Characterization of Goathill North Rock Pile, New Mexico

Abstract

Nineteen trenches were constructed in the Goathill North (GHN) rock pile as it was regraded in [2004][2005]. The regrading of GHN provided a rare opportunity to examine, map, and sample the interior of this rock pile. The GHN rock pile is stratified and the strata can be correlated down slope through the rock pile. Rock fragment lithology is generally consistent within mapped units and correlates well with mineralogy and chemistry. Detailed geologic mapping and sampling revealed mappable geologic units in the rock pile, which were correlated between benches and to opposite sides of each trench, and units were correlated down slope through the excavated trenches. Units were defined on the basis of particle size, color, texture, stratigraphic position, lithology, and other physical properties that could be determined in the field. Eighteen geologic units were differentiated, described, and sampled. The units also could be distinguished within the adjacent drill holes.Weathering is the set of physical and chemical changes, up to and including disintegration of rock by physical, chemical, and/or biological processes occurring at or near the earth's surface (e.g. in the vadose zone at temperatures less than or equal to approximately 70°C) that result in reductions of grain size, changes in cohesion or cementation, and changes in mineralogical composition. In this operational definition, there is no clear distinction between weathering and diagenesis, although the coarse-grained and subaerial nature of the mined rock pile restricts the nature of "diagenetic" reactions compared to, for example, those that might occur in a submarine, saturated system. From this mapping and analyses of samples, four different zones of weathering can be distinguished at GHN:• outer oxidized zone (included the surface, unit C, and geologic units I and J)• intermediate zone (included unit N)• inner, less oxidized, unweathered zone (included units K-W)• basal oxidized zone (included geologic units R and rubble zone). The regrading of GHN provided a rare opportunity to examine, map, and sample the interior of this rock pile. The GHN rock pile is stratified and the strata can be correlated down slope through the rock pile. Rock fragment lithology is generally consistent within mapped units and correlates well with mineralogy and chemistry. The majority of geochemical activity appears to be confined to the interface regions of the rock pile, but not all of these regions exhibit weathering effects. The areas most affected by weathering processes are the 1) surfaceatmosphere, 2) bedrock-rock pile (rubble zone) areas, and 3) interfaces between fine-and coarsegrained layers in the interior zones where water and/or humid air flow and oxidize the rock pile material. Weathering generally decreased from the outer edge of the rock pile to its interior and then increased toward the base of the rock pile. Where water and oxygen are available, pyrite and calcite are being dissolved by weathering to form gypsum, jarosite and other soluble Fe salts. Oxygen an...