Characterization of Goathill North Mine Rock Pile, Questa Molybdenum Mine, Questa, New Mexico

Abstract

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. Additional

Petrographic, Mineralogical and Chemical Characterization of Goathill North Mine Rock Pile, Questa Molybdenum Mine, Questa, New Mexico

“…End dumping results in these five zones of segregation (Fig. 3;Nichols, 1987;McLemore et al, 2005McLemore et al, , 2006:…”

“…The purpose of this report is to describe the mineralogical and chemical laboratory results of trench sampling within the interior of the stable portion of the GHN rock pile. The sampling techniques employed in this study differ from most studies because of the construction of wide trenches downslope through the rock pile as regrading progressed (McLemore et al, 2005(McLemore et al, , 2006.…”

“…The first step in assessing the long-term stability of the Questa mine rock piles is to collect and compile data from the physical, mineralogical, chemical, and geotechnical characterization of samples of earth materials contained in approximately 298 million metric tons of rock present in nine rock piles at the mine site. McLemore et al (2005McLemore et al ( , 2006 addressed the mapping, sampling, and preliminary laboratory results of trenching activities of GHN rock pile in 2004GHN rock pile in -2005. This report presents new petrographic, mineralogical, and chemical results.…”

“…The mine began production in 1919 from underground workings. Open-pit removal of overburden began in 1964; ore production from the open pit began in 1965 and ended in 1982, at which time approximately 73.5 million metric tons of ore with an average grade of 0.185% MoS 2 had been processed (Schilling, 1960(Schilling, , 1990Ross et al, 2002;McLemore and Mullen, 2004). Underground mining resumed in 1982 and used block caving mining methods to extract the deeper molybdenum ore. During the open-pit period of mining, approximately 317.5 million metric tons of overburden rock were stripped and deposited onto mountain slopes and into tributary valleys, thereby forming the rock piles examined in this study (URS Corporation, 2000).…”

“…The Questa rock piles were constructed primarily by haul truck end-dumping in high, single lifts, which involved the dumping of rock over the edge of the hill slopes (URS Corporation, 2000;McLemore et al, 2005McLemore et al, , 2006. End-dumping generally results in the segregation of materials with the finer-grained material at the top and coarser-grained material at the base (Fig.…”