GOLD MINERALIZATION AS A CONSEQUENCE OF CONTINENTAL COLLISION - AN EXAMPLE FROM THE SOUTHERN ALPS, NEW-ZEALAND

Formation of gold-quartz veins within the Alpine Schist near the Alpine Fault was synchronous with uplift. The temperature of vein formation was greater than 320-degrees-C at a depth of approximately 4-5 km. Vein precipitation was due to dilution and cooling of metamorphic metal-bearing fluids by cooler, less saline, meteoric fluids. Numerical modeling of the topographic and thermal processes associated with continental collision has identified the time and space scales necessary for the generation of the geochemical conditions which resulted in vein genesis. A predictable topographic pattern of approximately 1500 m high ridges normal to the plate boundary leading to the main divide is established within the first 500 ka of uplift producing the head-driven flow patterns of meteoric circulation. The high thermal inertia of a rapidly uplifted region provides the thermal driving force leading to buoyancy-driven convection of metamorphic fluids when the geothermal gradient is in excess of 50-degrees-C/km. Using boundary conditions derived from the Southern Alps, elevation of temperatures to > 300-degrees-C at 5 km occurs after approximately 1.8 Ma of uplift at which time gold formation may occur if the intersection of head- and buoyancy-driven fluids lies within greenschist facies rocks. Further uplift causes erosion of the greenschist facies rocks together with the gold deposits adjacent to the fault and migration of the loci of mineralization further into the uplifting block. The results of the examination of the Southern Alps gold regions are extended to other Phanerozoic and Archean deposits which are spatially related to major fault zones. The characteristic topographic and thermal behavior of major collisional zones provides the driving forces necessary for gold mineralization to occur in the absence of any igneous heat source.