1. Geology, Mining, and Water Quality
by Matthew A. Sares
One of the major aspects of our work in the Environmental Geology Section at CGS is understanding how geology affects water quality.
This presentation looks at an issue that is very important in Colorado -- geology, mining, and water quality.
In 1998 CGS completed an inventory of abandoned mines on US Forest Service lands throughout Colorado. As we catalogued environmental degradation related to these mines, it was abundantly clear that the geology of the mine ore deposits and gangue minerals were important in the water quality encountered at the site.
It also became evident that the surrounding geology is important in the water quality of the local watershed.

2. Colorado has abundant supplies of clean water, much of it in the form of snowmelt runoff from the Rocky Mountains.
Water takes on the character of the rock it passes over and through, as it flows into streams and ground-water aquifers.
The composition, or chemistry, of the water will reflect the geology of the area with which it has been in contact.
This generally does not change the water quality significantly enough to be a concern to humans, wildlife, aquatic organisms, or vegetation.
In some areas it does.
Streams in high, headwater areas can greatly reflect the local geology. If local geology is deleterious, stream water quality can be poor. Why? Stream flows are relatively small within individual tributaries; the chemistry of the “young” water has only been in contact with few types of rock.
The blending of water from different types of geology, which can balance water composition, doesn’t happen until further downstream.

3. Not surprisingly, our headwater areas are coincident with the Colorado Mineral Belt.
The density of mining along the mineral belt and other ore bearing areas has contributed to water quality degradation in some streams.
Primarily, the contaminants are acidity and metals, but may include other problems such as sedimentation.

4. Water - from rain and snowmelt
Acid Mine Drainage
Water - from rain and snowmelt +
Oxygen - from the air
Pyrite - from the mine
Reaction =
Sulfuric Acid
Acid Mine Drainage (AMD) is the term given to the primary means of water quality degradation at mine sites.
Essentially, water from rain and snowmelt and oxygen from the air interact with pyrite (or other sulfide minerals to form sulfuric acid.
Pyrite and other sulfide minerals are present in ore and associated minerals in many mining districts in Colorado.
Where does acid mine drainage come from?

5. Sources of Acid Mine Drainage (AMD)
Mine Effluent
Mine Effluent or drainage from the mine workings affects surface and ground water: They serve as collection areas for ground water, increasing residence time in rocks/veins with high metal concentrations. This results in elevated in metal concentrations in the water. Air and water moving through the workings can oxidize the sulfide minerals producing acid.
A mine adit (or tunnel) can act like a horizontal well or french drain drawing water into its workings as the water finds the least path of resistance through the mountain and downhill.
Burbank Mine northwest of Silverton.
Burbank Mine, San Juan Co.

6. Mine Dump Mill Tailings Lewis Mine and Mill, San Miguel Co.
This is the Lewis Mine and Mill near Telluride in San Miguel County.
This photo illustrates additional sources of acid mine drainage.
Mine dump material, although not consisting of “high-grade” ore, does contain high concentrations of gangue minerals, minerals that accompany the ore (commonly pyrite, chalcopyrite, bornite). The broken rock increases the surface area exposed to weathering.
Mine Tailings. Even though much of the valuable metal is removed during the milling process, mill tailings expose an even larger surface area to weathering. Significant amounts of certain metals can be left in the tails depending on the age and type of the mill.
Now its time to look more in depth at the chemistry of AMD.
Mill Tailings
Lewis Mine and Mill, San Miguel Co.

7. THE CHEMISTRY of ACID MINE DRAINAGE
Reaction 1: FeS2(s) + H2O + 7/2O2
Fe2+ + 2SO4 + 2H+
pyrite
water
sulfate
acid
Reaction 2:* Fe2+ + 1/4O2 + H+
Fe3+ + 1/2H2O
Reaction 3: FeS2(s)+ 8H2O + 14Fe3+
15Fe2+ + 2SO4 + 16H+
Primary Reactions
Reaction 1 >> In the presence of water and oxygen, pyrite is oxidized to ferrous iron and sulfate and releases 2 moles of acidity.
Reaction 2 >> Ferrous iron is oxidized to ferric iron,consuming 1 mole of acidity.
Secondary Reactions
Reaction 3 >> This step is catalyzed by the bacteria ______ ferroxidans. Pyrite is again oxidized, this time by the ferric iron ion. This produces 16 moles of acidity.
Reaction 4 >> Lastly, ferric iron dissociates water creating more acidity and precipitating the solid iron hydroxide. This is the reaction that results in “yellow boy” the orange precipitate seen in many mine drainages.
Reaction 4: Fe3+ + 3H2O
Fe(OH)3(s) + 3H+
* catalyzed by bacteria

8. Background Water Quality
There is another factor that needs to be considered in this photo and in water quality investigations related to mining.
What is the background water quality of the stream, before it reaches the mine?
Fortunately, at this mine site the background water quality is good with a near neutral or slightly alkaline pH. But that is not always the case!
Lewis Mine and Mill, San Miguel Co.

9. THE CHEMISTRY of ACID MINE ROCK DRAINAGE
Reaction 1: FeS2(s) + H2O + 7/2O2
Fe2+ + 2SO4 + 2H+
pyrite
water
sulfate
acid
Reaction 2:* Fe2+ + 1/4O2 + H+
Fe3+ + 1/2H2O
Reaction 3: FeS2(s)+ 8H2O + 14Fe3+
15Fe2+ + 2SO4 + 16H+
The chemistry of acid mine drainage can happen anywhere sulfide minerals are present, especially where no alkaline minerals are present to counteract the production of acid.
The broader term is Acid Rock Drainage and it is active in many areas of Colorado.
Reaction 4: Fe3+ + 3H2O
Fe(OH)3(s) + 3H+
* catalyzed by bacteria

10. Acid Rock Drainage (ARD) sources:
Abandoned mines
Natural springs and drainages in
hydrothermally altered areas
The primary sources of acid rock drainage in Colorado are:
1) abandoned mines and natural springs
2) drainages in hydrothermally altered areas.
Frequently these two sources occur in the same areas.
What is hydrothermal alteration??

11. East Mancos River headwaters,
Hydrothermal alteration is a process whereby hot water circulating within the earth changes the composition of rocks.
Intensely altered rocks have yellow, orange, and red colors from the oxidation of iron in pyrite.
Hydrothermal alteration is a process whereby hot water circulating within the earth changes the composition of rocks.
Intensely altered rocks have yellow, orange, and red colors from the oxidation of iron in pyrite and the presence of clay minerals (feldspar decomposition).
This photo is from the East Mancos headwaters, just west of the La Plata Mountains of Southwest Colorado.
The Allard igneous stock occurs along the drainage divide of the La Platas in this area. The stock carries a sub-economic copper deposit along with other metals.
The headwaters of this stream, above any mining, generally has an acidic pH= and is high in aluminum (Al), Iron (Fe), Copper (Cu), and Zinc (Zn).
East Mancos River headwaters,
Montezuma Co.

12. Hydrothermal Alteration
Feldspars >> Clays
Iron and Sulfur >> Pyrite
Hydrothermal alteration can take place in different settings, and styles. This diagram is indicative of the type of alteration found in the Creede, Colorado area.
Essentially a heat source in the crust heats meteoric water allowing it to load with various ions. As the water moves upward toward the surface and away from the heat source, minerals are formed and drop out of solution. In this way sulfides and other minerals can fill veins and become finely disseminated throughout large areas.
Iron and sulfur are fairly ubiquitous in hydrothermal waters. Just think of the smell of hot springs!
Alkaline minerals are depleted, and in high grade hydrothermal alteration, totally removed.
Feldspars can also be broken down into various clay minerals.
(mod. from Guilbert and Park, 1986)

13. A number of streams in eleven different headwater areas of Colorado have naturally high concentrations of metals and/or acidity, upstream of any significant human impacts. Rocks in these areas have been affected by intense “hydrothermal alteration” in the geologic past.
A number of areas in Colorado have streams with naturally high concentrations of metals and acidity above mining impacts.
All of these areas have been affected by hydrothermal alteration in the geologic past, usually during the Tertiary.
First we’ll look at two areas in the northern part of the state
Montezuma Stock near Keystone
Grizzly Peak caldera

14. This mountain lies in the headwaters of the North Fork South Platte River.
On the other side of the continental divide (ridge in picture), this area of hydrothermal alteration affects the headwaters of the Snake River.
Drainages containing no mines have high acidity with pH=
Predominantly iron and sulfate, some copper above stream standards in this area.
Red Cone, Park Co.

15. Red Mountain, Chaffee and Pitkin Co.
Springs
This mountain is just south of Independence Pass
One small dry prospect adit in area does not affect water quality.
Natural springs contain metal precipitates lining their drainages with white on left, yellow-orange on right. These are specific responses to the minerals in the water and the pH of the spring.
Note sharp contact of hyrothermally altered rock on right.
Next slide shows analytical results for spring on right. (pH=2.17)
Red Mountain, Chaffee and Pitkin Co.

16. Factor Above Aquatic Life Standard Constituent PEEKABOO GULCH SPRING
Natural acid rock drainage dissolves metals present in the rock, affecting downstream water quality. 1 10
100
1000
10000
Factor Above Aquatic Life Standard
Constituent
PEEKABOO GULCH SPRING
pH=2.17, conductance=2470 µS, Q=25 gpm
1,724 x
42 x
164 x
100 x
6 x
21 x Al Cd Cu Fe Mn Pb Ag Zn
SO4
Water sampled in 1994 from a Peekaboo Gulch spring greatly exceeded state standards in aluminum, cadmium, copper, iron, and zinc concentrations. Silver and sulfate concentrations also exceeded standards.

17. Next we’ll look at some areas in southern Colorado:
Upper Alamosa River, near Summitville
Two areas in the Silverton-Lake City caldera area

18. Lookout Mountain, Conejos Co.
This is Lookout Mountain which lies on the north side of the upper Alamosa River. To the left (west) of this mountain is Iron Creek; to the east Alum Creek, and Bitter Creek. Very descriptive names!!
These creeks all run very acidic, mostly between pH Alum Creek has pH~2 at its mouth with the Alamosa River.
This is all upstream of, and unaffected by, the Summitville Mine area.
Lookout Mountain, Conejos Co.

19. Ferrosinter deposits are characteristic of springs in hydrothermally altered areas.
The smooth rock in the upper left of this picture is actually a spring. Natural acid rock drainage emanating from this spring carries abundant metals that drop out of solution when exposed to oxygen in the atmosphere. The resulting deposit is called a ferrosinter.
Ferrosinters are cold water iron precipitate mounds at springs. Composed of predominantly of FeO-FeOH and AlOH precipitates.
The streambed below this natural acid rock drainage spring consists of ferricrete. The water quality is poor and the streambed is very hard, eliminating habitat for fish and invertebrate aquatic life.
Ferrricretes are river bed (alluvium) or hill slope (colluvium) material cemented by iron precipitates.
Iron Creek Lower Spring,
Conejos Co.

20. Some hydrothermally altered areas have high rates of erosion,
and are prone to debris flows and landslides
because bedrock has
been weakened.
Alum Creek, Conejos Co.
Slide
Headwall
I mentioned earlier that hydrothermal fluids can alter feldspars to clays. When this happens the strength of the rock is significantly compromised. In these altered areas, erosion rates are high.
The high rates of erosion are continually exposing sulfide minerals to water and oxygen, continuing the acid rock drainage cycle.
Landslides and mudslides (debris flows) can be found in areas of hydrothermally altered rocks.
Red Mountain #2, Ouray Co.

21. Slumgullion landslide is in an area of hydrothermally altered rock
Slumgullion landslide is in an area of hydrothermally altered rock. Movement and erosion continually expose acid-producing minerals. Drainage from the slide area is very acidic (pH  3.4).
Headwall
Colorado’s largest landslide is the Slumgullion landslide, southeast of Lake City. This landslide caused the formation of Lake San Cristobal.
Drainage along the body of the slide has pH~3.4 and is high in Aluminum, Iron, and Manganese.
Slumgullion Landslide near Lake City, Mineral Co.

22. Conclusions
Acid rock drainage is generated at mines and naturally where sulfide minerals are present and the buffering capacity of the water is exceeded.
AMD degradation can be acute because:
1) Mines act as collectors of groundwater
2) Water is in contact with high grade ore minerals
3) Mine dumps and tailings provide dramatically increased surface areas for the interaction of water, oxygen, and sulfide minerals.

23. Conclusions
Natural ARD is important in areas of intense hydrothermal alteration because:
1) Rocks contain disseminated sulfide minerals.
2) Rocks are depleted of buffering minerals.
3) Large volumes of rock are affected.
Both ARD sources must be characterized to determine realistic remediation goals.