Structural Control of Lead-Zinc Mineralization In Carbonate Sequences of Northern Alberta: A Contribution to the Carbonate-Hosted Pb-Zn (MVT) Targeted Geoscience Initiative

Field investigations of the Devonian carbonate stratigraphy and deformation pattern in the Interior Plains of northern Alberta, corroborated with existing data on MVT deposits and prospects in the Rocky Mountains fold-and-thrust belt and on the Pine Point MVT district, indicate that the MVT mineralization in carbonate sequences of the Western Canada Sedimentary Basin is epigenetic. All MVT occurrences are confined to a halo of late hydrothermal dolomitization invariably related to fractures or fault zones. Hydrothermal ore fluids were hotter than what could be reasonably obtained from burial or advecting basin fluids. Consequently, huge volumes of highly saline fluids invoked by basinal fluid flow genetic models are not critical factors for the MVT mineralization. Tectonically induced, secondary porosity of carbonate rocks appears to be the main trap for ore-bearing fluids. The occurrence of highly porous reefs is a favourable geologic feature for the development of MVT, but is not a critical factor.

Cambrian and Devonian platformal carbonate sequences of the WCSB that developed on the ancestral passive margin of North America are the primary hosts of MVT deposits and prospects. For the portion of the WCSB carbonate stratigraphy involved in Mesozoic orogenies, routinely invoked basinal fluid flow models for MVT deposits are irrelevant. MVT deposits and prospects in the Rocky Mountains fold-and-thrust belt appear to be related to the Laramide orogeny and/or to post-orogenic collapse.

The MVT mineralization in Paleozoic carbonate sequences involved in the Mesozoic foreland basin is related to zones of recurrent strain in the Precambrian basement. Pine Point district is structurally controlled from continental to deposit scale. It is located along the transcrustal Great Slave Lake Shear Zone with the individual deposits within the district distributed in linear trends above basement fault scarps. Saline water from successive basins may have repeatedly infiltrated a recurrent fault zone, extracted and transported metals in an open system, low- to medium-grade metamorphic environment; focussed within linear zones of strained/permeable basement; finally, the metal-bearing waters were released and reacted with the carbonate caps at the site of ore deposition, from prospect or mine to district scale. The likely driving forces for fluid flow may have been a combination of topography, thermal convection and hydraulic pumping.

Based on the inferred genetic relationships between zones of recurrent strain in the basement and overlying carbonate stratigraphy, several exploration target areas for MVT are identified in northeastern Alberta.