or sideri tic graywacke. In part def1ni tely silty. Typically contains scattered chert 

:;:; nodules. 

~-------------1------------------_, b Sider1t_!c; 

"' slats=/ 

Thin-banded, particularly evi- dent where oxidized. Color gray where unoxidized, green and ..., = reddish where oxidized • 

~------------;-----------------------~-------; 

Pi~;;~!~~=~ ~;:.!~r a!~ ~~~;:e~~~r!~~~~~rUnoxidized: 14-22 

Oxidized: 

20-40 

Distinguished from footwall gray slate-graywacke serie:.: by: 

1) higher soluble iron content (14-22%, as compared with 6-12%) 

2) absence of cleavage in finer- gral. ned facies. 3) hanging-wall graywacke is coarser-grained, thicker-bedded. 4) presence O·f chert, .as thin bed.s fragments, or nodules. 

Bedded members o!' lower hanging-wall sequence commonly oxidized to green, red, and buff colors. High iron con- tent dis tine ti ve. Chert bands char- acteristically greenish. Black silty slate (typical facies in northern part of map area) distinguished from footwall slate by occurrence of chert nodules, higher soluble iron content~ 

and as so cia t1on with bedded siderite 

and graywacke. 

_..;.._--..,..--------'- ~~c~~;~z:~~=s ~on5~~: 

Usually found with gradational, interbedded contact with bedded iron-formation. 

tact .relations highly 

LOCALITIES 

Outcrops in wt sec. 36 

(mauve, banded facies). In drill holes Nos. 4 

on Cyr-Lennox, :!,1 on 

~or~a~isil ~~c~a~p!~· plorations. Elsewhere indicated only by mag- netic anomalies. 

Present in most mine workings 
IRON-POIUIATIOI' I 

.. 

ftl'1able.; ma7 be inter- bedded and 8l"lldational, or abrupt. 

.... 

Abrupt but cont'oriDable con tact; grad& tional zone rarely more than a· t'eff. inches thick, 

140 

Iron-t'o~ tion \ I breccia 

(not ever,.- where preaent) 

Bedded iron- t'ormation 

Laminated graphitic alate 

.Anglllar chert t'ragalin ta and india tine.t, l'Ounded s1del'1 te t'ragaen ts in s1del'1tic mat.rix. RarelJ aeems tO fOl"'ll ore, though co111110nlJ oJ!:l- dized to hematite and white chert. 

Unoxidized: Interbande<l gra7 sU.- ei'ite and dark chert. Banda rare- lJ leas than l inch or 110re than 2 inches thick. Layers or gray to green s1derit1c; llilate not unoo1111110n. Rare seams or graphitic alate, gen- erally only inches thick. 

~ 

~!li Inte~anded sort hema- whl te to gra7 chert. 

2£!1!/ ftostlJ sort heUti te retain- ing banded atl'llcture or iron forma- tion.. 1 

11nalieared: Thin-banded to lllllll- nated, dirk gra7 to black. l"Jrlte abundant but rarely visible. Black, gloss7 graphitic streak, Sheared: GlossJ black; ' parts on ciosel7 spaced, irregular, slick-

!C GRAPBI'l'IC SLA'l'B - 20-40 .. ~pMt1c ohto 

. ensided surfaces. l!uch 110re graph! tic in appearance than unaheared rock. 

!lalaiv"e la~r, rarely more than 10 teet thick in this area. Graph• ltic alate fragaents in nonbedded graphitic matrix. Scattered trag- menta of gray alate and rare frag- ments of chert, 

~ I'J 

i 

~ 

Gradational. Graphlt!c and nongraphitf.c slate interbedded for se.veral feet. SBRICI'l'IC GRAY Unknown SLA'l'I-GRA'!WACD SBRIBS 

reccia 

peokled gra;,•) 

l'ot sut't'icien-tly 

explored to per- mit detailed sub- division 

-

Gra7, aericitic alate and fine- grained, thinlJ banded graywacke. Slate more abundant in upper part; 

fine-grained graywacke predomi- n•tea in lower part, 

--

y !1\ .. e are the c~on rang.,. of iron content. aa indicated b:y analyaea t'ro~:~ drUling, Ind1:.,1dual run• .. y dinrce 

condderably • bOth becau.. of original difference• and because of later . alteration, 

2/ Unit r:ay or may not be present. 

"'II Lower hanging -u may oontain any one or any combination of these unita, !/ Ore 11 not diltineuiahed from oxidized iron-fo~t1on on the mapa and crou sections i~ thil report. 

Unoltidized: 22-26 

Oxidized: 

S:IS- 50 

Ore: more than 50 

Roasted: 

15-25 Not roasted: 

2.:.6 

Slate: 

6.;.1;0 

Graywacke 8-12 

Breccia difficult to separate from graywacke with chert tragmen ts., in to which it ~~~&y.grade. D1Btinguished 

by lack of quartz grains, sli.ghtlt 

higher iron content, less granular texture 

Bedded iron-fol"'ll&tion M~t'!ngu1ahed by thin banding (layers i inch to 2 inches thick) , high soluble iron content, abundance of chert. Com- plex minor folding as seen under- ground characteristic and not co1111110n to any o the.r unit. · · 

Graphitic streak; low soluble iron content (when not roasted). Because of high percentage of fine pyrite, rock swells, eftloresces, and goes to powder a.fter exposure to air for a few years. 

~;:::e~~=~c!s:s~i:!~!.:~~:/f a:la te . 

contrasted to thin-bedded adjacent 

rocks). Hay be unrecognlz,.:ble be- cause of shearing or li'l.acking. 

Sla ty parts have gray streak and distinct, nongraphi tic sheen. Gray- . wacke finer-grained, more finely sorted than banging-wall graJWacke. Low iron content .distinctive. Thin- bedded facies generally show numer- our tiny faults, which in places 

give r i se to a pseudo-conglomerate structure. 

'-

All mine workings. 

Present ln most mine workings 

Shaft at lower levels or Berkshire and Zi-erman mines. In 111011 t easterly 

part of Spies mine. In drill boles Bo. 5 on CyrLennox, Boa. 7, 13, 15, 17 on DeGraltse properties. 
2. Cyr-Lennox area ( swi esec •· 36, pl. 1). 

Silty, gray to green, siderlt1c slate over- lain by graywacke with or without chert fragments. 

3. Barras area ( SwfNEf 8 ec .• 36, pl. . l) • Graywacke or silty gray, green or red slate. 

Deeply oxidized. · · 4. Bengal mine area (SE-i sec, ' 36, pl. 1). 

Graywacke with chert fragmeh ts; deeply oxi.., 

dized. 

5. Tully mine area (SEi sec. 36, pl~ 1). 

Probably oxidized graywa~ke • .. ·. .. · 6. Caspian mine ~:rea (sec. 1, pl. 3). 

(a) West, breccia of chert fragments in a dark, sideritic, somewhat graphitt,c matrix. 

(b) East, silty red to green slatl! with- scattered chert fragments. 

7. Ni Borland area. _(NWt sec • . 6,pl. 3). Graywacke with chert fragments. · a. Corry area (NEtswt sec. 6, pl. 3). 

Graywacke and silty slat~. L6cal~y with chert 

!ragmen ts • . ; · . 9. Buck mine area (SWfSWf ~ec. 6, pl. 3). Mostly massive graywacke, · locally gray- wacke '·With chert fragments. · ltJ. Zimmerman mine . area ( NW-f sec. 7, · pl. 3). Graywacke. Locally some interbedded 

graphitic slate.· · . ; 

11. DeGrasse area (NEt sec. 7, pl. 3). 

Graywacke, commonly interbedded with some black graphitic slate, · 12 •. Section 8 (pl. 3). Graywacke with some silty slate. · 

All the above-described rock ~ypes grade 

upward into massive fine- to coarse-:-grained graywacke which includes some silty gray slate 

and, not uncommonly, thin layers '?/ chert: and 

siderite. The hanging-wall gr~ywacke . and 

slate are marked by a high content of siderite, so that the unaltered rock has an average iron 

content of 16 to 18 percent, as comparea w1 th an average iron content of 6 to 12 .,pe:rcent in 

the footwall gray slate and· gr~ywao1ce~ In the 

graywacke, the siderite forms a matrix for 

poorly rounded, glassy quartz grains. The 

slates are s1der1t1c and gray, except -locally 

where graphitic; cleavage, such as' that in the 

t ootwall sericitic gr~y sl~te, · ip not present. 

In some parts of the ~rea, · graywacke ' with 

fragments of black slate occurs above the 

graywacke with chert fragments. The fra~ 

ments, as much as half an inch long, are rather angular, and of similar lithology to the 

footwall slates. The matrix in such rock is 

commonly slightly graphitic, and the soluble 

iron content lower than normal. The rock is 

readily distinguished from the footwall slate 

breccia by the lighter color of the matrix, , more massive character, greater hardness, and 

essentiallynon-pyritic character. 

'l'he thickness of the hanging-"wall g:ray- l'lacke series appears to be highly· variab.le; 

in places, it may possibly attain __ a true 

thickness of several hundred feet, but more commonly the true thickness appears to be on the order of 200 feet. · 

In most ~laces adjacent to oxldized ironforma-tion or ore, the hanging-wal:l. strata are altered to -ferruginous _rocks in which red and 

green colors predominate. Much of, the ;"Ook ie 

irregularly mottled, with the green portions 

apparently being alter_ed to hematitic -,ed. 

The green coloration is due to · a yery fine~ 

grained chlorite that hall replacedpart .OJ.' all 

of the original si~erite, and. the :red colorat1on is due to later alteration or repracement of the chlorite and remaining siderite to 

red earthy hema-t1 te. In many places, this 

alteration brings out a fine banding in the 

silty slates that in. the unoxidized rock is 

not particularly evident. Alteration to clay 

minerals, as yet unidentified, occurs locally, but in general probably is rare. 

Magnetic slate.-Above 'the graywacke and 

gray-slate hanging-wall series is a magnetic slate that is the .young'est rock in this area for which es;eripti~:m can be given. This 

unit, which is oneqf the moat distinctive 

sedimentary units inthe Iron River~Crystal 

Falls district, is' composed of at least two 

distinct ropk types. 

The iower ·member, above--the graywacke, is a gray, thinly-banded, sideri tic rock of 

cherty, appearance in . wh1 ch layers containing 

dissemiria ted_, 'finely errs talline magnet! te occur. Some parts -of ·this rock, however, con-· tain little or no :znagnetite. The rock is hard ' 

and tough and breaks with an almost conchoidal 

fracture, and the finely banded varieties 

shew, in places, contorted or minutely faulted structures that probably are due to slumping prior to conso·lidation. Where slightly 

oxidized, as in ~utcrops, the rock assumes a highly distinctive mauve or purple color. 

Above the banded gray rock is a rather mas- sive, dark, strongly magnetic rock. It is 

composed largely _of a very fine mica or chlorite-like mineral which may be stilpnomelane 

(an iron-rich silicate}, with abundant tiny 

crystals of magnetite .scattered throughout. 

The. magnetic slate (mauve-colored, banded 

facies) is exposed ih several places on the 

hill on which the town of Stambaugh is bui;Lt, 

and magnetic slate has been cut by several 

drill holes. Drill hole No. 14 (the most 

northwesterly hole · in <the NE-fSWi sec. 36, pl. 1) on the Cyr-Lennox Property entered magnetic slate after passingthrough grayw~cke (appar-

·ently overturned:); dril~ hole Borland No. 11 

1n the NE-1-NVfl. ·sec. 6 :(pl. 3) entered the mag- netic slate at ledge surface and cut an excelle_nt section l;>efore _i:mt'ering graywacke; drill 

hole No. 31 on the .Caspian property (sec. 1, 

pl. 3) entered a few feet of magn~tic slate 

after passing through iron-formation and some slate, as shown on cross section 0-C' (pl. 4); 

and drill ho.le No.7 .in the NEiSEf sec. 8 

(pl. 3) cut the magnetl.c slate at ledge sur- face. 

The iron content of' the 'magnetic slate 

varies from 15 to 25 percent (average about 19 

percent), the irori being in the form of magnetite, s1deri te' or soluble silicate (possi- bly stilpnomelan~). Because of this, and 

because . of the banded cherty character of some . taoies, the magnetic alate sometimes has been 

mistaken fof' unoxidii!ed iron-formation. The 

thickness of the magnetic slate in this area 1s not known; in the ~tai Falls area 1Q/, 

1t is about 100 feet. 

1EJ Pettijohn. F. J.~ op. cit. 

' 
The magnetic slate presents many aspects 

not clearly understood. Kany problems, such as the character of the original sediment and 

the origin of the stilpnomelane and magnetite, await f'urther iJ tudy. 

STRUC'!URE 

Structurally, the map area fo~~ part of 

the tightly-folded western apex of the Iron 

River-Orrstal Falls basin. There is, however, no proof that the basin is completely closed 

off in the IrOn River area. It is possible that this •apex• is only a constricted part of a basin which continues to the . west. ·The 

dominant trend of the ~olds varies from nearly 

east-west in the southeastern part of the area to nearly north~south in the northwest, but 

almost ever-ywhere c;ross folds, at nearly right angles to the major trend, are present. Overturning, both of folds and cross f'olds, is 

general. The direction of overturn is not 

consistent even in vertical section, although 

most of the cross folds are overturned to the 

south and east. Because of these complexities, it, is virtually impossible to depict adequately the structure by ~eans of two-dimensional 

maps and sections. A vertical section, arbitrarily chosen as ~o position, may completely fail to illustrate the structure. The section 

may cut some of the folds in positions where 

their axes are overturned on ercrss foldsJ, so that an anticline may show as an .ieolate·d lena 

pf footwall strata completely surrounded by 

tron-formation, or a syncline may show only a~ 

11n isolated Diass of hanging-wall ~tra~a . su~- : rounded by iron-formation. The sections shown on plate 4 were chosen, so far as possible, to 

illustrate the major structures without com- plications such as described above, ·and, as a result; are much Qimpler than the majority of 

sections drawn. · 

'l'he origin of the cross folds is amattel!' ·for some conjecture. Tb.e fact that the cross folds normally maintain an orientation approxi- mately at right angles to the dominant trend , · would appear to indicate some genetic relatiQ.:Q 

between the two sets. Also, the cross folds · do not have the persistence of strike that 

might be expected of structures which were developed in a period of deformation later 

than that which produced the dominant set. It 

is therefore tentatively concluded that both 

folds and cross folds were formed at approx1-... JD&tely the same time-at least during the same period of deformation. 

. Faults are common, and many have large ~pparent throws. Host appear to have developed either as attenuation of folds or as . adjustments to release stresses set bp by ro~. 

The faults are generally tight;-- commonly they are marked by vein quartz, or by squeezed · graphitic slate, or by both. Host appear to to~ impermeable barriers rather than channels 

tor movement of solutions. 

The structure of individual areas is discussed under •Summary descriptions •" 

_10 

ORE DEPOSITS 

'l'he ore deposits are composed mainly of 

soft red hematite, with subordinate hard, 

fine-e;rained hematite · ("blue ore") and. limo ... n1 te (goethite). The latter may be very hard 

and compact, or.earthy. Por the most part the ore bodies retain the structure of the original bedded chert-siderite rock, although where 

replacement has been nearly ~omplete, the 

structure may be hard to discern. Only locally does the ore show crushing or other evidence 

of slump. The ore· tends to be porous and 

vuggy and is always a water-bearer. The ore in the ground is generally calculated on the 

basis of 12 cubic feet to the ton, . whereas a hematite ore assaying 55 percent iron, if 

solid, should contain about 8 cubic feet to 

the ton (assuming a specific gravity of 5 .• 0 

for the hematite and 3.0 for the waste mater-

'tal). Although the ore bodies are rather ~egular in plan and section, the 9re bounda- ~es are, of course, gradational. 

Tb.e structural control o!' the ore is in - most places obvious. The ore commonly, but by no means always, lies directly on the footwall 

of graphitic slate along the keel of a syn,. cline. In this structural position, much o~ 

the li-on-formation may be converted . to ore;~ 

and ore may be present on minor anticlinal 

structures within the larger synclines. Tlle ore bod~es are more irregular and generallJ annaller on the limbs of an ore~bearing structure. TOe ore, if present, occurs on the 

structural footwall (as distinct from the 

stratigraphic footwall), so that on the over- turned limb Qt an overturned told the ore will 

be in the upper part (stratigraphically) of 

the iron-:f'ol'Jilation and rest on a · structural 

:f'oot"all o!' hanging-wall at·rata. In some . cross sections, an ore body may appear to l.ack 

structural control. This is usually only an apparent lack as an ex.llm11la tion or the maps generally will reveal that the eontroll1ng 

structure ·is a cross fold, and that the structut1ll footwall of the ore is sub-pal"&llel to, 

and outside of, the plane o!'section. 

The effect of cross folding of a syncline on the localization of ore is very pronounced insome places, the •lows• of a syncline-that is, the intersection of the major syncline with a cross siecline-being much more favor- able than the highs" of the same syncline at 

its intersection witha cross anticline. 

. Tb.e effect of strongl .. overturned footwall etrata that forms an · overhang" above a s~cl1ne of iron-formation is to inhibit oxidation, although, if the plunge of the fold is very steep, oxidation to ore may occur along the fold pis d&epi te the overhang of imperme ... able footwall. slate. The latter cori.di tiona ~revail at the Fogarty property in sec. l 

(see cross section B-B', pl. 4). 

Tllere has been no indica t1on thus far in 

the Iron River district of. the depth to which ore bodies might extend. Within the area discussed in this report, the deepest mine is the 

Buck, from which ore is mined at depths of 

about ·1,000 feet below the surface . However, a short dfstance west of the map area, ore is 

being mined from the H1awa tha mine more than 

2,000 feet below the surface. Kany ore bodies 

do, of course, bottom a t lesser depths, but in 

all of the mines thus far examined, the con ... trol is structural. 
An important differenc e ex .;. st:~ bet.ween ore bodies formed on a structurHl footwall of 

hanging-wa l l strata from tho :• e formed in nor- mal position on footwall"strata. Ore bodies 

resting on overturned stratigraphic hanging- wall strata will mtail" ou t with depth into 

oxidized iron-fo-rmation, inasmuch as the ihfolded hanging-wall strata must , of necess~ty, 

steepen with depth and then assume the normal 

spatial relation as a structural hanging wall 

to the iron-formation. In contrast , ore ~o ies on a structural (and stratigraphic) 

foo twall of graphitic slate will pormally con- tinue, with some breaks becau se of structural 

irregularities, down to t h e keel of the syncline and will not end until the bottom of the 

fold is reached. Some of these relations are illustrated in cross section D-D' (pl. 4), 

which although it ,does not sb.ow ore.::" shows 

stoped areas. It will be noted that the ore (as indicated by stopes) that rested on the 

overturned hanging-wall graywacke bottomed in 

iron-forma tion, and that a gap exis-ted betwe en the "hanging-wall ore~ and the "footwall ore." 

Of considerable interest, but apparently 

of little or no importance so far as the 

formation of the ore i s concerned, is evidence 

which points to an epoch of mild hydrothermal 

activity after the ore was formed. In both 

oxidized and unoxidized iron-formation (and in 

other rocks), veinlets and crystal-lined vugs occur which contain abundant small, bright 

crystals of chalcopyri te, specularite, pyrite, 

marcasite, barite, chlorite, calcite, quartz, 

and apatite (the latter found in only one specimen). The vugs characteristically 

develop in the chert layers, and in one place were found developed in t he chert fragments of 

compact , unsheared, oxidized iron-formation 

breccia. Had the oxidation that produced the 

hematite in the matrix of the breccia been 

later than the development and filling of the 

vugs, the delicate crystals, particularly 

those of chalcopyrite, certainly would have 

been either destroyeQ. or altered. 

The nature of the ore-forming process, 

particularly as to whether the solutions were of surface (meteoric) or deep-seated (hydrothermal) origin is a controversial point which 

cannot be dis cussed adequately here. However, 

in all places where sufficient data are available, lt i s clear that the ore is related to a structural footwall, and the writers see no evidence to war rant serious divergence from 

the concept, postulated fifty years ago, that 

the major factor has been the presence of 

meteoric waters that worked from the surface 

downward along the iron-formation, although 

divergence of thought exists as to the nature 

of the system and_the reactions. 

Some factors t hat should be considered in 

the search for ore are summarized below. It 

must be recognized, however, that these are tentative generalizations, and exceptions can be found~ 

1 . Because of the complex structure of 

the area, in particular the widespread over- turning of beds, the majority of deep holes 

drilled Yfithin the general belt of ironfo,nnation folds will encounter iron-formation 

regardless of what rock i s encountered at 

ledge. This is strikingly brougi:n. out oy the 

fact that almost every mine shaft in the Iron 

River district has entered iron-formation · despite all attempts to l!.void just such a con11 

tingency. 

2. Extensive l.il tera t:;ion of the hangingwall graywacke and slate i s common in the 

vicinity of the. larger ore bodies in the iron- ~ormation. In this altered zone, the ro cks are commonly light green, reddish, or bu~f, as a result of chloritization and hematization. 

Such alteration of the graywacke should be 

considered a favorable indication and the near- by iron-formation should b e well explored. 

3. The larger ore bodies are most common- ly to be found along the keel of a synclinal 

structure, which is either open (that is, not 

overturned) or only moderately overturned so that the footwall beds of the adjacent anticline do not "overhangtt the keel. However, if 

the folds have steep plunges, ore may be found even where the keel is overhung by the footwall slate. 

4. The "lows" of a syncline-that is, 

its intersection with cross synclines-are the 

most favorable locations for ore. Such cross folds may be located by detailed mapping of 

the plunges of minor folds in the area, or by 

t heir effect on adjacent parallel synclines 

which have been explored, or by alinement of 

magnetic anomalies. 

5. Ore bodies that occur on the limbs of a syncline a re generally more irregularly distributed and smaller than those on the keel of 

t he syncline. 

6. On the overturned limb of an over- turned syncline, the ore (if present) will be 

found in the upper part of the formation resting on the structural footwall of hanging-wall 

rocks. Such ore bodies will not continue downward to merge with keel ore bodies, but will 

bottom in oxidized iron-formation. Ore bodies on the normal limb will occur in the lower part 

of the iron-formation resting on the stratigraphic and structural footw~ll, and may be 

continuous into the keel ore bodies. Conversely, the position of ore may be used to determine whether the limb of a syncline is normal or.overturned. For example, if a horizontal 

drill hole successively cuts hanging-wall 

graywacke, ore, oxidized iron-formation, and 

footwall slate, the limb may be assumed to be 

overturned; if it successively cuts graywacke, 

oxidized iron-formation, ore, and footwall 

slate, the limb may be assumed to be normal. 

INTERPRETATION OF THE MAGNETIC DATA 

The zero magnetic value on the magil.etic 

maps ~ls. 1, 2} has approximately the value of 

the normal magnetic field of the earth in this 

latitude. Any appreciable deviation from this 

value may be considered to be due to an unusual 

magnetic property of the underlying rock. This 

deviation is termed a magnetic anomaly, and is 

expressed in magnetic uuits (gammas} above or below the zero value. The general concepts of 

interpretation of the magnetic data are simi-· 

lar to those expressed for the adjacent Ice 

Lake-Chicagon Creek area, and the following 

excerpt is taken from that report•!!/ 

. 11/ James, H. L., Clark, L D. , and Smith, 

L. E.,op. cit., PP• 6-7. 
• 

"The largest and most persistent anomalies are recorded over the magnetic siate, but 

broad areas o~ weak anomaly are recorded over the hanging-wall beds stratigraphically below 

the strongly magnetic horizon. * * * Geolog- ic interpretation o~ the anomalies has been pre~aced on ·the assumptions listed below: 

1. The major anomalies are produced 

by the magnetic slate. 

2. The magnetic slate is only rarely 

nonmagnetic, although the intensity o~ the anomaly it produces 

varies within wide limits. 

3. The hanging-wall beds below the 

magnetic slate are weakly and 

rather uni~ormly magnetic and, i~ 

present in su~~icient volume, 

give rise to measurable magnetic 

anomalies. 

4. The rock magnetism is predominantly o~ the induced type (that 

is, produced by and parallel to 

the earth's present magnetic ~ield), and residual magnetism 

(polarization) is relativ,ely subordinate. This assumption is ba~ed on tests with specimens on ~ield instruments, and on the 

general pattern o~ the magnetic 

values. It is ~urther assumed 

that, because o~ the steep dips o~ the ~ormations and the high 

angle o~ magnetic inclination 

(76°) in this latitude, the nega- . tive (lower) pole o~ most o~ the 

anomalous bodies has no appreci- able e~~ect at the sur~ace." 

Some special problems have arisen in the 

interpretation o~ this s~rvey that were not 

present (or at least not considered important) 

in the interpretation o~ the Ice Lake-Chicagon 

Creek survey. The ~act that the southeastern 

part o~ this map area (pl. 2) is adjacent to 

the greenstone basement which apparently dips 

beneath the younger rocks gives rise to one major point o~ di~~erence between the two areas. Inasmuch as the greenstone is in part 

strongly magnetic and is not at a uni~orm 

depth, the magnetic "datum" changes ~rom place 

to place; the observed magnetic value at any 

point is thus the sumo~ two components, one o~ which is due to the deep-seated basement 

rock, and one o~ which is due to the immediately underlying rock. In sees. 7, 8, and 9 

(pl. 2) a consistent, gradual rise is noted 

toward the south, and exploratory traverses 

beyond the map area show that the values gradually increase until the high anomaly o~ the 

strongly magnetic part o~ the greenstone is 

reached. Further, in sec. 8, very considerable di~~erences o~ value are obtained along 

the strike o~ the same beds with no indication 

that the di~~erence is due to change in the 

immediately underlying rocks. This gradual 

increase toward the east is attributed to a change in character of the basement rock. In 

other places, notably in the north hal~ o~ the area shown on pl •. 1, the magnetic datum appears 

to be lower than normal and most values on the 

map are negative. 

12 

Because o~ this complicating ~actor o~ a variable_ magnetic base level., the gamma value 0~ the magnetic determination in many parts of 

the area holds little significance, and, as a· result, the speci~ic magnetic contours cannot 

be assumed to be subparallel to geologic boundaries (as was assumed in parts of the adja·cent 

Ice Lake-Chicagon Creek area). Attempts have 

been made to separate graphically the basement 

anomaly ~rom that o~ the sedimentary rocks 

above, but these have been only partly success- ful. However, the problem appears to be a serious one only in the southeastern part of 

this area. Another dif~erence between the two areas is that in at least one place (Stambaugh 

Hill, in sec. 36, pl. 1) the magnetic slate is 

polarized, as s(:lown by the variability o~ 

values on the map, and by field tests with 

specimens. 

Aside ~rom the problems . discussed above, 

interpretation o~ the magnetic data has been 

similar to that in the report on the Ice LakeChicagon Creek area, from which this second 

excerpt is taken:~ 

"Interpretation o~ bedrock structure 

from the pattern of the magnetic data divides 

into two general classes: 

1. The magnetic anomaly is directly over the ledge outcrop o~ a bed of 

magnetic rock; as such, the crest 

line o~ the anomaly forms a map 

pattern similar to that of the bed 

and will so outline the structure. 

The anomaly is normally narrow, 

with a steep gradient. 

2. The magnetic anomaly is over a tight ~old o~ magnetic strata 

which does not contain higher non- magnetic beds. In this case, a single crest line ~ollows approximately the axis -'>f the fold. The 

trend of any particular bed will 

parallel the crest line of the 

anomaly only along the flanks of 

the structure; on the nose of the 

structure this trend is subparallel 

to the magnetic contours, and, at 

the axial line, the trend of the 

bed will be at right angles to the 

crest line of the anomaly. 

Inasmuch as cross-~olding is extremely common in this area, many o~ the synclines are spoon-shaped and isolated because of reversals 

of plunge. If the isolated 'spoon' contains 

tightly ~olded magnetic beds and is covered 

with an appreciable thickness of dri~t--so that 

closely spaced anomalies will merge--there will 

be a single crest line which will have a maxi- mum intensity over the deepest part of the 

trough, and which wfll decrease to zero in 

both directions along the axis. The anomaly is normally broad, with a relatively low gradi- • ent. I~ the syncline is not isolated and · widens so as to include nonmagnetic beds above 

the magnetic slate, the anomaly will separate 

into two parts. * * * · 

12/ James, H. L., Clark, . L. D., and Smith, 

L. E.,op. cit., PP• 7-8. 
Overtu1~ed structures are not uncommon he area, and these give rise to modificat l ons of the magnetic pattern outlined above. 

I n general, the crest of an anomaly above an overturned syncline containing magnetic beds 

is dis placed away from the axis of the syncline toward the overturned limb of the struc-· 

t ure . " 

As in much of the Ice Lake-Chicagon creek a r ea, most of the larger anomalies in this map a rea a re i nterpreted as being located ove~ 

ti.ght syncl ines of magnetic slate, so that the crest l i ne of the anomaly and the axis of the 

syncline roughly coincide. 

The chief value of the magnetic survey, 

aside from the location of areas of magnetic 

slat e, is in providing a "carry-over" from one area of physical exploration to another 

in the complete absence of outcrops. Without 

this independent evidence, it would be quite 

i mpossible to draw even tentative geologic 

bounda ri es between anything except closelyspac ed expl orations. In addition, the mag- n eti c survey helps in delineating the hanging- wa l l graywacke series; this is clearly shown 

by the broad area of low magnetic anomaly in sec. 6 (pl. 2) where drilling indicates that 

hanging-wall graywacke and allied rocks form 

most of the ledge surface. However, in parts 

of sees . 24, 25, and 36,(pl. 1), most drill 

holes have encountered hanging-wall graywacke, 

yet magnetic values are low. These apparent 

exceptions to the generalization that the 

hanging-wall graywacke series causes low but 

appreciable magnetic anomalies are not entirely under stood. Locally, as in the SEt sec. 36 

(pl. 1 ), i t may be due to oxidation; elsewhere 

the r eason i s not known, although it may be 

that because of tight folding with the ironformat·ion and footwall slates there is insufficient volume of the graywackes to cause appreciable anomaly. 

. SUMMARY DESCRIPTIONS 

Wiswt section 19, T. 43 N., R. 34 w., plate 1 

All of the exploration in that part of sec. 19 included in this report is within the 

NWiSWi. This tract is entered by the most 

easterly workings of the fourth level of the 

Spies mine, and has been explored by several 

underground holes from that level. The 23 

surface holes that have been sunk in the area r epresent three separate explorations, one by 

the H. A. Hanna Co., one by the Carpenter 

interests, and one (the most recent) by the 

Cleveland-Cliffs Iron Co. Only the core from 

the latter explo;ration, which was limited to 

the western part of the "forty11 , was available 

for examination. 

The major structural trena in the NWfSwt 

1s a l ittle east of north, and the dominant ructu1~e is the broad anticline, illustrated 

i n c ross section A-A' (pl. 4), that is entered 

by the workings of the fourth level Spies mine and defined by several underground holes that were drilled easterly to the east flank of the s t ructur e . This drilling also indicates that an anticl inal crosswarp centers approximately 

along the line of cross section, so that minor 

folds plunge both north and south from this 

axis . The inferred fault shown on the map 

(pl. 1) is based on drill-hole evidence. Other 

faults are almost certainly present but cannot 

be defined on the basis ~f present data. 

13 

None of the core from the surface d r ill 

holes in the central and eastern parts of the 

"forty" has been preserved, so that classification of the eastern part as "hanging-wall 

strata" is open to considerable doubt. 

The SWts~ sec. 19 has not been drilled. 

Magnetic values are low and it seems likely 

that iron-formation will be exposed at ledge 

in part, if not most, of the area. 

~section 30, T. 43 N., R. 34 W., plate 1 

No direct evidence concerning bedrock is 

available for the part of sec. 30 shown on the ·map, and very little evidence is available for 

adjoining areas. The definite anomaly in the swt~ indicate~ a fairly extensive synclinal basin of hanging-wall rocks in that area. The 

trend is nearly east-west, although in the 

adjacent area to the east,~ this same anom- aly shows a trend that is nearly no'rth-sou th. 

The low magnetic values in the vicinity 

of Ice Lake indicate that iron-formation or footwall strata may form much of the ledge sur- face in that area. 

st section 24, T. 43 N., R. 35 W •. , plate 1 

Except in the NEiSEt, which is entered by 

the workings of the Spies mine and explored in 

connection with that operation, the information 

in the south half of sec. 24 is too meager and 

scattered to permit anything except tentative 

conclusions. Magnetic values are generally 

low throughout the area, although weak positive 

anomalies are present in the W!swt and in the NiSE~. The magnetic su~ey was hampered in 

the northern part of the area by magnetic disturbances arising from the adjacent Spies mine. 

However, the values shown in that area may be 

considered reliable, as the stations were occupied on a day when the power was shut off 

at the mine • 

.Spies mine area (EiSEi-) .-The structural ~rends in the east Spies mine are nearly northpouth. The fold axes are steeply pitching, so that in vertical section the folds have an appearance of tightness that is more apparent 

than real (see cross section A-A', pl. 4). 

The iron-formation encountered in the fourth 

level is bounded on the east by a nearly vertical fault that has cut out most of the foot-

'(lall graphitic slate, so that the iron-formation is in contact with the footwall gray ~late and graywacke. This fault is shown on the cross section referred to above. 

The hanging-wall strata in the Ei sec. 24 are significantly different .from those found 

elsewhere in the ~p area Here the hangingwall succession commonly includes layers of 

massive, black, slightly graphitic slate, 

which typically contains scattered chert 

nodules. The rock superficially resembles 

footwall graphitic slate, from whi.ch it is 

distinguished by its nonpyritic character, 

thicker bedding, chert nodules, and higher 

soluble iron content. 

13/ James, H. L., Clark, L. D., and Smith, 

L. E.,op. cit. 
• 

The two drill holes near the . center of 

the SE~ both entered hanging-wall graywacke or slate at ledge surface, but the more southerly hole (a steep angle hole to, the south) was continued until it entered oxidiz~d ironformation. Low magnetic vaiues were obtained over the area of known hanging-wall rock, as shown on plate 2. As discussed under "Interpretation of magnetic data", such low values 

may be attributed in part to a change in char- _acter of the deep-seated basement rock so .that 

the local magnetic datum is lower tha~ normal • However, it is possible that the strata are so folded with iron-formation and footwall rocks 

that the hanging-wall rock is not present in 

sufficient volume to cause an appreciable , anomaly. 

swt sec. 24.-;The inform ion in this area is too scant to determine structural 

trends, although the weak positive anomaly in 

the western part suggests trends more nearly 

east-west than north-south. The anomaly presumably indicated a fairly deep synclinal 

basin of hanging-wall strata. The inferred ha~ging-wall strata would be separated by a belt of iron-formation f~om the footwall area in the workings and exploration of the SpiesVirgil mine to the north. · 

The five drill holes on the Carlson ex- ploration in the NE-fswt show oxidized ironformation in the southern part bounded on the 

north by hanging-wall graywacke and slate. 

Secition 25, T. 43 N., R. 35 W., plate 1 

Except for the Wiswt, which is to be 

described as part of a separate report, there 

has been very little exploration in the ·wt sec. 25, and none whatsoever in the east half. 

The magnetic survey reveals little. The weak 

positive anomalies in the east part of the 

south half of the section probably are due to 

hanging-wall strata, though no direct information is available. 

Five deep holes were drilled in the 

SwfNWf. The core was not available for exam- ination, but company records show that all the 

holes entered graywacke or slate at ledge and 

continued until oxidized iron-formation was reached. Magnetic values are low despite the 

drill-hole evidence that the ledge surface is 

predominantly hanging-wall graywacke and slate. 

The low level or magnetic values in part may 

be due to changes in the basement rock. However, other factors, such as oxidation, or folding so that the hanging-wall graywacke is 

not present in sufficient volume to cause appreciable anomaly, may be important. 

The drill hole in the NEtNWf was driven 

to 299 feet without reaching ledge; it reveals 

the existence of a remarkably deep trough in 

the ledge surface in that area. 

Section 36, T. 43 N., R. 35 W., plate 1 

The most prominent structural feature in sec. 36 is the major syncline of magnetic 

slate that forms the hill on which the town of 

Stambaugh is built. This structure, wh;tch 

doubtless contains numerous minor anticlin,es and synclines, trends a little west of r10rth. 

The magnetic slate (cherty, mauve facies) is 

exposed in low, glacially polished outcrops in 

14 

the immediate vicinit¥ of Stambaugh High 

School (in the SWtNW-f).~ and in outcrops .along the east side of NwfS~. Observed dips are nearly vertical.- The magnetic values over the 

slate are high and variable, and some of the 

rock is strongly polarized even in hand speci- men. A second, much shallower syncline of magnetic slate is indicated by the magnetic 

anomaly in the NE-f of the section; it trends 

nearly east and west. Otherparts of sec. 36 are discussed below. 

Gyr-Lennox properties (EiSWf).--The Cyr 

and Lennox mines are located on northerly 

trending structures which are essentially drag 

folds on the east flank of the Stambaugh Hill 

syncline. An anticlinal buckle of footwall is 

cut by some of the workings and by drill hole 

No. 5; it is separated by a shallow syncline 

from another footwall area on the east, as shown in cross section B-B' (pl. 4). The 

drill holes along the west side of the NE~Wl 

and SEtswt all en tared hanging-wall e;raywacke 

and slate at ledge, but hole No. 14 (the most 

northwesterly hole in the NE-fSwf), passed through graywacke at ledge and then entered an overturned syncline of magnetic slate. Easterly trending cross-folds are evident from the 

geologic pattern in the underground workings, 

particularly in the southern part of the 

Lennox property. 

Barras property (SiNE}) .--The Barras mine 

appears to have betn developed in iron-formation that occurs on northwesterly trending 

anticlinal folds with hanging-wall slate and 

graywacke. Core was available from all the 

surface drill holes; all entered deeply 

oxidized lower hanging-wall slate or graywacke 

at ledge surface. 

Bengal-Tully area (SEt) .--The' details of 

the geology of this area remain unsolved. In a broad sense, most of the area is anticlinal, so far as the iron-formation i .s concerned, · with iron-formation and hanging-wall strata 

forming most of the ledge surface. Most of 

the rock, including the hanging-wall strata, 

is so deeply oxidized that lithologic identification is difficult, and, in the case of the 

Tully surface drilling, for which only analyses were available, definite separation of the 

iron-formation and hanging-wall _ strata is 

nearly impossible. .. 

Very little footwall rock has been en- tered by either the mine workings or the drill 

holes. Most of the extensive drilling from 

the lower levels of the Benga·l mine cut a complex of oxidized iron-formation and oxidized hanging-wall graywacke with abundant 

chert breccia, although underground holes Nos. 

22, 25, and 26 (all deep holes drilled in a general southerly direction at steep angles) 

entered the footwall rock. 

The location of the ore in contact with 

hanging-wali strata, the extensive oxidation .of the latter, the obvious broad-scale anticlinal structure of the area, and the tailingout of ore bodies into oxidized iron-formation 

with depth strongly indicate that much of the ore of the Bengal and Tully properties occurs on the overturned limbs and axes of a series 

of overturned anticlinal folds. The structure 

is complicated by overturned cross-folds, 
which coupled with the difficulty in making 

lithologic identifications because of the deep o •lc,,tion, make structural delineation and the wlng of rational cross sections almost impossible. Though it has not been found possib le thus far to draw an illustrative cross section, the typical ore body now known in the area is believed to occur in an anticline of 

iron-formation, the axis of which is over- turned slightly past .vertical so that the st ruc tural form is that of a steeply-plunging 

syncline. Such an ore body may be expected to 

end in oxidized iron-formation when the pitch 

of the fo ld becomes goo or less. 

Section 1, T. 42 N., R. 35 W., plate 3 

The northern and eastern parts of sec. 1 are marked by projections of hanging-wall 

strata from the extensive areas of younger 

rock to the northwest and east. The dominant .trends are northwesterly, but these are 

crossed by several major cross-folds trending northeast. 

Relatively little is known of the geology 

of the west half of the section. In the NE~NWi, hanging-wall strata and iron-formation are inferred largely on the basis of angle 

holes drilled to the northeast. The extensive area of hanging-wall rock shown on the map in 

the S~NWl i s based in large part on the weak 

positive magnetic anomaly in that area (see 

pl. 2). Ho~ever, the inference is supported 

by informa tlon from the deep drill hole in the 

southeast part of the SE~NW! (M. A. Hanna hole 

No. Do-4-44) which, after entering unoxidized 

iron-formation at ledge surface, passed in and 

out o f typical.hanging-wall graywacke and 

slate, apparently in an overturned fold. 

Several shallow holes have been .drilled 

in the EiSWt and in the adjoining area to the east. The core from this drilling is not 

available for study. Presumably most of the area is underlain by footwall strata, but 

whether the "green rock" referred to in the 

records is greenstone or a slate is not known. 

Caspian mine area (NEi}.--The major ore structures of the Caspian mine are north- to 

northwesterly-trending synclines separated by 

tight anticlinal folds of footwall slate ·(see cross section G-G', pl. 4). A major cross syncline, trending northeasterly, crosses a short distance north of the Caspian No. 1 

shaft. This fold is strongly overturned to 

t he southeast, as indicated by the west to 

northwest dips recorded for the eastern part 

of the fourth-level workings and by the 

cutting of magnetic slate below iron-formation 

in hole No. 31 in the NEtNEi sec.l. A second cross syncline, also overturned to the southeast, is present in the most southerly part of 

the mine, Here, in the most southerly drift 

of the fourth level, hanging-wall graywacke that had a northeast strike and dipped northwest was entered, and drill hole No. 56, a horizontal hole to the southeast, entered 

iron-formation breccia after passing through 

grayNacke with chert fragments. 

· Morgan exploration (NW}SEi} .-Several holes· have been drilled in this area. Gore from four was located and examined; hanging-wall graywacke and unoxidized to partly oxidized ironformation breccia were present in each. The 

most northwesterly hole entered graphitic 

footwall slate at ledge and then passed into 

15 

unoxidized iron-formation breccia, indicating an overturned syncline. 

Cottrell ex !oration NE SE .--The 

Gottre 1 mine was developed on the northwest 

tip of a syncline that plunges southeasterly 

to merge with the Berkshire-Buck folds. Considerable drilling has been done in the 

remainder of the "forty", but no core has been 

retained and available desc~iptions are meager. 

A major anticline of footwall, which extends 

northeastward into the area north of the 

Berkshire mine, is present north and west of 

the Cottrell shaft. It is bounded successively on the northwest by iron-formation and an inferred mass of hanging-wall graywacke. 

Fogarty min-. ( SE}SEi}. --Although economically part of the Buck mine, the Fogarty area is a structural unit which warrants separate 

description. It is characterized by a series 

of tight, steeply-plunging folds overturned to 

the southwest (dips easterly) and striking 

northwest. Several of the broader synclines 

contain hanging-wall graywacke. The steep 

plunges, in places as much or more than goo, 

give the folds the exaggerated appearance of 

tightness shown in cross section E-E' (pl. 4). 

Most of the ore occurred as sheetlike masses resting on a structural footwall of 

either footwall slate or overturned hanging- wall graywacke. On the cross section it is seen that the iron-formation is very thin in some parts, apparently as a result of erosion 

at the end of iron-formation time. This 

thinning has resulted, in one place (see cross section), in a troughlike structure between 

hanging-wall and footwall strata in which the 

iron-formation was entirely converted to ore just as in the keel of a syncline. 

A complex, dike-lik~ mass of chloritized 

diabase is cut at the ninth-, tenth-, and 

eleventh-level stations of the Fogarty No. 1 

shaft. The dike dips easterly at these lower 

levels and in detail cuts the footwall strata 

in irregular fashion, but nothing else is 

known of its extent. It is a massive, soft, 

pale-green rock, locally with reddish spots 

where partly oxidized. It is composed largely 

of very fine-grained, green chlorite. 

An apparently' isolated syncline of partly 

oxidized iron-formation is revealed by outcrops in the west part of the section (see 

detail map on pl. 3). The southwestern boundary of the iron-formation is .apparently a fault, inasmuch as no graphitic slate appears 

to intervene between the formation and the 

slates to the southwest. 

Section 12, T. 42 N., R. 35 W., plate 3 

Geologic information in sec. 12 is limited to the area of the Youngs mine, in the . NEiNE-f, and the area immediately to the west. 

The mine was developed on the southeast con- tinuation of the synclines on the Fogarty 

property in sec. 1. As on the Fogarty, the 

folds trend northwesterly and are overturned 

to the southwest (as shown on cross section 

F-F', pl. 4) • Most of the ore occurred along 

the keel of the most southwesterly syncline, 

which on lower levels forms two separate 

"spoons" because of changes in plunge. 
' 

• 

The outcrops in the NEt of the section are of a fissile, micaceous to soapy slate, 

gray to limonitic yellow in color. .This rock 

is apparently stratigraphically deep in the 

footwall, below the sericitic slate and graywacke series. 

Section 5, T. 42 N., R. 34 W., plate 3 

There has been practically no exploration in section 5. Seven shallow, scattered 

churn drill holes provide all the direct 

information as to the nature of the bedrock, 

and no material was retained from four of 

these. 

The magnetic survey (see pl. 2) indicates 

that a large part of the area is underlain by 

magnetic slate and associated rocks. However, a broad belt of low magnetic values trends 

northeasterly through the northwest part of 

the section; it is interpreted as indicating a fold belt which will involve principally 

iron-formation and footwall strata, though 

such folds would doubtless contain hangingwall strata locally. Although the above is 

tb.e preferred explanation for the magnetic 

low, an alternate interpretation of the 

observed magnetic pattern may be suggested. 

The presence of the magnetic low as a border 

to the high magnetic anomaly could be con- strued as indicating that the magnetic slate 

(which is inferred from the high positive 

anomaly) is contained in a deep, northeasterlytrending syncline overturned to the southeast 

(dips northwest). The magnetic low would then 

be due to the negative effect of the lower 

pole of the syncline of magnetic slate. If 

this were true, mu~h of the area shown on plate 3 as possibly underlain by iron-formation might actually be underlain by hangingwall strata. 

The details of the structure, even on a broad scale, are necessarily obscure. The 

inferred iron-formation fold belt forms a northeast-trending anticlinal area between 

hanging-wall strata on the west and the east. 

The trend of the magnetic slate area to the 

east is in part northeast, but crest lines 

swing southeasterly to establish definite 

trends in that direction. The · complicated 

pattern of high magnetic anomalies doubtless 

indicates several folds which involve the 

magnetic slate, but detailed analysis cannot 

be made. 

Section 6, T. 42 N., R. 34 W., plate 3 

The central part of sec. 6 is underlain 

by graywacke, gray slate, and graywacke with 

chert fragments of the hanging-wall seri es below the magnetic slate. The iron-formation 

beneath the graywacke is probably at relatively shallow depths (perhaps within a few 

hundred feet) in much of this area, although 

in a few places higher magnetic values indicate the presence of magnetic slate in deeper 

folds. Structural trends are difficult to 

establish. 

The southeastern part of the section is · marked by a broad magnetic low which extends 

northeasterly into sec. 5. Practically no bedrock information is available, but it seems likely that iron-formation will be present at 

much of the ledge surface, probably in folds 

16 

that· also involve some hanging-wall strata 

and ·footwall strata at ledge. Iron-formation, 

if present, will be on a structure which, in a broad sense, is anticlinal, and whether or not 

it is oxidized 'is a matter of some doubt. 

Borland exploration (NiNWi) .-All the 

drill holes in this area cut iron~formation or hanging-wall strata; none entered the footwall 

slates. The most easterly l;lole, No. 11 of 

this exploration, entered magnetic slate a.t 

ledge surface and continued in it for more than 300 feet before entering hanging-wall 

gray slates, graywacke, and graywacke with 

chert fragments. Breccia, both in the ironformation and the graywacke, is abundant in 

the area and was cut in many of the holes, 

although silty ferruginous slates appear to 

predominate in the hanging wall immediately 

above the iron-formation. 

Sections indicate wholesale overturning on both northwesterly-trending folds and on cross folds. As a result, it has not been 

found possible to adequately rationalize details of the structure from the available data. 

Corry exploration (NEJSWf).-All the sur- face holes in this "forty" entered hangingwall strata at ledge surface. Massive, oxidized graywacke is dominant, although some ferruginous slate is commonly present in most 

places. The deeper holes and the underground wo.rkings ( wbi ch were opened as eastward extensions of the lower levels o.f the Berkshire 

mine) enter~d oxidized iron-formation, locally 

with some ore, and holes No. 8 (most north- weste~ly surface hole) and McEachern hole 

No. 14 (.from the first level o.f the Corry 

shaft) cut f ootwall strata about 500-600 :feet 

below surface. A long horizontal drill hole 

(Corry No. 14), drilled southeasterly from the 

"642" level of the workings, passed from 

hanging-wall graywacke into footwall slate at a point approximately 700 :feet northwest of 

the SE cor. NEtSWi sec. 6, at a depth of 750 

feet below the surface, and ended in footwall 

slate about 200 feet northwest of that corner. The contact oetween these two rocks is appar- " 

ently a fault which cuts out the iron-formation. 

Dominant trends in the area appear to be 

nearly east-west, at least in the western part; 

the folds are steep, possibly overturned 

slightly to the south. The iron-formation was well oxidized, but apparently thin - about 50 

to 75 feet thick. 

Berkshire mine (NWfSWi}~-Most of the ore bodies in the upper levels of the Berkshire 

mine were located on two east-west trending 

synclines overturned to the north. In the 

deeper workings, a tight buckle of footwall 

slate made its appearance as an "island" 

between these two northernmost synclines. On 

the "642" level this buckle connects with the 

main footwall slate area to the north and · divides the ·synclines (which in the upper 

levels had an east-west trend) into two major 

structures which assume a north to northeasterly trend. These synclines plunge to the south 

and connect with the major synclines of the 

Buck mine-;, 

North of the Berkshire shaft is a major 

anticline of footwall slate. The iron-:forma- ·tion on the north side of this anticline has 

been explored in but one place, namely, by a 
long hole (No. 3) extending northeast from the 11 3 f'3 " level to the northeast corner of the 

1 ;t: Jt sec~ 6. This hole, after cutting more than a thousand feet of folded footwall strata 

entered partly-oxidized iron-formation. ' 

Buck mine ( stswt) • -The Buck mine, part 

of which is within sec. 6, is one of the 

largest producers in the district. Operated 

from the Berkshire, Baltic, and Fogarty shafts, 

it is now on the eleventh level (1,000 feet 

below surface) and covers parts of seven "forties". 

The only ore on the upper levels of this 

part of the Buck mine occurred as northeasttrending bodies resting on a structural foot- wall of hanging-wall graywacke dipping to the 

northwest, as shown in cross section D-D' and 

E-E' (pl. 4). The"graywacke, which occurs in a northeast-trending, northeast-pitching, 

overturned cross fold, merges with l· he larger area of hanging-wall strata to the northeast 

(pl. 3). It will be noted from the magnetic 

map (pl. 2) that this area of graywacke is 

marked by slightly higher magnetic values 

than those over iron-formation and footwall 

rock, and that the zero isomagnetic contour is 

in general close to the iron-formation hanging- wall contact. Underground, this cross syncllne, together with others to the north which are subparallel, intersect northwest-trending 

sync lines about in the center of the swtswt 

to produce, on the tenth level, two isolated 

basins of iron-formation surrounded by footwall beds. The relations between the northwesterly-trending folds and the northeasterlytrending f olds are illustrated by cross sections D-D' and E-E 1 (pl. 4). 

On lower levels, the deep, northwesttrending synclines of the adjacent Fogarty 

property, that are overturned to the west, 

pass into the Buck anea, but the major structures containing iron-formation are in the 

eastern part of the sw!swt. These folds trend 

northwesterly. It will be noted from cross section D-D' that the ore bodies that rested on a structural footwall of graywacke in the 

upper levels died out with depth as the dips 

of the overturned graywacke steepened and then 

became normal, and that the ore did not connect 

with that resting on the footwall of the ma.jor troughs. 

A long horizontal hole (No. 40) drilled 

east from the tenth l ;vel to the north-south 

center line of sec . 6 shows that the eastern 

part of the swtswt at this depth is hangingwall graywacke which continues about 400 feet 

into the adjacent SE-fSWt, where it is in faul-t 

contact with footwall slate. Inasmuch as neither the trend nor the dip of this fault is 

known, it is not shown on the map (pl.3). The remainder of the hole is i n footwall for seve ral hundred feet and then enters unoxidized 

iron-formation b r eccia. 

The structure in the southeast 1·art of 

the swtswi is not clearly understood. The most soc the >. s ! erly surfac•: ho e, No . 23, en ,:e r·ed 

footwall :· phitic slate '· the <,, ce .nd 

continued in footwall to a d ·· pth of about 850 

feet. Be ·ow is, it was in graywacke, which 

resembles hanging-wall strata, to the bottom 

at l,lOO .feet. Thus far, no adequate structural explanation can be found which rati ~, nal ·· 

izes the presence of footwall slate at the sur- face with the ge'ology shown in the underground 

17 

workings, although a northeast-trending fault 

of large displacement may be indicated. Such, a fault might pass into bedding-plane shear 

to the southwest along the northeast-trending, 

tight folds of the Baltic mine. 

S~ction 7, T. 42 N., R. 34 W., plate 3 

The north half of sec. 7 is underlain 

predominantly by iron-formation and hanging- wall strata. The strong magnetic anomaly in 

the NEt is almost certainly due to underlying 

magnetic slate, for although no holes have 

been drilled in this section in the area of 

strong anomaly, a hole in the east part of sec. 8, drilled on the south flank of an anom- aly that is nearly continuous into the sec. 7 

high, cuts typical banded cherty magnetic 

slate. The magnetic low which borders the 

high positive anomaly is interpreted as indicating iron-formation at ledge surface. However, as for the sec. 5 magnetic low, it is 

possible that this magnetic low actually is 

due to the effect of the lower pole of a deep, 

overturned syncline of magnetic slate, the 

upper pole of which yields the high positive 

anomaly. 

The south half of the section appears to 

be almost entirely underlain by footwall 

strata, although questionable iron-formation was reported in one h ole in the NEiSWt. -Most 

of the remainder of the holes enteredwhat is 

described in the records as "gray slate." 

None of this core was available for examination. 

Baltic mine (NWiNWf) .-The. Baltic mine was developed on a series of tight, steeplyplunging, closely-spaced folds trending northeast, athwart the dominant northwest trends 

of the adjacent areas. Viewed broadly, the 

Baltic area is anticlinal at ledge surface, 

inasmuch as the folds bring footwall strata 

to the surface with iron-formation and hanging~ 

wall strata on either side. However, the 

synclines are very deep, so that 1 , 000 feet 

below the. surface some of them still contain 

iron-formation whereas adjacent synclines, 

which at the surface contained hanging-wall 

graywacke, at that depth have bottomed in the 

footwall slate. Cross section F-F' (pl. 4) 

shows some of these relations. 

The total width of the Baltic fold belt 

is not more than about 800 feet. The footwall 

strata separating the synclines form narrow, highly~sheared anticlines, so that in many 

respects, the Baltic folds are closely analogous to faults. There are indications that a 

fault (or faults) with this same northeasterly trend cuts the more competent strata to the 

northeast. · 

Zimmerman mine area (E.J..NW .-As shown in eross section G-G' pl. 4 , the Zimmerman mine was devel oped in a series of drag folds that .progressively _deepened to the north. In the 

Zimmerman mine itself, many of the folds are overturned to the south; north of this area, 

however, overturning to the north is indicated. 

Plunges are variable _, but j n the east part of 

the mine a rea, the folds plunge eastward into 

the adjacent DeGrasse property. 

' 

• 
J 

The north Zimmerman nforty" ( t he NEiNWt) was not entered by the Zimmerman mine workings 

but was entered by the tenth level of the Buck 

mine. Several deep holes have been drilled 

from the surface; most cut hanging-wall graywacke or slate at ledge surface and some e~­

tered iron-formation at depth. The deep basin 

of hanging-wall strata thus indicated is shown on the cross section. The ·iron-formation is 

believed to be present at surface in the 

northern part of the area partly because the 

rock encountered at ledge in drill hole No. 

C-43 (see cross section G-G') is a sideritic 

slate of the type commonly found immediately 

above the iron-formation. The two shallow 

holes in the northwest corner of the NEiNWi 

(drilled from the same location) cut what was described as "black slate and chert. n It is 

possible that thi~ is actually unoxidized 

iron-formation, inasmuch as much of the ironformation cut at depth by the tenth level of 

the Buck mine is unoxidized. 

It will be noted from the magnetic map 

(pl. 2) that the deep basin of hanging wall 

strata shown on cross section G-G' (pl. 4) 

does not give the uniformly higher magnetic 

values that might be expected. The reason for 

this is not clear; possibly it is due to the 

influence of basement rocks, but more likely 

it is due to cultural objects which make the 

magnetic determinations unreliable. 

DeGrasse area (SJNEt).--The geologic and 

structural features in the DeGrasse area are similar in most respects to those of the 

adjoining Zimmerman area to the west, and most 

of the mining was carried on as an eastward 

extension of the Zimmerman mine. The DeGrasse 

shaft was shallow, and no ore was produced from it. 

As shown on cross ·section H-H' (pl. 4), 

the mining operations were largely confined to a single, rather complex syncline that is one of a aeries of drag folds that deepen to the 

north. However, from the magnetic data, it is 

believed that an unexposed belt of 'iron-formation may reach ledge surface north of the ex- plorations and south of the high magnetic anom- aly. A single shallow churn-drill hole cut at 

ledge what was described as "black slate," 

which contained about 20 percent iron. 

Section 8, T. 42 N., R. 34 W., plate 3 

Two strong belts of magnetic anomaly cross the northern part of sec. 8 (see pl. 2); both are doubtless caused by the magnetic slate. 

The magnetic slate was cut at led~e in the most 

northerly drill hole in the NEfSEt. A magnetic low in the northwest corner of the section is 

interpreted to indicate iron-formation folds at 

ledge, as in the magnetic low in sec. 7, 

although the only hole drilled (a churn-drill 

hole) cut black slate. 

Deta.lled interpretation of the magnetic 

data in the southern part of the section is 

impossible because of the change in magnetic 

base level, discussed earlier under "Interpretation of the magnetic data." This change is believed to be due to change in character 

of the basement greenstone rocks. In the 

absence of help from the magnetics, little can be done other than to rationalize the available 

drill-hole data, such as was done on plate 3. 

Wl Section 9, T. 42 N., R. 34 W., plate 3 

Magnetic values in the northern threefourths of the Wi sec. 9 are high (see pl. 2), 

and the area is believed to be underlain 

entirely by hanging-wall strata and magnetic 

slate. The two drill holes in the NEtsWi cut 

typical hanging-wall graywacke. The drill 

hole located 1,400 feet south of the center sec. 9, is like the others in this area, a shallow hole, and only a few pieces of core were preserved. The rock is a banded, cherty, 

sideritic slate, which, except that it is non- magnetic, is very similar to the lower part of 

the magnetic slate elsewhere. As can be seen from the map, the hole is close to the crest 

line of a magnetic anomaly, so it seems likely that the hole was in a nonmagnetic zone within 

the magnetic slate. 

Three. drill holes in the SEiSWi entered 

footwall graphitic slate and one entered 

unoxidized iron-formation. South of the map 

area, other holes entered unoxidized ironformation. 

The four maps may _be obtained for $1.40 per set from 

the Director, U. S. Geological Survey, Washington 25, D. C. 

18