FLUID EVOLUTION, FLUID IMMISCIBILITY AND GOLD DEPOSITION DURING CRETACEOUS RECENT TECTONICS AND UPLIFT OF THE OTAGO AND ALPINE SCHIST, NEW-ZEALAND

The Otago and Alpine Schist metamorphic belts have been uplifted and experienced collisional, extensional, transcurrent and then renewed collisional tectonics over the past 120 Ma. Au-bearing veins have been deposited in greenschist-facies rocks at many different structural levels, from > 10-km depth to within a few hundred metres of the surface, in many structural settings during this long uplift history. Fluids which deposited these veins have had a remarkably similar composition throughout the tectonic history. Veins formed near to the brittle-ductile transition and in the lower parts of the brittle region were deposited by water with < 1 mole% CO2 and < 2 wt% dissolved salts. This fluid resembles the metamorphic fluid found in late-metamorphic veins. Veins at shallower levels formed from water with 3-6 mole% CO2 and up to 4 wt% dissolved salts. Fluid immiscibility occurred at shallow levels (pressure < 1400 bar, temperature < 220-degrees-C), and bulk CO2 content may have been up to 8 mole% in these fluids. Fluid sulphur content lay between 10(-4) and 10(-1) m throughout the schist pile. Fluid immiscibility separated H2S into the exsolved phase, decreasing the dissolved S content in shallow fluids by 1-2 orders of magnitude. Deposition of sulphides depleted the fluid S content also by 1-2 orders of magnitude at all structural levels. Fluid S depletion by either immiscibility or sulphide deposition, or both, may have caused Au precipitation in the veins. Sulphide deposition is the mechanism which occurred most widely, and is governed primarily by rock permeability, not rock chemistry.