Preliminary Status Report on Molycorp Goathill North Trenches, Questa, New Mexico

Abstract

Rarely do rock pile characterization methods allow for examination and sampling of the undisturbed interior of large rock piles in-situ. The regrading of the Goathill North (GHN) rock pile at the Molycorp Questa mine, New Mexico provided a unique opportunity to examine and sample the undisturbed, interior of a large rock pile through the construction of trenches cut into the rock pile as earth-moving progressed. Weekly during the regrading of the GHN rock pile, contractors excavated a trench to allow for sampling of rock pile material. Trenches typically had four benches, which were 1.5 m wide and did not exceed 1.2 m in height, to give an overall slope of 1.4 horizontal to 1.0 vertical within the trench. Each trench extended for a length sufficient to explore site conditions, maintain the regraded 2:1 slope, and ensure personnel safety. Once excavated, trench walls and benches were surveyed using a differential global positioning system. For every trench, maps and logs of each bench were created to describe the different "mine soil" units, including the thickness, dip and extent of the units. Units were defined based on color, grain size, 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 series of successively excavated trenches. The field sampling crew began sampling within each of the identified units after the unit boundaries were identified and mapped. The following in situ measurements were taken along either the horizontal or vertical surfaces of each exposed bench and along the base of the trench: sand cone (density), tensiometer (matric suction), gravimetric moisture content, grain size, infiltration, and nuclear gauge measurements (density, moisture content). Gravimetric water content samples were collected at the locations selected for the measurement of matric suction and infiltration tests. Samples were collected from each defined unit for geochemical, geotechnical (including shear box tests, slake durability tests), biological, and electron microprobe analyses. Channel sampling for pyrite reserve modeling was performed in short 1.5-m long horizontal slots using a rock hammer to chip material to be placed into a sample bag. Additional material from selected layers was collected for potential weathering-cell tests in the future. Typically, paste pH increases with distance from the outer, oxidized zone (west) towards the interior, unoxidized zone (east) of the GHN rock pile.

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

“…See McLemore et al (2005) for detailed explanation of zones. ................................. 10 FIGURE 4.…”

“…End dumping results in five physical zones of segregation (Fig. 3;Nichols, 1987;Quine, 1991;Morin et al, 1991Morin et al, , 1997Smith and Beckie, 2003;McLemore et al, 2005;Azam et al, 2006):…”

“…Conceptual model of the particle-size distribution of a rock pile constructed by end dumping over the crest of a natural slope of a hill, similar to the construction of GHN and many rock piles in the world (from field studies at GHN and Nichols, 1987;Quine, 1991;Morin et al, 1991Morin et al, , 1997Smith and Beckie, 2003;Azam et al, 2006). See McLemore et al (2005) for detailed explanation of zones.…”

“…Furthermore, the behavior of major elements and variations in paste pH and LOI (loss on ignition) within a soil profile and within the Questa rock piles can not be explained solely on the basis of weathering. In the Questa rock piles, variations in pre-mining lithology and hydrothermal alteration are important and must be quantified (McLemore et al, 2005(McLemore et al, , 2006a. The type and abundance of clay minerals within the Questa rock piles, unlike those within a natural weathered soil profile, are likely a result of pre-mining hydrothermal alteration and not necessarily a result of post-mining weathering (McLemore et al, 2006b;Donahue et al, 2007).…”

Lithologic atlas for the Questa Mine, Taos County, New Mexico

“…These faults and fractures not only provide pathways for modern fluids (Caine, 2006), but also provide for angular rock fragments when the material is blasted. These angular rock fragments when found in the rock pile provide for stronger material (McLemore et al, 2009a).…”