EFFECTS OF UPLIFT AND EROSION ON GRAVITY-DRIVEN FLOW SYSTEMS IN THE NORTHERN ALBERTA BASIN AND IMPLICATIONS FOR MVT MINERALIZATION AT PINE POINT

Geochemical relationships and absolute dating have correlated MVT Pb-Zn ore mineralization with gravity-driven flow systems initiated by tectonic rebound. Previously developed, steady-state, fixed-mesh, numerical models of such flow systems have predicted the formation of the Pine Point ore district, NWT. However, these simulations have not reconciled density-dependent flow, the presence of brines in aquifers today and the transience of the topography of the water table controlled by uplift and erosion rates in the foreland basin.

Regional-scale fluid flow models of the northern Alberta basin were developed to evaluate the evolution of regional fluid flow due to Laramide tectonic rebound and the implications for Pb-Zn mineralization at Pine Point. Measured permeabilities were assigned to the hydrostratigraphy along the Presqu'ile barrier reef in two-dimensional, deformable finite-element, fluid flow models. Two basic uplift and erosion scenarios were tested: 1) the basin is uplifted at a constant rate for a set period of time, after which the surface topography is eroded at a slower constant rate; and 2) uplift and erosion occur simultaneously at rates that decay exponentially over time. For both scenarios, a flat water table was assigned as an initial condition and the final configuration corresponded to present day conditions. Based on pre-erosion reconstructions, uplift rates of 0.5 to 1.5 mm/yr and erosion rates less than 0.15 mm/yr were defined at the westernmost edge of the foreland basin.

Flow rates in the Devonian aquifers gradually increase as the hydraulic gradient increases during uplift. When erosion commences, the flow rates quickly drop by two orders of magnitude as the gradient decays and underpressures develop in high diffusivity strata. Simultaneous uplift and erosion simulations predict gradual increases in flow rates until maximum uplift. After erosion and uplift rates become equal, flow rates decrease and finally become negative, marking a flow reversal caused by erosional underpressuring. Transient simulations of each case cannot predict fluxes adequate to explain Pine Point mineralization unless permeabilities vary through time or the regional flow system includes 100 km of the fold and thrust belt early in the simulations.