OVERVIEW OF CALCITE OPAL DEPOSITS AT OR NEAR THE PROPOSED HIGH-LEVEL NUCLEAR WASTE SITE, YUCCA MOUNTAIN, NEVADA, USA - PEDOGENIC, HYPOGENE,OR BOTH/

Calcite/opal deposits (GOD) at Yucca Mountain were studied with respec t to their regional and field geology, petrology and petrography, chem istry and isotopic geochemistry, and fluid inclusions. They were also compared with true pedogenic deposits (TPD), groundwater spring deposi ts (GSD), and calcite vein deposits (CVD) in the subsurface. Some of t he data are equivocal and can support either a hypogene or pedogenic o rigin for these deposits. However, Sr-, C-, and O-isotope, fluid inclu sion, and other data favor a hypogene interpretation. A hypothesis tha t may account for all currently available data is that the COD precipi tated from warm, CO2-rich water that episodically upwelled along fault s during the Pleistocene, and which, upon reaching the surface, flowed downslope within existing alluvial, colluvial, eluvial, or soil depos its. Being formed near, or on, the topographic surface, the COD acquir ed characteristics of pedogenic deposits. This subject relates to the suitability of Yucca Mountain as a high-level nuclear waste site.