geologic setting and history of mining in the animas river
TRANSCRIPT
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
Yager and Bove, 2007, Professional Paper 1651
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
Yager and Bove, 2007, Professional Paper 1651
Red Mountain mining district
Eureka mining district
South Silverton mining
district
Church et al., 2007, Professional Paper 1651
5,397 mine
shafts, adits,
tunnels, and
prospects
Church et al., 2007, Professional Paper 1651
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.
Church et al., 2007, Professional Paper 1651
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)