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Hidrotermal Processes and Mineral System - Franco Pirajno

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out by Pinti (2005) this indeed seems to be the case, because sulphates,
such as barite and anhydrite, are only locally present in Archaean rocks (e.g.
Van Kranendonk and Pirajno 2004; Van Kranendonk 2006), where volcanic
SO2 reacted withH2O at temperatures below 4008C to produceH2S andH2SO4,
without requiring oxygen.
1.2.1 Seawater
The principal characteristic of the water in the Earth’s oceans is their salinity.
Salinity largely derives from two sources: (1) hydrothermal discharges and (2)
weathering of continental crust materials. Evidence from both terrestrial and
marine environments indicates that the salinity of the early oceans had already
been established since the formation of the first large bodies of water more than
1.2 Origin of Water; Sea and Surface Waters 9
4 Ga ago. Thus the primeval oceans must have contained significant concentra-
tions of Cl–, Na+, Ca2+ and Mg2+, with Na+ and Cl– being the dominant
cation and anion respectively (Holland 1984, 2003).
A history of the composition of seawater through geological time is provided
by Holland (2003). This author examined the evidence from the rocks in the
Archaean (4.0–2.5 Ga), such as the 3.87 Ga Isua greenstone belt in Greenland,
in which banded iron formation (BIF) deposits are preserved. Holland (2003)
reasoned that, although metamorphosed, these BIF must have been deposited
in seawater because, as mentioned above, Fe2+ and the silica required to form
these rocks must have been dissolved in seawater after being cycled through
oceanic crust. However, it must be pointed out that some of the field evidence
from the Isua greenstone belt that has been regularly used for proposing early
life and seawater compositions was reappraised by Myers (2004), who cau-
tioned that the preserved examples of biogenic and sedimentary structures may
not be primary but could have resulted from later deformation events. During
the Mesoarchaean (3.7–3.0 Ga) the early oceans must have been saline. This is
corroborated by the presence of sedimentary deposits of evaporitic origin in
rock sequences of Archaean age, such as those of the Warrawoona group
(3.5 Ga) in Australia (Van Kranendonk and Pirajno 2004, Van Kranendonk
2006). In the rocks of this group, well-developed, rosette-shaped minerals
replaced by silica are interpreted to represent casts of evaporite minerals such
as gypsum. In fact, the only evidence that could give clues as to the composition
of Mesoarchaean seawater is provided by the mineralogy and composition of
the rocks from that period (Holland 2003). Minerals such as calcite, dolomite
and aragonite were dominant carbonate minerals in the Mesoarchaean. Side-
rite, on the other hand, was scarce and only found in BIF. The presence of these
carbonates suggests that seawater was supersaturated with respect to CaCO3
and CaMg(CO3)2. The lack of siderite in Archaean carbonate sequences implies
higher Fe2+ content of limestones and dolomites, which in turn implies a low-
oxygen atmosphere (Holland 2003). In the Neoarchaean (3.0–2.5 Ga) rocks of
the Fortescue andHamersley Groups in the Pilbara Craton ofWestern Australia
also provide us with some clues. Although, the presence of microbial biota
(cyanobacteria) indicate the presence of a localised photic zones, the atmo-
sphere must have had little or no oxygen because of detrital grains of pyrite,
uraninite, gersdorffite and siderite in siliciclastic sediments of the Pilbara Craton
(Rasmussen and Buick 1999), which means that the seawater must have been
largely anoxic. Again, this is confirmed by the mineralogy of BIF with the
dominance of Fe2+ minerals, greenalite and siderite, which were deposited in
shallow seas (subtidal, lagoonal and near-shore). The Proterozoic McArthur
rift basin in Australia, which contains giant stratabound Pb-Zn-Ag ore deposits
(see Chapter 8), contains sedimentary rocks (1.7–1.6 Ga, McArthur Group)
with pseudomorphs of anhydrite and gypsum nodules and crystals, halite casts
as well as remnants of sulphate minerals. Like for the Warrawoona Group in
the Pilbara, these evaporitic minerals were formed by evaporation of seawater
in shallow marine environments. The presence of these evaporitic minerals in
10 1 Water and Hydrothermal Fluids on Earth
the McArthur Group sediments indicates that the salinity of seawater then was
much the same as it is today.
However, some scientists believe that the composition of seawater has not
remained constant throughout the geological ages. Spencer and Hardie (1990),
for example, consider that the composition of seawater is controlled by the
mixing of twomajor inputs, namely Na-HCO3-SO4 from rivers and Ca-Cl from
hydrothermal discharges at mid-ocean ridges, oceanic plateaux and back arc
settings. Therefore the concentration of these ionic species in the sea could have
varied with time as a function of the intensity of hydrothermal discharges from
mid-ocean ridges and climatic changes. These authors argue that a smaller
development of mid-ocean ridges would result in lesser fluxes of Ca and Cl
and higher solubilities of Na+, Mg2+, SO4
–2 and HCO3
– from river input and
viceversa in the case of larger mid-ocean ridges. This would also affect the
composition of marine evaporites, in which aragonite rather than calcite and
Mg-rich salts would predominate for smaller fluxes, whilst calcite and K-rich
salts would predominate in the other case. Secular variations in seawater
chemistry have been modelled by Spencer and Hardie (1990) and Demicco
et al. (2005). The major controls of seawater chemistry depend on the rate of
input of solutes from rivers, cycling rates of seawater at mid-ocean ridges and
rates of carbonate production to return seawater (Demicco et al. 2005).
The composition of seawater is defined by salinity, which is the total amount
of dissolved solids per kilogram of water. For the student of hydrothermal ore
deposits it is important to be aware of the dissolved solids in seawater, bearing
in mind that these may have changed with time and may change in specific
environments, as for example, lagoonal, evaporitic ponds, anoxic deep basins.
Table 1.2 gives an average composition of seawater. The precipitates obtained
Table 1.2 Seawater composition; values on the left are from
Goldberg (1972); those on the right are from Seibold and Berger
(1982)
Ion Part per million
(ppm)
Ion Part per million
(ppm)
Ca2+ 410 Ca2+ 400
Mg2+ 1350 Mg2+ 1272
Na+ 10500 Na+ 10561
K+ 390 K+ 380
Cl- 19000 Cl- 18980
SO4
2– 2700 SO4
2– 2649
HCO3
– 142 SO4
2– 140
Br– 67 HCO3
– 65
Sr2+ 8 Br– 13
SiO2 6.4 SiO2 1
B 4.5 B 26
F 1.3 F 0.01
Total 34579 Total 34487
1.2 Origin of Water; Sea and Surface Waters 11
from the evaporation of seawater (evaporites) can constitute important non-
metallic orebodies. Leaching of ancient evaporites may influence the composi-
tion and precipitation of sulphide species, as is the case for the sulphide brines
formed in the Red Sea (see Chapter 4). Table 1.3 lists some of the more common
and important salts of marine evaporites.
One of the most remarkable characteristics of the oceans is that the concen-
tration of those elements that may be harmful to living organisms (e.g. As, Se,
Hg, Pb etc.) does not increase, as might be expected from the annual input from
the rivers. Whitfield (1982) explained the chemical budget of the oceans in terms
of a steady-state model; in other words, material is added and removed at a
constant rate. He also suggested that while the composition of seawater is largely
controlled by geological processes, the biotic masses may also influence modern-
day seas in their control the elements’ distribution, as indicated by the correlation
between the biological importance of an element and its concentration.
Seawater also contains gases in solution (O2, N2, CO2, Ar, H2S) as well as a
host of other elements including Li,