The effect of weathering on the acid-producing potential of the Goathill North Rock Pile, Questa mine, NM

摘要

The Goathill North (GHN) rock pile was constructed in 1964-74 and re-graded in 2004-05. GHN samples were subjected to static, petrographic, and chemical tests to characterize acidity and future acid-producing potential (AP). Samples that have higher concentrations of pyrite are more likely to have a higher AP. Static tests indicated that, although the rock-pile material contains acid consuming minerals, acid generation has occurred. Samples with rhyolite rock fragments have lower NP (neutralization potential) then samples with andesite rock fragments. Samples with rhyolite rock fragments had undergone hydrothermal quartz-sericite-pyrite (QSP) alteration, whereas samples with andesite rock fragments were propylitically altered; these hydrothermal alterations occurred after the molybdenum mineralization, but prior to mining. No single component controls the ABA (acid-base accounting) and NAG (net acid generation) tests. This lack of correlation between ABA and NAG tests, mineralogy, and chemistry is a result of 1) weathering reactions in the soil matrix producing precipitation of coatings surrounding the rock fragments and preventing further weathering of the rock fragments, especially around pyrite crystals, 2) a lack of the water available that is required for weathering in the 25-40 years since the formation of GHN rock pile, 3) non-uniform weathering of pyrite within the rock pile due to heterogeneous air flow, and 4) little or no weathering of the rock fragments. Also, the AP capacity of the rock-pile is determined by the combination of K-feldspar, calcite, smectite, illite, and pyrite, which are controlled in part by the lithology, pre-mining hydrothermal alteration, and post-mining weathering. The effects of pre-mining hydrothermal alteration and post-mining weathering both affect the static tests, emphasizing the need to perform detailed petrographic and mineralogic investigations along with the static tests to determine the AP of any mine waste material.