CHARACTERIZATION OF ACID DRAINAGE POTENTIAL FROM THE GOATHILL NORTH ROCK PILE OF THE QUESTA MOLYBDENUM MINE, QUESTA, NEW MEXCIO – INITIAL RESULTS

Characterization of the Goathill North mine rock pile is important to determining the extent to which acid drainage can be produced in the rock pile. This, in turn, contributes to determining the place and rate at which chemical weathering can occur within the pile. Samples of rock pile material were collected both from trenches that were dug in the Goathill North rock pile during its deconstruction and from boreholes drilled into the pile before deconstruction. Static tests: paste pH, acid-base accounting (ABA) and net acid generation (NAG), were performed on the samples. The paste pH tests were done to determine the present degree of acidity in the soil. The ABA and NAG tests were performed to predict the capacity of the soils to produce and /or neutralize acid.

Initial results indicate that the pH of the pile increases from about 2.2 at the outermost parts of the pile to about 5.7 at the interior of pile. Both the net neutralization potential and the ratio of neutralization potential to acid potential are lowest outside and highest inside the rock pile. In contrast, the net acid generation is highest outside and lowest inside the pile. These results indicate that there could be a higher proportion of acid-producing minerals such as pyrite and/or already-produced acid in the materials at the edge of the pile than in the materials within the pile. Alternatively, there could be a higher proportion of acid-consuming minerals such as carbonates and silicates within the pile than at its edges. These results can be attributed to a number of factors: the different lithologies of the stratigraphic units that comprise the rock pile; the amount of weathering and leaching that had occurred before the material was mined and dumped (deposited) onto the pile; and the difference between factors, such as oxygen concentration, moisture content and presence of bacteria, that influence weathering at the outside and inside of the rock pile.