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| Titel |
The Development of Redox-pH-fronts in Hydrothermal Systems |
| VerfasserIn |
Alison Ord, Bruce Hobbs, Dan Lester |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049323
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| Zusammenfassung |
| Most mineralised hydrothermal systems show the development of redox fronts with
mineralisation concentrated at or near the maximum gradient in redox. In many instances a
pH front is also spatially related to the redox front. These fronts occur at all spatial scales
from the regional scale where they define the position of mineral camps, to the local scale
where they define the positions of individual ore bodies, to the outcrop scale where they
define the localisation of grade to the micro scale where they define the positions of metal
concentration within say grains of pyrite or arsenopyrite. Such scale invariance (over perhaps
10 orders of magnitude) is reminiscent of critical systems and is characteristic of non-linear
systems not at equilibrium. One common explanation of the fronts is the mixing of two
or more fluids that originate in diverse chemical and physical environments. In
this paper we explore the proposition that a single fluid is involved and that the
fronts with this associated scale invariance arise from instabilities in a chemically
reacting advecting environment held far from equilibrium by the influx of fluid and
heat.
The keys to the development of both temporal and spatial instability in coupled
chemical reactions are (i) the presence of autocatalytic behaviour somewhere in the
system of reactions and/or (ii) the presence of some process such as mass diffusion,
deformation or fluid advection that physically moves chemical components relative to one
another and relative to some initial reference frame. The resulting instabilities can be
temporal in nature so that the chemical composition of the system oscillates in
time, spatial in nature so that gradients or fronts in chemical composition arise or in
the form of travelling waves so that waves of chemical change move through the
system.
We write the chemical equations commonly used in economic geology to describe the
deposition of metals or sulphides and the formation of alteration systems with the
additional equation Fe2++2Fe3+ -3Fe3++electron . This immediately produces a large
spectrum of unstable behaviour both in space and time. A variant of this equation is
2H++2Fe3+ -3Fe3++H2+electron which introduces a pH inter-relation with the redox
relation. Similar behaviour is introduced by including fluid advection to the standard
equations.
Our conclusion is that the rich variety of spatial patterns that are observed in
hydrothermal mineral systems can be explained, with exploration consequences, by chemical
instabilities arising from the influx of a single fluid. If autocatalytic behaviour is included
than an even richer spectrum of behaviour arises. The observed scale invariance follows
directly from this behaviour. |
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