CONTRASTING STABLE-ISOTOPE BEHAVIOR BETWEEN CALCITE AND DOLOMITE MARBLES, LONE-MOUNTAIN, NEVADA

Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events including: (1 ) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated wi th extension, uplift and intrusion of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isot opic compositions that can be divided into two groups, one with positi ve delta(13)C values from + 3.1 to + 1.4 parts per thousand (PDB) and high delta(18)O values from + 21.5 to + 15.8 parts per thousand (SMOW) , and the other with negative delta(13)C values from -3.3 to -3.6 part s per thousand loo and low delta(18)O values from + 16.9 to + 11.1 par ts per thousand. Marbles also contain minor amounts of quartz, muscovi te and phlogopite. Brown and blue luminescent, clear, smooth textured quartz grains from orange luminescent calcite marbles have high delta( 18)O values from + 23.9 to + 18.1 parts per thousand, while brown lumi nescent, opaque, rough textured quartz grains from red luminescent dol omite marbles typically have low delta(18)O values from + 2.0 to + 9.3 parts per thousand. The delta(18)O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks: with values b etween + 10.4 and + 14.4 parts per thousand, whereas mica delta D valu es are very depleted, varying from - 102 to - 156 parts per thousand. No significant lowering of the delta(18)O values of Lone Mountain carb onates is inferred to have occurred during metamorphism as a result of devolatilization reactions because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and quartz overgrowths in dolomite marbles, low delta D values of micas, negative delta(13)C values and low delta(18)O values of cal cite and dolomite, and depleted delta(18)O values of quartz from dolom ite marbles all indicate that meteoric fluids interacted with Lone Mou ntain marbles during the Tertiary, Partial oxygen isotopic exchange be tween calcite and low O-18 meteoric fluids lowered the delta(18)O valu es of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite fractionation s significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration, Partial oxygen isotop ic exchange between quartz and meteoric fluids also resulted in O-18 d epletion of quartz from dolomite marbles. This partial exchange was fa cilitated by an increase in the surface area of the quartz as a result of its dissolution by meteoric fluids. The negative delta(13)C values in carbonates result from the oxidation of organic material by meteor ic fluids following metamorphism. Stable isotopic data from Lone Mount ain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western Nevada in Tertiary time.