Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Holk, G. J. Articles by Taylor, H. P. Search for Related Content GeoRef GeoRef Citation

¹⁸O/¹⁶O Evidence for Contrasting Hydrothermal Regimes Involving Magmatic and Meteoric-Hydrothermal Waters at the Valhalla Metamorphic Core Complex, British Columbia^*

Gregory J. Holk^1, and Hugh P. Taylor, Jr.²

¹ Department of Geological Sciences and the Institute for Integrated Research in Materials, Environments, and Society, California State University, Long Beach, Long Beach, California 90840-3902
² Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

Corresponding author: e-mail, gholk{at}csulb.edu

Modeling of published oxygen isotope analyses of 115 whole-rock, quartz, and feldspar samples from the Valhalla metamorphic core complex provides evidence for three distinct stages of hydrothermal activity affecting the detachment faults and upper plate. (1) Lithostatically pressured aqueous fluids having ¹⁸O ~10 per mil were expelled from the lower plate and moved upward into the upper plate through an oblique-sinistral transfer zone between the southern terminus of the Columbia River detachment fault and the northern terminus of the Slocan Lake detachment fault. These fluids ascended through fractures and exchanged with Slocan Group shales and graywackes to produce the marked zonation of vein siderite ¹⁸O (11.4–18.4) observed in previous studies of the Sandon group of Ag-Pb-Zn lode deposits. (2) Later, as brittle deformation along the Slocan Lake detachment fault became the dominant tectonic process, these magmatic and/or metamorphic fluids intermittently mixed with even larger quantities of downward circulating, hydrostatically pressured meteoric-hydrothermal fluids. In the upper plate Nelson granodiorite adjacent to the Slocan Lake detachment fault, ¹⁸O values of quartz (8.9–11.9) and feldspar (–5.0 to +9.6) define a steep array in - space (slope = 25), suggesting that synextensional meteoric-hydrothermal activity along the Slocan Lake detachment fault (initial ¹⁸Owater ~–15 and T ~300°C) was relatively short-lived (~10⁶ yr), but very intense, with W/R_open-system = 1.05 and fluid flux = 0.1 to 3 m/yr. The ¹⁸O values of quartz (10.8–12.4) and feldspar (6.4–10.1) from lower plate greenschist mylonites indicate that small quantities of the low ¹⁸O meteoric-hydrothermal fluids penetrated into the lower plate. The systematic decrease in feldspar ¹⁸O toward the Slocan Lake detachment fault in both the upper and lower plates shows that these low ¹⁸O waters were channeled along the detachment fault. (3) Meteoric-hydrothermal activity along the Valkyr shear zone postdates ductile deformation and was associated with the emplacement of the Coryell plutons in the upper plate. The ¹⁸O values of quartz (7.6–11.0) and feldspar (0.0–8.6) from the Valkyr shear zone define a much shallower array in - space (slope = 2.3) than those at the Slocan Lake detachment fault, indicating a higher temperature and/or longer lived episode of hydrothermal activity. The geographic distribution of pronounced disequilibrium ¹⁸O effects in Coryell quartz (5.8–8.2) and feldspar (–4.4 to +7.4) indicates that meteoric-hydrothermal activity was most intense along inferred coeval extensional fracture zones that lie beneath Arrow Lake. This modeling suggests that transfer zones that accommodate deformation between detachment faults may be a very attractive exploration setting for lode-type mineral deposits.