Zeolitization of the Howson facies, Telkwa Formation, British Columbia

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Zeolites in the Howson facies of the Lower Jurassic Telkwa Formation, north-central British Columbia, include analcime, wairakite, laurnontite, thomsonite, mesolite, scolecite, stilbite, and heulandite. Electron microprobe anlyses of these zeolites show close agreement with ideal end-member compositions. The Howson facies zeolitization occurred in a regime of high temperature/pressure ratio typical of geothermal fields. Published stratigraphic reconstruction indicates a maximum burial depth during zeolitization of 4 kilometres. This corresponds to a maximum load of approximately 102 MPa. Comparison of the zeolite assemblage to published experimental data suggests a maximum temperature of 350°C for zeolitization of the Howson facies. Minimum pressures and temperatures could have been close to surface values. Zeolitization of the Howson facies was governed by physicochemical variables associated with the movement of fluids of variable temperature and composition through rock of varying permeability and composition. A high water/rock ratio is suggested by extensive occurrences of amygdules and groundmass replacement comprising up to 80% of the rock, veins and fracture breccia cements which in many instances bear no compositional relationship to the bulk rock composition, and the preferential alteration of initially more permeable strata. The complex stratigraphic distribution of zeolites in the Howson facies cannot be interpreted as a progressive dehydration with increasing depth and temperature. Paragenetic sequences can be explained only in part by a decreasing temperature with time. In some instances fracture permeability controlled the degree of hydration of the stable zeolite. Isolated groundmass zeolite occurrences were controlled by the composition of the host rock, whereas those occurrences associated with veins or fracture breccia cements involved external buffering of the fluid chemistry and mass transfer. Evolution of fluid chemistry with progressive interaction of meteoric water and volcanic rock can explain some, but not all, of the paragenetic sequences observed in the Howson facies. Changes in fluid chemistry associated with processes such as boiling can explain parts of paragenetic sequences such as late calcite in veins and fracture breccia cements. The regional distribution of zeolites within the Howson facies was controlled by changes in rock composition, temperature, and origin of the groundwater away from the volcanic centre toward the eastern boundary of the facies which is marked by a paleo-shoreline.
Bibliography: p. 283-302.
Dudley, J. S. (1983). Zeolitization of the Howson facies, Telkwa Formation, British Columbia (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/12837