Volatile and precious metal zoning in the Broadlands geothermal field, New Zealand

Abstract

Au, Ag, As, Sb, Tl, Se, Te, Co, and Bi have been determined by neutron activation analysis in samples from two deep diamond drill holes, hot-spring sinters, and precipitates from discharge waters from the Broadlands, New Zealand, geothermal field. Ore-grade amounts of Au and Ag are being deposited from the dilute hydrothermal waters at the surface, whereas erratically distributed sulfides (mainly pyrite with minor galena, sphalerite, and chalcopyrite) make up to 10 percent of the hydrothermally altered volcanic rocks at depth. Bore temperatures in BR 16 (the main hole studied) range from 280 degrees C at 1,400 meters depth to 60 degrees C at 80 meters depth. The data confirm the existence of a crude metalliferous zoning with As, Sb, Au, and Tl enriched in near-surface regions while Ag, Se, Te, Bi, Pb, Zn, Cu, and Co are concentrated mainly at depth.Deposition of Tl is occurring solely in response to decreasing solution temperatures while the other elements are being precipitated through the combined effects of decreasing temperatures and boiling within permeable zones. It is concluded that they are being transported as thio complexes which become unstable due to loss of H 2 S, CO 2 , and/or NH 3 during boiling. Trace elements may have been coprecipitated with major element sulfides through adsorption and incorporation into sulfide lattices. Solution/wall-rock interactions and oxidation processes appear to be unimportant depositional mechanisms at Broadlands, while the possibility that dilution has caused deposition cannot be properly evaluated.Tl enrichment halos are predicted around many base metal deposits. These could be more pervasive and uniform than those formed by other metals because Tl would have been deposited mainly in response to decreases in temperature away from hydrothermal vents, etc. Since Tl substitutes for K in potash-rich silicates, it may also survive metamorphic effects better than other volatile metals which might also form primary dispersion halos around ore deposits.