Hydrothermal Dolomitization Paradigms and the Manetoe Dolomite: Are All HTDs Fault-Related?

Abstract

Recent work on hydrothermal dolomite masses has emphasized their close relationship to structural deformation and faulting. The source of dolomitizing fluids is considered to have been intimately and primarily related to the development of extensional, or strike-slip, wrench-style faults. This paradigm has been recently challenged by workers who have documented many occurrences of Paleozoic-hosted hydrothermal petroleum reservoir dolomites in North America that bear little, if any, overt relationship to faults. The gas-bearing Manetoe Dolomite of northern Canada is another spectacular example of a hydrothermal dolomite that does not exhibit a general spatial relationship to known faults. This laterally extensive HDT is encountered in nearly 70 wells across more than 25,000 km² in the little deformed subsurface east of the Mackenzie Mountains, as well as in outcrop across Liard Plateau and Mackenzie Mountains where it has been mapped as a discontinuous “formation” across six 1:250,000 scale geologic map areas, or over more than 20,000 km². The Headless shale aquiclude exerted a strong hydrodynamic control on the upward circulation of dolomitizing brines in Late Devonian time east of the mountain front across Slave Plain. The much greater depth to hydrodynamic basement west of the mountain front may have engendered more vigorous circulation of dolomitizing brines, as indicated by fluid inclusion and isotopic data. This enhanced convective brine circulation, in conjunction with local attenuation of the Headless shale and the presence of carbonate shelf edge shoal grainstone facies, may have caused the development of the very thick Manetoe Dolomite masses, such as in the Liard gas fields and in the “Manetoe” shelf edge exposures at Ram River and Iverson Lake. Outcrop exposures of “HDT” zebra and boxwork fabrics in the Manetoe are best interpreted as the consequence of hydrothermal dissolution by heated evaporitic brines of seawater origin, and not as dilational breccia fabrics attendant upon extensional or wrench faulting.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016