FRACTURE-LINING MANGANESE OXIDE MINERALS IN SILICIC TUFF, YUCCA MOUNTAIN, NEVADA, USA

Yucca Mountain, Nevada, is currently under investigation to determine its suitability as a potential host for the first U.S. high-level nuclear waste repository. Yucca Mountain is composed of a thick sequence of silicic volcanic tuffs and subordinate lavas which in some areas have been extensively altered to zeolite minerals. Since fractures may act as transport pathways or conduits for fluids moving through and away from a potential repository, the nature of the minerals lining the walls of fractures is of great importance. Mn-oxide minerals in particular are important since they show strong retardation capabilities for transuranic elements such as U, Np, Pu and Am. This study focused on the mineralogy, chemistry and distribution of the Mn-oxides at Yucca Mountain using X-ray diffraction and optical and electron microscopy. The major Mn-oxide minerals identified were aurorite, cryptomelane/hollandite/coronadite, lithiophorite, pyrolusite, rancieite and todorokite. Mn-oxide mineralogy appears to be controlled both by stratigraphy and by position relative to the static water level (SWL). For example, rancieite was found predominantly in the Paintbrush Tuff both above and below the SWL. Aurorite, pyrolusite, todorokite and the cryptometane group minerals, although found in several stratigraphic units, occur only below the SWL. Additionally, mineral chemistry varies widely over short distances, suggesting that the chemical composition of the minerals must have been strongly influenced by local variations in water chemistry. Chemical analyses of the cryptomelane group minerals suggest the existence of al least partial solid solution between hollandite (Ba) and the isostructural minerals cryptomelane (K) and coronadite (Pb).