GEOELECTROCHEM1CAL
PHENOMENA
IN
PLACER
DEPSOITS
GEOELECTROCHEMICAL PHENOMENA IN PLACER DEPOSITS. PROBABLE ROLE OF ELECTROCHEMICAL FIELDS AS A
WHOLE FOR GENERATION OF REGOLITH.
ISSAI S GOLDBERG
IONEX Pty Limited Level I, 55 York Street, Sydney, N5 W; 2000, Australia
ABSTRACT
As a rule, interpretation of regolith formation is done at present without reference to electrical energy
Exceptions are studies that describe processes of oxidation and reduction in sulphide deposits
The three
examples considered in this article propose significant participation of electrical energy, and therefore
electrochemical processes in the transformation of geological environments
Key words: geoelectrochemistry, electrofiltration potential, liqUid-junction self-potential, dissolution, migration,
accumulation, ion
INTRODUCTiON
The
Significant transformation of rocks takes place in the huge
concentration gradients inevitably create
areas with regolith development between 30 0 N - 30°5
potential gradients within geological environments
gradients
of
pressure,
temperature,
and
electrical
A
wide spectrum of electrical energy form has been
(Smith, 1996) The rocks are reprocessed and altered as
revealed within earth and they manifest as filtration,
a whole to the depths of up to 500 meters and, in areas
diffusion-absorption, galvanic and others local electrical
with ancient crust, weathering alteration can be seen to
fields(Kunory et ai,
depths of 2-3km (rich oxidized ores of Krivorozhsky type)
1966, semenov,
1955, Keller and Frischknecht,
1968,sheriff,
1974) These natural
electrical fields may exist for a long time depending on
The largest supergene deposits of AI, Fe, Mn, Ni, Co, Ti,
the duration of the geological processes, which
Au and other metals are known in the areas of regolith
creating their The energy of such fields may be of the
development Their generation is often accompanied by
order of 10-100 joules/cm 2
segregation
into
monomineralic
forms
are
This
The main electrochemical processes operating in
phenomenon is associated with matter separation The
the
geological environment are associated with dissolution and
processes involved can probably proceed only with
deposition of matter, migration of dissolved components
energy input because any such concentration process
within an electric current field, as well as the accumulation
cannot be spontaneous (safronov, et ai, 1978)
of matter within specific parts of the field It is not possible
to cover the whole range of problems associated with the
Sources of energy
under surface and near-surface
conditions
include:
•
water exchange with mechanical transfer
active
role of electrical energy in the formation of regolith in this
paper These geological "problems" have been chosen for
of surface and underground
discussion to illustrate electrochemical concepts
waters in response to
The three examples to be discussed are:
pressure gradients (filtration of solutions);
•
•
solar
energy
processes;
which
induces
active
biologic
native elements (Voronin and Goldberg, 1972)
the importance of this type of energy,
•
manifested through living matter, was first noticed
•
Mineral genesis in deposits of placer gold and other
"Uphill" movement or migration of metals and their
accumulation compared with concentration gradient
by VI.Vernadsky as early as the 1920s (1955);
(Alekseev and Dukhanin, 1989; Alekseev, Goldberg
chemical potential gradients (especially interact with
and Dukhanin,1989a, 1989b)
near-surface oxidizing environments abyssal rocks
•
and the boundary between fresh and saline waters)
Formation
of
rich
oxidized
iron
ores
Krivorozhsky type (Goldberg et ai, 1988)
89
of the
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The filtration stream in the natural environment is
MINERAL GENESIS IN PLACER DEPOSITS OF
GOLD AND OTHER NATIVE ELEMENTS.
localized within some volume (riverbed, fractures in
There are number of features of placer deposits that, in the
rocks,
written opinion, have not been satisfactory explained For
potential value of a river stream ranges from tens to
mechanically weak zones) The electrofiltration
particular, whereas placer form in oxidizing environment
thousands of mV; especially so in areas of mountain
some the processes appear to be reducing in character
rivers - up to 1 V/km
Some characteristics of placer deposits are as follows:
1
One of the manifestations of this electrochemical
Native metals found in the earth's crust occur more
process is in charging electronic conductors A potential
often in placers than in bedrock;
2
difference is generated between the ends of such a
All placer metals (ZnD, FeD, SnO, PbD, BiD, CuD, AsD)
conductor, which is defined by the potential decrease in
except for gold, platinum group metals and mercury,
3
the field corresponding to the length of the conductor
- are unstable under normal surface conditions;
Electrochemical reactions caused by cathode reduction
Native metals in placer occur in mutual intergrowths,
and anode oxidation can start at definite potential values
that are unknown in bedrock sources These include
on the ends of the conductor The most typical reduction
Pt-Pb, Sn-Pt, Sn-Au and some other unusual metal
reactions are reactions that precipitate elements out of
associations (Vernadsky, 1955)
solutions like (Glasstone, 19S6 ):
Me+ + e- = MeD
Vernadsky (19SS) suggested of native metals directly
Examples of electronic conductors in placers are
in placers This was confirmed by finding secondary gold,
overgrown over primary gold
particles of gold, platinum, magnetite, graphite etc
The secondary gold is
Experimental investigations using models have been
usually developed on one side of a primary gold grain
carried out to define possible mechanisms for initiation
(Fig 1) Such asymmetric gold growth, which can be
of electrochemical processes on such particles by the
distinctly seen in microsections and has been described
in a number of
papers (Iabloneva,
1965),
electrofiltration field (Voronin and Goldberg, 1972)
has
nevertheless, never been explained
EXPERIMENTAL INVESTIGATION
An electrofiltration field was produced by forcing distilled
water through a column of quartz sand (0 1-0 2mm
fraction) 2cm long and lcm2 in cross-section (Fig 2)
Potentials were measured relative to standard calomel
electrodes using a high-ohm voltmeter
Current was
measured using a highly sensitive micro-ammeter Platinum
wire was used as an electron-conducting electrodes When
Figure 1: Dendritic branches oj "new gold" from
pressure from S to 120mm was measured (corresponding
placer deposit (after Iabloneva, 1965) 7x
to an elevation difference from 0 06 to 16m) the streaming
potential varied linearly from 0 1 to 1 3 V
The observations outlined above demonstrate a conflict
Electric current occurred within the column when platinum
between the oxidizing nature of the environment and
electrodes were connected up via a micro-ammeter
apparent reducing process observed in placers
The current value also varied linearly with pressure variation;
Let
us
consider
this
electrochemical point of
phenomenon
view
from
the
current varied from 1 x 10-8 to 4
X
10-7 A
Firstly let us discuss
some processes proceeding in alluvial deposits in river
valleys where
placers are being formed Movement of
solutions through porous media results in an electric
potential difference
as the streaming
This potential difference is known
potential
Bogoslovsky and Ogilvy,
Quarto, R , 1984)
(E f )
1972
(Semenov, 1968;
Schiavone, D
and
In Western geophysical literature it
is also known as the streaming
potential
Figure 2: Electrochemical Nucleus 1
Non-polarizable
measuring electrodes 2 Platlum electrodes 3 Quartz sand
4 Quartz membranes
or
electrofiltration potential (Sheriff, 1974)
9D
GEOELECTROCHEMICAL
PHENOMENA
IN
PLACER
DEPSOITS
hydrogene scale) At a current strength of 2 x 10-8 A the
DISCUSSION OF THE RESULTS.
The occurrence of an electric current in the conductor
potential on
indicates the induction of electrochemical reactions
anodic value of +0 116 V (or +0 336 V according to the
within it They are observable even at a low electric field
hydrogen scale - h s) At current strength of 1 x 10-7 A it
potential
was -02 V (or -OOSV according to h s) The defined
defined
The possibility of one reaction or another is
by
the
electrode
potential
and
the platinum cathode end changed to an
solution
potentials are sufficient for precipitation of elements with
composition In this experiment, the electrode potential,
reduction potentials more positive than the measured
measured on platinum prior to occurrence of the electric
field
was
values They include the whole group of elements from
Sno to Auo
equal to +0 12V (or +03V according to a
listed in Table 1
Table 1: Normal electrode potentials
(0, V
ElEaRODE REACTION
(0, V
-0763
Bi 3+ + 3 e = Bi O
+023
Cd 2+ +2e = Cdo
-0402
As 3+ + 3e = AsO
+030
C0 2+ +2e = CoO
-0.270
Cu 2+ + 2e = CuO
+034
Ni 2+ + 2e = NiO
-0230
Ag+ + e =AgO
+070
pb 2+ + 2e = pbO
-0127
pd 2+ + 2e=pdO
+0.83
Fe3+ + 3e = FeO
-0036
Hg2+ + 2e = HgO
+090
H+ + e = H201/2
0000
Pt2+
+120
Sn 4+ + 4e = Sn O
+0050
Au 3+ + 3e = Au O
+1.43
= Sb O
+0200
Au+ + e= AuO
+170
Ell<TRODE REAaION
Zn 2+ + 2e = Zn O
Sb3+ + 3e
+ 2e = PP
Normal electrode potentials are shown for standard
electronegative metals - silver and copper - occur less
conditions
the
commonly in placers Metals like lead and tin are very
concentration of metal ions in solution will differ from
rare and native zinc (most electronegative among native
that under standard conditions and the potential values
metals) is unique to these settings
However, under natural conditions,
in the table will be different (Cammann, 1979)
The
dependence of the electrode potential on ionic activity
As gold and platinum are most abundant in placers,
(activity approaches concentration in dilute solutions) is
reductive reactions with cathode precipitation proceed
defined by Nemst equation (Davies and James, 1976)
mainly on them
This equation can be used to calculate for example, the
Thus, appearance of exotic mineral
concretions is natural (PtO - Pbo; Sno -Au and others)
gold potential at ionic concentrations corresponding to
the average gold content in natural waters 0 002 ppb
(Levinson, 1974) The calculated potential (+ 1 06 V) is
The
much more positive than experimental values The gold
secondary gold on primary gold grains is explained by
phenomenon
of asymmetric
precipitation
of
content in waters in auriferous provinces is typically one
the fact that the process of precipitation in the electric
or two orders of magnitude higher than unmineralised
current field proceeds only on the cathode side of the
regions
(Giblin,
A,
1997),
and
in
such
cases
electrode conductor
precipitation from solutions is more likely
The electrochemical of mineral genesis
The application of the electrochemical
mechanism
has strict
"regularities" which leads to the following conclusions:
outlined above in placer deposits may be applied as
follows Abundance of native metals is defined by the
• new formation minerals of copper, silver, platinum and
values of their reduction potentials The most abundant
metals of the platinum group may be expected,
and
which, like gold,
most electropositive metals are in placers (gold,
platinum,
metals
of the
platinum
group)
primary minerals;
More
91
can be precipitated on their own
ISS
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• deposition of these new minerals can be asymmetric
Diffusive potential arises in the earth at the boundaries
where the asymmetry should correspond to the
of rocks of different composition, at the boundaries
electric field direction;
between layers in rocks of different composition and
• if the occurrence native zinc in placers is true, all other
salinity
or
concentration
differences
between
electropositive metals (chromium, cadmium, cobalt,
underground water and surface groundwater etc
nickel, antimony) could also be expected in the
value of diffusive potentials (DP) in the earth accounts
metallic form
The
for tens and, in some cases even hundreds of millivolts
• since the quantity of metal participating in the reaction
(Semenov, 1968)
(precipitation) is determined according to the second
Faraday law, then most
• Systems
new species (minerals)
with very high
DP
values (hundreds
formations might be expected in the most ancient
millivolts) in the earth arise under extreme condition:
placers and within
on the boundary between highly concentrated acid
areas with
the highest electric
potential gradient;
(>lM) or alkaline solutions and neutral ones (Bischoff
etal,1996)
• these patterns are true not only for gold and platinum
placers but also for placers of magnetite, titanium-
• Systems with DP values up to tens of mV arise on the
magnetite and others
boundaries between highly mineralized NaCI, Na2S04
and fresh waters
Study of these and other observations concerning the
The DP value comes to tens of mV
(25-35mV) when the difference in concentrations is
processes of mineral genesis in placers will allow
100-1000 times
estimation the scale of these processes and conditions
These conditions are typical for
geological environments
of their formation
• Systems with DP of several mV include boundaries
between weakly mineralized waters and fresh waters
MIGRATION OF MBTALS AND THBIR.
ACCUMULATION COMPARED WITH THB
CONCBNTRATION GRADIBNT ("UPHILL"
MOVBMBNT).
An "averaging effect" is usually observed in the earth It
An unexpected effect was noted when working with the
and selective sorption of individual ions
method of diffusive extraction (MDE), (Alekseev and
geophysical literature such potentials were named
Dukhanin,
diffusive-adsorptive
1989;
Alekseev,
Goldberg
is characterized both by different coefficients of diffusion
and
Dukhanin,1989a, 1989b) The method was intended for
use in the search for buried ore depOsits
(electrochemical SP)" (Sheriff, 1974) Their
follows from field observation,
( "element-collectors"), filled with electrolyte are placed
analyzed
for
chemical
In
English
values, as
sharply differ from
theoretical values calculated by the equation for water
into soil They are placed for some time and then the
is
1968)
geophysical literature they are known as "shale potential
According to
method capsules made of an electroneutral membrane
solution
(Semenov,
In Russian
solutions As a rule, such potentials are much higher than
elements
diffusive potentials and may be equal to hundreds mV
Accumulation of ions in the capsule solution is often
observed in concentrations exceeding their content in
the enVironment, i e an "uphill" movement takes place
Ion flows, which are
The concentration of metal in the "element-collectors" at
diffusive (diffusive-adsorptive) potentials, are described
higher concentrations than the in interstitial solution was
by the Nernst-Plank equation (Glasstone, 1956, Davies
considered by us to be due to electric potentials existing
and James,
on the boundary of the media This potential was called
geophysical
was named
literature
this
"liqUid-junction electrochemical
potential
SP
takes
into account
field
gradient
concentration gradient for transfer of components
(DP), and the aSSOCiated electric field was named
In
It
contribution of the electric
the "diffusive potential" in Russian geophysical literature
"diffusive electric field" (Semenov,1968)
1976)
moving in rocks in presence of
geological
English
literature
only
discusses
transfer
the
and
The
of
components under the influence of concentration
gradient
(self-potential)
(Sheriff, 1974)
The contribution of the electric field gradients
is usually not estimated
92
GEOELECTROCHEM1CAL
PHENOMENA
IN
PLACER
DEPS01TS
Experimental investigations have been performed to
The resulting values characterized DP between the
estimate ion concentration under the influence of
investigated solutions in the system The initial DP value,
diffusive potentials
arising in the system just after the capsule with acid had
EXPERIMENTAL INVESTIGATIONS.
Ion mass transport processes have been studied in
laboratories
time,
with
DP
values
decreased
synchronously
membrane and after 2 hours the difference in electric
investigated: a contact between two media of different
Two solutions of different
With
equalization of acid concentration either side of the
One common geological system was
composition
been put into the acid solution, was 110 mV
potential decreased to several millivolts
composition
Thus, the
and concentration, divided by a cellophane membrane
chosen model system provided DP life during a limited
served as a model of such system A composition and
period of time (2 hours)
concentration of solutions were selected in order to
create electric fields values similar to that occurring
Osmotic effects were also observed in the system,
naturally (With an absolute value of about 100 mV) Such
manifest as an increase in solution volume in the
a diffusive potential (DP - 100mV) was determined by
capsule
using the Henderson equation on the contact between
maximum volume increase in the capsule did not
001
exceed 10% of the initial volume
M
salt
solution
KF
and
1M
HN0 3 "
These effects were not significant because
These particular solutions were used for the initial
experiments
polyethylene
Salt solution was poured
glass;
a
into a
DISCUSSION OF THE RESULTS.
cellophane capsule with acid
solution was place into its center
Initially potassium concentration in the inner capsule
The experiments
increases and after 0 5 hours from the beginning of the
permitted the study of quantities of components that
experiment it becomes higher than the concentration of
moved through the membrane during the period of
observation
the ions in the original solution
To achieve this, the capsule was taken out
of the salt solution after a fixed time, cut and solutions
value and then begins to decrease
from it and from the glass were analyzed for all the
elements included
in
the
(Fig 3)
After three
hours the potassium concentration attains a maximum
Note that in open
natural systems the concentration process is sustained
system In the solutions,
concentration of hydrogen ions was determined by
over a long time Fluorine ions diffuse into the capsule at
titration with sodium hydroxide and methylorange, nitrate
a slower rate
than equivalent quantities of potassium
ions by
ions (Fig 3)
Only after 8 hours does concentration
the fluorocolorimetric method with a phenol-
disulphide
acid, potassium ions by flame photometry,
of potassium and
and fluorine ions by fluoride-selective ion electrode
During
the experiments,
differences
in
fluorine
ions
in
the system
become equal
electrical
The established difference in the behavior of potassium
potentials between the investigated solutions were
and fluorine ions corresponds to the variation DP The
measured with the help of saturated chlorine-silver
highest rate of potassium ion diffusion is observed at
electrodes DP values on the boundaries between non-
the initial moment when DP is the greatest The rate
polarized electrodes and solution were deduced from this
difference The calculations were carried out according to
of potassium ion diffusion decreases with decrease of
Henderson equation (Davies and James, 1976):
DP value
its
pH = 112 pKw - 112 pKA - 112 Ig c
When the DP value falls to several millivolts,
effect on
negligible and
Were:
the
potassium
influence, of their
- Ig c (W )
mass
transfer
process becomes
ions diffuse under the
concentration gradient from
- constant of ionization of H 20
capsule into the surrounding
- constant of ionization of acid
dynamics in the of behavior potassium ions
considered
according
to
salt
The
can be
Nemst-Plank equation as a
result of DP alteration in the system
93
solution
the
ISS
A
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G
0
L
D
B
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To confirm the relationship between the electrical field
generated and the rate of ion diffusion, the system was
modeled where DP values were
whole experiment
minimal during the
Potassium and fluorine ions were
transferred into distilled water The measured DP values
for such a system did not exceed several millivolts In
this case potassium and fluorine ions (Fig.3) diffuse
in equivalent quantities (or at a similar rate)
•
1
o 2
~ 3
t:,. 4
The behavior of magnesium and chlorine ions is similar
to that of potassium and fluorine ions during
their
Time,h
diffusion from the magnesium chloride solution into the
Figure 3: Change oj the relation oj concentration ions
(potassium and fluorine) In the capsule to concentration
into surrounding salt solution by diffuse
Potassium
the case where DPs at the initial moment are maximum
- in nitric acid solution (1)
in di.stilled water
Fluorine
nitric acid solution Experiments have been performed for
and decrease up to several mV after 2 hours and into
(2)
distilled water, when
- in nitric acid solution (3)
in distilled water
DPs during the whole time of the
experiment do not exceed several millivolts (Fig 4)
(4)
Thus, DPs arising in the system, result in speeding up
cation flow relative to anion flow Electric neutrality of
solutions is most likely maintained at the expense of
a reduced flow of nitrate ions from the acid solution
compared with hydrogen ions
The results demonstrate the posSibility for matter
accumulation provided that the boundaries between the
different
Such
•
o
1
2
media
are
conditions
are
kept
as
a
in
rule
non-equilibrium
maintained
in
geological environments
~3
.64
Description of ion diffusion processes versus
the
Time,h
gradient of their concentration by Fik's Law (Glasstone,
1956) is impossible for the cases shown in Fig 3 and 4
Change oj relation oj concentration ions
(magnesium and chlorine) in capsule to concentration into
surrounding salt solution by diffuse
Figure 4:
J1!Iagnesmm
-
Chlorine
- in nitric acid solution
In
nitric acid solution
in distilled water
In
distilled water
Separation of ions according to the sign of their charge
(1)
can proceed depending on the DP sign with zones of
(2)
(3)
local enrichment They can arise on the contacts of
(4)
rocks or natural waters different in composition; it may
be especially active near the boundaries between fresh
and salt waters and other contrasting media
94
GEOELECTROCHEMICAL
PHENOMENA
electrode
IN
PLACER
DEPSOITS
Content of iron oxides and pH value in this
FORMATION OF RICH OXIDIZED ORES OF
KRlVOROZHSKY TYPE.
solution were determined
Genetic conditions for the formation of the rich oxidized
varied (depending on the duration of the experiments)
iron ores of the Krivorozhsky type have been discussed
from 0 4 to 2mglL
Iron content in the solution
for a long time (Belevtsev et ai, 1981) There is also no
common
view
on
the
mechanism
for formation
X-ray diffraction analysis of the precipitate revealed two
of oxidation zones at depths of 2-3km or more, or on the
phases: goethite and magnetite The latter is precipitated
nature of processes responsible for intensive dissolution
on
and removal of quartz, carbonates, silicates on the
electroneutral edge of the rhombododecahedron
lone hand and oxidation and retention in place of
the
original
magnetite-electrode along the
The
content of ferrous iron in the magnetite lattice increased
magnetite (martite) and additional enrichment with iron
in the upper layer of the electrode from 24 to 27"10
- on the other At present, the chemical mechanism of
these processes is not well understood,
because,
A develop
though it was possible to increase quartz solubility in
produced
experiments, the main contradiction - dissolving and
removal of quartz and
retention of iron
electrochemical-reducing reaction is
magnetite ore bodies are electrical conductors
of rich
Basin
iron
ore
are significant in size
reached
in
2Fe304+4e = 6FeO + O2 - electrical stage,
- decay,
4FeO = Fe304 + aFeD
2aFeD+l 50 2 = Fe203
- chemical stage,
the Krivirozhsky
wide
interval
of
(1)
(2)
(3)
According to Bol, (1962), a FeD ( 2 ), produced in
be lOOO-llOOm
in length ; and extend 3000m down dip
of
combined with a
Their thickness may
200-240m, they may
connection different parts
the
chemical oxidizing reaction according to the scheme:
explanation of their genetiC formation
deposits
contradicts
explained by the gradation of the process, where
and it is reasonable to apply electrochemical laws for
The
which
expected reaction process This contradiction may be
(magnetite)
under natural conditions - was not solved At the same
time,
goethite (the complete iron oxide) is
simultaneously,
ore bodies
physical-chemical
chemical process, is stable and it is oxidized at the
In this
exist
expense of O2 (1) to goethite or hematite depending on
oxygen abundance of the medium and temperature This
in
conditions
These conditions include: variation in ion concentrations
gradated process proceeds on the surface of the
of ferrous and ferric iron and other potential-producing
electrode and so,
secondary magnetite
should
be
ions in the solution associated with irregular distribution
precipitated naturally according to the direction of the
of magnetite in ferruginous rocks ( Ryss,
electric field, and oxidized iron acquires the form of
1983)
From this condtions it follows that occurrence of
a
galvanic
element
in
the
system
pseudomorphic substitution
between
ferruginous quartzites and ore is possible
Anode area" The study of processes proceeding on the
The character of galvanic processes can be estimated
anode was carried out using the same method as for
based on the composition of rocks and types of reaction
cathode
proceeding within electrochemical "chain"
The chain
processes
proceed),
and
in
this
case magnetite was
naturally polarized on the anode Experiments lasted
consists of three parts: anode and cathode (where
electrode
reactions;
from 20 to 130 days without
intermediate,
magnetite
responsible for ionic processes Electrochemical studies
visible
changes
to
Iron content in solution of anode area
increased (from 1 3 to 5 0 mgll) From 25 to 50"10 of the
of every part of the system have been carried out to
soluble iron went into cathode area
estimate the role of the galvanic mechanism (Goldberg
The pH value
in
the anode area varied from 6 to 2
et ai, 1988)
Appearance of iron ions in solution points to dissolving
EXPERIMENTAL STUDIES.
of
Cathode area
Study of the processes proceeding on
precipitation near the anode and it is transferred to the
the cathode was performed by cathode polarization of
cathode area, where it is partially precipitated As a result
magnetite in a solution of 0001N NaCi
of
(pH = 5-6)
magnetite, but
such
a
a low
process
the
pH
value prevents iron
magnetite-anode
looks
Platinum, placed into another glass, connected with the
unchanged In more prolonged experiments the sign of
first one by a salt bridge,
magnetite dissolution should clearly appear
served as an additional
95
GOLDBERG
The intermediate area corresponds to the outer chain of
the galvanic element Here, electrochemical processes
proceed on minerals, mainly with ion-covalent bonds
(dielectrics and semiconductors)
impregnations and veinlets of
electronic conductivity,
If the rocks contain
ore
minerals
with
the cathode-anode processes
outlined above will develop on them
The
study
of
processes
in
ferruginous
+
rock
transformation within the electric current field,
was
carried out in an electrochemical cell (Fig 5) This cell
was invented in
1983
(Alekseev, Goldberg
and
1983); This method of concentration of
Dukhanin,
Figure 5: Electrochemical Nucleu.s 1 - Nucleus, 2 & 9 Platinum electrodes, 3 & 8 - Brim cells, 4 - Parchment
membmnes, 5 - Intermediate cells, 6 - Centml cell, 7 - Nonpolarizable measuring electrodes, 10 - Ammeter, 11 - Direct
current source, 12 - Voltmeter
elements was adopted as a principal of both laboratory
and field investigation by the CHIM method (Alekseev et
ai, 1983; Alekseev et ai, 1981)
Table 2:
Electrochemical extmction oj elements out oj carbonate-magnetite quartz
MASS 01 ELEMENT IXTRACTED OUT 01
1G OF ROCK (MIXT), MKG
ELEMENT
CATHOLITE
ANOLITE
TOTAL
MASS OF ELEMENT
IN IG OF INITIAL
ROCK (MINI), MG
RATE OF
DEGREE OF EXTRAG
EXTRAG
MEXT/MINI
MKG/H
xl00%
Si
10
I
6439
6540
1689
07
0.4
AL
6220
65
6280
14.2
07
4.4
Fe
6320
2.5
634.0
351.9
07
02
Co
4800.0
48000
12
I
53
390
Mg
35000
35100
16.2
38
21.7
100
The nucleus (1) consisted of the seven cells (3,5,8),
ions from the central cell It is one of the most efficient
which were divided by parchment membranes (4)
of method to impede diffusion of electrolytes
A
cathode and anode cells
pulverized, sieved and moist sample (25g) was placed in
the central cell (6)
from
Other cells (5) were filled with a
In the central cell, pH was approximately 6-7
001 N solution of UN0 3
It was
achieved by means of neutralisation of the hydroxyl ion
in the cathode cell by HN03 acid and the hydrogen ion
The platinum electrodes (2,9) were located in the outer
in the anode cell by UOH alkali The concentration of the
cells (3,8) and the current switched (10,11,12) on
acid in the cathode cell and the alkali in the anode cell
through these electrodes
was calculated to take account of the quantity of
The intermediate cells
between the electrodes cells and central cell were used
electricity and mass of the hydrogen and hydroxyl ions,
to ensure stable conditions for retention of the metals
which were created by the electrode processes
96
GEOELECTROCHEMICAL
PHENOMENA
IN
PLACER
DEPSOITS
Ferruginous quartzite was the object of investigation
Its rock composition is: magnetite - 40%, quartz - 30%,
practically at any depth In addition to maintenance of
siderite - 30% Chemical composition of the rock
Fe that is transported into the cathode area and
iron minerals, they are being enriched at the expense of
is:
siO%2 - 36 13%; Ti02 - 0.06%; AI 20%3 - 2 69; Fe20%3 -
precipitated in the alkaline medium in the form of oxides,
25 84; FeO - 22 0%; MnO - 0 3%; MgO -2,7%; CaO -
hydroxides, and siderite Simultaneously,
1 69%; K20 - 0 2%; Na20 - 0 18%; 5 - 006%; Pe205 003%; H20 - 007%; losses by ignition - 854%; 5 10049% The 0 1-0 2mm fraction was used in the
experiment, its moist was 25% The experiments were
migrate from this area to an anode area
content of quartz
silicon
ions
As a result the
in the ore bodies decreased or the
quartz depletes completely
performed at 200C ± 5 and atmospheric pressure
In the anode area of Fe and other cations deplete and
The electric field intensity (2 V/cm) was maintained
more si accumulates
constant in the cell central (6) (fig 5) The average
current density was 0.02 A/cm 2 Duration of the
data, the amount of si accumulation exceeds the
experiment was 900 hours
According to the experimental
amount of depleted Fe, and, as a consequence, dense,
Extraction of various
cemented silicified rocks will be
elements out of the rock was observed during the
part of the system
process of the experiments by means of regular (after
formed in the anodic
Another specific feature is the
presence of iron only in the form of magnetite Thus,
40-50 hours) analysis in the cathode and anode
cameras Ca and Mg were determined using Trilon - B;
unlike the cathode, the magnetite-anode is dissolved but
AI, si, and Fe - using photocolorimetry
stays unchanged as a mineral type
The results of the experiments are shown in Table 2
The intermediate area of the electrochemical chain is
They indicate that:
characterized
by
the
electrochemical
process
all the enumerated elements are extracted out of
extraction of si and Fe
rocks electrochemically;
zone are: the dissolution of carbonates and silicates,
si is transported into the anode area and all the other
significant leaching of quartz, martinization of magnetite
elements - mainly into the cathode area;
and its partial dissolution, and, as a consequence of
the
these processes,
rate of component extraction is in inverse
proportion to their content in the rock;
quartzites
The specific features of this
disintegration
Spatially,
of
disintegration
ferruginous
areas
are
- according to the degree of extraction the components
concentrated around rich ores This is associated with
are arranged in the row (Ca, Mg, AI, si, Fe) which
the smaller size of the cathode area as compared with
contrasts to that of their content in the initial rock
the anode, which is the reason for the higher current
density
and,
consequently,
more
A similar mechanism of dissolution is observed in other
electrochemical processes (Ryss, 1983)
intensive
A geological
rocks as well (Goldberg, 1979) The degree of extraction
analogy of the above mentioned part of the galvanic
means that the relationship between the mass of
sequence may be zones of leachedjaspilites - similar to
extracted component and its content in the original rock
formation of loose quartz bodies in sulphide deposits
is directly correlated
The electrochemical model of rich oxidized (martinite)
ore formation, developed from the experimental
DISCUSSION OF THE RESULTS.
The problem of the formation oxidized ores of martite
bodies The spatial position of the discussed zonality is
and hematite - martite composition could be described
determined by the structure of a natural galvanic element
by considering the experimental data
A geological analogy of
oxidized
ores of
composition
It may
The results from the experiments and average values
the cathode area should be
martite
be
and
of natural electric field intensity allow one to estimate the
hematite - martite
concluded
data,
generally corresponds to the geologic structure of ore
from
approximate
the
time
required
for
electrochemical
experiments that magnetite ores can be oxidized as a
transformation of thin-banded ferruginous quartzites and
result of the cathode process only at the expense of
ores resulting in formation of martinite ores with low quartz
oxygen in the electrochemical reduction of magnetite
content The time can be determined by the equation for
This means that deep zones of oxidation can be formed
electrochemical mass transfer (Goldberg, 1993):
97
GOLDBERG
T
General Conclusion
Smi lei UiESk
Regoliths have been described in many papers and there
Where:
is much factual material
Smi total quantity of silica, extracted in the process of
As a rule, interpretation of
regolith genesis is done at present without due account
electrochemical transformation of the rock;
Ci
of electrical energy. Exceptions are works describing
concentration of the electromobile silica form in the
matter flow;
phenomena
Ui
mobility of silica;
deposits (Sveshnikov, 1967; Govett 1973, Shvartzev,
E
electric field intensity;
1976, Ryss, 1983) It seems to
Sk
the area of electric current spreading;
examples outlined suggest significant participation of
T
time of electrochemical extraction
electrical energy and, correspondingly, electrochemical
processes
Mass of completely extracted quartz
of oxidation and reduction in sulphide
in
transformation
me
of
that the three
the
geological
(Smi) was
environment
calculated for the following rock volume: length along the
strike - 1000m; width - ZOOOm; thickness - lOcm
Electrical energy is particularly important
in the process of matter concentration In essence, the
This
volume corresponds approximately to several quartz
structure of the electric field itself creates regular areas
inter-layers in thin-banded quartz-magnetite quartzites
of dissolution (depletion) and concentration (supply)
With
these parameters, the total
SiOZ should
be
mass of extracted
5 x 1010g Concentration
of
These processes can move large volumes of rock or
the
develop in local settings according to mUlti-range
mobile silica form in the solution, according to the above
experimental data is 6 x 1O-6g/ml,
character of electric fields
i e close to silica
content in underground waters (Levinson, 1974)
I think that there are two main areas for investigations
based on electrochemical phenomena:
Silica mobility during quartz dissolution is taken to be
equal to n 10-6 mZNh (where n=l-lO) based on the
regolith as
experimental data (Goldberg,
deposits;
1993)
intensity is taken as 1O-3V/m
Electric field
an
object
for search for supergene
Such gradients are
regolith as a model for understanding of processes
common in anomalous areas of natural electric fields
(Semenov, 1968), connected with emf type sources
of matter redistribution, because the system is more
of galvanic elements The area of current spreading is
accessible for study under near-surface conditions
determined by the size of the reacting surface of a
magnetite
electrode
If
the electrode area is
comparable with the quantity of removed
SiOZ
Experimental data continue to accumulate and methods
per a
are being developed which will allow study these of
unit of its surface area, then the total surface of the
processes by taking into account geoelectrochemistry
electrode will be Z x 10-6 mZ But as electric reactions
are most active near electrodes, the size of the reacting
surface will be only half this Because of this the time for
electrochemical removal extraction of quartz is 5 x 108
ACKNOWLEDGEMENTS
years
Experimental investigation was carried out in the
The calculated time does not contradict that
defined by the electrochemical model and appears to
Department
be reasonable from a geological point of view
Rudgeophisica (St Petersburg, Russia) and author is
of
Geoelectrochemistry
within
NPO
indebted to his co-workers in particular Professor Yu S
The discussed electrochemical model closely describes
Ryss, Dr S A Alekseev, Dr D V Voronin, T V Basco and
the phenomena observed in deposits of the Krivorozhsky
type
ype
It
may
associated
A S Dukhanin I wish to thank Dr Neil Rutherford
be applied to other deposits of this t
with
Pre-Cambrian
deposits
and
Prof G.JS Govett for checking the English language of
of
the text and comments
ferruginous quartzites
98
GEOELECTROCHEMJCAL
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100