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Hidrotermal Processes and Mineral System - Franco Pirajno

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34 1 Water and Hydrothermal Fluids on Earth
feature of these discharge waters is their poor metal content compared with the
strong concentration in the precipitates (by a factor of up to 106), as may be the
case for Au and Ag. As previously mentioned, some of the critical factors for
this enrichment must be time and depositional rates, as well as the presence of
microbial life. In this respect an interesting calculation first published by
Weissberg et al. (1979) shows that the discharge water of one borehole con-
tained 0.04 ppb Au, or 0.00004 ppm, so that 32� 106 g of Au would be
contained in 800 km3 of discharge water (1 tonne of H2O = 1 m
3). Given that
a natural discharge rate of 1.6 � 106 kg/h (or 0.014 km3 yr–1) was recorded at
Wairakei, then some 800 km3 of thermal water would pass through the system
in 57 000 years. In 300 000 years a total of 4200 km3 of water would circulate
through the system, and given a suitable rate of deposition, an orebody contain-
ing 168 � 106 g of Au could result. Similar calculations carried out by Browne
(1986) indicate that those of Weissberg et al. may, in fact, be conservative. On
the basis of the Au content in scales deposited during discharge from a well,
Browne (1986) estimated a concentration of 1.5 g/t Au in the aquifer, and
envisaged that it would take only 1500 years to transport 32 � 106 g of Au
through the system. Studies of fumarolic gases of Colima volcano (Mexico) by
Taran et al. (2000) revealed the presence of several mineral precipitates, includ-
ing Au crystals up to 40 mm in size, in silica tubes that had been inserted into
8008C fumarolic vent. The mineral precipitates that these workers found in the
inner walls of the experimental silica tube reflected complex temperature zones,
namely: (1) 380–4508C amorphous silica, cristobalite, hematite and various
Na-K-V-Zn-Cu-S phases; (2) 450–5508C Au, cristobalite, hematite, Fe-Ti oxi-
des, tenorite, chalcopyrite; (3) 6008C Au, amorphous silica, cristobalite, hema-
tite, tenorite, WO3, barite, anglesite and various K-Cu-Pb-V-Cl-S phases; (4)
6808C cristobalite, tridymite, hematite, rutile, plattnerite (PbO2), anglesite, Cu-
Sn-Zn-Cl phases; (5) 7408C tridymite, hematite, fluorite, wolframite, K-Na-Pb-
Zn-s phase; (6) 8288C, tridymite, hematite, bunsenite, fluorite, Na-K-Pb-Cu-S
phases, Na-K-Ca-S phases, Fe-Ti-u-S phases, As-Sb-S phase and W-V-Co
phase. Gold was found to have precipitated within a narrow temperature
range of 550–6008C. Taran et al. (2000) calculated that within the above
temperature range, Au precipitation at high fSO2 corresponds to a Au concen-
tration of about 1 ng/kg. Since geothermal fields are known to have been active
for between 1 and 2 million years, the implications of these estimates, even if
taken conservatively, in consideration of less efficient deposition rates, are
obvious.
The exploration for geothermal energy received great impetus in the 1960 s
and 1970 s as a result of the increased oil prices and the political uncertainties in
the oil-producing countries. The result of this exploration, particularly in New
Zealand, the USA, southern Europe and Japan, was of incalculable benefit to
our knowledge of hydrothermal activity, processes of solution, transport and
deposition of metallic elements, effects of water-rock interaction, and in a
broader sense, the ore-forming processes related to the geothermal systems.
There is much literature on the subject, but for the present purpose the reader is
1.4 Hydrothermal Fluids 35
referred to White (1981), Weissberg et al. (1979), Ellis (1979), Ellis and Mahon
(1977), Seward (1979a,b) and Henley and Ellis (1983). Similarities between the
active geothermal systems and epithermal precious and base metal ore deposits
were noted many years ago by Lindgren (1933). It is now widely recognised that
epithermal deposits are essentially the result of the interaction of geothermal
waters with wall rocks, and that many epithermal deposits in the geological
record are in fact the fossil equivalent of ancient geothermal systems. Nowhere
is this in greater evidence than in New Zealand, where subduction-related
volcanic arcs, such as the active Taupo Volcanic Zone and the Coromandel
peninsula volcanics of Miocene-Pliocene age and associated Hauraki gold-
fields, afford a unique opportunity for examining active and fossil epithermal
systems geographically adjacent to one another.
In general, discharge of fluids at the surface is represented by near-neutral
chloride-rich hot spring waters, or it may be characterised by acid-sulphate
boiling pools. The latter are generally related to steam that separates from
deeper chloride-rich boiling fluids in vapour-dominated systems resulting in
fumarolic activity containing CO2 and H2S. Bicarbonate-rich waters are com-
mon in areas where carbonate rocks are present. Deeper in the system, beneath
the levels of boiling and atmospheric oxidation, waters are usually slightly
alkaline (pH 6–7) and weakly saline. Isotope systematic indicate that the waters
of geothermal systems are dominantly of meteoric origin with a possible minor
component being derived from magmatic sources. For the Yellowstone
geothermal system it is estimated that all the chloride waters discharged by
the hot springs could be derived from about 0.2–0.4% magmatic and
99.6–99.8% meteoric waters (Fournier 1989).
Carbonate-rich thermal waters are normally found in areas underlain by
calcareous rocks. Travertine is the CaCO3 sinter, its deposition due to exsolu-
tion of CO2 from the waters as they reach the surface. Travertine deposits are
abundant in the Latium region, central Italy, from where the name originated
(Lapis tiburtina). Low-chloride waters may contain bicarbonate in areas of H2S
and CO2–bearing seam. Bicarbonates are formed by reactions with wall rocks.
The study of degassing volcanoes has gained considerable momentum, par-
ticularly since the catastrophic CO2 eruption in 1986 from Lake Nyos (a crater
lake) in Cameroon. In this eruption CO2 gas welled up from the lake killing
about 1700 people and 5000 cattle. Fumaroles have been studied for nearly 130
years on Mount Vesuvius, where analyses carried out in the late 19th century
revealed high amounts of Cu, Pb, Fe and Sn, but the realisation of a link
with ore-forming systems is comparatively recent. In this respect the work of
Hedenquist et al. (1994) is of considerable importance. These