Alkaline sulphate fluids produced in a magmatic hydrothermal system

timescales of a few minutes, and individual springs discharge at rates of less than1 kg/s. The
Rembokola stream is dominated by thermal water contributions in the upstream part of the
catchment resulting in water temperatures up to 80°C. Combined discharge from the springs
was approximately 10–50 kg/s (susceptible to variations in rainfall and spring flow rate).
White siliceous sinter forms a crust over much of the streambed and sediments adjacent to the
hot springs.
2.2.2 Crater Wall Springs – Poghorovorughala
The Crater Wall is the largest and most vigorous of the hydrothermal areas, with springs,
spouters and fumaroles. It is in the upper reaches of the Poghorovorughala stream (Fig. 1), in
the south of the island. The area extends approximately 200 m along the steep-sided valley
from the crater rim, with less than 30 m lateral extent from the stream.
There is extensive fumarolic ground at the Crater Wall area, particularly on the northern side
of the stream valley. This is marked by kaolinite and abundant native sulphur at the surface
and black pyrite-bearing mud beneath. Boiling alkaline hot springs (pH 7–8) are common,
both pools within small depressions and as spouters on the valley wall, and in the stream bed
(marked by bubbling and boiling water as they discharge into the stream). There are a small
number of mud pots, and some small spouters. Many of the springs produce unusual
carbonate + opal-A + anhydrite deposits, as both layered and rounded (lobate) structures
surrounding springs and spouters. Many of the springs identified in 2005 were still active in
2006, although some had deposited sufficient travertine to block the conduits, thus greatly
reducing discharge. For unblocked springs, discharge rates were similar to those of the
Eastern area, estimated to be 0.1–1 kg/s. The combined discharge of the springs is similar to
that of the Eastern area (10–50 kg/s).
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