Characterization of Goathill North mine rock pile, Questa molybdenum mine, Questa, New Mexico

Abstract

Rarely do rock pile characterization methods allow for examination and sampling of the interior of large rock piles in-situ. The regrading of the Goathill North (GHN) rock pile at the Questa mine provided a unique opportunity to examine and sample the interior of a large rock pile through the construction of trenches cut into the rock pile as earth-moving progressed. Maps of each bench were created to document the different stratigraphic units, including the thickness, dip, and extent of the units. Units were defined based on grain size, color, and other physical properties. Units were correlated between benches and downward through the series of successively excavated trenches. Typically, paste pH increased with distance from the outer, oxidized zone (west) towards the interior units (east) of the GHN rock pile. The outer zone was oxidized (weathered) based upon the white and yellow coloring, low paste pH, presence of jarosite and gypsum, and absence of calcite. However, the oxidation/reduction (weathering) state in the interior zone is not yet determined. The base of the rock pile closest to the bedrock/colluvium surface represents the oldest part of the rock pile since it was laid down first. Portions of the base appeared to be nearly or as oxidized (weathered) as the outer, oxidized zone, suggesting that air and water flow along the basal interface occurred and possibly was an active weathering zone. Analyses of samples from unweathered, unoxidized drill core samples and from the GHN rock pile are similar in clay mineralogy as determined by XRD and electron microprobe analyses, which suggests that the majority of clay minerals in the GHN samples were derived from the original, pre-mined hydrothermal alteration and not post-mining weathering.