Modeling Applications and Reaction Pathways
Reaction Pathways and Geochemical Modeling
In natural settings, supergene processes and the generation of mine drainage involve complex interactions of surface and ground waters with a variety of minerals that weather at different rates. Thus, the stable isotope signature and inorganic chemistry of ground and surface waters around deposits or mine drainage represents an integrated signal from a mixture of individual minerals. The research topic focused on the disequilibrium processes by which stable isotope fractionations occur in low-temperature environments as a means of identifying reaction pathways. Emphasis was placed on ore and gangue mineral systems, especially sulfide and sulfate minerals, and in laboratory-based studies on the weathering of hypogene ores and mine wastes. Stable isotopes provide an unique means of tracking the behavior of individual compounds in the weathering process. Interpretations will involve computer simulations of reaction progress using a variety of available software. The results of this research are expected to have major implications for the origin of supergene ores and the generation of mine drainage, and will eventually result in incorporation of new efforts into other field-based projects.
Geochemical and modeling studies at the Valzinco abandoned Zn-Pb mine in central Virginia demonstrate the utility of this approach. The Valzinco abandoned mine waste was dominated by flotation tailings deposited near the headwaters of Knights Branch in the stream channel. The gangue mineralogy of the tailings is dominated by quartz, plagioclase feldspar, muscovite, and chlorite; the ore mineralogy comprises pyrite, sphalerite, galena, and chalcopyrite. Geochemical analyses of unimpacted waters, source waters seeping from the waste piles, and impacted reaches of Knights Branch downstream of the site document a stream impaired by acid mine drainage. Aquatic ecosystem toxicity standards are exceeded for Fe, Al, Cu, Pb, Zn, and Cd.
Oxygen isotope analyses of waters, dissolved sulfate, and secondary sulfate minerals from the site provide insights into sulfide oxidation pathways (Fig. 1). Paired analyses of dissolved sulfate (δ18OSO4) and water (δ18OH2O) can be interpreted in terms of the relative importance of water or dissolved atmospheric oxygen as the dominant sources of oxygen in the sulfate as described by the reactions:
FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+ (1).
FeS2 + 10 Fe3+ + 6 H2O + O2 → 11 Fe2+ + 2 SO42- + 12 H+ (2).
In Reaction 1, all of the oxygen to form sulfate is derived from water, whereas in Reaction 2, both atmospheric oxygen and water contribute to the resulting sulfate. The oxygen isotopic fractionation associated with these two reactions can be used to assess the relative importance of these two reactions to the evolution of acid mine drainage. Sulfate solely derived through Reaction 1 would be expected to fall along the dashed line labeled “0 %” on Figure 1; sulfate derived solely through Reaction 2 would be expected to fall along the line labeled “100 %” on Figure 1. Combinations of the two pathways are indicated by the dashed lines labeled “25 %”, “50 %”, and “75 %”. At Valzinco, the isotopic results indicate that there are two distinct sulfide oxidation environments. The first is represented by evaporating conditions in the shallow tailings environment (the efflorescent salts and evaporating puddle) where most of the sulfate oxygen is derived from water and ferric iron is the oxidant; the second is represented by a deeper setting in the tailings where sulfate oxygen is derived from dissolved oxygen and water and molecular oxygen is dominant oxidant. The isotopic data indicate that greater than 50 % of the sulfide oxidation is described by Reaction 2. In addition, similarity with discharge from the site in Knights Branch (VLZN-3, VLZN-11) indicates that the second environment is most important for producing most of the acid mine drainage issuing from the site.
Figure 1. Plot of δ18O from SO4 and H2O from Knights Branch site.
The geochemical ramifications of these two paths can be evaluated through reaction-path modeling using the Geochemist's Workbench® with rate constants for sulfide oxidation with either ferric iron or dissolved oxygen as the oxidant. Models were run assuming a residence time of up to one year with 1 kg of water reacting with 50 g of mine waste. Results indicate that with ferric iron as the oxidant, much lower pH values are achieved compared to those with dissolved oxygen as the oxidant (Fig. 2). The evolution of the hypothetical mine drainage with time reflects ongoing dissolution of both the silicate and sulfide starting minerals, and subsequent precipitation of secondary phases. The modeled drainage chemistry for both pathways compares favorably with data from water samples from the site for iron and sulfate, but those values derived from the dissolved oxygen rate constants tend to describe better the mass fluxes from the site, which is consistent with the oxygen isotope results.
Figure 2. Plots of pH and Fluid composition vs. Time and Iron and Sulfate concentrations vs. pH.
Piatak, N.M., Seal, R.R., II, Sanzolone, R.F., Lamothe, P.J., and Brown, Z.A., 2006, Preliminary results of sequential extraction experiments for selenium on mine waste and stream sediments from Vermont, Maine, and New Zealand: U.S. Geological Survey Open-File Report 2006-1184. Available online at http://pubs.usgs.gov/of/2006/1184/.
Abstracts, Presentations, and Posters for Professional Technical Meetings
Piatak, N.M., Seal, R.R., II, Sanzolone, R.F., Lamothe, P., and Brown, Z.A., 2006, Distribution of selenium in mine waste and stream sediments associated with abandoned base-metal mines based on sequential extraction experiments [abs.]: Geological Society of America 2006 Annual Meeting, Philadelphia, Pennsylvania, October 22-25, 2006, Abstracts with Programs, Vol. 38, No. 7, p. 424. (Abstract and poster) Available online at http://gsa.confex.com/gsa/2006AM/finalprogram/abstract_112822.htm.
Seal, R.R., II, Hammarstrom, J.M., and Piatak, N.M., 2005, USGS research related to the treatment of acid-mine drainage: U.S. Environmental Protection Agency Workshop. (Oral presentation)
Robert Seal II
954 USGS National Center
12201 Sunrise Valley Drive
Reston, VA 20192
Phone: (703) 648-6290