Contributions to 

General Geology 

1951

GEOLOGICAL SURVEY BULLETIN 973

WHO fiEOlOGIUU. SiWET

UNITED STATES G ENT PRINTING OFFICE, WASHINGTON : 1952
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director
CONTENTS

[The letters in parentheses preceding the titles are those used to designate the papers for separate publication]

Page

(A) Magnetic exploration for chromite, by H. E. Hawkes__--__--__---- 1

(B) The Coalwood coal field, Powder River County, Montana, by Robert

P. Bryson__________-_______-__--_--__--_------_------______ 23

(C) Dolomite deposit near Sloan, Nevada, by Charles Deiss--___-__--__ 107

(D) Quicksilver deposits in the Southern Pilot Mountains, Mineral County,

Nevada, by David A. Phoenix and James B. Cathcart___________ 143

(E) Quartz crystal deposits of western Arkansas, by A. E. J. EngeL _____ 173

(F) Fluorspar deposits of Burro Mountains and vicinity, New Mexico, by

Elliot Gillerman__________________..._..._..._..___ 261

ILLUSTRATIONS

Plate Page

1. Geologic map and cross sections of the Coalwood coal field, Powder

River County, Mont---__-__--_---_-----_---_---__-------- In pocket

2. Diagram showing correlations of coal beds in the Coalwood coal field 

and of named stratigraphic units in fields to the west, and to the 

east..___-_-_----_--__---__--------------------.i--_-_---_ In pocket

3. A, Lower member of the Fort Union overlying beds of the Hell Creek 

formation. B, Clinker of the Nicholls coal bed overlying beds of the 

Hell Creek formation, Coalwood coal field_-__-_____-___-___-__ 42

4. A, Clinker of the Sawyer coal bed overlying beds of the Tongue River 

member of the Fort Union formation. B, Clinker of the "X" coal 

bed, Coalwood coal field._______--__--________._-__--__________ 74

5. A, Clinker of the Broadus coal bed, with beds of the Tongue River 

member overlying beds of the lower member. B, Clinker and ash, 

formed by burning of the Broadus coal bed, resting on gray shale and 

carbonaceous shale, Coalwood coalfield__-__------_---_-__-_____ 75

6. Diagram showing correlations, ranges in thickness, and intervals

between coal beds in the Coalwood coal field.________________ In pocket

7. Sections of coal beds in T. 1 S., R. 49 E., Mont______________ In pocket

8. Sections of coal beds in T. 2 S., R. 49 E___.__.._______________ In pocket

9. Sections of coal beds in T. 3 S., R. 49 E_______________________ In pocket

10. Sections of coal beds inT. 1 S., R. 50 £-_____-___-__-____--___ In pocket

11. Sections of coal beds in T. 1 S., R. 53 E_______________________ In pocket

12. Sections of coal beds in T. 3 S., R. 53 E_________.________.____ In pocket

13. Geologic map of the dolomite deposit near Sloan, Nev_.________- In pocket

14. Structure sections, Sloan dolomite deposit. ._---.______-_______ In pocket

15. Views of the quarries at Sloan HilL_--___________________________ 112

16. A, Bench of the Anchor member of the Monte Cristo dolomite on the 

west side of Sloan Hill. B, Points and pits on the weathered surface 

of the Bullion member of the Monte Cristo dolomite at Sloan Hill__ 118

Or
IV CONTENTS

Plate . Page

17. A, Lower part of the Bird Spring formation on the northwest part of 

Sloan Hill. B, Part of a fault breccia zone in the Bullion member of 

the Monte Cristo dolomite at Sloan Hill__-__-_---_---_-__--_-_- 119

18. A, Small overthrust in the Crystal Pass limestone member of the 

Sultan limestone in the face of quarry 3, Sloan Hill. B, Drag fold 

on a small overthrust above the south tunnel in quarry 3__-_-__-__ 128

19. Geologic map and sections of the Pilot Mountains quicksilver district,

Mineral County, Nev____-___________-___---______________ In pocket

20. Geologic map of the underground workings of the Mina Development

Co. mine, Pilot Mountains district-_-_______-_-___-________- In pocket

21. Geologic map of the Mina Development Co. mine area__._______ In pocket

22. Isometric block diagram of the Mina Development Co. mine and

vicinity__________________________________________________ In pocket

23. Geologic map of the Reward mine and vicinity, Pilot Mountains district__________________________________________________ In pocket

24. Geologic map of the Inman mine and vicinity, Pilot Mountains district. -_--_---____--_____-____-__-_______________________ In pocket

25. Map of western Arkansas showing the distribution of the quartz

crystal deposits_________________________________________ In pocket

26. Geologic map and sections of the Fisher Mountain area, Montgomery

County, Ark_____________________________________________ In pocket

27. Geologic map and sections of the Chance area, Garland County, Ark, In pocket

28. Geologic map and sections of the Miller Mountain area, Garland

County, Ark_____________________________________________ In pocket

29. Crystal-studded quarzite slab from a fissure vein on Fisher Mountain. 198

30. Quartz veins in Stanley shale, western Arkansas_______-___-_----__ 199

31. Quartz vein in Stanley shale exposed along Arkansas Highway 9____ 214

32. Pile of larger crystals from the Dierks No. 4 mine (Blocker lead),

western Arkansas____________________________________________ 215

33. A, Quartz and adularia vein intergrowth developed in sandy shale on 

Miller Mountain. B, Quartz vein containing brecciated" fragments 

of shaly wall rocks rimmed with adularia-_--_-_--_--_-----___- 228

34. Quartz veins in the Crystal Mountain sandstone (quartzite) exposed

on Arkansas Highway 6_____________________________________ 229

35. Geologic map and sections of the West Chance quartz crystal area,

Garland County, Ark_.__-___-_--______________-___--- In pocket

36. Detailed geologic map and sections of part of Miller Mountain, Garland County, Ark_______._____.-_--__--__-__-_---------- In pocket

37. Map and sections showing workings of the Metals Reserve Company

on the north slope of Miller Mountain_____________________ In pocket

38. Geologic map and section of the Hamilton Hill quartz crystal area,

Garland County, Ark_____________________________________ In pocket

39. Geologic map and sections of Fisher Mountain and Fisher Spur,

Montgomery County, Ark____.__. ------- ---- In Pocket

40. Parts of mine workings at Fisher Mountain showing the character of

the exposed quartz veins_________:____--_----_---------- In pocket

41. Geologic map and section of the WilliS quartz Crystal mine, Saline

County, Ark__._._.-__. - In P°cket

42. Geologic map and section of the Marler mine and nearby quartz

crystal deposits, Saline County, Ark__ ___________________ In pocket

43. Geologic map and section of the Ellison-Dierks quartz crystal area,

Garland County, Ark___--__-__-_'_--_-_--_---_-__---_----_ In pocket

44. Geologic map and section of the W. TV Beard quartz crystal area,

Garland County, Ark_____-___-------_--------- ----- In pocket
CONTENTS V

Plate Page

45. Geologic map and section of the Monroe-Robbins quartz crystal area,

Montgomery County, Ark.________________________________ In pocket

46. Geologic map of the Burro Chief fluorspar deposit and vicinity,

Burro Mountains, Grant County, N. Mex___________________ In pocket

47. Underground level maps of the Burro Chief fluorspar mine, Burro

Mountains, Grant County, N. Mex_.__-____-______--______- In pocket

48. Cross sections through the Burro fluorspar mine, Burro Mountains,

Grant County, N. Mex_ ________._-_____-______--___._-- In pocket

49. Vertical longitudinal projection through the Burro Chief fluorspar

mine, Burro Mountains, Grant County, N. Mex__ ____________ In pocket

50. Geologic map of the Shrine fluorspar deposit and vicinity, Burro Mountains, Grant County, N. Mex_______________________________ In pocket

51. Geologic map of the California Gulch fluorspar deposits and vicinity,

Burro Mountains, Grant County, N. Mex___________________ In pocket

52. Geologic map of the Spar Hill and Pine Canyon fluorspar deposits and

vicinity, Burro Mountains, Grant County, N. Mex___________ In pocket

53. Geologic map of the Long Lost Brother fluorspar deposits and vicinity,

Grant County, N. Mex-__-_____________----_-_-__-_______- In pocket

54. Geologic map of the Moneymaker fluorspar deposit and vicinity, Burro

Mountains, Grant County, N. Mex.________________________ In pocket

55. Geologic map of the Bluebird fluorspar deposit and vicinity, Grant

County, N. Mex___---___________-_-__-__-__-_-_----_-__- In pocket

56. Map showing the fluorspar veins and associated faults in a part of T. 22

S., R. 15 W., Grant County, N. Mex_____________________ In pocket

57. Geologic map of the Continental fluorspar deposit and vicinity, Grant

County, N. Mex________________________________________ In pocket

58. Geologic map of the American fluorspar deposit and vicinity, Grant

County, N. Mex._______________________________________ In pocket

59. Geologic map of the Double Strike fluorspar deposit and vicinity,

Grant County, N. Mex________sl_-_-____-_______-_-----_- In pocket

F igure

1. Magnetic map of the Ray chrornite mine, John Day area, Grant

County, Org._-_-______._._....._..._....__...._.._ 10

2. Magnetic map of the southwest ore body, Chambers chromite mine,

John Day area_____________________________________________ 16

3. Index map showing the location of the Coalwood coal field, Mont____ 25

4. Diagram showing correlations of coal beds in the Coalwood coal field

and of stratigraphic units with those to north____________________ 33

5. Sketch map of part of southeastern Montana showing localities for

which analyses of coal are listed_______________________________ 67

6. Sections of coal beds in T. 2 S., R. 50 E., Mont.........._------- 88

7. Sections of coal beds in T. 3 S., R. 50 E., and.T. 2 S., R. 51 E.____ 90

8. Sections of coal beds in T. 1 S., R. 51 E________________________ 92

9. Sections of coal beds in T. 3 S., R. 51 E._.__'_________________ 94

10. Sections of coal beds in T. 1 S., R. 52 E______________.___________ 95

11. Sections of coal beds in T. 2 S., R. 52 E., and T. 3 S., R. 52 E___._ 97

12. Sections of coal beds in T. 2 S., R. 53 E__________________________ 99

13. Sections of coal bed in T. 2 S., R. 54 E., and T. 3 S., R. 54 E_______ 102

14. Index map showing the location of the dolomite deposit near Sloan,

Nev.___________ ____________________________________________ 109

15. Index map of Nevada, showing the location of the Pilot Mountains

district ____________________________________________________ 144

16. Geologic map of the Drew mine, Pilot Mountains district........... 161
VI CONTENTS

Plate ^ Page

17. Geologic map of the Mammoth mine, Pilot Mountains district_______ 164

18. Geologic map of the Hitt mine, Pilot Mountains district.___________ 166

19. Geologic map of the Betty mine, Pilot Mountains district___________ 169

20. Diagrammatic block sketch showing a deformed vein localized along

a thrust fault near Angling Pinnacle, Ark _______________________ 197

.21. Plan and sections of the quartz crystal-bearing vein zone at the Dierks

No. 4 mine (Blocker lead), western Arkansas ____________________ 202

22. Plan and section of quartz veins at the Diamond Drill Carbon Co.

No. 5 mine, western Arkansas_________________________________ 203

23. Faces (forms) present on simple crystals in the Arkansas district_____ 206

24. Block diagram of a comb crystal pocket in a quartz vein at the John 

Brown prospect, western Arkansas, and sections of three smoky 

quartz crystals______________________________________________ 222

25. Minerals and their approximate paragenesis in the Arkansas quartz

crystal district-______________________________________________ 228

26. Block diagram showing examples of vein constituents, characteristic 

of particular, wall rocks, which are "carried over" into the vein in 

an adjacent alien wall rock____________________________________ 234

27. Slightly deformed quartz crystal pocket at the Diamond Drill Carbon

Co. No. 5 cut, western Arkansas__________---_-___-____________ 236

28. Sketch of a deformed and partly dispersed comb crystal pocket in a 

quartz vein on the north slope of Miller Mountain, Garland County, 

Ark._____________________--__-__-_--_--------_------------_ 250

29. Geologic map and section of the Howard-Sheffield quartz crystal

mine, Montgomery County, Ark_______________________________ 257

30. Index map showing location of fluorspar deposits and major geologic 

features of the Burro Mountains and vicinity, Grant County, 

N. Mex__..______..---._____- _..-__._----_---.----_---- 262
Magnetic Exploration 

for Chromite

By H. E. HAWKES

CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

GEOLOGICAL SURVEY BULLETIN 973-A

A summary of the problem of 

magnetic exploration for 

chromite

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1951
UNITED STATES DEPARTMENT, OF THE INTERIOR

Oscar L. Chapman, Secretary \ - -'' --.

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 15 cents (paper cover)
......... ...... .... CONTENTS . . , ' '

. '-.... Page

Abstract __"___ ________________--___-_-___--_-__-_______--__-_______ 1

Introduction.._ ______.___-__----_--_-.-------_-____-__---__--.____ 1

Geology of chromite deposits__---___--___---_________.__-__--__..___ 2

Mineralogy of chromite-___________---_--_---_----.-_---_---_-.--_. 3

Composition__-____________._--_---_-_-________--_____-______. 3

Color and streak.___.___.___-_____._.__.______..______._. _ 4

Magnetic properties ____.___--___--__-_-___-_____-_-_______ 4

General mineralogical relationships_____________________________. 6

Interpretation of magnetic-survey data_-_----____-_-___-_____._______ 7

Susceptibility of common minerals_______________________________ 7

Field determination of magnetic properties. __-__-_---_____..__ 8

Magnetic field instruments__________-_---___----__---_____-_______ 9

Results of field surveys_-__-_________--_----__-_---_-_-.__-_-____-__ 9

Direct magnetic indication of chromite ore._______________________ 10

Ray mine, John Day area, Oreg_-__-____-___-___.___-_______ 10

Red Lodge district, Carbon County, Mont__________________ 10

Verbliuzhi Mountain, Ural region, U. S. S. R_______---_______ 11

Groenfontein No. 302 farm, Transvaal, South Africa__-_______ 11

Oregon (chromite-bearing beach placers)______________________ 11

Indication of favorable host rock________________________________ 12

Casper Mountain, Wyo____-_---_--____-_____-___-_-___-___ 12

Chrome Point, western Newfoundland.______________________ 12

Shoal Pond area, eastern Newfoundland___-________-______.__ 12

U. S. S. R_-_____-____-____-_____-___---__--____-_-___-_._ 13

Manitoba, Canada (Bird River complex)_____________________ 13

Unsuccessful application of magnetic methods..-._________________ 13

Castro mine, San Luis Obispo County, Calif__________________ 13

Grey Eagle mine, Glenn County, Calif _______________________ 14

Little Castle Creek valley, Shasta County, Calif._____________ 14

Cyclone Gap mine, Siskiyou County, Calif____________________ 14

Sourdough mine, Curry County, Oreg-_---___-_-_--_-___--___ 15

John Day area, Oreg----___-_-_--___-__---_---_--____-_____ 15

Wood mine, Lancaster County, Pa_________________________ 16

Thetford district, Quebec___________________________________ 16

U. S. S. R_-_._--__-__-_________-___---____--______.______ 17

Montana (Stillwater complex)_____________________________ 17

Summary. ________________________________________________________ 18

References cited-_______________________________________________1__ 18

Index. ________________________________________________ 21

ILLUSTRATIONS

FIGURE 1. Magnetic map of the Ray chromite mine, John Day area,

Grant County, Oreg__________________________________ 10

2. Magnetic map of the southwest ore body, Chambers chromite

mine, John Day area-_-_---_---___-__________-__-______ 16

m
IV CONTENTS

Page 

TABLE 1. Magnetic susceptibility of chromite.___-_._____._____-_-.__ 5

2. Some chemical and physical properties of chromite from Casper

Mountain, Wyo_.___.______._-_*____..__.________ ____ 6

3. Magnetic susceptibility of some common minerals_______ 8.
CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

MAGNETIC EXPLORATION FOR CHROMITE

By H. E. HAWKES

ABSTRACT

Present knowledge indicates that the magnetic susceptibility of chromite 

varies from practically nil, for chromite approaching the composition MgO.CrzOa, 

to high values for chromite approaching magnetite in composition. Magnetic 

prospecting may be useful in locating ore containing magnetic chromite but is 

of little help in the direct location of high-grade, nonmagnetic chromite. Under 

favorable conditions, geological information of value in prospecting for chromite 

can be obtained by magnetic surveys. However, although magnetic methods 

appear to give some promise in certain areas, no report of a discovery of 

commercial-grade chromite directly attributable to a magnetic survey has come 

to the attention of the author.

INTRODUCTION

This paper is an attempt to summarize the problem of magnetic 

exploration for chromite. It presents a review of the published 

literature on the subject together with the results of experimental 

surveys by the author in 1941 on some of the more important chromite 

deposits of the western United States.

In many respects, chromite is similar to magnetite'. Both minerals 

are iron-bearing spinels, are black and heavy, and form massive, 

monomineralic ore deposits. The strongly magnetic character of 

magnetite is well known, and magnetic methods of exploration for 

magnetite-bearing iron ores have met with outstanding success. If 

the mineralogical similarities between magnetite and chromite could 

be extended to include magnetic properties, magnetic methods of 

prospecting for chromite might be equally effective.

Chromite, however, is commonly nonmagnetic or only very weakly 

magnetic. Only in a few localities is it strongly magnetic. Nevertheless, magnetic methods have been applied to chromite prospecting 

far more often than would seem to be warranted by its relatively 

undistinguished magnetic properties. The writer knows of at least 

30 areas where magnetic surveys have been run, but not a single 

discovery of primary chromite of commercial value that could be 

credited to the magnetic data.

1
2 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951 

GEOLOGY OP CHROMITE DEPOSITS

The geologic occurrence of primary chromite is limited to what 

are generally regarded as segregation products of ultramafic magmas. 

Chromite is never found in the form of veins, shoots, or other hydrothermal deposits, lying outside the limits of the ultramafic mass. In 

this respect the geology of chromite is unlike that of any other 

common ore mineral, with the possible exception of titaniferous 

magnetite. The problem of prospecting for primary chromite, therefore, is twofold first, to locate bodies of the ultramafic host rock 

and, second, to locate the concentrations of chromite within the ultramafic bodies.

Two quite distinct varieties of chromite deposit are known. These 

have been called the "sackform" and the "stratiform" types, terms 

suggested by the characteristic shape of the deposits.

The sackform type is characteristic of the dunites and peridotites 

of certain geosynclinal belts such as the Coast Kange of California. 

The ore ranges from massive chromite to sparse disseminations of 

chromite grains in a silicate matrix. In addition to the chromite 

segregations, the dunite and peridotite country rock may contain 

about 1 percent chromite as an accessory mineral of no economic 

importance except as a source of placer ore. Deposits of chromite 

occur in the enclosing ultramafic rock as clusters, linear zones, or 

randomly scattered lenticular or irregular bodies. Sackform chromite 

bodies have been compared to plums in a plum pudding, as the distribution of the deposits in many localities appears to lack any 

perceptible control. Where the deposits tend to occur in clusters, 

prospecting may be guided by the probability that new discoveries 

are more likely to be found in the vicinity of known ore than at a 

distance from it. Observers have also noted that where the ultramafic mass containing the ore consists of both dunite and peridotite, 

the ore bodies characteristically occur only in the dunitic phase. Where 

the local magnetic properties of the dunite are notably different from 

those of the peridotite, favorable zones for mineralization can be 

indicated by the data of magnetic surveys. In many areas, however, 

the occurrence of dunite is limited to a sheath of negligible thickness 

separating the ore from the main mass of peridotite. In some localities, "ore zones" of unknown genetic significance have been indicated

by magnetic surveys. Possibly these are the reflection of variations 

in the degree of serpentinization that are related in some way to the 

presence of ore deposits.

The stratiform type of chromite deposit is found only in the lower 

ultramafic layers of differentiated norite or gabbro sills. Examples 

are the tabular chromitite bodies of the Still water complex in Montana, the Bird River complex in Manitoba, and the Bushveld complex
MAGNETIC EXPLORATION FOR CHROMITE 3

in South Africa. The chromitite layers commonly maintain a fairly 

consistent stratigraphic relationship to the primary igneous layering 

and to the marginal contacts of the sill. Prospecting for this type 

of chromite ore is thus simplified once the stratigraphic relationships 

have been .worked out, and lateral extensions or faulted segments can 

be located in much the same way that a coal seam is traced through 

areas of complex structure.

Serpentinization of the primary silicate minerals of the ultramafic 

country rock of both types of chromite is common. The process of 

alteration of olivine and pyroxene to serpentine producers very fine 

grained magnetite as a byproduct. Magnetite originating from 

processes of serpentinization may cause extraneous magnetic anomalies that interfere with direct magnetic observation of chromite ore. 

However, the resulting strong magnetism of the ultramafic country 

rock may be an advantage in prospecting, in that it makes possible 

the location of small isolated ultramafic bodies or the tracing of the 

contacts of larger ultramafic masses.

At Casper Mountain, Wyo., and near Red Lodge, Mont., the ultramafic rocks with their included chromite deposits have been invaded 

by later igneous intrusions and hydrothermal solutions. At Casper 

Mountain (Stephenson, H. K., 1940), the ultramafic rocks have been 

stoped out or metamorphosed almost beyond recognition, whereas at 

least some of the chromite ore has retained its original position and 

shape. The effect of metamorphism on the ore at this locality has 

been primarily the addition of 'iron to the chromite molecule and the 

alteration of the silicate gangue minerals. The high iron content of 

some of the Red Lodge chromites, however, is thought to be primary 

(James, 1946, p. 170).

Concentrations of chromite in placer deposits have been of commercial interest in at least one locality. Placer chromite commonly occurs 

in association with other heavy minerals such as magnetite, garnet, 

zircon, olivine, gold, and platinum.

MINERALOGY OF CHROMITE

COMPOSITION

Both chromite and magnetite are members of the isometric spinel 

group. The chemical formula for chromite is commonly written 

FeO.Cr2 03 . Inasmuch as chromite almost invariably contains magnesia, alumina, and ferric iron in addition to chromium and ferrous 

iron, the formula might more correctly be written (Mg,Fe)O.(Cr, 

Fe,Al) 2O8 . The work of Stevens (1944), involving 52 complete 

analyses of chromites from;the Western Hemisphere, indicates that 

complete isomorphism apparently does not exist within the limits of 

this, formula.. Compositions vary from magnetite, FeO.Fe2O3 ; to
4 CONTRIBUTIONS TO ECONOMIC GEOLOGY/ 1951

magnesiochromite, MgO.Cr2O8 ; and spinel, MgO.AJ 2O3 ; with small 

proportions of ferrochromite, FeO.Cr2O3 . Magnesioferrite, MgO.- 

Fe2O3, is not -an end member of naturally occurring chromite. For 

simplicity, the composition of chromite may be regarded as varying 

from magnetite to magnesiochromite and spinel, with some substitution 

of FeO for MgO. As one of these end members is highly magnetic 

(magnetite) and the other two essentially nonmagnetic (spinel and 

magnesiochromite), the magnetic properties of chromite may vary 

widely. This variation should be studied experimentally.

The composition of chromite is closely related to the nature of the 

ultramafic country rock and its subsequent metamorphic history. 

Sackform chromites from peridotites and dunites are high in chromium ; chromites from feldspar-bearing ultramafic rocks such as troctolite are commonly high in aluminum; stratiform chromites from 

differentiated norite sills are characteristically high in iron. The 

metamorphosed chromites from Casper Mountain and the somewhat 

similar chromites from Eed Lodge are extremely high in iron, some 

specimens approaching the composition of magnetite.

COLOR AND STREAK

The color of chromite in transmitted light varies from light brown 

or reddish brown to black. Stevens (personal communication) has 

observed that the streak of chromite may be greenish brown or red 

brown to black depending on composition. He found that when the 

composition lies between spinel and magnesiochromite the streak is 

greenish brown;'whenthe composition approaches magnesiochromite 

but is intermediate between that and magnetite the streak is red 

brown; as the composition of magnetite is approached the streak 

becomes darker. Thus an estimate of the composition of the chromite

can. be made by observing the color of the streak. 

MAGNETIC PROPERTIES

The magnetic properties of minerals can be compared quantitatively by determining experimentally a factor designated as the magnetic susceptibility, equal to the ratio of the intensity of magnetization

at the surface of the specimen to the intensity of the imposed magnetic field. Quantitative measurements of the magnetic susceptibility 

of chromite have been made by a number of investigators (table 1).

In connection with these determinations, it should be noted that 

many of the susceptibility determinations were made at a magnetizing 

field strength ranging from several hundred to 5,000 times that of the 

earth. Slichter (1929) has found that the susceptibility of some 

minerals can vary within very wide limits as a function of the magnetizing force; his experiments with magnetite and pyrrhotite showed 

that susceptibilities computed from field-survey data were 10 to 100
times greater than those determined under laboratory conditions. 

Slichter's findings may not apply to chromite, however, as H. K. 

Stephenson found that his highly magnetic chromites from Casper 

Mountain showed considerably higher susceptibilities at field strengths 

of 200 gauss than at 0.3 to 2.6 gauss (table 2).

TABLE 1. Magnetic susceptibility of chromite 

Locality

Do_._ ..................... .......

Do................................

Do................................

Do..... ...........................

Shoal Pond, Newfoundland ...........

Verbliuzhi Mountain (Urals), U. S.

S. R. (4 samples').

(15 samples). 

Varshavsk (Urals), U. S. S. R. (6

samples).

R. (14 samples).

Casper Mountain, Wyo. ..............

Kutarstan (Urals), U. S. S. R. (4

samples). 

Gologorsk, etc. (Urals), U. S. S. R.

(4 samples).

Stowbrldge, Newfoundland _____

Kurdistan, Transcaucasia, U. S. S.

R. (5 samples). 

Balsam Gap, N. C. ___ . _____ .

Blufl Head, Newfoundland .... ......

Montana (Stillwater complex) ......... 

Newfoundland (Blow-me-down complex). 

Burnt Hill, Newfoundland.. ____ .

Newfoundland (Blow-me-down complex). 

Stowbridge, Newfoundland ...........

Reference

_ ..do. ___ . _______ ..

.....do.........................

.....do.........................

.....do.........................

Snelgrove (1934) ___ .... __ .

Andreev (1937)... .............

Reich (1930).. _ .. __ .. _ ..

Andreev (1937). _ .. __ . ....

.....do.........................

Reich (1930) ..... ........

Andreev (1937) ................

Stephenson, H. K. (1940). .....

Andreev (1937) ................

.....do. ............... ......

Reich (1930)... ................

Stutzer et al. (1918)...... ......

Stephenson, H. K. (1940)... ...

.....do...... _ . __ . _ ... ....

Andreev (1937)-. ____ . ......

Stephenson, H. K. (1940).. .....do...................;.....

 do. . .

.....do... _ ...................

.....do. _____________

Field 

strength (in 

gauss ')

0.3-2.6 

0.3-2.6. ..

0.3-2.6 

0. 3-2.6.. ..... .

0.3-2.6 

0.3-2.6........

0.3-2.6........

0.5 ..

0. 5.. ......... .

0.3-2.6 

1,200-1,400 

2,500-. 

1,200-1,400 

2,600 .......

0.5.. .. 

220. 

2,500- 

2,500 

2,500... _ .

2,500--. 

2,500 .......... 

2,500-. _ -

1,200-1,400 

1,200-1,400 

1,200-1,400. 

SusceptibUity 

(in c. g. s. units 

X10«)

94,000.

64,000.

61, 000.

55,000.

27,000.

16, 500.

50 to 10,900.

8,800. 2

6,000 to 8,000. «

100 to 900.

50 to 900.

600 to 800.»

120 to 674.

600.'

574.

549.

537.

200 to 400.

370 to 450.

317.

200 to SOO.a

244.51.

232.

227.

100 to 200.

182.

180.

173. 

136.

127.

80.

57.

> One gauss (100,000 gammas) equals 1 oersted or l gilbert per centimeter. The strength of the natural 

magnetic field of the earth is about 0.5 gauss or 50,000 gammas.

' See also table 2.

' Semiquantitative estimate computed from deflection of magnetic system of magnetometer when 

specimen is held at fixed distance from magnetometer.

A notable feature of the data of tables 1 and 2 is the apparent 

variability in the susceptibility of chromite specimens collected from 

the same area or even from different parts of the same deposit. The 

susceptibilities of H. K. Stephenson's Casper Mountain suite, all collected within an area half a mile in length, vary widely. Duplicate 

specimens from Bluff Head, the Blow-me-down complex, Stowbridge, 

and Chrome Point, Newfoundland, all collected by Snelgrove but determined independently by Snelgrove and Stephenson, differ materially. Hence it is apparent that susceptibility determinations on individual specimens can be misleading.

919242 51 2~
CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

TABLE 2. Some chemical and physical properties of chromiie from Casper

Mountain, Wyo.

[Adapted from U. S. Geol. Survey press release of Sept. 14,1942; susceptibility at 200 gauss calculated by

H. K. Stevenson (1940, table 2)]

Sample 

No.

1. .... .

2.........

3.... .....

4. .-..

6.- ...

6. ..

1... ......

Specific 

gravity

4.65 

4.73 

4.76 

4.85 

4.92 

4.93 

4.99

Fe 

(percent)

26.76 

31.60 

46.00 

40.52 

51.76 

47.19 

58.40

Cr2 03 

(percent)

44.74 

42.7 

25.4 

35.39 

18.4 

28.2 

13.7

Color of very fine powder

.....do......... .... .. 

... ..do...... ...-..-......-

.....do.... .................

Relative 

darkness 

of color 

of powder

1 

3 

2

5 

4 

6

7

Susceptibility (in c. g. s. 

units X 10»)

At field 

strength of 

0.3 to 2.6 

gauss

600 

8,800 

27, 000 

61, 000 

64,000 

55,000 

94,000

At field 

strength, of 

200 gauss

1,600- 

14, 300 

. 44,000 127, ooa

216, 000 

117, 000 

267, 000

GENERAL MINERALOGICAL RELATIONSHIPS

Undoubtedly the magnetic properties of chromite are related either 

directly or indirectly to its other chemical and physical properties. 

Although no comprehensive investigation of the mutual relationship 

of the mineralogical properties of chromite has been reported, scattered observations by many different workers on various aspects of the 

problem seem to fit into a generally consistent picture.

Several workers have reported qualitative observations on the relation of the magnetic properties of chromite to its chemical composition. Singewald ,(1919), in a comparative study of the magnetic 

and nonmagnetic fractions of chrome-bearing sands from Maryland, 

found that in the magnetic fraction the ratio of chromic oxide to 

alumina was greater than 1:1, whereas in the nonmagnetic fraction 

the ratio was less than 1:1. This view has not been supported by 

most other workers, who have found evidence indicating that the 

magnetic properties of chromite are related more or less directly to 

the iron content of the chromite molecule. Stevens (1944, p. 26) observed that in samples high in iron the chromite was magnetic, making 

magnetic separation of chromite and magnetite impracticable. H. K.

Stephenson's data from Casper Mountain (table 2) point toward a 

definite relationship between increasing iron content and increasing 

susceptibility, but the problem here is confused by the presence of

finely intergrown hematite that could not be mechanically separated

from the chrornite.

Kitchen (1929), at Unst in Scotland; Dresser (1913), at Thetford,.

Quebec; Maxwell (1949), in New Caledonia; and the present writer,

at Little Castle Creek in California, have observed thin sections of 

chromite where the grains consist of a core of light-colored chromite 

surrounded by an opaque rim. The opaque phase may also be developed in a fine lacework pattern along apparent incipient fractures;
. MAGNETIC i EXPLORATION FOR CHROMITE ' 7-

leading into the light-colored core. Observations on some of this 

material indicate that the opaque phase is not only higher in iron but 

also relatively more magnetic. Dresser (1913, pp. 76-80) reported 

that magnetic separation of the crushed ore gave a clean black magnetic fraction and a fairly clean, translucent nonmagnetic fraction. 

Analysis showed that the magnetic fraction contained 15.66 percent 

FeO and 48.20 percent Cr2O3 , whereas the nonmagnetic fraction contained 13.94 percent FeO and 45.30 percent Cr2 03 . The present writer, 

in studying chromjte from Little Castle Creek, found that the opaque 

halos were magnetic in contrast to the nonmagnetic character of the 

translucent cores, but obtained no supporting chemical analyses. HJ 

K. Stephenson (1940), at Casper Mountain, did not observe the halo 

structure in thin section, but found that acid etching of polished surfaces brought out a similar halo pattern in which the material of the 

halo was more magnetic and more soluble in acid than that of the core. 

Bead tests of the halo mineral showed the presence of chromium; the 

etch reaction suggested a higher iron content in the halo.

James (1946, p. 171), in the Eed Lodge district of Montana, observed a variation in the opacity of the chromite, but did not report 

having seen the halo texture. He examined 14 representative samples 

and found that 5 were translucent and 9 were opaque. All five of the 

translucent chromites were nonmagnetic, and all four of the magnetic 

chromites were opaque. The former were high in chromium and low 

in iron, .whereas the reverse was true of the latter. Table 2 shows a 

similar relationship between opacity and magnetic properties in 

samples from Casper Mountain. The same suite of seven samples 

also shows a parallelism between magnetic susceptibility and specific 

gravity.

In summary, it has been found that wherever comparative observa-. 

tions on the various chemical and physical properties of chromite have 

been made, a rough parallelism exists between increasing iron content 

and increasing magnetic susceptibility.

INTERPRETATION OF MAGNETIC-SURVEY DATA

The significant measurement in a magnetic survey is the local deviation of the observed magnetic intensity from the average value within 

the area of the survey. Such deviations are termed "anomalies" in 

geophysical parlance and are the effect of the distortion of the normal 

magnetic field of the earth by local concentrations of magnetic minerals.

SUSCEPTIBILITY OF COMMON MINERALS

Magnetite; is by far the most strongly magnetic substance that 

qccurs naturally. The susceptibility of magnetite is. several times 

greater than that of the less common magnetic minerals ilmenite, pyr-
8 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

rhotite, and franklinite, and it is some 10,000 times more magnetic 

than most rock-fprming'minerals (table 3).

Other factors being held constant, the magnetic anomaly value due 

to the presence of a given mineral in an underlying deposit is approximately proportional to the product of the susceptibility of that mineral and its concentration in the deposit. Thus a body containing 0.01 

to 0.1 percent normal magnetite may give the same anomaly as a body 

of the same size and shape containing 100 percent normal chromite. 

By virtue of their magnetite content, serpentinized ultramafic rocks 

may have susceptibilities as high as 15,600 X10 ~6 c. g. s. units (Heiland, 1940; Haalck, 1934; Snelgrove, 1934).

Besides being the most strongly magnetic natural mineral, magnetite is by far the most common of the magnetic minerals. Except in 

the presence of large concentrations of one of the other magnetic minerals, therefore, it is safe to assume that the magnetic properties of a 

rock are controlled almost entirely by its magnetite content.

TABLE 3. Magnetic susceptibility of some common minerals 

[Data from Heiland (1940, table 35, pp. 310-311)]

Mineral

Pyrrhotite ________________________

Franklinite __________ .... .. _______

Pyrlte _______ . ____ .. _____ .... ____ ......

No. of specimens

10 

5 

1

1 

i i i 

1 

1 

(?)

77

Susceptibility (in c. g. s. units X 10 «)

Average

' 608,000 

166, 000 

125, 000 

35,640 3,200

240. 89 

133. 13 

12.23 

4.53

Range

151,500 to 1,620,000. 

30,740 to 252,000.

-1.07 to -1.2. 

57 to 94,000.1

1 Summary of data from table 1.

FIELD DETERMINATION OF MAGNETIC PROPERTIES

A working approximation of the magnetic properties of hand specimens can be made very easily in the field without recourse to tedious

laboratory tests. The most commonly used method is to bring the 

specimen to be tested close to the case of the Schmidt magnetometer 

and observe deflections of the magnetic system. Specimens of about 

the same size are rotated in all positions at a point over or under one

end of the magnetic system, and the average deflection of the system 

in scale divisions is noted. This value is a measure of the relative,

susceptibility of the specimens. Some specimens are magnetically

polarized and will deflect the system differently depending on the 

orientation of the specimen. The magnetic polarity can thus be measured by noting the difference between the maximum and the averagedeflection of the magnetic system.
MAGNETIC EXPLORATION FOR CHROMITE "9

In the absence of a magnetometer, very reliable information can 

;be obtained by swinging the hand specimen near the tip of the needle

-of a standard Brunton compass. Care should be taken to swing the 

specimen with the same frequency as the natural frequency of oscillation of the needle. The maximum deflection of the compass needle 

after a number of swings gives a useful measure of the magnetic properties of the specimen.

MAGNETIC FIELD INSTRUMENTS

The magnetic instrument used most commonly in chromite explora- 

"tion is the Schmidt-type vertical magnetometer. The operation and

-design of the magnetometer have been described by Joyce (1937);

-additional descriptive information can be found in most standard 

texts on geophysical prospecting. The operating sensitivity of the

-magnetometer is usually between 10 and 50 gammas per scale division. 

This sensitivity is usually considerably more than is required for the 

.average survey of ultramafic rocks, and field operation may be slowed

-down by frequent changes of the compensating magnet.

The magnetic dip needle, or mining compass (Stearn, 1929), has 

been used successfully in magnetic prospecting for chromite. The 

sensitivity of the dip needle in middle latitudes and at the optimum, 

setting of the counterweight is about 400 gammas per degree. This 

.sensitivity is entirely adequate to detect most of the significant anomalies encountered in chromite prospecting, where the magnetic relief is

-commonly about 1,000 gammas and may be as great as 20,000 gammas. 

The very much greater speed and ease of operation of the dip needle

-as compared with the magnetometer make it thoroughly satisfactory 

under most conditions, and a more widespread application of the 

instrument is highly recommended.

The airborne magnetometer lias been used with success in locating 

ultramafic masses (Hurley, 1949), but apparently has never been 

.applied to chromite prospecting. No record was found of the use of 

.any other magnetic instrument in chromite exploration.

RESULTS OF FIELD SURVEYS

Here follows a review of published reports on magnetic prospecting 

for chromite, together with the results of magnetic surveys by the 

present writer in some chromite-bearing areas of the western United 

States. Applications of magnetic methods to chromite prospecting 

may be considered promising where a direct magnetic indication of 

the chromite ore itself was found or where an indication of favorable 

.host rock was brought out by the magnetic data. Magnetic surveys are 

classified as unsuccessful where the chromite could not be detected 

^directly and where the additional geologic information provided by 

the magnetic data was of no value in prospecting.
10 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

Positive evidence that a magnetic feature can be correlated directly 

with chromite ore requires that comparative magnetic observations be 

made over barren country rock and over known deposits of ore. If 

the known ore has been largely mined out, the positive evidence may be 

lacking.

Ray* mine, John Day area, Oreg. The ore at the Ray. mine consists 

of a group of small lenses of chromite in dunite and peridotite country 

rock (Thayer, 1940, p. 102). Field tests of hand specimens with the

EXPLANATION

Chrome ore a, surface L

Chromite ore 

exposed underground 

165 150 

* . x Magnetometer observations in 

190 gammas above arbitrary datum

intervals up to 500 

gammas

XJ-»-i^-

Test pit or 

open-cut 

 ' 

Tunnel

125

Surveyed by H. E. Hawkes and T. P. Thayer 65

X

FIGURE 11. Magnetic map of the Ray chromite mine, John Day area, Grant County. Oreg.

magnetometer showed that the ore was moderately to highly magnetic, 

whereas the ultramafic country rock was uniformly nonmagnetic. A 

magnetometer survey of the area by the writer showed positive 

anomaly values ranging from 500 to 1,800 gammas over known or

probable ore. (See fig. 1.) The magnetic relief over the relatively

nonmagnetic country rock is about 200 gammas. No evidence of new

ore was found except for possible minor extensions of the known lenses 

in the immediate vicinity of the old workings.

; Red Lodge district, Carbon County, Mont. K magnetometer survey

was conducted by the U. S. Geological Survey in three chromiferoUS 

areas on Hellroaring Plateau and in one on Line Creek Plateau, near 

Red Lodge, Mont. (James, 1946) . Correlation of magnetic data with
MAGNETIC EXPLORATION FOR CHROMITE 11

known geology showed that very high anomalies were characteristic 

of known chromite ore, diabase, and magnetite-rich metamorphic 

rocks, whereas serpentine gave smaller anomalies and granite, quartzite, amphibolite, and porphyry gave still weaker magnetic intensity 

values. Although the method apparently was well suited to local 

magnetic and geologic conditions, no new chromite discoveries were 

made.

VerbliuaM Mountain, Ural region, U. S. S. R. Krasulin (1933) 

reports that a magnetic survey over a known large deposit revealed a 

well-defined anomaly of about 7,000 gammas above the magnetic background. Chromite bodies of less than 200 tons, however, gave no 

reaction.

Groenfontein No. 302 farm, Transvaal, South Africa. Weiss, Simpson, and Paver (1936) describe two 1,000-foot magnetic traverses 

across a series of thin layers of stratiform chromitite in pyroxenite of 

the Bushveld complex. Hand specimens of ore from these seams were 

slightly magnetic. A magnetometer survey showed that one of the 

three chromitite layers gave an anomaly of about 1,500 gammas on both 

traverse lines, whereas the other two layers gave no reaction whatever. 

The average magnetic relief of the pyroxenite country rock was about 

500 gammas. The authors conclude that the chromitite layers can be 

traced magnetically "only in areas where the magnetic intensity values 

over the country rock are normal or constant." Apparently this preliminary test was not followed up with systematic magnetic surveying, 

as no new discoveries were reported.

Oregon (chrorrvite-'bearing beach placers). E.L. Stephenson (1945) 

reports the results of magnetometer measurements over marine 

beach terraces of the Oregon coast near Bandon. The terrace deposits 

in places contain layers, up to 10 feet thick, of "black sands." These 

are concentrations of heavy minerals, mainly magnetite, ilmenite, 

chromite, garnet, olivine, zircon, and .pyroxene (Griggs, 1945). 

Although the principal economic constituent of the black sands is 

chromite, the deposits are magnetic by virtue of their magnetite content, which commonly ranges from 1 to 7 percent. Magnetometer surveys were run in the vicinity of eight separate black-sand occurrences. 

Stephenson concludes that "in general the anomalies associated with 

black sand are stronger and more uniform than other magnetic variations encountered in the immediate area. Exceptions are anomalies 

due to igneous or metamorphic rocks." The significant magnetic 

variations range from a few gammas, where the sand is at a depth of 

60 feet, up to 250 gammas for black sands within 2 or 3 feet of the 

surface. Magnetic indications in six localities pointed to the occurrence of previously unknown sand deposits; drilling in three of the 

areas resulted in the discovery of small new deposits or extensions of 

known deposits.
12 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

Magnetic exploration for placer deposits is an entirely different 

problem from exploration for primary chromite ore, and this example 

is included here only because the object of the search was chromite.

INDICATION OP FAVORABLE HOST BOCK

Serpentinized ultramafic rocks, because of their relatively high magnetite content, can almost always be distinguished from other common 

rocks by magnetic observations. In most areas, however, the general 

location and shape of the ultramafic masses are sufficiently well known 

from geologic observations, and additional magnetic evidence contributes little toward solving the problem of finding chromite. A few exceptions to this rule are noted, together with examples of apparent 

ore-bearing areas within the ultramafic rocks that can be distinguished 

magnetically.

Gasper Mountain, Wyo. E. L. Stephenson (1941; see also Stephenson, H. K, 1940; U. S. Geol. Survey, 1942; Horton, 1949) found a welldefined magnetic contrast between a chrome-bearing talc schist and a 

complex of less magnetic intrusive rocks. The published data (U. S. 

Geol. Survey, 1942) indicate that the magnetic pattern over the schist 

is characterized by greater magnetic relief, together with diagnostic 

contact effects, rather than by a uniformly positive magnetic anomaly. 

The contacts of the area of chromiferous schist inferred by the magnetic data were closely confirmed by later trenching work. Concentrations of chromite occur throughout the mass of the talc schist in a very 

irregular pattern. Although laboratory experiments on specimens of 

chromite showed that the chromite itself at this locality is commonly 

strongly magnetic (tables 1 and 2), the location and grade of deposits 

could not be directly correlated with the magnetometer data.

Chrome Point, western Newfoundland. Snelgrove (1934) presents 

a geologic map of the Chrome Point chromite area with magnetic dipneedle contours. Eight of the twelve chromite concentrations shown 

on the map are either enclosed by, or lie within 100 feet of, the 4°

dip-needle Contour. Susceptibility determinations on hand specimens

showed no significant magnetic contrast between ore and country rock. 

Shoal Pond area, eastern Newfoundland. Snelgroye (1934) reports 

that a dip-needle survey over the area of the chromite deposits at 

Chrome Hill near Shoal Pond gave a negative anomaly of 15° to 

22°. The 10° dip-needle contour encloses an area up to 300 feet wide

and at least 1,800 feet long that roughly coincides with the center of 

a belt of serpentinized dunite 500 feet wide contained in a large pyroxenite mass. Snelgrove's map shows that 12 of the 19 chromite concentrations of the area lie inside the <10° contour; the remainder lie 

within 100 feet of this contour. No explanation for the negative character of the anomaly is offered. Subsequent laboratory tests showed 

that a specimen of chromite from this deposit had a susceptibility of
MAGNETIC EXPLORATION FOR CHROMITE ...' ' 13

15,500 XIO'6 c. g. s. units, as contrasted with 590 XlO'6 for a specimen 

of pyroxenite from the same ultramafic mass. Although these susceptibility tests showed the ore to be highly magnetic, the field survey 

served only to outline the general area where chromite concentrations 

occur.

U. /S. S. R. fDnkov (1938) describes the results of four seasons 

of experimental magnetometer surveys at the Poltavsk, Khalilovsk, 

Shaidurovsk, Alapaevsk, Verkh-Neivinsk, and Akkarginsk chromite' 

deposits in the Ural region. He concludes that "(1) micromagnetic 

surveys make it possible to determine rapidly and cheaply the location 

of the contacts of ultramafic masses, and to distinguish these masses 

on the basis of their lithologic types (gabbro, peridotite, and dunite). 

It is also possible to differentiate some vein rocks; (2) ore-bearing 

zones cannot always be distinguished, as this depends on the geologic 

setting of the deposit; (3) at none of the deposits examined was it 

possible to distinguish the ore itself by micromagnetic observations."

Krasulin (1933) reports that, at the Zapivalovsk and Nadezhdinsk 

deposits in the Urals, the chromite occurs in zones of relatively less 

magnetic rocks that can be distinguished by magnetometer surveys.

Manitoba, Canada (Bird River- complex). The stratiform chromite 

ore of the Bird River gabbro sill occurs at a fairly constant stratigraphic position in the complex. This igneous complex is faulted and 

largely covered by a thick overburden, so that it is not always possible 

to trace the chromite layer by surface exposures. Bateman (1943) reports that exploration for chromite was "facilitated by a magnetometer survey that established the position of the gabbro-peridotite 

contact and hence of the chromite zone." He does not report any discoveries as a result of this technique.

UNSUCCESSFUL APPLICATION OF MAGNETIC METHODS

Castro mine, /San Lws Obispo County, Calif. Ore at the Castro 

mine (Smith and Griggs, 1944) occurs as grains of disseminated chromite in a serpentine matrix. The deposits are well-defined, almost 

horizontal, lenticular masses. Hand specimens of chromite were generally somewhat more magnetic than the ultramafic country rock. 

Two magnetometer traverses were run by the present author on the 

hill slope directly overlying the lower ore body, which has the form of 

a flat lens averaging 12 feet in thickness, most of it in place at the time 

of the visit. Where the magnetic observations were made, the top 

surface of the ore was 20 to 25 feet below the surface of the ground. 

The average of 14 magnetic observations directly over the known ore 

was 50 gammas lower than the average of 17 observations over barren 

ground in the immediate vicinity; the total magnetic relief was 200 

gammas. 
14 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

Grey Eagle mine, G-lenn County, Calif. Ore at the Grey Eagle 

mine (Rynearson and Wells, 1944) occurs as a lens of disseminated 

chromite, 50 feet in maximum thickness, enclosed in a sheath of serpentinized dunite up to 100 feet thick and 750 feet in total exposed length. 

The country rock of the chromite-dunite complex is peridotite. Minor 

faulting has broken the ore complex into a complicated series of fault 

blocks. Magnetometer field tests of hand specimens by the present 

author showed that the peridotite and the chromite ore are weakly to 

moderately magnetic, whereas the serpentinized dunite is relatively 

nonmagnetic. Generally low magnetic values were observed over and 

adjoining the ore, possibly as the result of the nonmagnetic character 

of the dunite as contrasted with the peridotite. This relationship was 

not sufficiently consistent, however, to be of value in prospecting.

Little Castle Creek valley, Shasta County, Calif. In the spring of 

1941 the author conducted a magnetometer survey of the area southwest of the Castle Crags mine on the south side of the Little Castle 

Creek valley. Chromite ore in the Little Castle Creek area occurs 

as lenses or irregular masses of disseminated to massive ore, scattered 

over a linear distance of almost a mile in a general southwesterly direction. The deposit at the Castle Crags mine, which is the southernmost deposit of the group, was a 15,000-ton lens of massive chromite 

that was almost entirely mined out in 1917. Thus it was not possible 

to make trial magnetic observations over known ore.

Tests of hand specimens showed that massive ore from the main 

deposit is relatively magnetic as compared with most specimens of 

country rock. Thin-section study showed that the grains of chromite 

consisted commonly of opaque, magnetic halos surrounding cores of

translucent nonmagnetic chromite.

The survey revealed a fairly complex pattern of anomalies, with a 

total relief of about 5,000 gammas. -A prominent feature of the area

is a zone of positive magnetic intensity extending for at least 800 feet 

southwest of the Castle Crags mine and almost coincident with the continuation of the line along which the known deposits of the area are

clistribixtecL On the chance that this anomaly might be the indication

of a chromite-bearing zone, an extensive program of surface trenching

 was recommended and carried out* Except for a few tons of disseminated chromite uncovered 50 feet to one side of the magnetic zone, 

the results of the surface work were entirely negative (Matson, 1949). 

No work has been done to check the possibility of a chromite deposit at 

depth.

Cyclone Gap mine, Siskiyou\ C&imty, Calif. Ore at the Cyclone 

Gap mine occurred as a cluster of six closely spaced pods in serpentinized peridotite and dunite. These pods yielded a total *of 2,000 

tons of chromite. Many short gash dikes of diorite cut the ultramafic 

rocks; in one place a dike was seen cutting a chromite pod (F. G.
MAGNETIC EXPLORATION FOR CHROMITE 15

Wells, personal communication). Magnetometer tests of hand specimens showed that the country rock is generally highly magnetic, 

whereas the chromite ore is nonmagnetic. It is possible that some 

magnetite may have been developed by metamorphism along the diorite dikes. In 1941, when the author examined the deposit, the ore 

was almost mined out, so that no magnetic readings could be made 

directly over known ore. A magnetometer traverse passing within a 

few feet of the end of one of the deposits gave a highly irregular curve 

with a magnetic relief of over 4,000 gammas, but it showed no apparent 

relation to the position of the ore.

Sourdough mine, Curry County, Oreg. In the Sourdough area, 

massive chromite occurs as layers from a few inches to several feet 

thick in a matrix of dunite, which in turn is contained in peridotite 

(Wells, Page, and James, 1940). The degree of serpentinization of 

both dunite and peridotite ranges from slight to intense. According 

to Lee (1938), "the dunite and chromite were not magnetic; on the 

other hand the peridotite was very highly magnetic and much of it 

was magnetically polarized." Lee found "similar magnetic anomalies" over the zone of mineralization on three profile traverses, but 

was unable to distinguish individual lenses of ore. Additional magnetometer observations by the present author showed that these 

anomalies consisted of a zone 50 to 100 feet wide of relatively high 

magnetic relief. Similar areas of high relief were observed over 

barren peridotite country rock, so that their value as a guide to 

chromite ore is questionable.

John Day area, Oreg. The southwest ore body at the Chambers 

mine (Thayer, 1940, pp. 96-98) is a somewhat irregular lens of 

medium-grade to massive chromite in a matrix of serpentinized dunite. 

The dunite in turn is enclosed in serpentinized peridotite, which comprises the bulk of the ultramafic mass. Magnetometer measurements 

over this deposit were made by the present author in 1941, at which 

time virtually none of the ore had been removed. A magnetic map 

of the area of the southwest ore body (fig. 2) showed a complex distribution of anomalies, but no apparent correlation between the magnetic pattern and the location of the ore body. A single magnetic 

traverse across the center of the northwest ore body, which also has 

been drilled but not mined, likewise showed no magnetic indication 

of the ore.

The author ran magnetometer traverses across known ore at the 

Iron King mine, the Marks and Thompson mine, and the upper and 

lower deposits at the Dry Camp mine. Specimens of slightly or 

moderately magnetic ore were found at all these deposits, although in 

each area specimens of serpentinized peridotite and dunite from the 

immediately adjacent country rock were as magnetic as the ore speci-
CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

SECTION THROUGH DIAMONDDRILL HOLES 2 AND 4

EXPLANATION

Chromite ore

480

x

Magnetometer observations 

in gammas above arbitrary 

datum

_ 

"^- 300 

Lines of equal magnetic

intensity at 100-gamma 235 

intervals up to 500 gammas

P-°-"-»-3Q

Diamond-drill hole projected 

to surface, showing inclination

Geology and drill data by 

T. P. Thayer

FIGURE 2. Magnetic map of the southwest ore body, Chambers chromite 

Grant County, Oreg.

mine,

mens or more so. The data failed to show any systematic correlation 

between magnetic anomalies and the location of known ore. Even 

where small anomalies occurred over ore, still higher values were 

usually found over barren country rock in the immediate vicinity. 

At the Iron King mine, for instance, the average of eight readings over 

known ore was only 270 gammas higher than the average of 36 readings over barren country rock, whereas the total magnetic relief was 

more than 3,200 gammas. Similarly, at the Dry Camp mine, the 

average of 13 readings over known ore was 40 gammas higher .than 

the average of 25 readings over barren country rock, where the total 

relief was 560 gammas. It seems safe to conclude, therefore, that 

with the exception of the Bay mine, mentioned previously, the chromite deposits of the John Day area do not lend themselves readily 

to magnetic exploration.

Wood mine^ La/ncaster Comity, Pa. Ore at the Wood mine (McIntosh. and Mosier, 1948) occurs as nocUiles and pockets of massive,

compact chromite in serpentinized ultramafic rocks. A magnetometer 

and gravimeter survey by the Geological Survey over an area measuring 6,000 by 2,000 feet indicated several anomalies. These were subsequently diamond-drilled, with negative results.

Thetford district, Quebec. Miller (1932) reports that magnetometer surveys at Caribou Lake near Thetford, Quebec, showed high 

( anomalies in the vicinity of chrome ore. The magnetic properties of
MAGNETIC EXPLORATION FOR CHROMITE 17

two dozen hand specimens were tested with a pocket magnet, and 

"in every case the effect produced by the country rock was larger 

than that by the chromite." Miller expressed considerable doubt, 

therefore, as to whether the anomalies can be related directly to the 

chromite. Experimental surveys were run at the Beaver mine and at 

Black Lake with similar negative results.

U. S. jS. R. Bagratuni (1933), in a description of the Gei-Dara 

chromite deposits in Kurdistan, reports that hand specimens of gabbro, 

peridotite, and dunite caused deflections of the magnetic balance of 

one scale division or less when brought close to the magnetometer 

case, whereas specimens of chromite caused a deflection of several 

scale divisions. On this evidence a magnetometer survey in the GeiDara area was recommended, and several anomalies were found and 

interpreted as the effect of buried chromite deposits. Bagratuni did 

not report on the success of the project, as the anomalies were being 

drilled at the time the report was written. In a later report Andreev 

(1937) cites susceptibility data for samples from Gei-Dara, expressed 

as c. g. s. units times 106, as follows: chromite, 270 (average of 15 

determinations); serpentinized dunite, 1,530 (average of 11 determinations) ; serpentinized peridotite, 2,200 (average of 17 determinations). Inasmuch as Andreev makes no mention of any discoveries at Gei-Dara as a result of the earlier work, it might safely 

be presumed that none were made.

Speaking in general, Andreev (1937) states that "trial magnetic 

surveys over many magnetite deposits of the Urals and Transcaucasia, to test the possible application of the method for direct detec- 

.tion of chromite, have led to the definite conclusion that magnetic 

prospecting methods cannot be used for this purpose. Observations 

taken directly over known chromite outcrops show that the majority 

of ore bodies give no magnetic indication. In some other cases, ore 

bodies give both positive and weakly negative anomalies, where the 

character and sign of the anomaly is not consistent even within the 

limits of the same mining.district. This variability may be explained 

by the extremely variable magnetic susceptibility of chromite and its 

country rock together with the factor of permanent magnetization."

Montana (Stillwater complex}. The stratiform ore of the Stillwater igneous complex generally lies near the center of the ultramafic 

zone in the lower part of the complex (Peoples and Howland, 1940). 

In the West Benbow area, Mont., the ore occurs as a single seam of 

chromitite about 10,000 feet in strike length and commonly ranging 

from 2 to 6 feet in width. Experimental magnetic and resistivity 

surveys over the western end of this area were conducted in 1941 by 

Hans Lundberg, consulting geophysicist. According to Wimmler 

(1948), "the results of these surveys showed that the chr.omite bands
18 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

gave no specific indications by either method. Faults could, however, be recorded where serpentinization had occurred along theni, 

but that information was of little value."- The magnetic relief in the 

West Benbow area is about 20,000 gammas, and from a study of hand 

specimens it appears to be entirely the effect of secondary magnetite 

developed along serpentinized faults and fissures.

SUMMARY ;

From a review of the literature and from field investigations by 

the author it is apparent that magnetic methods of exploration for 

primary chromite can be successfully applied only under very special 

geologic conditions. The magnetic indication may take the form of a 

direct magnetic response from the chromite ore itself or of an indication of favorable host rock which may serve as an indirect guide in 

the search for ore.

For the direct detection of chromite it is necessary that the chromite 

be characteristically magnetic and that the country rock be uniformly 

nonmagnetic. ' These conditions presuppose a rare combination of 

circumstances that can be expected only in exceptional cases.

Ultramafic rocks can usually be distinguished from other common 

rock types by magnetic methods. Thus, if a knowledge of the location, 

contacts, or shape of the ultramafic host rocks is helpful in prospecting, 

magnetic methods can be of indirect help in chromite exploration. 

In some areas ore-bearing zones within the ultramafic mass can apparently be located and mapped by magnetic methods, providing, the 

prospector with another clue.

In the majority of areas described in the literature or examined by 

the author, magnetic methods showed no promise as a means of locating chromite bodies. Even in the areas where "successful" results 

were reported, the surveys apparently were successful only in that 

the method showed promise. The author knows of no discovery of 

a commercial deposit of primary chromite resulting either directly 

or indirectly from magnetic-survey .work.

Andreev, B. A., 1937, Application of geophysical methods for prospecting chromite deposits [in Russian]; Central Geol, and Prosy. Irist. Trans. lOOj 

34pp:

Bagfatuni, E., 1933, Chromite deposits in Gei-Dara, Kurdistan [in Russian]: 

Razvedka Nedr 19, pp. 19-21.

Bateman, J. D., 1943, Bird Kiver chromite deposits, Manitoba: Canadian 

Inst. Min. Metallurgy Trans., yol; 46, pp. 154-183. . . . Dresser,. J.. A.,, 1913, Preliminary .report on the serpentine and associated rocks 

of southern Quebec: Canada Geol. Survey Mem. 22.

Griggs, A. B., 1945, Chromite-bearing sands of the southern part of the coast 

of Oregon: U. S. Geol. Survey Bull. 945-E, pp. 113-150.
MAGNETIC EXPLORATION FOR CHROMITE . 19

Haalck, H., 1934, Lehrbuch der angewandten Geopnysik, Berlin, Gebriider

.Borntraeger. . . . Heiland, C. A., 1940, Geophysical exploration, New York, Prentice-Hall, Inc. 

Hitchen, C. S.,.1929, Unst and its chromite deposits: Mining Mag. .[London], vol.

40, pp. 18-24. . Horton, F. W., and Allsman, P. T., 1949, Investigation of Casper Mountain chro- 

, . mite deposits, Natrona County, Wyo.: U. S. Bur. Mines Kept. Inv. 4512,

26 pp. 

Hurley, P. M., 1949, Airborne magnetic survey in Maine: Eng. and Min. Jour.,

vol. 150, no. 8, pp. 52-55. , . . 

fDnkov, A. A., 1938; Geophysical methods of prospecting for chromite deposits

[in Russian]: Ural Sci. Research Inst. Geology, Prosp., and Econ. Mineralogy Trans., Geology and Geophysics, no. 1, pp. 127-131, Sverdlovsk. 

James, H. L., 1946, Chromite deposits near Red Lodge, Carbon County, Mont.:

U. S. Geol. Survey Bull. 945-F, pp. 151-189. 

Joyce, J. W., 1937, Manual on geophysical prospecting with the magnetometer,

Washington, Hobart Publishing Co. (reprinted 1948). . 

Krasulin, , 1933, Some results of geophysical work in the Urals in 1932

[in Russian]: Razvedka Nedr 5-6, pp. 18-21. ... 

Lee, F. W., 1938, Preliminary geophysical investigation on the Sourdough chromite deposit in Oregon, in Chromite deposits in Oregon: Oregon State

Dept. Geol. Min. Ind. Bull. 9, pp. 8-10. 

Mclntosh, F. K., and Mosier, McHenry, 1948, Investigation of Wood chromite

mine area, Lancaster County, Pa.: U. S. Bur. Mines Rept. Inv. 4383, 5 pp. 

Matson, E. J., 1949, Investigation of Little Castle Creek chromite deposit,

Shasta County, Calif.: U. S. Bur. Mines Rept. Inv. 4516, 6 pp. 

Maxwell, J. C., 1949, Some chromite occurrences in New Caledonia: Econ.

Geology, vol. 44, pp. 525-544. 

Miller, A. H., 1932, Studies of geophysical methods, 1930: Part III, Gravitational

and magnetometric investigations: Canada Geol. Survey Mem. 170, pp.

99-118. 

Peoples, J. W., and Howland, A. L., 1940, Chromite deposits of the eastern part

of the Stillwater complex, Stillwater County, Mont.: U. S. Geol. Survey

Bull. 922-N, pp. 371-416. 

Reich, H., 1930, Erfahrungen mit geophysikalischen Methoden in Sudafrika:

Metall u. Erz, vol. 27, pp. 287-291. 

Rynearson, G. A., and Wells, F. G., 1944, Geology of the Grey Eagle and some

nearby chromite deposits in Glenn County, Calif.: U. S. Geol. Survey Bull.

945-A, pp. 1-22. 

Singewald, J. T., Jr., 1919, Maryland sand chrome ore: Econ. Geology, vol. 14,

pp. 189-199. 

Slichter, L. B., 1929, Certain aspects of magnetic surveying: Am. Inst. Min. Met.

Eng. Trans., Geophysical Prospecting, pp. 238-260. 

Smith, C. T., and Griggs, A. B., 1944, Chromite deposits near San Luis Obispo, San

Luis Obispo County, Calif.: U. S. Geol. Survey Bull. 945-B, pp. 23-44. 

Snelgrove, A. K., 1934, Chromite deposits of Newfoundland: Newfoundland

Dept. Nat. Resources Bull. 1, 26 pp. 

Steam, N. H., 1929, The dip needle as a geologic instrument: Am. Inst. Min. Met.

Eng. Trans., Geophysical Prospecting, pp. 345-363. 

Stephenson, E. L., 1941, Geophysical and geological investigations of the Casper

Mountain chromite deposit, Wyo.: Washington Acad. Sci. Jour., vol. 31,

p. 170. 

 , 1945, Magnetometer surveys on black sands of the Oregon coast: U. S.

Bur. Mines Rept. Inv. 3814,18 pp.
20 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1951

Stephenson, H. K., 1940, Contributions to the mineralogy of chromite based on

the chromite deposit of Casper Mountain, Wyo. (Princeton Univ. Ph.D.

dissertation). . . . . . Stevens, B. E., 1944, Composition of some chromites of the Western Hemisphere:

Am. Mineralogist, vol. 29, pp. 1-34. 

Stutzer, F., Gross, W., and Boraemann, K., 1918, Uber magnetischen Eigenschaften der Zinkblende und einiger anderer Mineralien: Metall u. Erz, vol. 15,

no. 1, pp. 1-9. 

Thayer, T. P., 1940, Chromite deposits of Grant County, Oreg.: U. S. Geol. Survey

Bull. 922-D, pp. 75-113. 

U. S. Geological Survey, 1942, Chromite deposit of Casper Mountain, Natrona

County, Wyo., 2 pp. (press release dated Sept. 14,1942). 

Weiss, Oscar, Simpson, D. J., and Paver, G. L., 1936, Some magnetometric and

gravimetric surveys in the Transvaal: South Africa Geol. Survey Bull.

7, p. 27.

Wells, F. G., Page, L. E., and James, H. L., 1940, Chromite deposits in the Sourdough area, Curry County, and the Briggs Creek area, Josephine County,

Oreg.: U. S. Geol. Survey Bull. 933-P, pp. 461-496.

Wimmler, N. L., 1948, Investigation of chromite deposits of the Stillwater complex, Stillwater and Sweetgrass Counties, Mont.: U. S. Bur. Mines Rept. 

Inv. 4368, 41 pp.
INDEX

Page 

Bird River complex______...... .... 13

Brunton compass, use of._....... 9

California, magnetic exploration in....... 13-15

Casper Mountain, Wyo____.._ . 6,12 

Castro mine, Calif...' ...................... 13

Chambers mine, Oreg___...... . 15

Chemical composition of chromite_ 3-4 

relation of magnetic properties to.... 5-7

Chrome Point, Newfoundland____ ... 12 

Color of chromite._........ 4

Cyclone Gap mine, Calif______.__... 14-15

Dip needle, use of....__. ...... 9

Dry Camp mine, Oreg............. ...... 15-16

Field methods................................ 8-9

Qel-Dara deposits, Kurdistan...__ 17

Geology'of'depositSr:. . 2-3

Grey Eagle mine, Calif....._..__. .. 14

Groenfontein No. 302 farm, Transvaal.. 11

Instruments used___________ .. 9 

Interpretation of data 7-9 

Iron content of chromite, relation to magnetic

susceptibility..... . 6-7 

Iron King mine, Oreg--- 15-16

John Day area, Oreg . .. 10,15-16 

Kurdistan, magnetic exploration in.. 17 

Little Castle Creek valley, Calif........ . 14

Magnetic properties, of chromite..-. 4-6 

of common minerals.. __ 7-8 

Magnetite, chromite compared with.. 1,3-4 

Magnetometer, use of. 8-9 

Manitoba, magnetic exploration in. 13 

Marks and Thompson mine, Oreg... 15-16 

Mineralogy of chromite_.... 3-7 

Montana, magnetic exploration in. . 10-11,17-18

Page 

Newfoundland, magnetic exploration in...... 12-13

Occurrence of ore_____. ___._____ 2-3 

Opacity in chromite__________. __ 6^7 

Oregon, magnetic exploration in__ 10,11-12,15-16

Pennsylvania, magnetic exploration in... 16 

Placer deposits......... . . ........ 3,11-12

Purpose of study__.. 1

Quebec, magnetic exploration In_____.... 16-17

Ray mine, Oreg_____ 10

Red Lodge district, Mont______.... .. 10-11

References cited._...._...._...._._ 18-20

Results of surveys______._... _... 9-18

successful indication of host rock_____ 12-13

successful indication of ore_ . . 10-12

unsuccessful application of methods 13-18

Sackform 'deposits ..v^. 2

Serpentinization.. 3

Shoal Pond area, Newfoundland 12-13

Sourdough mine, Oreg.._._._ . 15

Stillwater complex___._____ .... 17-18

Stratiform deposits____._____ .... 2-3

Streak.... .... . 4

Susceptibility, of chromite. 4-6

of common minerals..._____ ... 7-8

Thetford district, Quebec. 16-17 

Transcaucasia, magnetic exploration in....... 17

Transvaal, magnetic exploration in_____ 11 

Types of deposit. ... . 2-3

Ural region, magnetic exploration in 11,13,17 

Verbliuzhl Mountain____... . .... 11

West Benbow area, Mont 17-18 

Wood mine, Pa_______.__.... 16 

Wyoming, magnetic exploration in 12

21

O