| Title | TRACE METALS AND VOLATILES ZONING IN ACTIVE GEOTHERMAL SYSTEMS |
|---|---|
| Authors | I. Chambefort and J.H. Dilles |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Taupo Volcanic Zone, lithogeochemistry, trace elements zoning, white mica major and trace contents. |
| Abstract | New insights concerning chemical zoning in active geothermal systems of the Taupo Volcanic Zone refine the chemical footprint of both dilute meteoric-dominated and potential magmatic-hydrothermal fluids. Whole rock lithogeochemical analyses (via 4-acid digestion with Au super trace) of altered rocks and in situ major and trace element analyses on white mica at the Ngatamariki, Rotokawa, and Ohaaki geothermal systems, show major variation with depth, due to the influence of past hydrothermal activity (i.e. at Ngatamariki where a shallow intrusion degassed at 0.7Ma), natural variability of the reservoir rocks, and current active fluid-rock interactions. Whole rock concentrations of Au, Tl, Bi, Sn, Ag, Se, Te, generally increase upward toward the paleosurface, where they are 10-100 times greater than near the intrusions that may be the source of metals. At Ngatamariki, samples distal to or post-dating this magmatic-hydrothermal alteration consistently contain less Au, Ag, As, Sb, Bi, and Te than samples of rock affected by the intrusion-related alteration, highlighting the intrusive complex as the likely source of the metals. In contrast to the pre-modern Ngatamariki enrichment, active geothermal fields do not show enrichment in Bi, Se, Mo and Te, and only rarely are enriched in Ag and Tl but show clearly zoning in As, Au, Sb, Sr, Li and Cs. Major and trace element analyses of the white micas are dependent on temperature, water/rock ratio and fluid composition. White micas composition varies from illite to muscovite or phengite with depth with increase in K and Al. Clay trace element content is independent of the illite stoichiometry, and volatile and trace metal element contents correlate well with the whole rock chemical footprint. ‘Shallow’ illite is enriched in As, Te, Cl, S, Li, and B, whereas deeper samples show enrichment in Ba and Tl. Variation in trace and volatile contents between the different site results from the host rock, heat source and magmatic derived fluids. |