CONTROLS ON OROGENIC GOLD ENDOWMENT IN THE BLUE MOUNTAINS, EASTERN OREGON, USA

The Blue Mountains contain the richest gold fields in the State of Oregon, accounting for ~75% of pre-1900 gold production (Lindgren, 1901), and probably more thereafter. Most gold within the Blue Mountains was produced from orogenic (or lode) gold veins and associated placer deposits. However, the geodynamic, regional, and deposit scale controls on lode gold deposition within the Blue Mountains are not understood. I present a scale-integrated mineral system that links gold mobilization, transport, deposition, and exposure to protracted tectonism outboard and along the western North American margin. Gold production by mine (in ounces) was estimated from written records 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 tectonic framework. Argillite, schist, and slate formed within an accretionary wedge or fore-arc host approximately 74% (~1.1 M oz) of the total recorded ounces of gold. Unique geochemical, geomechanical, and hydrological properties made these excellent hosts. These properties were predetermined by both depositional setting (deep ocean basin) and metamorphic conditions (lower greenschist facies). Approximately 55% (~0.9 M oz) of recorded lode gold was mined from veins striking 030° ± 15°. A reconstructed paleo-stress field suggests that these were left lateral faults. The faults were likely part of a margin-parallel shear zone that partitioned the strike slip component of oblique convergence between oceanic plates and magmatic arcs. Well-endowed lodes also tend to strike parallel to nearby contacts with the granodioritic plutons and/or terrane-bounding faults and dip steeply (~70°). At both the district and the deposit scales, minor deviations in lode/vein strike form local high-grade sites (i.e., dilatant zones). The intersection lines between the veins, host rock fabrics, and secondary veins likely control the pitch of high-grade ore shoots.