Langhorne Smith and Richard Nyahay, Reservoir Characterization Group, New York State Museum, Room 3140 CEC, Albany, NY 12230, [email protected]

Geochemical and fluid inclusion analysis confirm that most or all dolomite found in the Trenton and upper parts of the Black River Group in the northern Appalachian Basin is of a structurally controlled hydrothermal origin. A hydrothermal origin for the dolomites can be demonstrated where primary dolomite fluid inclusion homogenization temperatures exceed maximum burial temperatures by at least 20° C. Using these criteria, virtually all of the dolomite found in the Trenton and upper parts of the Black River in Kentucky, Ohio, Michigan and Ontario is of a hydrothermal origin. This is a very large volume of dolomite (hundreds or possibly thousands of cubic kilometers).

Using published data on the amount of fluid required to dolomitize limestone, more than one hundred trillion barrels of saline brine would likely be required to make that amount of dolomite. That is probably more fluid than is contained in the Appalachian and Michigan Basins combined.

Mass balance models for this dolomitization must be able to explain the source of the magnesium and the volume of fluid required to make this amount of dolomite. One possible solution is recycling of fluid where fluids flow up active faults and then back down where they are reheated and recharged with magnesium or "forced episodic convection." Another possibility is that seawater slowly recharges the aquifers that source the hydrothermal fluids over a broad area and that the salinity increases due to heating. It may also be that some dolomitization occurs at very shallow depths, due to mixing of hydrothermal fluids and seawater. In any case, hydrothermal dolomitization must now be considered to be an option for widespread pervasive dolomitization.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York