Hydrothermal dolomite - a product of poor definition and imagination

The latest dolomite bandwagon is the "hydrothermal dolomite model". In its present form, this bandwagon is doomed or at least very much overstated for at least two reasons: (1) there are several definitions of hydrothermal, and hardly any author specifics which one s/he is using; (2) very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition, except the worst. As presently applied, the term "hydrothermal dolomite" is confusing and/or meaningless. We suggest to use White's [Geol. Soc. Amer. Bull. 68 (1957) 1637] time-honored definition of "hydrothermal" as "aqueous solutions that are warm or hot relative to its surrounding environment", with no genetic implications regarding the fluid source. Hence, a dolomite should be called hydrothertual only if it can be demonstrated to have formed at a higher than ambient temperature, regardless of fluid source or drive. Furthermore, this definition does not carry a lower or upper temperature limit. Even a dolomite formed at 40 degreesC could be hydrothermal, By extension, dolomites formed at temperatures lower than ambient are not hydrothermal, even if they formed at a rather high temperature. For example, groundwater may penetrate a rock sequence through a highly permeable pathway, such that it is heated to 150 degreesC at a depth where the surrounding rock has a temperature of 250 degreesC. We suggest to call dolomite formed from this water "hydrofrigid". Dolomite formed in or near thermal equilibrium with the surrounding rocks may be called "geothermal". Furthermore, not all saddle dolomite formation requires advection (fluid flow) to transport Mg. Saddle dolomite can be formed in at least three ways, i.e., from advection, local redistribution of older dolomite during stylolitization, and as a by-product of thermochemical sulfate reduction in a closed or semi-closed system. Only the first and the last of these three possibilities have a chance of being hydrothermal. Almost all dolomites and dolostones in the Western Canada Sedimentary Basin have recently been (re-)interpreted as hydrothermal. Applying the rationale outlined above reveals, however, that this basin contains very little hydrothermal dolomite. Rather, most dolomites in this basin, and almost all dolostones south of the Peace River Arch, are geothermal, and/or the proof of a hydrothertual origin has not been made. This has important implications beyond the various case studies at hand, as attempts to tic dolomitization to orogenic events become moot, at least in the southern part of the basin. (C) 2002 Elsevier Science B.V. All tights reserved.