Sharon F. Diehl

What's weathering? Mineralogy and field leach studies in mine waste, Leadville and Montezuma mining districts, Colorado

Weathering is important in the development of rock fabrics that control porosity in mine-waste materials, and in turn, porosity affects metal transport through and from mine-waste piles into watersheds. Mine-waste piles are dynamic physical and chemical systems as evidenced by remnant Fe-oxide boxwork structures after sulfide minerals, development of alteration rinds and etch pits on grains, and precipitation of secondary minerals under low temperature conditions. These microscale changes in the mine-waste materials are the result of partial to total dissolution of sulfide and other minerals. Mine-waste materials from the Dinero, Lower Chatauqua, and Saints John sites, Leadville and Montezuma mining districts, Colorado, exhibit rock fabrics that indicate that weathering products, e.g., Fe oxyhydroxides, jarosite, and clays, have been transported in suspension through the waste piles and deposited in voids and as coatings on rock fragments. Microscale characterization of weathered, partially dissolved minerals lends insight into the source of leachable metals in these mine-waste sites. Mineralogic studies show that galena in the Lower Chatauqua waste is enriched in Ag. Qualitative and semiquantitative microanalysis of weathered, altered galena grains from all three sites show that the Ag-bearing galena is more susceptible to dissolution. It is not surprising, then, that solutions experimentally leached from Lower Chatauqua waste are higher in Pb (2310 ppb) compared to leachates from the Dinero (31 ppb) and Saints John (1360 ppb) wastes. The mobility of metals is increased at acidic pH. Using the USGS Field Leach Test protocol, leachate derived from the Dinero waste has a pH of 3 and high concentrations of Al (443 ppb), Fe (441 ppb), and Zn (7970 ppb). Leachate from Sts. John tailings has a pH about 4 and high concentrations of Mn (1520 ppb), Zn (2240 ppb), and Pb (1360 ppb). Leachate from the Lower Chatauqua waste has an intermediate pH of 5, but in addition to the high Pb level already mentioned, it contains high levels of K (1.9 ppm), Mn (6720 ppb), and Zn (1550 ppb). The high concentration of metals, despite the intermediate pH of the leachate, may be explained by acidic microenvironments that exist at the surfaces of sulfide minerals, where sulfurand iron-oxidizing microbes may flourish. It is at the reactive mineral-oxygen-water interface where metals are released and low-pH sulfate precipitates such as jarosite-beudantite form. Additional

The use of synthetic jarosite as an analog for natural jarosite

The presence of jarosite in soil or mining waste is an indicator of acidic sulfate-rich conditions. Physical and chemical properties of synthetic jarosites are commonly used as analogs in laboratory studies to determine solubility and acid-generation of naturally occurring jarosites. In our work we have mineralogically and chemically characterized both natural and synthetic jarosites. Analysis of 32 natural hydrothermal and supergene K- and Na-jarosites indicates no (< 5 mole %) solid solution between K and Na end members. Instead, our detailed study of cell dimensions and composition reveals discrete mixtures of K and Na end members. Hydronium-bearing jarosite was detected in only one natural sample, and it appears that hydronium-bearing jarosites are metastable. Although the presence of hydronium in jarosite cannot be directly measured, we found that when synthetic hydronium-bearing jarosites are heated at 120°C for 78 days or 240°C for 24 hours, Fe(OH)SO4 is formed. The Fe(OH)SO4 is easily detected by X-ray diffraction and, hence, can be used as a post-mortem indicator of the presence of hydronium jarosite. Results from our synthetic jarosite studies indicate that natural metastable hydronium-bearing jarosite or iron-deficient forms of natural jarosite likely play an important role in acid generation in some mining wastes, but are not accurately represented by synthetic jarosite prepared by commonly used methods. The widespread practice of heating to at least 110°C after jarosite synthesis appears to drive off structural waters from protonated hydroxyl sites, which changes the properties of the jarosite. Therefore, synthetic jarosite should not be heated above 95 o C if it is to be used as an analog

Sources of acid and metals from the weathering of the Dinero waste pile, Lake Fork watershed, Leadville, Colorado

Abstract. Two trenches were dug into the south Dinero mine-waste pile near Leadville, Colorado, to study the weathering of rock fragments and the mineralogic sources of metal contaminants in the surrounding wetland and Lake Fork Watershed. Water seeping from the base of the south Dinero waste-rock pile was pH 2.9, whereas leachate from a composite sample of the rock waste was pH 3.3. The waste pile was mostly devoid of vegetation, open to infiltration of precipitation, and saturated at the base because of placement in the wetland. The south mine-waste pile is composed of poorly sorted material, ranging from boulder-size to fine-grained rock fragments. The trenches showed both matrix-supported and clast-supported zones, with faint horizontal color banding, suggesting zonation of Fe oxides. Secondary minerals such as jarosite and gypsum occurred throughout the depth of the trenches. Infiltration of water and transport of dissolved material through the pile is evidenced by optically continuous secondary mineral deposits that fill or line voids. Iron-sulfate material exhibits microlaminations with shrinkage cracking and preferential dissolution of microlayers that evidence drying and wetting events. In addition to fluids, submicron-sized to very fine-grained particles such as jarosite are transported through channel ways in the pile. Rock fragments are coated with a mixture of clay, jarosite, and manganese oxides. Dissolution of minerals is a primary source of metals. Skeletal remnants of grains, outlined by Fe-oxide minerals, are common. Potassium jarosite is the most abundant jarosite phase, but Pb- and Ag-bearing jarosite are common. Grain-sized clusters of jarosite suggest that entire sulfide grains were replaced by very fine-grained jarosite crystals. The waste piles were removed from the wetland and reclaimed upslope in 2003. This was an opportunity to test methods to identify sources of acid and metals and metal transport processes within a waste pile. A series of entrapment ponds, lined with limestone rip rap, was created where the mine waste was once situated. A flooded adit discharges low-pH metal-bearing waters into the ponds. A white (Zn, Mn)-sulfate precipitate was observed in 2003 around the edges of the most distal pond. Key Words: mine waste, dissolution, jarosite, anglesite, microlamination, leachate -----------------------------------------

TRACE-METAL SOURCES AND THEIR RELEASE FROM MINE WASTES: EXAMPLES FROM HUMIDITY CELL TESTS OF HARD-ROCK MINE WASTE AND FROM WARRIOR BASIN COAL

To assess the potential impact of metal and acid contamination from mine-waste piles, it is important to identify the mineralogic source of trace metals and their mode of occurrence. Microscopic analysis of mine-waste samples from both hard-rock and coalmine waste samples demonstrate a microstructural control, as well as mineralogic control, on the source and release of trace metals into local water systems. The samples discussed herein show multiple periods of sulfide mineralization with varying concentrations of trace metals. In the first case study, two proprietary hard-rock mine-waste samples exposed to a series of humidity cell tests (which simulate intense chemical weathering conditions) generated acid and released trace metals. Some trace elements of interest were: arsenic (45-120 ppm), copper (60-320 ppm), and zinc (30-2,500 ppm). Untested and humidity cell-exposed samples were studied by X-ray diffraction, scanning electron microscope with energy dispersive X-ray (SEM/EDX), and electron microprobe analysis. Studies of one sample set revealed arsenic-bearing pyrite in early iron- and magnesium-rich carbonate-filled microveins, and iron-, copper-, arsenic-, antimony-bearing sulfides in later crosscutting silica-filled microveins. Post humidity cell tests indicated that the carbonate minerals were removed by leaching in the humidity cells, exposing pyrite to oxidative conditions. However, sulfides in the silica-filled veins were more protected. Therefore, the trace metals contained in the sulfides within the silica-filled microveins may be released to the surface and (or) ground water system more slowly over a greater time period. In the second case study, trace metal-rich pyrite-bearing coals from the Warrior Basin, Alabama were analyzed. Arsenic-bearing pyrite was observed in a late-stage pyrite phase in microfaults and microveins that crosscut earlier arsenic-

Determining the toxicity potential of mine-waste piles

When assessing the environmental impact from mining operations, an immediate question arises about potential impact and toxicity of mine-waste piles. This question is particularly difficult to assess for waste piles on abandoned mine lands in the western United States and coal-waste piles in the eastern United States. In many of these situations, there is no water in direct contact with the piles, except during meteorological events, yet it appears that the pile has caused significant ecological disturbance. For the past several years, scientists at the Colorado School of Mines and the U.S. Geological Survey have been studying the toxicity potential of waste-rock piles. Simple and practical methods have been developed for determining the potential of a waste- rock pile to cause significant contamination. For example, quick inexpensive field leaching tests have been developed that offer an evaluation of acid and trace-metal release from mine-waste material. Additionally, two-dimensional hydrologic and erosion models might be used to assess acid and metal sources and sinks. Such methods are presented for evaluating mine-waste piles from watershed scale, site scale, and microscopic scale, using geophysical, geochemical, and mineralogical methods. Current methods used to determine bioaccessibility and bioavailability of metals from wastes, such as extraction techniques, are described and assessed. Case studies with field and laboratory data illustrate these methods. These applications are used as the basis for a simple decision tree that has been developed to assess the potential impact of a waste-rock pile, and the scientific background that serves as the basis for decisions. Workshop Time: 8:30 am – 4:00 pm, June 1, 2003 Workshop Organizers: Dr. Thomas R. Wildeman Dr. Kathleen S. Smith Dept. of Chemistry & Geochemistry U.S. Geological Survey Colorado School of Mines M.S. 973, Denver Federal Cntr. Golden, CO 80401 Denver, CO 80225-0046 Phone: 303-273-3642 Phone: 303-236-5788 E mail: twildema@mines.edu E mail: ksmith@usgs.gov