Mineralogical, Chemical, and Isotopic Characterization of Fracture-Coating Minerals in Borehole Samples from Western Pahute Mesa and Oasis Valley, Nevada

Benedict, F C ; Rose, T P ; Zhou, X

Lawrence Livermore National Laboratory

关键词: Calderas;    Hydrothermal Alteration;    Coatings;    Boreholes;    Rare Earths;   

DOI  :  10.2172/15006451
RP-ID  :  UCRL-ID-152919
RP-ID  :  W-7405-ENG-48
RP-ID  :  15006451

美国|英语

来源: UNT Digital Library

【 摘 要 】

This report summarizes the results of a mineralogical and geochemical investigation of fracture-coating phases obtained from archived borehole core and cuttings samples from the western Pahute Mesa-Oasis Valley region. The objective is to provide data needed to validate UGTA flow and transport models for this region. Fracture-lining minerals were characterized using micrographic techniques (SEM-EDS), and selected calcite samples were analyzed for their stable isotope ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) and rare earth element (REE) abundances. The main conclusions are as follows: (1) The distribution of fracture-lining mineral phases is a function of primary rock type, the style and degree of syn-depositional alteration, effects of post-depositional hydrothermal alteration, and fracture location relative to recharge waters (in the unsaturated zone) or through going groundwater (in the saturated zone). (2) Fracture-lining minerals within the welded tuff aquifers (principally the Timber Mountain and Paintbrush Tuffs) are characterized by the assemblage calcite + chalcedony + Fe- and Mn-oxyhydroxides + mixed illite/smectite (in approximate decreasing order of abundance). The predominant mode of host rock alteration is quartzofeldspathic. (3) Interbedded rhyolitic lava flow aquifers are characterized by the fracture-lining assemblage chalcedony + mixed illite/smectite + Fe- and Mn-oxyhydroxides {+-} calcite {+-} quartz {+-} K-feldspar (in approximate decreasing order of abundance). These include lava flow aquifers from the Thirsty Canyon, Beatty Wash, Paintbrush, and Quartz Mountain groups. The predominant mode of host rock alteration is quartzofeldspathic. (4) Fracture-lining zeolite minerals are abundant only within one of the basaltic lava flow aquifers (Trachyte of Ribbon Cliff) where they occur with chalcedony + calcite + clay minerals. (5) Stable isotope analyses ({sup 13}C/{sup 12}C and {sup 18}O/{sup 16}O) of secondary calcite samples were used to deduce the origin and temperature of formation of the calcite. These data are also useful for constraining the geochemical evolution of dissolved inorganic carbon in groundwater flowpath models. Two general types of secondary calcite are recognized on the basis of temperature conditions. (6) Low- to moderate-temperature calcite (<45 C) was deposited from local recharge or from ambient regional groundwater flow. It contains carbon from either pedogenic (soil zone) or carbonate bedrock sources, and occurs at depths up to {approx}2200 ft. It commonly forms transparent, euhedral crystals that may reflect recent precipitation from groundwater, especially in ER-EC-4 and -7. (7) High-temperature calcite ({approx}50 to 116 C) was deposited from groundwater that previously equilibrated with carbonate bedrock, and is observed at depths greater than {approx}1900 ft. It typically occurs as dense, opaque crystalline veins. It is most prevalent in wells located within the moat of the Timber Mountain caldera (particularly ER-EC-1, -5 and -6), and was probably deposited under hydrothermal conditions following earlier periods of volcanic activity. (8) Variations in REE patterns for secondary calcite reflect the influence of groundwater chemistry, host rock chemistry, the carbonate source, and the physiochemical conditions of deposition. The partitioning of LREE relative to HREE is influenced by water chemistry (e.g. complexing with HCO{sub 3}{sup -} and SO{sub 4}{sup 2-} ions) and by the preferential substitution of LREE for Ca{sup 2+} in the calcite crystal lattice. (9) Changes in the oxidation state of Ce and Eu are reflected as anomalies in chondrite- and shale-normalized REE plots, and are useful indicators of redox conditions during calcite formation. Strong negative Ce and Eu anomalies are most pronounced in samples from relatively shallow depths, but are not observed in all ''shallow'' samples. In general, comparative studies of REE patterns in secondary calcite with REE patterns in groundwater are needed to determine the applicability of these data for constraining reactive flow and transport models.