TRACE ELEMENT GEOCHEMISTRY OF CARBONATE CEMENTS AND FRACTURE FILLS IN FORELAND BASIN SANDSTONES, WIND RIVER AND PICEANCE BASINS

Authigenic calcite and ferroan dolomite are ubiquitous (though variable in amount) in foreland basin sandstones along the eastern side of the central Rocky Mountains. Locally, these minerals are sufficiently abundant to exert significant control on both "matrix" and fracture porosity in oil and gas reservoirs, and thus, there is a practical motivation for delineating the controls on carbonate precipitation in these sandstones. Comparison of trace element contents in intergranular cements and associated fracture fills shows that, within any single sample, these contrasting forms of authigenic carbonate are broadly similar in composition and were likely emplaced by the same pore fluid. Examples of both early, near-syndepositional, and late, deep-subsurface, carbonate emplacement are observed, and in both cases, associated cements and fracture fills may have similar compositions. Exceptions are exposure-related calcite caliche and speleothems that coat the interiors of some open fractures in outcrop but have no counterpart filling primary or secondary pores within the sandstone. In general, cements and fracture fills also manifest similar patterns of co-variation among trace elements. For example, calcites in both cements and fracture fills typically show a positive co-variation between Mg and Fe and high concentrations of Mn (>0.5 mole % MnCO3) and Sr (>0.2 mole %) are restricted to calcites containing little Mg (<0.8 mole %). In ferroan dolomites, highest Mn contents (>1 mole %) correlate with the most Ca-rich compositions (> 57 mole %). The significance of such elemental patterns remains uncertain, but the similarity of elemental trends in associated cements and fracture fills further supports the idea that these different forms of authigenic carbonate record a common history of fluid/rock interaction.