FORMATION OF OROGENIC GOLD DEPOSITS BY SHORT-LIVED PULSES OF METAMORPHIC FLUIDS DRAINED BY MAJOR CRUSTAL SHEAR ZONES

Orogenic gold deposits are typically adjacent crustal shear zones which themselves are devoid of significant mineralization. Mapping of fluid flow using oxygen isotopes and 3D modeling of transport and reaction in the Val-d'Or goldfield (Abitibi Belt, Quebec) show that the crustal shear zones were hydraulic drains across a low permeability crustal layer. The crustal shear zones draw gold-bearing hydrothermal fluids from the network of smaller shear zones that host mineralization. About 1000 km³ of hydrothermal fluid could be released from devolatilization of a 1000 km² rock column less than 10 km thick during rapid burial in a convergent orogen. Coupled thermal-mechanical models suggest that such a rock column would cross a devolatilization isotherm in less than 5 Ma. Using reasonable permeability, 3D modeling indicates that this amount of fluid can be transported in about 10⁵ a. This short time span is compatible with other estimates for duration of the Val-d'Or hydrothermal system based on solubility (10⁴-10⁵ a), dating of mineralization (< 1 Ma) in the Juneau district, and modeling of crustal dewatering (10³-10⁵ a).

Rapid plate convergence events will bury a pile of volcano-sedimentary rocks causing a short-lived pulse of metamorphic fluid to rise along zones of transient permeability. This results in Connolly's fluid wave that propagates upward carrying gold to the mineralized area. Earthquakes along crustal shear zones cause dilation near jogs that draws fluids and deposit gold in an interconnected network of shear zones. Fluid flow is arrested by an impermeable barrier separating the hydrostatic and lithostatic fluid regimes. The high, local fluid-rock ratios require that fluids flow out of the vein field through the evolving and interconnected network of shear zones and by advection through the rock matrix. Breaches in the impermeable barrier along the crustal shear zones allow fluid flow out of the goldfield.