Open-file Report - 545
Questa Rock Pile Weathering and Stablity Project: The effect of particle size fractions on chemistry, mineralogy, and acid potential of the Questa Rock piles, Taos County, New Mexico

Understanding the effect of mineralogy, chemistry, and acid potential of different particle size fractions is important to understand mobility and availability of minerals to dissolution and oxidation during weathering. The purpose of this report is to describe the differences in mineralogy, chemistry, and acid potential between different particle size fractions of samples from the Questa rock piles, debris flows, and alteration scars. There were differences between the particle size fractions. The finer-size fraction, typically <25% of the sample, generally is higher in trace elements and certain minerals than the coarse-size fraction. Total clay minerals, gypsum, and jarosite are higher in the finer-size fractions. In some samples, calcite and pyrite decreases in the finer-size fractions. The finer-size fractions were more acid producing than the coarser-size fractions. The finersize fractions had higher net acid producing potential (NAPP) and lower paste pH. Conversely, the coarser-size fractions had lower NAPP. For some samples, paste pH values decreases from the coarse-size fraction to the fine-size fraction, but not all samples show a large decrease. There was not a consistent trend in the chemistry and mineralogy between size fractions for all the samples observed. Al2O₃ shows little change with different size fractions. For most of the samples, concentrations of feldspar, pyrite, Na2O and K2O decreased from coarse to fine fractions, while the concentration of FeO, CaO, and SO₄ and total sulfates (gypsum + jarosite) increased with decreasing size fraction for most samples. Silicate dissolution can be recognized from coarse- to fine-size particles, as indicated by the decrease in feldspar, Na2O and K2O from coarse- to fine-size fractions. Similar trends were found by Graf (2008) in the Hansen alteration scar. Collectively, these results are consistent with weathering being more pronounced in the fine-size fraction than the coarse-size fraction. The dissolution of pyrite, calcite, and to a lesser extent some combination of chlorite, illite, feldspars, smectite, and other silicate minerals are the predominant chemical reactions that results in the precipitation of gypsum, jarosite, soluble efflorescent salts, and Fe oxide/hydroxide minerals, mostly in the fine-size fractions, although some authigenic gypsum crystals can be quite large. No single process is responsible for the differences in composition between size fractions. The effects of primary igneous crystallization, pre-mining hydrothermal alteration, and post-mining emplacement into the rock pile and subsequent weathering affect the composition of each size fraction. This emphasizes the need to determine not only the composition of the different size fractions, but also perform detailed mineralogy and petrology investigations to understand the processes involved in controlling the compositional differences between size fractions. Although the results are generally consistent, there still is some variation between samples and it is recommended that composition of different particle sizes be examined at other sites.

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