HOWARD'S PASS PB-ZN SEDEX MODEL NEEDS REVISITING: NEW INTERPRETATION OF DEFORMATION HISTORY

Zinc and lead mineralization was discovered at Howard’s Pass in Silurian shale of the Selwyn Basin in the 1970’s and is the largest undeveloped Zn deposit in the world. It is currently considered to be a classic example of a sedimentary exhalative (SEDEX) deposit. The existing model for the Howard’s Pass deposits holds that metals were exhaled onto the seafloor and deformed while sediments were still water-saturated, during a period of local compressional tectonism speculated to have interrupted the overall extensional regime of the Silurian. In this model, later Cordilleran-wide orogenesis produced further folding, faulting, low-grade metamorphism and minor remobilization of pyrite along slaty cleavage, but did not significantly affect sulphide textures and distribution. New insights from recent detailed surface bedrock mapping indicate that an alternative interpretation is required.

We propose that the existing distribution of rock types is primarily controlled by thrust imbrication, not simply by folding. Northeast-verging thrusts are proposed to root into a detachment zone observed at Howard’s Pass, herein termed the Howard’s Pass décollement. The Howard’s Pass décollement displays significant ductile strain. Above the décollement, a series of imbricated thrust faults disrupt the clastic-dominated lead-zinc ore and stratigraphic succession. This zone of complex deformation is capped by a thrust that is the roof of a complexly formed duplex, and above which less shortening has been accommodated. We suggest that the pressure solution cleavage, and associated folds and transposition, formed 250-300 Ma after deposition of sediments.

The best structural interpretation for the Howard’s Pass district is that layer-parallel shortening during Jura-Cretaceous orogenesis resulted in a regional-scale duplex structure. The bulk of the deformation is Jura-Cretaceous, not the Silurian in age.

Recent studies on ‘SEDEX’ deposits of the Selwyn Basin and world-wide suggest that, in some cases, there is little evidence for exhalation and that the bulk of the Pb-Zn sulphide minerals precipitated below the seafloor. These studies, together with the structural re-interpretation of Howard’s Pass demanded by our work, highlight the need to re-examine the currently accepted models for all ‘SEDEX’ in the Selwyn Basin.