TRACE METALS AND VOLATILES ZONING IN ACTIVE GEOTHERMAL SYSTEMS

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.