U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2016–5089–D

Prepared in cooperation with the Bureau of Land Management

Geology and Mineral Resources of the North-Central Montana 

Sagebrush Focal Area

Chapter D of

Mineral Resources of the Sagebrush Focal Areas of Idaho, Montana, Nevada, 

Oregon, Utah, and Wyoming
Cover. Photograph of a low, dark-colored bench, the outcrop expression of a bentonite bed, in north-central Montana 

(U.S. Geological Survey photograph by Michael L. Zientek).
Geology and Mineral Resources of the 

North-Central Montana Sagebrush 

Focal Area

By Jeffrey L. Mauk, Michael L. Zientek, B. Carter Hearn, Jr., Heather L. Parks, 

M. Christopher Jenkins, Eric D. Anderson, Mary Ellen Benson, Donald I. Bleiwas, 

Jacob DeAngelo, Paul D. Denning, Connie L. Dicken, Ronald M. Drake II, 

Gregory L. Fernette, Helen W. Folger, Stuart A. Giles, Jonathan M.G. Glen, 

Matthew Granitto, Jon E. Haacke, John D. Horton, Karen D. Kelley, Joyce A. 

Ober, Barnaby W. Rockwell, Carma A. San Juan, Elizabeth S. Sangine, Peter N. 

Schweitzer, Brian N. Shaffer, Steven M. Smith, Colin F. Williams, and 

Douglas B. Yager

Chapter D of

Mineral Resources of the Sagebrush Focal Areas of Idaho, Montana, 

Nevada, Oregon, Utah, and Wyoming

Edited by Warren C. Day, Thomas P. Frost, Jane M. Hammarstrom, and Michael L. Zientek

Prepared in cooperation with the Bureau of Land Management

Scientific Investigations Report 2016–5089–D

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Mauk, J.L., Zientek, M.L., Hearn, B.C., Jr., Parks, H.L., Jenkins, M.C., Anderson, E.D., Benson, M.E., Bleiwas, D.I., 

DeAngelo, J., Denning, P.D., Dicken, C.L., Drake, R.M., II, Fernette, G.L., Folger, H.W., Giles, S.A., Glen, J.M.G., 

Granitto, M., Haacke, J.E., Horton, J.D., Kelley, K.D., Ober, J.A., Rockwell, B.W., San Juan, C.A., Sangine, E.S., 

Schweitzer, P.N., Shaffer, B.N., Smith, S.M., Williams, C.F., and Yager, D.B., 2016, Geology and mineral 

resources of the North-Central Montana Sagebrush Focal Area: U.S. Geological Survey Scientific Investigations 

Report 2016–5089–D, 104 p., http://dx.doi.org/10.3133/sir20165089D.

ISSN 2328-0328 (online)
iii

Acknowledgments

We thank Scott Murrellwright and Adam Merrill of the Bureau of Land Management (BLM) 

for helpful comments that improved the Introduction of this report. We thank Dick Berg of the 

Montana Bureau of Mines and Geology (retired) for valuable discussions on industrial minerals 

in Montana and the study area, and for providing information on the kaolinite in the Whitemud 

Formation in Saskatchewan. Jay Gunderson of the Montana Bureau of Mines and Geology, provided assistance in finding and utilizing the Oil and Gas field outlines from the Montana Board 

of Oil and Gas Conservation. We acknowledge Gregory Fesko of the Montana State Office of the 

BLM for confirming the lack of potential coal development in the study area.

We thank Eastern Washington University student interns Derek De Haas, Alyssa Doloughan, 

Cassandra Hennings, and also Duc Nguyen, who is a U.S. Geological Survey volunteer, for their 

help with compiling data, GIS work, and figure creation.

We thank personnel from the Montana Bureau of Mines and Geology, the BLM in Montana, and 

the Montana Department of Environmental Quality for their assistance in providing geological information, and for their helpful feedback on the preliminary tracts that we developed for 

the study area. From the Montana Bureau of Mines and Geology, we particularly thank John 

Metesh, Thomas Patton, Jay Gunderson, Peggy Delaney, Phyllis Hargrave, and Stan Korzeb. 

From the BLM in Montana, we particularly thank Dave Williams, Craig Towery, Joan Gabelman, 

and Nate Arave. From the Montana Department of Environmental Quality, we particularly thank 

Warren McCullough, Robert Cronholm, Herb Rolfes, Wayne Jepson, Ed Coleman, Chris Yde, and 

Chris Cronin.

We thank David Crouse of Imerys for providing a figure illustrating the stratigraphy of the 

bentonite beds on their property. Marilyn Wegweiser and Brent Bestram of the BLM National 

Minerals Testing Laboratory in Worland, Wyoming, provided valuable discussions on bentonite 

geology and testing. Gregg Menge of Lewistown, Montana, provided a site visit of the bentonite 

property near Glasgow for Mike Zientek and Craig Towery.

We thank Regan Austin, Chris Holm-Denoma, and Mark Gettings for careful reviews that helped 

improve this manuscript.
iv

Contents

Acknowledgments........................................................................................................................................iii

Executive Summary.......................................................................................................................................1

Mining and Mineral Activity in the Area...........................................................................................1

Leasable ........................................................................................................................................1

Locatable.......................................................................................................................................2

Salable...........................................................................................................................................2

Introduction.....................................................................................................................................................2

Lands Involved.......................................................................................................................................3

Organization of this Report and Terminology ...................................................................................3

Who Did the Work?........................................................................................................................................6

Description of Geology..................................................................................................................................7

Physiography and Ecoregions............................................................................................................7

Regional Geology and Tectonic Setting............................................................................................9

Precambrian Basement Terranes .............................................................................................9

Structure........................................................................................................................................9

Latest Cretaceous to Paleocene Intrusive Centers .............................................................10

Middle Eocene Intrusive and Eruptive Centers ....................................................................11

Missouri Breaks Diatremes .....................................................................................................11

Stratigraphy .........................................................................................................................................11

Pleistocene Glacial Deposits...................................................................................................11

Paleocene Sedimentary Rocks ...............................................................................................11

Cretaceous Sedimentary Rocks..............................................................................................16

Jurassic Sedimentary Rocks ...................................................................................................18

Mississippian Sedimentary Rocks..........................................................................................18

Devonian Sedimentary Rocks..................................................................................................18

Ordovician Sedimentary Rocks ...............................................................................................18

Cambrian Sedimentary Rocks .................................................................................................19

Precambrian Sedimentary Rocks ...........................................................................................19

Leasable Minerals .......................................................................................................................................19

Potash...................................................................................................................................................19

Coal........................................................................................................................................................19

Geothermal...........................................................................................................................................22

Geology and Occurrence .........................................................................................................22

Exploration and Development..................................................................................................22

Results of Previous Assessments...........................................................................................22

Oil and Gas...........................................................................................................................................24

Phosphate ............................................................................................................................................26

Locatable Minerals......................................................................................................................................27

Mineral Systems and Mineral Deposit Types ................................................................................27

Mineral-Resources Potential............................................................................................................27

Known Locatable Mineral Deposits ................................................................................................27

Historic Production....................................................................................................................32
v

Mining Claims and Permits ......................................................................................................32

Metallic Locatable Minerals.............................................................................................................36

Hydrothermal-Volcanic Rock Associated System ........................................................................36

Geology and Occurrence in the Study Area .........................................................................36

Potential for Occurrence..........................................................................................................36

Epithermal Deposits: Tract MTEP01 (Epithermal)........................................................38

Economic Analysis of the Deposit Types ...............................................................................38

Other Hydrothermal Mineral Deposits ............................................................................................38

Surficial-Mechanical (Placer) Mineral System.............................................................................39

Mineral Description...................................................................................................................39

Geology and Occurrence in the Study Area .........................................................................39

Potential for Occurrence..........................................................................................................39

Placer Deposits: Tract MTPL01 (Placer).......................................................................39

Economic Analysis of the Deposit Types ...............................................................................39

Nonmetallic Locatable Minerals......................................................................................................42

Sedimentary System (Formed During or After the Conversion from Sediment to 

Sedimentary Rock): Bentonite Deposit Type ....................................................................42

Mineral Description...................................................................................................................42

Geology and Occurrence in the Study Area .........................................................................42

Geology of the Bentonite Beds in the Malta and Glasgow Areas............................45

Exploration and Mining Activity ..............................................................................................53

Potential for Occurrence..........................................................................................................56

Geostatistical Modeling of the X and Y Beds in the Bearpaw Shale, Malta and 

Glasgow areas: Tracts MTB06 to MTB09........................................................57

Occurrence of Bentonite Outside the Malta and Glasgow Areas............................57

Stratigraphic and Facies Modeling to Map the Bentonitic Interval in the 

Bearpaw Shale: Tracts MTB01 to MTB03..............................................57

Bentonite in the Fox Hills and Hell Creek Formations: Tracts MTB04 and 

MTB05 ..........................................................................................................62

Mineral Potential of Bentonitic Units Outside the Malta and Glasgow Areas.......62

Economic Analysis of the Deposit Type.................................................................................63

Magmatic System: Diatreme-Hosted Diamond Deposit Type.....................................................68

Mineral Description...................................................................................................................69

Geology and Occurrence in the Study Area .........................................................................69

Missouri Breaks Diatremes ............................................................................................72

Grassrange Intrusions .....................................................................................................72

Exploration and Mining Activity ..............................................................................................72

Williams Kimberlites.........................................................................................................75

Homestead Kimberlite......................................................................................................76

Other Areas in the Missouri River Breaks Field ..........................................................76

Undiscovered Diatremes.................................................................................................76

Potential for Occurrence..........................................................................................................81

Diamond Deposits Tracts: MTD01 (Diamond 1), MTD02 (Diamond 2), and MTD03 

(Diamond 3) ...................................................................................................................81

Economic Analysis of the Deposit Types ...............................................................................81

Salable Minerals..........................................................................................................................................83

Sand and Gravel..................................................................................................................................83
vi

Clay Minerals.......................................................................................................................................83

Gemstones: Petrified Wood ..............................................................................................................84

Mineral Economics......................................................................................................................................84

Strategic and Critical Mineral Materials ...............................................................................84

References Cited..........................................................................................................................................88

Appendix 1. Mineral Potential Classification System..........................................................................100

Level of Potential...............................................................................................................................100

Level of Certainty ..............................................................................................................................100

Reference Cited.................................................................................................................................100

Appendix 2. Mineral-Potential Assessment Tracts for Locatable Minerals in the North-Central 

Montana Sagebrush Focal Area................................................................................................102

Reference Cited.................................................................................................................................102

Appendix 3. Oil and Gas Plays and Assessment Units in the North-Central Montana Sagebrush 

Focal Area......................................................................................................................................103

Appendix 4. Parameters for the Geostatistical Model of the Lower Contact of the Upper Y 

Bentonite Bed in the Malta and Glasgow Areas, Montana ..................................................104

Figures

1. Surface land management map of the North-Central Montana Sagebrush 

Focal Area......................................................................................................................................4

2. Map showing the physiographic ecoregions of the North-Central Montana 

Sagebrush Focal Area .................................................................................................................8

3. Maps showing tectonic and basement terrane setting and geologic setting of 

the North-Central Montana Sagebrush Focal Area..............................................................12

4. Map showing geology of the North-Central Montana Sagebrush Focal Area ................14

5. Lithostratigraphic column for the North-Central Montana Sagebrush Focal Area, 

showing stratigraphy exposed in Continental Oil Company’s South Zortman 

well 1, Phillips County, Montana ..............................................................................................17

6. Map showing the approximate extent of the Williston Basin and evaporite 

and carbonate rocks of the Elk Point Basin ...........................................................................20

7. Map of the North-Central Montana Sagebrush Focal Area showing areas 

with coal-bearing formations ...................................................................................................21

8. Map showing the location of thermal springs and wells, overlain on the 

logistic regression results for geothermal favorability for the North-Central 

Montana Sagebrush Focal Area ..............................................................................................23

9. Map showing the locations of low-temperature thermal springs and wells 

overlain on the estimated temperature at 6 km depth, for the North-Central 

Montana Sagebrush Focal Area ..............................................................................................25

10. Map showing State oil and gas provinces and exploration and well type for 

the North-Central Montana Sagebrush Focal Area..............................................................28

11. Maps showing active and closed mine claims for locatable commodities in 

the North-Central Montana Sagebrush Focal Area..............................................................30

12. Map showing 43 CFR 3809 notices and plans of operations for locatable 

commodities in the North-Central Montana Sagebrush Focal Area .................................33

13. Maps showing bentonite claims and deposits for locatable commodities 

in the North-Central Montana Sagebrush Focal Area .........................................................34

14. Mineral potential map for epithermal deposits in the North-Central Montana

 Sagebrush Focal Area ..............................................................................................................37
vii

15. Mineral potential map for placer deposits in the North-Central Montana 

Sagebrush Focal Area ...............................................................................................................40

16. Map showing geology of the Little Rocky Mountains and the epithermal 

and placer tracts in the northwest part of the North-Central Montana 

Sagebrush Focal Area ..............................................................................................................41

17. Map showing extent of Cretaceous sedimentary rocks that formed in the 

Western Interior Seaway with locations of mining districts and bentonite 

deposits, north-central United States and south-central Canada .....................................43

18. Stratigraphic column showing lithology with bentonite layers in the Hardin 

District, Montana ........................................................................................................................44

19. Schematic cross section showing the geology of the Montana Group from 

northwest to southeast Montana.............................................................................................46

20. Stratigraphic columns showing bentonitic intervals in the Bearpaw Shale, 

Montana and Wyoming. ............................................................................................................47

21. Maps showing the location of bentonite occurrences in the Bearpaw Shale 

in the North-Central Montana Sagebrush Focal Area .........................................................48

22. Imagery, bentonite geology, and claims in the Malta area, Montana...............................50

23. Generalized stratigraphy of bentonite beds in the Glasgow area, Montana ...................52

24. Maps showing predicted distribution of the X and Y bentonite beds in the 

Malta and Glasgow areas, Montana.......................................................................................58

25. Maps showing prediction errors in the geostatistical surface used to generate 

the tracts shown in figure 27 for Malta and Glasgow areas, Montana .................................60

26. Mineral potential map for sedimentary bentonite deposits in the bentonitic 

interval in the Bearpaw Shale and in the Fox Hills and Hell Creek Formations, 

in the North-Central Montana Sagebrush Focal Area .........................................................64

27. Mineral potential map for sedimentary bentonite deposits in the NorthCentral Montana Sagebrush Focal Area ................................................................................66

28. Graph showing steel, bentonite, and crude oil prices and the number of 

drilling rigs in the United States from 1950 to 2013 ...............................................................68

29. Map showing the location of diatremes and related igneous rocks in the 

study area and the surrounding region...................................................................................70

30. Detailed map showing the Williams cluster of kimberlite occurrences, 

geology, and location of claims................................................................................................73

31. Map showing analytic signal results from a high-resolution aeromagnetic 

survey by Anaconda Minerals Company and the location of known and 

inferred diatremes ......................................................................................................................74

32. Diagram showing calculated and inferred pressure-temperature-depth 

estimates for upper mantle xenoliths from diatremes in the study area 

and nearby ...................................................................................................................................75

33. Mineral potential map for diamond deposits in the North-Central Montana 

Sagebrush Focal Area ...............................................................................................................82

34. Map showing known deposits of salable minerals and active mineral material 

permit sites in the North-Central Montana Sagebrush Focal Area ...................................85

35. Map showing known sand and gravel deposits, active mineral material permit 

sites, and surficial deposits in the North-Central Montana Sagebrush Focal Area .......86

1-1. Matrix showing the classification system used for qualitative mineralresource potential for locatable minerals in the Sagebrush MineralResource Assessment.............................................................................................................101
viii

Tables

1. Status and number of mining claims, mineral leases, mineral material sales 

sites, and 43 CFR 3809 notices and plans of operations in the proposed 

withdrawal area within the North-Central Montana Sagebrush Focal Area.....................6

2. Occurrence of identified low-temperature geothermal systems in the NorthCentral Montana Focal Area.....................................................................................................24

3. Statistics of geothermal favorability and temperature at 6-km depth for the

North-Central Montana Focal Area, and for the entire Western United States ..............26

4. Summary of mining claims for locatable minerals in the proposed withdrawal 

area within the North-Central Montana Sagebrush Focal Area ........................................32

5. Summary of status and number of 43 CFR 3809 notices and plans of operations 

for locatable minerals in the proposed withdrawal area within the NorthCentral Montana Sagebrush Focal Area ................................................................................32

6. Active 43 CFR 3809 notices and plans of operations summarized by commodity

in the proposed withdrawal area within the North-Central Montana Sagebrush 

Focal Area....................................................................................................................................32

7. Summary of bentonite-related exploration and mining activities in the NorthCentral Montana Sagebrush Focal Area ................................................................................54

8. Bentonite pit locations in and near the North-Central Montana Sagebrush 

Focal Area....................................................................................................................................55

9. “Reserves” stated by American Colloid Company in 1981 for properties in 

the Malta area, Montana...........................................................................................................56

10. Location, size, and rock type of Missouri River Breaks diatremes, major dikes, 

and magnetic anomalies ...........................................................................................................77

11. Active Bureau of Land Management mineral material authorizations in the 

proposed withdrawal area within the North-Central Montana Sagebrush 

Focal Area....................................................................................................................................83

4-1. Parameters used in creating the geostatistical model for the lower contact 

of the Y bentonite bed in the Malta and Glasgow areas, Montana..................................104
ix

[U.S. customary units to International System of Units]

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

square inch (in2

) 6.452 square centimeter (cm2)

square mile (mi2

) 259.0 hectare (ha)

square mile (mi2

) 2.590 square kilometer (km2) 

Volume

barrel (bbl; petroleum, 1 barrel=42 gal) 0.1590 cubic meter (m3

) 

Mass

ounce, troy (oz) 31.103 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 metric ton (t) 

ton, long (2,240 lb) 1.016 metric ton (t) 

[International System of Units to U.S. customary units]

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 247.1 acre

square centimeter (cm2

) 0.001076 square foot (ft2)

square meter (m2

) 10.76 square foot (ft2) 

square centimeter (cm2

) 0.1550 square inch (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 6.290 barrel (petroleum, 1 barrel = 42 gal)

cubic meter (m3

) 264.2 gallon (gal) 

cubic meter (m3

) 0.0002642 million gallons (Mgal) 

cubic centimeter (cm3

) 0.06102 cubic inch (in3) 
x

Multiply By To obtain

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

Mass

gram (g) 0.032 ounce, troy (oz)

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

metric ton (t) 1.102 ton, short [2,000 lb]

metric ton (t) 0.9842 ton, long [2,240 lb]

Density

kilogram per cubic meter (kg/m3

) 0.06242 pound per cubic foot (lb/ft3) 

gram per cubic centimeter (g/cm3

) 62.4220 pound per cubic foot (lb/ft3) 

Supplemental Information

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as 

°F = (1.8 × °C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as 

°C = (°F – 32) / 1.8.

Datum

Vertical coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.
xi

Abbreviations

Ad andradite (iron-rich garnet)

AMIS Automated Minerals Information System

AOC Assessment Oversight Committee

API American Petroleum Institute

AR “as received”

ASTER Advanced Spaceborne Thermal Emission and Reflection Radiometer

ASTM American Society for Testing and Materials

AU assessment unit (oil and gas)

BCF, BCFG billion cubic feet (of gas) 

BHT bottom-hole temperature

BLM Bureau of Land Management

BRW Bear River Watershed (Sagebrush Focal Area)

BV “best value”

CAGR compound annual growth rate

CAI conodont alteration index

CFR Code of Federal Regulations 

CIGS copper-indium-gallium selenide

CSA Canadian Standards Association

CSM clay-sulfate-mica

CSAMT controlled-source audiofrequency magnetotellurics

DLA Defense Logistics Agency

DOD U.S. Department of Defense

DOE U.S. Department of Energy

DOI U.S. Department of the Interior

EGS enhanced geothermal system

EIS Environmental Impact Statement

ETM+ Landsat 7 Enhanced Thematic Mapper Plus

FLPMA Federal Land Policy and Management Act of 1976

f.o.b. free on board

FWS U.S. Fish and Wildlife Service

g/t gram per metric ton

Ga giga-annum or billions of years ago
xii

GFTZ Great Falls Tectonic Zone

GHC Geo-Heat Center

GIS geographic information system

HSSR Hydrogeochemical and Stream Sediment Reconnaissance

IDW inverse distance weighted

LEDs light-emitting diodes

LLD lower limit(s) of determination

m.y. millions of years

Ma mega-annum or millions of years ago

MAS Minerals Availability System

MCF thousand cubic feet of gas

MILS Mineral Industry Location System

MMBO million barrels of oil

MMBNGL million barrels of natural gas liquids

MOP muriate of potash

Moz million troy ounces

MRDS Mineral Resources Data System

Mt million metric tons

MTU metric ton unit

MVT Mississippi-Valley-type

MW megawatt

MWe megawatt electricity

NASGLP North American Soils Geochemical Landscape Project

NBMG Nevada Bureau of Mines and Geology

NDS National Defense Stockpile

NEPA National Environmental Policy Act of 1989

NGDB National Geochemical Database

NGL natural gas liquids

NGS National Geochemical Survey

NMIC USGS National Minerals Information Center

NOAA National Oceanic and Atmospheric Administration

NOGA USGS National Oil and Gas Assessment

NURE National Uranium Resource Evaluation

NWR National Wildlife Refuge

opt troy ounce per short ton
xiii

oz troy ounce 

PGE platinum-group element

PGM platinum-group metal

PLSS Public Land Survey System

ppm parts per million

REE rare earth element

REOE rare earth oxide equivalent(s)

RMOTC Rocky Mountain Oilfield Testing Center

ROD Record of Decision

RTP reduction-to-the-pole or reduced-to-pole

SaMiRA Sagebrush Mineral-Resource Assessment

SEDAR Canadian System for Electronic Document Analysis and Retrieval

SEDEX sedimentary exhalative

SFA Sagebrush Focal Area

SG specific gravity

SI structural index

SOP sulfate of potash

SWIR shortwave-infrared (region of the electromagnetic spectrum)

t metric ton

TCM Tax Court Memorandum

Th/K thorium/potassium ratio

TMI total magnetic intensity

TOMS Topographically Occurring Mine Symbols

TPS total petroleum system

UMOS Utah Mineral Occurrence System

USBM former U.S. Bureau of Mines

USFS U.S. Forest Service

USGS U.S. Geological Survey

USMIN USGS Mineral Deposit Database

VMS volcanogenic massive sulfide

wt.% weight percent

WYO Southwestern and South-Central Wyoming (Sagebrush Focal Area)
xiv

Chemical Symbols and Formulas Used 

Ag silver Gd gadolinium

Al aluminum Ge germanium

Al2

O3 aluminum oxide H hydrogen

Ar argon H2

S hydrogen sulfide

As arsenic Hf hafnium

Au gold Hg mercury

B boron Ho holmium

Ba barium In indium

BaSO4 barium sulfate Ir iridium

Be beryllium K potassium

Bi bismuth K2

O potassium oxide

Br bromine La lanthanum

C carbon Li lithium

Corg organic carbon Lu lutetium

CO carbon monoxide Mg magnesium

CO2 carbon dioxide MgO magnesium oxide

Ca calcium Mn manganese

CaO calcium oxide Mo molybdenum

Cd cadmium N nitrogen

Ce cerium NH3 ammonia

CH4 methane Na sodium

Cl chlorine NaCl sodium chloride

Co cobalt Na2

O sodium oxide

Cr chromium Nb niobium

Cs cesium Nd neodymium

Cu copper Ni nickel

Dy dysprosium O oxygen

Er erbium Os osmium

Eu europium P phosphorous

F fluorine P4 elemental (white) phosphorus

Fe iron PO4 phosphate

Fe2

O3 ferric iron oxide P2O5 phosphorous pentoxide

Ga gallium Pb lead

GaAs gallium arsenide Pd palladium

GaN gallium nitride Pr praseodymium
xv

Pt platinum Te tellurium

Rb rubidium Th thorium

Re rhenium Ti titanium

Rh rhodium TiO2 titanium dioxide

Ru ruthenium Tm thulium

S sulfur Tl thallium

Sb antimony U uranium

Sc scandium U3

O8 triuranium octaoxide (yellowcake)

Se selenium V vanadium

Si silicon V2

O5 vanadium pentoxide

SiO2 silicon dioxide (silica) W tungsten

Sm samarium WO3 tungsten trioxide

Sn tin Y yttrium

Sr strontium Yb ytterbium

Ta tantalum Zn zinc

Tb terbium Zr zirconium

Mineral Formulas Used 

adularia KAlSi

3

O8

alunite KAl3(SO4)

2

(OH)

6

andradite (garnet) Ca3Fe3+

2

(SiO4)

3

ankerite Ca(Fe,Mg,Mn)(CO3)

2

argentite Ag2S

arsenopyrite FeAsS

barite BaSO4

bornite Cu5FeS4

cassiterite SnO2

chalcocite Cu2S

chalcopyrite CuFeS2

cinnabar HgS

clinoptilolite (zeolite) (Ca,Na,K)2-3Al3

(Al,Si)2Si13O36·12(H2O)

coffinite U[SiO4

,(OH)4]

corderoite Hg3

S2Cl2

dolomite CaMg(CO3)

2

erionite (zeolite) (Ca,Na,K)10[Al10Si26O72]·~30H2O
xvi

fluorite CaF

2

galena PbS

hectorite (smectite clay) Na

3

(Mg,Li)

3

Si

4

O10(F,OH)

2

hematite Fe

2

O

3

ilmenite FeTiO

3

kaolinite Al

2

Si

2

O

5

(OH)

4

leucite K(AlSi

2

O

6

)

magnetite Fe

3

O

4

molybdenite MoS

2

molybdite MoO

3

monazite (Ce,La,Th,Nd)PO

4

montmorillonite (Na,Ca)0.33(Al,Mg)

2

(Si

4

O10)(OH)

2 · nH2

O

nepheline Na

3

KAl

4

Si

4

O16

opal SiO

2

·nH

2

O

phillipsite (zeolite) (Ca,Na,K)4-7[Al4-7Si2-9

O32]·12H

2

O

powellite CaMoO

4

pyrite FeS

2

quartz SiO

2

rutile TiO

2

scheelite CaWO

4

siderite FeCO

3

sillimanite Al

2

SiO

5

sylvite KCl

sylvinite KCl+NaCl

sphalerite (Zn,Fe)S

staurolite Fe

2

Al

9

Si

4

O23(OH)

stilpnomelane (K,Ca,Na)(Fe2+,Mg,Fe3+

)

8

(Si,Al)12(O,OH)27·nH

2

O

tetrahedrite (Cu,Fe,Ag,Zn)12Sb

4

S13

uraninite UO

2

xenotime YPO

4

zircon ZrSiO

4
Geology and Mineral Resources of the North-Central 

Montana Sagebrush Focal Area

By Jeffrey L. Mauk, Michael L. Zientek, B. Carter Hearn, Jr., Heather L. Parks, M. Christopher Jenkins, 

Eric D. Anderson, Mary Ellen Benson, Donald I. Bleiwas, Jacob DeAngelo, Paul D. Denning, Connie L. 

Dicken, Ronald M. Drake II, Gregory L. Fernette, Helen W. Folger, Stuart A. Giles, Jonathan M.G. Glen, 

Matthew Granitto, Jon E. Haacke, John D. Horton, Karen D. Kelley, Joyce A. Ober, Barnaby W. Rockwell, 

Carma A. San Juan, Elizabeth S. Sangine, Peter N. Schweitzer, Brian N. Shaffer, Steven M. Smith, 

Colin F. Williams, and Douglas B. Yager

Executive Summary

The U.S. Department of the Interior has proposed to 

withdraw approximately 10 million acres of Federal lands 

from mineral entry (subject to valid existing rights) from 

12 million acres of lands defined as Sagebrush Focal Areas 

(SFAs) in Idaho, Montana, Nevada, Oregon, Utah, and Wyoming (for further discussion on the lands involved see Day 

and others, 2016). The purpose of the proposed action is to 

protect the greater sage-grouse (Centrocercus urophasianus) 

and its habitat from potential adverse effects of locatable 

mineral exploration and mining. To inform the decision on 

whether to withdraw the SFAs from mineral entry, the Bureau 

of Land Management (BLM) requires a mineral-resource 

assessment be completed to identify mineral resources within 

the proposed area of withdrawal. The USGS Sagebrush 

Mineral-Resource Assessment (SaMiRA) project was initiated 

in November 2015 and supported by the BLM to (1) assess 

locatable mineral-resource potential and (2) to describe leasable and salable mineral resources for the seven SFAs and 

Nevada additions. This report summarizes the current status 

of locatable, leasable, and salable mineral commodities and 

assesses the potential of locatable minerals in the NorthCentral Montana Focal Area.

The proposed withdrawal area that is evaluated in this 

report is located in north-central Montana, and includes 

parts of Fergus, Petroleum, Phillips, and Valley Counties. 

The proposed withdrawal area encompasses 3,545 km2

(876,036 acres), of which, the BLM manages 3,430 km2

(847,571 acres).

The study area is along both sides of the Missouri River, 

in the Northern Great Plains physiographic province. The 

Little Rocky Mountains are just outside the northwestern part 

of the study area, and the Judith Mountains are nearby, to the 

southwest of the study area.

The vast majority of the study area is underlain by 

sedimentary rocks. The Cretaceous Bearpaw Shale is the most 

widespread formation; it crops out over most of the study 

area, and younger Cretaceous to Paleocene sedimentary rocks 

crop out locally in the southeastern part of the study area. 

Pleistocene glacial deposits are found locally in the study area, 

predominantly in the northern part, and Quaternary deposits 

are widespread in river and stream valleys.

The Little Rocky Mountains host Cretaceous to Paleocene 

alkaline intrusive rocks that have domed and uplifted the older 

sedimentary rocks that they intruded. The study area also 

includes part of the Missouri River Breaks diatremes, which 

are Eocene mantle-derived alkaline ultramafic diatremes 

and dikes.

Mining and Mineral Activity in the Area

The focal area does not appear to contain significant 

concentrations of leasable minerals. However, there has been 

production of locatable and salable minerals from the study 

area, and there is further potential for these in the focal area.

Leasable

Leasable fluid minerals include oil and gas and geothermal resources, and leasable solid minerals include coal, oil 

shale, native asphalt, phosphate, sodium, potash, potassium, 

and sulfur. Oil shale, native asphalt, sodium, potash, potassium, and sulfur do not occur in significant concentrations 

within or near the study area.

There are no coal or geothermal leases in the proposed 

withdrawal area, and there are 16 non-energy solid mineral 

leases in the proposed withdrawal area, all of which are 

closed. Owing to the thinness, lack of lateral continuity, poor 

quality, and depth of cover, the coal beds in the study area are 

not likely to generate economic interest in the future. There are 

no known moderate- to high-temperature geothermal systems 

in the area, and the area does not have significant enhanced 

geothermal systems potential.

There are 1,133 oil and gas leases in the proposed withdrawal area; all but five of these are closed. There has not been 

any significant hydrocarbon production within the study area, 

and the overall potential appears to be low.
2 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Locatable

Locatable minerals are those for which the right to 

explore, develop, and extract on Federal land open to mineral 

entry is established by the location of lode or placer mining 

claims. Locatable minerals include metallic minerals, 

industrial minerals, and uncommon varieties of sand, gravel, 

stone, pumice, pumicite, and cinders. Based on the geology 

of the study area, and past production in the study area and 

nearby, the study area has potential for gold, silver, bentonite, 

and diamonds.

Gold and silver may occur in epithermal gold-silver 

deposits and placer gold deposits. The Zortman and Landusky 

epithermal deposits in the Little Rocky Mountains outside 

the study area produced 2.5 million troy ounces (oz) of gold 

(78 metric tons [t]) and 20.7 million oz of silver (644 t), but 

the igneous rocks that host these deposits are exposed in 

only a small part of the northwestern part of the study area, 

and there has been no significant production from epithermal 

deposits in the study area. Available geological, geochemical, 

and geophysical data indicate that the potential for epithermal 

deposits in the study area is low.

Placer gold deposits occur downstream of some hard 

rock gold deposits, but historic production of gold from placer 

deposits in the Zortman-Landusky District was reportedly only 

326 oz of gold (10 kg), which is insufficient for any modern, 

large-scale placer operation. Furthermore, the gold in the 

hard rock deposits occurs predominantly in solid solution in 

pyrite and in telluride minerals. Gold in these minerals is not 

recoverable by conventional placer operations. Available data 

indicate that the potential for gold placer deposits in the study 

area is low, and there are no gold placer tracts in the proposed 

withdrawal area.

Bentonite exploration and mining activity has been 

ongoing in the study area for at least 60 years. More than 1.7 Mt 

of bentonite has been produced from small open pits near 

Glasgow and Malta. There are two pending plans of operations for bentonite in the proposed withdrawal area and one 

approved plan of operations. Predictive models developed in 

this report show that the study area contains approximately 

280 km2

 (69,190 acres) where bentonite is potentially mineable because there are relatively thick bentonite beds that are 

at or near the surface. Approximately 124 km2

 (30,640 acres) 

of this high-potential area is in the proposed withdrawal area.

The Missouri River Breaks diatreme field partly overlaps 

the study area. Kimberlite-hosted diamond deposits in diatremes 

are one of the two main sources of diamonds from hard rock 

mines. However, in the study area, few of the diatremes 

are kimberlite, and the kimberlite diatremes lack indicator 

minerals that would suggest that they may contain diamond. 

Therefore, the potential for diamond deposits in the study area 

is low. There are no known kimberlites in the proposed withdrawal area, and therefore the potential for diamond deposits 

is low.

Salable

Salable minerals include sand and gravel, aggregates, 

dimension stone, petrified wood, cinders, clay, pumice, and 

pumicite. Sand and gravel have been produced in the study 

area, and there are also closed claims for petrified wood in the 

study area.

The sedimentary rocks that form the majority of the 

bedrock in the study area commonly contain abundant clay 

minerals, including bentonite. These minerals make the rock 

soft and crushable, and the clay minerals are expandable, so 

these rocks are not favorable sources for sand and gravel, 

which require hard, durable, chemically inert materials. Consequently, sand and gravel have predominantly been produced 

in the northern part of the study area, where glacial deposits 

with hard, durable materials are thickest.

The southern part of the study area contains a locality 

outside the proposed withdrawal area that appears to correspond to an occurrence of petrified wood in the Hell Creek 

Formation. However, the quality of petrified wood in the study 

area is low, and is not like the high-quality samples from 

southwestern Montana or Arizona. Furthermore, the lack of 

other claims in the study area, and the absence of previous 

assessments or citations in the published literature, suggest 

that significant resources of gem-quality petrified wood are 

unlikely to be present in the study area.

Introduction

The U.S. Department of the Interior “has approved an 

application to withdraw approximately 10 million acres of 

public and National Forest System lands identified as Sagebrush Focal Areas in Idaho, Montana, Nevada, Oregon, Utah, 

and Wyoming from location and entry under the United States 

mining laws to protect the Greater Sage-Grouse and its habitat 

from adverse effects of locatable mineral exploration and 

mining, subject to valid existing rights” (U.S. Department of 

the Interior, 2015a). A withdrawal does not affect valid existing rights. The lands proposed for withdrawal are part of the 

seven Sagebrush Focal Areas (Bureau of Land Management, 

2015b, c).

The purpose of this report is to summarize the current 

status of locatable, leasable, and salable mineral commodities 

and to assess the potential of locatable minerals in the NorthCentral Montana Focal Area in Montana. This report was prepared as required by the Federal Land Policy and Management 

Act of 1976, 43 U.S.C. 1701–1785 (FLPMA), for an application for withdrawal of lands. This report follows guidance 

provided in BLM Manual Sections 3031 and 3060 (Bureau of 

Land Management, 1985, 1994) for mineral assessments and 

mineral reports. The information and interpretations provided 

herein relied on the best publically available data and information sourced from Federal and State agencies, academic 

literature, and company reports.
Introduction 3

Lands Involved

This report describes the mineral potential of the proposed 

withdrawal area within the North-Central Montana Sagebrush 

Focal Area. The proposed withdrawal area is located in 

Montana, and includes parts of Fergus, Petroleum, Phillips, 

and Valley Counties. The proposed withdrawal area encompasses 3,545 km2

 (876,036 acres), of which, the BLM manages 3,430 km2

 (847,571 acres; fig. 1). The study area, which 

encompasses 10,424 km2

 (2,575,704 acres), is made up of 

all the townships that include areas proposed for withdrawal. 

The townships include those first described by the BLM 

(Bureau of Land Management, 2015d), as formally proposed 

by the U.S. Department of the Interior (2015a, b), and further 

amended by the BLM through correspondence (Anthony 

Titolo, BLM, written commun., April 22, 2016). The adjacent 

Public Land Survey System (PLSS) township boundaries were 

dissolved to define the study area and then modified to exclude 

Native American lands, which are sovereign and exempt 

from withdrawal.

The BLM LR2000 database shows 2,177 mining claims 

in the proposed withdrawal area within the North-Central 

Montana Sagebrush Focal Area; 1,490 of these are closed and 

the remaining 687 are active (table 1). These are described 

more fully in the Locatable Minerals section of this report. 

There are no coal or geothermal leases in the proposed withdrawal area, and there are 16 non-energy solid mineral leases 

in the proposed withdrawal area, all of which are closed. There 

are 1,133 oil and gas leases in the proposed withdrawal area; 

all but five of these are closed (table 1). There are 15 mineral 

materials sales sites in the proposed withdrawal area; one 

of these is authorized, and the rest are closed or expired. 

As discussed in the Salable Minerals section of this report, 

these relate to sand and gravel operations. There are eleven 

43 CFR 3809 notices and plans of operations in the proposed 

withdrawal area, and, as discussed in the Locatable Minerals 

section of this report, these all relate to bentonite.

Organization of this Report and Terminology

The outline of this report is based on guidance published 

in the BLM Manual Sections 3060 and 3031 (Bureau of Land 

Management, 1985, 1994). To the extent possible, we organized the information in this report to reflect BLM technical 

and legal language and use the legal classification of minerals 

recognized by BLM. Potential for occurrence is only discussed 

for locatable minerals.

Several schemes are used to classify types of minerals in 

scientific and technical literature. For example, a distinction 

is made between materials from which metals are extracted 

(metallic) and those that are not used as a source of metal or 

energy (nonmetallic or industrial). Another scheme differentiates material that is extracted from solid rock (lode) from that 

which was concentrated by moving water in sediment (placer). 

Common variety minerals do not possess a distinct or special 

value. Uncommon variety minerals have unique commercial 

value. Strategic and critical minerals are distinguished according to their importance to the Nation. Other classification 

schemes distinguish material based on the ultimate source of 

the valuable material—magma, hydrothermal fluid, surficial 

water, or weathered material.

The BLM differentiates locatable, leasable, and salable 

minerals; these terms are based on U.S. mining laws and 

Departmental decisions, and are not widely used in scientific 

literature. Locatable minerals in the United States are those 

that may be acquired under the General Mining Act of 1872, 

30 U.S.C. 22–54, as amended. Locatable minerals include 

materials from which metallic and nonmetallic minerals are 

mined, industrial minerals, and certain varieties of mineral 

materials if they are uncommon because they possess a distinct and special value. Leasable minerals for Federal lands 

or a federally retained mineral interest in lands in the United 

States refer to commodities acquired through the Mineral 

Leasing Act of 1920 (30 U.S.C. 181 et seq.), as amended; the 

Geothermal Steam Act of 1970 (30 U.S.C. 1001 et seq.), as 

amended; or the Mineral Leasing Act for Acquired Lands of 

1947 (30 U.S.C. 351 et seq.), as amended. Examples of leasable minerals include oil, gas, coal, oil shale, sodium, potash, 

phosphate, and all minerals within acquired lands. Salable 

minerals on Federal lands are those sold by sales contract from 

the Federal Government, and by free use permit to governmental agencies and nonprofit organizations. The applicable 

statute is the Mineral Materials Act of 1947 (30 U.S.C. 601 et 

seq.), as amended. Salable minerals are generally common 

varieties of materials; examples include construction materials 

and aggregates such as sand, gravel, cinders, roadbed, and 

ballast material.

Rights to locatable minerals are established by the location (or staking) of lode or placer mining claims. Acquisition 

of leasable minerals is by application for a government lease 

and permits to mine or explore after lease issuance. Salable 

minerals on Federal lands are sold by sales contract. Surface 

disturbance associated with locatable mineral development 

must comply with surface-management regulations (43 CFR 

3809). Table 1 summarizes information on mining claims, 

leases, and salable mineral sites, along with surface-management 

(43 CFR 3809) authorizations in the Focal Area.

The mining laws applicable to Federal lands of the United 

States were not developed with specific knowledge of geology 

or types of mineral materials. Even so, the various legal types 

of minerals do have some broad geologic associations. Leasable minerals include areally extensive types of valuable earth 

materials most commonly occurring in sedimentary basins: 

oil and gas fields, coal fields, oil shales, bedded deposits of 

soluble sodium and potassium salts, and bedded deposits of 

phosphorite. Salable minerals are common earth materials that 

are widely distributed and have low unit value. They must be 

obtained near where the need exists. If the material is not leasable or salable, it is locatable. Most, but not all, of this type of 

material occurs in spatially restricted areas.
4 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

Roy

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

191

2

191

191

Fort Belknap Reservation

Charles M. Russell National Wildlife Refuge

Upper Missouri 

River Breaks

National Monument

Little Rocky

Mountains

Missouri

River

A

0 10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors. Surface 

management data from BLM, 2015.

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983.

Figure 1. Surface land management map of the North-Central Montana Sagebrush Focal Area. BLM, Bureau of Land Management; 

USGS, U.S. Geological Survey. 
Introduction 5

107°0'

Fort Peck LakeFort Peck Lake

Glasgow

Jordan

GARFIELD COUNTY

VALLEY COUNTY

2

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

B

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Focal area (only in B)

Surface management status (only in A)

Bureau of Indian Affairs

Bureau of Land Management

Bureau of Reclamation

Department of Defense

State

U.S. Fish and Wildlife

Upper Missouri River Breaks 

 National Monument
6 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 1. Status and number of mining claims, mineral leases, mineral material sales sites, and 43 CFR 3809 notices and plans of 

operations in the proposed withdrawal area within the North-Central Montana Sagebrush Focal Area.

[Source: Bureau of Land Management (BLM) LR2000 database, March 6, 2016. The number of cases is for the complete section that includes a proposed 

withdrawal area. ND, no data]

Type Mineral 

type

Number 

of unique 

cases

Active Authorized Pending Closed Cancelled Expired Rejected Withdrawn

Mining claims Locatable 2,177 687 ND ND 1,490 ND ND ND ND

Coal leases Leasable 0 ND ND ND ND ND ND ND ND

Geothermal leases Leasable 0 ND ND ND ND ND ND ND ND

Non-energy solid 

mineral leases

Leasable 16 ND ND ND 16 ND ND ND ND

Oil and gas leases Leasable 1,133 ND ND 5 1,128 ND ND ND ND

Mineral materials 

sales sites

Salable 15 ND 1 ND 11 ND 3 ND ND

Surface-management 

plans

Locatable 11 ND 1 2 8 ND ND ND ND

In this report, we use three terms to refer to the area 

under investigation: (1) Focal Area, (2) study area, and 

(3) proposed withdrawal area.

1. The Sagebrush Focal Areas have been identified by the 

BLM and the U.S. Forest Service as important landscape 

blocks with high breeding-population densities of sagegrouse and existing high-quality sagebrush. The NorthCentral Montana Sagebrush Focal Area is the term used 

for the identified Focal Area that is the subject of this 

report.

2. Because the lands that are within the Focal Area have 

complex boundaries, the study area has been expanded 

outward from the Focal Areas to the nearest survey 

township boundary, where each survey township is a 

square parcel of land of 36 mi2

 (93.2 km2). This simplifies the approach from a geological basis, because it 

allows us to examine a large, relatively contiguous block 

of land.

3. For the Sagebrush Focal Areas, the proposed withdrawal 

areas are a subset of the Focal Areas, so some parts of 

the Focal Areas are proposed for withdrawal, and other 

parts are not.

In this report, we use Focal Area for the titles of figures 

and tables to show the link with the Sagebrush Focal Areas 

that have been identified by the BLM and the U.S. Forest Service. In most of the text, and within the figures, we focus on 

the study area and the proposed withdrawal areas. The former 

provides the larger perspective that is most useful for evaluating the geological, geochemical, geophysical, and remote 

sensing data that underpin our work (Anderson and Ponce, 

2016; Rockwell, 2016; Smith and others, 2016), and that 

together provide the foundations for our resource assessments. 

The proposed withdrawal areas can then be compared with 

the larger scale evaluations that we describe within the 

study area.

A glossary of terms is provided in Day and others (2016). 

In some parts of this report, a brief discussion is provided to 

clarify usage of specific terms and to relate how concentrations 

of valuable earth materials conform to the legal definitions 

that determine their ownership and development. However, for 

other terms, such as “minerals”, the intended meaning must be 

inferred from context.

Who Did the Work?

This report represents contributions from a multidisciplinary team of U.S. Geological Survey (USGS) geologists, 

geophysicists, geochemists, mineral commodity specialists, 

and geographic information system (GIS) experts. These geoscientists reached out to personnel from the Montana Bureau 

of Mines and Geology, the BLM in Montana, and the Montana Department of Environmental Quality to gather the most 

recent information on geology and mineral resources. Representatives from these groups also provided helpful feedback 

on the preliminary tracts that we developed for the study area.

Jeffrey L. Mauk, Michael L. Zientek, and B. Carter 

Hearn, Jr., wrote the majority of the text in this report, and 

Michael L. Zientek created the outline used for the project 

when preparing the assessment reports. Heather L. Parks and 

M. Christopher Jenkins undertook much of the GIS work that 

underpins this report, including working with others to create 

most of the tracts in this report, and Heather produced the 

figures.

For the assessment of locatable minerals, Jeffrey L. 

Mauk took the lead for assessing gold potential in epithermal and placer deposits with assistance from Stuart A. Giles 

and Heather L. Parks (GIS support and tract creation). Eric 

Anderson modeled depth-to-source of geophysical anomalies 

to determine if any could represent shallow intrusions that 
Description of Geology 7

may host epithermal deposits. Michael L. Zientek was primarily 

responsible for the bentonite assessment, with Heather L. 

Parks and M. Christopher Jenkins compiling data, interpreting satellite imagery, and doing geospatial calculations for 

the assessment. B. Carter Hearn, Jr., evaluated diamond 

potential in the study area with assistance from Eric D. 

Anderson (interpretation of aeromagnetic data for unmapped 

diatremes), Heather L. Parks (GIS support, tract creation, and 

writing), and Michael L. Zientek (GIS support, tract creation, 

and writing).

The Leasable Minerals section of this report was produced by several authors: Douglas B. Yager wrote the Potash 

section; Brian N. Shaffer and Jon E. Haacke wrote the Coal 

section; Colin F. Williams, Jonathan M.G. Glen, and Jacob 

DeAngelo wrote the Geothermal section; Ronald M. Drake II 

wrote the Oil and Gas section; and Mary Ellen Benson worked 

together with Jeffrey L. Mauk to write the Phosphate section.

The Salable Minerals section was put together by Jeffrey 

L. Mauk with assistance from Heather L. Parks (GIS support 

and figure creation). Joyce A. Ober provided thoughtful, 

timely, and valuable feedback that helped improve this section 

of the report.

Connie L. Dicken compiled and delivered information 

from the BLM, which included information on locatable, 

leasable, and salable commodities. She provided the summary tables required for the report and synthesized the BLM 

data spatially so it could be used in the assessment. Her work 

guided our analysis of what commodities should be assessed, 

and provided some level of validation when we created our 

mineral potential maps.

Paul D. Denning, Stuart A. Giles, John D. Horton, and 

Carma A. San Juan provided assistance with GIS at various 

critical points in the project. Eric D. Anderson compiled the 

geophysical results in this report, and worked with us to use 

legacy data in novel ways to help inform the assessment for 

metallic and non-metallic minerals. Helen W. Folger, Matthew 

Granitto, Karen D. Kelley, and Steven M. Smith compiled the 

geochemical data that underpin parts of this report, provided 

specialty databases, reviewed and commented on parts of this 

report, and provided deep insight into the interpretation of 

the geochemical data in the study area. Barnaby W. Rockwell 

provided expert knowledge of remote sensing data, which 

helped to inform and validate the tracts for bentonite. Donald 

I. Bleiwas and Elizabeth S. Sangine contributed commodity 

information and expertise, and Don provided helpful feedback 

on our preliminary tracts. Gregory L. Fernette and Peter N. 

Schweitzer made available the USGS Mineral Deposit Database project (USMIN) and the Mineral Resources Data System 

(MRDS), and Greg provided helpful discussions about past 

production and possible resources in the study area.

Description of Geology

This section describes the physiography and geology of 

the study area and nearby.

Physiography and Ecoregions

The study area is in north-central Montana, along both 

sides of the Missouri River, in the Northern Great Plains 

physiographic province (fig. 2). The study area is close to the 

Little Rocky Mountains on the northwest, and close to the 

Judith Mountains on the southwest. Havre, Montana, is about 

180 km to the northwest and Lewistown, Montana, is about 

120 km to the southwest. Access to the study area can be difficult because most of the roads are unimproved tracks across 

sedimentary rocks that contain bentonite—a soft clay mineral 

with the ability to absorb large quantities of water and change 

volume. A small amount of rain may immobilize even fourwheel drive vehicles.

Dominant topographic features include flat-top ridges 

and plateaus, cut by winding coulees, many of which are 

precipitously steep and narrow, although some have broad 

flood plains. Elevations range between 1,400 and 600 m. All 

streams in the area are intermittent and discharge into the 

Missouri River or the Milk River (fig. 2). Temperatures may 

exceed 40 °C in the summer and drop below −40 °C in the 

winter. Thunderstorms are common from spring through fall, 

accompanied by winds that occasionally exceed 60 km per 

hour. A thin snow cover persists during the winter and spring 

months, but accumulation may be more than 1 m deep after 

large storms.

The study area contains three level III ecoregions: the 

Middle Rockies, the Northwestern Glaciated Plains, and the 

Northwestern Great Plains (Woods and others, 2002). A small 

part of the Scattered Eastern Igneous Core Mountains subdivision of the Middle Rockies ecoregion extends south into the 

study area from the Little Rocky Mountains. This subdivision 

is mostly wooded and, though dry, receives more precipitation 

(40 to >80 cm annually) than other parts of the study area. 

Two subdivisions of the Northwestern Glaciated Plains are 

present in the study area: the Glaciated Northern Grasslands 

cover most of the northern part of the study area, and the 

Foothill Grasslands surround the Little Rocky Mountains in 

the northwest. The Glaciated Northern Grasslands is a divided 

plain dominated by rangeland, with agriculture taking place 

on intermittent gravel benches and on the Milk River Valley’s 

irrigated alluvial soils. The Foothill Grasslands slope from 

the mountains in the northwest of the study area down to the 

plains, with sporadic hills and buttes cut by streams fed from 

the mountains; there is less precipitation there than in the 

mountains above, and more than in the plains below. Three 

subdivisions of the Northwestern Great Plains are present in 

the study area: the River Breaks in the southeastern part of the 

study area, the Missouri Breaks Woodland-Scrubland in the 

southwestern and south-central areas, and the Montana Central 

Grassland in the far southwestern as well as in the centraleastern areas. The River Breaks comprises a series of steeply 

eroded terraces and uplands that lead down into the Missouri 

and Yellowstone River systems below. The Missouri Breaks 

Woodland-Scrubland is similarly eroded, with clayey soils 

weathered from Cretaceous sedimentary rock (Woods and 

others, 2002).
8 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

!

!

!

!

!

!

!

Fort Peck aL ekFort Peck aL ek

Milk River

Missouri

i R ver

Little Rocky

Mountains

Roy

Hays

Malta

Glasgow

Lodge Pole

Bears Paw Mountains

Judith Mountains

Jordan

107°0' 108°0' 109°0'

48°0'

47°30'

Missouri Plateau, Glaciated Missouri Plateau, Unglaciated

Great

Plains

Interior Plains

Area of map

Scattered Eastern Igneous-Core MountainsGlaciated Northern GrasslandsFoothill GrasslandNon-calcareous Foothill Grassland

River Breaks

Missouri Breaks Woodland-ScrublandMontana Central Grasslands

USGS study area boundary

EXPLANATION

Ecoregions

Interior Plains (index map only)

Great Plains (index map only)

Missouri Plateau, Glaciated (index map only)Missouri Plateau, Unglaciated (index map only)Ecoregions within Missouri Plateau, Glaciated  and Missouri Plateau, Unglaciated

Middle RockiesNorthwestern Glaciated PlainsNorthwestern Great Plains

10 20 30 MILES 0

0 10 20 30 KILOMETERS

UNITED STATES 

MEXICO

CANADA

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

Figure 2. Map showing the physiographic ecoregions of the North-Central Montana Sagebrush Focal Area (modified from Trimble, 1980; Woods and others, 2002). USGS, 

U.S. Geological Survey.
Description of Geology 9

Regional Geology and Tectonic Setting

Precambrian Basement Terranes

Most of the study area is underlain by the Wyoming 

Craton–Wyoming Province terrane of Archean age, at depths 

of about 1.5 km. Wyoming Craton metamorphic rocks are 

exposed in the Little Rocky Mountains as a result of uplift 

adjacent to and above Paleocene syenite porphyry intrusions 

(Knechtel, 1959). The Wyoming Craton terrane is bound 

to the northwest by the Great Falls Tectonic Zone (GFTZ) 

(fig. 3) and to the northeast by the north-northwest-trending 

Paleoproterozoic Trans-Hudson orogen/Dakota domain. These 

are intracratonic mobile belts that transitioned into Paleoproterozoic suture zones, marking the collision of the Archean 

Medicine Hat block, the Archean Wyoming craton, and the 

Archean Superior terrane (for example, Mueller and others, 

2005; Lund and others, 2015, and references therein; fig. 3A). 

East of the Rocky Mountain front in Montana, the GFTZ 

is entirely concealed by Paleozoic and Mesozoic sedimentary rocks, has been approximately defined by geophysical 

anomalies, and shows no northeast-trending faults in surface 

exposures. The GFTZ is shown with various trends and widths 

in recent publications (in other words, N. 25° E., 65 km wide, 

Gorman and others, 2002; N. 55° E., 90 km wide, Carlson and 

others, 2004; N. 50° E., 135 km wide, Barnhart and others, 

2012). In the Wyoming Craton, GFTZ, and southern part of 

the Medicine Hat block, seismic profiles show that the crust 

is abnormally thick, as much as 60 km, and the lower crust 

consists of 10–30 km of dense, high-velocity 7.0–7.8 km/s 

material (Gorman and others, 2002). On a different scale, past 

studies have inferred separate Precambrian basement blocks 

on the basis of contrasting aeromagnetic and gravity patterns, 

with inferred block boundaries between areas of contrasting 

orientation of parallel magnetic anomaly patterns, or where 

such parallel patterns end against areas without any preferred 

anomaly orientation (Zietz and others, 1968; Zietz and 

others, 1980).

Isotopic age data show additional complexities of the 

terranes and collision zones. Margins of Archean cratons 

have been affected by younger Paleoproterozoic metamorphic 

events in the 1.8–1.7 Ga range, whereas U-Pb zircon ages 

show evidence for Archean ancestry (Davis and others, 1995). 

Exposed Precambrian felsic gneisses and mafic amphibolite in 

the Little Rocky Mountains have Archean Rb/Sr isochron ages 

but have biotite and amphibole K/Ar ages in the 1.8–1.1 Ga range 

(Peterman, 1981). Age data for monazites in deep crustal 

xenoliths show multiple Paleoproterozoic and Mesoproterozoic metamorphic events (Barnhart and others, 2012).

Xenoliths of lower crustal mafic and felsic granulites 

from dikes, plugs, and diatremes provide age data for the 

underlying concealed Precambrian basement. Xenoliths 

from the southeastern Bears Paw Mountains localities 

record a series of heating events at 2.2–2.1 Ga, 1.5 Ga, 

and 1.3 Ga, whereas zircon cores have 207Pb/206Pb ages of 

3.0–2.5 Ga (Barnhart and others, 2012). Xenoliths from 

the Archean Medicine Hat block show dates of ~1.8 Ga 

and neodymium isochron dates of 1.7–1.5 Ga, representing 

reheating of Archean crust or Proterozoic additions to the 

lower crust (Davis and others, 1995; Irving and others, 1997; 

Gifford, 2015).

Structure

Most of the study area and surrounding region show 

bedding with low-angle dips in outcrops, owing to gentle 

monoclinal flexures, anticlines, and synclines in the exposed 

Mesozoic and Paleocene sedimentary formations (fig. 3B). 

The eastern part of the study area is on the western flank of 

the Williston Basin (fig. 3). Formations are strongly uplifted 

around and above intrusions in the Little Rocky Mountains 

and Judith Mountains, which each contain closely spaced 

intrusions that produced a series of nested, coalescing domes 

(fig. 4).

In central and south-central Montana, several subparallel, 

en-echelon fault zones trending N. 70–80° W. reflect buried 

Precambrian basement faults (figs. 3, 4, though owing to their 

lack of surface exposure, none of the faults are labeled or 

described on the maps). From north to south are the Cat Creek 

fault zone (lineament), a poorly developed zone east-southeast 

of Grass Range, the Devils Basin–Big Wall anticlinal trend, 

and the Lake Basin fault zone. Some fault zones have been 

repeatedly active from the Proterozoic to the Mesozoic, controlling deposition, facies, source rocks, and accumulations 

of oil and gas. The Cat Creek fault zone and the Lake Basin 

fault zone have been the north and south boundaries of the 

Central Montana Trough depositional basin (fig. 3; Norwood, 

1965). The Cat Creek fault zone at depth extends at least to 

longitude 110° W. based on subsurface data, showing offset 

of Mississippian and Devonian beds (southwest side down) at 

1,500 m below mean terrain elevation (Culver, 1982). The Cat 

Creek fault zone approximately marks the northern boundary 

of the eastward depositional salient of the Proterozoic Belt 

Supergroup (Winston, 1986). In the Mesozoic, much of the 

Central Montana Trough was uplifted (referred to as the 

Central Montana Uplift or Central Montana Platform), with 

the development of an east-southeast band of folds and domes 

(Norwood, 1965).

Other prominent faults are in the N. 45–55° E.-trending, 

200-km-long Weldon–Brockton–Froid fault zone, which lies 

to the east and southeast of the study area (Colton and Bateman, 

1956; Colton, 1963). Faults in the Weldon–Brockton–Froid 

zone have displaced Pleistocene glacial outwash deposits of 

late Wisconsin age, and alluvial-colluvial deposits and thus are 

relatively young (Colton and Bateman, 1956; Colton, 1963). 

Displacement of bedrock is as much as 60 m. A 40-km-long, 

N. 65° W.-trending fault zone bounds the northeast side of 

Freedom Dome, which is 20 km south of Jordan and south of 

the study area. Within the study area, 1:100,000-scale geologic 

maps show shorter normal faults and fault zones that trend 

~N. 45° E., and fewer short 1–3-km-long normal faults trending N. 30–60° W. These faults displace Upper Cretaceous 
10 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

and Paleocene beds about 6 to 30 m (Porter and Wilde, 1993, 

2001; Wilde and Bergantino, 2004a, b).

Southwest of the Little Rocky Mountains, the western 

part of the study area contains remarkable, complex shallow 

gravity-slide structures (fig. 4). The Missouri River Breaks are 

crossed by many long fold-and-fault structures. These foldand-fault structures are due to gravity sliding of the middle 

Eocene Bearpaw Mountains volcanic pile off of a central eastwest antiformal uplift (Bearpaw Mountains arch) containing 

multiple intrusion-cored domes, sills, and dikes (Hearn, 1976). 

The gravity slides also involved the underlying pre-volcanic 

Upper Cretaceous, Paleocene, and lower Eocene sedimentary 

formations. The deepest slide plane typically is in or near the 

Cretaceous Greenhorn Formation, but locally is deeper, in the 

Cretaceous Mowry Shale. The volcanic pile and subjacent 

sedimentary formations slid northward on the north side of 

the arch and slid southward on the south side of the arch. In 

the headward-proximal zones of the detachment sheets, pullapart graben structures developed, with volcanic rocks and 

pre-volcanic formations down-faulted as large tilted blocks or 

half grabens. In the distal zone across much of the Missouri 

Breaks, the sliding sheets locked, folded, and broke, forming 

reverse-fault antiformal structures approximately 1–2 km 

wide, spaced 3–5 km apart, with intervening zones of nearly 

horizontal sedimentary formations. Based on geophysical drill 

hole logs, distinctive marker beds below the basal slide plane 

show dips of 5–20 m per kilometer (Hearn, 1976).

The fold-and-fault zones of the gravity-slide complex 

can have three structural types: circumferential, radial, and 

oblique, based on their geometry in plan view, relative to the 

approximately circular area of the Bears Paw Mountains.

1. Structures circumferential to the Bears Paw Mountains 

are faulted folds that contain reverse faults and show the 

results of compression in the slide sheets.

2. Structures radial to the Bears Paw Mountains are tear 

faults that probably have strike-slip displacement, but 

the amount of strike-slip offset is difficult to document 

because there are no offset markers. The radial tear faults 

mark boundaries between separate gravity-slide sheets. 

Thus, on each side of such radial fault zones, circumferential faulted folds probably developed independently 

and cannot be correlated as lateral offsets. Tear faults 

may show zones of folding and uplift, but those zones 

are generally narrower than in circumferential structures.

3. Structures that are oblique (neither circumferential nor 

radial) show characteristics of both circumferential 

and radial faults. The oblique structures can change 

along strike from faulted-fold type, to narrow zones of 

steeply dipping faults that contain thin slices of uplifted 

formations.

The timing of the uplift, volcanism, and gravity sliding 

in the Bears Paw Mountains area is uncertain. The span of 

volcanism is 53 to 49 Ma by conventional K/Ar techniques 

(Marvin and others, 1980), with some supporting paleomagnetic 

data. In the lowermost Bearpaw Mountains volcanic rocks, 

local mafic or mixed mafic-felsic pyroclastic deposits are 

involved in down faulting that is probably related to early 

gravity sliding. The youngest units of the Bearpaw Mountains 

volcanic rocks apparently are unconformable over earlier 

volcanic rocks, but are also faulted and tilted along the border 

of the arch. All of the diatremes and associated dikes in the 

Missouri Breaks area are younger than the gravity sliding, 

based on crosscutting relations. Conventional K/Ar ages of the 

diatremes and dikes are 52 to 47 Ma, which is slightly younger 

than Bears Paw Mountains igneous activity (Marvin and 

others, 1980). Several diatremes contain pebbles and cobbles 

of Bearpaw Mountains intrusive and volcanic rocks, and one, 

the Lone Tree Ridge diatreme, contains down-faulted slices of 

clastic outwash deposits derived from erosion of the Bearpaw 

Mountains volcanic rocks. These clastic deposits are conglomeratic sandstones that contain pebbles of mafic phonolite and 

latite, and contain distinctive clasts of mafic analcime phonolite, porphyritic syenite, and tinguaite from the two youngest 

series in the Bears Paw Mountains. The sand-size fraction is 

rich in dark green augitic clinopyroxene, typical of presentday stream sediments coming from the Bearpaw Mountains 

volcanic fields (B. Carter Hearn, Jr., field observations).

Latest Cretaceous to Paleocene Intrusive 

Centers

Igneous rocks are mainly in two age groups: latest 

Cretaceous to Paleocene, and middle Eocene. Some outlying 

isolated intrusions are undated or are considerably younger, 

such as the Smoky Butte lamproite, which is dated at 27 Ma 

(Marvin and others, 1980; O’Brien and others, 1995).

Igneous rocks in the four intrusion-cored uplifts of the 

Little Rocky, Judith, and North and South Moccasin Mountains (fig. 3B) have ages from latest Cretaceous to Paleocene, 

68 to 60 Ma. Igneous rocks generally are a sequence of syenite 

porphyries, quartz syenite porphyries, aegirine trachytes, or 

aegirine syenite porphyries. All four intrusion-cored uplifts 

have gold-silver mineralized areas of altered porphyries that 

are past producers. Skarn deposits are lacking or rare. In the 

Judith Mountains, three gold-silver-bearing skarn deposits 

occur in the Linster Peak dome in Mississippian limestone 

(Woodward, 1995). No skarn deposits are known in the Little 

Rocky or Moccasin Mountains (Weed and Pirsson, 1896; 

Wampler, 1994).

The intrusive sequence in the Little Rocky Mountains 

is quartz-phyric syenite porphyries, then syenite porphyries, 

followed by late aegirine trachyte and syenite porphyry 

dikes. The gold-silver mineralization is associated in part 

with the latest dikes (Wilson and Kyser, 1988; Rogers and 

Enders, 1990).
Description of Geology 11

Middle Eocene Intrusive and Eruptive Centers

The Bears Paw Mountains, Highwood Mountains, Eagle 

Buttes, Little Belt Mountains, and Sweet Grass Hills (110 km 

west-northwest of Havre) are all middle Eocene in age, ranging 

from approximately 53 to 49 Ma (Marvin and others, 1973; 

Marvin and others, 1980). Only the Bears Paw and Highwood 

Mountains have volcanic flows and fragmental deposits, in 

addition to their intrusive equivalents. Known mineralization 

is minor compared to the Little Rocky, Judith, and Moccasin 

Mountains (Bateman and others, 1977; Bateman and 

Yamamoto, 1978).

Missouri Breaks Diatremes

The Missouri Breaks diatremes are named for the Missouri River Breaks, the intricately eroded badlands area along 

the Missouri River between Fort Benton and Fort Peck in 

north-central Montana where many of these bodies occur (fig. 2). 

These diatremes and associated intrusions provide evidence 

for mechanisms of eruption and emplacement of volatile-rich 

magmas, and demonstrate the genetic connections among 

melnoitic, kimberlitic, and carbonatitic magmas.

Diatremes and dikes produced by mantle-derived alkalic 

ultramafic magmas occur in a 140-km-long band that trends 

N. 80° E. from Haystack Butte on the southeast side of the 

Highwood Mountains to the southeast side of the Little Rocky 

Mountains (Hearn, 1968, 1979). Approximately 50 diatremes 

(including satellitic pipes) and 15 major dikes are concentrated 

in the eastern half of the band (fig. 3B). Additional undiscovered 

diatremes could be concealed by terrace gravels and glacial 

deposits in the western half of the band. Additional diatremes 

and dikes could be present beneath surficial deposits east of 

the Little Rocky Mountains, although existing geologic maps 

show no igneous features. Diatremes were emplaced between 

52 and 47 Ma (Marvin and others, 1980), following most, if 

not all, of the igneous activity in the Bears Paw Mountains, 

Highwood Mountains, and Eagle Buttes. Many of the diatremes contain inclusions of peripheral down-faulted slices 

of early Eocene Wasatch Formation, and some contain clasts 

of some of the youngest intrusive rocks (51 to 49 Ma) of the 

Bearpaw Mountains igneous complex. Some diatremes cut 

across the distal faults and folds in the gravity-slide sheets.

The dominant trends of dikes, and of some groups of 

three to four diatremes, are from about N. 60° E. to N. 45° E., 

more northerly than the N. 80° E. overall trend of the band. 

These dike trends probably reflect only the shallow stress patterns and fracture alignments within the gravity-slide sheets, 

normal to the dominant directions of movement (S. 30° E. to 

S. 45° E.). However, the overall N. 80° E. trend of the band 

of diatremes represents a major zone of access of deep-source 

magmas, and thus indicates the deeper stress field and trend 

of a zone of weakness in the lithosphere. This trend differs 

somewhat from aeromagnetic trends (N. 30–60° E., Zietz 

and others, 1980) and gravity anomaly trends (Peterson and 

Rambo, 1967; Smith, 1970) in the Precambrian basement 

terrane, as well as from trends of major tectonic lineaments 

(Thomas, 1974; O’Neill and Lopez, 1985).

Stratigraphy

The vast majority of the study area is underlain by 

sedimentary rocks. The Cretaceous Bearpaw Shale is the most 

widespread formation; it crops out at the surface over most of 

the study area (fig. 4). The Cretaceous Judith River Formation 

crops out in the northeastern part of the study area; the Cretaceous Fox Hills and Hell Creek Formations and the Paleocene 

Fort Union Formation crop out locally in the southeastern part 

of the study area. Pleistocene glacial deposits and Quaternary 

alluvium, sand, and gravel are also widely distributed throughout the study area, where they typically are found in current 

and former river and stream valleys.

Pleistocene Glacial Deposits

Continental glaciers of Late Illinoian age (~190–127 ka) 

and Late Wisconsin age (~24–11 ka) (Davis and others, 2006) 

advanced from the north and northwest, damming and diverting the existing major drainages. This produced large glacial 

lakes, which then overtopped topographic divides and eroded 

new channels. Ground moraine, glaciofluvial deposits, and icemargin features are common (Knechtel, 1959; Davis and others, 2006). Glaciers wrapped around the Bears Paw and Little 

Rocky Mountains, but did not entirely surround them. A large 

glacial lake, Lake Musselshell, formed south of the Laurentide 

ice sheet; the lake transported ice-rafted boulders and till 

that are now isolated occurrences in unglaciated areas (Davis 

and others, 2006). Large flat-floored valleys that are glacial 

meltwater channels are locally present. Younger postglacial 

drainages have reoccupied the preglacial courses in some 

areas. Larger areas of ice-rafted till have produced problems 

in delineating the maximum extent of glacial ice advance (for 

example, Gowan, 2013, and references therein).

Paleocene Sedimentary Rocks

The Fort Union Formation contains gray to greenish-gray 

smectitic shale and mudstone, and gray to yellow, very fineto medium-grained sandstone. The unit locally contains coal 

beds. The formation is approximately 150 m thick in the study 

area, though elsewhere it ranges to 200 m thick (Wilde and 

Vuke, 2004a).
12 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

"

"

"

"

"

MONTANA

WYOMING

WELDON - BROCKTON - FROID FAULT ZONE

Little Rocky

Mountains

Highwood

Mountains

Cat Creek

Lake Basin Fault Zone

C E N T R A L M O N T A N A T R O U G H

Medicine Hat block

Wyoming craton

A

WILLISTON BASIN

Rocky Mountain Front

Little Belt

Mountains

Fault Zone

112° 108° 104°

48°

46°

MONTANA

IDAHO

WYOMING

OREGON

NEVADA

WASHINGTON

UTAH

ALBERTA SASKATCHEWAN

COLORADO

Area of map A

Area of map B

Study area

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

0 40 80 120 160 KILOMETERS

0 40 80 MILES

Fault

Southwestern margin of 

 Williston Basin

USGS study area boundary

EXPLANATION

Medicine Hat block

Trans-Hudson orogen

Wyoming craton

Paleoproterozoic and Archean 

 rocks

Mesoproterozoic rocks (includes 

 Belt Basin)

Wallace terrane

Imbricately intercalated rocks

Tectonically shortened rocks

Great Falls Tectonic Zone

Bedrock exposure

Inferred basement

Paleoproterozoic orogens

Archean cratons

Wallace Terrane

GREAT FALLS TECTONIC ZONE

Figure 3. Maps showing tectonic and basement terrane setting (A) and geologic setting (B, facing page) of the North-Central 

Montana Sagebrush Focal Area (modified from Dobbin and Erdmann, 1955; St. John, 1999; Sims and others, 2004; Vuke and 

others, 2007; Woolley and Kjarsgaard, 2008; Garrity and Soller, 2009; Faure, 2010; U.S. Geological Survey, 2016b). USGS, U.S. 

Geological Survey.
Description of Geology 13

Missouri River

Milk River

Devil’s B asin – Big Wall

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Marysville

Havre

Butte

Hardin Laurel

Helena

Glasgow

Bozeman

Belgrade Billings

Cut Bank

Lewistown

Livingston

Deer Lodge

Great Falls

Little Rocky

Mountains

Grassrange

intrusions

North Moccasin

Mountains

South Moccasin

Mountains

Little Belt

Mountains

Highwood

Mountains

Bowdoin

Dome

Porcupine

Dome

Freedom

Grass Range Dome

Bearpaw Mountains arch

Rocky Mountain Front

Cat Creek Fault Zone

Rocky Boy’s

Stock

Fort Peck Lake

Deerborn River

B

Lake Basin Fault Zone

C E N T R A L M O N T A N A T R O U G H

Eagle Buttes

112° 108°

48°

46°

Musselshell River

Yellowstone River

Bighorn River

Marais River

Cat Creek anticline

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

0 30 60 90 120 KILOMETERS

0 30 60 MILES

Missouri River Breaks intrusions

USGS study area boundary

Fault

Quaternary deposits

Tertiary intrusive igneous rocks

Tertiary volcanic igneous rocks

Neogene sediments and sedimentary rocks

Paleogene and undivided Tertiary sedimentary rocks

Cretaceous to Tertiary volcanic igneous rocks

Cretaceous to Tertiary intrusive igneous rocks

Late Cretaceous igneous and sedimentary rocks

Late Cretaceous sedimentary rocks

Triassic to Early Cretaceous sedimentary rocks

Paleozoic sedimentary rocks

Mesoproterozoic metamorphic and sedimentary

 rocks and associated sills

Paleoproterozoic rocks

Archean rocks

Domes

Anticline

Rocky Boy’s Stock

Other intrusive centers, diatremes, 

 and dikes

EXPLANATION
14 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

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Roy

Hays

Malta

Lodge Pole

Missouri

River

10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 0

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Little Rocky Mountains

Figure 4. Map showing geology of the North-Central Montana Sagebrush Focal Area (modified from Vuke and others, 

2007). The gravity-slide structures referred to in the text are the array of faults to the west and southwest of the Little 

Rocky Mountains. USGS, U.S. Geological Survey.
Description of Geology 15

107°0'

!

!

Fort Peck Lake

Glasgow

Jordan

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

EXPLANATION

USGS study area boundary

Fault

Surficial deposits

Eocene and younger sedimentary rocks

Tertiary plutonic igneous rocks

Tertiary volcanic igneous rocks

Paleocene Fort Union Formation

Hell Creek and Fox Hills Formations

Bearpaw Shale

Judith River Formation and Claggett Shale

Lower to Upper Cretaceous Fall River and 

 Eagle Sandstones

Upper Jurassic to Lower Cretaceous sedimentary 

 rocks

Triassic and older sedimentary rocks

Precambrian metamorphic rocks

Upper Cretaceous
16 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Cretaceous Sedimentary Rocks

Cretaceous rocks form the bedrock throughout the majority of the study area, and are also volumetrically important in 

the subsurface. These include, from youngest to oldest, the 

Hell Creek Formation, Fox Hills Formation, Bearpaw Shale, 

Judith River Formation, Claggett Shale, Eagle Sandstone, 

Telegraph Creek Formation, Niobrara Formation, Carlile 

Shale, Greenhorn Formation, Belle Fourche Shale, Mowry 

Shale, Thermopolis Shale, Fall River Sandstone, and Kootenai 

Formation (fig. 5, however, the Hell Creek and Fox Hills 

Formations are not shown).

The Hell Creek Formation is predominantly gray to lightbrown sandstone, silty shale, and mudstone; it locally contains 

thin, lenticular coal beds or carbonaceous shale. The sandstone 

is fine to medium grained, and calcium-carbonate cemented 

concretions are typical in the fine-grained sandstone. The Hell 

Creek Formation is Late Cretaceous in the study area, though 

elsewhere it ranges to Paleocene in age, and locally it contains 

the osmium- and iridium-rich Chicxulub impact claystone 

(Moore and others, 2014). The contact with the underlying 

Fox Hills Formation may be either gradational or erosional. 

The average thickness in the study area is approximately 90 m 

(Johnson and Smith, 1964; Bergantino, 1999; Wilde and 

Vuke, 2004a).

The Upper Cretaceous Fox Hills Formation is lightbrown to light-gray, thin- to thick-bedded, micaceous, fine- to 

medium-grained sandstone with ferruginous concretions in 

the upper part and thin-bedded siltstone and silty shale in the 

lower part. The average thickness in the study area is approximately 30 m (Knechtel, 1959; Porter and Wilde, 1993; Wilde 

and Vuke, 2004b).

The Bearpaw Shale is a medium- to dark-gray, fissile 

shale and mudstone that underlies low, sage-covered, gently 

rolling topography across most of the study area; it also forms 

the high bluffs and broken topography known as the “Missouri 

Breaks” along the Missouri River. Thin white bentonite layers 

are common throughout the Bearpaw, and in places these are 

thick enough to form the bentonite deposits that are described 

elsewhere in this report. Swelling clay minerals may locally 

produce “popcorn” weathering. Calcareous concretions are 

common in the unit, and in places they form mappable marker 

horizons. In most places in the study area, the top of the 

Bearpaw Shale has been eroded, but the average thickness of 

the entire unit in the region is approximately 350 m (Knechtel, 

1959; Bergantino, 1999; Wilde and Vuke, 2004b).

The Judith River Formation is made up of light-colored 

interbedded sandstone, siltstone, sandy mudstone, claystone, 

and shale that record deposition in marine and terrestrial environments. The unit thickens westward in the study area, and 

ranges from 15 to 180 m thick (Knechtel, 1959; Rice, 1979; 

Porter and Wilde, 2001). Recent work in the western part of 

the study area has provided new insights into the stratigraphy 

and sedimentology of the Judith River Formation and has 

established the age of the contact between the Judith River 

Formation and Bearpaw Shale at approximately 75.2 Ma, 

based on 40Ar/39Ar dates of a bentonite unit at the base of the 

Bearpaw Shale (Rogers and others, 2016).

The Claggett Shale is dark-gray, thinly bedded shale with 

calcareous concretions, numerous bentonite beds and some 

thin, lenticular, siltstone and fine-grained sandstone beds. The 

Ardmore Bentonite Bed, which is a 9- to 12-m-thick zone of 

bentonite beds, occurs near the base of the formation and is a 

regionally significant marker horizon. The Claggett Shale thins 

eastward in the study area, but it is on average approximately 

130 m thick (Alverson, 1965; Wilde and Vuke, 2004b).

The Eagle Sandstone contains two units in the study area. 

The upper unit contains light-gray sandy shale, siltstone, and 

thin sandstone beds. The lower unit contains a cross-stratified, 

fine-grained, light-yellow to buff-weathering sandstone unit 

that is mapped elsewhere as the Virgelle Sandstone. The total 

thickness of the Eagle Sandstone in the study area ranges from 

60 to 90 m (Rice, 1976; Porter and Wilde, 2001; Wilde and 

Vuke, 2004b).

The Telegraph Creek Formation consists of light-gray 

to yellowish-gray sandy shale, siltstone, and thin-bedded 

sandstone with ironstone concretions in its lower part. The unit 

is approximately 50 m thick (Johnson and Smith, 1964; Wilde 

and Vuke, 2004b).

The Niobrara Formation contains calcareous and noncalcareous gray to dark-gray shale, siltstone, thin bentonite beds, 

and gray to brown calcareous or ferruginous concretions. The 

middle part of the formation contains the MacGowan Concretionary Bed, which is grayish-brown concretionary dolostone 

and limestone with phosphatic pellets and gray to black chert 

pebbles. The overall thickness in the study area is poorly constrained, but it may range to 100 m (Wilde and Vuke, 2004b; 

Vuke and others, 2007). This marine formation has a rich 

collection of vertebrate and invertebrate fossils, which provide 

well-defined biostratigraphic zones. Recent geochronology, 

using paired U-Pb analyses of zircon and 40Ar/39Ar analyses 

of sanidine from ash beds, has provided refined ages of 89.75 

± 0.38 Ma for the Turonian-Coniacian boundary, 86.49 ± 0.44 

Ma for the Coniacian-Santonian boundary, and 84.19 ± 0.38 

Ma for the Santonian-Campanian boundary (Sageman and 

others, 2014).

The Carlile Shale is a marine, dark-gray, noncalcareous, 

sandy shale. The upper two-thirds of the formation contain 

abundant calcareous concretions. The overall thickness in the 

study area is approximately 90 m (Wilde and Vuke, 2004b; 

Vuke and others, 2007).

The Greenhorn Formation is marine, gray to light-gray 

calcareous shale and shaly marl with local thin beds of limestone 

that range from 13 to 65 m thick in the study area (Rice, 1979; 

Wilde and Vuke, 2004b; Vuke and others, 2007).

The Belle Fourche Shale is a medium-gray to black 

marine shale that is up to 90 m thick in the study area. It 

contains ironstone concretions and numerous bentonite beds 

(Rice, 1979; Vuke and others, 2007).

The Mowry Shale is light-gray to dark-gray siliceous 

shale and subordinate thin-bedded, gray siltstone or very finegrained sandstone. Bentonite is common in the formation, and 
Leasable Minerals 17

EXPLANATION

Surficial deposits

Shale

Siltstone

Sandstone

Limestone

Dolomite

Sandy shale Bentonite

83

37

83

46

92

27

182

55

80

23

100

175

25

46

55

130

53

84

120

117

154

80

Thickness,

in meters

Cretaceous Jurassic Mississippian Devonian Ordovician

Madison Group Montana Group Ellis Group

Lithology

Surficial deposits

Bearpaw Shale

Judith River

Formation

Claggett Shale

Eagle Sandstone

Niobrara Formation

Carlile Shale

Greenhorn Formation

Belle Fourche Shale

Telegraph Creek Formation

Mowry Shale

Thermopolis Shale

Fall River Sandstone

Kootenai Formation

Morrison Formation

Swift Formation

Rierdon Formation

Mission Canyon

Limestone

Lodgepole

Limestone

Three Forks Shale (?)

Jefferson Formation

Maywood Formation

Bighorn Dolomite

Emerson Formation

Group

System

Formation 

and member

Figure 5. Lithostratigraphic column for the North-Central Montana Sagebrush Focal Area, showing 

stratigraphy exposed in Continental Oil Company’s South Zortman well 1 (dry hole SZ-1), Phillips County, 

Montana (modified from Knechtel, 1959).
18 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

the Clay Spur Bentonite Bed is a widespread marker bed. The 

Mowry Shale is marine in origin, and it ranges from 30 to 140 m 

thick in the study area (Knechtel, 1959; Rice, 1979; Vuke and 

others, 2007).

The Thermopolis Shale is dark-gray to black marine shale 

with subordinate siltstone beds. The unit contains many thin 

bentonite beds and also many small ferruginous concretions. 

It ranges from 70 to 180 m thick in the study area (Knechtel, 

1959; Vuke and others, 2007).

The Fall River Sandstone is light-gray to brown, fine- to 

medium-grained, quartzose sandstone with thin interbeds of 

dark-gray shale and siltstone. The unit was deposited in a 

nearshore marine environment, and it ranges from 11 to 30 m 

thick in the study area (Rice, 1979; Vuke and others, 2007).

The Kootenai Formation has a basal sandstone member 

that is commonly conglomeratic, and rests unconformably 

on the underlying Morrison Formation. The upper member is 

mottled, light- to dark-gray siltstone and shale with interstratified sandstone. The Kootenai ranges from 60 to 160 m thick in 

the study area (Knechtel, 1959; Rice, 1979).

Jurassic Sedimentary Rocks

Jurassic sedimentary rocks in the study area include the 

Morrison, Swift, and Rierdon Formations. These units contain 

an important lithostratigraphic change from younger shale and 

sandstone to older limestone; this transition marks a change 

from a marine shelf environment, where the limestone formed, 

to a younger terrestrial to shallow-marine environment, where 

the sandstone and shale formed.

The Morrison Formation is light-gray mudstone with 

subordinate sandstone and siltstone beds, and thin beds 

of coal. It ranges from 10 to 60 m thick in the study area, 

and it was deposited in lacustrine and fluvial environments 

(Knechtel, 1959; Rice, 1979; Vuke and others, 2007).

The Swift Formation has a light- to dark-gray shale basal 

member that is overlain by an upper member containing lightgray to greenish-gray glauconitic, flaggy-bedded, commonly 

fossiliferous, fine-grained sandstone with subordinate dark 

gray shale interbeds. The unit is 30 to 120 m thick, and it was 

deposited in a shallow marine environment.

The contact between the Swift Formation and the underlying Rierdon Formation marks an important transition from 

predominantly shaly siliciclastic units above to predominantly 

limestone and dolomite units below (Knechtel, 1959; Rice, 

1979; Vuke and others, 2007).

The Rierdon Formation is light- to dark-gray, locally 

fossiliferous limestone. The lowermost part of the unit consists 

of greenish gray calcareous shale. The Rierdon Formation 

was deposited in lagoonal and marine-shelf environments, and 

it ranges from 30 to 60 m thick in the study area (Knechtel, 

1959; Rice, 1979; Vuke and others, 2007).

Mississippian Sedimentary Rocks

The Mission Canyon Limestone is gray, relatively pure, 

massive to locally crossbedded limestone with chert beds and 

nodules, and solution cavities. The unit formed in a shallow 

marine environment, and it ranges from 120 to 220 m thick in 

the study area (Knechtel, 1959; Rice, 1979; Vuke and others, 

2007).

The Lodgepole Limestone is dark- to light-gray, 

predominantly thin-bedded to massive limestone that typically 

contains chert lenses and interstratified shale partings. The unit 

formed in a shallow marine environment, and it ranges from 

160 to 200 m thick in the study area (Knechtel, 1959; Rice, 

1979; Vuke and others, 2007).

Devonian Sedimentary Rocks

The Three Forks Shale contains light-gray to light-green 

dolomitic to calcareous shale and siltstone that is locally 

sandy. The unit formed in marine to restricted-marine environments, and it ranges from 20 to 30 m thick in the study area 

(Knechtel, 1959; Rice, 1979; Vuke and others, 2007).

The Jefferson Formation has the 10- to 20-m-thick 

Birdbear Member in its upper part. The Birdbear is light- to 

medium-gray, sucrosic dolomite. The lower part of the Jefferson 

Formation, which forms approximately 90 percent of the unit, 

consists of dark-gray to brownish-gray finely crystalline limestone. The Jefferson Formation formed in a marine environment, and it ranges to 200 m thick in the study area (Knechtel, 

1959; Rice, 1979; Vuke and others, 2007).

The Maywood Formation is gray to green to red to yellow 

to brown thin-bedded dolomitic limestone and dolomite that 

are interstratified with calcareous shale. This unit was deposited in a shallow marine environment, and it is approximately 

40 to 50 m thick in the study area (Knechtel, 1959; Vuke and 

others, 2007).

Ordovician Sedimentary Rocks

The Bighorn Dolomite is dappled-gray to very pale 

orange crystalline dolomitic limestone that locally contains 

sandstone near its base. The unit formed in a marine shelf 

environment, and it ranges from 20 to 80 m thick in the study 

area (Knechtel, 1959; Rice, 1979; Vuke and others, 2007).

The Emerson Formation is dark-gray to green shale with 

upward-increasing interbeds of light-gray to white glauconitic 

sandstone, sandy and glauconitic marlstone, and fossiliferous 

limestone. It ranges from 290 to 340 m thick in the study area 

(Knechtel, 1959; Rice, 1979).
Leasable Minerals 19

Cambrian Sedimentary Rocks

The Flathead Sandstone consists mainly of light-gray, 

green, and tan sandstone. It is partly argillaceous, and contains 

some interbedded fine-grained conglomerate. The thickness 

is approximately 15 m in the Little Rocky Mountains 

(Knechtel, 1959).

Precambrian Sedimentary Rocks

Exposed Precambrian rocks in the Little Rocky Mountains 

are metasedimentary and metavolcanic (Knechtel, 1959; 

Peterman, 1981). The metasedimentary rocks are felsic 

gneisses and schists that contain quartz, potassium feldspar, 

and plagioclase, with varying amounts of biotite, muscovite, 

chlorite, hornblende, garnet, and kyanite, and with accessory 

apatite, zircon, magnetite, and pyrite. The metavolcanic rocks 

are mafic hornblende-plagioclase and hornblende-plagioclasegarnet gneisses that are generally fine grained.

Leasable Minerals

Leasable minerals include leasable fluid and solid minerals (Mineral Leasing Act, as amended, February 1920; 43 CFR 

3000–3599, 1990). Leasable fluid minerals include oil and gas, 

and geothermal resources, and leasable solid minerals include 

coal, oil shale, native asphalt, phosphate, sodium, potash, 

potassium, and sulfur (appendix 1 in Day and others, 2016). 

Oil shale, native asphalt, sodium, potassium, and sulfur do not 

occur within or near the study area, so they are not considered 

further. This section describes potash, coal, geothermal, oil 

and gas, and phosphate occurrences.

Potash

No significant potash salt occurrences are known within 

the North-Central Montana Sagebrush study area. The nearest 

potash resources are more than 100 km to the northeast in 

Paleozoic strata near the western margin of the Williston Basin 

(fig. 6; Anderson and Swinehart, 1979; Kruger, 2014). A potash tract referred to as the Belle Plaine was identified by Orris 

and others (2014) in the northwestern part of North Dakota 

and the northeastern part of Montana that coincide with the 

Williston Basin.

The Williston Basin potash deposits are an extension 

of large potash deposits that are mined extensively in the 

Elk Point Basin in Saskatchewan Province, Canada (fig. 6). 

Potash resources in the Williston Basin are possibly the largest 

known, but largely unexploited potash resources in the United 

States and are in the Devonian Prairie Formation of the Elk 

Point Group (Kruger, 2014). The great depth (>3 km) of the 

Prairie Formation within parts of the Williston Basin would 

limit potash recovery to in place solution mining. The Williston 

Basin potash deposits are of the marine, stratabound type that 

formed during retreat and evaporation of a Devonian seaway 

(Peterson and MacCary, 1987). These types of deposits, which 

formed during evaporation of Paleozoic intracontinental 

seaways, tend to be the largest sources of potash mined today. 

In comparison, the Quaternary, closed-basin brine potash 

sources, such as those that are associated with the Pleistocene 

Lake Bonneville and Holocene Great Salt Lake in Utah 

(Warren, 2010), are much smaller.

Coal

The USGS has not conducted a formal quantitative coal 

resource assessment for the North-Central Montana Sagebrush 

study area. However, a series of four USGS Leasable Mineral 

and Waterpower Land Classifications maps indicated large 

areas of coal-bearing formations surrounding and somewhat 

overlapping the study area (Bateman and Lutz, 1976; Bateman 

and Allen, 1977, 1978; Bateman and others, 1977). The 

more recent “Coal fields in the conterminous United States” 

(East, 2013) more precisely defined the coal-bearing areas. 

The report shows that the westernmost part of the study 

area falls within the North-Central coal region (fig 7). Four 

additional coal-bearing areas are outside of the study area 

but partially overlap the southwest, southeast, and east parts 

of the map area shown in figure 7: the Great Falls coal field, 

Garfield County coal field, Powder River coal region, and 

the Fort Union coal region, respectively. These areas contain 

coal of lignite to bituminous rank in Jurassic to Tertiary 

sedimentary rocks. As discussed below, the coal beds within 

the study area are not likely to generate economic interest in 

the future. 

The coal beds in the North-Central coal region are Late 

Cretaceous in age, and are subbituminous in rank (Combo and 

others, 1949; Wilde, 2010). The coal beds in the North-Central 

coal region are restricted to two coal-bearing formations—the 

Judith River Formation and the Eagle Sandstone—that are 

separated stratigraphically by marine shales and thin sandstone 

beds of the Claggett Shale. Brief descriptions of the Judith 

River Formation and the Eagle Sandstone are provided by 

Vuke and others (2007). Both units have thin, laterally discontinuous coal beds that are interbedded with sandstones, sandy 

shales, and carbonaceous shale. In Montana, the Judith River 

Formation reaches a maximum thickness of approximately 

300 m. Most of the coal in the Judith River Formation occurs 

in two thin discontinuous beds in the upper 20 m of the formation where the thickness of the two coal beds ranges from 0.8 

to 2.0 m of impure coal containing bone coal partings; these 

coal beds have been mined for local ranch use, predominantly 

for home heating (U.S. Geological Survey and Montana 

Bureau of Mines, 1963).

In Montana, the Eagle Sandstone reaches a maximum 

thickness of approximately 150 m, and it contains thin, laterally discontinuous lenses of coal in a 25 m layer of carbonaceous shale in the upper part of the unit (Vuke and others, 

2007). Owing to the thinness and poor quality of the coal, 

mining has been limited to that needed for local use. Coal 

beds in the Eagle Sandstone near the Bears Paw Mountains 
20 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

LAKE

WINNIPEG

MONTANA

ALBERTA

MANITOBA

SASKATCHEWAN

NORTH DAKOTA

IDAHO SOUTH DAKOTA

WYOMING

vi Riruossi M

re Saska ct hewan River Fargo Regina Calgary Winnipeg Bismarck

Billings

Missoula

Edmonton

Red Deer

Saskatoon

Lethbridge

Grand

Forks

Great Falls

Medicine Hat

100°0' 110°0'

54°0'46°0' APPROXIMATE OUTLINE OF WILLISTON BASIN

0 80 160 240 MILES0 80 160 240 320 KILOMETERS

Boundary data from San Juan and others (2016). Political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

EXPLANATION

USGS study area boundary

AnhydritePotash, halite, and anhydriteHalite and anhydrite

Elk Point Basin

Approximate outline of Williston Basin

UNITED STATES

OF AMERICA

CANADA

Carbonate rocks, anhydrite and  minor basin edge clastic rocks

PACIFIC

OCEAN

Study area Map area LAKE MANITOBA

Figure 6. Map showing the approximate extent of the Williston Basin and evaporite and carbonate rocks of the Elk Point Basin (modified from Worsley and Fuzesy, 1979; 

St. John, 1999). USGS, U.S. Geological Survey.
Leasable Minerals 21

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

Roy

Hays

Malta

Jordan

Glasgow

Lodge Pole

2

191

191

200

107°0' 109°0' 108°0'

48°0'

47°30'

NORTH

DAKOTA

SASKATCHEWAN

SOUTH

DAKOTA

WYOMING

NEBRASKA

MONTANA

IDAHO

ALBERTA

CANADA

Study area

Area of map

0 10 20 30 MILES0 20 40 KILOMETERS

40 Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

EXPLANATION

Proposed withdrawal  area

North-Central

Powder River

Garfield CountyGreat Falls

Fort Union

USGS study area boundary

Coal field

Coal region

Missouri i R ver

Little Rocky

Mountains

Fort Peck

aL

ek

Fort Peck

aL

ek

Figure 7. Map of the North-Central Montana Sagebrush Focal Area showing areas with coal-bearing formations (from East, 2013). USGS, U.S. Geological Survey.
22 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

have not been mined since 1925 (U.S. Geological Survey and 

Montana Bureau of Mines, 1963).

The coal beds of the Judith River Formation and the 

Eagle Sandstone within the study area are exposed at the 

surface along a narrow zone where Cretaceous rocks are 

upturned along the margin of the uplift associated with the 

doming and emplacement of the syenite in the Little Rocky 

Mountains. Away from the Little Rocky Mountains uplift area, 

to the north, east, and south, the Judith River Formation and 

the Eagle Sandstone are relatively flat lying and underlie the 

Bearpaw Shale (commonly 350 m thick in the study area) and 

Quaternary alluvium deposits (fig. 5; Porter and Wilde, 2001; 

Vuke and others, 2007). Owing to the thinness, lack of lateral 

continuity, poor quality, and depth of cover, the coal beds 

in both the Judith River Formation and the Eagle Sandstone 

within the study area are not likely to generate economic 

interest in the future.

Geothermal

This section characterizes the nature and occurrence 

of low-temperature geothermal resources, the geothermal 

resource potential, or favorability, of moderate- to hightemperature hydrothermal systems, and the resource potential 

for enhanced geothermal systems (EGSs) within the NorthCentral Montana Sagebrush Focal Area. Enhanced geothermal 

systems are geothermal resources that possess the necessary 

heat, but lack sufficient permeability to generate electric 

power. In these systems, a geothermal reservoir is engineered 

(through hydraulic, chemical, or thermal stimulation) to create 

sufficient permeability to support fluid circulation that can 

effectively mine heat from the surrounding rock.

Geology and Occurrence

Inventories of geothermal occurrences in Montana (Reed 

and others, 1983; Lienau and Ross, 1996; Metesh, 2000) 

identify only one thermal spring, Landusky Plunge Springs, 

with a temperature of 24 °C, and four thermal water wells with 

temperatures ranging from 16 to 27 °C within the study area. 

There are several other warm springs and wells in and near 

the Little Rocky Mountains (fig. 8), but there are no known 

moderate- to high-temperature geothermal systems in the area. 

The nearest known moderate- to high-temperature geothermal 

system is more than 200 km to the west of the study area at 

Marysville, Montana. This relative absence of geothermal 

manifestations is consistent with the relatively low values of 

favorability for the occurrence of moderate- and high-temperature 

geothermal systems across the region spanned by the study 

area (fig. 8). This low favorability reflects the relative absence 

of factors associated with the formation of moderate- and 

high-temperature geothermal systems, such as seismicity, 

Quaternary faulting, Quaternary magmatic activity, a dominantly extensional crustal stress regime, or high crustal heat 

flow (Williams and DeAngelo, 2008).

Exploration and Development

Exploration for geothermal systems within and near the 

study area have been limited to three crustal heat flow measurements approximately 100–150 km to the south and four 

measurements approximately 150–200 km to the west (Williams 

and DeAngelo, 2011; locations shown on figure J2 of Glen and 

others, 2016). The three measurements to the south range from 

43 to 51 mW/m2

, with an average of 46 mW/m2. The four 

measurements to the west are more variable, ranging from 67 

to 100 mW/m2

, with an average of 77 mW/m2. Based on these 

relatively distant observations alone, it is uncertain what the 

average value of crustal heat flow is for the region covered by 

the study area.

A compilation of bottom-hole temperature (BHT) records 

from oil and gas exploration wells and estimates of geothermal 

gradients within the associated sedimentary formations has 

been published by Gunderson (2011). For wells in the study 

area, BHTs range from 73 to 85 °C over a depth interval from 

2,300 to 2,800 m (Gunderson, 2011). Although measured 

BHTs typically underestimate true formation temperatures, 

corrections to these data will be on the order of 10 to 15 °C 

hotter. These results are consistent with the observation from 

the sparse heat flow data that the study area is not characterized by high temperatures at shallow depths typically required 

for commercial geothermal operations. Implications for lower 

temperature geothermal applications are discussed below.

Results of Previous Assessments

A previous USGS geothermal resource assessment (Williams

and others, 2008) did not locate any identified moderate- and 

high-temperature geothermal systems within the study area. 

The assessment for undiscovered resources was based in part 

on a series of GIS logistic regression analyses through which 

geothermal potential was modeled using a weighted combination of evidence layers derived from mappable geologic and 

tectonic features available in digital databases (Williams and 

DeAngelo, 2008; Williams, Reed, and others, 2009). Figure 

8 illustrates the distribution of relative geothermal potential 

from these analyses across the region encompassing the 

study area. The low values are consistent with the absence 

of any identified moderate- and high-temperature 

geothermal systems.

Low-temperature geothermal resources of this region 

were assessed by Reed (1983), with inventories of lowtemperature springs and wells published by Reed and others 

(1983), Lienau and Ross (1996), and Metesh (2000). As noted 

above, one thermal spring and four thermal wells are located 

within the study area (table 2). In addition, Reed (1983) 

considered the low-temperature potential associated with 

thermal aquifers in the deeper sedimentary units of the region. 

These aquifers could be exploited locally for direct use applications, and it is conceivable that under more favorable commercial conditions, geothermal power projects similar to the 

Rocky Mountain Oilfield Testing Center-Powder River Basin 
Leasable Minerals 23

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!

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY PETROLEUM COUNTY

Fort Peck

aL

ek

Fort Peck

aL

ek

Roy

Hays

Malta

Jordan

Glasgow

Lodge Pole

28

26

20

24

Landusky Plunge

Springs

Missouri i R ver

Little Rocky

Mountains

2

191

191

200

107°0' 109°0' 108°0'

48°0'

47°30'

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

EXPLANATION

USGS study area boundaryProposed withdrawal  areaGeothermal favorability

0.1–0.5<0.1

! Low-temperature thermal spring, 

 showing temperature (°C)Low-temperature thermal well ^

26

0 10 20 30 MILES0 20 40

40

KILOMETERS 60

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

Figure 8. Map showing the location of thermal springs and wells, overlain on the logistic regression results for geothermal favorability for the North-Central Montana 

Sagebrush Focal Area (from Williams and others, 2008). Warm springs show temperatures in degrees Celsius. The units for geothermal favorability represent the ratio of posterior probability to prior probability for each cell as derived from a combination of logistic regression models for the occurrence of geothermal systems. The cool colors indicate low favorability for geothermal systems. USGS, U.S. Geological Survey.
24 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 2. Occurrence of identified low-temperature geothermal systems in the North-Central Montana 

Focal Area.

[See figures 8 and 9 for a plot of the locations of identified geothermal systems. Data based on results from Williams and others 

(2008). Name is system name. Type is data type (well or spring). Sources: USGS, Reed and others (1983); Geo-Heat Center (GHC), 

Boyd (2002)]

Name/owner and location Type Source

Mean 

temperature 

(°C)

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Landusky Plunge springs Spring USGS 24 47.843 -108.599

John Matovich; 23 mi SW Sun Prairie, Mont. Well GHC 16 47.683 -108.07

Merlin Busenbark; 1 mi S of Valentine, Mont. Well GHC 27 47.299 -108.421

Jim McCollum; 10 mi NW of Mathison 

Ranch, Mont.

Well GHC 18.8 47.582 -108.718

Whitmayer Assoc.; 4.5 mi SE of Sun Prairie 

School, Mont.

Well GHC 15.6 47.819 -107.629

demonstration project in Wyoming described by Anderson 

(2010) could be developed in Montana. However, no site-specific 

assessment has been conducted in the study area to identify 

permeable thermal aquifers that are associated with oil and gas 

production for which the produced waters are delivered in sufficient quantity and temperature to produce electric power.

In the provisional assessment of EGS resource potential 

(Williams and others, 2008), the cutoff for EGS viability was 

established as a minimum temperature of 150 °C at a depth 

of 6 km (see also Williams and DeAngelo, 2011). Any region 

below that temperature-depth cutoff was not included in the 

assessment. The region covered by the study area falls at or 

just below that cutoff on a national-scale model of temperature at a depth of 6 km (fig. 9, table 3). Although an updated 

version of the model with refined spatial corrections for variations of thermal properties in sedimentary basins (as discussed 

by Williams and DeAngelo [2011]) moves the average into 

the range of 150 to 175 oC, this region cannot be considered as 

having significant EGS resource potential.

Oil and Gas

The North-Central Montana Sagebrush Study Area lies 

within the North-Central Montana and Williston Basin oil 

and gas provinces. According to IHS Enerdeq Well data (IHS 

Energy Group, 2016), there has been no significant oil or gas 

production within the study area. However, there has been gas 

produced adjacent to the study area in fields such as Leroy, 

Vandalia, Ashfield, Bowdoin, and Strater (fig. 10). Bowdoin 

field is the oldest, and it is just north of the study area. It was 

discovered in 1913, and it produces biogenic gas from Cretaceous reservoirs (Dyman, 1995).

The Central-Montana area has been extensively assessed 

for potential hydrocarbon resources. The Williston Basin 

Province, which overlaps the eastern side of the study area, 

was assessed in 1995, and five plays, which are known or 

postulated oil and gas accumulations, were defined in this 

area (Peterson and Schmoker, 1995). These plays include the 

(1) Madison (Mississippian), (2) Red River (Ordovician), 

(3) Middle and Upper Devonian (Pre-Bakken–Post-Prairie 

Salt), (4) Pre-Prairie Middle Devonian and Silurian, and 

(5) Post-Madison through Triassic Clastics. The estimated 

mean resources for these plays within the Williston Basin 

Province are 344 million barrels of oil (MMBO), 99.6 million 

barrels of natural gas liquids (MMBNGL), 374.3 billion cubic 

feet of associated-dissolved gas (BCFG), and 500.2 BCFG of 

non-associated gas (Peterson and Schmoker, 1995). Although 

none of these plays lie directly on the study area, they are 

mentioned owing to their proximity to the study area.

Oil and gas assessments were also conducted by the 

USGS in 1995 in the North Central-Montana Province, which 

overlies the majority of the study area (fig. 10). In 1995, the 

cumulative conventional production in the province was more 

than 440 million MMBO and 1.1 trillion cubic feet of gas, 

and it included the following plays (Dyman, 1995) within the 

sagebrush study area: (1) Cambrian-Ordovician Sandstones, 

(2) Red River Carbonates, (3) Bakken Shale Fracture Systems, 

(4) Devonian-Mississippian Carbonates, (5) Tyler Sandstone, 

(6) Fractured-Faulted Carbonates in Anticlines, (7) JurassicCretaceous Sandstones, (8) Shallow Cretaceous Biogenic Gas, 

(9) Northern Great Plains Biogenic Gas Play with “Moderate 

Potential” (Suffield Block Analog), and (10) Northern Great 

Plains Biogenic Gas Play with “Low Potential”. The assessed 

undiscovered mean resources for these plays are 270 MMBO, 

840 BCFG, and 10 MMBNGL (Dyman, 1995).

In 2000, the USGS assessed the undiscovered biogenic 

(microbial) continuous gas resource potential in the NorthCentral Montana Province and estimated a mean of 6,192 billion 

cubic feet of shallow gas (U.S. Geological Survey NorthCentral Montana Province Assessment Team, 2008). This 

large area includes most of central and eastern Montana. 

Within this assessment, there are seven different assessment 
Leasable Minerals 25

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PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY PETROLEUM COUNTY

Roy

Hays

Malta

Jordan

Glasgow

Lodge Pole

28

26

20

24

Fort Peck

aL

ek

Fort Peck

aL

ek

Missouri i R ver

2

191

191

200

Little Rocky

Mountains

107°0' 109°0' 108°0'

48°0'

47°30'

0 10 20 30 MILES0 20 40

40

KILOMETERS 60

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

EXPLANATION

USGS study area boundaryProposed withdrawal  areaTemperature (°C) at 6 km

175.1 – 200150.1 – 175125.1 – 150100.1 – 125

! Low-temperature thermal spring, 

 showing temperature (°C)Low-temperature thermal well ^

26

Figure 9. Map showing the locations of low-temperature thermal springs and wells overlain on the estimated temperature at 6 km depth, for the North-Central Montana 

Sagebrush Focal Area (modified from Metesh, 2000; Williams and DeAngelo, 2011). Warm springs show temperatures in degrees Celsius. USGS, U.S. Geological Survey.
26 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 3. Statistics of geothermal favorability and temperature at 6-km depth for the North-Central Montana Focal Area, and for the 

entire Western United States.

[Geothermal favorability based on logistic regression analysis results of the 2008 USGS national geothermal assessment (Williams and others, 2008). 

Temperature at 6 km determined from Williams and DeAngelo (2011). NCI, North-Central Montana Focal Area; WUS, Western United States]

Area

Mean 

value of 

favorability

Minimum 

value of 

favorability

Maximum 

value of 

favorability

Standard 

deviation of 

favorability

Mean 

temperature at 

6 km depth (°C)

Minimum 

temperature at 

6 km depth (°C)

Maximum 

temperature at 

6 km depth (°C)

Standard 

deviation of 

temperature at 

6 km depth (°C)

NCI 0.13789559 0.05211746 0.25275114 0.0810685 148.7 138.7 158.2 4.2

WUS 1.21126121 0.01127062 58.6081276 2.04851707 181.7 86.2 290.2 34.5

units (AUs) including the (1) Judith River Formation, (2) Eagle 

Sandstone and Claggett Shale East, (3) Eagle Sandstone and 

Claggett Shale West, (4) Niobrara-Carlile, (5) GreenhornLower Belle Fourche, (6) Greenhorn-Upper Belle Fourche, 

and (7) Bowdoin Dome AUs. The Judith River Formation AU 

was estimated to contain the most undiscovered biogenic gas 

resource with a mean of 1,855 BCFG (U.S. Geological Survey 

North-Central Montana Province Assessment Team, 2008).

The USGS also assessed conventional resources in the 

area and within the Williston Basin Province in 2010. Some of 

the these AUs overlap the North Central-Montana study area 

and include the (1) Mission Canyon-Charles, (2) Lodgepole, 

(3) Red River Fairway, (4) Interlake-Stonewall-Stony Mountain, 

and (5) Winnipeg-Deadwood AUs (see chapters 3 and 4 of 

U.S. Geological Survey Williston Basin Province Assessment 

Team, 2011). The assessed undiscovered mean resources for 

these AUs are 112 MMBO, 451 BCFG, and 50 MMBNGL. 

The study area only covers a small part of these AUs (U.S. 

Geological Survey Williston Basin Province Assessment 

Team, 2011).

All of the 115 PLSS townships that contain proposed 

withdrawal areas are associated with at least one play or AU 

(appendix 3). Although there are many potential resources 

in the area, there has not been any significant hydrocarbon 

production within the study area; the overall potential appears 

to be low. For more details about the oil and gas resource 

assessment of this area, please see the aforementioned USGS 

oil and gas assessment reports (Dyman, 1995; Peterson and 

Schmoker, 1995; U.S. Geological Survey North-Central 

Montana Province Assessment Team, 2008; U.S. Geological 

Survey Williston Basin Province Assessment Team, 2011).

Phosphate

Phosphate is classified by BLM as a non-energy solid 

leasable mineral and is leased under the Mineral Leasing Act 

of 1920, as amended, and is not directly related to energy 

production. Phosphate is administered by BLM pursuant to 

43 CFR 3500—“Leasing of solid minerals other than coal and 

oil shale” regulations.

Phosphorus is an essential element for plant and animal 

nutrition, and it is indispensable for world food production 

(Van Vuuren and others, 2010; Ashley and others, 2011). It is 

mined from phosphate rock that is converted to phosphoric 

acid, which is the basis of many fertilizer and non-fertilizer 

products (Cordell and White, 2014).

Sedimentary phosphate deposits (phosphorites) form 

in marine shelf-slope or epeiric settings along the margins 

of continents in middle to low latitudes where nutrient-rich, 

O2

-depleted bottom waters are upwelled from depths of 100 

to 600 m and delivered to the photic zone, promoting primary 

productivity that concentrates phosphate in the remains of 

organisms at all levels of the food web. The phosphate is 

released through respiration of biogenic debris as it settles 

through the water column. In settings of low siliciclastic 

detrital input and low carbonate production, and in suboxic to 

reducing to denitrifying conditions that may include iron redox 

cycling at or beneath the bottom water-substrate interface, 

microbially mitigated early diagenetic authigenic phosphate 

mineralization occurs as crusts, hardgrounds, laminae, nodules, or pellets within the organic-rich sediments (Glenn and 

others, 1994; Föllmi, 1996; Moyle and Piper, 2004; Piper 

and Perkins, 2004; Filippelli, 2011). Subsequent to burial and 

exhumation, phosphate-enriched grains and fragments are 

commonly hydraulically and biologically reworked, concentrated, and re-deposited to form granular phosphorites (Glenn 

and others, 1994). Such reworking at sustained intervals of 

widespread erosion commonly results in nodular lag deposits 

at the base of unconformable sedimentary units (Glenn and 

others, 1994; Filippelli, 2011).

Major phosphorite deposits in the United States are 

related to zones of oceanic upwelling that took place (1) along 

the western margin of North America during the Permian, 

which formed the western phosphate field in Wyoming, Idaho, 

Montana, and Utah and (2) off the southeastern North American coast in the Miocene, which formed phosphate deposits in 

Florida and the Carolinas (Sheldon, 1981; Zientek and Orris, 

2005; Filippelli, 2011).

The study area lacks Permian units such as the Phosphoria Formation, which are important sources of phosphorite in 

southeastern Idaho and adjoining States (Piper and Perkins, 

2014). Instead, phosphate is confined to part of the Niobrara 

Formation, where it occurs in a concretionary dolostone and 

limestone bed that contains phosphatic pellets and gray to 

black chert pebbles, or as a phosphatic pebble bed that is as 

much as 10 cm thick (Cobban and others, 1976; Vuke and 
Locatable Minerals 27

others, 2007). These thin and minor occurrences are insignificant compared to the phosphorite deposits that are mined elsewhere in the United States, such as the Permian Phosphoria 

Formation or Miocene deposits of Florida, and consequently 

the study area lacks rocks that may be prospective for phosphate deposits.

There is no known exploration or mining activity for 

phosphate in the north-central Montana study area. There are 

no known USGS assessments for phosphate in the north-central 

Montana study area.

Locatable Minerals

The BLM defines locatable minerals as those minerals 

not leasable under the Mineral Leasing Acts and not salable 

under the Mineral Materials Act of 1947 (43 CFR 3830.11). 

Generally this includes valuable deposits of metallic minerals 

and industrial minerals (appendix 2 in Day and others, 2016).

Based on the geology of the study area, records of past 

production and current exploration, and consideration of 

geochemical, geophysical, and remote sensing data, four commodities need to be assessed as locatable minerals in the study 

area: gold, silver, bentonite, and diamond. The other locatable 

minerals listed in appendix 2 of Day and others (2016) have 

no production records in the study area, and the study area 

lacks the necessary geologic settings to host the mineral 

deposit types where these minerals occur (appendix 3 in Day 

and others, 2016). Therefore, these other locatable minerals 

are not considered further.

Mineral Systems and Mineral Deposit Types

The four commodities, bentonite, diamond, gold, and 

silver, that need to be assessed as locatable minerals in the 

study area occur in different mineral systems and deposit types 

(Hammarstrom and Zientek, 2016; appendix 3 in Day and 

others, 2016). Gold and silver occur in epithermal gold-silver 

(mercury) deposits that are part of the hydrothermal-volcanic 

rock associated system and in placer and paleoplacer gold 

deposits that are part of the surficial-mechanical (placer) 

mineral system. Bentonite deposits are part of the sedimentary 

system (formed during or after the conversion from sediment 

to sedimentary rock). Diamond occurs in diatreme-hosted 

diamond deposits that are part of the magmatic system 

(appendix 3 in Day and others, 2016). In this report, we base 

our assessments on the geologic models that have been developed for these mineral deposit types in the published literature, 

and as summarized elsewhere in this report (Hammarstrom 

and Zientek, 2016; appendix 3 in Day and others, 2016).

Mineral-Resources Potential

The BLM defines a “potential mineral resource” as the 

potential for the occurrence of a concentration of mineral 

resources. This term does not imply that there is potential 

for development or extraction of valuable mineral resources 

(Bureau of Land Management, 1985). The favorability for 

mineral resources can be evaluated by using multiple lines of 

evidence, including potential host rocks, geologic structure, 

alteration, geochemistry, geophysics, mineral deposits and 

occurrences, mineralogy, depth to basement, and other considerations (Bureau of Land Management, 1985).

The BLM uses qualitative rankings for mineral-resource 

assessments, ranking mineral-resource potential as high (H), 

moderate (M), low (L), no (O), and not determined (ND). 

For high potential, multiple lines of evidence indicate high 

potential for accumulation of mineral resources, and evidence 

indicates that mineral concentration has taken place. For moderate potential, multiple lines of evidence indicate a reasonable 

likelihood for accumulation of mineral resources. For low 

potential, the geologic environment and the inferred geologic 

processes indicate low potential for accumulation of mineral 

resources. “No potential” is used where the geologic environment, the inferred geologic processes, and the lack of mineral 

occurrences do not indicate potential for accumulation of 

mineral resources. For example, there is no oil potential in an 

area where the only rocks are unfractured Precambrian granite. 

“Not determined” is used where there is a lack of useful data, 

and this notation does not require a level-of-certainty qualifier 

(Goudarzi, 1984; Bureau of Land Management, 1985; appendix 2 

in Day and others, 2016).

For mineral-resource assessments, the BLM recognizes 

four levels of certainty: A, B, C, and D, where A is the least 

certain, and D is the most certain (Bureau of Land Management, 1985). For level A, the available data are not adequate 

to support or refute the possible existence of mineral resources 

within the area. For level B, the available data provide indirect 

evidence to support or refute the possible existence of mineral resources. For level C, the available data provide direct 

evidence but are quantitatively minimal to support or refute 

the possible existence of mineral resources. For level D, the 

available data provide abundant direct and indirect evidence 

to support or refute the possible existence of mineral resources 

(Goudarzi, 1984; Bureau of Land Management, 1985; appendix 2 

in Day and others, 2016).

Known Locatable Mineral Deposits

The BLM LR2000 database shows no active lode claims 

in the Focal Area and 411 closed lode claims (table 4). The 

lode claims occur in the western part of the study area, south 

of the Little Rocky Mountains, and predominantly represent 

claims that were staked for exploration for epithermal and diamond deposits (fig. 11). Figure 11A shows some active claims 
28 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

!

!

!

!

!

!

!

!

!

!

!

!

PHILLIPS COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

Roy

Hays

Malta

Harlem

Hilger

Lewistown

Lodge Pole

Fort Belknap Agency

White Bear

Assiniboine

Creek

Winifred

Armells

Strater

Cat Creek

Sawtooth Mountain

Milk River

Sherard

Ashfield 

Leroy

Tiger Ridge

Bowes

Black Coulee Three Mile

East Bowes

Southeast

Southwest Cherry Patch

Cherry Patch

Two Forks

191

191

2

109°0' 108°0' 

48°30'

48°0'

47°30'

Little Rocky

Mountains

0 10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Missouri

River

Figure 10. Map showing State oil and gas provinces and exploration and well type (IHS Energy Group, 2016) for the 

North-Central Montana Sagebrush Focal Area. USGS, U.S. Geological Survey.
Locatable Minerals 29

!

!

!

GARFIELD COUNTY

VALLEY COUNTY

Jordan

Glasgow

Swanson

Creek

Bowdoin

Vandalia

2

200

Fort Peck Lake

Fort Peck Lake

107°0'

MONTANA

IDAHO

ALBERTA SASKATCHEWAN

WYOMING

MANITOBA

NORTH DAKOTA

SOUTH DAKOTA

NEBRASKA

BRITISH

COLUMBIA

Area of map

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Oil and gas fields

Williston Basin

Oil/Gas Province (on index map only)

Well type summary for 1/4 mile cells

Oil

Gas

Oil and gas

Dry

North Central-Montana
30 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

109°0' 108°0' 

191

191

47°45

48°15'

47°45

48°15'

Hays

Malta

Lodge Pole

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

BLAINE COUNTY

PHILLIPS COUNTY

BLAINE COUNTY

A

B

10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 0

Boundary data from San Juan and others (2016). Roads 

and political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

 Datum of 1983

Figure 11. Maps showing active and closed mine claims for locatable commodities in the North-Central Montana Sagebrush Focal 

Area. A, Lode claims (modified from Dicken and San Juan, 2016a, b). B, Placer claims (modified from Dicken and San Juan, 2016a, b). 

USGS, U.S. Geological Survey.
Locatable Minerals 31

107°0'

107°0'

2

2

Fort Peck LakeFort Peck Lake

Fort Peck LakeFort Peck Lake

Glasgow

Glasgow

VALLEY COUNTY

VALLEY COUNTY

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Area of maps

A and B

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Number of active mining claims

1 - 5

6 - 11

12 - 16

17 - 22

23 - 31

Closed mining claims
32 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

in this area that occur within the study area, but lie outside the 

proposed withdrawal area.

The BLM LR2000 database shows 687 active placer 

claims in the Focal Area, and 1,079 closed placer claims 

(table 4). The majority of placer claims in the North-Central 

Montana Sagebrush Focal Area represent former and current 

exploration for bentonite deposits, and they occur generally in 

the northern part of the study area (fig. 11B).

Historic Production

Production of locatable minerals from the study area and 

nearby has come from epithermal deposits that produced gold 

and silver, and from bentonite deposits. Production figures and 

estimates for these deposit types are presented below, in the 

Hydrothermal-Volcanic Rock Associated System section, and 

the Bentonite section.

Mining Claims and Permits

The BLM LR2000 database shows six notices for the 

proposed withdrawal area of the North-Central Montana 

Sagebrush Focal Area (table 5). These notices are all closed, 

and there are no active notices in the proposed withdrawal area 

(table 6).

The BLM LR2000 database shows five plans of operations 

for the proposed withdrawal area of the North-Central Montana 

Sagebrush Focal Area (table 5). One plan of operations is 

authorized, two are pending, and two are closed (table 6). All 

active plans of operations in the proposed withdrawal area 

relate to bentonite (table 6; figs. 12, 13).

Table 4. Summary of mining claims for locatable minerals in the proposed withdrawal area within the North-Central Montana 

Sagebrush Focal Area.

[Source: Bureau of Land Management (BLM) LR2000 database, March 6, 2016. The number of cases is for the complete section that includes a proposed 

withdrawal area]

Area Active lode claims Closed lode claims Active placer claims Closed placer claims

Cases containing proposed 

withdrawal area

0 411 687 1,079

Table 5. Summary of status and number of 43 CFR 3809 notices and plans of operations for locatable minerals in the proposed 

withdrawal area within the North-Central Montana Sagebrush Focal Area.

[Source: Bureau of Land Management (BLM) LR2000 database, March 6, 2016. The number of cases is for the complete section that includes a proposed 

withdrawal area. ND, no data]

Permit type

Number 

of unique 

cases

Active Authorized Pending Closed Cancelled Expired Rejected Withdrawn

Plans of 

operations

5 ND 1 2 2 ND ND ND ND

Notices of 

intent

6 ND ND ND 6 ND ND ND ND

Table 6. Active 43 CFR 3809 notices and plans of operations summarized by commodity in the proposed withdrawal area within the 

North-Central Montana Sagebrush Focal Area.

[Source: Bureau of Land Management (BLM) LR2000 database, March 6, 2016. The number of cases is for the complete section that includes a proposed 

withdrawal area]

Commodity Number of active notices Number of active plans of operations

Clay, Bentonite 0 3
Locatable Minerals 33

107°0' 108°0' 109°0'

48°0'

47°30'

Fort Peck

aL

ek

Fort Peck

aL

ek

Roy

Hays

Malta

Glasgow

Lodge Pole

Jordan

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY PETROLEUM COUNTY

191

2

191

191 2 191

191

191

200

Little Rocky

Mountains

Missourii R ver

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Authorized or pending bentonite

Locatable minerals (undefined commodity)Gold

Closed or expired

BLM plans of operations and notices of intent

Bentonite

10 20 30 MILES 0

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

Missouri River

2

Figure 12. Map showing 43 CFR 3809 notices and plans of operations for locatable commodities in the North-Central Montana Sagebrush Focal Area (modified from Dicken and San Juan, 2016a, b). BLM, Bureau of Land Management; USGS, U.S. Geological Survey.
34 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

Roy

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

Little Rocky

Mountains

Missouri

River

Area of map B

109°0' 

48°0'

Hays

Lodge Pole

BLAINE COUNTY

A

B

10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 0

Boundary data from San Juan and others (2016). Political 

data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Figure 13. Maps showing bentonite claims and deposits for locatable commodities in the North-Central Montana 

Sagebrush Focal Area. A, The entire focal area. B, A detailed view of the area containing claims (Nathaniel Arave, Bureau of 

Land Management, Billings, Mont., written commun., 2016). USGS, U.S. Geological Survey.
Locatable Minerals 35

107°0'

Fort Peck LakeFort Peck Lake

Glasgow

Jordan

GARFIELD COUNTY

VALLEY COUNTY

MONTANA

IDAHO

ALBERTA SASKATCHEWAN

WYOMING

MANITOBA

NORTH DAKOTA

SOUTH DAKOTA

NEBRASKA

BRITISH

COLUMBIA

Area of map A Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Bentonite pit

Bentonite placer claims (patented 

 and unpatented)

108°0' 107°0'

Malta

PHILLIPS COUNTY
36 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Metallic Locatable Minerals

Based on geologic setting, past production, and current 

and past exploration activities, the only metallic minerals 

that are potentially locatable in the North-Central Montana 

study area are gold and silver, which may occur in epithermal 

and placer deposits in the study area. There are no records of 

production from any other types of metallic mineral deposits 

in the study area, and to the best of our knowledge, there has 

been no significant exploration for any other types of metallic 

mineral deposits in the study area.

The sections below describe epithermal and placer deposits, 

and then estimate the potential and certainty for locating these 

types of deposits in the study area. For the sake of completeness, 

we also include a brief discussion of other types of hydrothermal 

deposits that commonly occur with epithermal deposits, and 

we evaluate whether these other deposits, such as porphyry 

and skarn deposits, might occur in the study area.

Hydrothermal-Volcanic Rock Associated System

The hydrothermal-volcanic rock associated system contains a number of different types of mineral deposits, including 

epithermal gold-silver (mercury) deposits (appendix 3 in Day 

and others, 2016), which are referred to herein as epithermal 

deposits; this is the only deposit type in this system that is 

known to occur near the study area. Epithermal deposits form 

within 1 km of the surface of the Earth, and they typically 

occur in volcanic arcs at convergent plate margins in continental and oceanic settings, in continental-margin back arcs, in 

rifts, and in intraplate environments (Hedenquist and Lowenstern, 

1994; Sillitoe and Hedenquist, 2003; Sillitoe, 2008). Epithermal 

deposits form vein, stockwork, disseminated, and replacement 

deposits that are principally mined for gold and silver. 

Byproducts may include lead, zinc, copper, and (or) mercury, 

but in most deposits, these are not recovered (Simmons and 

others, 2005).

Epithermal deposits most commonly occur in alkaline 

to calc-alkaline volcanic rocks and hypabyssal intrusions that 

range in composition from mafic to felsic. Hosts for some 

deposits are silica-undersaturated, highly alkali-rich (Na2

O + 

K2

O) rocks ranging from felsic phonolites to ultramafic 

lamprophyres (Jensen and Barton, 2000). Most epithermal 

deposits are Cenozoic in age, which reflects preferential 

preservation of these shallowly formed deposits in tectonically 

active regions (Kesler and Wilkinson, 2009). Epithermal 

deposits have characteristic ore and alteration mineralogy, 

and these produce characteristic geochemical and geophysical 

signatures (for example, Lindgren, 1933; Warren and others, 

2007; Morrell and others, 2011; Saunders and others, 2014), as 

summarized in appendix 3 of Day and others (2016).

Geology and Occurrence in the Study Area

There are no known epithermal deposits in the study 

area. However, the Zortman and Landusky deposits in the 

Little Rocky Mountains are within 25 km of the study area, 

and together, they form a large epithermal deposit (fig. 14). 

From the 1880s to 1951, underground mining of high-grade 

epithermal veins in the Little Rocky Mountains produced 

380,000 ounces (oz) of gold (Wilson and Kyser, 1988). Mining resumed in the area in the 1970s with the introduction of 

gold and silver recovery from heap leaching using cyanide 

recovery techniques. This led to recovery of an additional 

2.5 million oz (Moz) of gold and 20.7 Moz of silver from the 

Zortman and Landusky deposits (Maehl, 2002), so together the 

Zortman and Landusky deposits form an orebody that is nearly 

world-class (>3.2 Moz of gold, Singer, 1995) in size.

The deposits occur in quartz monzonite and syenite 

that are part of the suite of Tertiary alkaline igneous rocks 

in the Little Rocky Mountains (Knechtel, 1959). Normative 

abundances of quartz, orthoclase, and albite calculated from 

the whole-rock chemical compositions of unaltered intrusive 

rocks from the Zortman-Landusky mine areas yield alkali 

syenite, quartz syenite, and quartz monzonite compositions 

(Wilson and Kyser, 1988). Hydrothermal alteration includes 

widespread illite and pyrite alteration and more localized silicification and quartz stringers, and the groundmass of altered 

intrusive rocks contains variable amounts of pyrite, kaolinite, 

montmorillonite, and fluorite (Wilson and Kyser, 1988; Rogers 

and Enders, 1990; Wampler, 1994). In the unweathered rock, 

the vast majority of the gold occurs in pyrite, and minor gold 

also occurs in telluride minerals and electrum or as native 

gold. The gold that resides in pyrite and in telluride minerals is 

not recoverable by conventional cyanide recovery techniques, 

so the Zortman and Landusky deposits mined only oxidized 

material where native gold occurs in fractures with hematite, 

goethite, and, more rarely, jarosite (Wilson and Kyser, 1988; 

Williams, Gabelman, and others, 2009).

Potential for Occurrence

The potential for epithermal deposits in the Little Rocky 

Mountains has been previously assessed by delineation of a 

Known Mineral Deposit Area (KMDA) by U.S. Bureau of 

Mines geologists and engineers for the Inventory of Land-Use 

Restraints Program (ILURP), which was active from 1983 

through 1995 (Parks and others, 2016a, b). These tracts are 

similar to tracts that were previously delineated by Bateman 

and Lutz (1976) and Box and others (1996). These tracts, 

which are outside the study area, include the Zortman and 

Landusky deposits, which together produced nearly 2.9 Moz 

of gold since the 1880s.
Locatable Minerals 37

107°0' 108°0' 109°0'

48°0'

47°30'

Roy

Hays

Malta

Glasgow

Lodge Pole

Jordan

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

191

2

2

191

191

200

Little Rocky

Mountains

Fort Peck

aL

ek

Fort Peck

aL

ek

Missourii R ver

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

0 10 20 30 MILES0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Assessment tract type—Epithermal gold-silver (mercury)

Epithermal tract (MTEP01), low 

 potential with certainty level B

2

2

191

191

191

MTEP01

Figure 14. Mineral potential map for epithermal deposits in the North-Central Montana Sagebrush Focal Area. USGS, U.S. Geological Survey.
38 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Epithermal Deposits: Tract MTEP01 (Epithermal)

This is a low-potential, certainty level B tract for epithermal deposits that envelops the area that is likely underlain by 

the igneous rocks of the Little Rocky Mountains (appendix 2; 

fig. 14). The MTEP01 tract lies within the study area, but the 

tract overlies only one small area that is specifically proposed 

for withdrawal (fig. 14; San Juan and others, 2016).

About 20 named domes occur in the low hills on the 

southwest, south, and southeast flanks of the Little Rocky 

Mountains. The National Uranium Resource Evaluation 

(NURE) aeromagnetic data show magnetic anomalies that 

indicate that the low hills likely overlie concealed igneous 

intrusive rocks, similar to those exposed in the Little Rocky 

Mountains (Hill and others, 2009). In the main coalescingdome complex of the higher part of the Little Rocky Mountains, 

syenite porphyries are mostly in fault contact with postCambrian formations, implying that the depth of bed-parallel 

intrusion was in or below the Cambrian formations. Only one 

outlying dome has exposed syenite porphyry, and there, the 

porphyry occurs beneath Cambrian strata (Knechtel, 1944, 

1959). Therefore, the intrusions that underlie the domes in the 

study area are presumably sill-like or laccolithic, and are probably in or below the Cambrian sedimentary rocks.

Some of the domes were drilled for oil and gas prior to 

1944, and are shown on the Knechtel (1944) map. Nine drill 

holes were relatively shallow (depth 250 m or less); three 

holes (depth 300–600 m) reached the Mississippian Mission 

Canyon Limestone. Syenite porphyry intrusions were not 

encountered in those depth ranges. Gas was found in three 

holes (Knechtel, 1944).

Part of the MTEP01 tract is covered by high-resolution 

aeromagnetic data that were acquired by Anaconda Minerals 

Company in 1981 (Anderson and others, 2016). These data 

clearly indicate that intrusions underlie domes in the sedimentary 

rocks at the outer margins of the Little Rocky Mountains. 

However, these data do not indicate whether any of these 

intrusions are altered, so they do not increase the certainty or 

potential of the tract.

This is a low-potential tract because there is evidence 

that intrusive rocks that host nearby epithermal deposits occur 

within the tract boundary; however, except for one exposure, 

those intrusive rocks lie at depth in the study area (fig. 4), 

and there are no data to indicate that those intrusive rocks are 

altered or mineralized.

At the time of this report, there do not appear to be any 

active claims, notices, or plans of operations related to potential epithermal mineralization inside the study area, despite 

activity in the past. At the time of this report, there is a ban 

on extraction of gold by cyanide recovery techniques in the 

State of Montana. Heap leaching is the preferred extraction 

technique for these types of deposits, so the ban on extraction 

of gold by cyanide recovery techniques likely contributes to 

the reduced exploration activity.

Economic Analysis of the Deposit Types

On a global basis, epithermal deposits are an important 

source of gold and silver (Simmons and others, 2005; 

Saunders and others, 2014). The Zortman-Landusky District 

has large epithermal deposits, and therefore the igneous rocks 

of the Little Rocky Mountains that lie outside the study area 

have had very significant past production of gold and silver. 

However, because the potential host rocks for epithermal 

deposits lie almost exclusively under cover in the study area, 

and because there are no data to indicate that these rocks are 

altered or mineralized, the potential for epithermal deposits 

is considered low within the study area. Furthermore, the 

vast majority of that area with low potential is outside of the 

proposed withdrawal area (fig. 14). Moreover, the current 

ban on extraction of gold by cyanide recovery techniques in 

the State of Montana would have to be lifted before mining 

of epithermal gold-silver deposits could be considered in the 

area. Because the potential for epithermal deposits is considered to be none for most of the proposed withdrawal area, and 

because of the current ban on cyanide recovery techniques, it 

is unlikely that extraction of gold and silver from epithermal 

deposits in the proposed withdrawal area could have a significant economic impact in the future.

Other Hydrothermal Mineral Deposits

Epithermal gold deposits form from hydrothermal fluids 

at shallow levels in the Earth’s crust in areas that are characterized by active volcanism, active plutonic rock formation, or 

both. Other hydrothermal deposits, such as porphyry and skarn 

deposits, form from magmatic hydrothermal processes in this 

environment, so the potential for these other deposit types 

must be considered herein.

There is little contact metamorphic skarn in the Little 

Rocky Mountains, even where intrusive rocks are in contact 

with carbonate rocks (Weed and Pirsson, 1896; Wampler, 

1994). Furthermore, we are not aware of any occurrences in 

the Little Rocky Mountains that would indicate that there may 

be skarn mineralization in the area (Meinert and others, 2005). 

Similarly, there are no reported occurrences of porphyry copper, porphyry molybdenum, or iron oxide copper-gold mineralization in the Little Rocky Mountains, even though alkaline 

intrusive rocks can host large porphyry and iron oxide coppergold deposits (Seedorff and others, 2005; Williams and others, 

2005). Because there are no reported occurrences of porphyry 

or skarn mineralization in the Little Rocky Mountains where 

the main mass of intrusive rocks are exposed, the potential for 

these deposits inside the study area is considered to be none.

It is probable that the sedimentary rocks inside the study 

area conceal latest Cretaceous to Paleocene intrusive igneous 

rocks, and that these rocks may contain magmatic hydrothermal 

mineralization. However, depth to source modeling of the 
Locatable Minerals 39

available aeromagnetic data indicates that the majority of 

the magnetic anomalies can be attributed to variability in 

basement rocks at more than 1 km depth (Anderson and 

Ponce, 2016). The exception to this is an area 45 km southeast of the Little Rocky Mountains that may have magnetic 

sources as shallow as 500 m. However, with (1) no supporting evidence, (2) no known surface structure or alteration 

effects from a shallow intrusion, (3) no subsurface data that 

could allow assessment of potential mineralization within 

this possible intrusive complex, and (4) no evidence of 

deep-seated hydrothermal mineralization in the area, such as 

porphyry, iron oxide copper-gold, or skarn deposits, we lack 

any basis on which to draw a tract over this possible buried 

intrusive complex.

Surficial-Mechanical (Placer) Mineral System

The surficial-mechanical (placer) mineral system contains 

a number of different types of mineral deposits (appendix 3 in 

Day and others, 2016). Placer and paleoplacer gold deposits, 

which are referred to herein as placer deposits, occur in the 

study area, but other types of deposits in the surficial-mechanical 

(placer) mineral system are not known to occur within the 

study area. Placer deposits generally occur as alluvial deposits 

and as coastal deposits; the latter are briefly described as 

“heavy mineral placer” deposits in appendix 3 of Day and 

others (2016). Placer deposits are a principal source of zirconium and a primary source of titanium for the TiO2

 pigments 

industry, but these elements are almost entirely produced from 

heavy-mineral sands deposited in coastal environments (Force, 

1991; Garnett and Bassett, 2005; Van Gosen and others, 2014). 

Heavy mineral placers are significant sources of gold in some 

areas of the Unites States, such as Alaska, and also in some 

countries elsewhere (Yeend and Shawe, 1989; Youngson and 

Craw, 1995; Jones and others, 2015).

Mineral Description

Although placer deposits can be important sources for a 

variety of minerals such as ilmenite (FeTiO3

), rutile (TiO2), 

zircon ((Zr,Hf,U)SiO4

), monazite ((Ce,La,Y,Th)PO4), xenotime (YPO4

), garnet, cassiterite (SnO2), sillimanite (Al2SiO5), 

staurolite (Fe2

Al9Si4O23(OH)), and platinum-group minerals, 

the majority of the alluvial placer deposits worldwide are 

worked to recover gold. Given the occurrence of the large 

epithermal gold-silver deposits of the Zortman-Landusky 

District, just outside the study area, the placer gold potential of 

the study area must be considered.

Geology and Occurrence in the Study Area

From the early 1880s to 1951, placer deposits in the 

Zortman-Landusky District produced 326 oz of gold (10 kg), 

which is three orders of magnitude less than the production 

from lode deposits (380,000 oz of gold) during the same time 

period (Rogers and Enders, 1990). The very low yields from 

placer deposits reflect the fact that the vast majority of the 

gold in nearby epithermal deposits occurs in solid solution 

in pyrite and in telluride minerals, and not as native gold or 

electrum (Wilson and Kyser, 1988), which are required for 

productive gold placer deposits.

At this time, there do not appear to be any active claims, 

notices, or plans of operations related to potential placer 

deposits inside the study area.

Potential for Occurrence

Placer deposit sites in the Little Rocky Mountains were 

described by Bateman and Lutz (1976). To the best of our 

knowledge, there are no previous assessments for placer 

deposits in the study area (Parks and others, 2016a, b).

Placer Deposits: Tract MTPL01 (Placer)

Tract MTPL01 (Placer) is a low potential, certainty 

level B tract for placer deposits (appendix 2). The tract was 

constructed by enveloping Quaternary alluvial deposits that 

extend downstream for a distance of 10 km from potential 

lode sources of gold at the Zortman and Landusky deposits 

(figs. 15, 16). This tract does not overlie any areas that are 

specifically proposed for withdrawal (fig. 15; San Juan and 

others, 2016).

Economic Analysis of the Deposit Types

Although placer deposits can be an important source of 

gold, the lack of significant past production and the lack of 

suitable source material to form large and productive deposits 

suggest that the potential for economically significant placer 

deposits in the study area is low to none. Because the placer 

tracts do not overlie any area that has been proposed for 

withdrawal, the potential for economically significant placer 

deposits in the proposed withdrawal area is considered to 

be none.
40 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area 107°0' 108°0' 109°0' 48°0' 47°30' Roy Hays Malta Glasgow Lodge Pole Jordan PHILLIPS COUNTY GARFIELD COUNTYVALLEY COUNTY BLAINE COUNTY FERGUS COUNTY

PETROLEUM COUNTY

191

2

2

191

191

200

Little Rocky

Mountains

Fort Peck aL ekFort Peck aL ek

Missouri

i R ver

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

0 10 20 30 MILES0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

2

2

191

191

191

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Assessment tract type—Placer and paleoplacer gold

Placer tract (MTPL01), low 

 potential with certainty level B

MTPL01

Figure 15. Mineral potential map for placer deposits in the North-Central Montana Sagebrush Focal Area (modified from Knechtel, 1959; Montana State Library, 2016a). USGS, U.S. Geological Survey.
Locatable Minerals 41

Study Area

Study Area

Study Area

"

"

108°45' 108°30'

48°0'

47°50'

Landusky 

Zortman

Hays

Lodge Pole

Little Rocky Mountains

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Area of map

Study area

0 2 4 6 MILES

0 2 4 6 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

EXPLANATION

Epithermal deposit

Patented claim

Placer tract (MTPL01)

Epithermal tract (MTEP01)

Surficial deposits

Tertiary igneous rocks

Bearpaw Shale

Judith River Formation and Claggett Shale

Lower to Upper Cretaceous Fall River 

 and Eagle Sandstones

Upper Jurassic to Lower Cretaceous 

 sedimentary rocks

Triassic and older sedimentary rocks

Precambrian metamorphic rocks

USGS study area boundary Upper Cretaceous

MTEP01

MTPL01 MTEP01

Figure 16. Map showing geology of Little Rocky Mountains and the epithermal and placer tracts in the northwest part of the 

North-Central Montana Sagebrush Focal Area (modified from Knechtel, 1959; Vuke and others, 2007). USGS, U.S. Geological 

Survey.
42 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Nonmetallic Locatable Minerals

Based on geologic setting, past production, and current 

and past exploration activities, the only nonmetallic minerals 

that are potentially locatable in the North-Central Montana 

study area are bentonite and diamond, which may occur in 

deposits in the study area. This section describes these two 

types of deposits, and provides estimates of the potential and 

certainty for locating these types of deposits in the study area.

Sedimentary System (Formed During or After 

the Conversion from Sediment to Sedimentary 

Rock): Bentonite Deposit Type

Bentonite is a claystone formed by the chemical alteration of glassy volcanic ash (Neundorf and others, 2005) most 

commonly found interbedded with shallow marine shales and 

limestones. The most common clay minerals in bentonite are 

members of the smectite mineral group, especially the mineral 

montmorillonite ((Na,Ca)0.33(Al,Mg)2

(Si4O10)(OH)2 · nH2O). 

The properties of bentonite are dictated by the properties of 

the dominant clay minerals (Eisenhour and Brown, 2009). 

Bentonites are divided into sodium-bentonite (dominated by 

sodium montmorillonite), which is also called swelling bentonite, 

and calcium-bentonite (dominated by calcium montmorillonite), 

which is non-swelling bentonite. Other varieties of bentonite 

are dominated by other smectite minerals, for example, hectorite 

(Na0.3(Mg,Li)3

Si4O10(OH)2) (Harben and Kužvart, 1996; 

Harben, 2002). Sodium-bentonite can absorb large amounts of 

water and form viscous, thixotropic clay. In contrast, calciumbentonite has low water absorption and swelling capabilities 

and cannot stay suspended in water.

Bentonite is an important raw material required for the 

extraction and production of energy and steel resources and is 

called the “clay with 1,000 uses” (Feick, 2016). Bentonite is 

commonly used in drilling fluids, in metal castings, in petwaste absorbents, and as a binder for making iron-ore pellets 

(Eisenhour and Brown, 2009). It is also used as an agent for 

clarifying and removing colors from oils, as a desiccant, as a 

flocculent for papermaking, and in sealants (Grim and Güven, 

1978; Inglethorpe and others, 1993; Eisenhour and Brown, 

2009; Dlamini, 2015).

Mineral Description

Bentonite forms by the alteration of volcanic ash and 

tuffs deposited in shallow oceans or brackish ponds and lakes 

(Eisenhour and Brown, 2009; Wilson, 2013). Sodium bentonite-type clay deposits are derived from ash that was deposited 

in shallow marine environments. Calcium bentonite-type clays 

form in freshwater settings (Alther, 2004). Bentonite layers 

commonly contain trace amounts of minerals, such as feldspar, 

biotite, quartz, and pyroxene that were accessory phases in the 

original volcanic material. Several key processes are necessary to form an accumulation of bentonite that has economic 

potential (Wilson, 2013). First, ash and tuff of rhyolitic to 

dacitic composition are deposited in shallow saline waters. 

During diagenesis, glassy material in the ash and tuff are 

chemically altered to montmorillonite and other clays. Beds 

are typically less than 1 m thick. The bentonite layers must be 

exhumed; only those layers within about 15 m of the surface 

form deposits that can be developed.

Bentonite is mined from open pits that rarely extend to 

depths of 15 m. Maximum strip ratios are about 10:1. Bentonite 

mined from open pits is blended, ground, dried, and processed 

into various products at mills. Close proximity to transportation 

routes, such as highways and railroads, is an important consideration in determining if a near-surface bentonite layer can be 

economically developed (Miller and others, 1979; Eisenhour 

and Brown, 2009; Sutherland and Drean, 2014).

Both the grade and the quality of the bentonite must be 

considered for different industrial applications (Inglethorpe 

and others, 1993). The grade is defined as the smectite content 

of the bentonite, whereas quality is related to the physicochemical properties of the clay, and is a measure of likely 

industrial performance.

Geology and Occurrence in the Study Area

Cretaceous bentonite deposits occur in marine shales 

that accumulated in shallow seas covering the present-day 

midcontinent region of North America (Western Interior 

Seaway; fig. 17). Deposits of bentonite are found in Wyoming, 

Montana, South Dakota, Alberta, and Saskatchewan in rocks 

of the Colorado Group (Lower Cretaceous) and Montana 

Group (Upper Cretaceous), and their equivalents. From lowest 

to highest in the Cretaceous System, layers of bentonite in the 

Hardin District of Montana are designated from A through W, 

with some prominent layers having proper names (fig. 18). 

Most sodium bentonite production in the United States is from 

the Clay Spur Bentonite Bed in the Upper Cretaceous Mowry 

Shale in Wyoming and adjacent parts of Montana. In other 

places in Montana, sodium bentonite has been mined from the 

(1) Soap Creek Bentonite Bed in the Cody Shale, Hardin District; (2) X and Y beds in the Bearpaw Shale in the Malta and 

Glasgow areas; and (3) X and Y beds in the Bearpaw Shale in 

the Ingomar-Vananda area (fig. 17; Hosterman and Patterson, 

1992). The North-Central Montana Sagebrush Focal Area 

largely covers the Malta and Glasgow areas of Hosterman and 

Patterson (1992).

The Late Cretaceous Bearpaw Shale of the Montana 

Group is a dark-gray montmorillonite-rich shale that crops 

out over much of northern Montana. A bentonite-rich interval 

in the Bearpaw Shale is recognized in stratigraphic sections 

ranging from north-central Montana to central Wyoming. In a 

dry oil and gas hole southeast of the Little Rocky Mountains 

(fig. 5), there are several beds of bentonite in a 6-m-thick zone 

in the Bearpaw Shale, about 37 m above the top of the Judith 

River Formation. Knechtel (1959) proposed that this interval 

is approximately equivalent to the Monument Hill Bentonitic Member of the Pierre Shale of the Black Hills region 
Locatable Minerals 43

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!

!!!

!

!

!

!

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!

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!!

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110° W 105° W

50° N

45° N

120° W 100° W

50° N

30° N

EXPLANATION

0 50 100 150 200 KILOMETERS

0 50 100 MILES

Boundary data from San Juan and others (2016). Political 

boundaries from U.S. Department of State (2009)

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

MONTANA

IDAHO

WYOMING

UTAH

Southern Black Hills

(Colony District)

COLORADO

ALBERTA SASKATCHEWAN

CANADA

UNITED STATES OF

AMERICA

Eastern Bighorn

Basin District

Kaycee District

Northern Black Hills

(Clay Spur District)

PACIFIC 

OCEAN

Map area

Ingomar-Vananda

area

Hardin District

Malta area Glasgow area

Study area

Cretaceous sedimentary rocks of the 

 Western Interior Seaway

Bentonite areas of interest

USGS study area boundary

Location of bentonite producer, 

 past producer, or prospect

!

Mining district

Figure 17. Map showing extent of Cretaceous sedimentary rocks that formed in the Western Interior Seaway 

with locations of mining districts and bentonite deposits, north-central United States and south-central Canada. 

Geology modified from Garrity and Soller (2009). Mining districts from Knechtel and Patterson (1956), Hosterman 

and Patterson (1992), and Sutherland and Drean (2014). Bentonite locations with production from U.S. Geological 

Survey (2016a). USGS, U.S. Geological Survey.
44 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Group

Series

System

473

394

474

280

410

867

375

367

200±

700±

350±

Hell Creek Formation

116

Montana

Judith River Formation

Upper Cretaceous

Cody Shale

Colorado

Cretaceous

Lower Cretaceous

Thickness,

in feet Section Formationand Member

Bearpaw Shale

Upper

unnamed

member

Parkman

Sandstone

Member

Claggett

Shale

Member

Unnamed sandy

shale member

equivalent to

Eagle Sandstone

Telegraph

Creek Member

Niobrara

Shale

Member

Carlile

Shale

Member

Greenhorn

Calcareous Member

Belle

Fourche

Member

Mowry

Shale

Thermopolis

Shale

Bearpaw Shale

Judith River

885

255

Formation Thickness,

in feet

X

W

V

U

T

R

S

Q

P

O

N

M

L

Soap Creek

J

I

Clay Spur

G

F

E

D

C

A

B

Shale

Sandstone

Bentonite

Sandy shale

Siltstone

EXPLANATION

Figure 18. Stratigraphic column showing lithology with bentonite layers in the Hardin District, Montana. 

Modified from Knechtel and Patterson (1956).
Locatable Minerals 45

and also to a bentonite-rich part of the Bearpaw Shale in the 

Hardin District, Montana (fig. 17). The extraordinary lateral 

continuity of the bentonite-bearing interval in the middle 

part of the Bearpaw Shale in Montana is shown on a section 

that runs west to east across central and eastern Montana, 

between the Dearborn River and the Porcupine Dome (fig. 19, 

section A–A’; Gill and Cobban, 1973). The bentonitic interval 

is likely correlative with the bentonite beds in the medial part 

of the Bearpaw Shale in the Kaycee District, Wyoming, almost 

470 km to the south (figs. 17, 20).

Within the bentonitic interval of the Bearpaw Shale, 

two prominent bentonite beds are exposed in two regions in 

the northern part of the study area (fig. 21A). These beds are 

the most important accumulations of bentonite in the focal 

area. In the western area (Malta), the two beds are exposed 

for about 48 km on strike along Beaver Creek, Big Warm 

Creek, and Wild Horse Creek in Phillips County, about 40 km 

south of Malta. To the east, in the Glasgow area, beds extend 

about 21 km on strike and are exposed along Brazil Creek and 

the North and South Forks of Little Beaver Creek in Valley 

County, approximately 24–32 km west-southwest of Glasgow.

To the south, in the Charles M. Russell National Wildlife 

Refuge, a laterally continuous bentonite bed, named the Siparyann bed, occurs approximately 24–61 m above the base of 

the Bearpaw Shale (fig. 21B; Miller and others, 1979). The bed 

crops out for at least 48 km along strike, from the west end 

of the refuge to the east. Near the west end of the refuge, the 

bed is 0.3 m thick; farther east, it is as much as 1.8 m thick. 

This bed is certainly within the 6-m-thick interval containing 

bentonite beds in the Bearpaw Shale described by Knechtel 

(1959) and may correlate with the two distinct bentonite beds 

in the Malta and Glasgow areas (Miller and others, 1979).

Also within the Charles M. Russell National Wildlife 

Refuge, Miller and others (1979) described and sampled 

bentonite layers and montmorillonite-rich silty shales within 

the Fox Hills and Hell Creek Formations, upsection of the 

Bearpaw Shale. Some of these layers could be equivalent to 

the volcanic ash near Linton, North Dakota (Manz, 1962). 

Within the refuge, these units appear to lack lateral continuity, 

are thin, and are siltier than the thicker bentonite beds in the 

Bearpaw Shale.

Geology of the Bentonite Beds in the Malta and Glasgow 

Areas

The Malta and Glasgow areas are underlain by Bearpaw 

Shale, though it is covered, in part, by alluvial and glacial 

deposits. In areas where bedrock is exposed and bentonite is 

weathered, it has a characteristic “popcorn texture” that can 

be distinguished at ground level. In areas of gentle dip, Berg 

(1969) noticed that beds of bentonite form small terraces 

because of their resistance to erosion, and that some exposures 

of bentonite can be traced by their lack of vegetation and by a 

color difference with surrounding rocks. Areas of hummocky 

glacial till and alluvial cover are readily distinguished in most 

areas. With some background knowledge, the distribution of 

the bentonite beds can be mapped on satellite-images (fig. 22).

Mineral reports for mineral patent applications provide 

the most information about the geology of the two prominent 

bentonite beds, X and Y, in the Malta area (figs. 21 and 22; 

Bigsby and Sollid, 1975; Durst, 1981). In the area studied by 

Bigsby and Sollid, the lower bed, X, is 0.3–1.2 m thick, with 

an average thickness of 0.79 m. The upper bed, Y, is 0.46–1.1 m 

thick, with an average thickness of 0.71 m. The lower bed is 

3.7–5.5 m below the upper bed, with an average separation of 

4.3 m. Both beds are thicker than the 0.6-m minimum limit 

that is usually considered minable (Bigsby and Sollid, 1975). 

The average overburden thickness for the evaluated claims is 

6 m on the upper bed and 8.5 m on the lower bed. Stripping 

ratios are generally less than 10:1. In the area examined by 

Durst (1981), the upper bed, is typically 0.3–0.6 m thick, and 

the lower bed is 0.76–1.2 m thick. The layers are separated 

by approximately 3 m of bentonite-rich shale. Furthermore, a 

third, orange bentonite bed, which is a few centimeters thick, 

is 6–15 m below the lower bed (Durst, 1981).

The upper and lower beds in the Malta area (Bigsby and 

Sollid, 1975; Durst, 1981) can be distinguished by the presence 

or absence of biotite grains and calcareous-clay concentrations. 

Numerous biotite grains are scattered through the lower bed, 

giving it a speckled appearance. The upper bed is characterized by the presence of calcareous-clay concretions. The 

concretions are white-colored cementations of the bentonite 

that appear to have grown in the bed, starting at the base and 

spreading upwards and outwards in a mushroom shape. Where 

the upper bed has been removed by erosion and the basal 

bedding plane of the upper bed is exposed, the concentrations 

appear in outcrop and on satellite imagery as circular white 

areas as great as 6 m in diameter (fig.22).

Two prominent bentonite beds also occur in the Glasgow 

area (figs. 21, 23). The lower and upper beds are 0.58–0.73 m 

and 0.63–0.79 m thick, respectively (David Crouse, Imerys, 

written commun., 2016). The upper bed is about 3.5–4.7 m 

below the lower bed. Biotite is present in the lower bed and 

calcareous concretions occur in the upper bed, just as in the 

Malta area.
46 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area 200 400 600

0

FEET

50100150

MONTANA

A

A′ 

METERS0

0 20 40 60

0 30 60

MILES

KILOMETERS

EXPLANATION

Volcanic-rich rocks

Welded tuff, ash

Nonmarine rocks

Marine sandstoneMarine shaleCalcareous rocks

Approximate midpoint of selected ammonite zones

Bentonite

Baculites compressus cuneatus

Baculites grandis Baculites eliasi

Baculites mclearni

Ardmore Bentonite Bed

Scaphites hippocrepis

Montana Group

Montana Group

Hell Creek

Formation

(part)

Hell Creek

Formation

(part)

Two Medicine

Formation

Virgelle

Sandstone

Telegraph Creek

Formation

St. Mary River

Formation (part)

Colorado Shale

(part)

Dearborn

River

Bruno

Siding Shawmut Lavina Mosby Porcupine Dome

A

A′ 

Judith River

Formation

Eagle Sandstone

Virgelle Sandstone Member

Horsethief Sandstone Lennep Sandstone

Kb

Cody Shale

(part)

Colorado Shale

(part)

Hell Creek

Formation

(part)

Fox Hills

Formation

Bearpaw

Shale (Kb)

Parkman

Sandstone

Claggett

Shale

Gammon

Shale

Niobrara

Formation

Study area

Figure 19. Schematic cross section showing the geology of the Montana Group from northwest to southeast Montana (

A

–

A’). Prominent bentonite beds occur in the Gammon 

Shale and Claggett Shale in the lower part of the group and in the Bearpaw Shale in the upper part. Modified from Gill and Cobban (1973).
Locatable Minerals 47

Outcrop section of Bearpaw Shale,

Kaycee district, northeastern

Natrona County, Wyoming

Bearpaw Shale South Zortman well 1, 

Phillips County, Montana

(Dry hole SZ-1)

00

100

50

25

50

150

FEET METERS

A

B EXPLANATION

Judith River 

Formation

Judith River 

Formation

Study area

A

B

MONTANA

WYOMING

Shale

Sandstone

Bentonite

Sandy shale

Siltstone

Figure 20. Stratigraphic columns showing bentonitic intervals in the Bearpaw Shale, Montana and 

Wyoming. A, Stratigraphic column from Continental Oil Company’s South Zortman well 1 (dry hole SZ-1), 

Phillips County, Montana. Modified from Knechtel (1959). B, Stratigraphic column from an outcrop 

section, Kaycee District, Natrona County, Wyoming. Modified from Merewether (1996, figure 19).
48 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

PHILLIPS COUNTY

Malta

2

191

191

South Fork Little

 Beaver Creek

North Fork Little

Beaver Creek

Big Warm Creek

Beaver Creek

Beaver Creek

Beaver Creek

Wild Horse Creek

BURLINGTON NORTHERN/SANTA FE

Glasgow area

Malta area

Big Warm Creek

Charles M. Russell 

National 

Wildlife Refuge

108°0' 

48°30'

48°0'

0 5 10 15 MILES

0 5 10 15 KILOMETERS

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

A 

Figure 21. Map showing the location of bentonite occurrences in the Bearpaw Shale in the North-Central Montana Sagebrush 

Focal Area. A, Patented and placer claims located for bentonite resources in the Malta and Glasgow areas, Montana. Most of the 

bentonite exploration and development activity in the North-Central Montana Sagebrush Focal Area has been concentrated in these 

two regions. Claim data compiled from records provided by Nathanial Arave, Bureau of Land Management, Billings, Montana. B, 

Outcrop distribution of the Siparyann bentonite bed in the western part of the Charles M. Russell National Wildlife Refuge. Modified 

from Miller and others (1979). USGS, U.S. Geological Survey.
Locatable Minerals 49

GARFIELD COUNTY

Glasgow

2

Brazil Creek

Beaver Creek

Fort Peck LakeFort Peck Lake

Glasgow bentonite

plant 

107°0'

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Area of map A

Study area

Area of map B

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Placer claim located for bentonite

Patented placer claim

Area with bentonite resource

Mined out bentonite pit

Charles M. Russell National Wildlife Refuge

Siparyann bentonite bed

PHILLIPS COUNTY

FERGUS COUNTY PETROLEUM

COUNTY

£¤191

Charles M. Russell 

National Wildlife Refuge

B 108°40' 108°30' 108°20'

47°40'

47°35'
50 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

108°05' 108°00' 107°55'

48°05'

48°00'

PHILLIPS COUNTY

Area of maps A and B

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Area of map C

Study area

Calcareous concretions

0 1 2 3 MILES

0 1 2 3 4 KILOMETERS

Political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

A B

C

Imagery copyright © 2014 Esri and its licensors. 

EXPLANATION

American Colloid Co. 

 active placer claims

Figure 22. Imagery, bentonite geology, and claims in the Malta area, Montana. A, Satellite image. B, Interpretations of areas 

underlain by glacial till and the X and Y bentonite beds. Circular white features in the areas underlain by the Y bentonite bed 

are calcareous concretions. C, Index map showing location of the image in relation to American Colloid Co. placer 
Locatable Minerals 51

0 100 200 FEET300

0 25 50 75 METERS

B

Imagery copyright © 2014 Esri and its licensors.

D

EXPLANATION

Glacial till

Bentonite

Bed X

Bed Y

Bed Y

Bed X

Rolling prarie and

 present land 

 surface

Coulee bottom Bearpaw Shale

Pre-glacial land 

 surface

Bed thickness

~2.5 ft.

EXPLANATION

Bentonite

Glacial till

Marine shale

VERTICALLY EXAGGERATED

Bed X

Bed Y

mining claims located for bentonite. Claim data compiled from records provided by Nathanial Arave, Bureau of Land Management, 

Billings, Montana. D, Schematic cross section showing relation of landforms developed on geologic and surficial units in the Malta 

area. Modified from Bigsby and Sollid (1975).
52 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

MONTANA

IDAHO

ALBERTA

WYOMING

SASKATCHEWAN

EXPLANATION

Political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Map area

107°10' 107°00'

48°10'

48°05'

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

Section 11

Bent Hill

Bent #4

Study area

A

B

1.02

Soil

Upper bentonite, Y bed

Lower bentonite, X bed

Shale

10.90

2.06

11.74

2.40

Average

thickness

(ft)

Section 11

1.90

11.60

2.20

7.10

0.87

Bent Hill

0.98

2.45

2.60

15.48

2.11

Bent #4

Average

thickness

(ft)

Average

thickness

(ft)

Hard clay/bentonite (waste)

Brown clay (overburden)

VALLEY COUNTY

Location of stratigraphic section

Imerys active placer claims

Figure 23. Generalized stratigraphy of bentonite beds in the Glasgow area, Montana. A, Map showing location of stratigraphic 

columns in relation to active placer claims located for bentonite. B, Stratigraphic columns showing thickness and stratigraphic 

separation of the lower and upper bentonite layers. Map and sections modified from David Crouse, Imerys, written commun., 

April 2016.
Locatable Minerals 53

Hosterman and Patterson (1992) correlate these two 

beds in the bentonite-rich interval in the Bearpaw Shale in the 

Malta and Glasgow areas (fig. 21) to the X and Y beds in the 

Ingomar-Vananda area in south-central Montana (fig. 17; Berg, 

1970). Throughout this report, the upper bed in the Malta 

and Glasgow areas is referred to as “X”, and the lower bed as 

“Y”. In the Ingomar-Vananda area, Berg (1970) describes two 

bentonite beds that are 0.9–1.2 m thick, and are separated by 

approximately 3 m of montmorillonite-rich shale. The lower of 

the two beds (X bed) is 90–120 cm thick, pale-olive color on 

a fresh moist surface, light gray on a dry surface, and contains 

flakes of biotite that are visible in hand samples. The upper 

bed (Y bed) is 76–107 cm thick, greenish yellow, and does not 

contain visible biotite phenocrysts in hand samples. The most 

characteristic feature of the Y bed is buff-colored calcareous 

concretions composed of calcite and montmorillonite. The 

concretions are generally ovoid and range from 0.3 to 5.5 m 

in diameter. The concretions are concentrated at the base of 

the bentonite bed, and in some places, form a continuous layer 

several centimeters thick. Similarities in the thickness of the 

beds and their stratigraphic separation, association of biotite 

with the lower bed, and concentrations within the upper bed 

support the interpreted correlation.

In the Hardin District (figs. 17, 18), Knechtel and Patterson 

(1956) describe beds V, W, and X but do not describe a Y bentonite

bed. The bentonite beds are notably thicker in this district than 

in the Ingomar-Vananda and Malta-Glasgow areas. The V bed 

is as much as 6.8 m thick; calcareous concretions 2.4–3.6 m in 

diameter are abundant on the exposed surfaces of this bed. The 

W bed consists of 0.9–2.4 m of light-colored bentonite. The 

X bed is as much as 2.1 m thick and locally contains large, 

gray, calcareous concentrations.

Exploration and Mining Activity

Bentonite exploration and mining activity has been 

ongoing in the Malta and Glasgow areas for at least 60 years 

(table 7). The locations of active placer claims and mineral 

patents are shown on figure 21. The spatial database of active 

placer claims1

 located for bentonite was derived from records 

provided by the BLM office in Billings, Montana. A spatial 

database showing the location of mineral patents for bentonite 

was derived from records provided by the BLM office in Billings 

and a Web page for General Land Office records (Bureau of 

Land Management, 2016a). Using mineral occurrence and 

permit databases, we located 21 open pits that were previously 

mined for bentonite, and we created a database from spatially 

referenced satellite imagery (figs. 13, 21; table 8).

Bentonite was discovered in the Glasgow area in 1955; 

we could not find information regarding when bentonite was 

1

Under the 1872 Mining Law, placer claims originally included only deposits of mineral-bearing sand and gravel containing free gold or other detrital 

minerals. By congressional acts and judicial interpretations, many nonmetallic 

bedded or layered deposits, such as gypsum and bentonite, are also located as 

placer claims (Bureau of Land Management, 2011).

found in the Malta area. There is almost no information on the 

exploration activity that occurred between discovery and when 

mining first took place in 1976 (Glasgow) and 1978 (Malta). 

Mapping, drilling, and sampling programs must have been 

conducted but this information is not publicly available. Based 

on their exploration work, American Colloid submitted applications and received patents on more than 200 placer claims 

between 1978 and 1984 that cover about 15.7 km2

 (Bureau of 

Land Management, 2016a).

Mining continued until about 1986, at which time American Colloid had processed approximately 0.91 million metric 

tons (Mt) of bentonite from their plant in Malta. Federal Bentonite processed less than 0.91 Mt of bentonite at their plant 

near Glasgow and shipped another 163,000 metric tons of bulk 

material by rail. Federal Bentonite produced 838,000 metric 

tons between 1978 and 1986 (Gregg Menge, oral commun., 

2016). Bentonite was extracted from at least 21 open-pit mines 

in the Malta and Glasgow areas. A rail spur linked the plant 

near Glasgow to the Burlington Northern-Santa Fe railroad 

that runs across northern Montana.

Following closure of the mines near Glasgow in about 

1986, the Brazil Creek Bentonite Company and its affiliates 

dropped all their placer bentonite claims in the Glasgow area. 

The area was staked again by Gregg Menge of Lewistown, 

Montana beginning in the early 1990s. As of 2016, there are 

450 active bentonite placer claims in the Glasgow area that 

cover approximately 36 km2

. A plan of operations to produce bentonite on the Bent Number 4 claim was submitted 

to the BLM in 2015 and approved in 2016. The claims in the 

Glasgow area are currently held by Imerys, a company specializing in the production and processing of industrial minerals (David Crouse, Imerys, written commun., April 2016).

After the plant at Malta was closed and demolished, 

placer claims appear to have been dropped and re-acquired 

at least twice. As of 2016, American Colloid Company has 

320 active bentonite claims in the Malta area that cover about 

25 km2

. No recent applications have been submitted to the 

BLM under 43 CFR 3809.

For the Malta area, mineral-resource estimates must 

have been calculated as part of the mineral patent and validity 

examination process. Only two BLM mineral reports covering 76 claims were available for this study. American Colloid 

Company estimated 3.1 Mt bentonite for the upper and lower 

beds in one group of 38 claims that cover 2.9 km2

 (Bigsby and 

Sollid, 1975). The tonnages were calculated on a dry basis 

using 1.6 metric tons dry bentonite per cubic meter of in-place 

material. For a separate group of 38 placer claims, the tonnage estimate provided by American Colloid was 1.03 Mt for 

an unspecified acreage (Durst, 1981). Tonnage was calculated 

using approximate bulk density of 1.6 metric tons dry bentonite per cubic meter (Durst, 1981). The report by Durst (1981) 

also provides a “reserve” statement from American Colloid at 

the time the report was prepared. They estimated 25.2 Mt in 

the field, which corresponds to 18.8 Mt of processed bentonite 

(table 9).
54 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 7. Summary of bentonite-related exploration and mining activities in the North-Central Montana Sagebrush Focal Area.

[TCM, Tax Court Memorandum; IBLA, Interior Board of Land Appeals; Mt, million metric ton]

Date Text Reference

1955 Robert Hansen discovered an unknown amount of bentonite on public 

lands in 1955.

Hansen v. Commissioner (34 TCM 1488 

[1975])

1958 The Brazil Creek Bentonite Company was formed to mine the bentonite 

discovered near Brazil Creek, southwest of Glasgow. The company 

staked 100 mining claims.

Hansen v. Commissioner (34 TCM 1488 

[1975])

1965 Brazil Creek Bentonite Company granted Archer-Daniels-Midland Company an option to purchase the bentonite on its claims. Subsequently, 

Ashland Oil and Refining Company acquired all interests of ArcherDaniels-Midland. Federal Bentonite, in turn, acquired all the interests 

of Ashland.

Hansen v. Commissioner (34 TCM 1488 

[1975])

1976 Federal Bentonite, a division of Aurora Industries, Aurora Illinois, opened 

a small bentonite processing plant southeast of Glasgow. The bentonite 

claims were leased from Brazil Creek Mining Company.

Bureau of Land Management (1980)

1976 Surface-management regulations (43 CFR 3809) were published as a 

proposed rulemaking.

Bureau of Land Management (1980, 2000)

1978 American Colloid Company opened a bentonite processing plant in Malta, 

Montana.

Pederson (2004); Bureau of Land Management 

(1980)

1978 to 1984 American Colloid patented 209 mining claims. Bureau of Land Management (2016a)

1979 Federal Bentonite processing plant closed after processing less than 0.91 

Mt tons of bentonite.

Bureau of Land Management (2015a); Bureau 

of Land Management (1992)

1982 Lawsuit clarified status of bentonite as locatable mineral. United States v. Kaycee Bentonite Corp. (64 

IBLA 183, 233 [1982])

1983 to 1985 After the Malta plant closed, Federal Bentonite mined, solar dried, and 

shipped in bulk by rail approximately 163,000 metric tons of bentonite.

Bureau of Land Management (1992)

1986 American Colloid closed and demolished the Malta plant after processing 

approximately 0.91 Mt tons of bentonite.

Bureau of Land Management (1980); Pederson 

(2004)

1988 The 228 unpatented claims located for bentonite in the Brazil Creek area 

were abandoned.

Bureau of Land Management (1992); Causey 

(2011)

1993 Annual maintenance fee required for unpatented mining claims. 30 U.S.C. 28f - Fee

1993 Placer claims in the Malta area abandoned. Causey (2011)

1993–2007 Bentonite claims re-staked in the Brazil Creek area by Gregg Menge. Causey (2011); Gregg Menge, oral commun., 

March 2016.

1994 to 1995 Moratorium on patent applications. Bureau of Land Management (2010)

2005 The Montana Department of Revenue enacted the Bentonite Production 

and Royalty Tax on the gross yield of bentonite produced replacing the 

tax on mines net proceeds similar to other miscellaneous minerals.

Montana Tax Foundation Inc. (2007); Montana 

Department of Revenue (2014); Montana 

Legislative Services (2015)

2006 Placer claims in the Malta area re-staked. Causey (2011)

2008 and 2009 The global recession and corresponding cuts in steel production caused a 

drop in price and reduced demand for bentonite.

Virta (2010)

2010 Placer claims in the Malta area abandoned. Causey (2011)

2013 American Colloid recorded placer claims in the Malta area. Recordation receipts in BLM office, Billing, 

Montana.

2014 S and B Industrial Minerals North America submitted opencut mining 

plan of operations for Bent Hill to the State of Montana

Montana Department of Environmental Quality 

(2016)

2015 S and B Industrial Minerals North America submitted a plan of operations 

to the BLM for Bentonite #4 site.

Bureau of Land Management (2015a)

2015 Imerys, a French multinational company which specializes in the production and processing of industrial minerals acquired S and B Minerals. Imerys (2015)

2016 Plan of operations to mine bentonite at the Bentonite #4 site approved. KLTZ (2015)
Locatable Minerals 55

Table 8. Bentonite pit locations in and near the North-Central Montana Sagebrush Focal Area.—Continued

[BLM, Bureau of Land Management; ND, no data; PWA, proposed withdrawal area; T, township; R, range; S, section]

Surfacemanagement 

agency or 

owner

Deposit name Claim name Claimant

Proposed 

withdrawal 

area (PWA)

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Township-Range 

Sections Aliquot

Private land ND ND ND Farther than 

500 m from 

PWA

47.3706 108.7823 T19N R23E S33 NWNW

Private land ND ND ND Farther than 

500 m from 

PWA

47.3716 108.7836 T19N R23E S33 NENE

Federal 

(BLM)

Bentonite 

Number 6 

Mine

ND ND Completely 

within PWA

48.0791 107.0102 T27N R36.5E S24 L5

Federal 

(BLM)

Bentonite 

Number 5 

Mine

P46 Imerys Completely 

within PWA

48.0885 107.1116 T27N R36E S17 SWSE

Federal 

(BLM) 

and private land

ND PAUL25, 

PAUL26

American 

Colloid 

Co.

Within 500 m 

of PWA

48.0328 107.7719 T26N R31E S06 SESE, SENESE

Federal 

(BLM)

Bentonite 

Number 4 

Mine

ND ND Completely 

within PWA

48.0991 107.0094 T27N R36.5E S13 L1

Private land ND ND ND Within 500 m 

from PWA

48.0340 107.7837 T26N R31E S06 NESW, SWSW, 

SESW, SESENW, 

NWNWSE, 

NENWSE, SWNWSE, SENWSE

Federal 

(BLM)

ND ND ND Completely 

within PWA

48.1038 107.0313 T27N R36E S12 SESW

Federal 

(BLM)

ND B4 Imerys Completely 

within PWA

48.1079 107.0356 T27N R36E S12 NWSW

Federal 

(BLM)

Bentonite 

Number 3 

Mine

ND ND Completely 

within PWA

48.1103 107.0120 T27N R36.5E S12 L4

Federal 

(BLM)

Bentonite 

Number 3 

Mine

ND ND Completely 

within PWA

48.1110 107.0134 T27N R36.5E S12 L3

Federal 

(BLM)

ND P61, B19 Imerys Completely 

within PWA

48.1036 107.1202 T27N R36E S08 SESW, SWSW

Federal 

(BLM)

ND R95, R373, 

R374

Imerys Completely 

within PWA

48.1046 107.1039 T27N R36E S08; 

T27 R36E S09

SESE; SWSW

Federal 

(BLM)

ND R78 Imerys Completely 

within PWA

48.1039 107.1659 T27N R35E S12 SWSW

Federal 

(BLM)

ND ND ND Completely 

within PWA

48.1156 107.0176 T27N R36E S12; 

T27N R36.5E 

S12

NENE; L2

Federal 

(BLM)

Bentonite 

Number 1 

Mine

B43, R75 Imerys Completely 

within PWA

48.1022 107.1834 T27N R35E S11; 

T27N R35E 

S14

SWSW, SESW; 

NWNW, NENW
56 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 8. Bentonite pit locations in and near the North-Central Montana Sagebrush Focal Area.—Continued

[BLM, Bureau of Land Management; ND, no data; PWA, proposed withdrawal area; T, township; R, range; S, section]

Surfacemanagement 

agency or 

owner

Deposit name Claim name Claimant

Proposed 

withdrawal 

area (PWA)

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Township-Range 

Sections Aliquot

Federal 

(BLM)

ND R121, R120 Imerys Completely 

within PWA

48.1159 107.0223 T27N R36E S01; 

T27N R36E 

S12

SWSE, SESE; 

NWNE, NENE

Federal 

(BLM)

ND R109, R107 Imerys Completely 

within PWA

48.1173 107.0549 T27N R36E S20 SWSW, SESW

Federal 

(BLM) 

and private land

ND LEK 74, 

LEK73

American 

Colloid 

Co.

Within 500 m 

of PWA

48.1246 108.1904 T27N R27E S02; 

T27N R27E 

S01

NESENE, 

SESENE; 

NWSWNW, 

NESWNW, 

SWSWNW, 

SESWNW, 

NWSW, SENW, 

NWNESW, 

NENESW, 

SWNESW, 

SENESW, 

SWNE, NWSE

Federal 

(BLM)

ND ND ND Completely 

within PWA

48.1278 108.2140 T27N R27E S02 SWNW, SENW

Federal 

(BLM) 

and private land

ND LEK 68, 

LEK69

American 

Colloid 

Co.

Within 500 m 

of PWA

48.1278 108.2055 T27N R27E S02 NENW, SENW, 

NWNE, SWNE, 

NWSENE

Table 9. “Reserves” stated by American Colloid Company in 

1981 for properties in the Malta area, Montana (Durst, 1981).

[Reserves are in quotes because it is unknown if the tonnages solely 

represent that part of the mineral inventory that is economic. Processing 

1.34 metric tons (Mt) of bentonite in the field yields a metric ton of 

processed bentonite]

Land type Bentonite in the field 

(Mt)

Processed bentonite 

(Mt)

Unpatented claims 17 12.7

Private land 3.92 2.92

Private lease 4.13 3.08

State lease 0.181 0.135

Total 25.2 18.8

Calculating tonnage for flat-lying beds of bentonite is 

fairly straightforward—it is the product of the area of the 

unit, its thickness, and the bulk density of the ore material 

(volume × bulk density). The only complication is the effect 

of moisture content on the bulk density used for bentonite. 

Calculated tonnage of bentonite in the Malta area assumed an 

original moisture content of 31 percent (2.1 metric tons per cubic 

meter), a reduced moisture content of 13 percent (1.83 metric 

tons per cubic meter), and 90 percent recovery:

Calculated tonnage = (1.647 metric tons) × (area [m2

]) × 

(average bed thickness [m]) (Durst, 1981).

Potential for Occurrence

There are no known maps that show the distribution of 

bentonite-rich intervals in the middle part of the Bearpaw 

Shale or the important bentonite beds, X and Y, in the MaltaGlasgow area. There are maps that show the Siparyann bed in 

the Charles M. Russell National Wildlife Refuge, which are 

appropriate for the scale of this assessment. For this report, 

geologic models were developed to estimate the distribution of 

the bentonite-bearing rocks. When considering mineral potential, 

we focused on identifying areas where thick (~0.6 m) bentonite beds could be mined using open-pit methods with relatively 

low stripping ratios (~10:1 or less).

Nine bentonite tracts, MTB01 to MTB09, were delineated (appendix 2; San Juan and others, 2016). The lower X 

and upper Y bentonite beds in the Glasgow and Malta areas 

were assessed in detail (figs. 24 and 25). Outside these two 

areas, we completed a generalized evaluation of the bentonitic 

interval in the Bearpaw Shale and bentonite beds in the Fox 

Hills and Hell Creek Formations in the rest of the study area 

(fig. 26). For the final mineral potential map and GIS, the 
Locatable Minerals 57

detailed and generalized assessments were integrated (fig. 27). 

Generalized results were removed from the areas where the X 

and Y beds in the Bearpaw Shale in the Malta and Glasgow 

areas were studied in detail (fig. 26) and the detailed assessment results shown in figures 24 and 25 were inserted. The 

final potential results included in the GIS database (San Juan 

and others, 2016) are shown in figure 27. The following sections describe the assessment process for the various areas. 

Geostatistical Modeling of the X and Y Beds in the 

Bearpaw Shale, Malta and Glasgow areas: Tracts MTB06 

to MTB09

For the Malta and Glasgow areas, predictive models 

based on the distribution of concretions associated with the 

Y bentonite bed were created to estimate where the X and Y 

bentonite beds are exposed, or under less than 3.6 m of cover. 

The concretions form bright, gray to white, circular features as 

great as 6 m in diameter on exposed bedding planes (fig. 22). 

Using GIS software, a geologist digitized the distribution of 

concretions, as seen on the satellite images, as short line segments. Each line segment represents one or more concretions 

along the base of the Y bed. The line file segments were converted to points and an elevation for each point was extracted 

from a 30-m digital elevation model. The distribution of 

concretions mapped by geologists using GIS software gives 

a quick overview of where bentonite beds are present and the 

point information was used to create a geostatistical model of 

the lower contact of the Y bed, as described below.

Using the point data for the distribution of Y-bed concretions as input for empirical Bayesian kriging, geostatistical 

models (prediction surface and error map) for the base of the 

Y bed were created separately for the Malta and Glasgow 

areas. Model parameters are summarized in appendix 4. The 

geostatistical models, which represent the lower contact of 

the Y bed, were converted to raster files, and then subtracted 

from the 30-m digital elevation model. On maps, the value of 

zero would approximate the location of the base of the Y bed 

where exposed at the surface. Results of the raster calculation 

were classified into two categories: (1) one that represents a 

typical elevation distance between the base of the upper and 

lower bentonite beds and (2) one that represents the Y bed 

and an amount of overburden consistent with a low strip ratio 

(3.6 m) (fig. 24). A map of the distribution of the two model 

categories shows a close relation to the distribution of patented 

and active placer claims for bentonite in the two areas. This 

is to be expected if the model is successful. Error maps show 

best results near observation points used to construct the 

geostatistical model (fig. 25). In addition to predicting what 

we know, the model shows that the two bentonite beds should 

extend well beyond the current distribution of mining claims. 

In the Malta area, the model predicts about 158 km2

 where the 

X and Y beds are at or near the surface (about 24 km2

 in the 

proposed withdrawal area). In the Glasgow area, the model 

predicts about 120 km2

 where the X and Y beds are at or near 

the surface within the study area (about 100 km2

 in the proposed withdrawal area). By comparison, American Colloid has 

patented and active placer claims covering 41 km2 in the Malta 

area, and Imerys has placer claims covering approximately 36 km2

in the Glasgow area. Model results for the X and Y bentonite 

beds were converted to mineral potential tracts, clipped to the 

USGS study area boundary, and included in the mineral potential tracts database (MTB06 to MTB09, appendix 2; fig. 25; 

San Juan and others, 2016).

Occurrence of Bentonite Outside the Malta and Glasgow 

Areas

Bentonite is an important locatable mineral in the study 

area. Unfortunately, no detailed geologic maps exist in the 

public domain that define the distribution of bentonite beds. 

Therefore, we developed the following model to help provide 

a modern assessment.

For the study area outside the Malta and Glasgow area, 

a different strategy was used to delineate bentonite tracts 

because the bentonite beds and their associated concretions 

were harder to map on satellite images. For the bentonite-rich 

interval in the middle part of the Bearpaw Shale, a stratigraphic 

and geostatistical model was constructed to predict the 

outcrop extent. For bentonite in the Fox Hills and Hell Creek 

Formations, geologic map units were extracted from published 

1:100,000-scale maps to show where bentonite could occur.

Stratigraphic and Facies Modeling to Map the Bentonitic 

Interval in the Bearpaw Shale: Tracts MTB01 to MTB03

A model was developed using stratigraphic information, 

interpolation, and raster mathematics to map the extent of the 

bentonite-rich interval that contains the X and Y bentonite beds 

in the Bearpaw Shale. The basic strategy was to use stratigraphic 

information in well data to create surfaces representing the 

upper and lower contacts of the bentonitic interval and to use 

GIS software to intersect these surfaces with the ground surface to show where they should crop out.

The bentonitic interval was not recorded in the logs for 

oil and gas wells that cut the Bearpaw Shale in the study area 

(Ronald M. Drake II, written commun., 2016), with the exception of Continental Oil Company’s South Zortman well 1 (dry 

hole SZ-1; figs. 5, 20; plate 53 in Knechtel, 1959). The elevation of the top of the Judith River Formation was available in 

the wells, and was used to create a surface that represented 

that contact. Using the stratigraphic information in SZ-1, we 

added elevation to the Judith River Formation surface to create 

surfaces representing the bottom and top of the bentonite-rich 

interval as measured in dry hole SZ-1. The process for creating the surfaces is described below.

The elevation of the top of the Judith River Formation 

in oil and gas wells in and near the study area was derived 

from databases obtained from the Montana Board of Oil and 

Gas Conservation (2016). Formation tops were recorded as 

down-hole depths; elevations were calculated by subtracting 

the depth from the collar elevation of the well. About 1,500 drill 

holes intersected the top of the Judith River Formation in and 

near the study area. Most intersections were in oil and gas 

fields west, northwest, and north of the study area (fig. 10). 

Only 30 holes intersected the top of the Judith River Formation in the study area.
58 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

107°10' 

48°10'

48°5'

B

108°20' 108°0' 

48°10'

48°0'

A

0 2 4 6 MILES

0 2 4 6 8 KILOMETERS

Base modified from U.S. Geological Survey 

DEM data, 2016. Boundary data from San 

Juan and others (2016). Roads and political 

data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic 

Projection. Central meridian, 108° W., 

latitude of origin, 36.5° N. North American 

Datum of 1983

Figure 24. Maps showing predicted distribution of the X and Y bentonite beds in the Malta, A, and Glasgow, B, areas, 

Montana. Category –15 to 3 represents the area where the interval between the bottom of the X bed and the top of the Y 

bed could be exposed. Category 3.1 to 15 represents the area where the Y bed is overlain by less than 3.6 m of overburden. 

USGS, U.S. Geological Survey.
Locatable Minerals 59

107°0' 106°50'

107°40'

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Area of map A Area of map B

Study area

EXPLANATION

Patented bentonite claims

Bentonite placer claims

Bentonite layer depth (from surface, in feet)

-15–3

3.1–15

USGS study area boundary
60 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

107°10' 

48°10'

48°5'

B

108°20' 108°0' 

48°10'

48°0'

A

0 2 4 6 MILES

0 2 4 6 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

MTB08

MTB09

MTB06

MTB09

MTB07

MT

MTB07

Figure 25. Maps showing prediction errors in the geostatistical surface used to generate the tracts shown in figure 27 for 

Malta, A, and Glasgow, B, areas, Montana. Tracts MTB06 through MTB09 are all high potential, but their certainties range from 

A to D. Errors increase away from control points. USGS, U.S. Geological Survey.
Locatable Minerals 61

107°0' 106°50'

107°40'

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Area of map A Area of map B

Study area

EXPLANATION

Bentonite layer 15 - 15 ft depth

Assessment tract type—Bentonite

Outside study area

Prediction error

Control points for base of upper 

 bentonite layer

MTB09 - Certainty A

MTB08 - Certainty B

MTB07 - Certainty C

MTB06 - Certainty D

USGS study area boundary

1.57–23.5

23.5–36.6

36.6–53.2

53.2–152

B06

MTB08
62 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Geostatistical methods (kriging) were used to generate surfaces for the top of the Judith River Formation using 

the entire dataset and just those drillholes in the study area. 

Using simple kriging, model errors were not acceptable; mean 

standardized was not equal to or close to zero, and root mean 

square standardized was not equal to or close to one. Given 

the limited data in the study area and the model errors for 

the geostatistical approach, we decided to use a deterministic 

method to create a continuous surface for the top of the Judith 

River Formation. The ArcGIS Topo to Raster tool was used to 

create the surface for the elevation of the upper contact of the 

Judith River Formation.

Using raster calculations in ArcGIS, the stratigraphic 

distances from the top of the Judith River Formation to the 

lower and upper contacts of the bentonitic interval in well 

SZ-1 were added to the Judith River elevation surface to create 

hypothetical elevation surfaces for the bottom and top of the 

bentonite-rich interval. Using raster calculations in ArcGIS, 

the bentonite surfaces were subtracted from a 90-m digital 

elevation model to estimate where the stratigraphic surfaces 

should intersect the ground surface. Where the stratigraphic 

and the ground surfaces intersect, the subtraction raster has a 

value of zero. The zero values were plotted on a map to show 

where the bentonitic surfaces should crop out.

The model results were then compared with the maps of 

mineral patents and active placer claims located for bentonite, 

as well as the maps we created showing the distribution of 

concretions associated with bentonite beds in the Malta and 

Glasgow areas. The model performed well for areas near dry 

hole SZ-1, but poorly to the east.

The model was not good because the assumption of 

constant thickness from the top of the Judith River Formation 

to the base of the bentonitic interval in the Bearpaw Shale was 

incorrect. The Bearpaw Shale is a transgressive marine unit 

that conformably overlies the Judith River Formation (Knechtel, 1959; Gill and Cobban, 1973). During the Cretaceous Period, 

sea level rose and oceans transgressed across the central part 

of North America, creating the Western Interior Seaway (fig. 

17). Nearshore, coarser grained clastic material, such as sand, 

was deposited; offshore, fine-grained material, such as silt and 

clay, accumulated. During marine transgressions, the shoreline 

moves towards higher ground, and with it, the coarser grained 

sedimentary facies. The Bearpaw Shale was deposited during 

the last substantial marine transgression in the Cretaceous 

Period. This resulted in the movement of deepwater environments (shales of the Bearpaw) to areas formerly characterized 

by nearshore, shallow water conditions (sandstones of the 

Judith River Formation). As a result, the Bearpaw Shale forms 

a western-thinning wedge of shale (thinner towards the western shore of the Western Interior Seaway and thicker towards 

the deeper part of the basin) and the contact between the Judith 

River Formation and the Bearpaw Shale becomes younger to 

the west. Therefore, the distance between the top of the Judith 

River Formation and an overlying time-horizon, such as a 

bentonite layer, is not constant. The separation distance should 

increase towards the east, away from the western shore of the 

Western Interior Seaway.

We applied a simple linear correction to account for the 

increasing stratigraphic separation between the top of the 

Judith River Formation and the bentonitic interval in deeper 

parts of the Western Interior Seaway (fig. 20). A section of the 

Bearpaw Shale in the Kaycee District in Wyoming provided 

an initial estimate of values to use to estimate the distance 

between the top of the Judith River Formation and the Bearpaw Shale bentonite-rich interval. We assumed the shoreline 

was approximately north-south in Bearpaw-Shale time and 

used the separation distances in well SZ-1 and the Kaycee 

District to create a raster file of separation values to add elevation of the top of the Judith River Formation. After applying a 

facies correction, the predicted outcrop pattern for the bentonite-rich interval was consistent with mapped distribution of 

concretions and mining claims. In the areas where bedrock is 

exposed, interpretation of surficial mineralogy from Advanced 

Spaceborne Thermal Emission and Reflection Radiometer 

(ASTER) imagery confirmed the presence of smectite in those 

areas where the model indicated the bentonite-rich interval 

should crop out (Rockwell and others, 2015; Rockwell, 2016). 

The model also predicted that the bentonite-rich interval in the 

Bearpaw Shale to be exposed along the Missouri River over 

the area where the Siparyann bentonite bed is known to occur 

in the Charles M. Russell National Wildlife Refuge (fig. 21B; 

Miller and others, 1979).

Bentonite in the Fox Hills and Hell Creek Formations: Tracts 

MTB04 and MTB05

Bentonite is also present in the Fox Hills and Hell Creek 

Formations that crop out the southeastern part of the study 

area. For the potential map, the distribution of these map units 

was extracted from 1:100,000-scale geologic maps provided 

by the Montana Bureau of Mines and Geology (2016). No 

modeling was required because the existing maps were sufficient to show the distribution of rocks with bentonite potential; 

steps to determine potential and certainty of these tracts are 

described in the following section.

Mineral Potential of Bentonitic Units Outside the Malta 

and Glasgow Areas

In order to determine mineral potential and certainty, 

these models and maps were combined with a surficial geologic 

map that distinguishes areas underlain by weathered bedrock 

from areas overlain by overburden of glacial till or alluvial 

deposits. The amount of overburden determines if a bentonite 

bed is a potential resource. As noted previously, bentonite is 

mined from open pits that rarely extend to depths of 15 m with 

maximum strip ratios of about 10:1. Only those beds close to 

the surface have any potential to be exploited. The models and 

approaches described in the previous section predict where 

bentonite-bearing units should crop out. If they are covered 

by younger surficial sediments or are in an area of steep terrain, 

there could be too much overburden for the beds to be a 

potential resource.
Locatable Minerals 63

In order to integrate cover and overburden into the potential 

map, a surficial map was derived from soil maps prepared by 

the Natural Resources Conservation Service (Montana State 

Library, 2016b). Soil map databases for the five counties that 

intersect the study area were queried to extract the parent 

material of the soil map units. The soil-map-unit descriptions 

were used to code parent material if the information was missing from the database. The data for the counties was merged, 

dissolved, and clipped to create the map used in the analysis. 

The Natural Resources Conservation Service provides detailed 

descriptive names for parent material based on the grain size 

of the parent material and (or) the lithology of the bedrock 

source. These specific, descriptive names were grouped into 

fewer units based upon common characteristics. For example, 

parent material called clayey alluvium, coarse-loamy alluvium, 

gravelly alluvium, loamy alluvium, sandy alluvium, and silty 

alluvium were grouped into a category we called alluvium. 

Complex parent material descriptions were recoded to create 

a spatial database with fewer than a dozen categories of 

parent material.

The distribution of surficial units was merged with the 

modeled outcrop distribution of the bentonite-rich interval 

in the Bearpaw Shale and the known extent of the Fox Hills 

and Hell Creek Formations to create the potential map for the 

study area outside the Malta and Glasgow areas (fig. 26; tracts 

MTB01 to MTB05, appendix 2; San Juan and others, 2016). 

The assumption behind combining these two themes is that 

areas covered by glacial till and alluvium could have lower 

potential because the till and alluvium may be too thick, or 

alluvial or glacial processes could have removed the bentonite 

beds by erosion (fig. 22D).

Terrain also affects the amount of overburden and the 

resource potential of bentonite beds. Throughout the study 

area, the bentonite beds are relatively flat-lying, with dips 

of 1–2o. In the Malta and Glasgow areas, bentonite beds are 

exposed and the slope is sufficiently low such that the amount 

of overlying rock does not increase substantially over considerable lateral distances. In parts of the study area near the Missouri River, the bentonite beds are exposed in steeper valleys 

and are 60–90 m below the upland surface (Miller and others, 

1979). If the bed is followed into the hillside, the amount of 

overlying rock will increase substantially over short distances. 

Because of the geometry of the layers relative to the upland 

surface and the valley walls, the stripping ratio could range 

from 0:1 to 50:1 in little more than a kilometer. Therefore, 

areas where the bentonite-rich interval is predicted to occur, 

and which are underlain by weathered bedrock with shallow 

slope, were ranked high potential (tract MTB01, appendix 2, 

fig. 26; San Juan and others, 2016). Areas where the bentoniterich interval is predicted to be present and exposed, but with 

steep slopes and poor accessibility, were ranked “moderate 

potential” (tracts MTB02 and MTB04, appendix 2, fig. 26; 

San Juan and others, 2016). Areas where the bentonite-rich 

interval is predicted to be present, but is covered by extensive 

surficial deposits, are ranked “low potential” (tracts MTB03 

and MTB05, appendix 2, fig. 26; San Juan and others, 2016). 

Ideally, this assessment should consider the grade of 

the bentonite beds (the amount of montmorillonite present), 

the physicochemical properties of the clay minerals, and the 

presence of any deleterious material, such as gypsum, that 

would affect end use. General statements about the quality 

of a particular bentonite deposit are difficult to make because 

of the amenability of bentonite to blending or treatment with 

additives to enhance its properties. New uses continue to be 

identified (for example, Carrado and Komadel, 2009; Gates 

and others, 2009; Güven, 2009) and standards for testing 

continue to be updated (American Petroleum Institute, 2011). 

Previous standards used to evaluate the use of bentonite as a 

taconite binder have been eliminated and not replaced (ASTM 

International, 1997). The old and limited publically available 

data on the grade and quality of the bentonite in the study area 

(Berg, 1969; Bigsby and Sollid, 1975; Miller and others, 1979; 

Durst, 1981) really cannot be used to make conclusions about 

its potential value now and in the future.

Economic Analysis of the Deposit Type

The most important bentonite resource in the study area 

is associated with the X and Y bentonite beds in the Malta 

and Glasgow areas, and their extensions. The bentonite beds 

are more than a meter thick, are continuous, and occur in 

areas with relatively low relief that would have relatively low 

stripping ratios if the beds were covered by overburden. These 

beds are also close to a major rail line that could transport 

material to markets.

The Siparyann bed in the Bearpaw Shale is relatively 

thick and continuous (fig. 21B), but it occurs in an area where 

the amount of overburden exceeds the economic strip ratio. 

The steep terrain where the bed is exposed limits areas with 

low overburden to long narrow strips along the sides of steep 

valleys (Miller and others, 1979).

The bentonites in the Fox Hills and Hell Creek Formations are laterally discontinuous and are in areas with limited 

accessibility (Miller and others, 1979). The Hell Creek bentonites have been used as sealants for stock ponds and irrigation 

ditches by local ranchers, but many other deposits of material 

with comparable quality exist nearby.

The domestic and global commodity summary for bentonite (appendix 5 in Day and others, 2016; Bleiwas, 2016) 

indicates that bentonite is an important commodity now and 

will continue to be in the future. As an example, from 1990 to 

2014 the compound annual growth rate in domestic production 

was approximately 1.4 percent. Given the broad range of uses 

for bentonite and the growth of demand and production, one 

could ask why there has been little bentonite produced from 

the Malta and Glasgow areas in the 60 years since the beds 

were discovered.
64 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

Roy

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

191

2

191

191

Charles M. Russell National Wildlife Refuge

Little Rocky

Mountains

Missouri

River

MTB01

MTB03

MTB03

MTB02

MTB03

MTB04

MTB05

10 20 30 MILES

0 10 20 30 KILOMETERS

0

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Figure 26. Mineral potential map for sedimentary bentonite deposits in the bentonitic interval in the Bearpaw Shale 

and in the Fox Hills and Hell Creek Formations, in the North-Central Montana Sagebrush Focal Area. The potential map 

is based on combining data themes that map the possible distribution of the bentonitic interval based on stratigraphic, 

facies, and terrain analysis with a surficial map of the study area. USGS, U.S. Geological Survey.
Locatable Minerals 65

107°0'

Fort Peck LakeFort Peck Lake

Glasgow

Jordan

GARFIELD COUNTY

VALLEY COUNTY

BURLINGTON NORTHERN/SANTA FEBURLINGTON NORTHERN/SANTA FE

2

200

MTB03

MTB01

MTB04

MTB05

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Low potential with certainty level D

High potential with certainty level D

Moderate potential with certainty level D

Charles M. Russell National Wildlife Refuge

Assessment tract type—Bentonite

Glasgow and Malta study areas
66 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

Roy

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

191

2

191

191

Charles M. Russell National Wildlife Refuge

Little Rocky

Mountains

Missouri

River

10 20 30 MILES

0 10 20 30 KILOMETERS

0

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

MTB06-09

MTB01

MTB05

MTB02

MTB03

MTB05

MTB04

MTB04

Figure 27. Mineral potential map for sedimentary bentonite deposits in the North-Central Montana Sagebrush Focal 

Area. This map shows how the two approaches used to assess bentonite in the study area were integrated for the 

geographic information system database that accompanies the assessment reports (San Juan and others, 2016). 

USGS, U.S. Geological Survey.
Locatable Minerals 67

107°0'

Fort Peck LakeFort Peck Lake

Glasgow

Jordan

GARFIELD COUNTY

VALLEY COUNTY

BURLINGTON NORTHERN/SANTA FEBURLINGTON NORTHERN/SANTA FE

2

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Low potential with certainty level D

High potential with certainty levels A, B, C, and D

Moderate potential with certainty level D

Charles M. Russell National Wildlife Refuge

Assessment tract type—Bentonite

Glasgow and Malta study areas

MTB06-09

MTB01

MTB04

5
68 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

One reason could be related to broader trends in the 

energy and steel industries. The price of bentonite in the last 

60 years is broadly correlated to the price of oil, rig counts, 

and the price of steel (fig. 28). As the industry values of oil 

and steel wax and wane, so does the demand for bentonite. 

The only time bentonite was produced in the study area was 

during the early 1980s when crude oil and steel prices were 

both relatively high. Other reasons for low production include 

changes to the legislative and regulatory environment as summarized in table 7. For example, imposition of maintenance 

fees for mining claims, and a moratorium on mineral patents 

more than likely suppressed exploration and mining activity 

in the study area, particularly when combined with a downturn in the oil and steel industries. A new State law enacted in 

2005, which reduced the tax burden on bentonite production, 

made it more likely that bentonite could be mined in the area 

(Montana Tax Foundation Inc., 2007; Montana Department of 

Revenue, 2014; Montana Legislative Services, 2015).

Future development will likely be affected by the energy 

and steel markets, the tax burden imposed by government, and 

the overall regulatory environment.

Magmatic System: Diatreme-Hosted Diamond 

Deposit Type

Diamond is a crystalline form of elemental carbon that 

forms at extremely high temperature and pressure conditions 

that are possible only very deep in the Earth’s upper mantle. 

Large diamonds, particularly large diamonds without flaws, 

and colored diamonds are extremely rare and are very valuable 

as gemstones. The majority of diamonds, however, are small, 

0

50

100

150

200

250

US dollars, constant 2013

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

Rig count

1950 1960 1970 1980 1990 2000 2010

Year

Steel (dollars/ton)

Bentonite (dollars/ton)

Crude oil prices (dollars/barrel)

Rig count

EXPLANATION

25

75

125

175

225

Figure 28. Graph showing steel, bentonite, and crude oil prices and the number of drilling rigs in the 

United States from 1950 to 2013. Prices are recalculated to constant dollars as of 2013. Modified from U.S. 

Geological Survey (2014), Macrotrends LLC (2016), and U.S. Energy Information Administration (2015).
Locatable Minerals 69

flawed, and contain dark impurities. These small non-gem 

diamonds have a variety of industrial uses as abrasives, in drill 

bits, and to coat saw blades for stone and concrete cutting.

Mineral Description

Kimberlite and lamproite diatremes and hypabyssal 

intrusions are the only two primary sources of diamonds in 

the world (for example, Groat and others, 2014; appendix 3 

in Day and others, 2016, and references therein). The age of 

diamond formation is much older than the age of magmatism 

that transports diamond to the surface (Gurney and others, 

2010). Occurrence of diamond in kimberlite or lamproite 

depends on two important factors: (1) formation and preservation of diamond in upper mantle rocks and (2) rapid transport 

of diamond to avoid conversion to graphite or oxidation to 

CO or CO2

 during the upward movement of kimberlitic and 

lamproitic magmas through diamond-bearing mantle rocks to 

the surface (Mitchell, 1993; Gurney and others, 2010; Stachel 

and Luth, 2015).

Diamonds occur in mantle keels, the thickened parts 

of the lithospheric mantle that underlie continental crust of 

Archean cratons, at depths below the graphite-diamond transition zone. Kimberlitic and lamproitic magmas are derived 

by small amounts of melting deep within the mantle or in 

the asthenosphere, have relatively high volatile content, are 

MgO-rich, and are less oxidized than basaltic magma. These 

low-viscosity, volatile-rich melts erupt rapidly by hydraulically fracturing the overlying rocks; the ascent of these melts 

is thought to be one of Earth’s most dynamic volcanic processes (Shirey and Shigley, 2013). The rapid upward transport 

of diamond crystals prevents them from being completely 

dissolved and resorbed into the melt. Even so, most diamond 

crystals show external patterns of resorption.

Within Archean cratons, kimberlites and lamproites occur 

in clusters, fields, and provinces (Mitchell, 1986). Kimberlites 

and related rocks are commonly associated with major lineaments and fault zones that correspond to major zones of 

weakness in the lithosphere; these zones appear to control the 

ascent from the upper mantle. The tectonic forces responsible 

for triggering mantle melting to produce kimberlite are still 

debated (Helmstaedt and Gurney, 1994; Heaman and others, 

2003). The presence of mantle keels under the continents 

appears to affect where kimberlitic volcanism takes place by 

slowing the rise of upwelling mantle, at about the depth where 

carbonate-bearing peridotite could begin to melt (Shirey and 

Shigley, 2013). Under these conditions, volatile-rich melts 

separate to form volatile-enriched kimberlite magma that 

migrates rapidly upwards through the crust. The location of 

intrusions at or near the surface is determined by local and 

regional geologic and structural features, such as proximity to 

faults and mafic dike intersections (Mitchell, 1986).

Worldwide, only 1 in 10 kimberlites is diamond-bearing, 

and only 1 in 10 diamond-bearing kimberlites is economically 

mineable (Janse and Sheahan, 1995). In their global compilation, Janse and Sheahan (1995) recognized 5,000 kimberlite 

intrusions—500 are diamondiferous, 50 have been or are 

being mined, and 15 are large active mines. Now, more than 

10,000 kimberlites are known worldwide (Gurney and others, 2010), but only about 1 percent are commercially viable 

diamond mines.

Kimberlites and lamproites are mixtures of xenoliths of 

crustal rocks and mantle, minerals separated from disaggregated 

xenoliths, phenocryst minerals, alteration minerals, and 

pieces of preexisting kimberlitic or lamproitic rock (Shirey 

and Shigley, 2013). Even if all conditions are met, diamonds 

are always trace minerals in the host kimberlite and can be 

difficult to find. Instead, indicator minerals that are geologically 

associated with diamonds, but are much more abundant, are 

used to explore for diamond deposits. Indicator minerals 

are derived from upper mantle peridotites and other upper 

mantle inclusions. For example, mantle-derived garnets show 

compositional variations that allow them to be put into groups. 

The term G10 is used for, calcium-undersaturated (subcalcic), 

or harzburgitic peridotitic garnets, whereas G9 represents 

calcium-saturated or lherzolitic peridotitic garnets (Grütter and 

others, 2004). Indicators for diamonds derived from peridotite 

are G10 pyrope garnets with enriched Cr and depleted Ca, 

chromite with enriched Cr and Mg and depleted Al and Ti, 

diopside with enriched Cr and Al, and orthopyroxene with 

enriched Mg/(Mg+Fe) ratios. For diamonds derived from 

eclogite, Cr-poor garnets with enriched Na and Ti and diopside 

with enriched Na are common indicator minerals (Shirey and 

Shigley, 2013).

Geology and Occurrence in the Study Area

The Central Montana alkaline province contains Late 

Cretaceous to Eocene alkalic rocks that include clusters of 

diatremes, dikes, and plugs that range in composition from 

alkaline ultramafic to carbonatites (fig. 29). A few of the 

occurrences are kimberlitic in affinity. The study area includes 

part of the Missouri River Breaks diatreme field, and the cluster of Grassrange intrusions is immediately south of the study 

area. Because of the presence of kimberlitic rocks, both areas 

have been explored for diamond.
70 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

FERGUS COUNTY

BLAINE COUNTY

CHOUTEAU COUNTY

HILL COUNTY

JUDITH BASIN COUNTY

PETROLEUM COUNTY

CASCADE COUNTY

LIBERTY COUNTY

MEAGHER COUNTY MUSSELSHELL COUNTY

WHEATLAND COUNTY

GOLDEN

VALLEY

COUNTY

Havre

Lewistown

Great Falls

Lodge Pole

Hays

Roy 

Homestead

Grassrange

intrusions

Macdougal Springs

Haystack Butte

Eagle Buttes

Highwood 

Mountains

Little Belt Mountains

110°0' 10

48°0'

47°0'

M

Williams cluster

Bullwhacker Coulee

0 20 40 60 MILES

0 20 40 60 KILOMETERS

Shaded-relief data copyright © 2008 Esri and its licensors. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

MTD01

MTD02

MTD03

Figure 29. Map showing the location of diatremes and related igneous rocks in the study area and the surrounding 

region (modified from Hearn, 1979, 2004; McGee and Hearn, 1989; Irving and Hearn, 2003; Faure, 2010; Anderson and 

others, 2016). Map also shows mineral potential for diamond deposits in the North-Central Montana Sagebrush Focal 

Area. USGS, U.S. Geological Survey.
Locatable Minerals 71

Other intrusive centers, diatremes, 

 and dikes

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

ROSEBUD COUNTY

Glasgow

Malta

Jordan

Fort Peck LakeFort Peck Lake

Gold Butte

Smoky Butte

08°0'

Missouri

River

IDAHO

MONTANA

WYOMING

OREGON

NEVADA

WASHINGTON

UTAH

ALBERTA

Study area

Map area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

Certainty level B

Certainty level C

Certainty level D

Connoly Coulee baked sandstone

Diatreme

Diatreme inferred from geophysics

Major or unusual dikes and plugs

Extent of Anaconda aeromagnetic survey

Missouri River Breaks intrusions

Low potential for diamonds

Assessment tract type—Diatreme-hosted diamond

MTD03
72 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Missouri Breaks Diatremes

The Missouri River Breaks diatremes form an eastnortheast-trending band of ~50 ultramafic alkalic diatremes 

(including satellitic pipes) and additional dikes and plugs, 

which range from 52 to 47 Ma, in the Missouri River Breaks 

area of north-central Montana (Hearn, 1968, 1979; Marvin and 

others, 1980). The diatremes and intrusions were produced by 

alkalic ultramafic magmas that mainly crystallized to melnoites 

and lesser amounts of other rocks such as kimberlites, aillikites, 

and carbonatites. Their fine-grained fresh equivalents, which 

would correspond to potassic olivine nephelinite or olivine 

melilitite, are typically altered to secondary minerals in bedded pyroclastic deposits within the diatremes. In other parts of 

the world, similar alkalic ultramafic magmas occur in the same 

broad geographic areas as kimberlites, and may have been 

parental to kimberlites (Dawson, 1980). Although the alkalic 

ultramafic character of the Missouri River Breaks diatremes 

indicates kimberlitic affinity, only the Williams diatremes, 

which are one cluster of four closely spaced occurrences, 

can be termed true kimberlite on the basis of the presence of 

the typical kimberlite indicator minerals chromian pyrope 

garnet, chromian diopside, enstatite, and magnesian ilmenite 

(Dawson, 1980).

The Williams cluster of kimberlite occurrences is located 

on the west side of Thornhill Butte (also known as Cyprian 

Butte, Sippary Ann Butte, and Sipparyann Butte), a syenitecored, faulted domal uplift on the southwest side of the Little 

Rocky Mountains (fig. 30). The largest occurrence, Williams 

1, is 350 by 200 m, has an irregularly rounded shape, and 

shows a flat-topped positive magnetic anomaly (Hearn and 

McGee, 1984; Irving and Hearn, 2003). Williams 2 is a breccia body about 100 by 50 m with two short dike offshoots in 

uplifted Paleozoic limestone. Williams 3 is a nearly circular, 

25-m pipe, with 10 percent kimberlite breccia and a large 

descended block of Telegraph Creek Formation that fills the 

rest of the pipe area. Williams 4 is a N. 32° E.-trending dike 

that is 380 m long and 10–30 m wide, which contains massive 

kimberlite and kimberlite breccia. Williams 1 and 4 contain 

xenoliths of garnet peridotite, garnet-spinel peridotite, spinel 

peridotite, and megacrysts (>1 cm) of garnet, chromian diopside, and magnesian ilmenite from the upper mantle (Hearn 

and McGee, 1984).

The only other Missouri River Breaks diatreme containing upper mantle garnet peridotites is the Macdougal Springs 

diatreme, which is 10 km northwest of the study area (McGee 

and Hearn, 1989). Partially altered xenoliths of garnet peridotite, 

garnet-spinel peridotite, and spinel peridotite contain fresh 

garnet, spinel, chromian diopside, and phlogopite, but all 

olivine and enstatite have been replaced by quartz and calcite. 

All garnets are purple to red G9 chromian pyrope garnets, and 

no G10 garnets are present.

Grassrange Intrusions

The Grassrange intrusions include aillikite, alnoite, 

carbonatite, kimberlite, and monchiquite, and occur as dikes, 

plugs, diatremes, and rare sills (Hearn, 2004). All of the 

Grassrange intrusions are south of the study area (fig. 29), 

but are discussed here because one of the occurrences is 

kimberlite that has been commercially tested, and many of 

the other rock types are similar to those of the Missouri River 

Breaks occurrences. Only the Homestead occurrence is kimberlite;

the other intrusions are not.

The Homestead kimberlite has an irregular rounded shape 

(430 by 130 m) forms low hills in lower Colorado Group 

beds, and contains massive kimberlite and kimberlite breccia. 

The Homestead kimberlite has a marked positive magnetic 

anomaly. The kimberlite locally contains abundant xenoliths 

of garnet peridotite, garnet-spinel peridotite, spinel peridotite, 

and fewer websterites; megacrysts are absent (Hearn, 2004).

Exploration and Mining Activity

On a continental scale, geological and geophysical 

modeling can help identify Archean terranes with a history 

conducive to preserving diamonds at depth (Griffin and others, 

2013). Geologic mapping, geophysical surveys, geochemical 

prospecting, and mineral studies aimed at identifying the presence of diamond-bearing mantle at depth are commonly used 

to evaluate specific targets.

Geophysical methods can accurately detect and map 

kimberlite and lamproite pipes (Macnae, 1995). Airborne magnetic surveys are the most cost effective; local anomalies may 

be of normal or reversed polarity compared to a non-magnetic 

background, depending on the remanent magnetization of the 

body. Airborne electromagnetic surveys are effective in detecting weathered or crater-facies pipes, and they can be used to 

discriminate targets where other nearby features may cause 

magnetic anomalies (Anderson and Ponce, 2016). In 1981, 

Anaconda Minerals Company contracted an aeromagnetic 

survey over part of the Missouri River Breaks field; the survey 

was roughly centered on the location of the Williams cluster 

of kimberlite diatremes (fig. 29). The analytic signal computed 

from the survey shows many of the known diatremes are 

characterized by short-wavelength, high-amplitude anomalies 

(fig. 31; Anderson and others, 2016).

Exploration for kimberlites in the field commonly is 

based on the occurrence of indicator minerals in soils, stream 

sediments, and glacial deposits in order to locate in-place, 

upstream, or up-ice kimberlite occurrences. Indicator minerals 

are recovered in heavy-mineral concentrates derived by panning samples in the field, separating material in a laboratory 

using heavy liquids, or using a hydraulic gravity-separation 

technique such as the Wilfley table. Chromian pyrope garnet 

and chromian diopside can be recognized in the field by their 

distinctive colors—purple to lavender for chromian pyrope 

garnet and bright (“traffic-light”) green for chromian diopside. 

Magnesian ilmenite, although not as distinctive, commonly 

has a shiny metallic gray appearance, and is non-magnetic or 

less magnetic than the usually abundant magnetite grains in 

heavy-mineral concentrates. The presence of high-chromium, 

low-calcium pyrope garnets (referred to as G10) in 
Locatable Minerals 73

Williams 4

Williams 3

Williams 2

Williams 1

108°42'0" 108°41'30"

47°51'15"

47°51'0"

9

10

5 1

6

2

7

3

4 8

11

12

13

1

PHILLIPS COUNTY

FERGUS COUNTY

BLAINE COUNTY

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Map area

Study area

EXPLANATION

Greenhorn Formation through 

 Skull Creek Shale

Claggett Shale

Eagle Sandstone

Judith River Formation

Kootenai Formation

Niobrara Formation and Carlile 

 Shale

Telegraph Creek Formation

Paleozoic rocks

Terrace and pediment gravel 

 deposits

Alluvial and colluvial deposits

Landslide and talus deposits

Massive kimberlite and 

 kimberlite breccia

Wasatch Formation

Star of Phillips lode claims

Thornhill Butte

Inferred contact of kimberlite

 diatremes

0 100 200 300 400 METERS

0 400 800 1,200 FEET

Political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. 

Central meridian, 108° W., latitude of origin, 36.5° N. 

North American Datum of 1983

Figure 30. Detailed map showing the Williams cluster of kimberlite occurrences, geology, and location of claims (modified 

from Hearn and McGee, 1983; Nathaniel Arave, Bureau of Land Management, Billings, Montana, written commun., 2016).
74 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

108°50' 108°30'

48°0'

47°50' Williams cluster

0 3 6 9 12 KILOMETERS

0 3 6 MILES

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Political data 

copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

MONTANA

IDAHO

WYOMING

ALBERTA SASKATCHEWAN

Study area

Area of map

Diatreme

Diatreme inferred from geophysics

Major or unusual dikes and plugs

USGS study area boundary Missouri River Breaks intrusions

Cow Creek Wilderness Study Area

Reduced-to-pole magnetics (nT/m)

0.0002

0.0121

0.0173

0.0231

0.0439

1.5130

0.0068

0.0095

0.0329

0.0270

0.0201

0.0147

EXPLANATION

Figure 31. Map showing analytic signal results from a high-resolution aeromagnetic survey by Anaconda Minerals Company 

(Anderson and others, 2016) and the location of known and inferred diatremes. Known and inferred diatremes are small 

features, typically less than 0.5 km across, which are characterized by short-wavelength, high-amplitude anomalies. USGS, 

U.S. Geological Survey. nT/m, nanotesla per meter.
Locatable Minerals 75

heavy-mineral concentrates from kimberlite is favorable for 

occurrence of diamond (for example, Grütter and others, 2004; 

Groat and others, 2014, and references therein).

In addition, pressure and temperature relations in the 

mantle can be deduced from the composition of minerals. 

Equilibrium temperature and pressure conditions for the upper 

mantle sources of garnet peridotite xenoliths can be calculated 

from a variety of mineral compositions, using several formulas 

based on mineral syntheses and thermodynamic calculations. 

Temperatures and pressures estimated from mineral compositions 

in xenoliths can be plotted relative to the graphite-diamond 

stability boundary to evaluate whether diamond-bearing 

mantle rocks underlie the region (fig. 32).

Exploration activity on the Missouri River Breaks 

diatremes between 1976 and 2010 can be inferred from the 

compilation of mining claim information (Causey, 2011). The 

number of lode claims on diatremes increased considerably 

between 1976 and 1977. The number of claims continued to 

gradually increase, peaking in 2003. The number of claims 

decreased beginning in 2004, and continued to decrease 

through 2010. By 2016, the only active claims in the study 

area were on the Williams cluster of kimberlites (Star of 

Phillips lode claims 1–13), and on the Saskatchewan Butte 2 

diatreme (HAL 40 lode).

Williams Kimberlites

The Williams 1 and 4 kimberlites were thoroughly 

explored in 1979–1980 by Anaconda Minerals Company 

(Ledford, 1981). An on-site separation plant to recover heavymineral concentrates processed 312 metric tons of kimberlite 

(several cubic meters of kimberlite from each of 31 pits and 

trenches) from the Williams 1 pipe and 27 metric tons from 

the Williams 4 dike. No macro-diamonds (>1 mm) or microdiamonds (<1 mm) were recovered.

Concentrates from the Williams kimberlites contain no 

G10 garnets, and the Williams garnet peridotite xenoliths 

similarly lack G10 garnets. Temperatures and pressures estimated from mineral compositions plotted in comparison to the 

graphite-diamond stability boundary show that the Williams 

garnet peridotites overlap the graphite-diamond boundary 

(fig. 32; Hearn and McGee, 1984).

Temperature, in degrees Celsius

Depth, in kilometers

Pressure, in gigapascals

0

40

80

120

160

200

240

1

2

3

4

5

6

7

500 700 900 1,100 1,300 1,500

Williams

 high T-P

Williams

 low T-P

Spinel peridotites

Macdougal

 Springs

Porcupine

 Dome

Homestead

Diamond

Oceanic lithosphere

Non-cratonic continental lithosphere

Cratonic lithosphere

Graphite

Asthenosphere

Figure 32. Diagram showing calculated and inferred pressure-temperature-depth 

estimates for upper mantle xenoliths from diatremes in the study area and nearby 

(modified from Hearn, 2004; Shirey and Shigley, 2013). T, temperature; P, pressure.
76 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

According to BLM records in 2016, 13 active lode claims 

cover the Williams kimberlite occurrences. They are referred 

to as Star of Phillips 1 to 13 (fig. 30).

Homestead Kimberlite

The Homestead kimberlite has been thoroughly evaluated 

by Delta Mining and Exploration. One micro-diamond 

was recovered in 1999 from a 45-kg sample processed in 

Saskatchewan Research Council Geoanalytical Laboratories 

(Ellsworth, 2000). In 2007, a 30.7-metric-ton bulk sample 

was collected from four trenches and processed in the same 

lab (Apex Geoscience Ltd., 2008). No macro-diamonds were 

recovered; the micro-diamond content was not reported (Delta 

Mining and Exploration Corp., 2008).

In several panned heavy-mineral concentrates from the 

Homestead kimberlite, ~2–12 percent of purple chromianpyrope garnets are G10 garnets. Microprobe analyses show 

that 3 out of 30 analyzed Homestead garnet peridotite xenoliths contain G10 garnets (Hearn, 2004). However, those G10 

garnets do not show the enriched chromium contents paired 

with the depleted calcium contents that are more favorable 

for economic diamond potential (Grütter and others, 2004). 

Homestead garnet peridotites plot in the graphite field (fig. 32), 

including the three garnet peridotites with G10 garnet compositions (Hearn, 2004).

Other Areas in the Missouri River Breaks Field

Elsewhere in or near the study area, diatremes and some 

stream sediments and terrace gravels have been sampled for 

the kimberlite indicator minerals chromian-pyrope garnet, 

chromian-diopside, and magnesian-ilmenite as part of a program to assess the Cow Creek Wilderness Study areas (Miller, 

1986; Mytton and others, 1988). A total of 169 grab samples 

were collected at 8 sites (6 diatremes and 2 dikes, Miller, 

1986). Chromian pyrope garnet and (or) chromian diopside 

were found at five of the eight sites. Twenty-five garnets from 

five diatremes were analyzed by electron microprobe. Only 

one is chromium pyrope, G9, from Saskatchewan Butte 2 

diatreme; the other 24 garnets are almandines and andradites, 

and are not specifically kimberlite-related. No diamonds were 

found in the small volume of material sampled. Large-scale 

sampling at each site, required to test for the presence and 

grade of diamond, was beyond the scope of their study. One 

placer sample, from Coyote Coulee, contained chromium 

pyropes and chromium diopsides possibly derived from 

Williams kimberlites, although the sample site currently 

is topographically isolated from direct drainage from the 

Williams area.

Two G9 chromian pyrope garnets were found in panned 

concentrates from the Big Slide diatreme (Hearn and McGee, 

1984). Ten purple chromium pyropes from Saskatchewan 

Butte 2 diatreme concentrates have been analyzed by B. Carter 

Hearn, Jr.; eight garnets are G9 compositions, and two are G10 

compositions. No garnet peridotite xenoliths have been found 

in Saskatchewan Butte 2 diatreme.

The indicator mineral chromium diopside occurs in concentrates from Big Slide 1 and Squaw Creek diatremes. These 

two diatremes, and the Bullwhacker Coulee diatreme, contain 

small partly altered xenoliths of spinel peridotite, which 

contain chromium diopside, and, if disaggregated, can be a 

source of chromium diopside in concentrates. Spinel peridotite 

xenoliths indicate derivation from ~45 to 75 km depth, which 

is too shallow for diamond stability.

Undiscovered Diatremes

Undiscovered diatremes could exist in or near the study 

area. Aeromagnetic surveys are commonly used to locate 

possible kimberlites. Widely spaced aeromagnetic flight lines 

will only locate diatremes if the aircraft passes directly over 

the feature. The NURE flight line data (Hill and others, 2009) 

were reviewed, and five possible diatreme locations were 

identified (table 10). Two are within the study area but outside 

the proposed withdrawal area. Six of the seven diatremes that 

show as small positive anomalies on the higher resolution 

Anaconda aeromagnetic maps (Anderson and others, 2016; 

Anderson and Ponce, 2016) were located and mapped in the 

field in 1987–1991 by B. Carter Hearn Jr., on the basis of an 

earlier version of those maps. The seventh diatreme is totally 

concealed beneath Quaternary pediment gravel deposits. All 

seven are outside the study area.

In areas of weathered bedrock, diatremes can be positive 

topographic features if solid, resistant kimberlite ring dikes 

or plugs are present. Some diatremes show no topographic 

expression, and a few are negative topographic features owing 

to lower erosional resistance compared to the wall rocks. In 

areas of cover, it is unlikely that there would be any topographical 

indication of diatremes. About 48 percent of the study area is 

covered by surficial deposits, including alluvium and glacial 

till; surficial deposits cover about 39 percent of the proposed 

withdrawal area.

In aerial imagery, clues to the presence of diatremes are 

intricately rilled topography (in the pyroclastic fill of diatremes), 

light-colored areas in gray marine Bearpaw Shale, and redor pink-colored areas of Wasatch Formation claystones and 

siltstones (the only red beds in the stratigraphy above the 

Lower Cretaceous Kootenai Formation). On-site clues are 

green, dark-gray, or black olivine-bearing igneous rock as 

float or in outcrop; green to gray soil colors (pyroclastic fill); 

red and green argillite pebbles (Belt Supergroup clasts from 

early Eocene Wasatch Formation conglomerates); pebbles 

of bleached syenite porphyry (from conglomerate beds in 

Paleocene Fort Union Formation, derived from the Little 

Rocky Mountains); and clasts derived from the volcanic rocks 

of the Bears Paw Mountains (analcime phonolite, porphyritic 

syenite, and tinguaite).
Locatable Minerals 77

Table 10. Location, size, and rock type of Missouri River Breaks diatremes, major dikes, and magnetic anomalies.—Continued

[Diatremes consist of sites where fragmental deposits are the major or substantial part of intrusion; major references size and length; if rock type is not specifically known from other sources, it is called “melnoite.” na, not available]

Name Symbol Size 

(meters) County

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Rock type Notes

Diatremes

Baker MonumentBM 78×62 Fergus 47.6828 108.9750 “melnoite”

Barnard Ridge BR 190×120 Blaine 47.8203 109.2455 “melnoite”

Beauchamp 

Creek

BC 73×46 Phillips 47.8797 108.4147 “melnoite”

Bergum BE 225×155 Fergus 47.6894 109.2889 “melnoite”

Big Slide 1 BS1 170×135 Blaine 47.8303 109.2678 “melnoite” Chromian pyrope garnet and chromian 

diopside occur in concentrates from Big 

Slide 1 diatreme (Hearn and McGee, 1984). 

Two grains of G9 garnet were analyzed. 

Sparse, small, partly altered spinel dunite 

and harzburgite xenoliths occur in east 

breccia pipe in Big Slide 1 diatreme.

Big Slide 2 BS2 75×30 Blaine 47.8297 109.2700 “melnoite”

Bird Rapids 1 

northeast

BR1 160×30 Fergus 47.7630 109.1842 “melnoite”

Bird Rapids 2 

southwest

BR2 76×37 Fergus 47.7619 109.1869 “melnoite”

Black Butte 1 BB1 270×210 Blaine 47.8428 109.1769 Bedded 

pyroclastic 

deposits; 

central breccia; plug of 

melnoite

Black Butte 2 

north

BB2 21×18 Blaine 47.8953 109.1783 “melnoite”

Black Butte 3 

southwest

BB3 150×67 Blaine 47.8397 109.1805 “melnoite”

Black Rock BR 145×115 Blaine 47.8558 109.0472 “melnoite”

Boundary BO 180×120 Blaine 47.9255 108.8855 “melnoite”

Bullwhacker 

Coulee

BU 330×240 Blaine 47.8078 109.0275 “melnoite”

Burnt Wagon BW 115×61 Blaine 47.8955 108.9669 “melnoite”

Button Butte BB 90×40 Fergus 47.5036 108.5050 “melnoite”

Cabin Creek C 49×47 Phillips 47.8914 108.8061 “melnoite”

Cole CO 420×200 Blaine 47.9361 108.8597 “melnoite”

Crazyman 

Coulee

CC 100×80 Blaine 47.9692 108.8272 “melnoite”

Ervin Ridge 1 E1 200×200 Blaine 47.8175 109.1250 “melnoite”

Ervin Ridge 2 

southwest

E2 40×30 Blaine 47.8150 109.1286 “melnoite”

Gilmore G 130×90 Blaine 47.8633 109.0033 “melnoite”

Hay Coulee H 230×205 Blaine 47.9128 108.9586 “melnoite” Studied by Miller (1986). Referred to as 

HAL 21, 22 claims.
78 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 10. Location, size, and rock type of Missouri River Breaks diatremes, major dikes, and magnetic anomalies.—Continued

[Diatremes consist of sites where fragmental deposits are the major or substantial part of intrusion; major references size and length; if rock type is not specifically known from other sources, it is called “melnoite.” na, not available]

Name Symbol Size 

(meters) County

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Rock type Notes

Haystack Butte HA 75×60 Chouteau47.6372 110.2222 “melnoite”

Joslin Bench JB 200×170 Phillips 47.8828 108.8069 “melnoite”

Lieurance 1 L1 395×240 Phillips 47.8939 108.8778 “melnoite”

Lieurance 2 

east

L2 69×40 Phillips 47.8639 108.8786 “melnoite”

Lieurance 3 

west

L3 280×120 Blaine 47.8900 108.8936 “melnoite”

Lone Tree 

Ridge

LT 375×280 Blaine 47.8317 109.2164 “melnoite”

Macdougal 

Springs

MD 290×190 Blaine 47.9344 108.9200 Pyroclastic tuff 

deposits; 

4 breccia 

units; 2 aillikite intrusions. Irving 

and Hearn 

(2003).

Macdougal Springs diatreme has garnet peridotite xenoliths (and spinel, and spinelgarnet peridotites; nearly all xenoliths are 

partly altered with olivine and orthopyroxene altered to calcite and quartz).

Miller Coulee MI 150×150 Phillips 47.9139 108.8064 “melnoite”

Saskatchewan 

Butte 1

SB1 280×245 Phillips 47.8622 108.8333 “melnoite” Studied by Miller (1986). Referred to as 

HAL 1 claim.

Saskatchewan 

Butte 2

SB2 130×33 Phillips 47.8608 108.8236 “melnoite” Studied by Miller (1986). Referred to as 

HAL 40 claim. Chromian pyrope garnet 

and chromian diopside occur in concentrates. Ten chromian pyropes from Saskatchewan Butte 2 were analyzed by B. 

Carter Hearn Jr. Eight have G9 compositions and two have G10 compositions.

Shellenberger SH 520×285 Blaine 47.8319 108.9008 “melnoite” Studied by Miller (1986). Referred to as 

HAL 2, 3 claims.

Squaw Creek SC 550×200 Blaine 47.8955 108.9180 “melnoite” Studied by Miller (1986). Referred to as 

HAL 23, 24 claims. Sparse, small, partly 

altered spinel dunite and harzburgite 

xenoliths occur in small dikes in Squaw 

Creek diatreme.

Sturgeon 1 

west

S1 95×88 Blaine 47.8078 109.0725 “melnoite”

Sturgeon 2 

central

S2 160×105 Blaine 47.8100 109.0678 “melnoite”

Sturgeon 3 east S3 325×185 Blaine 47.8108 109.0630 “melnoite”

Sturgeon 4 NE S4 up to 60×30 Blaine 47.8069 109.0494 “melnoite” Several

Tick T 235×130 Blaine 47.8597 109.2428 “melnoite”

Williams 1 W1 350×250 Phillips 47.8508 108.6925 kimberlite 

(Hearn and 

McGee, 

1984; Irving 

and Hearn, 

2003)

Concentrates and garnet peridotite xenoliths 

from the Williams kimberlites contain no 

G10 garnets.
Locatable Minerals 79

Table 10. Location, size, and rock type of Missouri River Breaks diatremes, major dikes, and magnetic anomalies.—Continued

[Diatremes consist of sites where fragmental deposits are the major or substantial part of intrusion; major references size and length; if rock type is not specifically known from other sources, it is called “melnoite.” na, not available]

Name Symbol Size 

(meters) County

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Rock type Notes

Williams 2 

northeast

W2 120×40 Phillips 47.8578 108.6900 kimberlite 

(Hearn and 

McGee, 

1984; Irving 

and Hearn, 

2003)

Concentrates and garnet peridotite xenoliths 

from the Williams kimberlites contain no 

G10 garnets.

Williams 3 

west

W3 34×28 Phillips 47.8522 108.6947 kimberlite 

(Hearn and 

McGee, 

1984; Irving 

and Hearn, 

2003)

Concentrates and garnet peridotite xenoliths 

from the Williams kimberlites contain no 

G10 garnets.

Williams 4 

dike

W4 390×37 Phillips 47.8525 108.7030 kimberlite 

(Hearn and 

McGee, 

1984; Irving 

and Hearn, 

2003)

Concentrates and garnet peridotite xenoliths 

from the Williams kimberlites contain no 

G10 garnets.

Major or unusual dikes and plugs

Barnard Ridge 

northeast 1

BRd1 480 Blaine 47.8222 109.2422 “melnoite”

Barnard Ridge 

northeast 2

BRd2 500 Blaine 47.8242 109.2361 “melnoite”

Big Slide NE BSd 240 Blaine 47.8333 109.2583 carbonatite

Bird Rapids BRd 4160 Blaine 47.7958 109.1472 “melnoite”

Lion Coulee LCd 1170 Blaine 47.8522 109.1633 “melnoite”

Black Butte 

south

BBsd 450 Blaine 47.8389 109.1786 “melnoite”

Bull Creek BCd 480 Phillips 47.8705 108.7372 carbonatite

Bullwhacker 

Coulee

BUd1 3310 Blaine 47.8278 109.0917 “melnoite” Discontinuous

Bullwhacker 

Coulee north

BUd2 880 Blaine 47.8467 109.0722 “melnoite”

Bullwhacker 

diatreme SW

BUd3 800 Blaine 47.8055 109.0319 “melnoite”

Burnt Wagon BWd 2610 Blaine 47.8789 108.9842 “melnoite”

Castle Rock CRd 400 Blaine 47.8633 108.9000 “melnoite” Several (~8) features; Studied by Miller 

(1986).

Ervin Ridge 

south 1

Esd1 850 Blaine 47.8133 109.1361 carbonatite

Ervin Ridge 

south 2

Esd2 220 Blaine 47.8103 109.1397 alnöite

Ervin Ridge 

north

End 500 Blaine 47.8230 109.1347 carbonatite

Gilmore north 

1

Gd1 880 Blaine 47.8667 109.0055 “melnoite”
80 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Table 10. Location, size, and rock type of Missouri River Breaks diatremes, major dikes, and magnetic anomalies.—Continued

[Diatremes consist of sites where fragmental deposits are the major or substantial part of intrusion; major references size and length; if rock type is not specifically known from other sources, it is called “melnoite.” na, not available]

Name Symbol Size 

(meters) County

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Rock type Notes

Gilmore west 2 Gd2 250 Blaine 47.8628 109.0055 “melnoite”

Gilmore east 3 Gd3 480 Blaine 47.8636 108.9992 “melnoite”

Gilmore east 

arcuate 4

Gd4 950 Blaine 47.8653 109.0005 “melnoite”

Lone Tree 

Ridge NE

LTd 600 Blaine 47.8403 109.2103 “melnoite”

Miller Coulee MId 620 Phillips 47.9111 108.8033 carbonatite

Mud Creek 1 

NE

M2 up to 21×9 Phillips 47.8847 108.6569 “melnoite” 5 plugs

Mud Creek 1 

SW

M1 24×7 Phillips 47.8828 108.6594 “melnoite” Sill, plug

Ricker Butte R up to 

200×12

Phillips 47.8819 108.3750 alnöite (Irving 

and Hearn, 

2003)

4 sills/dikes

Shellenberger 

Divide

SDd 210 Blaine 47.8433 108.8717 carbonatite Studied by Miller (1986).

Sturgeon Island Sd 930 Blaine 47.8000 109.0917 “melnoite” Discontinuous

Thorsen THd 350 Phillips 47.8944 108.7730 “melnoite”

Winifred East WEd 1470 Fergus 47.5808 109.1625 “melnoite” Zone

Zortman 1 east Z1p 335×75 Phillips 47.9144 108.5172 alnöite (Irving 

and Hearn, 

2003)

Plug

Zortman 2 west Z2p 21×12 Phillips 47.9130 108.5244 alnöite Plug

Anomalous features

Hidden HI 250–400 Blaine 47.9394 108.7744 Magnetic anomaly on Anaconda aeromagnetic survey; probable diatreme; estimated 

size based on anomalies over mapped 

diatremes.

na na na Blaine 47.9671 108.8576 Anomaly on Anaconda aeromagnetic survey

na na na Blaine 47.9784 108.8369 Anomaly on Anaconda aeromagnetic survey

na na na Phillips 47.9066 108.7869 Anomaly on Anaconda aeromagnetic survey

na na na Phillips 47.8778 108.8269 Anomaly on Anaconda aeromagnetic survey

na na na Blaine 47.8316 108.9642 Anomaly on Anaconda aeromagnetic survey

na na na Phillips 47.8667 108.7000 Anomaly on Anaconda aeromagnetic survey. 

Ground magnetic survey shows 90 gamma 

positive anomaly, unknown source. No 

igneous rock, no diatreme fill, no baking 

of Cretaceous bedrock.

na na na Blaine 47.8057 109.0174 Aeromagnetic spike on NURE flight line

na na na Fergus 47.7189 109.0011 Aeromagnetic spike on NURE flight line

Connoly 

Coulee

na na Petroleum47.3656 108.2744 Baked sandstone anomaly
Locatable Minerals 81

Potential for Occurrence

The study area includes part of the Missouri River Breaks 

field, so the study area was assessed for diamond (Tracts 

MTD01 through MTD03, appendix 2; fig. 33; San Juan and 

others, 2016). For this assessment, the rank of mineral potential is based on the regional geologic setting, the occurrence of 

the appropriate rock type (kimberlite or lamproite), the presence of diamond, the presence of indicator minerals that are 

consistent with the kimberlitic magma cutting upper mantle 

rocks that are within the diamond stability field, and the global 

rarity of diamond in known pipes (appendix 2).

Nearly all diamond-bearing kimberlites and lamproites 

occur in regions above Archean cratons (“Clifford’s Rule”, as 

modified by Janse, 1994). Known diatremes in the study area 

occur above the sections of the GFTZ that contains 1.8–1.7 Ga 

crust, which was derived from addition of igneous material or 

from metamorphic re-equilibration of Archean terrane in that 

zone (Peterman, 1981; Carlson and others, 2004; Sims and 

others, 2004; Gifford, 2010; Barnhart and others, 2012). Using 

the southeastern boundary of the GFTZ as shown in Barnhart 

and others (2012), both the Williams kimberlite cluster and the 

Homestead kimberlite lie above the GFTZ (fig. 3A), a Paleoproterozoic suture zone between two Archean terranes—Wyoming 

and Medicine Hat. The suturing event may have affected the 

cratonic root beneath the Archean cratons, reducing the potential for diamond occurrence and diamond survival.

Other parts of the study area that overlie the Wyoming 

Craton basement could have been more favorable for diamond, if kimberlite or lamproite magmas had been available 

for rapid transport. The Smoky Butte lamproite, south of the 

study area (fig. 3B), is not an olivine-rich lamproite, which 

would be more favorable for diamond occurrence. The history 

of diamond evaluation for Smoky Butte is unknown to the 

authors.

One kimberlite cluster (Williams) occurs in the study 

area; no macro-diamonds were found in commercial processing of 312 metric tons of kimberlite. One kimberlite (Homestead) occurs in the Grassrange area, 35 km south of the study 

area; one micro-diamond was recovered in early exploration, 

but later commercial processing of 30 metric tons of kimberlite 

found no macro-diamonds.

Group 9 chromian-pyrope garnets occur in garnet peridotite xenoliths and in heavy-mineral concentrates from Williams 

kimberlites. Group 10 chromian-pyrope garnets favorable for 

diamond occurrence are lacking. The Homestead kimberlite 

contains 2 to 12 percent G10 garnets in the garnet population 

in heavy-mineral concentrates, and 3 of 30 xenoliths sampled 

contain G10 garnets; these G10 garnets do not have the more 

enriched chromium contents and depleted calcium contents 

that would be favorable for diamond occurrence. The calculated equilibrium temperature-pressure field for Williams, 

Homestead, and Macdougal Springs garnet peridotite xenoliths overlaps the graphite-diamond stability boundary. However, xenolith temperature-pressure conditions are not definitive for prediction of diamond association. Macdougal Springs 

data may be less accurate because the xenoliths are partially 

altered, and the former presence of orthopyroxene is assumed.

Taken together, all available data indicate that the 

diamond potential of the study area is low. Tract delineation 

is based on the tendency of diatremes and related rocks to 

occur in clusters. We used the distances between intrusions 

and intrusion density to create mineral potential tracts. As 

described in Hammarstrom and Zientek (2016), two Esri 

ArcGIS tools were used to construct the tracts (fig. 33): (1) the 

“Generate Near Table” tool (to compute the “maximum near 

distance”), and (2) the “Kernel Density” tool. 

First, the distance between the centers (centroids) of 

every diatreme were determined using ArcMap’s “Generate 

Near Table” tool. The greatest near-distance is 70.3 km 

between two diatremes, and we used this value as the radius 

from which to draw a boundary (buffer) around all the known 

diatremes. This boundary forms the outer extent of the tract 

with the lowest level of confidence (B), which is tract MTD01 

(fig. 33; appendix 2; San Juan and others, 2016). Second, in 

order to map areas with certainty levels C and D, the kernel 

density surface (a raster file) was created in ArcGIS and classified 

into three groups using a geometrical interval classification 

scheme. The boundary between the D and C certainty levels 

ranges from 2,510 m to 5,235 m to the center of the nearest 

diatreme, and the boundary between the C and B certainty levels ranges from 3,616 m to 8,291 m to the center of the nearest 

diatreme (fig. 33; appendix 2; San Juan and others, 2016).

Both this study and the Utah-Wyoming study (Wilson 

and others, 2016) assessed diamond potential. There are other 

methods and approaches that could be used to create the tract 

maps and different parameters could be selected for this procedure. The reason for using this approach was for consistency 

in representing mineral potential tracts for the same deposit 

type between different areas.

Diamond Deposits Tracts: MTD01 (Diamond 1), 

MTD02 (Diamond 2), and MTD03 (Diamond 3)

These are low-potential tracts for diamond deposits, 

with certainty levels of D, C, and B, respectively. Parts of 

these tracts overlie areas that are specifically proposed for 

withdrawal.

Economic Analysis of the Deposit Types

Kimberlite-hosted diamond deposits in diatremes and 

dikes are one of the two main sources of diamonds from hard 

rock mines. However, in the study area, the lack of any past 

production, the absence of macro-diamonds, the scarcity of 

kimberlite, the lack of G10 garnets that are enriched in chromium and depleted in calcium, and the young age of the basement terrane all suggest that the potential for diamond deposits 

in the study area is low. There are no known kimberlites in the 

proposed withdrawal area, so the potential for diamond deposits 

in the proposed withdrawal area is also low.
82 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area 107°0' 108°0' 109°0' 48°0' 47°30' Roy Hays Malta Glasgow Lodge Pole Jordan PHILLIPS COUNTY GARFIELD COUNTY VALLEY COUNTY BLAINE COUNTY

FERGUS COUNTY PETROLEUM COUNTY

200

Fort Peck aL

ek

Fort Peck aL

ek

MTD03

MTD03

MTD02

MTD02

MTD02

MTD02

MTD02

MTD02

MTD01

Williams cluster

MTD01

MTD01

MTD01

MTD01

MTD01

Little Rocky

Mountains

Missouri

i R ver

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal area

10 20 30 MILES 0

0 10 20 30 KILOMETERS

Diamond tract, low potential

Assessment tract type

—

 Diatreme-hosted diamond

MTD01MTD02MTD03

Low potential with certainty level DLow potential with certainty level CLow potential with certainty level B

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

2

191

191

Figure 33. Mineral potential map for diamond deposits in the North-Central Montana Sagebrush Focal Area. USGS, U.S. Geological Survey.
Salable Minerals 83

Salable Minerals

Salable minerals, also known as mineral materials, 

include common varieties of sand, gravel, decorative stone, 

dimension stone, pumice, clay, and rock (appendix 1 in Day 

and others, 2016). These materials are typically used in 

various construction, agriculture, and decorative building or 

landscaping applications. Management of salable minerals in 

the planning area must comply with the Mineral Materials Act 

of 1947 (30 U.S.C. 601 et seq.), and all other relevant Federal 

and State laws.

In the study area, the most important salable minerals are 

sand and gravel. There has also been evaluation of clay. There 

has been no known production of decorative stone, dimension 

stone, pumice, or rock, and there are no known sources for 

these materials.

Sand and Gravel

The March 6, 2016, BLM LR2000 database shows one 

approved site for sand and gravel in the proposed withdrawal 

area within the North-Central Montana Sagebrush Focal Area 

(table 11).

Table 11. Active Bureau of Land Management mineral material 

authorizations in the proposed withdrawal area within the NorthCentral Montana Sagebrush Focal Area.

[Source: Bureau of Land Management (BLM) LR2000 database, March 6, 

2016. The number of cases is for the complete section that includes a 

proposed withdrawal area]

Commodity Number of 

approved sites

Number of 

pending sites

Sand and gravel 1 0

Sand and gravel have been produced from several locations within the study area (fig. 34). Most sand and gravel are 

used for construction purposes, and the majority is used for 

aggregate. This usage requires materials that are hard, durable, 

chemically nonreactive, and low in alkalis, and that do not 

contain deleterious minerals such as swelling clays. Because 

transport is typically the most substantial cost in producing 

aggregate, the source of the materials must be close to where 

they are needed.

The Bearpaw Shale, Judith River Formation, and 

Claggett Shale, which cover the majority of the study area, are 

not viable sources for aggregate because their rocks are soft, 

not durable, and commonly contain bentonite. Consequently, 

sand and gravel in the study area have been produced from 

glacial deposits and from alluvial deposits, where harder rocks 

and minerals are selectively preserved, and deleterious clay 

minerals are winnowed out (fig. 35).

The National Minerals Information Center of the USGS 

has confirmed that sand and gravel have been produced from 

operations in Fergus, Phillips, and Valley counties, and crushed 

stone has been produced from Fergus County. However, specific 

production figures are withheld to avoid disclosing company 

proprietary data, and the listed operations do not lie within 

the study area. No sand and gravel production was listed for 

Petroleum County.

Locations of active mines within the Focal Area were 

compiled from the Montana Department of Environmental 

Quality Opencut Mining Program permit data (Fernette, 

Bellora, and others, 2016; Fernette, Schweitzer, and Sangine, 

2016). There are a total of 18 active mines within the proposed 

withdrawal area, and 15 of these are sand and gravel operations 

(Fernette, Bellora, and others, 2016; Fernette, Schweitzer, and 

Sangine, 2016).

In some places outside the study area, the Late Cretaceous 

Fox Hills and Hell Creek Formations contain friable sandstones 

that are a source of sand. These formations have been quarried, 

crushed, and mixed with asphalt for use as paving material on 

highways (Johnson and Smith, 1964).

Clay Minerals

Clay minerals considered here include kaolinite and 

common clay; bentonite in the study area is described and 

discussed in the locatable minerals section of this report.

Common clay accounts for most of the world clay production, but not its value. In most places, common clay is a 

mixture of clay minerals. Common clay is used to manufacture 

brick, drain and roof tile, sewer pipe, and other construction 

materials. Common clay deposits are typically derived from 

shales that are low in iron and calcium.

Kaolinite is a clay mineral with the chemical composition Al2

Si2O5(OH)4. Kaolinite is used as (1) a filler and coater 

in paper, rubber, paint, and food; (2) the main component in 

refractories and fine china; and (3) a catalyst in petroleum 

refining and other manufacturing processes.

There are no verified records of production of kaolinite 

or common clay from the study area. Two studies suggest that 

kaolinite may occur in the study area, but the potential for 

significant deposits is low. Sahinen and others (1958) sampled 

and analyzed pure white kaolinite in 5- to 30-cm-thick beds 

of the Ellis Formation on the south side of the Little Rocky 

Mountains in sec. 12, T. 24 N., R. 24 E. This is white, highly 

refractory clay, with fair plasticity and green strength. The 

amount of kaolinite at this locality is not large (Sahinen and 

others, 1958).

Elsewhere in the study area, the Colgate Member of the 

Fox Hills Formation is a kaolinitic sandstone that is a potential 

source of kaolinite. The Whitemud Formation in Saskatchewan is the lateral equivalent of the Fox Hills Formation. The 

suitability of the Whitemud Formation as a source of kaolinite 

for paper coating was evaluated, and Pruett (1988) concluded 

that the economics were questionable because the kaolinite 

from the Whitemud has high abrasivity that prevents use in 

paper products, the yield of kaolinite from the Whitemud was 

low, and the kaolinite quality varies within the Whitemud. 
84 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Further work on the kaolinite in the Fox Hills Formation may 

be warranted, but results from the Whitemud Formation are 

not encouraging, and there are no known records of production 

of kaolinite from within the study area.

Gemstones: Petrified Wood

Petrified wood locally occurs in the Paleocene Fort Union 

Formation, in the Cretaceous sedimentary rocks of the upper 

part of the Judith River Formation, and in the Hell Creek Formation. However, the petrified wood is fairly drab; light tan, 

gray, or white; white-weathering; and incompletely petrified, 

with black carbonaceous areas. Locally, the petrified wood can 

have fine-grained drusy quartz coatings on fractures. The petrified 

wood in the bedrock of the study area is not at all similar to 

the multi-colored petrified wood from Arizona and southwest 

Montana (B. Carter Hearn, Jr., written commun., 2016).

The southern part of the study area contains a locality 

outside the proposed withdrawal area that is listed in the BLM 

database as “expired: non-precious gemstone”. Comparison of 

the location to the geologic map suggests that this corresponds 

to an occurrence of petrified wood in the Hell Creek Formation. Given the overall quality of petrified wood in the study 

area, the lack of other claims in the area, and the absence of 

previous assessments or citations in the published literature 

(Parks and others, 2016a, b), it seems unlikely that there are 

significant resources of gem-quality petrified wood in the 

study area.

Mineral Economics

Strategic and Critical Mineral Materials

As described and discussed elsewhere, the locatable 

minerals considered for the North-Central Montana Sagebrush 

Focal Area are gold, silver, bentonite, and diamond. None 

of these are considered to be strategic or critical minerals 

(British Geological Survey, 2012; appendix 4 in Day and 

others, 2016).

Outside the study area, including localities in the 

Bears Paw Mountains and on the Rocky Boy’s Reservation, 

carbonatites of the Central Montana alkaline province have been 

examined for REE potential, (Pecora, 1963; Lindsey and others, 

1977; McNary, 1981). On a global basis, carbonatites are the 

preeminent host rock for REE deposits, including the Bayan 

Obo deposit in China, which is currently the largest REE 

deposit worldwide (Yang and others, 2011; Chakhmouradian and 

Zaitsev, 2012).

We did not evaluate the likelihood of REE deposits in the 

study area for the following reasons:

1. In the Little Rocky Mountains, which are outside the 

study area, carbonatites are a minor component of the 

igneous rock suite.

2. The only known igneous rocks that crop out in the 

study area are syenite and alkali ultramafic rocks in the 

Missouri Breaks diatremes, and neither rock type is a 

favored host for REE deposits.

3. The only igneous rocks that crop out in the proposed 

withdrawal area are part of the Missouri River Breaks 

diatreme complex, and these bodies are too small to host 

a large REE deposit.

4. As expected for alkaline igneous rocks, available 

geochemical data show REE enrichment in many of the 

igneous rocks that were collected in the study area, but 

none of these samples contain strongly anomalous concentrations of REEs (Smith and others, 2016).

5. There has been no production of REEs from the study 

area, to the best of our knowledge there has been no 

extensive prospecting for REE deposits, and there are no 

previous assessments for REEs in the study area (Parks 

and others, 2016a, b).

Taken together, these factors indicate that the potential 

for REE deposits in the study area is low.
Mineral Economics 85

107°0' 108°0' 109°0'

48°0'

47°30'

Fort Peck L

ka e

Fort Peck L

ka e

Roy

Hays

Malta Glasgow

Lodge Pole

Jordan

PHILLIPS COUNTY

GARFIELD COUNTY

VALLEY COUNTY

BLAINE COUNTY

FERGUS COUNTY PETROLEUM COUNTY

191

2

2

191

191

200

Little Rocky

Mountains

200

Missourii R ver

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

Map area

Study area

EXPLANATION

USGS study area boundary

Proposed withdrawal areaBLM mineral material sites with case  type closed, expired, or rejected

Gemstone

. Sand and gravel occurrences

Sand and gravel

0 10 20 30MILES

0 10 20 30KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. Boundary data from San Juan and others (2016). Roads and political data copyright © 2014 Esri and its licensorsUSA Contiguous Albers Equal Area Conic Projection. Central meridian, 108° W., latitude of origin, 36.5° N. North American Datum of 1983

..

.

.

.

.

.

.

.

.

.

.

..

.

.

...

.

.

.

2

2

191

191

191

Figure 34. Map showing known deposits of salable minerals and active mineral material permit sites in the North-Central Montana Sagebrush Focal Area (modified from Dicken and San Juan, 2016a, b; Fernette, Bellora, and others, 2016; Fernette, Schweitzer, and Sangine, 2016). BLM, Bureau of Land Management; USGS, U.S. Geological Survey.
86 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

109°0' 108°0' 

48°0'

47°30'

Roy

Hays

Malta

Lodge Pole

PHILLIPS COUNTY

GARFIELD COUNTY

BLAINE COUNTY

FERGUS COUNTY

PETROLEUM COUNTY

191

2

191

191

Missouri

River

0 10 20 30 MILES

0 10 20 30 KILOMETERS

Base modified from U.S. Geological Survey DEM data, 2016. 

Boundary data from San Juan and others (2016). Roads and 

political data copyright © 2014 Esri and its licensors

USA Contiguous Albers Equal Area Conic Projection. Central 

meridian, 108° W., latitude of origin, 36.5° N. North American 

Datum of 1983

Little Rocky

Mountains

.

.

.

.

.

.

.

.

..

.

.

.

.

.

.

.

Figure 35. Map showing known sand and gravel deposits, active mineral material permit sites, and surficial 

deposits in the North-Central Montana Sagebrush Focal Area (modified from Dicken and San Juan, 2016a, b; 

Fernette, Bellora and others, 2016; Fernette, Schweitzer, and Sangine, 2016; Montana State Library, 2016b). BLM, 

Bureau of Land Management; USGS, U.S. Geological Survey.
Mineral Economics 87

107°0'

Fort Peck LakeFort Peck Lake

Glasgow

Jordan

GARFIELD COUNTY

VALLEY COUNTY

2

200

IDAHO

MONTANA

WYOMING

OREGON

NEVADA UTAH

WASHINGTON

COLORADO

Map area

Study area

EXPLANATION

USGS study area boundary

BLM mineral material sites with case type 

 closed, expired, or rejected

Gemstone

Sand and gravel

. Sand and gravel occurrences

Man made feature

Alluvium or colluvium

Denied access

Till, eolian deposits, glacial outwash, or 

 glaciolacustrine deposits

Residuum

..

.

.

.
88 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

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K.-Y., 2011, Mesoproterozoic carbonatitic magmatism in 

the Bayan Obo deposit, Inner Mongolia, North China—

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rare earth elements: Ore Geology Reviews, v. 40, no. 1, 

p. 122–131.

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U.S. Geological Survey Bulletin 1857-G, 19 p.

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deposits during regional uplift, central Otago, New Zealand: 

Economic Geology, v. 90, no. 4, p. 731–745.

Zientek, M.L., and Orris, G.J., 2005, Geology and nonfuel 

mineral deposits of the United States: U.S. Geological 

Survey Open-File Report 2005–1294A, 172 p.

Zietz, I., Gilbert, F.P., and Snyder, S.L., 1980, Aeromagnetic 

map of Montana, in color: U.S. Geological Survey, Geophysics Map GP-934, scale 1:1,000,000.

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Appendixes 1–4
100 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

The approach to classification of the qualitative mineralresource potential for locatable minerals followed that 

prescribed in Bureau of Land Management (BLM) Manual 

Sections 3031 and 3060 defined originally by Goudarzi (1984) 

(fig. 1-1).

Level of Potential

N. The geologic environment, the inferred geologic 

processes, and the lack of mineral occurrences do not indicate 

potential for accumulation of mineral resources.

L. The geologic environment and the inferred geologic 

processes indicate low potential for accumulation of mineral 

resources.

M. The geologic environment, the inferred geologic 

processes, and the reported mineral occurrences or valid 

geochemical/geophysical anomaly indicate moderate potential 

for accumulation of mineral resources.

H. The geologic environment, the inferred geologic 

processes, the reported mineral occurrences and (or) valid 

geochemical/geophysical anomaly, and the known mines or 

deposits indicate high potential for accumulation of mineral 

resources. The “known mines and deposits” do not have to be 

within the area that is being classified, but have to be within 

the same type of geologic environment.

ND. Minerals potential not determined due to lack of useful data. This does not require a level of certainty qualifier.

Level of Certainty

A. The available data are insufficient and (or) cannot 

be considered as direct or indirect evidence to support or 

refute the possible existence of mineral resources within the 

respective area.

B. The available data provide indirect evidence to support 

or refute the possible existence of mineral resources.

C. The available data provide direct but quantitatively 

minimal evidence to support or refute the possible existence of 

mineral resources.

D. The available data provide abundant direct and 

indirect evidence to support or refute the possible existence 

of mineral resources.

For the determination of “no potential” use N/D. This 

class shall be seldom used, and when used it should be for a 

specific commodity only. For example, if the available data 

show that the surface and subsurface types of rock in the 

respective area are batholithic (igneous intrusive), one can 

conclude, with reasonable certainty, that the area does not 

have potential for coal.

As used in this classification, potential refers to potential 

for the presence (occurrence) of a concentration of one or 

more energy and (or) mineral resources. It does not refer to 

or imply potential for development and (or) extraction of the 

mineral resource(s). It does not imply that the potential concentration is or may be economic, that is, could be extracted 

profitably.

Reference Cited

Goudarzi, G.H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey 

Open-File Report 84–787, 41 p., accessed December 9, 

2015, at https://pubs.er.usgs.gov/publication/ofr84787.

Appendix 1. Mineral Potential Classification System
Appendix 1 101

Level of certainty

A B C D

N

L

M

H

Level of resource potential

H/A

M/A

L/A

H/B

M/B

L/B

H/C

M/C

L/C

H/D

M/D

L/D

N/D

High potential with 

insufficient evidence

Moderate potential 

 with insufficient 

evidence

Low potential with 

insufficient evidence

High potential with 

indirect evidence

Moderate potential 

 with indirect evidence

Low potential with 

indirect evidence

High potential with 

direct evidence

Moderate potential

with direct evidence

Low potential with 

direct evidence

High potential with 

abundant direct and

indirect evidence

Moderate potential with 

abundant direct and

indirect evidence

Low potential with 

abundant direct and

indirect evidence

No potential

Current production/significant inventor.

Significant past production.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Historical mining.

Historical claims.

No active notices or mine plans.

No active exploration. 

No claims.

No other applicable data.

Permissive host rocks +/- previous assessment 

Permissive host rocks +/- previous assessment

Permissive host rocks +/- previous assessment 

Few active claims.

Historical mining. 

No USMIN active exploration. 

No active notices or mine plans.

Minor past production.

Attractive exploration targets.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Attractive exploration targets.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Contains 2 or more of the following:

Current production/significant inventory.

Significant past production.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Contains 1 or more of the following:

Minor past production.

Attractive exploration targets.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Attractive exploration targets.

Active or pending notices or mine plans.

Numerous active claims.

USMIN active exploration.

Contains 2 or more of the following: Contains 2 or more of the following:

Contains 1 or more of the following: Contains 1 or more of the following:

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Prospects, geochemical anomaly,

geophysical anomaly, and (or) 

related deposit type.

Reserved for a specific type of 

resource in a well-defined area. For 

example, it is appropriate to say that 

there is no oil potential in an area 

where the only rocks present are 

unfractured Precambrian granite, 

but the term ”low” is appropriate if 

there is a slight possibility for the 

presence of resources.

Figure 1-1. Matrix showing the classification system used for qualitative mineral-resource potential for 

locatable minerals in the Sagebrush Mineral-Resource Assessment (see text for abbreviations).
102 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Appendix 2. Mineral-Potential Assessment Tracts for Locatable 

Minerals in the North-Central Montana Sagebrush Focal Area

This appendix is available online only as an Excel (.xlsx) 

table at http://dx.doi.org/10.3133/sir20165089D. The table 

lists mineral-potential assessment tracts for locatable minerals 

in the North-Central Montana Sagebrush Focal Area. San Juan 

and others (2016) provide a geographic information system 

(GIS) database that shows the tracts that are included within 

this report.

Reference Cited

San Juan, C.A., Horton, J.D., Parks, H.L., Mihalasky, M.J., 

Anderson, E.D., Benson, M.E., Box, S.E., Cossette, P.M., 

Denning, P.D., Giles, S.A., Hall, S.M., Hayes, T.S., Hearn, 

B.C., Jr., Hofstra, A.H., John, D.A., Ludington, S., Lund, 

K., Mauk, J.L., Robinson, G.R., Jr., Rockwell, B.W., 

Rytuba, J.J., Smith, S.M., Stillings, L.L., Van Gosen, B.S., 

Vikre, P.G., Wallis, J.C., Wilson, A.B., Zientek, M.L., and 

Zürcher, L., 2016, Locatable mineral assessment tracts for 

the U.S. Geological Survey Sagebrush Mineral-Resource 

Assessment Project: U.S. Geological Survey data release, 

http://dx.doi.org/10.5066/F7833Q4R.
Appendix 3 103

Appendix 3. Oil and Gas Plays and Assessment Units in the 

North-Central Montana Sagebrush Focal Area

This appendix is available online only as an Excel 

(.xlsx) table at http://dx.doi.org/10.3133/sir20165089D. The 

table relates Public Land Survey System (PLSS) townships 

to U.S. Geological Survey (USGS) reports on oil and gas 

resource assessments for assessment units or plays in the 

North-Central Montana Sagebrush Focal Area. If an assessment 

was conducted in 1995 then the terminology used was “play.” 

After 1995, the terminology became “assessment unit” (AU). 

In many cases, more than one play or AU may be present 

within a given PLSS township. The table lists each PLSS 

township and the associated play or AU, the name of the 

play or AU, the related USGS publication title, and a link to 

the published USGS geologic assessment report. The USGS 

reports include many additional details regarding the source 

rocks, reservoir rocks, type of trap, reservoir properties, and 

resource potential.
104 Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area

Appendix 4. Parameters for the Geostatistical Model of the 

Lower Contact of the Upper Y Bentonite Bed in the Malta and 

Glasgow Areas, Montana

A geostatistical model representing the base of the 

Y bentonite bed was created using the Geostatistical Analyst 

tool in Esri ArcGIS 10.3.1 for desktop. The input for the 

model was point data representing the elevation of the base 

of the bed based on interpretation of satellite imagery. In 

areas where the Bearpaw Shale is exposed, large concretions 

occurring along the base of the Y bentonite bed were mapped 

by a geologist using satellite imagery. Using GIS software, 

the geologist digitized the distribution of concretions as short 

line segments directly from the satellite images. The line 

segments represent one or more concretions along the base 

of the bed. The line file segments were converted to points 

and an elevation for the point was extracted from a 30-m 

digital elevation model. These points were the input for the 

geostatistical model.

In ArcGIS, the geostatistical model for the base of the 

Y bed was created using empirical Bayesian kriging. The 

number of points in the input dataset and the model parameters 

are summarized in table A1. Both a prediction surface and a 

prediction standard error map were created.

Table 4-1. Parameters used in creating the geostatistical model for the lower contact of the Y bentonite bed in the Malta 

and Glasgow areas, Montana.

Item Malta area Glasgow area

Number of records in point feature class 1,035 799

Data field Elevation, feet Elevation, feet

Tool Geostatistical Analyst, geostatistical 

wizard

Geostatistical Analyst, geostatistical 

wizard

Method Empirical Bayesian Kriging Empirical Bayesian Kriging

Output types Prediction

Prediction standard error map

Prediction

Prediction standard error map

Transformation type None None

Semivariogram model type Power Power

Subset size 50 50

Overlap factor 3 2

Number of simulations 100 100

Searching neighborhood Standard circular Standard circular

Neighbors to include 15 15

Include at least 10 10

Sector type Full Full

Radius 248.166966768327 181.373355827947

Angle 0 0

Mean standardized (should be close 

to zero)

0.003239687619914229 0.016799063853327953

Root-mean-square standardized 

(should be close to one)

0.940889424436895 0.9325571467441592
Menlo Park and Lafayette Publishing Service Centers

Manuscript approval date June 21, 2016

Edited by Regan Austin

Layout by Jimmy Banton
Mauk and others—Geology and Mineral Resources of the North-Central Montana Sagebrush Focal Area—Scientific Investigations Report 2016–5089–D

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20165089D