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95-289, "Origins of Secondary Silica with Yucca Mountain, Nye

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95-289, ORIGINS OF SECONDARY SILICA WITHIN YUCCA MOUNTAIN, NYE COUNTY, SOUTHWESTERN NEVADA by Richard J. Moscati and Joseph F. Whelan U.S. GEOLOGICAL SURVEY Open-File Report 95-289 Prepared in cooperation with the NEVADA OPERATIONS OFFICE, U.S. DEPARTMENT OF ENERGY, under Interagency Agreement DE-AI08-92NV1 0874 Denver, Colorado 1996 CONTENTS A b stract ................................................................................................................................................................................. ..........................................I Introduction ......................................... ,...............,......................................,.................................;...................................... ....... I A cknow ledgm ents ................................................................................................................................................................. ............................................I Geologic setting .............................................................................................................................. 3 Silica and calcite petrology ................................................................................................................................................... ..........................................3 Analytical methods ........................................................................................................................... 5 Discussion of origins of silica ............................................................................................................................................... ..........................................5 Pedogenic silica ........................................................................................................................................................... ............................................9 Unsaturated zone silica ................................................................................................................... 9 "Hydrothermal" quartz ................................................................................................................................................ I..........................................11 Conclusions ........................................................................................................................................................................... ............................................12 Selected references................................................................................................................................................................ ............................................13 FIGURES 1. Maps showing location of Yucca Mountain region and potential repository site, sampled boreholes, Trench 14, and the Bullfrog Mine ........................................................................................................ 2 2. Approximately north-south. generalized cross section of Yucca Mountain showing geology, water table, and boreholes ............................................................................................................................................... ............................................4 3-7. Graphs showing: 3. Delta 13C (crosses) and o818 (circles) values of secondary calcite from all boreholes with respect to location of the water table .................................................................................................. ....................................7 4. Variationpetro.ogy38 in 1000 ISn (mineral-water) with temperature for 180 in silica and calcite 5. Delta 180 values of pedogenic and borehol secondary silica samples 10 6. Calculated temperatures, assuming 5180 water = -12.59ce, of Trench 14 soil zone opals (diamonds), unsaturated zone opals (circles), and unsaturated zone calcite (crosses) ............................................................... 11 7. Delta 1e0 values of the relatively lighter Bullfrog Mine and saturated zone silica samples relative to those in Trench 14 and the unsaturated zone .......................................................... 12 TABLE 1. Secondary silicaand calciteoo 80 values frompedogenic unsaturated zone,saturated zone,oand Bullfrog xi n eat and nearsamples Yucca Mountain, Nevada ...................................... 6 CONTENTS III Origins of Secondary Silica Within Yucca Mountain, Nye County, Southwestern Nevada; . ByRichard J. Moscati andJoseph F. Whelan Abstract conditions is being tested through stable oxygen- isotope studies of lithophysal tridymite/quartz The accuracy of predictions of the hydro- mixtures. Should this hypothesis be verified, min- logic response of Yucca Mountain to future cli- eralogic analysis by X-ray diffraction theoretically mate depends largely on how well relations would permit reconstruction of past maximum between past climate and hydrology can be water-table elevations. resolved. To advance this reconstruction, second- ary minerals in and near Yucca Mountain, depos- INTRODUCTION ited by ground waters that originated both as surficial recharge at Yucca Mountain and from Yucca Mountain is being evaluated by the regional aquifers, are being studied to determine U.S. Department of Energy for suitability as a potential past ground-water sources and chemistries. high-level nuclear-waste repository. This report pre- sents a summary of preliminary studies by the Preliminary data on stable oxygen isotopes U.S. Geological Survey (USGS) of the stable isotopic indicate that, although silica (opal, quartz, and compositions and formation temperatures of secondary chalcedony) and calcite and have formed in simi- silica mineralization from pedogenic, unsaturated zone lar settings and from somewhat similar fluids, we (UZ), saturated zone (SZ), and moderate-temperature have found no compelling evidence of coprecipi- hydrothermal settings in and near Yucca Mountain in tation or formation from identical fluids. If veri- Nye County, southwestern Nevada (fig. 1). Stable- fied by further analyses, this precludes the use of isotope analyses of secondary silica and coexisting cal- silica-calcite mineral pairs for precise geother- cite, recovered from near-surface pedogenic deposits, mometry. The preliminary data also indicate that fracture coatings, lithophysal cavity fillings, and veins, opal and calcite occurrences in pedogenic and can provide a record of the temperature and sources of unsaturated-zone settings are invariably compati- ground water within the mountain. In particular, where ble with formation under modem ambient surface stable-isotope studies indicate that the silica precipi- tated in equilibrium with another phase, such as calcite, or subsurface temperatures. Pedogenic calcite and both the temperature of formation and the isotopic opal appear to require somewhat different fluid composition of the fluid, hence its hydrologic history, chemistries and these differences likely reflect dif- can be uniquely inferred. In addition, these studies ferent climate states. Opal may, therefore, provide may also provide tools for determining past water-table an easily datable (Uranium/IThorium method) sec- elevations and for reconstructing past hydrology and ondary phase in paleosols and in the unsaturated climate. Herein, the term silica is used informally to zone that records major climate transitions. refer to that group of minerals and mineraloids contain- Silica and calcite stable-isotope studies are ing only SiO2. being integrated with soil geochemical modeling. This modeling will define the soil geochemical ACKNOWLEDGMENTS condition (climate) leading to opal or calcite dep- osition and to the transfer functions that may apply NVe would like to thank Schbn Levy (Los Alamos at the meteoric soil unsaturated-zone interfaces. National Laboratory) for a careful and critical review Additional study of pedogenic and unsaturated- o.thih; manuscript. Silica stable isotopic analyses were provided by Carol Gent (U.S. Geological Survey, zone silica is needed to support these models. Denver, Colorado). Thanks to E.A. McConnaughey The hypothesis that the transformation of (Science Applications International Corporation, vapor-phase tridymite to quartz requires saturated Denver, Colorado) for valuable discussions, and to Abstract 1 37' <,* .SA/\ BULLFO MINE DEATH VA O1- ecr j~\ \ ) ' /Arnargosa - ,~~~~~~~~~~ 1062MILE I~~~~~ S 10 15 KILOMETERS 3605Z W [-2 1 36'52' Boc 116 EPLNAIO Z Trench 14j aitbus Gou 36s~~~~e 0 V 0 Cz ~~~~~~~~Alluvium and colluvium Pg uswX W e s ? f ¢ IJm4 TimberlMountain Group t 31G 9749Si \ %t i 9 ~~UE-25 a#5* Borehole location and _t Lx= number o 1 2KILC6.1ETERS I I 0 1 2 MILES Figure 1. Location of Yucca Mountain region and potential repository site, sampled boreholes, Trench 14, and the BulTfrog Mine. 2 Origins of Secondary Sillca Within Yucca Mountain, Nye County, Southwestern Nevada David Vaniman (Los Alamos National Laboratory) for zone (SZ) and moderate-temperature hydrothermal providing pure separates of opal from Trench 14 sam- deposits from the Bullfrog Mine (near Beatty, Nevada, ples. fig. 1). The pedogenic calcite from Trench 14 is com- GEOLOGIC SETTING monly micritic. It forms displacive intergranular Yucca Mountain is located within the southwest- cements, laminated soil horizons and fracture fillings, ern Nevada volcanic field (Christiansen and others, and coatings on colluvial cobbles and pebbles within 2 the thin soils around Yucca Mountain. Opaline silica,
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