Hydrothermal Alteration of Dolomite: Yet Another Strike Against the Hydrothermal Dolomite Band Wagon

Hans G. Machel¹ and Jeff Lonnee^2
1 University of Alberta, Edmonton, AB
² Shell International E&P, Houston, TX

Hydrothermal alteration of preexisting "matrix dolomites" has been identified in Middle Devonian dolostones of the Clarke Lake gas field, British Columbia, Canada. Hydrothermal alteration encompasses dissolution, recrystallization, and saddle dolomite formation, as well as several distinctive geochemical characteristics (fluid inclusion data, stable and Sr-isotope data). Saddle dolomite occurs both as a replacement and as a cement, but it is not the product of replacement of limestone, hence, saddle dolomite formation was not dolomitization sensu stricto. Proof that the alteration was hydrothermal is provided by comparison of fluid inclusion homogenization temperatures with maximum burial temperatures.

These findings have important implications beyond the Clarke Lake gas field. Firstly, textural characteristics such as those found at Clarke Lake are widespread, yet they have been commonly misinterpreted as evidence of hydrothermal dolomitization. The fluids that created the observed textures and associated geochemical patterns were not Mg-rich brines that replaced limestones, which necessitates large water-rock ratios and/or sustained fluid flow, which also necessitate unusually high permeabilities and heat sources. Rather, the fluids that formed these rocks were dilute fluids initially undersaturated with respect to dolomite, and the water-rock ratios were likely rather low. Thus, hydrothermal alteration of this type can occur in semi-closed systems, and permeabilities as well as heat sources may be moderate to low.

These finding further compromise the popular band wagon of hydrothermal dolomitization. Also, a so-called "TSR-HTD furnace model"(thermochemical sulfate reduction - hydrothermal dolomitization) is found to be conceptually and geochemically incorrect.