Abstract

The Howard’s Pass district of the Selwyn basin in eastern Yukon and western Northwest Territories, Canada, includes a total of 15 sediment-hosted Zn-Pb deposits and is one of the largest undeveloped Zn-Pb districts in the world.

Understanding the structure is crucial for future exploration efforts and new discoveries. The current genetic model holds that syngenetic mineralization, exhaled during the Silurian, was deformed while sediments were still water saturated, and although deformed again into an open syncline during the Mesozoic Cordilleran orogen, the distribution of the ore was not significantly affected. Based on recent structural mapping of the entire district, it is proposed here that the map pattern is primarily controlled by thrusting, not simply by folding, and forms a duplex structure. Imbricated thrusts root into a flat-lying detachment surface, termed the Howard’s Pass décollement, which forms the floor thrust of the duplex and displays significant ductile strain. Above this floor thrust, a series of imbricated thrust faults disrupt both mineralization and stratigraphic succession. Sulfide minerals (galena, sphalerite, and pyrite) are concentrated and remobilized along a pressure solution cleavage that is well developed in zones of high strain. The duplex is capped by a flat-lying upper detachment (roof thrust) above which less shortening has been accommodated. It is suggested here that the duplex and associated fabrics (pressure solution cleavage, transposition, folds, and faults) formed 250 to 300 m.y. after deposition of sediments, during Jurassic-Cretaceous layer-parallel shortening of the Cordilleran orogen, which significantly affected the distribution of the mineralization.

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