Guidebook on the Geology, History, and 

Surface-Water Contamination and 

Remediation in the Area from 

Denver to Idaho Springs, Colorado 

U.S. GEOLOGICAL SURVEY CIRCULAR 1097 
Cover: 

View of Clear Creek Canyon prior to 1935 from Lookout Mountain. Railroad tracks 

are visible alongside the creek. Photograph by L. C. McClure, Denver Public 

Library Western History Collection (printed with permission). 
Guidebook on the Geology, History, and 

Surface-Water Contamination and 

Remediation in the Area from 

Denver to Idaho Springs, Colorado 

Edited by K.C. Stewart and R.C. Severson 

Written contributions by 

U.S. Geological Survey 

Walter H. Ficklin 

Helen W. Folger 

Wallace R. Hansen 

Ralph R. Shroba 

Kathleen S. Smith 

Kathleen C. Stewart 

Photographic contributions by 

John M. Epperson 

Larry P. Gough 

Thomas Kohnen 

Geoffrey S. Plumlee 

Richard B. Tripp 

Colorado School of Mines 

Ronald R. Cohen 

John C. Emerick 

Ronald W. Klusman 

Thomas R. Wildeman 

Colorado School of Mines 

Colorado Department of Transportation 

Denver Post 

Denver Public Library 

Idaho Springs Historical Society 

U.S. Geological Survey 

Graphics by Michael F. Duncan, Margo L. Johnson, Shawn K. Smith, and Richard P. Walker 

Editorial assistance by Thomas Kohnen. 

U.S. Geological Survey Circular 1097 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1994 
/ 

U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director 

Free on application to U.S. Geological Survey, Map Distribution 

Box 25286, MS 306, Federal Center 

Denver, CO 80225 

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government 

Library of Congress Cataloging-in-Publication Data 

Guidebook on the geology, history, and surface-water contamination and remediation in the 

area from Denver to Idaho Springs, Colorado I edited by K.C. Stewart and 

R.C. Severson ; wrilten contributions by Walter H. Ficklin ... [et al.] ; photographic 

contributions by John M. Epperson ... [et al.] ; graphics by Michael F. Duncan ... 

[et al.] ; editorial assistance by Thomas Kohnen. 

p. cm.-(U.S. Geological Survey circular; 1097) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.4/2:1097 

1. Environmental geology-Front Range (Colo. and Wyo.) 2. Environmental 

geology-Colorado-Denver Region. 3. Acid mine drainage-Front Range (Colo. 

and Wyo.) 4. Acid mine drainage-Colorado-Denver Region. I. Stewart, 

Kathleen C. II. Severson, R.C. (Ronald Charles), 1945- . III. Series. 

QE9~.F7G85 1994 

557.88'6--dc20 93-48416 

CIP 
Figure 1. 

2. 

3. 

4-7. 

CONTENTS 

Introduction ................................................................................................................... . 

Tour Log-Denver to Idaho Springs and return, by Ralph R. Shroba, 

Kathleen C. Stewart, and Wallace R. Hansen............................................................ 3 

Historical notes, by Kathleen C. Stewart ....................................................................... 23 

Geology and mineralization of the Idaho Springs and Central City Mining 

Districts, by Helen W. Folger..................................................................................... 33 

Generation and interactions of mine drainage at the Argo Tunnel, 

by Kathleen S. Smith.................................................................................................. 39 

Influence of acid-mine drainage on Clear Creek, Colorado, 

by Walter H. Ficklin and Kathleen S. Smith.............................................................. 43 

Constructed wetland treatment of acid mine discharge at Idaho Springs, Colorado, 

by John C. Emerick, Thomas R. Wildeman, Ronald R. Cohen, and 

Ronald W. Klusman................................................................................................... 49 

ILLUSTRATIONS 

Map of guidebook route .................................................................................................................................. . 

Photograph taken in 1940 of South Table Mountain showing Cretaceous-Tertiary boundary ...................... . 

Stylized oblique areal view of the Colorado Front Range looking west from Denver toward 

Central City and Idaho Springs ...................................................................................................................... . 

Photographs showing: 

4: Landslide with prominent tension fractures that disrupted a portion of the eastbound lanes of 

1-70 in 1984 ........................................................................................................................................... . 

3 

6 

7 

8 

5. The Dakota hogback looking south from 1-70 showing tilted strata...................................................... 9 

6. Mount Vernon wind gap......................................................................................................................... 10 

7. Roadcut exposing Precambrian gneiss and pegmatite in Mount Vernon Canyon.................................. 12 

8. Map showing the major northwest-trending faults that occur in the Front Range uplift between 

9. 

10--13. 

Golden and Idaho Springs............................................................................................................................... 14 

Map of the Idaho Springs area......................................................................................................................... 15 

Photographs showing: 

10. ClearCreekRoad, 1872 .......................................................................................................................... 17 

11. Narrow gauge railroad tracks at the junction of Clear Creek and North Clear Creek............................ 18 

12. Seven-ton rock that fell on U.S. Highway 6 in May 1991. .................................................................... 19 

13. · Beaver Creek railroad station and dance pavilion in the late 1800's..................................................... 20 

14. Portrait of George A. Jackson.......................................................................................................................... 23 

15. Map showing Argo and Big Five Tunnels in relation to Idaho Springs 

16-21. 

22. 

and Central City............................................................................................................................................... 24 

Photographs showing: 

16. Early ore mining underground................................................................................................................ 25 

17. Argo Tunnel buildings, dump, and ore loading chute prior to 1910 ...................................................... 26 

18. Argo Tunnel in operation during the 1930's.......................................................................................... 27 

19. Ore cars of the Big Five Tunnel on their way to the mill....................................................................... 28 

20. Charley Tayler with his water wheel sometime after its construction in 1893 ...................................... 29 

21. The Idaho Springs Natatorium in the early 1900's................................................................................. 30 

Map outlining the Front Range in Colorado and showing the location of the Central City and 

Idaho Springs Mining Districts in the Colorado Mineral Belt ............ ... ............. ............................................ 33 

III 
IV CONTENTS 

23-25. Photographs showing: 

23. Location of an ore vein marked by a hammer........................................................................................ 34 

24. Unusual occurrence of a gold crystal resting on the edge of a galena crystal........................................ 35 

25. Hydraulic dredge mining placer deposits along Clear Creek sometime after 1930 ............................... 36 

26. Map outlining principal zones of ore mineralization according to Sims (1989) ............................................. 37 

27. Diagrammatic representation of mine drainage reactions ........................................ ............. .......................... 40 

28. Photograph ofTorreys Peak taken in 1873 ..................................................................................................... 44 

29. Map oflocation of sample collection sites along Clear Creek........................................................................ 44 

30. Photograph of research chemist measuring pH with a portable meter ............................................................ 45 

31. Photograph of typical filter apparatus for water samples ................................................................................ 45 

32-35. Graphs showing: 

32. Influence of various inflows on the dissolved and total concentrations of iron, manganese, copper, 

and zinc in water collected from Clear Creek in June 1986................................................................... 46 

33. Influence of various inflows on the dissolved and total concentrations of iron, manganese, copper, 

and zinc in water collected from Clear Creek in October 1986 ............................................................. 46 

34. Proportion of suspended load of copper and zinc versus stream water pH downstream of the 

Argo Tunnel drainage into Clear Creek in June and October 1986 ....................................................... 48 

35. Distribution of loosely bound arsenic in Clear Creek sediment from near the Continental Divide 

to its mouth on the South Platte River.................................................................................................... 48 

36. Plan view and cross section ofthe Big Five Tunnel constructed wetland looking west................................. 51 

37. Photograph showing view toward the west of cattails growing in cells A, B, and C of the 

constructed wetland......................................................................................................................................... 52 

38. Cutaway view of the modified design of cell A ......... ............................................ ......................................... 52 

39. Cutaway view of the modified design of a cell B subcell ............................................................................... 52 

40. Location of the treatment cells of the Big Five Tunnel system as of spring, 1991 ......................................... 53 

41. Graph showing treatment effectiveness of cell C ................................................... ....................... .................. 54 

TABLES 

1. Stratigraphic units in the Greater Denver area............................................................................................................... 4 

2. Dissolved constituents in Argo Tunnel water, August 1985, and Clear Creek, October 1985...................................... 47 

3. Average concentrations and daily output for selected metals in the Big Five Tunnel effluent...................................... 50 

4. Treatment effectiveness of all cells at the Big Five Tunnel constructed wetland site for selected metals ................. ... 53 
Guidebook on the Geology, History, and Surface-Water 

Contamination and Remediation in the Area from Denver to 

Idaho Springs, Colorado 

INTRODUCTION 

This guidebook for a 1-day drive in the Front Range 

west of Denver, Colorado, includes commentary on mountain landscapes, notes on the colorful mining history of the 

area, and discussions on the geology and some miningrelated environmental hazards in the Idaho Springs area. It 

was originally prepared as U.S. Geological Survey OpenFile Report 91-426 for a field trip sponsored by Division 

A-5 (Environmental Quality) of the Soil Science Society of 

America on October 10, 1991. That original report has 

been revised and expanded to make this circular more 

useful as a self-contained guide. A pamphlet entitled 

"Mountains and Plains: Denver's Geologic Setting" is a 

complimentary general interest publication that is available 

free of charge from map sales in building 810 on the 

Denver Federal Center, Lakewood, Colorado. The pamphlet contains useful background information on the 

geologic setting of Denver and nearby portions of the Front 

Range. 

A few precautions and suggestions are offered to make 

the trip safe and enjoyable. Idaho Springs is about 7,500 feet 

above sea level and the lower oxygen content of the air 

(about 50 percent that of sea level) may adversely affect 

people with heart or breathing problems. The wetlands are 

"wet" so use suitable footwear. The daily weather patterns 

are changeable. Mornings in Denver may be sunny and 

warm, but afternoons in Idaho Springs may be cloudy, 

windy, and cool with rain or snow; be prepared for all types 

of mountain weather. 

As you travel westward from Denver into the mountains, successively older rocks are crossed. By studying the 

descriptions at the various mileage points and by taking time 

at each described STOP, you will get an excellent overview 

of the geologic events that shaped the Denver Basin and the 

mountains to the west. After traveling some 18 miles into the 

mountains, you will reach the old mining town of Idaho 

Springs. Here, you can learn about the two longest tunnels in 

the area-the Argo and Big Five Tunnels-and how the 

mine drainage affects the water of Clear Creek. 

The Argo Tunnel near the east end of Idaho Springs 

drains part of the large Central City and Idaho Springs hardrock mining districts and is a source of acid water (which 

contains arsenic and other heavy metals) that drains into 

Clear Creek. At the west end ofldaho Springs is the Big Five 

Tunnel. Near the tunnel entrance an experimental constructed wetland demonstrates technology for the remediation of acid-mine drainage and removal of arsenic and heavy 

metals. Take an opportunity to view this experimental 

method for treatment of mine drainage. For access to the 

wetlands, prior arrangements must be made with the Department of Environmental Sciences and Engineering Ecology 

or Department of Chemistry and Geochemistry, Colorado 

School of Mines in Golden, Colorado. Otherwise, the wetlands may be viewed from the road. Permission to enter the 

Argo Tunnel property may be obtained from Jim Maxwell of 

Idaho Springs, who owns the hot springs resort. The tunnel 

may also be viewed from public roads. 

After you leave Idaho Springs, the road log guides you 

down Clear Creek canyon, providing an opportunity to get a 

closer look at the various Precambrian rocks while traveling 

through one of Colorado's picturesque canyons. 

50' 

40' 

TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

By Ralph R. Shroba, Kathleen C. Stewart, and Wallace R. Hansen 

Begin at the Denver Metro Convention and Visitor's Bureau, comer 

of Colfax Avenue and Court Place (fig. 1). Denver stands on late Pleistocene 

terrace deposits composed of glacial outwash (Trimble and Machette, 1979). Building 

excavations in these deposits have sometimes uncovered mammoth bones and gold. Both 

Cherry Creek and the South Platte River (fig. 1) repeatedly flooded lower downtown 

Denver after it was founded in the late 1850's, but flood-control dams built about 100 

years later have helped·to reduce flooding. In this area Interstate 25 (1-25) is mostly on 

Holocene alluvium deposited by Cherry Creek and the South Platte River. The South 

Platte River has been extensively channelized by backfilling and restructuring to minimize flooding and improve land values. Consequently, the river no longer meanders 

freely across its floodplain here as it does northeast of Denver where it is not confined by 

flood control structures (Hansen and others, 1976). 

Table I shows the stratigraphic units that are exposed in the Denver area and are 

mentioned in this log. 

Proceed west on Colfax 3 blocks to Speer Boulevard (follow signs to 

1-25 north). Tum right and follow Speer Boulevard 1.2 miles and exit right 

onto 1-25 northbound. Speer Boulevard parallels Cherry Creek. The overpass crosses 

railroad tracks, Holocene alluvium, and the Platte River. 

Proceed 2 miles north on 1-25. Take Exit 214B onto 1-70 westbound. 

Proceed west on 1-70. 

At the present-day interchange of 1-25 and 1-70 stood the former Boston and 

Colorado Smelting Works. The smelter transferred operations from Black Hawk to Denver 

in 1879, where it processed ore from the Idaho Springs-Central City Mining Districts and 

other parts of the State (Hale, 1936). 

The route of 1-70 is mostly on thin deposits of Holocene alluvium and eolian1 sand 

(Trimble and Machette, 1979). 

1

Windblown. 

105° 30' 

~,i: ::::>Z oj=> 

~ 8FRONT LJ.!IZ 

O::cn LJ.!\o 

Uo:: 

o::LJ.! 

~~~ 

RANGE 

0 5 Miles I I., •' I I 

0 5 Kilometers 

~i: ::::>z 

0::::> 

uo 

zu 

55ffi ffi> 

tt15 LJ.!Cl 

..., 

105° 00' 

Figure 1. Map of guidebook route. Abbreviations: NTM, North Table Mountain; STM, South Table Mountain; Tl, T2, T3, Tunnels 

I, 2, and 3. 

3 
4 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Table 1. Stratigraphic units in the Greater Denver area (modified from Hansen and 

Crosby, 1982). 

Divisions of Geologic Time Stratigraphic Unit Age (millions of 

years) 

d: Gl Gl Lower level allUVIUms (terrace oeposits); c w,.. c Gl upper level alluviums (pediment gravels and 1-a: Gl "0 (J associated deposits); windblown sand, silt, a:<( (J c 0 

:§z ~ ns.~ clay; other surficial deposits (landslide, ::c ..9! colluvium, talus) 0 a. 

Pliocene :.·> '~ ----1.6----

{.) 1.,':','::: ::<. .,,,,,,,,,,,,,,,, .. 

2 Miocene ! :'•'•:: , •.• , •. , ,, ' ,, "<·····:• > ···<' \:::., :.<' ----5----

0 Oligocene 1 • \ •• • r .... ,L:.:,:.:,i (t············') i···' < 

---24----

z >- w a: Eocene :•:•' ': ':'"' ,,.,., .. ,: .. :.,,".,,::.: .. : .... · ... ----38----

{.) <( 

~ ---55---- Gl Green Mountain w c Conglomerate 1- Gl 

(J 

0 

Gl ~ ' Table Mtn. Shoshonite- iii Gl <11 a. > E c Lava flows, intrusives i 0 -~ I I < 

----66----

ou. ':.'>:· C/ 

Arapahoe Formation 

Laramie Formation 

(/) Fox Hills Sandstone ::> Upper 0 

w Pierre Shale 

{.) 

~ Niobrara Formation 

a: 

{.) Carlile Shale 

{.) c Gl 

6 0-

Greenhorn Limestone c <11 

N Gl.<: 

0 Graneros Shale Ol(/) 

(/) 

w 

~ Lower Dakota Group ----138----

Morrison Formation 

{.) Upper Ui Ralston Creek Formation (/) 

<( 

a: Middle .,,':' >: c .. , ., ( ( ::> .:·.: .. :., . ...., ... ,.,... ····::·•'. ' ~;i•· 

Lower )t' .. :,.,.,,, .. ,,,,,.. :,·,: .•...•.•. : •. , .. ,:.,, .. , ..· 

1~1. 07•···<?:'•'·:········~·····'·,•·,:.·:·•,•:''•······: ----205----

TRIASSIC ' 

Lykins Formation ----240----

z <( 

:ij 

a: Lyons Formation w 

a. 

{.) ----290----

2 PENNSYL· Fountain Formation 

VAN IAN 

@ .,: i 

..• ;' :\·,···,···········,·················,··········· 

----330---

MISSISSIPPIAN <'•' ...I <( 

a. DEVONIAN i ::? { <···· ) > 

SILURIAN :.<>>.,., .. :::····· ······:•:··••:··>:········: ··:· ,, ..•... :ii) ;:: .:,.,.. ·:· < ·:,,,, ' :••: •.. : '<'.."•': 

ORDOVICIAN . . ) .. ... ,.,::., >< i,.,.::.,.:. 

CAMBRIAN < . > { •·:········· > \ ~ i i::>>>:::: : ·,'. . .. :,.,.: .. ,:: :/: ... ----570----

Pikes Peak GranHe. Sliver Plume Quartz Monzo· 

nile. Boulder Creek Granodlortte. and associated 

PRECAMBRIAN Intrusive masses; pegmatHe lenses. dikes. and 

other bodies related to major Intrusives 

Metamorphic rocks. Including gneiss. schist. 

quartzlle. amphibolite. homblendHe. and others 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

Mileage 

0.0 Road log begins. Look for exit sign of Federal Boulevard attached to Pecos St. overpass. Just beyond the overpass, 

the highway route cuts into the underlying Denver Formation (table I) (Hansen 

and others, 1976). 

2.4 Exit 1-70 at Exit 271A and Sheridan Boulevard. Tum right (north) at 

exit, proceed north on Sheridan Boulevard (Colorado Highway 95) for 

half a block and tum left (west) into Inspiration Point Park (at Ames 

Way and Benton Way). Follow left road (Ames Way) to parking area 

and walk west about I/4 mile to the overlook. 

2.5 STOP 1. Inspiration Point Park overlook has outstanding views of Denver and the Front Range. Although now sheathed in concrete and asphalt, 

Inspiration Point is capped by a small remnant of middle Pleistocene alluvium 

that was deposited on a pediment2 surface eroded on the D~nver Formation (Upper Cretaceous and Paleocene). The alluvium protected the underlying pediment 

surface as erosion proceeded to remove the surrounding, unprotected pediment; 

now this remnant alluvium is preserved well above modern stream levels. The 

valley of Clear Creek to the north and west oflnspiration Point contains late Pleistocene and Holocene alluvial gravel as much as 30 feet thick. 

Much of the urban development visible from Inspiration Point postdates World 

War II. The valley of Clear Creek once contained extensive vegetable farms, but 

in the past 40 years they have largely been displaced by gravel pits, homes, and 

commercial establishments. A few farms are still visible from 1-70. Some of the 

lakes produced by the gravel mining have been converted to recreational uses; 

other gravel pits have been backfilled with solid waste. Much of the gravel along 

Clear Creek still remains but is not likely to be mined because of urban growth 

(Hansen and others, 1976). 

The flat tops of North and South Table Mountains to the west (fig. 1), near 

Golden, are capped by Paleocene lava flows (shoshonite). The shoshonite flows 

are interbedded with and, locally, overlie the Denver Formation. Above and far 

west of the Table Mountains are Mount Evans and the Continental Divide. Rolling topography in the middle distance cuts into a very broad but dissected late 

Eocene erosion surface (Scott and Taylor, 1986). Both Table Mountains and the 

late Eocene surface have a southward tilt, perhaps caused by middle Tertiary 

warping. On a clear day, Pikes Peak is visible 60 miles to the south and Longs 

Peak is visible 45 miles to the northwest (Hansen and others, 1976). 

Follow path around to the right to historic marker. In 1850 Cherokee Indians 

en route to California and led by Louis Ralston discovered gold in Ralston Creek 

gravel near its confluence with Clear Creek. The discovery site is out of view 

north of the highway bridge that crosses the small visible portion of Clear Creek. 

Fur traders in the early 1800's referred to Clear Creek as "Vasquez" Fork after 

trader Louis Vasquez. Vasquez built a fur-trading fort in 1835 on the South Platte 

River about 40 miles northeast of Denver (Hafen, 1964). 

3.1 Leave Inspiration Point Park, tum right (south) onto Sheridan Boulevard, go one block toW. 48th Avenue, tum right (west), and follow 

signs for 1-70 west along W. 48th Avenue to Harlan Street. 

2

Broad, gently sloping erosion surface formed on bedrock near the base of a mountain front. 

5 
6 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

3.8 Re-enter westbound 1-70 at the intersection of Harlan Street and 

W. 48th Avenue. For about the next 5.5 miles the route traverses mostly late 

Pleistocene and Holocene alluvium of Clear Creek (Trimble and Machette, 1979). 

4.4 Cross Clear Creek. 

4.9 Cross over Wadsworth Boulevard. On the left (south), middle Pleistocene 

alluvium overlies bluffs formed in the Denver Formation (Hansen and others, 

1976). 

8.5 Cross Clear Creek. 

8.6 North and South Table Mountains and a large gravel pit on the right. 

Since World War II, gold has been recovered as a byproduct of these gravelmining operations (Van Hom, 1976). 

10.6 South Table Mountain on the right. Near the southeast comer of South 

Table Mountain, the Cretaceous-Tertiary boundary (table 1) is exposed below a 

sequence of gray beds in the Denver Formation, about 150 feet below the base of 

the lava flow (fig. 2) (Brown, 1943; Van Hom, 1976). Brown found remains of 

Paleocene flora above the boundary and fragments of dinosaur bones below it. 

The boundary thus marks the end of the age of dinosaurs in this area. 

12.7 Cross U.S. Highway 6. On the left, Green Mountain is capped by Green 

Mountain Conglomerate (Paleocene). 

Figure 2. Photograph taken in 1940 of South Table Mountain showing Cretaceous-Tertiary boundary (U.S. Geological Survey photograph by R.W. Brown, in Brown, 1943). View is to the north of southeast side of the mountain. 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

13.6 Two-lane exit for C-470. Note clay pit scars along the prominent ridge (Dakota 

hogback) to the west and some remnants of clay mining in the valley between 

Green Mountain and the ridge. 

14.3 The Golden Fault dips beneath the mountain front. It is one of the major faults 

involved in uplift of the Front Range (fig. 3). 

14.4 Stay in right lane to take Morrison Exit 259. 

14.5 Approximate location of 1984Iandslide that disrupted a short portion of the eastbound lanes ofl-70 (fig. 4). 

14.7 Take Morrison Exit 259 to Colorado Highway 26. Tum right at traffic 

light and park in Jot north of 1-70. 

STOP 2. The roadcut through the hogback exposes a sequence of east-dipping 

sedimentary rocks of the Dakota Group, Morrison Formation, and Ralston Creek 

Formation (fig. 5; table 1) (Trimble and Machette, 1979). Steep dips in these rocks 

7 

Figure 3. Stylized oblique areal view of the Colorado Front Range looking west from Denver toward Central City and Idaho Springs. 

Cross section in lower left-hand comer shows approximate location of the Golden Fault. Major faulting during Laramide time uplifted the 

crystalline rocks of the Precambrian core and subsequent erosion left only remnants of the overlying Paleozoic and Mesozoic sedimentary 

rocks along its eastern flank. Travel route of I-70 can be seen. 
8 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 4. Landslide with prominent tension fractures that disrupted a portion of the eastbound lanes ofl-70 in 1984. Colorado Department of Transportation photograph (printed with permission). 

are caused by sharp folding along the east flank of the Front Range uplift and 

resulting drag along the Golden Fault (fig. 3). Rocks on both sides are sharply 

flexed, even overturned. The major faulting occurred during Laramide time3 but 

seismic instruments in Golden detect mild on-going activity (Kirkham and 

Rogers, 1978). The rocks with descriptive markers can be viewed in detail from 

the foot path that starts at the parking area. 

Green Mountain is visible to the east. Below the conglomerate cap, unstable 

colluvial4 slopes and expansive soils derived from the Denver Formation have 

posed problems for housing developments on Green Mountain (Robinson and 

others, 1976). Expansive soils are widespread in those parts of the Denver area 

that are underlain by the Pierre Shale and the Denver Formation (table 1). They 

contain clay that swells or expands when the mineral lattice absorbs water. This 

swelling may cause severe damage to light structures, such as houses and highways, unless adequate precautions are taken during construction. Builders and 

engineers commonly call the clay "bentonite," but to soil scientists it is 

"smectite." The landslide in figure 4 resulted from failure of roadfill placed on 

expansive shale and mine tailings (Larry McKenzie, Colorado Department of 

Transportation, oral commun., 1991 ). 

3

Late Cretaceous to Eocene time (Tweto, 1975). 4

Composed of loose rock or unconsolidated material deposited by rainwash or gravity at the base of gentle 

slopes or hillsides. 
TOUR LOO-DENVER TO IDAHO SPRINGS AND RETURN 9 

Figure 5. The Dakota hogback looking south from 1-70 showing tilted strata. Lines demark the Dakota Group and the Morrison Formation (U.S. Geological Survey photograph by W.R. Hansen, July, 1972; Hansen and Crosby, 1982). 

The clay pits that were visible from 1-70 in the Dakota Group are near the top 

of the hogback. Non-expansive clays have been mined along the foothills of the 

Front Range for over 100 years. These non-expansive clays are suitable to make 

brick, tile, sewer pipe, and pottery. Clays were obtained chiefly from the Laramie 

Formation, but also from the Arapahoe Formation, Dakota Group, and Benton and 

Pierre Shales (table 1). Pits in the Dakota Group previously supplied refractory 

(ceramic) clays, but industrial ceramics manufactured in the Golden area today 

(figs. 1 and 3) use clay blends mostly from out of state (Waag, 1952; Hansen and 

Crosby, 1982). 

From the west end of the roadcut to the entrance of Mount Vernon Canyon, 

1-70 crosses Jurassic to Pennsylvanian sedimentary rocks-the oldest exposed 

sedimentary rocks in the Denver area (Reed and others, 1987). Dinosaur bones 

are abundant in Mesozoic rocks in the Denver area. They were first discovered 

in 1877 in the Morrison Formation (Upper Jurassic) about 2 miles south of here 

(Lockley, 1988). The Morrison Formation is visible on the west end of the 

roadcut (fig. 5). 

The steeply tilted red rocks south of 1-70 and west of the hogback are 

resistant conglomeratic sandstones of the Fountain Formation (Pennsylvanian 

and Lower Permian). These sandstones are well exposed in Red Rocks Park 

about 2 miles to the south (fig. 3). Between this formation and the underlying 

Precambrian rocks there is a gap in geologic time of about 240 million years 

(table 1). 
10 GUIDEBOOK: GEOLOOY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Colorado Highway 26, which passes under I-70, was the route of the Denver, 

Auraria, and Colorado Wagon Road. Entrepreneurs built this freight road up 

Mount Vernon Canyon in 1859 to transport supplies for miners in Central City 

(Scott, 1976). 

About a half mile to the southeast (not visible from here) is a large notch in the 

hogback. This notch is the Mount Vernon wind gap (fig. 6). The gap will be visible to the east (to the left) just after you re-enter 1-70. 

As the Rocky Mountains rose over the last 67 million years, many creeks and 

rivers gradually cut through Paleozoic and Mesozoic rocks into the underlying 

Precambrian rocks, following ancient eastward-flowing stream courses. During 

the Eocene (38-55 million years ago; table 1), the uplift slowed or stopped. As a 

result, a broad, sloping plain or pediment developed on what was then mountains 

of low relief-a late Eocene erosion surface. Starting about 28 million years ago 

(Oligocene) the rate of uplift increased (Scott and Taylor, 1986). From the 

Miocene through Pliocene (24-1.6 million years), the Rockies continued to rise. 

Between 1 and 2 million years ago, the creek of Mount Vernon Canyon still flowed 

across the hogback through the wind gap. To the south, however, Bear Creek, with 

its larger drainage basin, was down cutting at a much faster rate. One of the minor 

tributaries of Bear Creek relatively quickly cut its way northward through the siltstones and claystones of the Morrison, Ralston Creek, and Lykins Formations 

until it reached Mount Vernon Creek. Mount Vernon Creek was then diverted. As 

Figure 6. View to the southeast of the Mount Vernon wind gap. Mount Vernon Creek flowed eastward cutting through the hogback 

until a quickly downcutting tributary of Bear Creek diverted the streamflow southward (photograph by Thomas Kohnen). 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

downcutting continued in response to further uplift, the notch you see today was 

left (fig. 6). This action of one stream capturing the waters of another is called 

stream piracy (Scott, 1972). 

Exit parking lot, tum left onto Highway 26 and right at traffic light. 

15.3 Re-enter 1-70 westbound. 

15.5 Look back to the left to see the Mount Vernon wind gap. 

15.6 Highway enters the mouth of Mount Vernon Canyon. Rocks exposed along the 

route between the canyon mouth and Idaho Springs are mostly felsic5 gneiss, 

biotite gneiss, hornblende gneiss6, and amphibolite 7 of Precambrian age (Bryant 

and others, 1981). At the mountain front these Precambrian rocks are oxidized to 

great depths, perhaps from extensive weathering before the overlying Pennsylvanian rocks were deposited, but farther up the canyon, near-vertical roadcuts 

expose fresh, unweathered rock. 

17.2 Roadcut exposing unweathered Precambrian rock and pegmatite8 dikes (fig. 7). 

Notice vertical drill-hole scars made during highway construction. 

Low terraces visible along Mount Vernon Creek and its tributaries were 

entrenched by flood waters after intense thunderstorms dropped 8.5 inches of rain 

in 3 hours at the head of Mount Vernon Canyon on September 2, 1938 (Follansbee 

and Sawyer, 1948; Hansen and others, 1978). 

20.4 The head of Mount Vernon Canyon is near Exit 254. 

20.5 Exit 1-70 just west of Exit 254 at the Buffalo Herd Overlook and park. 

STOP 3. Panoramic View. In 1912, the City of Denver acquired land on 

both sides of the highway here for additions to the Denver Mountain Parks 

System. Bison were imported in 1913 from Yellowstone National Park and their 

descendants still graze here in addition to elk and deer. 

Victory Pass on the old wagon road lies just to the east. The Patrick House, a 

former stage stop built in 1860, stands in the trees on the hill to the southwest, 

and the old wagon road parallels 1-70 in the trees. A barn to shelter horses after 

the ascent of Mt. Vernon Canyon once stood where the windmill is today (Marty 

Homola, Denver Mountain Parks, oral commun., 1991). 

To the west is a view of the late Eocene erosion surface (Scott and Taylor, 

1986) and the high peaks along the Continental Divide. For the next 7 miles, 

I-70 crosses rolling topography of the dissected late Eocene erosion surface. 

Remnants of the surface north of Clear Creek Canyon are locally visible from 

several points along 1-70. Most of the bedrock here is deeply weathered, 

making it easy to excavate for mountain homesites, roads, and utility corridors (Schmidt and Pierce, 1976). Ridges capped by Pliocene or Miocene 

gravels that were deposited in the ancestral channel of Clear Creek are 

5 A light-colored igneous rock composed mostly of light-colored feldspars and silica (quartz). 6

Banded metamorphic rock formed by high temperature and pressure. Mineral names, such as biotite and 

hornblende, are used to describe major components or a distinctive composition of a gneiss. 7 Amphibole-rich rock. 

8

Very coarse grained igneous rocks that fomlS veins and dikes in older igneous rocks. 

11 
12 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

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TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

occasionally visible. The ancient channel is several hundred feet below the 

late Eocene erosion surface and 400 to 600 feet above the present stream. 

Remnants of the gravel extend discontinuously eastward almost to the mouth 

of the canyon, where they lie several hundred feet above the projection of the 

highest Pleistocene pediment surfaces in the Colorado Piedmont (fig. 1) (Reed 

and others, 1987). Roadcuts in the bedrock expose interlayered felsic gneiss, 

amphibolite, and hornblende gneiss cut by dikes of pink granite9 and pegmatite (Reed and others, 1987). 

Proceed west and re-enter westbound 1-70. 

21.3 View to the right, just before the Chief Hosa Exit (Exit 253), of Pliocene 

or Miocene gravel in the ancestral channel of Clear Creek, below the late Eocene 

erosion surface, forms the large east-west trending ridge. 

26.2 Cross county line into Clear Creek County,just beyond Exit 248. 

Large subdivision on the left. Residential developments in the foothills of the 

Front Range commonly experience problems obtaining adequate water for drinking and waste disposal (Hansen and Crosby, 1982). 

27.5 Overpass, Aoyd Hill summit is just ahead. The northwest-trending Floyd 

Hill fault occupies the saddle at the summit (fig. 8). It is one of many northwesttrending faults in Colorado that have had recurrent movement since Precambrian 

time (Tweto, 1979; Kirkham and Rogers, 1978; Hansen and Crosby, 1982). 

27.8 Descend Aoyd Hill into Clear Creek Canyon. Across the canyon, the 

ridge north of Clear Creek is capped by Pliocene or Miocene gravel. Roadcuts 

below the summit of Floyd Hill expose the same interlayered gneiss, amphibolite, 

pink granite dikes and pegmatite mentioned previously (Reed and others, 1987). 

29.6 Large rockslide on the south side (left) of Clear Creek just above the junction 

ofl-70 and U.S. Highway 6. It is moving along foliation 10 in the biotite gneiss 

bedrock (Robinson and others, 1976). 

29.8 Across Clear Creek, large roadcuts on the right expose migmatiticll biotite 

gneiss and schist, 12 which contain pods13 and deformed layers of pegmatite (Reed 

and others, 1987). 

32.0 East portal of the highway tunnel. Well-exposed felsic gneiss and amphibolites crop out to the right of the tunnel (Reed and others, 1987). 

32.5 Roadcut exposes late Pleistocene glacial outwash gravel. 

33.0 Exit 1-70 at Exit 241A toward Idaho Springs. Follow Business 70 about 

1 mile west to a small municipal park on the south side of Clear Creek across 

from the Argo Mill (fig. 9).-[Refer to the section by Stewart in this report 

entitled "Historical Notes" for information on the Idaho Springs area. (p. 23)] 

9 A light-colored, coarse-grained intrusive rock consisting primarily of quartz and feldspar. 10Pianes formed by the flattening and alignment of minerals in metamorphic rock. 11Composite of igneous and metamorphic rock whose crystalline structure can be distinguished with the 

unaided eye or a hand lens. 12Layered crystalline rock formed by low-grade metamorphism. 13Eiongate bodies enclosed in schist and parallel to the lines of deformation. 

13 
14 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

4 5 MILES 

EXPLANATION 

D Precambrian rocks 

------'"- U Fault-Arrows show apparent direction of 

------ D relative movement. U, upthrown side; 

D, downthrown side 

Figure 8. Map showing the major northwest-trending faults that occur in the Front Range uplift between 

Golden and Idaho Springs (adapted from Sheridan and others, 1972). 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

33.9 STOP 4A. Argo Tunnel. -[Refer to the sections in this report on the economic geology of the area by Folger (p. 33), chemistry of acid mine drainage by 

Smith (p. 39), and the tunnel effluent by Ficklin and Smith (p. 43).] The tunnel 

drainage may be viewed by walking across the bridge and along the road toward 

the mill. 

Leave municipal park and proceed west on Business 70. Virginia Canyon 

can be seen to the right [See section by Stewart entitled "Historical Notes" that 

discusses Idaho Springs (p. 23).] 

34.2 Cross Clear Creek. -Much of downtown Idaho Springs is built on late Pleistocene outwash gravel. The lower limit of Pleistocene glaciation in the valley of 

Clear Creek is marked by subdued moraines near Lawson about 6 miles west of 

Idaho Springs (Bryant and others, 1981). 

35.2 Tum left (south) onto 1st Avenue. Park on right (west) side of the 

street. 

STOP 48. -If arrangements have not been made to view the wetlands with 

Colorado School of Mines personnel, walk back to Business 70, turn left, and walk 

toward the 1-70 overpass to observe the constructed wetlands and the portal of the 

Big Five Tunnel on the left within the fenced enclosure. [Refer to the sections in 

this report by Emerick and others entitled "Constructed wetland treatment of acid 

mine discharge at Idaho Springs." (p. 49) and by Stewart entitled "Historical 

Notes, Big Five Tunnel" (p. 26).] 

Leave the wetland. Tum left (east) onto Miner Street and left (north) 

onto 2nd Avenue. Proceed north on 2nd Avenue to Business 70. 

35.4 Tum right (east) onto Business 70 (Colorado Boulevard) and proceed 

east. 

36.0 Tum right (south) onto 13th Avenue and proceed south. 

Idaho Springs 

0.5 MILE 

0.5 KILOMETER 

Figure 9. Map of the Idaho Springs area. 

15 
16 GUIDEBOOK: GEOLOOY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

36.1 Tum left (east) onto Miner Street and proceed east on Miner Street.-

(You can tum right onto 17th A venue and park. Follow the footpath under 1-70 to 

see the Charley Tayler Water Wheel. [Refer to the section in this report by Stewart 

entitled "Historical Notes, Charley Tayler Water Wheel" (p. 28).] Remember to 

adjust your' mileage for this stop.) 

36.4 Cross Clear Creek. 

36.5 Tum right (south) onto Soda Creek Road. Proceed south on Soda 

Creek Road. 

36.7 Hot springs resort. -[Refer to the section in this report by Stewart entitled 

"Historical Notes" concerning "Hot Springs." (p. 29)] 

Retrace route to the intersection of Soda Creek Road and Miner Street. 

37.0 Tum left (west) onto Miner Street and proceed west. 

37.3 Intersection of Miner Street and 14th Avenue. -Note the stone mill 

(arrastre) on the left in front of the library on the northwest corner of Miner Street 

and 14th Avenue. (Depicted on p. 32.) This Spanish-style mill was used to grind 

gold ore in the Idaho Springs area (Cox, 1989). 

37.4 Intersection of Miner Street and 13th Avenue. Tum left (south) onto 

13th Avenue (Colorado State Highway 103), cross bridge, and tum left 

to enter eastbound 1-70.- Straight ahead (south) is the valley of Chicago 

Creek where George Jackson made his gold discovery in 1859. 

37.7 Waterwheel on the right. -To the left (north), across the valley, Virginia 

Canyon can be seen. 

42.0 Exit 1-70 left at Exit 244 and follow U.S. Highway 6 down Clear Creek 

Canyon. 

42.4 Rock quarry on the left. -As gravel resources along Clear Creek in and near 

Denver have been depleted or converted to other use, entrepreneurs have been 

able to develop quarries like this one for crushed rock. 

42.6 Just east of the junction of 1-70 and U.S. Highway 6, the road passes from migmatitic biotite schist and gneiss through a sequence of interlayered calc-silicate 14 

gneiss, hornblende gneiss, and amphibolite, and into a large body of amphibolite. 

43.3 Bed of the narrow-gauge Colorado Central Railroad on the opposite side of Clear 

Creek. The bed is frequently visible throughout the canyon (Reed and others, 

1987). The Colorado Central Railroad built this route to Black Hawk along the 

former route of the Clear Creek Road in 1872 (fig. 10) (Scott, 1976; Hauck, 1972). 

The Colorado and Southern Railroad bought the route in 1899 and abandoned it 

in 1941 (Morgan, 1974). 

43.4 Cinder block building on the south side of Clear Creek is the site of a former 

radium baths resort. 

14Consisting mainly of calcium-bearing silicates formed by metamorphism of impure limestone or 

dolomite. 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 17 

Figure 10. Clear Creek Road, 1872. Ties to be 

laid for railroad are at edge of the embankment 

(U.S. Geological Survey photograph by W.H. 

Jackson). 

45.0 Intersection with Colorado Highway 119. -Highway 119 heads northwest 

to Black Hawk. Area along this curve to the right is a landslide that has been 

slipping down the bedrock intermittently and crowding the road for many years. 

45.1 Cross county line into Jefferson County. -Confluence of North Clear 

Creek with Clear Creek. The Colorado Central Railroad built a water tower and 

station at this location (fig. 11). Tracks from mining areas on both forks of Clear 

Creek joined here and continued eastward to Golden (Hauck, 1972). Just downstream from the road intersection is one of four sites proposed for dams on Clear 

Creek (Colorado Water Resources and Power Development Authority, oral commun., 1991). Continue east on U.S. Highway 6. 

To the left, along Highway 119, can be seen interlayered amphibolite and 

biotite-quartz-plagioclase15 gneiss cut by a prominent light tan pegmatite just 

above the road (about 6 to 10 feet) (Hutchinson and others, 1976). 

45.7 Dredge tailings are conspicuous here and there for the next 2 miles downstream. 

Steep canyon walls give way in places to broader, gentler slopes along fault zones 

that are more easily weathered and eroded than the adjacent bedrock. The dominant bedrock in the canyon is quartz16 plagioclase gneiss interlayered with hornblende gneiss. Pegmatites that are locally exposed in the canyon walls are readily 

distinguished by their very light color. 

15 A feldspar with the general formula of (Na, Ca)Al(Si, Al)Si20 8• 

~ineral composed of Si02• 
18 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

46.5 Large pegmatite on north side of road. 

46.6 Approximate location where a 7-ton boulder landed on the highway in May 1991 

(fig. 12). 

47.5 Clear Creek Canyon historical marker in pullout on the left. -Nearly 

400 feet of Pliocene(?) or Miocene gravel, about 1,000 feet above 

present-day stream level, caps the ridge (to the southwest) on the south 

side of Clear Creek (to right) (Reed and others, 1987). 

In back of and to the right of the marker is part of the fracture zone of the Blackhawk Fault. This fault is one of the major northwest-trending faults in the region 

(fig. 8). The fracture zone extends to the southeast and crosses road before the next 

curve to the east. The Blackhawk Fault can be traced for over 22 miles in the Front 

Range. Just northwest of here the fault begins bending toward a more easterly 

trend. To the southeast, the fault passes beneath the Pliocene(?) gravel capped 

ridge and can be traced along a portion of Mount Vernon Canyon (Sheridan and 

others, 1972). 

49.7 Just before Tunnel 3 -note the cribbing along Clear Creek that supported the 

bed of the Colorado Central Railroad. Rocks exposed along here are still predominantly amphibolite and biotite-quartz-plagioclase gneiss. 

Figure 11. Narrow gauge railroad tracks at the junction of Clear Creek and North Clear Creek. Photograph by Collier. Printed courtesy 

of Denver Public Library Western History Collection. 
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 

Figure 12. Seven-ton rock that fell on U.S. Highway 6 in May 1991. Denver Post photograph by John Epperson (printed 

with permission). 

50.2 Just upstream from the east portal of Tunnel 2, the music at the Beaver Brook 

dance pavilion entertained tourists on day excursions from Denver in the 1880's 

(fig. 13). The nearby railroad station supplied water for locomotives (Hauck, 

1972). For about the next mile east of Tunnel 2 the rocks are predominantly the 

darker migmatites. 

51.8 Small side valley on the left is the former site of the Guy Gulch railroad station 

that was built in 1885. Just downstream from here is another proposed dam site. 

To the right can be seen gneissic granodiorite. 

52.5 The Junction Ranch breccia 17reef system (another major northwest-trending 

fault) (fig. 8) passes beneath the road at this curve. The term "breccia reef' refers 

to the wide zone of broken rock that is found in these fault zones. -The quarry 

on the north side of the road -cuts into this breccia zone exposing the 

yellowish-brown staining of the rocks. Many fractures here have coatings of 

calcite. This fault, originating in the Precambrian time, can be traced for over 25 

miles in the Front Range (Sheridan and others, 1972). North of Clear Creek, the 

Junction Ranch fault breccia zone has a maximum width of 200 feet, but to the 

south, between Clear Creek and Mount Vernon Canyon the breccia zone is over 

1, I 00 feet wide. 

17Coarse-grained rock composed of angular broken rock fragments held together hy a fine-grained matrix. 

19 
20 GUIDEBOOK: GEOLOOY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 13. Beaver Creek railroad station and dance pavilion in the late 1800's. Printed courtesy of Denver Public Library Western History 

Collection. 

55.3 Diversion dam on the right supplies water to the Welsh ditch for irrigation of 

Denver's western suburbs. After passing through Tunnell, Welsh ditch is visible 

across Clear Creek. 

56.6 Canyon mouth and end of road log. At the traffic light you can proceed straight ahead onto Highway 58 that returns you to eastbound 

1-70, or you can tum right onto U.S. Highway 6 (6th Avenue). 

,.-. ~. ··-
TOUR LOG-DENVER TO IDAHO SPRINGS AND RETURN 21 

REFERENCES 

Brown, R.W., 1943, Cretaceous-Tertiary boundary in the Denver 

Basin, Colorado: Geological Society of America Bulletin, 

v. 54, n. I, p. 65-86. 

Bryant, Bruce, McGrew, L.W., and Wobus, R.A., 1981, Geologic 

map of the Denver 1 °x2° quadrangle, north-central Colorado: 

U.S. Geological Survey Miscellaneous Investigations Series 

Map 1-1163, scale 1 :250,000. 

Chronic, Halka, 1980, Roadside Geology of Colorado: Missoula, 

Montana, Mountain Press Publishing Co., 334 p. 

Cox, Terry, 1989, Inside the Mountains: Boulder, Colorado, Pruett 

Publishing Company, 117 p. 

Follansbee, Robert, and Sawyer, L.R., 1948, Floods in Colorado: 

U.S. Geological Survey Water-Supply Paper 997, 151 p. 

Hafen, Leroy, 1964, Fort Vasquez: State Historical Society of 

Colorado, 14 p. 

Hale, Jesse D., 1936, The first successful smelter in Colorado: The 

Colorado Magazine, v. 13, p. 161-167. 

Hansen, W.R., Colton, R.B., Lindvall, R.M., and Rogers, W.P., 

1976, Greater Denver urban geology field trip, in R.C. Epis, 

and R.J. Weimer, eds., Studies in Colorado field geology: Professional Contributions of Colorado School of Mines no. 8, 

p. 505-511. 

Hansen, W.R., Chronic, John, and Matelock, John, 1978, Climatography of the Front Range Urban Corridor and vicinity, Colorado: U.S. Geological Survey Professional Paper 1019, 59 p. 

Hansen, W.R., and Crosby, E.J., 1982, Environmental geology of 

the Front Range Urban Corridor and vicinity, Colorado with a 

section on Physical properties and performance characteristics 

of surficial deposits and rock units in the Greater Denver area 

by Ralph R. Shroba: U.S. Geological Survey Professional 

Paper 1230,99 p. 

Hauck, Cornelius W., 1972, Narrow gauge to Central and Silver 

Plume: Colorado Rail Annual Number Ten: Golden, Colorado, 

Colorado Railroad Museum, 224 p. 

Hutchinson, R.M., White, W.H., Ward, A.D., and Ranta, D.E., 

1976, Precambrian structure and early Tertiary ore deposits of 

the Central City-Idaho Springs mining district (Au, Ag, Pb, Cu, 

Zn) and the middle Tertiary intrusive complex at the Henderson mine (Mo, W), Front Range, Colrado, in Epis, R.C., and 

Weimer, R.J., eds., Studies in Colorado Field Geology: Professional Contributions of Colorado School of Mines, no. 8, 

p. 460-476. 

Kirkham, R.M., and Rogers, W.P., 1978, Earthquake potential in 

Colorado: Colorado Geological Survey Open-File Report, 

131 p. 

Lockley, M.G., 1988, Dinosaurs near Denver, in G.S. Holden and 

R.E. Tafoya, eds., Geological Society of America field trip 

guidebook 1988, Centennial Meeting, Denver, Colorado: Professional Contributions of Colorado School of Mines, no. 12, 

p. 288-299. 

Morgan, Gary, 1974, Three foot rails: A quick history of the Colorado Central Railroad: Colorado Springs, Colorado, Little 

London Press, 38 p. 

Reed, J.C., Jr., Bryant, Bruce, Grose, T.L.T., Sims, P.K., Bookstrom, A.A.. Beaty, D.W., and Mallory, W.W., 1987, Geology 

and mineral resources of central Colorado: Guidebook for a 

field trip sponsored by the Colorado Scientific Society for the 

Rocky Mountain Section, Geological Society of America, 

Boulder, Colorado, May 5 and 6, 1987, 33 p. 

Robinson, C.S., Gallant, W.A., and Cochran, D.M., 1976, Land use 

and engineering geology of the Front Range Colorado, in R.C. 

Epis, and R.J. Weimer, eds., Studies in Colorado field geology: 

Professional Contributions of Colorado School of Mines no. 8, 

p. 486-504. 

Schmidt, P.W., and Pierce, K.L., 1976, Mapping of Mountain soils 

west of Denver, Colorado, for land-use planning in Coates, 

O.R., ed., Geomorphology and engineering: Stroudsburg, Pa., 

Dowden, Hutchinson. and Ross, Inc., p. 43-54. 

Scott, G. R., 1972, Geologic map of the Morrison Quadrangle, 

Jefferson County, Colorado: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-790-A. 

Scott, G.R., 1976, Historic trail map of the greater Denver area, 

Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map 1-856-G, scale 1:100,000. 

Scott, G.R., and Taylor, R.B., 1986, Map showing late Eocene erosion surface, Oligocene-Miocene Paleovalleys, and Tertiary 

deposits in the Pueblo, Denver, and Greeley 1 °X2° quadrangles, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1626, scale I :250,000. 

Sheridan, D.M., Reed, J.C., and Bryant, Bruce, 1972, Geologic 

map of the Evergreen Quadrangle, Jefferson County, Colorado: U.S. Geological Survey Miscellaneous Investigations 

Map 1-786-A. 

Trimble, D.E .• and Machette, M.N., 1979, Geology of the Greater 

Denver area, Front Range Urban Corridor, Colorado: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

1-856-H, scale 1:100,000. 

Trimble, D.E., Scott, G.R .• and Hansen, W.R., 1992, Mountains 

and Plains: Denver's Geologic Setting: unnumbered U.S. Geological Survey special publication, 23 p. 

Tweto, Ogden, 1975, Laramide (Late Cretaceous-early Tertiary) 

orogeny in the southern Rocky Mountains: in Curtis, B.F., ed, 

Cenozoic history of the Southern Rocky Mountains: Geological Society of America Memoir 144, p. 1-44. 

--1979, Geologic Map of Colorado, scale 1:500,000. 

Van Hom, Richard, 1976, Geology of the Golden quadrangle, Colorado: U.S. Geological Survey Professional Paper 872, 116 p. 

Waag~. K.M., 1952, Clay deposits of the Denver-Golden area, 

Colorado: Colorado Scientific Society Proceedings, v. 15, 

no. 9, p. 373-390. 

HISTORICAL NOTES 

By Kathleen C. Stewart 

IDAHO SPRINGS 

Before the discovery of gold brought an influx of 

miners to the valley of Idaho Springs, abundant grass 

attracted herds of bighorn sheep to graze. George A. Jackson (fig. 14) could, therefore, survive by hunting as he 

sought placer gold in January 1859. He camped at the mouth 

of Soda Creek (by Soda Creek Road, fig. 9) to take advantage of the hot springs, ate "plenty fat meat," and shot wolverines and mountain lions who also wanted his meat. After 

camping there several nights and exploring a few miles to 

the west, Jackson moved his camp one drainage upstream to 

Chicago Creek. He built a raging fire on the bar to thaw the 

gravel for panning and on January 7, 1859, he discovered 

gold. Five days later with an injured dog and moccasins 

shredded by Clear Creek ice, he limped into camp near 

present-day Golden and told his partner, Tom Golden, the 

news. Because of the winter conditions and the difficult 

accessibility of the area, they were unable to return until 

April (Hollister, 1867; Jackson, 1935). 

Only a month after Jackson's return, news came from 

Denver of John Gregory's lode discovery near Central 

Figure 14. Portrait of George A. Jackson. Printed courtesy of 

Denver Public Library Western History Collection. 

City-just over the mountains to the north. Men scrambled 

up Virginia Canyon, where the roads were later built (Virginia Canyon Road and Prospectors Trail), in a mad rush to 

reach and confirm the discoveries on the other side (fig. 15). 

Soon both valleys were filled with placer claims or "poor 

man's diggings." Around Idaho Springs every available 

gravel bar with room to stand was taken-Spanish Bar to the 

west, Illinois Bar, Payne's Bar and Idaho Bar in addition to 

Jackson's Bar (Hall, 1889; Hall, 1891; Mathews, 1940). 

Over the next 2 years the individual camps blurred into 

one another. In 1860, F.W. Beebee moved from Illinois Bar 

to Idaho Bar and opened a hotel where some of the early 

bachelor miners could get out of their tent for a couple of 

days. People also came to "spectate" and see if all they had 

heard was true. Across the street was the log cabin saloon 

where miners "squandered their hard earned dust in fiery 

liquids, suggestive of insanity and murder" (Hall, 1891). The 

Colorado Territorial Legislature made "ldahoe" the county 

seat of newly formed Clear Creek County in 1861, but the 

seat was moved to Georgetown in 1867, after silver discoveries there an~ in nearby Silver Plume (Hall, 1891; Bauer 

and others, 1990; Wright, 1986). 

The placer mines survived until 1865, but they were 

gradually replaced by ore mining (fig. 16). By 1861 several 

stamp mills for crushing ore were already in the area, including a "20-stamper" on Spanish Bar near the Stanley Mill 

west of Idaho Springs. The difficulty of recovering gold 

from ores increased, however, as the near-surface, weathered 

ores gave out and only the deeper sulfide ores or "sulfurets" 

remained. The more resistant ores were uneconomical to 

mine until Nathania! P. Hill built a smelter in Black Hawk in 

1867-1868. Gold could be recovered by the new process, but 

ores had to be transported over the mountains by horses or 

mules (by the Virginia Canyon Road)-a very strenuous 

operation (fig. 15). After the railroad reached Idaho Springs 

in 1877, ores could be carried by rail. Shortly afterward, Hill 

transferred his smelting operations to Denver in order to process ores from several areas and to benefit from lower transportation costs for coke (Cushman and Waterman, 1876; 

Hall, 1891; Hale, 1936). 

ARGO MILL AND TUNNEL 

By the 1890's many of the mines south of Central City 

had become so deep it was no longer economical to pump 

water or hoist blasted material to the surface (Cox, 1989). 

Because Clear Creek at Idaho Springs was nearly I ,000 feet 

23 
24 GUIDEBOOK: GEOLOOY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

/ 

~- ~~ 

Gold C~n-:-K~n:~s Shaft/ 

0 Gilson Gulch 

School 

1 MILE 

1 KILOMETER 

N 

1 

Figure 15. Map showing Argo and Big Five Tunnels in relation to Idaho Springs and Central City. 

Adapted from Terry Cox (printed with permission). 

lower than Gregory Gulch by Central City, Samuel Newhouse and some associates had the idea of boring a tunnel 

through the mountains to drain mines and transport ores at 

the lower level. Articles of incorporation for the Argo Mining, Drainage, Transportation and Tunnel Company were 

filed with the Secretary of State in January 1893. The Idaho 

Springs News noted in October that a road had been built and 

that Newhouse was "preparing for work on the tunnel" 

(Secretary of State Incorporation Recordings, 1893; Idaho 

Springs News, 1893). Locally, it was referred to simply as 

the Newhouse tunnel (fig. 17). Mines operating through the 

tunnel paid drainage and transportation fees to the company. 

The tunnel reached its maximum length of 4.16 miles and 

average depth of 1500 feet below the surface by 1910, 

extending just north of Prosser Gulch west of Central City 

(fig. 15) (Bastin and Hill, 1917). 
HISTORICAL NOTES 25 

Figure 16. Early ore mining underground (probably before 1890 in Clear Creek, Gilpin, or Boulder County). These miners had only 

candles for light and no safety equipment. It was a cold, muddy occupation-note mud and water on miners and tunnel floor. This photograph also shows the amount of timbering necessary to support tunnel walls for access to deeper areas of the mine. Photograph by L.C. 

McClure, printed courtesy of Denver Public Library Western History Collection. 

The company re-organized with new financing in 1899 

and by 1910 the volume of ore being processed had become 

so large that stockholders decided to build their own mill. 

Articles of incorporation for the Argo Reduction and Ore 

Purchasing Company were filed with the Secretary of State 

in July 1912, and by 1913 the mill was in operation with 

special features to process the low-grade ores (The Denver 

Republican, 1899; Secretary of State Incorporation Recordings, 1912; The Idaho Springs Siftings-News, 1912, 1913). 

The tunnel also benefited schools indirectly. Around 

1905, a young teacher, Beatrice Rule, rode the train slightly 

over a mile into the tunnel and took the platform hoist up the 

Gem shaft to her first job at the Gilson Gulch School (fig. 15) 

(History of Clear Creek County, 1986). 

Business waned after World War I due to disputes with 

mine owners. The tunnel was cleaned and dewatered in 

1926 (fig. 18) and was eventually acquired by George 

Collins, owner of some mines in Nevadaville. He continued 

to work the tunnel with the goal of intersecting the Kansas 

shaft (fig. 15), which had a good vein but had been allowed 

to flood since the 1880's and was therefore no longer mined. 

If he could drain the Kansas shaft through the Argo Tunnel, 

the vein would be his for the taking. The task of trying to 

intersect such deep workings was dangerous-it demanded 

work in dark, cold, wet conditions with foul air thrown in 

for good measure. Miners winced with every squirt of water 

when charges were set. They quit when they had a chance. 

When water pressure did not increase after working a stope 

(step-like excavation) for some weeks, Collins decided to 

use I 0-minute fuses instead of the 30-minute fuses used previously. The decision was fatal to four men. On the afternoon of January 19, 1943, about 4 o'clock, power went out 

and a huge roar was heard. One man raced 200 yards to the 

entrance escaping in waist-deep water. According to Merle 

Sowell, after several minutes, water was flowing to the top 

and shooting across Clear Creek into downtown Idaho 
26 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 17. View of the Argo Tunnel buildings, dump, and ore 

loading chute. Its local designation as the "Newhouse Tunnel" is 

apparent from the sign on the side of the loading chute. Photograph 

was taken prior to 1910 looking west from the hillside behind the 

current mill location. The mine buildings and dump are gone today, 

but the dirt road (with wooden gate) still exists. The three houses 

Springs. When the water subsided, the body of one miner 

was found covered with mud and gravel-three other bodies 

were found later. The tunnel was subsequently closed to 

mining because of lawsuits, and it has not been used for 

mining since, although attempts have been made to clean it 

out (The Denver Post, 1943; Sowell, 1974). 

BIG FIVE TUNNEL 

The origin of the "Big Five" is shrouded in mysterymen and companies become confused. The minutes of the 

Dew Drop Mine in Boulder County taken on September 2, 

just above and to the left of the loading chute still stand just south 

of Miner Street. The valley of Chicago Creek, site of Jackson's 

gold discovery, can be seen on the left in the distance. The valley 

of Clear Creek can also be seen on the right. Photograph by L.C. 

McClure, printed courtesy of Denver Public Library Western History Collection. 

1892, state that a certificate of organization for the Orphan 

Boy Extension Mining and Milling Company, also in 

Boulder County, was filed with the Secretary of State in 

1892. The incorporators were five men-N.C. Merrill, 

Sidney Williams, Charles H. West, Frank C. Smith, and 

Edwin E. lves; but their board of directors included four 

additional people (one of them the sister-in-law of Merrill). 

By 1893, the name of William P. Daniels appears on the 

board of directors and on letterheads for the two companies. 

A letterhead mailed in February 1898, shows five companies 

listed under "The Big Five"-The Dew Drop Mining 

Company, The Dew Drop Mill Company, the Adit Mining 

Company, The Adit Tunnel Company, and The Ni Wot 
HISTORICAL NOTES 27 

Mining Company, all in Boulder County. Minutes for the 

Big Five Tunnel, Ore Reduction and Transportation Company state that the charter was filed with the Secretary of 

State of Colorado in August 1900. The first shovelful of dirt 

was thrown for their "Central Tunnel" in December of that 

year, 7 years after the start of the Argo Tunnel (The Denver 

Times, 1900; Big Five Tunnel, 1906). 

Negotiations had been rather secretive, but the tunnel 

was to be excavated in direct competition with the Argo 

(fig. 15). According to advertising, the "Big Five" had 

acquired ownership of all mining claims to be intersected, 

thereby circumventing the bookwork needed to keep track of 

fees and handling charges used by the Argo (fig. 18). They 

also advertised they would encounter ore within half a mile, 

in contrast to the Argo, which had blasted for a mile and a 

half before cutting into a good vein (The Big Five in Pen and 

Pencil, 1901). 

By 1901, the Big Five consisted of seven or eight major 

stockholding companies and several minor ones. In addition 

to tunnels in Boulder, Gilpin, and Clear Creek Counties, they 

operated several in the San Juan Mountains in southern 

Colorado. The names of Merrill and Daniels continually 

appear on letterheads, minutes, and boards of directors, in 

various combinations with other names that appear and 

disappear (Big Five Tunnel, 1906). 

Despite grandiose claims to stockholders, the Big Five 

evidently had an image problem. In a letter dated July 1901, 

N. C. Merrill admonishes Wm. P. Daniels to erect billboards 

in addition to the big "5" carved in the stone over the tunnel. 

He says, "When they receive it (advertising circular) they 

have never heard of the Big Five" (Merrill, 1901). It seems 

that even the mine dump of the Argo was better publicity 

than the Big Five's small, inconspicuous bore. 

They also had legal problems. A stockholder from 

Boston sued the company in February 1904 to prevent incorporation of yet another company and to be allowed to examine the books of the Big Five. A receiver was appointed and 

mining operations ceased. In March, the Colorado Supreme 

Court issued an injunction to allow operations to continue, 

but news of the Big Five disappeared from mining journals 

after November 1904 (Mining Reporter, 1904 a, b, c). The 

tunnel was extended to about 9000 feet in the 1930's and was 

Figure 18. Argo Tunnel in operation during the 1930's. Printed courtesy of Denver Public Library Western History Collection. 
28 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

mined some in the 1960's. It is now owned by George 

Groves, Jr., who lives in New York City. The Big Five is 

scarcely referred to in historical accounts of Idaho Springs 

and mostly in reference only to the hot springs, another of 

Merrill's ventures. Although fire destroyed the remnants of 

Big Five properties in Boulder County at Ward in 1970, 

some buildings and rails from the Big Five Tunnel are still in 

place in Idaho Springs (fig. 19) (AI Hoy!, Clear Creek 

Mining Company, oral commun., 1991; Rocky Mountain 

News, 1970). 

CHARLEY TAYLER WATER WHEEL 

Charley Tayler built his original wheel in 1893 (fig. 20) 

to power a stamp mill on Ute Creek, a small drainage that 

empties into Chicago Creek about 4 miles southwest of 

Idaho Springs on the way to Mount Evans. 

In 1945, the wheel was donated to the town of Idaho 

Springs. It was dismantled and reassembled in its present 

location by the Chamber of Commerce as a tourist attraction. 

In 1973, the Henderson Mine made drawings of the individual pieces and rebuilt the wheel using much of the old 

structure. By this time the wheel had deteriorated badly, and 

the partial reconstruction ended up serving only as a temporary measure, because it soon ground to a halt again. In 1988, 

Idaho Springs residents started a major reconstruction effort. 

Using donated time, materials, and expertise, massive concrete foundations were poured, new supports were constructed, and the hub and axle were hoisted into place. On the 

basis of drawings from the previous reconstruction, new 

wooden components were attached to the hub with many of 

the original metal parts. The hub and axle constitute fully 

one-quarter of the total weight of 6000 pounds. Because of 

its tremendous weight, the wheel is furnished with solderlike bearings that prevent wear of the axle and can be poured 

in place when replacement is needed. The wheel is also 

equipped with special fittings for greasing from ground 

level. 

Water flowing to the wheel and the waterfall to its 

left come from an overflow pipe of the water supply to 

Idaho Springs. The water is diverted from its natural path 

either to the falls, the wheel, or both. During the winter, 

all water flow is diverted to the falls to prevent ice damage 

to the wheel (Wendell Upright, Colorado Department of 

Figure 19. Ore cars of the Big Five Tunnel on their way to the mill. Adit is barely visible behind building on the left. Photograph is 

taken between 1900 and 1910 from the south side of Clear Creek looking north. 1-70 now obscures this view, crossing the tracks above 

the stairway at the level of the road in the background. Shed visible at far left no longer exists, but one behind it and the four houses still 

stand. Denver Public Library Western History Collection (printed with permission). 
HISTORICAL NOTES 29 

Figure 20. Charley Tayler (far right) with his water wheel sometime after its construction in 1893. Printed courtesy of Idaho Springs 

Historical Society. 

Transportation, and Bruce Bell, Idaho Springs Historical 

Society, written commun., 1991). 

HOT SPRINGS 

The hot springs Jackson used were known to Ute, 

Arapahoe, Apache, Kiowa, and Comanche Iedians because 

possession of the territory had changed hands many times 

prior to its discovery by gold seekers. The best theories of the 

origin of the name "Idaho" are based on corruption of Indian 

words. In 1861, the area was "ceded" to the United States by 

the Arapahoe and Cheyenne Indians and soon thereafter 

became part of Colorado Territory (Mathews, 1940; Dawson, 1954; Erickson and Smith, 1985). The first attempt to 

commercialize the springs came 2 years later when Dr. E.M. 

Cummings acquired the land and built a wooden bathhouse. 

From that time forward people paid to use the springs but at 

least they were indoors and out of the cold. In 1866, Harrison 

Montague bought the property and built a more substantial 

stone structure. Montague's son-in-Jaw replaced the bathhouse in 1899 with the "natatorium" (fig. 21). In this era it 

became popular for tourists to stop and drink the mineral 

water (Hall, 1891; The Denver Times, 1899). 

Operators of the Big Five Tunnel bought the property 

in early 1900 and excavated a tunnel network in the mountain to further commercialize the hot springs. That network 

is still present today. About 1910, a property-holding company acquired the operation and in the 1920's attempted to 

develop it as a sanitarium for polio called "The Radium 
30 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 21. The Idaho Springs Natatorium in the early 1900's. Printed courtesy of Denver Public Library Western History Collection. 

Hot Springs" (N.C. Merrill was still on the board). Advertising stated, "Come and Drink the Water. It is FREE. 

The Radium Water, taken internally, radiates outward, as if 

the sun were shining out from inside" (Radium Hot Springs 

Records, 1924-1930). 

During this time the main hotel was built. The names 

of many celebrities have appeared in guest books over the 

years-Frank and Jesse James, H.A.W. Tabor, and Walt 

Whitman, to mention a few (History of Clear Creek County, 

1986; Radium Hot Springs Records, 1924-1930). Today the 

hot springs is still a commercial resort. 

ACKNOWLEDGMENTS 

I would like to thank the following for their help with 

this history: Marjorie Bell of Idaho Springs was a very 

patient initial and continuing source of information; Mary 

Jane Loevlie, also of Idaho Springs, kindly obtained the 

print of Charlie Tayler; the entire staff of the Denver Public 

Library's Western History Collection was generous with 

their help to obtain documentation and photographs; the 

staff of the U.S. Geological Survey and Bureau of Mines 

Libraries helped obtain and reproduce photographs and 

documentation; the staff of the Colorado State Archives 

assisted with document verification; and my sister Mary 

Stewart edited the first draft. 

REFERENCES 

Bastin, Edson S., and Hill, James T., 1917, Economic Geology of 

Gilpin County and Adjacent Parts of Clear Creek and Boulder 

Counties, Colorado: U.S. Geological Survey Professional 

Paper 94, 379 p. 

Bauer, William, H., Ozment, James L., and Willard, John H., 1990, 

Colorado Post Offices 1859-1989: Golden, Colorado, Colorado Railroad Museum, 280 p. 

Big Five Tunnel, 1906, Ore Reduction and Transportation 

Company Records, 1892-1906: Western History Collection, Denver Public Library. [Collection of records and 

correspondence] 

Cox, Terry, 1989, Inside the Mountains: Boulder, Colorado, Pruett 

Publishing Company, 117 p. 

Cushman, Samuel and Waterman, J.P., 1876, The Gold Mines of 

Gilpin County, Colorado: Historical, Descriptive and Statistical: Central City, Colorado, Register Steam Printing House, 

136 p. 
HISTORICAL NOTES 31 

Dawson, J. Frank, 1954, Place names in Colorado: Denver, Colorado, Golden Bell Press, 72 p. 

Erickson, Kenneth A. and Smith, Albert W., 1985, Atlas of Colorado: Boulder, Colorado, Colorado Associated University 

Press, 73 p. 

Hale, Jesse D., 1936, The first successful smelter in Colorado: The 

Colorado Magazine, v. 13, p. 161-167. 

Hall, Frank, 1889, History of the State of Colorado: Chicago, Illinois, Blakely Printing Company, v. 1, 564 p. 

---1891, History of the State of Colorado: Chicago, Illinois, 

Blakely Printing Company, v. 3, 518 p. 

History of Clear Creek County-Tailings, Tracks, and Tommyknockers, 1986: Denver, Colorado, Specialty Publications, 

Inc., 451 p. 

Hollister, Ovando J., 1867, The Mines of Colorado: Springfield, 

Massachusetts, Samuel Bowles and Company, 450 p. 

Idaho Springs News, 1893, Mining and Milling: Idaho Springs 

News, October 13, 1893, p. 4. 

Jackson, George A., 1935, Diary 1858-1859, in L.R. Hafen (ed.): 

The Colorado Magazine, v. 12, p. 202-214. 

Mathews, Ruth Estelle, 1940, A Study of Colorado Place Names: 

Masters Thesis, Stanford University, 473 p. 

Merrill, N.C., July 25, 1901, personal letter to Wm. P. Daniels: 

Denver Public Library, Western History Collection [collection 

of records and correspondence]. 

Mining Reporter, 1904a, The Big Five litigation: Denver, Colorado, Mining Reporter Publishing Company, February 11, 

1904, p. 138. 

---1904b, Receiver appointed for Big Five Mining Co.: 

Denver, Colorado, Mining Reporter Publishing Company, 

February 25, 1904. p. 195. 

---1904c, Mining News, Clear Creek County: Denver, Colorado, Mining Reporter Publishing Company, November 17, 

1904, p. 533. 

Radium Hot Springs Records, 1924-1930, Idaho Springs, Colorado: Denver Public Library, Western History Collection [collection of records and correspondence]. 

Rocky Mountain News, 1970, Big Five mine at Ward destroyed in 

fire: Rocky Mountain News, August 8, 1970, p. 44C. 

Secretary of State Incorporation Recordings, 1893: Colorado State 

Archives, v. 33, p. 474. 

--1912, Colorado State Archives, v. 157, p. 477. 

Sowell, Merle, 1974, Four men drowned on Saturday in January 

1943: Front Range Journal, August 12, p.l2. 

The Big Five in Pen and Pencil, "V," October, 1901: Denver, Colorado, Carson-Harper Company, (unpaged). 

The Denver Post, 1943, Argo tunnel flood traps four and one body 

is recovered: The Denver Post, January 19, 1943, p. I. 

The Denver Republican, 1899, Re-organization of the Newhouse 

Tunnel Company: The Denver Republican, April 14, 1899, 

p. 8, col. 3. 

The Denver Times, 1899, Idaho Springs, a health resort and mining 

center: The Denver Times, May 21, 1899. p. 17. 

---1900, The Big Five at Idaho Springs: The Denver Times, 

December 30, 1900, pt. 2, p. 3, col. 5. 

The Idaho Springs Siftings-News, 1912, The mining outlook for 

1913: The Idaho Springs Siftings-News, December 28, 1912, 

no page. 

--1913, Purchase of ores F.O.B. Argo Mill: The Idaho 

Springs Siftings-News. February 8, 1913, no page. 

Wright, Jim, 1986, Clear Creek County in History of Clear Creek 

County: Tailings, Tracks and Tommyknockcrs: Denver, Colorado, Specialty Publications, Inc., 451 p. 
32 

Arrastre 
GEOLOGY AND MINERALIZATION OF THE 

IDAHO SPRINGS AND CENTRAL CITY MINING DISTRICTS 

By Helen W. Folger 

In the years between 1860 and 1950, the Central City 

and Idaho Springs Mining Districts produced much gold and 

silver from sulfide ores. The ore deposits formed principally 

along fault fissures in Precambrian granites, gneiss, and 

schist, during Tertiary time. 

IDAHO SPRINGS AND CENTRAL CITY 

MINING DISTRICTS 

The mining towns ofldaho Springs and Central City are 

located approximately 30 miles due west of Denver in the 

Front Range of the Rocky Mountains. The Front Range 

includes the area from the Wet Mountains on the south, 

northward along the Arkansas River to the Laramie Range, 

and from the Park, Mosquito, and Sawatch Ranges on the 

west, eastward to the Denver Basin and Great Plains 

(fig. 22). The core of the Front Range is composed of Precambrian granites, gneiss, and schist that were uplifted during Late Cretaceous time along a north-south-trending 

anticline. The overlying Paleozoic and Mesozoic sedimentary rocks have since been eroded away along the crest leaving steeply dipping beds along its flanks (fig. 3). The Idaho 

Springs and Central City Mining Districts lie in the Colorado 

Mineral Belt-a zone of mineralization that runs northeast to 

southwest from Boulder through Breckenridge to the San 

Juan Mountains in southwest Colorado. 

The most favorable host rocks for mineral deposition 

were the Precambrian migmatites18 (including the Idaho 

Springs Formation), hornblende gneiss, and the Boulder 

Creek Granodiorite, Silver Plume, and Pikes Peak Granites 

(table 1). Openings and fissures formed by the faulting 

(fig. 8) became ideal sites for mineral deposition by fluids 

ascending from deep magma19 sources. Sulfide-bearing 

quartz veins and stockwork20 formed along east and northeast-trending faults near intersections with northwesttrending faults. In most instances the northwest-trending 

18 A composite rock composed of igneous and metammphic 

materials. 

19Molten rock. 200re composed of closely spaced irregular-shaped veinlets in a host 

rock. The whole rock is milled as opposed to just the vein material. 

w 

(!) 

z 

<( 

a: 

:c 

u 

~ 

~ (J) I 

\.. 

105°00' ··-rr··-·· I 

I 

I 

I 

I 

I 

I 

- -

-__j GREAT I PLAINS I .. 

1-

z 

0 

a: 

u. 

0 10 20 30 MILES 

I I I I I 

Figure 22. Map outlining the Front Range in Colorado and 

showing the location of the Central City and Idaho Springs 

Mining Districts in the Colorado Mineral Belt (modified from 

Sims, 1960). 

faults influenced the ore-fluid movement through the area, 

but are themselves unmineralized (Lovering and Goddard, 

1950). 

33 
34 GUIDEBOOK: GEOLOOY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 23. Location of an ore vein marked by a hammer. This is probably a pyritic ore vein in Clear Creek or Gilpin County. Photograph 

was taken in the mid-nineteenth century-note flame of burning candle that was used for light. Printed courtesy of Denver Public Library 

Western History Collection. 
GEOLOGY AND MINERALIZATION OF THE IDAHO SPRINGS AND CENTRAL CITY MINING DISTRICTS 35 

PRIMARY ORES 

Ores of the Central City and Idaho Springs Mining 

Districts were mined principally for gold and silver, 

although substantial amounts of copper, lead, zinc, and 

uranium were also recovered. In 1917, Bastin and Hill 

published the first comprehensive report on the two districts. 

Their report described the geology and production and gave 

detailed descriptions of the ores mined in the district. They 

classified the ores according to mineralogy into six types 

(from oldest to youngest): uranium ores, pyritic ores21 , 

galena-sphalerite ores22, composite ores, telluride ores23, 

and placer <Jeposits. 

The primary uranium ore mineral is uraninite24 (U02). 

Uraninite is found in a gray-quartz gangue25 with smaller 

amounts of chalcopyrite (CuFeS2) and pyrite (FeS2). Both 

pyritic and galena-sphalerite ores frequently contain recoverable amounts of uraninite. Sims and others (1963) believe 

that uranium mineralization took place prior the main sulfide 

(pyritic) mineralization. It occurs sporadically with other 

sulfides throughout the districts. 

Pyritic ores are the most widely distributed in the district (fig. 23). Their main constituents are pyrite (FeS2) and 

gangue with smaller amounts of chalcopyrite (CuFeS2), tennantite (Cu12As4S13), gold (Au), and occasionally enargite 

(Cu3AsS4). Small amounts of galena (PbS), sphalerite 

(ZnS), molybdenite (MoS2), and native bismuth (Bi) also 

occurred in some deposits. Gold and silver are most often 

associated with the chalcopyrite and enargite minerals (Lovering and Goddard, 1950). 

The galena-sphalerite ores were deposited later than the 

pyritic ores. These ores are composed predominantly of 

galena, sphalerite, and smaller amounts of pyrite. Some 

chalcopyrite, tennantite, and bornite (Cu5FeS4) are also 

present. Similar to pyritic ores, the gold is associated more 

with chalcopyrite and tennantite than with pyrite (fig. 24). In 

contrast to the pyritic ores, the galena-sphalerite ores are 

poorer in gold and copper but richer in silver. Quartz 

(Si02), calcite (CaC03), and siderite (FeC03) are the most 

common gangue minerals, but rhodochrosite (MnC03) and 

barite (BaS04) are abundant in some veins. 

The composite ores are composed of both the pyritic 

ores and the galena-sphalerite ores. Composite ores are 

thought to be the result of double mineralization aloncr a frac- e. 

ture or vein where renewed fault movement created new 

space for later ore deposition (Lovering and Goddard, 1950). 

As faults were reactivated, pyritic ores were broken and 

overlain by the galena-sphalerite ores. In some veins signif21 0res that contain a large percentage of pyrite (FeS2). 

220res that contain abundant galena (PbS} and sphalerite (ZnS). 230res that contain the element tellurium. 24AJso known as pitchblende. 25The valueless mineral or rock associated with ores. 

Figure 24. Unusual occurrence of a gold crystal resting on the 

edge of a galena crystal. Specimen idnetified as coming from the 

Leadville Mining District. Photograph by Richard B. Tripp, U.S. 

Geological Survey. 

icant brecciation26 of the pyritic ores occurred prior to the 

galena-sphalerite ore deposition. The gold plus silver content of these ores varies considerably from deposit to deposit. 

Telluride ores are composed of gold, silver, and tellurium in a gangue of quartz, fluorite (CaF2), calcite, and 

fine-grained pyrite. They are a later stage of mineralization 

than the pyritic and galena-sphalerite type ores and formed 

networks along fractures and in the adjacent wall rock. 

Only a few telluride-bearing veins that occur along a narrow fault zone northeast of Central City are found in this 

district. Lovering and Goddard (1950) suggested that they 

were rare because only a few faults tapped the source of the 

gold-telluride enriched ore fluids. Some of the tellurides 

identified include altaite (PbTe), krennerite (AuTe2), petzite 

(Au3AgTe2), and sylvanite ((Au,Agh Te4). 

Placer deposits were formed by the weathering of 

pyritic gold ores and were the first deposits to be discovered 

and worked by early miners. Surface exposures of goldbearing sulfides disintegrated because of weathering processes, thereby freeing the gold. Flowing water concentrated 

the free gold into pockets or bars of gravel along stream bottoms. Large-scale placer mining did not become economically important until the 1930's, when the first dry dredge 

worked on Clear Creek east of Idaho Springs and later on 

North Clear Creek (fig. 25). It is estimated that more than 

100,000 fine ounces of gold and 158 fine ounces of silver 

were recovered from placers in these districts. 

26Breaking and crushing into angular fragments. 
36 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 25. Hydraulic dredge mining placer deposits along Clear Creek sometime between 1930 and 1941. Printed courtesy of Denver 

Public Library Western History Collection. 

RECENT STUDIES 

In 1950, Lovering and Goddard studied the paragenetic 

sequence27 of mineralization in the Idaho Springs and 

Central City Districts and described how it related to other 

deposits in the Mineral Belt. They found a close association 

between the mineralization of these two districts. In 1963, 

Sims and others, recognized that certain sulfide minerals 

formed concentric zones around the mining districts (fig. 

26). The zonal pattern is believed to have resulted from the 

upward and outward movement of hydrothermal fluids28 

from a source beneath the central zone (Sims and others, 

1963). The different zones reflect the transitions from a core 

deficient in base-metals29 and rich in pyrite to a peripheral 

27The sequential ordering of mineral deposition in an ore deposit. 28Heated watery fluids that carry elevated concentrations of some 

elements. Ore deposits form by precipitation of these elements as minerals. 29Base-metals generally refers to the transition elements in the periodic chart, in particular Cu, Pb, and Zn. 

zone where base-metal sulfides predominate and pyrite is 

absent. Gold occurs mostly in and near the intermediate 

zone, and silver increases outward from the intermediate 

zone to the peripheral zone (Sims and Barton, 1962). 

ORE ENRICHMENT 

After the hydrothermal system waned, lower temperature ground waters further enriched ores in gold and silver. 

These hydrochemical processes were less important for 

copper and negligible for lead and zinc. Oxidized surface 

waters dissolved the sulfides and carried the metals downward to the water table. Gold enrichment, therefore, 

occurred mostly in the areas directly above and below the 

water table. This resulted in a dramatic increase in the content of native (elemental) gold in ores of the oxidized zone 

(Bastin and Hill, 1917). 

Secondary silver enrichment occurred mostly in the 

galena-sphalerite ores at or below the level of the water 
GEOLOGY AND MINERALIZATION OF THE IDAHO SPRINGS AND CENTRAL CITY MINING DISTRICTS 37 

table. Enrichment may extend 100-200 feet below groundwater level with silver concentrations decreasing with 

increasing depth. In the oxidized zone above the water table 

the ores were somewhat depleted in silver. The silver was 

deposited mostly as proustite (Ag3AsS3)-rarely as native 

silver. 

Copper enrichment was confined entirely to the 

ground-water zone of the pyritic ores (Bastin and Hill, 

1917). Ores in the oxidized zone were somewhat depleted in 

copper. The principle secondary copper mineral formed 

beneath the water table is chalcocite (Cu2S). 

THE ARGO AND BIG FIVE TUNNELS 

The hills around Idaho Springs and Central City are 

honey-combed with adits, shafts, and tunnels. Most are longabandoned because of the low quality of the remaining ores. 

Two of these tunnels are the focus of this field trip-the 

Argo and the Big Five (fig. 15). 

The Argo Tunnel (or Newhouse Tunnel) was designed 

to intersect at depth many of the veins that outcropped on the 

surface (Bastin and Hill, 1917), to drain the Central City 

mine workings, and to provide an easier and cheaper way to 

0 

DATUM IS MEAN SEA LEVEL 

I MILES 

-..A.ppn:oxim>IIO limit of 

productive veins 

Figure 26. Map outlining principal zones of ore mineralization according to Sims (1989). Ore veins are classified based on their content 

of base metals (lead, zinc, and copper). The central zone contains pyrite veins without base metals that grade into veins with abundant 

copper minerals. Veins of the intermediate zone contain more lead and zinc and less copper. The peripheral zone contains mostly lead and 

zinc sulfides without pyrite. Gold and silver are most abundant in veins of the core and intermediate zones (adapted from Sims, 1989). 
38 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

transport ores to the mills. Most veins were of low grade 

where they were cut by the tunnel, although several profitable ones were found. The feeling persists today that the 

tunnel proved the deep veins were of poor quality and disappointing grade (Lovering and Goddard, 1950). The Argo 

Tunnel cuts schist of the Idaho Springs Formation, Precambrian granite gneiss, and Tertiary bostonite and monzonite3° 

porphyry31 dikes. The Argo also intercepts veins of pyritic, 

galena-sphalerite, composite, and telluride ores. According 

to the zonal configuration of Sims and others (1963), the 

tunnel cuts both the pyrite~rich central zone and the basemetal-rich intermediate zones. 

The Big Five Tunnel cuts mostly schist of the Idaho 

Springs Formation, and some granite pegmatite and dikes of 

monzonite and bostonite. Most mineralization contains 

small amounts of finely disseminated pyrite or quartz, 

galena, and sphalerite. The tunnel cuts the Shafter vein with 

abundant composite ores, and the Bellrnan and Lake veins 

with abundant pyritic ores. Quartz and sericite32 are common alteration minerals found in the wall rocks (Idaho 

Springs schist). Quartz, pyrite, galena, sphalerite, chalcopyrite, tennantite, and various carbonate minerals are common 

in these veins. The tunnel cut ores of the peripheral, interme30Bostonite and monzonite are both types of igneous rocks formed 

below the Earth's surface. They differ from each other in their grain sizes 

and feldspar contents. 31 Igneous rock with larger crystals set in smaller groundmass. 

32A white, fine-grained, high-potassium mica occurring in small 

scales and flakes. 

Sphalerite 

diate, and central zones as described by Sims and others 

(1963). 

REFERENCES 

Bastin, Edson S., and Hill, James M., 1917, Economic geology of 

Gilpin County and adjacent part of Clear Creek and Boulder 

Counties, Colorado: U.S. Geological Survey Professional 

Paper 94, 379 p. 

Lovering, T.S., and Goddard, E.N., 1950, Geology and ore deposits 

of the Front Range, Colorado: U.S. Geological Survey Professional Paper 223, 319 p. 

Sims, PaulK., 1960, Geology of Central City-Idaho Springs area, 

Front Range, Colorado, in Weimer, Robert J., and Haun, John 

D., eds., Guide to the Geology of Colorado: Denver, Geological Society of America, Rocky Mountain Association of Geologists, and the Colorado Scientific Society, p. 279-285. 

Sims, P.K., 1989, Central City and Idaho Springs Districts, Front 

Range, Colorado, in Bryant, Bruce, Beaty, David W., Mineral 

deposits and geology of Central Colorado: 28th International 

Geological Congress, Field Trip Guidebook Tl29, p. 15-19. 

Sims, P.K. and Barton, P.B., 1962, Hypogene zoning and ore genesis, Central City District, Colorado, in Engel, A.E., James, 

H.L., and Leonard, B.F., eds., Petrologic studies; Buddington 

volume: Boulder. Colorado, Geological Society of America, 

p. 373-395. 

Sims, P.K., Drake, A.A., Jr., and Tooker, E.W., 1963, Economic 

geology of the Central City District, Gilpin County, Colorado: 

U.S. Geological Survey Professional Paper 359, 231 p. 

Siderite 
GENERATION AND INTERACTIONS OF MINE DRAINAGE 

AT THE ARGO TUNNEL 

By Kathleen S. Smith 

The ores of many mining districts, such as those around 

Idaho Springs, contain abundant sulfide (sulfur-bearing) 

minerals. When mining exposes these minerals to water, 

dissolved oxygen, and ferric (Fe3+) iron, chemical reactions 

take place that can produce high concentrations of dissolved 

metals (such as zinc, copper, and lead), ferrous (Fe2+) iron, 

sulfate, and acid (H+) (fig. 27, reaction A). Certain kinds of 

bacteria catalyze (and increase the rate ot) these reactions. 

The acid produced can attack other minerals in surrounding 

rocks and dissolve elements, such as calcium, aluminum, silicon, and manganese. Water involved in these reactions can 

become more acidic than vinegar (acetic acid) or lemon juice 

(citric acid) (pH<4). After migration, further reactions may 

form iron-rich precipitates and more acid (fig. 27, reaction 

B). A more detailed discussion of the generation and interactions of mine drainage is given in the following sections. 

GENERATION OF MINE DRAINAGE 

Although the solubility of sulfide minerals in water is 

generally low, they are unstable in the presence of dissolved 

oxygen and/or ferric iron. Both dissolved oxygen and ferric 

iron can oxidize sulfide minerals, such as pyrite (also known 

as fool's gold), as illustrated in the following reactions: 

FeS2(s) + 3.S02(aq) + Hp~Fe + + 2SOl- + 2H+ (1) 

FeS2(s) + 14Fe3+ + 8H 0~15Fe + + 2Soi- + 16H+ (2) 

where FeS2(s) is pyrite, 0 2(aq) is dissolved oxygen, H20 is 

water, Fe2+ is dissolved ferrous iron, soi-is dissolved sulfate, H+ is dissolved proton (responsible for acidity), and 

Fe3

+ is dissolved ferric iron (Garrels and Thompson, 1960). 

In the pH range from 2 to 7, pyrite oxidation is favored by 

reaction 2 rather than reaction 1 (Moses and others, 1987; 

Moses and Herman, 1991 ). Note that eight times more acidity is produced when ferric iron oxidizes pyrite (reaction 2) 

than when dissolved oxygen serves as the oxidant (reaction 

1) (16 moles compared to 2 moles ofH+ per mole ofFeS2(s)>· 

When ferric iron is the oxidant (reaction 2), the role of dissolved oxygen in the system appears to be limited to the 

replenishment of ferric iron by the following reaction: 

(3) 

This reaction is the rate-limiting step in the generation 

of mine drainage in the pH range from 2 to 9 (Moses and others, 1987; Moses and Herman, 1991). 

Certain kinds of bacteria, called Thiobacillus and Leptospirillum, are able to derive energy from the conversion of 

ferrous to ferric iron (Harrison, 1984). Reaction 3 proceeds 

slowly at pH values <4. However, in the presence of these 

bacteria, conversion of ferrous to ferric iron (reaction 3) can 

proceed a million times faster than in the absence of bacteria 

(Singer and Stumm, 1970). These bacteria can thrive in environments where mining has taken place as long as a minimal 

amount of oxygen is present. Therefore, the production of 

mine drainage can be a rapid, self-propagating process that 

continues as long as oxygen, water, and pyrite are available 

(Nordstrom and others, 1979). Several other factors can 

affect the oxidation rate; these include surface area of the 

sulfide minerals, type and composition of the sulfide minerals, the presence of acid-neutralizing minerals (especially 

calcite and dolomite), and temperature and salinity of the 

ground water. 

The following diagram summarizes the chemical and 

biological factors involved in the self-propagating generation of mine drainage from pyrite oxidation. 

replenished by bacteria 

(in the presence of oxygen) 

i 

Pyrite + Fe3+ + Water~Fe + +Sulfate+ Acid 

Dissolved oxygen and ferric iron can oxidize sulfide 

minerals that contain metals other than iron. In mines 

drained by the Argo Tunnel, oxidation of ·sphalerite (ZnS), 

chalcopyrite (CuFeS2), and galena (PbS) releases zinc (Zn), 

copper (Cu), and lead (Pb) into the water. 

The acid produced by some of the oxidation reactions 

discussed here can also attack minerals of surrounding rocks 

and dissolve their constituents. In the Argo Tunnel area, 

minerals in the rocks contain primarily aluminum, silicon, 

calcium, potassium, and manganese. Drainage from the 

tunnel, therefore, also contains elevated concentrations of 

these elements. 

A more general term commonly used for the reactions 

discussed in this section is "rock drainage." 

39 
40 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

I@ @I Pyrite (FeS2) 

I' --.1 Oxygen (02) 

~ Water (H20) 

Reaction A* 

Pyrite + Water + (Fe3+ or Oxygen) - Fe 2+ + Sulfate + Acid 

Reaction B * 

Fe3

+ + Water - Iron Hydroxides + Acid 

*Bacteria that convert Fe2+ to Fe3+ 

can greatly increase the rate of 

acid drainage generation. 

Figure 27. Diagrammatic representation of mine drainage reactions. Reactions begin as a result of mineral exposure to water and 

dissolved oxygen (diagram by Shawn Smith). 

INTERACTION OF MINE DRAINAGE 

DURING MIGRATION 

When mine-drainage waters migrate away from their 

site of generation, further reactions can take place. Moderate 

to highly acidic waters continue to dissolve constituents 

from most materials with which they make contact, thereby 

increasing the dissolved concentration of some elements. 

The dissolved concentration of other elements, however, 

decreases. For example, waters carrying elevated concentrations of iron frequently form iron-rich precipitates upon 

exposure to oxygen, or in response to a change in temperature, water chemistry, or increase in pH (Nordstrom, 1982). 

The following reactions demonstrate the formation of solid 

iron hydroxide from dissolved iron: 

Fe3+ + 3H20 ~Fe(OH)3(s) + 3H+ (4) 

Fe2+ + 2.5H20 + 0.2502(aq)~Fe(0Hh(s) + 2H+ (5) 

where Fe(OH)3(s) is solid iron hydroxide. Note that reactions 4 and 5 produce acidity. The precipitates may also 

sequester dissolved metals (such as copper) as they form. 

In acidic drainages, such as that from the Argo Tunnel, 

iron-rich precipitates form red-orange-yellow coatings on 

the stream bottom and on the surface of bacteria, algae, and 

moss that live in the mine-drainage waters. The organisms 

here form long streamers on the bottom. It is thought that the 

precipitates are made up of a mixture of iron, hydroxide, 

oxygen, and sulfate. Geochemists call these precipitates 

"hydrous ferric oxides" or "ferric oxyhydroxysulfate" or 

"ferrihydrite" (Bigham and others, 1990); miners called 

them "yellow boy." 

When relatively uncontaminated waters mix with and 

dilute mine-drainage waters changes take place. Some elements precipitate, some are sequestered by particulates in the 

water or on the stream bottom, and some remain dissolved. 

Where the Argo Tunnel drainage flows into Clear Creek, 

iron-rich precipitates (red-orange) form just downstream 

from the inflow. Manganese-rich precipitates (black) can 

often be observed a bit farther downstream (100 to 200m). 

Such elements and precipitates introduced into streams and 

rivers from mine drainage can adversely affect the quality, 

ecology, and chemistry of the receiving waters for many 

kilometers downstream (Moore and Luoma, 1990; Ficklin 

and Smith, 1993, this Circular). 

REFERENCES 

Bigham, J.M., Schwertmann, U., Carlson, L., and Murad, E., 1990, 

A poorly crystallized oxyhydroxysulfate of iron formed by 

bacterial oxidation of Fe(II) in acid mine waters: Geochimica 

et Cosmochimica Acta, v. 54, p. 2743-2758. 

Ficklin, W.H., and Smith, K.S., 1993, Influence of mine drainage 

on Clear Creek, Colorado: U.S. Geological Survey Circular 

1097, p. 43-48. 

Garrels, R.M., and Thompson, M.E., 1960, Oxidation of pyrite in 

ferric sulfate solution: American Journal of Science, v. 258, 

p. 57--67. 
GENERATION AND INTERACTIONS OF MINE DRAINAGE AT THE ARGO TUNNEL 41 

Harrison, A.P., Jr., 1984, The acidophilic thiobacilli and other acidophilic bacteria that share their habitat: Annual Reviews in 

Microbiology, v. 38, p. 265-292. 

Moore, J.N., and Luoma, S.N., 1990, Hazardous wastes from largescale metal extraction-A case study: Environmental Science 

and Technology, v. 24, p. 1278-1285. 

Moses, C.O., and Herman, J.S., 1991, Pyrite oxidation at circumneutral pH: Geochimica et Cosmochimica Acta, v. 55, 

p. 471-482. 

Moses, C.O., Nordstrom, D.K., Herman, J.S., and Mills, A.L., 

1987, Aqueous pyrite oxidation by dissolved oxygen and by 

ferric iron: Geochimica et Cosmochimica Acta, v. 51, 

p. 1561-1571. 

Nordstrom, O.K., 1982, Aqueous pyrite oxidation and the consequent formation of secondary iron minerals, in J.A. Kittrick, 

D.S. Fanning, L.R. Hossner, eds., Acid Sulfate Weathering, 

SSSA Special Publication Number 10: Madison, Wisconsin, 

Soil Science Society of America, p. 37-56. 

Nordstrom, O.K .• Jenne, E.A., and Ball, J.W., 1979, Redox equilibria of iron in acid mine waters, in E.A. Jenne, ed., Chemical 

Modeling in Aqueous Systems-Speciation, Sorption, Solubility, and Kinetics, ACS Symposium Series 93: Washington, 

D.C., American Chemical Society, p. 51-79. 

Singer, P.C., and Stumm, W., 1970, Acid mine drainage-The rate 

determining step: Science, v. 167, p. 1121-1123. 

Argo Mill 
Pyrite 

Tetrahedrite 

Chalcopyrite 
INFLUENCE OF MINE DRAINAGE ON 

CLEAR CREEK, COLORADO 

By Walter H. Ficklin t and Kathleen S. Smith 

Mine drainage is a serious environmental problem in 

Colorado. The State of Colorado has determined that over 

1,300 miles of its streams are impacted by metals as a result 

of drainage from mining areas. This mine drainage is often 

acidic and generally contains high concentrations of dissolved metals and metalloids. These high concentrations are 

caused both by the oxidation of metallic sulfides and by the 

acid dissolution of the country rock (see Smith, this Circular, 

p. 39). Oxidation of metallic sulfides can cause elevated concentrations of elements, such as iron (Fe), zinc (Zn), copper 

(Cu), lead (Pb), cadmium (Cd) and arsenic (As). Acid dissolution of the country rock can result in the release of the following elements: aluminum (AI), manganese (Mn), silicon 

(Si), calcium (Ca), magnesium (Mg), sodium (Na), and 

potassium (K). 

Additional information on the extent of contamination 

due to mine drainage in Colorado can be found in Wentz 

(1974). Kimball and Wetherbee (1988) reported on the 

instream chemical reactions of acidic mine water entering a 

neutral stream. A very technical discussion of the chemistry 

of natural waters can be found in Stumm and Morgan (1981 ). 

Ferguson and Gavis (1972) discussed the chemistry of 

arsenic in natural waters. Benjamin and Leckie (1981) discussed pH-dependent adsorption of metals on hydrous iron 

oxide. 

CHARACTERIZATION OF WATER 

QUALITY IN CLEAR CREEK 

Amazingly, 13 different streams in Colorado are called 

Clear Creek. The particular Clear Creek that we are interested in originates at the Continental Divide west of Georgetown, Colorado, in the pristine alpine environment found at 

11,000 feet of elevation near Torreys Peak (fig. 28). Along 

its path through the Front Range of the Rocky Mountains, 

Clear Creek traverses a section of the Colorado Mineral Belt. 

Many old mines and mine dumps can be seen along Interstate Highway 70, which follows the course of Clear Creek 

throughout most of Clear Creek County, Colorado. Eventually, Clear Creek empties into the South Platte River just 

north of Denver (fig. 1). During periods of low flow most of 

tJanuary 13, 1937 to October 11, 1993. 

the water is removed from the stream at Golden, Colorado, 

for domestic, agricultural, and industrial use. 

Acidic metal-rich drainage from the Argo Tunnel in the 

City of Idaho Springs (fig. 29) enters Clear Creek and has a 

profound effect on the concentration of dissolved constituents, the amount of suspended sediment, and the concentration of metals in the streambed sediment. The concentration 

of several constituents found in the Argo Tunnel water is 

presented in table 2. The South Fork of Clear Creek receives 

waters draining the Argentine Mining District near the city 

of Georgetown. The West Fork of Clear Creek receives mine 

drainage from an area near the town of Empire. The North 

Fork of Clear Creek (North Clear Creek) passes through the 

Central City Mining District (figs. I and 29). The waters and 

sediments of the North Fork are particularly affected by mining and add additional metals to Clear Creek. 

In June of 1986, during high flow, we collected 27 

water and sediment samples from Clear Creek at about 2-

mile intervals from above Georgetown near Torreys Peak 

to its mouth on the South Platte River. Sample locations are 

shown in figure 29. The distribution of samples covered 

almost the entire length of the stream. In October of that 

same year, when low flow was re-established, we collected 

samples at the same locations (fig. 30). Field observations 

included pH, temperature, and conductivity. 

Metals and metalloids are transported in streams and 

rivers both as dissolved constituents and as suspended sediment. For this study, dissolved metal concentrations were 

determined from samples filtered through a 0.45-micrometer 

filter (fig. 31 ); total metal concentrations were determined 

from unfiltered samples. Both filtered and unfiltered 

samples were acidified. The amount of an element carried in 

the suspended phase was calculated from the difference 

between concentrations in the unfiltered (total concentration) water and the filtered (dissolved concentration) water. 

Graphical results for dissolved and total concentrations 

of iron, manganese, copper, and zinc are presented in figure 

32 for June 1986 and in figure 33 for October 1986. The 

results demonstrate the addition of iron, manganese, copper, 

and zinc to the stream system by the acidic metal-rich water 

of the Argo Tunnel. Elevated concentrations of these metals 

are transported by Clear Creek to its confluence with the 

South Platte River, especially during high flow. The graphs 

also show the partitioning of these metals between the 

43 
44 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 28. View of Torreys Peak looking toward the west. Peaks of the Continental Divide are visible behind it on the horizon. Clear 

Creek is out of view to the right. (U.S. Geological Survey photograph by W. H. Jackson, I 873.) 

Lo:J 

Figure 29. Location of sample collection sites along Clear 

Creek. Samples were taken from near the Continental Divide to 

within 3 miles of its confluence with the South Platte River. 
INFLUENCE OF MINE DRAINAGE ON CLEAR CREEK, COLORADO 45 

Figure 30. U.S. Geological Survey research chemist Walter H. 

Ficklin measuring pH with a portable meter. Walt died suddenly on 

October 11, 1993, after 25 years with the U.S. Geological Survey 

(photograph by Larry P. Gough). 

Figure 31. Typical filter apparatus for water samples. Raw water sample from jug passes 

through filter held in tripod-like structure and is collected in bottles (photograph by Geoffrey 

S. Plumlee). 
46 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

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ARGO TUNNEL 

INFLOW 

TOR REVS 

PEAK 

SOUTH 

PLATTE RIVER 

50.---.---.-.--.---.-r--.---ro-r---, 

--------- UNFILTERED 

D----------<J FILTERED (0.45 Jlm) 

TORREYS 

PEAK 

--------- UNFILTERED 

D----------<l FILTERED (0.45 Jlm) 

TORREYS 

PEAK 

TORREYS 

PEAK 

5 10 20 

MILES 

COPPER 

SOUTH 

PLATTE RIVER 

ZINC 

SOUTH 

PLATTE RIVER 

SOUTH 

PLATTE RIVER 

Figure 32. Influence of various inflows on the dissolved 

(passed through 0.45 Jlm filter) and total (unfiltered) concentrations of iron, manganese, copper, and zinc in water collected from 

Clear Creek in June 1986. Dissolved concentrations of iron were 

below the determination limit of 0.1 mg!L and are not shown in 

the graph. Jlm=micrometer; mg/L=milligrams per liter (roughly 

equivalent to parts per million). 

--------- UNFILTERED 

TOR REVS 

PEAK 

SOUTH 

PLATTE RIVER 

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5 10 20 

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30 

SOUTH 

PLATTE RIVER 

40 50 

GOLDEN 

SOUTH 

PLATTE RIVER 

SOUTH 

PLATTE RIVER 

Figure 33. Influence of various inflows on the dissolved 

(passed through 0.45 Jlm filter) and total (unfiltered) concentrations of iron, manganese, copper, and zinc in water collected 

from Clear Creek in October 1986. Dissolved concentrations of 

iron were below the determination limit of 0.1 mg/L and are not 

shown in the graph. Sec figure 32 and table 2 for definitions of 

various units. 
INFLUENCE OF MINE DRAINAGE ON CLEAR CREEK, COLORADO 47 

Table 2. Dissolved constituents in Argo Tunnel water, August 1985, and Clear Creek, 

October 1985. 

Constituent, 

concentration 

Sulfate, 11g/L' 

Calcium, 11g/L 

Magnesium, 11g/L 

Iron, 11g/L 

Manganese, 11g/L 

Sodium, 11g/L 

Zinc, 11g/L 

Copper, 11g/L 

Potassium, 11g/L 

Molybdenum, )lg/L 

Arsenic, )lg/L 

pH, std. units 

Argo Tunnel 

water 

2,000,000 

400,000 

110,000 

37,000 

31,000 

19,000 

9,600 

5,400 

4,000 

170 

120 

3.2 

Clear Creek at Argo Tunnel 

Upstream Downstream 

29,000 44,000 

14,000 15,000 

3,000 3,300 

<100 <100 

640 1,100 

8,300 8,700 

190 430 

18 14 

1,700 1,700 

no data no data 

<1 <1 

7.5 7.6 

'Micrograms per liter (equivalent to parts per billion). 

dissolved and suspended phases. In each case, part of the 

total concentration is accounted for by suspended materialthe difference between the unfiltered and filtered lines for a 

given element. Iron is present primarily in the suspended 

phase, and manganese, copper, and zinc are present in both 

dissolved and suspended phases. Note that much more manganese, copper, and zinc are present in the suspended phase 

downstream from the Argo Tunnel inflow. This indicates 

that reactions at the Argo Tunnel inflow cause metals to be 

partitioned to the suspended sediment phase. 

FACTORS INFLUENCING METAL 

TRANSPORT IN CLEAR CREEK 

Dissolved metal concentrations decrease in Clear Creek 

downstream from the Argo Tunnel inflow due to oxidation, 

precipitation, and adsorption reactions and due to dilution 

effects. (See "Generation and Interactions of Mine Drainage 

at the Argo Tunnel" by Smith, p. 39.) Many metals become 

adsorbed onto hydrous oxides of iron, aluminum, and manganese and are then transported with the oxides. The pH 

range for adsorption is different for each metal and each type 

of oxide. Adsorption of positively charged metal cations 

(such as copper and zinc) generally increases as the pH of the 

stream water increases. Figure 34 demonstrates that the proportion of copper and zinc associated with the suspended 

phase increases with increasing stream water pH in Clear 

Creek in June 1986 downstream from the Argo Tunnel. This 

indicates that both copper and zinc may be adsorbed onto 

hydrous oxides that precipitate when drainage from the Argo 

Tunnel enters Clear Creek. However, this pH-dependent partitioning to the suspended phase is not apparent in data from 

October 1986. 

Argo Tunnel water also contributes dissolved arsenic to 

Clear Creek; but below the tunnel inflow, dissolved arsenic 

cannot be detected (table 2). Figure 35 shows that just downstream from the Argo Tunnel at least some of the arsenic 

becomes loosely bound (soluble in dilute hydrochloric acid) 

to streambed sediment. A similar increase in sediment-borne 

arsenic is seen at the confluence of the North Fork of Clear 

Creek with Clear Creek. Hydrous iron oxides associated 

with suspended material may transport arsenic as well. 

CONCLUSIONS 

Argo Tunnel water affects the quality of Clear Creek 

water from its inflow at Idaho Springs until its further dilution when it mixes with South Platte River water-a distance 
48 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

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ARGO TUNNEL TO SOUTH PLATIE RIVER 

JUNE, 1986 t:. 

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STREAM pH 

ARGO TUNNEL TO SOUTH PLATIE RIVER 

OCTOBER. 1986 

I~ Copper 

s e&, 

Zinc 

0 

0 6. 

0 0 0 

6. ~ e 6. 6. t:.lf 0 A A A 6. 

A A 

7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 

STREAM pH 

Figure 34. Proportion of suspended load of copper and zinc 

versus stream water pH downstream from the Argo Tunnel drainage into Clear Creek in June and October 1986. 

of nearly 35 miles. Dissolved and suspended-sediment concentrations of iron, manganese, copper, and zinc increase 

due to the inflow of the Argo Tunnel. In Clear Creek, iron 

from the Argo Tunnel is transported primarily in the suspended phase; manganese, copper, and zinc are transported 

in both dissolved and suspended phases. Loosely bound 

arsenic appears to be associated with the streambed sediment along Clear Creek, showing a dramatic increase in 

concentration just downstream from confluences with the 

30,-----------------------, 

Arsenic in Clear Creek sediment 

10 -----------------

30 20 10 

MILES FROM THE SOUTH PLATTE RIVER 

Figure 35. Distribution of loosely bound arsenic in Clear Creek 

sediment from near the Continental Divide to its confluence with 

the South Platte River. 

Argo Tunnel drainage and with the North Fork of Clear 

Creek. 

REFERENCES 

Benjamin, M.M., and Leckie, J.O., 1981, Competitive adsorption 

of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide: 

Journal of Colloid and Interface Science, v. 83, p. 410-419. 

Ferguson, J.F., and Gavis, Jerome, 1972, A review of the arsenic 

cycle in natural waters: Water Research, v. 6, p. 1259-1274. 

Kimball, B.A., and Wetherbee, G.A., 1988, Instream chemical 

reactions of acid mine water entering a neutral stream near 

Leadville, Colorado, in G.E. Mallard and S.E. Ragone, eds., 

Proceedings of the Technical Meeting, Phoenix, Arizona, 

September 26-30, 1988, U.S. Geological Survey Toxic Substances Hydrology Program: U.S. Geological Survey Water 

Resources Investigations Report 88-4420, p. 567. 

Smith, K.S., 1994, Generation and interactions of mine drainage at 

the Argo Tunnel: U.S. Geological Survey Circular 1097, 

p. 39--41. 

Stumm, Werner, and Morgan, J.J., 1981, Aquatic Chemistry, 2nd 

Edition: New York. John Wiley and Sons, Inc., 780 p. 

Wentz, D.A., 1974, Effect of mine drainage on the quality of 

streams in Colorado 1972-73: Colorado Water Resources 

Circular No. 21, Colorado Water Conservation Board, Denver, 

Colorado, 117 p. 
CONSTRUCTED WETLAND TREATMENT OF ACID MINE 

DISCHARGE AT IDAHO SPRINGS, COLORADO 

By John C. Emerick, Thomas R. Wildeman, Ronald R. Cohen, and Ronald W. Klusman 

ABSTRACT 

Acid mine drainage affects the water quality and 

aquatic resources of many streams and rivers in the mineral 

belt of the Southern Rocky Mountains. In an effort to 

develop low-cost effective methods to treat mine discharge 

using constructed wetlands, a pilot-scale treatment system 

has been maintained at the Big Five Tunnel in Idaho Springs, 

Colorado, since October 1987. This experimental system 

has been used to assess the importance of substrate type, 

flow characteristics, vegetation, and other factors on treatment effectiveness. During the life of the pilot system, modifications and additions have been made in an effort to refine 

knowledge of treatment processes and to develop design criteria for large-scale systems. This system has been effective 

for removal of most metals studied, including iron, zinc, and 

copper. In general, results indicate that treatment systems 

incorporating a mechanism to force vertical flow through the 

substrate are more effective than systems relying on lateral 

flow. Low flow rates provide more effective metal removal 

than high flow rates. 

INTRODUCTION 

Waters with low pH and high metal concentrations 

associated with drainage from mineralized zones have one of 

the most serious impacts on surface water in the Rocky 

Mountain region. These waters form as a result of geochemical processes involving the oxidation of metal sulfides 

found in mineralized rock. Reactions are initiated by exposure of the rock to moisture and oxygen. This occurs naturally in places where the host rock is fractured and results in 

so-called "iron seeps" or "iron bogs," whose presence has 

been used in geochemical exploration for minerals. Manmade situations cause greater concern. Where sulfide-bearing rock has been exposed in underground mine workings, or 

at the surface in mine dumps or mill tailings, the geochemical sulfide weathering processes have been accelerated. 

These conditions often promote the formation of acid drainage and acid leachate from surface waste piles. (See "Influence of Mine Drainage on Clear Creek, Colorado" by Ficklin 

and Smith, 1993, p. 43). In both cases, the major environmental concerns are to maintain the quality of the receiving 

waters for fisheries habitat, and to avoid potential threat to 

human populations. 

State and federal agencies are particularly concerned 

with inactive mines because of the lack of adequate treatment, poor records of ownership, and the large numbers of 

these sites in western mineral belts. In Colorado, several 

mining districts have been placed on the National Priority 

List for action under the Comprehensive Environmental 

Response, Compensation, and Liability Act of 1980 (known 

as CERCLA or Superfund), including the Idarado Mine near 

Telluride, the Eagle Mine near Minturn, the California Gulch 

district near Leadville, and the Central City-Idaho Springs 

district. 

These problems have highlighted the need for an effective and relatively inexpensive technology to treat acid mine 

drainage and acid leachate from surface waste piles. 

Although various methods of conventional treatment exist, 

such technologies have either a high equipment cost, a high 

maintenance cost, or both. For several years we have been 

examining the potential of using natural biogeochemical 

processes in a constructed wetland scheme to treat acid mine 

drainage. Coal mining regions of the Eastern United States 

have used this technology with some success, but its effectiveness for remediation of metal mine discharges in mountain climates has not been established with certainty. 

In 1987, as part of the remedial investigation connected 

with the Central City-Idaho Springs Superfund Project, 

scientists constructed an experimental pilot-scale wetland to 

evaluate its effectiveness and to determine the most important biogeochemical processes involved in metal removal 

and remediation of acid conditions. The Environmental 

Protection Agency (EPA) initially funded the work through 

a subcontract with Camp, Dresser, and McKee, and later 

through the EPA's Emerging Technology Program. The 

experimental wetland was constructed at the portal of the 

Big Five Tunnel, at the western edge of Idaho Springs, 

Colorado. 

This report summarizes results of some studies at the 

wetland and presents perspectives on the design of operational systems. It is not intended to provide a detailed 

description of methods, or even a synopsis of all the studies 

that have been conducted at the site; much of that information is available in a book by Wildeman and others (1992). 

Detailed discussions on the role of vascular plants and bacteria can be found in a book by Hammer (1989). Herlihy and 

49 
50 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Mills (1985) and Wakao and others (1979) address the role 

of bacteria in treatment of mine drainage. Also of special 

interest are reports by Kerndorf and Schnitzer (1980), who 

discuss metal sorption to humic acids, and by Benjamin and 

Leckie (1981), who discuss metal sorption to hydrous iron 

oxides. 

SITE CONDITIONS AND 

CONSTRUCTION OF THE 

ORIGINAL TREATMENT SYSTEM 

Excavation of the Big Five Tunnel began in December 

1900 as a drainage tunnel for several gold and silver mines 

in the Idaho Springs district. The site is located in the valley 

bottom at an elevation of approximately 2,280 m (7,600 ft) 

and less than I 00 m from Clear Creek. Because of its location, the climate is cool, and cold air drainage from the surrounding slopes can produce freezing conditions even during 

the summer. Water flow from the tunnel is relatively constant throughout the year, averaging 1.34 L/sec33. The 

chemical characteristics also are relatively uniform over 

time. Table 3 shows mean concentrations for the more 

important metals in the drainage. Mean temperature is 

approximately 13°C, and pH is usually about 2.6. The major 

anion is sulfate, averaging 2,100 mg/L. 

Construction of the treatment system was completed in 

the fall of 1987 (fig. 36). In its original configuration, the 

system was 3.05 m (10ft) wide by 18.3 m (60ft) long, with 

a foundation constructed of concrete and wood to allow a 

total depth of 1.22 m (4 ft). The system was divided into 

three equally sized cells, A, B, and C, each 6.1 m (20ft) long. 

Each cell was lined with Hypalon, an acid-resistant liner 

fabric, to prevent leakage. 

Several media were tried in the cells. Cell A was filled 

with mushroom compost consisting of approximately 50 

percent animal manure and 50 percent barley mash wastes 

from a local brewery. Cells B and C were filled with a substrate consisting of equal parts of peat, aged steer manure, 

and decomposed wood shavings and sawdust. In addition to 

the above substrate, the bottom of cell C was filled to a depth 

of 12 em (5 in) with limestone rock prior to laying the 

organic substrate. Following installation of the organic substrate, several species of aquatic plants were transplanted 

into each cell (fig. 37)-primarily cattail (Typha /atifolia), 

sedges (Carex aquatilis and C. utricu/ata), and arctic rush 

(Juncus arcticus). These plants were obtained from nearby 

wetland areas and from Shadow Mountain Reservoir near 

Grand Lake, Colorado. 

Flow into and through the cells was regulated by a system of polyvinyl chloride (PVC) pipe and valves. The pipes 

33Liters per second, a liter of liquid is slightly more than a quart. 

Table 3. Average concentrations and daily output (loading) 

for selected metals in the Big Five Tunnel effluent. 

Element Concentration 

(~g/Ll) 

Iron -----------------51,000 

Manganese ---------- 29,000 

Aluminum -------------14,000 

Zinc ----------------------8,200 

Copper----------~--------1 ,400 

Nickel----------------------240 

Lead------------------------ 40 

Cadmium------------------- 27 

Chromium------------------ 14 

Arsenic --------------------8 

Silver------------------------6 

Daily output 

(kg/day2) 

5.6 

3.1 

1.5 

0.91 

0.14 

0.03 

0.01 

<0.01 

<0.01 

<0.01 

<0.01 

1Micrograms per liter (equivalent to parts per billion). 

2Kilograms per day, a kilogram is about 2.2 pounds. 

carried mine drainage from the portal to a 30-cm (12-in) 

wide rock basket at the head of each cell. The basket 

extended across the entire width of the cell and was held in 

place with plastic fencing. This configuration allowed water 

to be distributed over the entire surface of the substrate and 

to flow laterally through the substrate toward the drain at the 

downstream end of each cell. Saturation of each cell was 

controlled by overflow into a standpipe attached to the drain. 

Water eventually percolated to the top of the substrate, 

where it was drawn off by the standpipe. A PVC drain pipe 

was installed at the downstream end of each cell. The initial 

flow rate of 3.8 L/min (1 gal/min) was selected according to 

guidelines published by the U.S. Bureau of Mines from 

similar investigations on treatment systems in eastern coal 

mining areas. 

During most of the project, monthly samples of water, 

substrates, and plants were collected for analysis. Substrates 

and plants were analyzed for major cations and trace elements; water samples were analyzed for principal anions, 

cations, trace elements, and nutrients (phosphate and 

nitrate). 
CONSTRUCfED WETLAND TREATMENT OF ACID MINE DISCHARGE AT IDAHO SPRINGS, COLORADO 51 

~' OUTLETS TOP VIEW 

I - I 

c B A 

PORTAL 

- ' 

, .. ---- , INLETS ,--- ' INLET ~ 

, , , 

., , 

, , , 

, , , , , 

N 

SAMPLING 

WELLS 

.. .. .. .. ' ... ' ... ' INLET ... 

--~: ·~~- ~;~\_..,.,.... 

Figure 36. Plan view and cross section of the Big Five Tunnel constructed wetland looking west 

Several conclusions resulted from the first year of operation. First, we found an unacceptable degree of "shortcircuiting" and channelization taking place. This allowed the 

mine drainage to rise to the surface close to the rock box and 

flow over the top of the cell to the drain. Secondly, flow rates 

appeared to be too high for the cells. We therefore felt that 

the drainage had insufficient contact with the substrate and 

that more metal could be removed by the system. 

DESIGN MODIFICATIONS 

During 1988 and 1989, several changes were made. 

Cell A was rebuilt with a system of longitudinal walls to 

increase the flow path and with transverse redistribution 

baffles that were designed to collect water from the top 

surface and return it to the bottom (fig. 38). 

Cell B was divided into two 3.05-m by 3.05-m (10-ft by 

10-ft) cells that were redesigned to promote vertical flow 

(fig. 39). Each half of cell B was fitted with a perforated PVC 

distribution system at the top and at the bottom of each cell 

so that mine drainage could either be admitted to the top of 

the cell and drawn off the bottom (downflow system), or be 

admitted to the bottom of the cell and drawn off the top 

(upflow system). A plastic stock tank was added to the inlet 

circuit of each subcell to allow additional precipitation of 

ferric oxyhydroxides from the mine drainage before entry 

into the cell, thereby minimizing clogging of the system. 

Changes were also made in the substrates. The original peatbased substrate was replaced by mushroom compost identical to that used in cell A, and no vegetation was transplanted 

into cell B after it was rebuilt. 

At the same time of cell B reconstruction, two new 

cells were built (fig. 37). Cell D was designed to further 

treat the effluent of cell A by surface flow. Its dimensions 

are approximately 1.22 m (4ft) wide by 12.2 m (40ft) long 

by 45 em (1.5 ft) deep. Cell E was designed as a circular 

downflow cell, approximately 3.05 m (10ft) in diameter by 

0.61 m (2ft) deep. It was constructed by excavating a small 

impoundment, lining it with Hypalon, installing a PVC 

drain at the bottom under a layer of gravel, and placing 

filter fabric over the gravel. Both cells D and cell E were 

filled with the peat-based substrate and were planted with 

vegetation removed from cell B. Figure 40 shows the 

approximate location of all cells at the Big Five Tunnel site. 

SUMMARY OF RESULTS 

Although several interrelated studies on the hydrology, 

microbiology, geochemistry, and vegetation were conducted, only a general summary of cell behavior will be 
52 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

Figure 37. View toward the west of cattails growing in cells A, B, and C of the constructed wetland. Individual cells are not discernible 

in this photograph. However, the acid-resistant liner fabric, Hypalon, can be seen. Colorado School of Mines photograph (printed with 

permission). 

Figure 38. Cutaway view of the modified design of cell A. 

discussed here. Table 4 presents data from the 11-month 

period following the last of the major modifications and 

shows the percentage removal of manganese, iron, copper, 

and zinc from the drainage by each cell. 

Cell A, in spite of its reconstruction, continued to show 

poor removal of manganese and iron. This was due in part 

to the design of the modifications. Some leakage occurred 

NUM 

Figure 39. Cutaway view of the modified design of a cell B subcell. 
TOP 

VIEW 

CONSTRUCTED WETLAND TREATMENT OF ACID MINE DISCHARGE AT IDAHO SPRINGS, COLORADO 

Table 4. Treatment effectiveness of all cells at the Big Five Tunnel constructed wetland site for 

selected metals. Data are means of monthly observations and analyses of samples collected during 

the period from October 1989 through August 1990. Values are shown as the percentage of total 

metal removed from the mine drainage. 

Cell Nl Mn Fe Zn Cu Effluent Flow 

Percentage Removal pH (L/min4

) 

A (Lateral flow) 11 0.2 36 61 83 4.7 1.0 

D (Surface flow)2 7 13 73 99 100 6.3 0.84 

B (Upflow hal03 

First 6 months 10 18 54 52 65 5.1 0.63 

Last 6 months 5 34 91 97 100 6.8 0.34 

B (Downflow halO 11 22 76 40 83 5.4 0.50 

C (Lateral flow) 9 2 50 35 65 4.4 1.0 

E (Downflow) 11 10 86 100 97 6.2 0.51 

1

N is the number of monthly observations during the period. Although the total period 

of observation was 11 months, the number of monthly observations was fewer for cells 

C and D (due to winter freezing problems) and for the upflow half of cell B (flow 

problems). 

2

Cell D was receiving the effluent from cell A. 

3

Problems with vertical channelization and flow were experienced during the first 6 

months of operation. The second row of values reflects the operation of the cell during 

the last 5 months after the problems were rectified. 

4

Liters per minute. 

53 

around the walls, and clogging of screened ports in the 

redistribution baffles limited flow in some sections of the 

system. 

PORTAL_____. 

The modifications to cell B which incorporated a 

vertical flow design produced promising results. It 

removed more manganese in general than the other cells, 

as well as a higher proportion of the other elements. The 

downflow subcell initially performed better. When flow 

problems were solved in the up flow subcell, however, this 

cell seemed to produce the best overall performance. 

Figure 40. Location of the treatment cells of the Big Five Tunnel 

system as of spring 1991. 

Cell C is the only cell that remained unmodified 

since the treatment system was constructed. Although 

cell C did not perform as well as the others, it was in operation longer than the rest, and therefore the most data has 

been accumulated for this cell. Figure 41 (top) shows the 

percentage removal of manganese, iron, and zinc by cell 

C from November 1987 through August 1990. The 
54 GUIDEBOOK: GEOLOGY AND SURFACE-WATER CONTAMINATION, DENVER-IDAHO SPRINGS, COLO. 

...J 

<t 6 60 

~ 

w 

a: 

...J 

~ w 

~ 

1-

z 

w 

u 

a: 

w 

a.. 

3: 

0 

ii 1 

. ' I I .. ""\ 

I I ,' I t 

~ .. / Flow!'~ ,' ', " /~ \ ,, ',, .. ' '\,',-.. ~ ._, ,, 

'' ,' '-- .. -.. OL-~~-L~~L-L-~-L~~--L-L-~~·-L~--' 

11 1 3 5 7 9 11 1 9 11 1 3 5 7 

~ ~ ~ 

MONTH/YEAR 

Figure 41. Graph showing treatment effectiveness of cell C. Metal 

values are shown as percentage of removal compared to concentrations in the mine drainage. 

bottom part of figure 41 shows the variation in pH and effluent flow rate for the same time period. The flow rate for the 

first 7 months is probably not as constant as shown because 

it was monitored and adjusted only every few weeks, and the 

inlet valve was frequently clogged. Beginning in July 1988, 

water samples and flow rate measurements were taken 

simultaneously prior to cleaning of the inlet valve. 

Figure 41 shows a relatively clear inverse relationship 

between flow rate and pH. Lower flow rates are consistent 

with higher pH, showing removal of acid. Lower flow rates 

also correlate with higher percent removal of iron and zinc. 

Although not shown in this figure, the pattern of copper 

removal is similar to that of zinc. These relationships have 

been observed in other cells as well. 

Manganese behaves differently from the other elements, principally because it precipitates more effectively in 

an oxidizing environment, rather than in the reducing 

(oxygen-poor) environment of this treatment system. Manganese will not precipitate in a reducing environment unless 

the pH in the wetland increases significantly. 

Ceil D, which received the output of cell A, showed the 

greatest efficiency for removal of most metals. However, 

cell D often froze during the winter because it is a relatively 

shallow surface flow cell. 

Cell E originally was constructed to utilize substrate 

and plants left over from the reconstruction of cells B and D. 

It has been rather effective throughout most of its operation 

at removing metals and raising pH of the mine drainage. 

This is significant because it was less expensive to construct 

and it may represent a more realistic prototype of future 

small treatment systems. 

CONCLUSIONS 

The studies ofthe past 3 years of the pilot treatment system have provided a data base to support several conclusions, although ongoing work at Colorado School of Mines 

and elsewhere may produce results that will modify some 

statements below. 

The two principal methods by which the wetlands treat 

acid mine drainage are (1) by metal and hydrogen ion (H+or 

proton) adsorption, and (2) by transformation of metals into 

insoluble compounds. Adsorption of metals onto an organic 

substrate increases the residence time as a transitional phase 

but is not important for long-term retention. Adsorption 

will, however, provide a readily available source of metals 

for bacterial reactions. Adsorption of metals and protons 

will also help buffer the wetland to sudden pH changes 

(mostly acidic). 

The major metal removal in the wetland takes place by 

the microbially mediated process called dissimilatory sulfate 

reduction. Certain bacteria use sulfate (S04) present in the 

mine drainage instead of oxygen (02) as a terminal electron 

receptor for cellular reactions. These reactions produce 

hydrogen sulfide, (H2S), which precipitates metals as metal 

sulfides thereby removing them from solution. The reactions proceed as long as a reducing environment is maintained and other anaerobic bacteria are present that use 

organic carbon as a food source. The decomposition of 

organic matter by anaerobic bacteria releases ammonia 

(NH3), bicarbonate ions (HC03 -), and methane (CH4) all of 

which raise the pH sufficiently for the sulfate reduction to 

occur. 

Metals, such as copper and zinc, can be removed effectively by these processes, iron somewhat less effectively, 

and pH can be increased on a long-term basis. Iron removal 

is a key aspect in the functioning of the system due to the role 

of iron in forming acidity (fig. 27). In addition, iron plays a 

key role by its absorption of other metals on its oxides. 

Plants function mainly to stabilize the substrate 

surface, reduce channelization, and provide a source of 

organic matter for the microbes. Many plant species would 

probably work well in constructed wetlands, as long as they 

can tolerate saturated soils, low pH, and high metal concentrations. Ideally, the plants should have high biomass production to provide a suitable source of carbohydrates for 
CONSTRUCIED WETLAND TREATMENT OF ACID MINE DISCHARGE AT IDAHO SPRINGS, COLORADO 55 

microbial populations. Although plants absorb metals 

through their roots, they are relatively unimportant for 

direct long-term removal of metals. Most of the metals that 

are absorbed by the plant tissues return to the soil when the 

plants die and decompose. Of the species studied at the Big 

Five wetland, cattails worked best. Plants may pump oxygen into the upper part of the wetland, however, thereby 

producing oxidizing conditions and decreasing the amount 

of sulfate reduction. 

Substrates that buffer to pH 5.5 or higher enhance 

sulfate reduction and allow a new wetland to function effectively from the beginning. The minimum desirable limits of 

hydraulic conductivity (rate of water movement) for substrates is 10-5 crnlsec.34 The use of peat is, therefore, not 

advisable because of its tendency to buffer the aqueous 

phase to low pH's (<5.5) and its low hydraulic conductivity 

(<10-5 em/sec). 

Flow rates were the major limiting factor in our wetland 

treatment. Lower flow rates were better for metal removal 

and reduction of acidic "shock" by Big Five Tunnel drainage 

water to the micro-organisms responsible for sulfate reduction. A vertical saturated-flow design was also more effective than one relying on lateral flow. Circulation through the 

substrate was poor in the lateral systems because water 

tended to flow on the surface or cause channelization in the 

substrate thereby limiting contact of the mine drainage water 

with the substrate. Both problems were minimized in vertical 

flow systems. 

Removal efficiency depends strongly on the loading 

factor-a function of flow rate and surface area available 

for reactions to take place. More than 24 m2/L/min 

(1000 ft2/gal/min) was needed for reasonable removal at 

the Big Five system, assuming a substrate depth of 0.61-

1 m (2-3 ft). Mine drainage with different metal and 

acid concentrations would require a different volume of 

substrate. This can be determined by bench-scale tests 

(laboratory simulation of field tests). 

Plumbing should be kept simple to avoid clogging by 

hydrous ferric oxides; some parts of the treatment cell may 

need insulation to prevent freezing. Sharp bends and vertical rises are particularly vulnerable to accumulations of iron 

34Centimeters per second, there are about 2.5 centimeters per 

inch. 

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication September 28, 1993 

Graphics by Michael F. Duncan, Margo L. Johnson, Shawn K. 

Smith, Richard P. Walker, Norma J. Maes, Arthur L. Isom, and 

Carol Quesenberry 

Edited by Thomas Kohnen 

Photocomposition by Marie Melone 

*'u.s. ~~OFFICE: 1994-573-038/86034 

precipitates. To minimize freezing, the plumbing of the 

entire system should be placed in areas that receive winter 

sun. 

ACKNOWLEDGMENTS 

Many people have contributed to the studies at the Big 

Five Project. These include Dr. David Updegraff, Dr. E.A. 

Howard, Robin Madel, Margaret Staub, Wafa Batal, Judy 

Bolis, Larry Chang, John Deitz, Pat Keller, Leslie Laudon, 

Peter Lemke, Steve Machemer, Mahani Mamat, Leslie 

Moe, Dee Mohamed, Nita Mohdnoordin, Tom Oliver, 

Scott Plummer, Julie Reynolds, and Chris Sellstone, of the 

Colorado School of Mines; Dr. John Gormley and James 

Gusek of Denver Knight Piesold; Dr. Rick Chappell, Susan 

Morea, and Dr. Roger Olsen of Camp, Dresser, and 

McKee; Dr. Edward R. Bates, Holly Fliniau, James 

Kreissl, Dr. James Lazorchek, and Mark Smith of the U.S. 

Environmental Protection Agency; and Rick Brown, Jeff 

Deckler, and Jeb Love of the Colorado Department of 

Health. The involvement and cooperation of all these individuals is appreciated and attests to the multidisciplinary 

nature of this project. 

REFERENCES 

Benjamin, M.M., and Leckie, J.O., 1981, Competitive adsorption 

of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide: Jour· 

nal of Colloid and Interface Science, v. 83, p. 410-419. 

Hammer, Donald, A., ed., 1989, Constructed wetlands for wastewater treatment-municipal, industrial, and agricultural: Boca 

Raton, Florida, Lewis Publishers, Inc., 831 p. 

Herlihy, Alan T., and Mills, Aaron L., 1985, Sulfate reduction in 

freshwater sediments receiving acid mine drainage: Applied 

and Environmental Microbiology, v. 49, p. 179-186. 

Kemdorf, H. and Schnitzer, M., 1980, Sorption of metals onto 

humic acid: Geochimica et Cosmochimica Acta, v. 44, 

p. 1701-1708. 

Wakao, Norio, Takahashi, T., Sakuria, Y., and Shiota, H., 1979, A 

treatment of acid mine water using sulfate-reducing bacteria: 

Journal of Fermentation Technology, v. 57, p. 455-452. 

Wildeman. T.R., Brodie, G.A., and Gusek, J.J., 1992, Wetland 

Design for Mining Operations: Vancouver, B.C., Bitek Publishers, 400 p.