THE STRUCTURAL FRAMEWORK OF GOLD-RICH MINERALIZING SYSTEMS

Gold is concentrated from crustal background of <5ppb to economic values of 0.5-100ppm by a wide variety of geological processes. At high temperatures (400-1200^oC) magmatic-hydrothermal saline brines and vapors produce Au-rich deposits typically with economic concentrations of other metals (Cu +/- Mo +/- Ag). At intermediate temperatures (200-400^oC) dilute fluids, CO₂-rich in some cases, produce Au deposits over paleodepths of >10km to <100m with Au as the main economic commodity. At similar temperatures, Au-rich base metal deposits form on or immediately below the seafloor in VMS systems. At the surface, weathering enriches Au in laterite and liberates Au for subsequent concentration by fluvial processes. No other commodity shows such a range of formation conditions and deposit types.

Structure is a key factor in subsurface Au deposits and operates at three scales: (i) regional kinematic frameworks control the location of major mineralizing systems; (ii) intermediate, typically second order structures provide pathways for fluids and hence localize mineralization; (iii) local stress fields control vein formation, the distribution of grade, and other geometallurgical characteristics that determine mining and processing costs. The overall framework may reflect pre-mineral, syn-mineral and post-mineral deformation with the structural character of each potential event influenced by paleodepth and the change from ductile to brittle regimes. Magmatic activity influences the depth of the brittle-ductile transition and fluid pressure and wallrock reactions in some systems provide or modify pathways becoming more significant than regional kinematics.

At regional scales compressional and transpressional settings control the majority of deposit types but transtension and true extension (rifting) are characteristic of some high level deposits and seafloor environments. At the deposit scale, local controls are highly variable and rarely relate directly to crustal-scale structures or dilational jogs in these structures. Surface mapping integrated with geophysical, geochemical and drilling information are critical to developing a temporally and spatially constrained framework which is consistent with the mineral system and anticipated paleodepth and hence can be used predictively.