DOLOMITIZATION OF THE CAMBRIAN CARBONATE PLATFORM, SOUTHERN CANADIAN ROCKY-MOUNTAINS - DOLOMITE FRONT GEOMETRY, FLUID INCLUSION GEOCHEMISTRY, ISOTOPIC SIGNATURE, AND HYDROGEOLOGIC MODELING STUDIES

There are two generations of dolomite in the Cathedral Formation in th e southern Canadian Rocky Mountains. A first generation dolomite is a finely-crystalline material that preserved sedimentary structures. The geometries of this dolomite are sheet-like strataform layers tens of meters thick. However, the beginnings and ends of first generation dol omite sheets are not visible. Primary fluid inclusions from this gener ation of dolomite have homogenization temperatures (Th) that average a pprox 110 degrees C and final ice melting temperatures (Tm) of approx -25 degrees C. The most important replacement dolomite in the Cathedra l Formation is a second generation dolomite-a white, coarsely-crystall ine material that destroyed primary sedimentary textures where it repl aced limestones and was a ''saddle'' dolomite where it is a fracture-f ining cement. There are two distinct geometries of the second generati on dolomite bodies: (1) vertical pipes and dikes with scales on the or der of tens of meters that contain ''zebra dolomite'' breccia cores an d are restricted to the paleoplatform margin and (2) strataform sheets tens of meters thick that extend 5 to 30 km into the platform interio r where they pinch out into unaltered limestones. The homogenization t emperatures (Th) of fluid inclusions in these dolomites decreases from 200 degrees C at the margin to 110 degrees C tens of kilometers on th e platforms, whereas the final ice melting temperatures (Tm) of fluid inclusions in these dolomites remain constant at about -20 degrees C. The delta(18)O values for unaltered limestones and first generation do lomites are in the range of -10 to -15 permil whereas the delta(18)O v alues for second generation dolomites are in the range of -20 permil. The delta(13)C values fbr unaltered limestones and first generation do lomites are 1 to -2 permil whereas the delta(13)C values for second ge neration dolomites are -1 to -4 permil. Brines trapped in the inclusio ns appear to have a Ca++/Mg++ of 2:1 based on crushing-leaching experi ments. The origin of the first generation dolomite is problematic and may involve seawater, sinking evaporite waters, or basinal brines. How ever, for the second generation dolomite, the patterns of dolomite/ li mestone transition, the fluid inclusion data, and isotope ratios toget her suggest that hot (200 degrees C), saline brines (10-25 percent) mi grated updip at the platform margin and eastward for tens of kilometer s into the Cathedral Formation shelf carbonates. Temperatures decrease d from approx 200 degrees C at the margin to 110 degrees C tens of kil ometers back onto the platform. The now patterns were dominantly horiz ontal and strata-bound across large portions of the platform, except a t the western margin where local vertical, cross-formational flow was channeled upward through conduits (perhaps of hydrofracture origin). T he salinity, delta D compositions of inclusion-fluid, and delta(18)O v alues Of dolomitizing fluids indicated that the basinal brines (probab ly residual evaporite waters) were diluted by meteoric water as the do lomitization proceeded. Extensive, sheet-like dolomite bodies of white , fabric destroying dolomite were also observed in the overlying Eldon , Pika, and Waterfowl Formations, suggesting similar flow patterns acr oss the entire Middle Cambrian platform. Geological relationships indi cate that this main dolomitization event occurred between Middle Silur ian and Late Devonian time. Numerical simulations of: (1) topographica lly-induced forced convection, and (2) thermally-driven free convectio n can be compared to the patterns measured and deduced from the field relationships. The forced convective models appear better able to expl ain the distribution of dolomites in the field area.