Rapid natural acid weathering, physical erosion, and debris-flow hazards in scar areas developed on hydrothermally-altered rocks along the Red River Valley near Questa, New Mexico, USA

Prominent erosional scars on hillslopes along the Red River Valley have steep, denuded slopes and associated down-gradient debris fans. The scars developed naturally on stockwork-veined, quartz-sericite-pyrite （QSP）-altered volcanic rocks proximal to porphyry-Mo deposits, including those mined at the Molycorp＇s Questa mine. Downcutting of the Red River, following development of the Rio Grande rift to the west, initiated scar formation. Mineralized rocks in the scars were decomposed chemically （by pyrite oxidation and acid weathering） and physically （by freeze-thaw action and volume expansion from secondary gypsum formation） to produce a weathered veneer of sand- to pebble-size fragments of unweathered rock in a matrix of secondary gypsum, clays, and jarosite. Unoxidized QSP-altered bedrock crops out in steep scar gully bottoms, indicating that the weathered veneer is at most 3-30 m thick. Acidic （pH 2.6 to 4.4）, metal-rich waters develop in the weathered veneer and near-surface bedrock of the scars. Carbonates in the unweathered, QSP-altered bedrock are sufficient to neutralize acid in deeper ground waters, which have near-neutral pH （5.7 to 7.6） and lower metal contents. The scars grow along steep headwalls by slope failure of the weathered veneer.

Rapid natural acid weathering, physical erosion, and debris-flow hazards in scar areas developed on hydrothermally-altered rocks along the Red River Valley near Questa, New Mexico, USA