CONCENTRATIONS AND FLUXES OF PRECIOUS AND RELATED TRACE METALS IN HYDROTHERMAL SYSTEMS, NORTH ISLAND, NEW ZEALAND: IMPLICATIONS FOR ARC METALLOGENY

We determined the concentrations of gold and related metals in deep hydrothermal solutions (~1 km depth, 200 to >300° C) from six geothermal systems and calculated the fluxes of metals for a 250 km long segment of volcanic arc in the Taupo Volcanic Zone, New Zealand. Deep samples (~1 km) of hydrothermal solutions were obtained from geothermal wells with a titanium down-hole sampler, and the solutions were analyzed by ICP-MS. The epithermal environments (<1 km depth) of these six systems are hosted by rhyolite-dacite volcanic deposits, but on the basis of gas chemistry, the three systems on the east side of the arc are intruded at depth by andesite magma whereas the three in the central and west side of the arc are intruded by basalt. Metal concentrations range from <0.1 to 23 ppb Au, 2.7 to 2400 ppb Ag, 100 to 5000 ppb As,15 to 540 ppb Sb, and 0.7 to 80 ppb Hg. The corresponding hydrothermal metal fluxes for the six systems range from <0.2 to >40 kg/yr Au, <10 to >5000 kg/yr Ag, <100 to >30,000 kg/yr As, <40 to >7000 kg/yr Sb, and <5 to >200 kg/yr Hg. While gold and silver concentrations show a broad positive correlation, the concentrations of As, Sb, and Hg vary independent of the precious metals. Speciation calculations indicate that deep solutions are undersaturated in gold and also to some extent undersaturated in silver. The results indicate that the supply of precious metals in deep hydrothermal solutions is influenced by the intrusion of magmas, which also control the convective heat transfer and flux of metals. Two of the geothermal systems (one eastern and one western) have the highest hydrothermal fluxes of gold and/or silver known and can supply enough metal in ~50,000 years or less, assuming no change in concentration or flow rate, to match the inventories of the largest Au-Ag ore deposits in the world, although so far, no ore mineralization has been found in these systems. If the huge amounts of hydrothermal precious metals (80 to 163 kg Au/yr; 6800 to 13,850 kg Ag/yr) that flow through the Taupo Volcanic Zone are representative of all arcs throughout time, then fluid focussing and efficient metal deposition are the most important processes governing the formation of gold and silver ore deposits.