ZANGRILLI, Paul,
Geol. Dept., Colgate Univ., Hamilton, NY 13346, pzangrilli@center.colgate.edu
Lower and Middle Devonian carbonate rocks exposed near Catskill New York contain hydrothermal veins formed during Alleghenian(?) folding and thrusting. Veins are developed in fractures associated with bedding-plane thrusts and minor folds. Quartz and calcite crystals in veins are often deformed, but late, macroscopically undeformed crystals are present. In thin section, quartz crystals exhibit growth-zonation marked by primary methane-water inclusions. Later, secondary two-phase aqueous inclusion trains are present, as well as single-phase methane inclusions. These later inclusions mark subtle post-crystallization deformation that is not obvious in hand sample.
Homogenization temperatures of texturally primary methane-water inclusions in quartz range from 220-260°C (mean ~240°C). Single-phase methane inclusions form a vapor bubble during cooling and subsequently homogenize between -84 and -80°C indicating nearly pure methane. Melting temperatures of water ice range from -3.1 to -1.4°C, corresponding to average salinities of 3.6 wt.% NaCl equivalent. Methane-water inclusions form clathrate on freezing; clathrate melt temperatures range from 22.0 to 25.1°C. Oxygen isotope values of co-existing quartz (delta18O SMOW 21.9 - 21.5) allow calculation of equilibrium temperatures of crystallization of 234-279°, using the constants of Sharp and Kirschner (1994).
Clathrate melting temperatures, and estimation of volume proportions of water and methane, permit calculation of methatne-water inclusion isochores, using the constants of Zhang and Franz (1992). Given that Th of methane-water (~240°) and isotope geothermometers (~250°) are similar, the temperature of quartz crystallization is assumed to be approximately 245 ± 15°C. From inspection of the isochores calculated from clathrate data, the fluid pressure during quartz crystallization is estimated to range from 1600-1900 bars.
Calculation of the delta18O of the waters from which quartz and calcite precipitated, using the fluid inclusion Th, yield values of +14 to +16 SMOW, suggesting that while the waters were of normal marine salinity, the fluids had undergone rock-water equilibration during burial. These data suggest that the vein-mineralizing fluids were derived from connate waters within the immediately adjacent sedimentary rocks during deformation.
