MICROSTRUCTURAL AND MINERALOGIC CONTROLS ON THE RELEASE OF TRACE ELEMENTS AND ACID; SOLID-PHASE ANALYSES OF MINE WASTES FROM HUMIDITY CELL TESTS

Two proprietary mine-waste samples were subjected to well-oxygenated conditions and rinsed weekly in humidity cell tests to: (1) determine the long-term potential of waste to generate acid and (2) follow trace-metal release. Potential acid producing minerals in the mine-waste samples are pyrite, tetrahedrite-tennanite, and jarosite. Unleached samples have the following potentially toxic trace elements: As (45-120 ppm), Cu (60-320 ppm), and Zn (30-2,500 ppm).

The first mine-waste sample exhibits multiple stages of deformation and sulfide mineralization. Early Mg-rich carbonate- and siderite-filled microveins host pyrite and arsenic-bearing pyrite. Silica-filled microveins that host Cu-, As-, and Sb-bearing sulfides crosscut the carbonate microveins. Post-mortem XRD examination of leached material indicates that most of the carbonate minerals dissolved during the course of the humidity cell test. Therefore, under natural exposure conditions, the carbonate minerals could be expected to go into solution, thus exposing pyrite to oxidation, and releasing arsenic into ground-water and surface-water systems. Conversely, silica-filled microveins and associated sulfides were still intact in the leached material. Therefore, the trace metals contained in the sulfides within the siliceous microveins may be released to the ground-water system more slowly over a greater time period.

The second mine-waste sample was jarosite-rich and contained very little pyrite, so we examined jarosite as a potential source of acid generation. There were few differences between fresh and leached samples of this material. Electron-beam analyses consistently revealed less than stoichiometric K:Fe ratios (<1:3), indicating a K deficiency. In the absence of any other detectable cation, we presume H₃O⁺is present to make up that deficiency and is responsible for acidic leachates generated by this material.