Ores, mining, and the legacy
Environmental Geology lecture outline
Figure of the Hibbing Iron Mine in Minnesota. You are viewing
about one third of the mine which is some 6 miles long. The red coloration
is due to oxidation of the iron ore. Open pit mines such as this one often excavate beneath the water table. Like a hole deep enough in beach sand they can often
fill up with water, and can lower the groundwater table in the area when they are active. Dewatering is an important part of mine operations.
Where and why of ore deposits = discipline of Economic Geology.
Importance of metals and ores to society:
- stone age, iron age, bronze age.
- now in the aluminum, silicon or composite age.
This is a figure showing some of the tailings (waste rock) associated with Hibbing
iron mine in Minnesota. The blue arrows point to the long ridge that is
composed totally of waste material from the Hibbing mine. The student stands
on more tailings. Much of it is sand/gravel overburden, and relatively
benign, but this gives you an idea of the volume of earth material involved. For many metallic deposits the percent of rock that is the ore mineral is a few percent by volume, and the remaining amount is waste rock.
Cradle to grave perspective on metallic resources
Useful framework = cradle to grave environmental perspective includes exploration, extraction, processing (often smelting), manufacturing, disposal and/or recycling.
- exploration: usually minimally invasive; consists of some mix of geologic mapping, geophysical mapping, and drilling.
- extraction: their different mining methods,
each with their own suite of environmental concerns.
extraction - associated potential environmental concerns include the following:
- open pit.
- strip mine.
- underground mining.
- solution mining.
- water quality of run off from tailings (e.g. acid mine drainage, aresenic contamination). Understanding the geologic association of minerals and chemical composition particularly useful - e.g. silver ore
often has a lot of arsenic associated with it, and zinc can be associated with lead. Thus one can predict what possible contaminants to monitor and mitigate for.
- failure of waste mine impoundments releasing contamination to surface and ground water.
- surface collapse over underground mines.
- mine safety (e.g., collapse, radon exposure)
- lowering of groundwater table (wells running dry, ground subsidence, sinkhole collapse in some cases).
- land reclaimation for surface mining and for tailings piles for any type of mining:
- mine land reclamation mandated in the U.S.:
- many different approaches and can include:
- recontouring spoils piles into desired topographic configuration.
- chemical neutralization (e.g. adding limestone to the spoils).
- caps and seals(to reduce leakage into spoils).
- placement of topsoil and revegetation.
- installation of monitoring wells.
- adds to the cost of doing business.
- Tailings on the upper reach of the Arkansas River in Colorado. The mineralogy of the tailings coupled with an increased surface area and porosity due to the crushing often leads to the production of acidic waters laden with various metals for certain types of mines. Image source: http://toxics.usgs.gov/photo_gallery/UpperArk.html
- processing - associated environmental concerns:
- acidic emissions from smelting (e.g. Ducktown, Tennessee and Sudbury,
- energy used and waste byproducts.
compounds used (e.g. mercury for gold).
- Above is view of a collapse pit above mine workings at Ducktown and to right is some of the brassy, gold mineral, pyrite, that weathers to produce acidic waters, and when smelted releases sulfur to become acid rain. Images from: http://pubs.usgs.gov/gip/ocoee2/#mining
- use and proper disposal: potential health effects
(Pb, Hg), recycling.
Legacy of abandoned old mines
In Montana and Colorado are particularly well know, 1000s of sites across the country that were mined before the era of environmental awareness.
2015 Colorado - Gold King Mine waste water spill - https://en.wikipedia.org/wiki/2015_Gold_King_Mine_waste_water_spill .
Animas river in Colorado 24 hours after mine release. Image source: https://en.wikipedia.org/wiki/2015_Gold_King_Mine_waste_water_spill#/media/File:Animas_River_spill_2015-08-06.JPG .
USGS site on a related initiative - http://www.usgs.gov/themes/factsheet/095-99/ .
Mine in Montana, which prides itself on its trout streams. Image source: http://toxics.usgs.gov/photo_gallery/metals_variation_all.html
Two specific examples of ores
Aluminum as an example:
- geologic formation factors: the recipe for forming bauxite (an Al rich clay deposit that is the primary ore for Al) is a)Al rich source rock (e.g. a granite), b) intense
chemical weathering (tropical conditions), and c) geologic preservation of intensely weathered material.
- a strategically critical mineral (why?).
- major global deposits:
- U.S. has fraction of percent of world's reserves.
- processing, environmental concerns and the why of recycling:
- 5-6 tons of bauxite ore --> 1 ton of metal.
- 1300 lbs of coke per ton required.
- 244*10E6 Btu required, roughly equal to 17 tons of
good coal for one ton of Al produced.
- recycling saves 90% of the energy.
- Norway a major producer (because of abundant hydroelectric power).
Uranium as an example:
- NW Nebraska's deposits: Crow Butte Mine..
- initial lease site 3,000 acres, leases on additional
- geologic aspects:
- White River Group strata (rich in volcanic ash).
- basal conglomerate unit - Chadron aquifer, 400-800'
- underlying impermeable Pierre Shale.
- normal faults in the area.
- U in pores, replacing organics: carnotite: complex
- U precipitates under reducing conditions and is mobile
under oxidizing conditions.
- overlying Brule aquifer. Pump tests indicate minimal
connection (where tested) between ore horizon and aquifers higher in the section.
- mining technique used:
- solution mining.
- pump out more than inject so net flow towards and
not away from the site.
- sodium bicarbonate and hydrogen peroxide used as mobilizing
- history of mining site:
- in operation some 15 years.
- about million pounds of U3O8 (yellowcake) per year.
- environmental concerns:
- surface spills.
- contamination of other aquifers (primarily the Brule aquifer).
- induced seismicity (none produced to date).
- several million dollar trust fund posted with the state.
- three accidents to date:
- pump malfunction caused 7,000-8000 gallon spill; DEQ
- 23,000 gallon spill, some of which reached local creek.
Below detectable limits of radium in water.
- leak into Brule aquifer, 100,000 to 300,000 gallons
(this part of aquifer unused).
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