New Mexico Geological Society Annual Spring Meeting — Abstracts

Petrographic examination of a series of soil samples collected during deconstruction of a rock pile at the Questa Mine reveals differences in characteristics such as color and minerology. Twenty-one soil samples were collected at five foot intervals along one bench. This traverse crosses eight different mappable units, defined primarily by color, stratigraphic position, and grain size. Two of these units are classified as oxidized, one is intermediate, and five are unoxidized. The color of secondary iron oxide minerals is variable among the twenty-one samples and includes black, orange, red, yellow, and brown material.

Gypsum is present in all of the soils and occurs in different habitats. Stable isotope studies demonstrate that this petrographic observation can be used to distinguish gypsum as either detrital or authigenic. A preponderance of clear, prismatic, authigenic gypsum grains is an indicator of sulfide weathering after the material was emplaced in the mine rock pile.

Variations in the abundance of carbonate in the soil samples exist and preliminary data suggest a positive correlation between carbonate abundance and paste pH values. Seven samples contain no detectable carbonate and have paste pH values between 2.2 and 3.7, whereas four samples contain abundant carbonate and have paste pH values between 8.4 and 9.6. The remaining ten samples contain trace in minor amounts of carbonate and have paste pH values ranging from 3.3 to 9.6. All four samples with abundant carbonate are from unoxidized units.