geologic setting and history of mining in the animas river

Geologic Setting and History

of Mining in the Animas River

Watershed, southern Colorado

Virginia T. McLemore

New Mexico Bureau of Geology and Mineral

Resources, New Mexico Tech, Socorro,

NM

On August 5, 2015, EPA personnel caused the

release of wastewater at the Gold King mine,

Colo. into the Animas River

• Gold King adit • Animas River

Gold King mine

Church et al. (2007)

Gobla et al., 2015

Purpose

• To provide geologic and historical

background of the Gold King-Animas River

spill and mining in the Animas River

watershed

• To address the path forward in preventing

similar spills or where do we go from

here?

(from Church et al., 2007; Church, 2014;

Gobla et al., 2015 among other references)

Location and topography

• Northernmost headwaters of the Animas River watershed in San Juan County, Colo

• Drainage basins of the Animas River at and upstream from Silverton, Colo

• Two main tributaries, Cement and Mineral Creeks

• Silverton is 9,305 ft and some of the mountain peaks in the headwaters rise to more than 13,800 ft

• 150 sq miles

Church et al., 2007, Professional Paper 1651

Geological Setting

• Late Oligocene volcanic centers that erupted many cubic miles of lava and volcanic tuff and formed the Silverton caldera (27.6 Ma), which is nested within the San Juan caldera (28.2 Ma)

• Followed by multiple episodes of hydrothermal activity that produced widespread areas of pyrite alteration and quartz-pyrite-metals veins

• Most of the host rock is very fractured and permeable=complex groundwater flow paths

Yager and Bove, 2007, Professional Paper 1651

Gold King mine

Geologic setting

• Subsequent weathering of hydrothermally

altered rock, particularly the acid sulfate and

the quartz-pyrite-sericite alteration that was

extensive in the watershed, is a natural

source of source of metals and acidity

(Church et al. 2007, among other studies)

• Not all elevated concentrations of metals

and low-pH conditions found in streams

are the result of historical mining


Gold King mine

In 1874, it was noted that water in both Cement and Mineral Creeks

contained iron sulfate,making the water undrinkable

(Rhonda, 1876)

WHAT IS WEATHERING?

• Physical, chemical,

and/or biological

processes at or near the

earth’s surface that

result in reductions of

grain size, changes in

cohesion or

cementation, and

change in mineralogical

composition that convert

rocks to soils and can

produce acid drainage

• Occurs in the country

rocks before mining

• Occurs in the mine

waste materials after

mining

WHAT IS WEATHERING?

• Weathering is simply the consequence of

exposing rocks to the conditions at the

Earth’s surface

– fairly low temperatures

– low pressures

– organic activity

– Requires chemically active substances such

as water and the gases of the atmosphere

Factors affecting pyrite oxidation

=generation of acid drainage (AD)

• Oxygen concentration

• Temperature

• pH

• Pyrite surface area

• Concentration of ferric iron (Fe+3)

• The presence of bacteria

• Water

Time Scales of Weathering

and Generation of AD

• Unaltered (fresh) rocks (age 28-11 Ma)

• Vein deposits (age 28-11 Ma)

• Other land forms and surfaces (such as

debris flows, ages unknown)

• Ferricretes and other stream terrace

deposits (thousands to million years)

• Mine features (0-140 years)

Mining History

History of mining in the Animas

River Watershed

• Early mining by the Spanish (presence of arrastra—used to recover gold and silver)

• Rediscovered about 1860

• Development begin in 1871 with the Little Giant mine

• Extensive mineral exploration did not begin until after the signing of the Bernot Treaty with the Ute Tribe in 1873

• Production increased with construction of the railroad to Silverton in July 1882 – Smelter in Durango

Legacy Mines

• 1872 mining law was a product of gold rushes and other mining booms in the west during the 1800s

• Allows US citizens and firms to explore for minerals and establish rights to federal lands

• Contains no environmental provisions

• Required that every mining claim have a discovery hole or shaft and annual assessment work = many mine features that we see today

• Assessment work requirement abolished in 1990s

US Environmental Regulations

• 1935 Executive Order – Stopped discharge of mine waste and tailings in

waterways—did not always apply to arroyos

• Federal Lands Regulation 1960s-70s

• 1970s Federal Regulations

– Clean Water Act (CWA)

– Federal Surface Mining Control and

Reclamation Act (SMCRA)

– Federal Land Policy and Management Act

(FLPMA)

– Among other legislation

Note

Miners were not required to reclaim their

sites until the 1990s and most followed

acceptable mining practices of the time.

It is important to recognize that these early

miners were not breaking any laws, because

there were no laws to break.

Another Caution

• Only a limited amount of Federal land has any mineral resource potential

– Mineral deposits form by natural geologic processes in specific places in the world

– Mineral deposits are not always found in the most convenient places in the world

• Most mining claims do/did not result in active producing mines

– For every ~1000 promising prospect areas, only 1 may make it to production

• =results many mine features

Sunnyside mine

• Reuben J. McNutt staked on August 18, 1873

• In 1896, the Silverton Northern Railroad was extended to Eureka, Colorado, to serve the Sunnyside mine

• American Tunnel in 1959 gave new life to the Sunnyside mine, which operated from 1961-1991

• blowout in 1978 that drained Lake Emma and required two years to repair

• 7 million short tons of ore over its life span of 118 years

Gold King mine• Production started in 1886

• In 1899, the Silverton, Gladstone, and Northerly Railroad was built to Gladstone

• 665,000 tons of ore averaging 14.65 g/t gold,

• 1886-1917, production amounted to 74.32 g/ton Ag, 0.71% Pb, and 0.52% Cu

• Mining operations ended around 1923

• Exploration in the 1970s

• 7 levels– Level 1 at 12,160 ft

– Lower Gold King portal 10,617 ft = American Tunnel and drained higher workings

Gobla et al., 2015; map prepared by Kirstin Brown

Gold King mine

Sunnyside mine


Red Mountain mining district

Eureka mining district

South Silverton mining

district


5,397 mine

shafts, adits,

tunnels, and

prospects


Mills, tailings,

smelters

By 2007, more than $20,000,000

had been spent on remediation in

the watershed (Church et al., 2007;

Church, 2014). Both private and

public (EPA, USFS, State of

Colorado, BLM) funds have been

used for these remediation efforts.

Challenges for reclamation efforts• Complex land ownership (USFS, BLM, orphaned properties

held by San Juan County for delinquent taxes, private)

• Issues of financial liability

• Technical approaches needed to mitigate acid mine drainage without resorting to costly treatment facilities

• The need for adequate space to build new repositories to move those mine wastes that cannot be mitigated in place

• Effect of reclamation on tourism in a historical mining district such as Silverton

• Understanding the groundwater system, which affects all the mines in the area and the potential for work on one mine affecting conditions at another

• What are the pre-mining conditions

• What should a long-term monitoring plan consist of? What should we test for? What media should be tested (water, crops, fish, plants)? What are the pathways? What is safe and what is not safe?

(Church et al., 2007; Church, 2014; Gobla et al., 2015)

geologic setting and history of mining in the animas river

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