LEACHING CHARACTERISTICS OF COMPOSITED MATERIALS FROM MINE WASTE-ROCK PILES AND NATURALLY ALTERED AREAS NEAR QUESTA, NEW MEXICO

Leaching studies provide insights into the relative contributions of natural erosional scar areas and mine waste-rock piles to ground- and surface-water composition in mineralized areas near Questa, New Mexico. The scars are developed in acid-generating pyrite-rich rocks, which are also present in some waste-rock piles. However, other waste-rock piles contain less pyrite and greater amounts of carbonate, reflecting an origin deeper in the mineralized systems. Surficial <2 mm size-fraction composite samples from five waste-rock piles and nine scar areas were subjected to two sequential leaching procedures. The first was a short-duration (5-min shaking and 10-min settling) deionized water leach intended to leach readily soluble materials. This was immediately followed by an 18-hour end-over-end rotation deionized water leaching procedure on the same sample. All the scar leachates exhibited low pH (pH < 4.1), but concentrations of leachable metals varied significantly between the scar areas sampled. Under these low-pH conditions, many metals can be mobilized and remain in solution; however, anionic species (such as Mo) tend to be less mobile under acidic conditions. All the waste-rock leachates were at least mildly acidic, and some exhibited increasing pH with increasing leaching time and agitation, which indicates that some pH buffering capacity exists in some of the waste-rock piles. Metal concentrations in the waste-rock leachates generally do not exceed the upper range of concentrations of the same metals in the scar leachates. One notable exception is Mo, which is significantly higher in the waste-rock leachates compared with the scar leachates. The pH values in the waste-rock leachates span the pH range for some pH-dependent solubility and metal-attenuation reactions. Hence, as pH increased in the waste-rock leachates, concentrations of several metals decreased with increasing time and agitation. Similar pH-dependent reactions may take place upon migration of the leachates in the mine waste-rock piles.