Structurally controlled hydrothermal dolomite, Eganville-Douglas Paleozoic outlier, Ottawa-Bonnechere graben, eastern Ontario

In Upper Ordovician (Trenton-equivalent) limestone of the Eganville-Douglas Paleozoic outlier, ∼50 km northwest of Ottawa, Ontario, sub-vertical bodies (up to 10s of metres in width) of replacement dolomite and vein dolomite are well exposed in a limestone quarry adjacent to the Bonnechère River. Additional minor dolomitization has been discovered at one other nearby locality. The region lies within the Ottawa Bonnechere graben, a cratonic-scale fault system originating in the Neoproterozoic. Dolomitization shows a lateral gradation from 100% replacement (+ porosity) in cores to isolated crystals in a non-porous skeletal-peloidal grainstone along the margins of the dolomite zones. Saddle dolomite occurs as fracture and vein-fill dolomite that post-dates the replacement dolomite. A WNW-ESE basement fault controls the axis of the Bonnechère River immediately to the north of the quarry. Zones of dolomite, and associated dolomite veins originating as part of local strike-slip faults, display orientations that fit the expected extension and strike-slip components, respectively, of structural fabric associated with a regional wrench-style fault system. δ 18O values for replacement dolomite (-12.71 to -13.78‰) and saddle dolomite (-12.2 to -12.73‰) are negative compared to the estimated value for Upper Ordovician shallow-water marine dolomite. Fluid inclusion data, in combination with oxygen isotope data, suggest that fluids responsible for dolomitization were hot (87.2-115°C) brines of Ca-Mg-Cl origin. 87Sr/86Sr isotope ratios for replacement dolomite plot above the seawater curve for the Late Ordovician indicating that the fluids from which they precipitated had interacted with strata containing more radiogenic 87Sr. Potential sources are siliciclastics or Precambrian gneissic rocks, if they underlay the limestone in the outlier. The estimated temperature appears to be 10-20°C warmer than the expected burial temperature based on previous work evaluating conodont alteration indices. Such structurally-controlled hydrothermal alteration requires tectonism well within the North American craton, and it likely occurred during one of two periods of orogenic development along the Laurentian margin: (1) in the Late Ordovician, associated with Taconic orogenesis; and, (2) in the Late Devonian, associated with Acadian orogenesis. Both require more elevated geothermal gradients (∼100° and ∼56°C/km, respectively) than previous studies have estimated assuming a stable craton-interior basin. As the region lay within the Ottawa-Bonnechere Graben, this style of dolomitization likely highlights an unstable structural corridor, with elevated heat flow. The nature of dolomitization fits with more regionally distributed occurrences in adjacent basins (in Michigan, New York State), although the precise timing (Late Ordovician vs Late Devonian) remains uncertain.