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Understanding "Bacterial Leach" Technology
For the last two hundred years the exploration for
potential ore reserves has intensified on an
exponential basis. The result of this increase in
effort has been to largely discover and identify the
easily available ore bodies. In the late 20th Century
it became obvious to exploration companies that if
they were to survive into the 21st Century there
would have to be considerable research into new
technologies to discover so called buried or “blind” ore bodies. These ore bodies occur at significantly
greater depth than the ones being mined today and
have a negligible surface expression in terms of
geochemical signature. In addition these ore bodies
are often also geophysically blind thus increasing
the difficulty of their discovery. The task of the
exploration geochemist in the 21st Century is
therefore to make available to the exploration
company drilling targets that, on the balance of
probability, will result in the development of
economic mines.
Traditional geochemical investigations:
They rely on the use of a chemical signature in
soil, sediment or primary rock to identify a
mineralized area. The chemistry involved in the
discovery of signatures largely revolves around the
use of mineral acid extractions, fusions and
cupellation associated with gold fire assay
exploration protocols for gold and mineral based
chemistry. In the last ten years there has been an
increase in the use of partial chemical leaches
which rely on the fact that mineralization at depth
will be subjected to reaction with hydrothermal and
juvenile solutions permeating through the host
rocks. These solutions will eventually reach the
surface and impose a subtle hydromorphic anomaly in
the soil. However, the identification of this
hydromorphic anomaly is extremely difficult because
the concentration of the hydromorphically-imposed
ions is extremely small when compared to the normal
background levels of elements present in “normal” soils or sediments.
Geochemical leach techniques:
Have been developed to try and preferentially remove
the “easily soluble” metal ions and leave the more
stable ions behind. The theory is that the more
stable ions are directly integrated into the lattice
of soil and sediment forming minerals and as such
their presence is not related to emplacement via
leaching of buried minerals, transportation and
adsorption. However, there is a potential problem
with this approach. Depending on the size of the
particles being leached, the nearness of underground
waster, the season (rainy or dry) and associated
minerals, there can be a significant vectoring of
the levels of elements leached. Under certain
circumstances, it is possible to leach more from a
fine-grained, soil not associated with
mineralization than from a coarser soil associated
with mineralization. Obviously this would lead to a
significantly ambiguous interpretation of data. One
further problem is the amount of water movement in
and through the soil or sediment. Drainage patterns
affect this and so too does the amount of rainfall
and the nearness to a rainfall event that the
samples were collected. Nonetheless, the technique
has achieved some success.
The physico-chemical regimes that are associated
with these events are complicated and may differ
from area to area and from mineral type to mineral
type. Ultimately when the transporting solution
reaches the surface it will deposit dissolved ions
as a nano-layer around a locus. This locus is
usually a soil or sediment particle. This process
takes place continuously with new molecular layers
building up on top of each other.
After deposition changes take place in the newly
formed nano-layer. It is now exposed to free oxygen,
to a range of pH changes and to a cocktail of other
elements which affect its composition. New minerals
will be deposition on the surface of these
aggregations on a continuous basis building up a
coating of once soluble compounds over the surface
of particles. This is the coating that most leach
techniques try to selectively remove. However,
soluble metals can be accumulated around the surface
of grains as a result of the drying of rain
solubilized molecules. These latter sub-aerially
deposited metals are not related to any buried
mineralization but can often be extremely high in
concentration and originate from solubilization of a
significant amount of material over a wide area.
Hence they only represent an integrated picture of
the mineral chemistry of the surface of a wide area.
Selective leaching:
Selective Leaching is not chemically sophisticated
and is subjected to normal laws of chemistry
including reaction rate dynamics. This implies that
even less soluble molecules (in this case those that
have been deposited some time previously) will, in
time, also be leached. As can be seen again,
anomalous interpretations can result from the use of
this data.
However, a more detailed understanding of the way in
which elements are leached from primary ore, and
then transported through a sediment or soil column,
facilitates the understanding of what is necessary
in a leach technique to obtain unambiguous results.
Essentially what happens is that the elements in
solution in migrating fluids are precipitated and
redissolved a number of times on their way to the
surface. The solutions, on reaching the surface
deposit their soluble molecular load as a
crystalline or amorphous residue. The process is
repeated again and again and so a coating is built
up around the grains of a supporting sediment. If it
were possible to remove this coating only, as a
molecular layer, it would be possible to identify
only the elemental assemblage related to the last
crystallization event. This elemental assemblage
would be certain to be associated with deposition
from solutions that had been in contact with a
buried ore body because sampling time could be
stipulated and defined to ensure this, simply by not
sampling during or within five days of a rainstorm.
A technique that would facilitate this approach
would be certain to yield results that could
therefore be unambiguously interpreted to indicate a
buried ore body or otherwise.
That is the first problem, the second is to then
have a viable analytical technique that is sensitive
enough to be able to quantitatively determine the
extremely low levels of elements present in such a
leach solution. The third problem is to ensure that
leaching is performed using extremely high purity
chemicals (to reduce blank levels) and in an
environment that is chemically clean (i.e. a clean
room, a facility which is often not appropriate or
available in most chemical laboratories). There is
also a fourth problem, the physical chemistry of all
element is not the same. Not only is it not the same
it is extremely different and is affected by the
presence or otherwise of associated elements. It is
necessary therefore to be able to interpret the data
on the basis, not only of elemental concentrations,
but also on the basis of inter-element associations.
Furthermore, because the levels of these leached
elements are so low the chance of adsorptive
precipitation is high and consequently it is
necessary to ensure that they are in some way
complexed or sufficiently acidified to reduce
adsorption and precipitation.
Use of extra acids as preservatives:
This introduces the possibility of increasing baseline levels and thus reducing the resolution of the
technique and missing the subtle anomalies that the
leach techniques are designed to find. Only in these
ways is it possible to truly identify the presence
or otherwise of buried ore bodies. All four of these
problems have been solved using the bacterial leach
technique.
The bacterial leach technique:
Samples are collected and returned to the laboratory where they are exposed to a solution containing modified bacteria. The bacteria become intimately associated with the surface of the ore grains, enabling the bio-magnification of metals present in the grain coating. After a set point in time, the solutions are analysed using highly sensitive analytical instrumentation; inductively coupled plasma mass spectrometer (ICP-MS). This instruments has the capability of analysing a wide range of elements (>60) simultaneously. As the extraction process is carried out in an entirely sealed environment, the potential for contamination is avoided. This leaching technology has application in the search for kimberlitic, base metal and precious metals deposits. (See
Flow Chart).
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