The Role of Water in Controlling Metal Concentration in the Crust
B.W.D. YARDLEY, A.J. BENNETT, D.A. BANKS
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
SUMMARY – Water behaviour in the crust is dominated by both the temperature and the direction of
change of temperature of its host rocks. It is a major factor in crustal deformation and its interactions with
rocks dictate both fluid chemistry and sites of focussed fluid flow. The concentrations of a range of metals
in crustal fluids have been investigated using a database of analyses of both fluids sampled by drilling and
fluid inclusions. Results show that concentrations of many metals depend primarily on temperature and the
salinity of the fluid, and there is remarkably little variation that can be ascribed to the effects of buffering by
rocks with different silicate mineral assemblages.
1. INTRODUCTION
Although much of the Earth’s crust contains little
or no free water for much of the time, water is
nevertheless crucial to both the physical and
chemical evolution of the crust. In the accessible
uppermost part of the crust, many rocks contain
free water in pores or fractures, and if suitable
hydraulic gradients exist, large fluxes are possible.
The amount of water decreases in general with
depth in the crust, but while this reflects
compaction in most sedimentary sequences, the
lack of water in deep crystalline rocks arises
because the rocks themselves are a fluid sink,
consuming water as original high-temperature
minerals are converted to hydrous phases stable at
lower temperatures. This contribution explores the
larger scale implications of fluid-rock interactions
during heating and cooling of the crust for
localization of deformation and fluid flow. It
presents data compiled from a range of sources to
show that the compositions of fluids from a wide
range of settings vary systematically as a result of
silicate buffering, and also argues that, because of
the effects of water on crustal rheology, the
probability of these fluids being focused to form
ore zones is greatest in cooled crustal rocks.
2. WATER AND CRUSTAL RHEOLOGY
From the point of view of crustal rheology, it is the
presence or absence of a free water phase that is
critical. Water is reused as it facilitates deformation
mechanisms including both intercrystalline and
intracrystalline processes. Perhaps the most
universal generalization that can be made about
orogenic deformation is that sedimentary rocks
undergoing burial and heating undergo pervasive
ductile deformation and the development of
tectonic fabrics at very low metamorphic grades,
and may continue to undergo pervasive
deformation
throughout
progressive
metamorphism. At low metamorphic grades
however, large igneous intrusions within the
sequence are often undeformed. In contrast, the
same metamorphic rocks do not undergo pervasive
deformation during cooling, even while still at
very high temperatures, which is why prograde
structures and assemblages are often so well
preserved. Instead deformation is localized into
discrete fractures and shear zones, where it is often
accompanied by retrograde metamorphism.
Deformation in the crust is a response to stresses,
but also depends on rock strength. Over the past 20
years it has become apparent that stresses in stable
continental areas are often higher than in zones of
deformation, simply because the rocks are stronger.
If we combine this observation with the
recognition that rock strength is very strongly
dependent on the presence or absence of water, the
consistent deformation pattern of orogenesis can
be explained. Pervasive prograde deformation
reflects pervasive release of water from the
breakdown of hydrous minerals during heating,
while localized retrograde deformation arises from
local loss of rock strength due to the infiltration of
water into rocks that were left completely dry by
this stage in the metamorphic cycle. The source of
water and mechanism of infiltration are difficult to
resolve, but the basic physical picture of a prograde
part of the cycle in which fluid escapes from rocks
pervasively, at near-lithostatic pressures, followed
by a retrograde period in which cooled rocks are
underpressured, and undergo deformation and
retrogression in response to infiltration, seems
robust.
The implication of this physical view of the
behavior of water in the crust is that chemical
effects of crustal fluid flow are in general likely to
be more significant when water infiltrates cooled
rocks than during prograde metamorphism. This is
because retrograde infiltration occurs along
discrete pathways, which are reused, whereas
prograde fluid production is pervasive. In addition,
overall rates of prograde fluid release are very slow,
because they are controlled by regional heat supply.
The principal exceptions to this generalization are
skarn production, and settings where changes in
thermal gradient and uplift mean that prograde
metamorphism occurs beneath cooled crystalline
rocks.
3. MINERAL EQUILIBRIA AND METAL
LOADS OF FLUIDS
Bottrell and Yardley 1989, Heinrich et al. 1992,
Campbell et al. 1995, Kasai et al.1996,
Kamenetsky et al. 2002, Smith et al. 1996,
unpublished analyses of SW England ore fluids).
If it is correct that fluid pressures in much of the
crust approach values controlled by mineral
equilibria, then it follows that the chemistry of the
fluids also must be controlled by mineral equilibria.
The dominant ligand in most crustal fluids is
chloride, since chloride behaves conservatively in
all but halite-bearing rocks, fluid compositions can
be evaluated to a first approximation for their
variations with salinity and with temperature.
Figure 1 demonstrates the variation in Zn with
salinity for a sequence of data sets of fluids formed
at very different temperatures. Since the scales are
logarithmic, analytical uncertainties with some of
the methods are not a first order consideration.
Despite the wide range of silicate assemblages
with which these fluids may have equilibrated, it is
clear that there are consistent trends within each
data set, and that data sets corresponding to
progressively higher temperatures define parallel
trends at progressively higher Zn concentrations.
The temperature effect is clarified in Figure 2, for
which the Zn/Cl2 ratio (defined from the slope of
the trends in Figure 1) is plotted against 1/T. This
plot shows the major effect of temperature on the
Zn-contents of crustal brines, with significantly
higher Zn levels at higher temperatures, even
through the range of oilfield data.
100000
-1.5
-2.5
log mol (Zn/Cl2)
The recent development of microanalytical
techniques has resulted in the generation of a
number of datasets of fluid compositions from
metamorphic and igneous settings to complement
the information about fluid chemistry available
from the results of deep drilling. As a result, it is
possible to find datasets that compare the
chemistry of fluids from the same setting with very
different salinities, and also to compare fluids with
similar salinities but equilibrated with silicate rock
or magma at very different temperatures. We have
compiled a data set of a wide range of natural high
salinity fluids, for which a reasonably wide range
of elements had been analysed, and have used it to
investigate systematic variations in fluid
chemistry.
-3.5
-4.5
10000
-5.5
Zn (ppm)
1000
100
-6.5
0.0005
0.001
0.0015
0.0021/T (K)
0.0025
0.003
0.0035
0.004
10
Figure 2. Plot of Zn/Cl2 (mol ratio) versus 1/T for a
range of analyzed fluids described in the caption to
Figure 1. Note the continuous variation in Zn
content with 1/T even though the fluids plotted
range from low-T shield brines to magmatic brines.
1
0.1
0.01
10000
100000
1000000
Cl (ppm)
Figure 1. Correlation between Zn and temperature
for a range of analyzed brines from drill hole
sampling and fluid inclusion analyses. Filled
symbols: directly sampled fluids, open symbols:
fluid inclusion analyses. Squares - shield brines
(Fritz and Frape 1987), circles - sedimentary brines
(Carpenter et al. 1974, Land et al. 1988, Connolly
et al. 1990, unpublished analyses from N. England
limestone-hosted
orebodies,
triangles
metamorphic and geothermal brines (Banks et al.
2000, McCaig et al. 2000, Meere et al. 1997, Munz
et al. 1995, Williams and McKibben 1989),
diamonds - magmatic fluids (Audetat et al. 2000,
The data set for Zn is typical of other transition
metals for which sufficient data is available to
make the comparisons. Fe, Mn and Pb behave
similarly, although Cu may not.
These plots demonstrate a remarkably systematic
variation in transition metal concentrations in
fluids ranging from deep shield and sedimentary
brines to fluids exsolved from magmas at high-T.
The only reasonable explanation for this pattern is
that the other variables that potentially affect metal
solubility, such as pH, redox and H2S fugacity, are
in fact buffered within quite narrow limits over the
entire range of silicate rock variation that has been
encountered by any of the fluids compiled. It can
be seen that the normal crustal fluid encountered in
relatively high-T environments will become an
anomalously metal-rich ore fluid once it has
cooled . However the variation in metal contents
encountered at a single temperature, after
correction for salinity, is sufficiently large to allow
for the possibility of >90% of the dissolved metal
load (eg Zn) of a fluid being precipitated at
constant temperature due to changes in pH etc.
4. DISCUSSION AND CONCLUSIONS
Combining observations on the influence of
aqueous fluids on rock deformation with the
evidence of the controls on the metal contents of
crustal fluids leads to some general observations
on the factors leading to metal ore concentrations
in the crust, valid at least for transition metals such
as Fe, Zn and Pb. Focusing of fluid flow is more
likely to develop in strong rocks that fracture, and
for crystalline rocks this condition will generally
be met in rock bodies that have cooled from their
original formation, not in those undergoing
progressive dehydration. Fluids within crustal
rocks equilibrate with them and develop metal
concentrations that, for many elements, reflect
salinity and especially temperature. It follows that
the down-temperature part of the fluid flow path is
where ores of these transition metals are most
likely to form. It can be seen from the data now
available that magmatic ores form from brines with
remarkably high metal concentrations in many
cases, so that large fluid volumes are not required.
Sedimentary basin brines may also form ores, but
here it is evident that deep, hot basins that retain a
high porosity occupied by a concentrated brine are
the most likely source of ore fluids.
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