Geochemistry of sinters of the alkaline chloride hot springs of the Rotorua Geothermal Field

Sinters around the active geothermal springs at Rotorua consist of amorphous opal-A (except those around the Ngararatutara spring, which have in addition various amounts of chalcedony). Fossil sinters consist of opal-CT. Trace element analyses of the sinters show enrichment of As, Ga, Tl, Ge, Hg, Sb, W, Mo, Ag and occasionally Au. The fossil sinters have in comparison a much lower water content and are in comparison depleted of Al, Fe, Mg, Ca, Na, K and P and the traces of As, Ga, Tl, Ag, Sb, Mo and Pb, all due to leaching of sulphates or destruction of sulphides. Ge, Hg and W are not depleted. The presence of up to 17 ppm Ge in the SiO4 lattice is indicative of rapid drop in temperature and pressure of the geothermal fluids.