Crustal Imaging and Characterization Team

Task 5: Predictive Experimental Studies of Sulfide Oxidation Processes

Task Objectives

The objective of our work is to understand and ultimately quantify the behavior and distribution of environmentally significant elements as they are dispersed from their sources through the environment. Sources of elements in mineralized terranes include natural and anthropogenic entities such as ore deposits, mine tailings, and waste rock. Characterization of these source materials involves identifying the solid phases where the toxic elements reside, the speciation of the toxic elements (e.g., oxidation state and type of bonding to the mineral structure), and the rates and mechanisms of release from the mineral structure. Once released from their sources, physical, inorganic, and biogeochemical processes act to re-distribute and transform both dissolved and particulate forms of the elements.

Our specific objective is to determine rates and mechanisms of sulfide mineral oxidation and toxic element release due to chemical and microbiological processes. Currently, the rates of inorganic oxidation of some common sulfide minerals are known under certain ranges of conditions, but much more needs to be done, especially with alteration of specific minerals and chemical reactions mediated by microorganisms. In addition, little has been done experimentally to determine the behavior of potentially toxic trace elements that exist as intergrown refractory minerals and non-sulfide mineral inclusions (usually silicates or oxides), in discrete inclusions of sulfides or sulfosalts, in defect sites, or in solidsolution within the host minerals. Trace minerals containing toxic elements may have very irregular distribution in the host sulfides.

Statement of Work

This year, we plan to:

1. continue laboratory experimental work on chalcopyrite (plus covellite and chalcocite) and sphalerite oxidation in acidic solutions (pH = 1-4). Much has already been accomplished with set of multiple inorganic and biotic experiments for each mineral. Additional experiments at higher Cl and with different oxidants are needed to finish a comprehensive data set.
2. continue work on mineralogy and distribution of trace elements and isotopes in Cu and Zn minerals used in our experimental studies.
3. continue work on oxidation of cinnabar (HgS) in acidic solutions.
4. continue a comprehensive study of transition metal isotopes and sulfur isotopes in acid mine drainage environments.
5. continue work on oxidation of sulfide from sediment-hosted volcanogenic massive sulfide deposits in the transitional marine environment of Prince William Sound, AK, and in the Vermont Copper Belt drainage and pit lake environments.

Highlights & Key Findings

Highlights of ongoing studies include:

1. laboratory experimental determination of rates of chalcopyrite and sphalerite oxidation in acidic solutions under biotic and abiotic conditions;
2. analysis of trace metal and sulfur isotope distribution in chalcopyrite and sphalerite using microanalytical techniques so that metals and sulfur in experimental solutions can be related to specific sites in starting minerals and so that partition coefficients can be established;
3. discovery of matrix effects related to ion microprobe analyses of sulfur isotopes in sphalerite;
4. discovery of acidic metal-rich fluids related to sulfide mine-waste oxidation in marine waters of Prince William Sound, AK.

The abandoned Valzinco mine site is a fairly simple mine-waste complex that permits several methods for predicting mine drainage chemistry to be predicted. The Valzinco deposit was a Kuroko-type massive sulfide ore body that was mined intermittently during the first half of the 20th century. The entire ore body was accessed by underground mining methods, which resulted in a limited amount of waste rock at the site. The ore body was below the water table such that there is no known discharge of mine drainage from the underground workings. Thus, the only significant mine waste on the site is flotation tailings, which have a fairly uniform grain size and mineralogy comprising quartz, chlorite, albitic feldspar, pyrite, sphalerite, galena, and chalcopyrite. The tailings were deposited in the stream channel. The mine drainage discharging from the site was sampled on a quarterly basis for two years immediately below the site (VLZN-3) and approximately 1.1 km downstream (VLZN-11).

Estimates of mine drainage compositions were made using two distinct methods:

1. Synthetic precipitation leaching protocol (SPLP) of mine waste samples (Hageman & Briggs, 2000) using both in situ and fluvial tailings samples.
2. Kinetic modeling of water-mine waste reactions using the Geochemist's Workbench®.

The Geochemist's Workbench® simulations used rate constants for both sulfide oxidation via ferric iron and molecular oxygen. Stable isotope data from dissolved sulfate in mine drainage from the site suggest that both reaction pathways are equally important at the site. Results indicate that both the SPLP tests and the computer modeling yield reasonable estimates of the bulk composition in terms of pH and dissolved constituents of drainage discharging from the site. The simulations using ferric iron rates constants tend to have lower pH values and higher dissolved concentrations, which is not unexpected considering that isotopic data indicate that this pathway only accounts for roughly 50% of the sulfide oxidation.

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