Insights to High Temperature Geothermal Systems (New Zealand) from Trace Metal Aqueous Chemistry

Abstract

In the period 2001-2005, we deployed a titanium downhole sampler to obtain deep geothermal water samples for trace metal analyses from high temperature geothermal systems in the North Island, New Zealand. Samples were collected from production and monitor wells at ~1 km depth over the temperature range 200-330°C from Broadlands-Ohaaki, Kawerau, Mokai, Ngawha, Ngatamariki, Rotokawa, and Wairakei. Water samples and field blanks were analysed for As, Ag, Au, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, Te, Tl, V, W, and Zn by ICP-MS analysis at CSIRO, and the results show considerable variability. Most data are in the range of 0.1 to 1000 ppb, but Ag, As, Cu, Sb, Ni, Zn, and Te attain concentrations more than 1000 ppb in deep thermal waters from some systems. The concentrations of many metals range 1-2 orders of magnitude variation, but Au, Ag, and Te range 3 orders of magnitude concentration. The variability in metal concentrations shows a weak geospatial correlation and a modest to poor correlation with the concentrations of deep sourced constituents like Cl and B. Arsenic and antimony is an exception with As/Sb ~ 3 to 5 in the Taupo Volcanic Zone, but As/Sb=0.2 at Ngawha. The results show that metal supply is heterogeneous and controlled deep in the system, by a combination of intruding magmas and local country rocks.