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

Abstract

Rarely do rock pile characterization studies allow for petrographic, mineralogical, and geochemical characterization of the undisturbed interior of large rock piles in-situ. The regrading of the stable portion of Goathill North (GHN) rock pile at the Molycorp Questa molybdenum mine, New Mexico, provided an opportunity to examine and sample rock pile material in-situ through the construction of trenches cut into the rock pile as earth-moving progressed. Maps of each bench were created to describe the different stratigraphic units, including the thickness, dip, and extent of the units. Units were defined based on grain size, color, texture, stratigraphic position, and other physical properties that could be determined in the field. Units were correlated between benches and to both sides of each trench, and several units were correlated downward through the excavated trenches in the rock pile. Characterization of original rock pile material was accomplished by petrographic, mineralogic, and chemical analysis and includes descriptions of rock fragment lithology, mineralogy, texture, and alteration type and intensity. Rock fragment lithology is generally consistent within mapped units and correlates well with mineralogy and chemistry. Typically, paste pH and carbonate abundance increases with distance from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile. Conversely, authigenic gypsum, which is likely a product of in-situ weathering, usually exhibits a decrease in abundance from the outer to the inner portion of the rock pile. CaO and Sr also typically increase in concentration from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile, suggesting dissolution of feldspars and calcite to form gypsum in the outer, oxidized portions of the rock pile. Clay mineral and electron microprobe analyses of soil samples and unweathered drill core imply that the majority of clay minerals in the GHN samples are derived from the original premining hydrothermal alteration and not by oxidation of minerals within the rock pile.

Geology, mineral resources, and geoarchaeology of the Montoya Butte quadrangle, including the Ojo Caliente No. 2 mining district, Socorro County, New Mexico

“…Selected samples collected for this project were analyzed for major and trace elements by X-ray fluorescence (XRF) at the Washington State University; selected samples were analyzed for Be, Li, and U by induced coupled plasma spectroscopy (ICP) at the NMBGMR chemistry laboratory (Appendices 3, 8). Laboratory methods and analytical precision are described in McLemore and Frey (2009) and Johnson et al (1999).…”

“…Samples of clay materials also were analyzed by instrumental neutron activation analysis (INNA). Clay mineralogy was determined by X-ray Diffraction methods (XRD) using the procedure by Hall (2004;McLemore and Frey, 2009;Appendix 5). Age determinations of three rock samples from the Montoya Butte quadrangle were determined by 40 Ar/ 39 Ar (Appendix 6); laboratory methods are described by McLemore et al (1995) and http://geoinfo.nmt.edu/labs/argon/home.html.…”

Literature review of other rock piles: Characterization, weathering, and stability

“…The upper portion of the Questa rock piles tends to be more soil-like (matrix-supported), whereas the lower portions tend to be rock-like (cobble-supported). The underlying base of the Questa rock piles is coarse rock and typically cobble supported (McLemore et al, 2008c). The resulting layers are locally at or near the angle of repose and subparallel to the original slope angle that was very steep.…”

Bulking and Subsequent Self-Weight and Saturation Settlements, and Geotechnical Stability of High Coal Mine Spoil Piles

“…According to Williams and Zou (1991), density and strength of the spoil are the two most critical properties of the spoil from a stability perspective. McLemore et al (2009) somewhat agreed, suggesting that one of the primary geotechnical parameters required for predicting slope stability is shear strength.…”

“…Only a handful of characterisation programmes have been reported in the literature. Table 5-1 compares some characteristics of coal mine spoil from this study, against those reported in the literature (Egretli & Singh 1988;Filipowicz & Borys 2007;Little 2008;McLemore et al 2009). The specific gravity ranges obtained from this study compare well with Fityus, Robertson and Simmons (2014) for a 'typical Australian mine spoil' of 2.59 t/m 3 .…”