COMPARISON OF MATRIX, FOSSIL, AND VEIN MINERAL ASSEMBLAGES IN THE BURGESS SHALE, BRITISH COLUMBIA, CANADA

Fossils of both soft-bodied and skeletal organisms preserved within the Burgess Shale are commonly composed of authigenic sheet-silicate bearing assemblages. This study documented modal percentages of mineral components of skeletal fossils (primarily trilobites) along with the enclosing matrix and neighboring vein material from 12 samples from quarry localities near Field, British Columbia. Minerals within the very fine-grained metamorphosed shales were identified by digital combination of elemental x-ray maps. The freeware Multi-Spec was used to determine modal abundances through graphical analysis of digitally-combined x-ray maps. In general, ratios of sheet silicates (chlorite and muscovite) exhibited in fossils tend to correlate with those within the surrounding matrix. Exceptions to this trend tend toward fossil-fill assemblages that are significantly richer in muscovite than the matrix. Fossils and matrix exhibit no correlation between modal abundance of either quartz or calcite. Chlorite compositions (Mg:Fe ratios) within fossils correspond to those within the enclosing matrix. Three mineralogically distinct veinlet sets cut the samples: 1) tightly folded pre-metamorphic veinlets oblique to bedding and composed of fine-grained opaque minerals; 2) weakly folded syn- to late metamorphic veinlets perpendicular to bedding and composed of chlorite-muscovite±quartz±calcite; and 3) post-metamorphic bedding parallel veinlets composed of calcite. The pre- to syn-metamorphic veinlets are composed predominantly of chlorite, and sheet silicate ratios do not vary with matrix composition. Vein-chlorites are consistent in atomic composition and contrast with the composition of chlorites in the surrounding matrix and neighboring fossils. Patterns in mineral abundance and atomic composition suggest that fossil-fill assemblages were controlled primarily by matrix composition. Metamorphic fluids were not involved in the growth of fossil-fill minerals, and these authigenic minerals were emplaced prior to the syn- to late metamorphic veinlets. The tendency towards muscovite-rich assemblages in trilobite fossils is similar to that documented in preliminary organism-scale elemental mapping of soft-bodied fossil fauna, suggesting a similar mechanism of authigenic mineral growth.