STRATIGRAPHY AND WEATHERING HISTORY OF THE GOATHILL NORTH MINE ROCK PILE, QUESTA MOLYBDENUM MINE, NEW MEXICO - SOME INITIAL RESULTS

At the Questa mine, New Mexico, geologic units exposed in trenches dug during the deconstruction of the Goathill North rock pile were mapped primarily by color, stratigraphic position, and grain size. It is important to understand whether these variations are due to differences in the original material used to construct the rock pile or due to superimposed mineralogic and geochemical changes that have occurred since the rock pile was deposited. A series of mine soil samples, which consist of loosely consolidated rock fragments and finer-grained matrix, were collected along a transect that extended from the outer margin of a rock pile trench toward the center and deepest part of the rock pile. Eight different geologic units were mapped along this transect. It is possible to recognize some mapped units based on their rock fragment lithology (primarily andesite and rhyolitic tuff) and degree of alteration, both of which are characteristics inherent to rock pile material. This finding suggests that the observed zonation can in part result from variations in the original material used to construct the rock pile.

In addition, authigenic gypsum and carbonate-mineral abundances can be used to study superimposed mineralogic and geochemical changes. Stable isotope studies demonstrate that clear, prismatic gypsum grains are authigenic and indicate that weathering of pyrite occurred after the rock pile material was deposited. The abundance of authigenic gypsum is highest in samples near the surface of the rock pile and exhibits a general decreasing trend inward. Conversely, carbonate-mineral abundance increases toward the inside of the rock pile. These results confirm that weathering intensity is greatest on the outer margin of the rock pile and imply that authigenic gypsum forms at the expense of carbonate. Furthermore, samples with abundant carbonate (n=4) have paste pH values between 8.4 and 9.6, whereas those with no detectable carbonate (n=7) have paste pH values between 2.2 and 3.7. This indicates that carbonate at least partially controls pH by neutralization of produced acid.