SOIL AND COLLUVIUM CHEMISTRY AS AN INDICATOR OF AMD-PRODUCING SULFIDE DEPOSITS

Over the past 3 years, highway construction in Pennsylvania has inadvertently uncovered sulfide mineral-bearing bedrock, resulting in acid mine drainage (AMD). This has occurred at at least three localities, and costs of remedial efforts may reach over $40 million. The type of standard subsurface analyses typically used to recognize/avoid these costly situations, utilizing numerous deep core borings, is in its own right very expensive. To minimize both risk of environmental degradation and exploration costs for future road construction will require a phased approach during which the first relatively inexpensive phase examines the likelihood of occurrence of sulfide bearing minerals. Sampling and analysis of colluvium and soil overlying and downslope of bedrock of potential concern—obtainable through shallow borings and test pits—is much less costly than extensive rock coring, and results from this kind of analysis can be used to determine if more thorough subsurface investigation is warranted.

To test this idea, we completed an analysis of soil and colluvium from the toe of Bald Eagle Mountain (Centre County, PA) near the Skytop AMD site and downslope of known sulfide bearing mineral deposits in Centre County. Discrete intervals of core and air rotary boring samples were analyzed for total metal concentrations. Within discrete samples, increased concentrations of two or more metals likely reflect weathering of specific sulfide mineral horizons. For example, increased concentrations of zinc occur in the same samples as increased concentrations of lead, suggesting parent material containing lead-zinc sulfide deposits, similar to hydrothermal deposits well known from Sinking Valley (approximately 15 miles away) but hereto unreported in the Skytop vicinity. Likewise, concentration trends for cobalt and chromium are similar (possibly related to weathering of pyritic source rock), whereas the concentration trend in arsenic seems to be a hybrid of the lead-zinc and cobalt-chromium trend. Cobbles from uppermost colluvium and soil, in which concentrations of metals may be an order of magnitude higher than grab samples of colluvium, indicate that sulfide vein-rich horizons are a likely source for much of the metal. Thus, the chemistry of the colluvium may be diagnostic for first-order analysis of bedrock mineralogy.