Mineralization Indicators of Boiling in Two Modern Low-Sulfidation Epithermal Environments: the Broadlands-Ohaaki and the Waiotapu Geothermal Systems, New Zealand

ABSTRACT
Hydrothermal alteration studies of Quaternary felsic volcanic
host rocks at Broadlands-Ohaaki and Waiotapu delineate three distinct
mineral assemblages that are products of boiling in modern
epithermal environments. Each assemblage reflects a specific water
type, (i.e., deeply derived chloride water, and peripheral CO2-rich
and superficial acid-sulfate steam-heated waters) and a specific
hydrological setting (i.e., free fluid ascent, stagnant margin and the
water table).
At Broadlands-Ohaaki, vertically drilled wells (400 to ~2600 m
deep) tap deeply derived chloride waters (>250¡C) to form free fluid
paths to the surface. Platy calcite, electrum-chalcopyrite, and colloform-
crustiform banded amorphous silica deposit in succession at
discrete sites over the whole vertical path length of boiling (>500 m)
and fluid ascent in the well. Rapid cooling and slow reaction kinetics
appear to inhibit deposition of adularia, however, though it is
common with quartz and platy calcite in subvertical veins in drill
cores. The flow conditions in geothermal wells thus resemble those
which form epithermal veins
Steam-heated CO2-rich waters (~150¡C) occur on the stagnant
margins of an upflow zone and at shallow depths below the water
table. The resulting hydrolytic alteration comprises clays (smectite,
kaolinite, illite, chlorite) and carbonates (calcite, siderite), and these
may extend to 1000 m depth. Alteration intensifies towards the margin
of the upflow zone, but the rocks are typically barren of precious
metals, arsenic and antimony. The distribution of temperature sensitive
smectite and illite reflect isotherms and the location of an
upflow zone.
Steam-heated acid-sulfate waters (<100¡C) occur extensively at, and near, the water table at Waiotapu. The resulting alteration comprises alunite, kaolinite, sulfur, marcasite, pyrite and opal, which is also barren of precious metals, arsenic and antimony. Intense alteration by alunite, kaolinite and opal occurs directly beneath and within a few meters of the water table. Their deposition results from neutralization of acid waters. In active epithermal environments, domains of boiling in active epithermal environments and their mineral by-products occur over length scales ranging from <100 to >1000 m distance. The main boiling
indicators include: 1) lattice textures and adularia and crustiformcolloform
banding in veins; 2) extensive clay-carbonate alteration on
the periphery of the upflow zone; 3) acid alteration zoned below tabular
horizons of massive opal that reflect the water table. These features
can assist explorers to assess the level of erosion and the location
of boiling upflow zones in low sulfidation prospects.