FORMATION OF COMPLEX FIBROUS CALCITE VEINS IN UPPER TRIASSIC STRATA OF WRANGELLIA TERRAIN, BRITISH-COLUMBIA, CANADA

Fibrous calcite veins are ubiquitous throughout the thinly bedded, org anic-rich Upper Triassic marine mudrocks of the Queen Charlotte Island s and their lateral equivalents on Vancouver Island. These veins show variable and complex morphologies and can be grouped into several type s: (a) simple; (b) anastomosing or composite; (c) boxwork; and (4) pol ygonal network oriented normal to bedding. Field, petrographic, and ge ochemical evidence suggest that vein opening, resulting from hydraulic fracturing due to elevated pore-fluid pressures, was an early phenome non and occurred prior to significant compaction of the host sediments . Calcite fibers in the veins are up to 30 mm long and commonly orient ed perpendicular to the wall but locally display conical structures. F ibrous calcites, with the exception of those in boxwork veins, are gen erally non-ferroan and dull to very weakly orange luminescent. The box work calcites are ferroan, zoned and show dull luminescence with some bright rims. delta(18)O values range from -8.2 to -21.6 parts per thou sand (PDB) and delta(13)C values range from 2.0 to -4.4 parts per thou sand (PDB). Although some variations are present among the different m orphological types of calcite veins, oxygen and carbon isotopic values display important variations when compared geographically. The most d epleted oxygen and carbon isotopic values are those of boxwork calcite and they are associated with areas where the effects of early Mesozoi c plutonism were most severe. Precipitation of boxwork fibrous calcite s is interpreted to have been related to hydrothermal discharge into u nconsolidated host sediment, rather than to later burial. Although the hydrothermal influence on the formation of vein calcite is related to geological events specific to the Wrangellia Terrain, this study prov ides an alternative mechanism for the generation of fibrous calcite ve ins and demonstrates the local importance of hydrothermal input in the evolution of pore-water chemistry.