Orogenic gold in the Blue Mountains, eastern Oregon, USA

Abstract

At least 75% of gold produced in the State of Oregon came from the Blue Mountains. Most gold within the Blue Mountains was mined from orogenic gold veins and associated placer deposits. However, the geodynamic, regional, and deposit scale controls on orogenic gold mineralization within the Blue Mountains are not understood. Here, the mobilization, transport, deposition, and exposure of golds are linked to protracted tectonism of the western North American Cordillera through a scale-integrated mineral system. Gold production by mine (t) was estimated from public domain documents of production value (USD) and production time period (years). Lode endowment was then analyzed against criteria such as host rocks, vein structure, and location within the regional tectonic framework. Argillite, schist, and slate formed in accretionary wedge and fore-arc basins host the majority (∼60%; ∼29 t) of the total recorded gold production. Unique geochemical, geomechanical, and hydrological properties made these rock types excellent hosts. These properties were predetermined by both depositional setting (deep ocean basin) and metamorphic conditions (lower greenschist facies). Nearly half (50–55 %; ∼25.5 t) of recorded lode gold production was mined from lodes striking 030° ± 15° azimuth. A reconstructed paleo-stress field supports that these were left lateral faults. It is speculated that these were part of a margin-parallel shear zone that partitioned the strike slip component of oblique convergence between subducting oceanic plates and associated magmatic arcs. At both the district and the deposit scales, minor deviations in vein strike form local high-grade sites (i.e., dilatant zones). Such dilatant zones within major crustal shear zones could also explain the spatial, but not fluid-genetic association of granitic plutons with lode gold. The intersection lines between the veins, host rock fabrics, and secondary veins likely control the locations, thicknesses, and pitch of high-grade ore shoots.