Geochemical Exploration
Techniques

Geochemical Exploration?
Searching geochemical anomaly
– Searching geochemical anomaly
– Elts disperse over large area around their source
N l di t ib ti f th l t i th
– Normal distribution of the elements in the
geochemical media of interest
Element distribution in d/t parts of the earth
– Element distribution in d/t parts of the earth
– Geochemical association of elements
B h i f l t i th diff t
– Behavior of elements in the different
environments
Sampling
– Sampling
– Preparation
A l i
– Analysis
– Interpretation

Basic Principles: a refresher
Basic Principles: a refresher

th
he
eart
n
of
th
osition
compo
mical
Chem

Surficial Environment
Mi l
Soluble Salts
Mineral
Breakdown
Residual Minerals
Oxides & Clays
Sedimentation
Diagenesis
Uplift
Erosion
Sedimentary Rocks
Igneous Rocks Sedimentary Ores
Hydrothermal Ores
Metamorphism
Crystallization
Magmatic Ores
Mantle
M lti
Magma
Metamorphic Ores
Metamorphic Rocks
Deep-Seated Environment
Mantle
Material
Melting

Geochemical Environments
Deep Seated
• magmatic-
Surficial
• weathering -
g
metamorphic
• High P & T
g
sedimentation
• low P & T
• low free O2
• restricted fluid
• free O2, CO2 H2O
• free fluid movement
movement
most ores formed most ores destroyed

The Dispersion Process
Dispersion
Transport
Mobile
phase
Immobile phase Matrix
Environment
of mobilization
Environment
of deposition
of mobilization

Dispersion
The process that takes an element and
moves it from one environment to another,
,
generally decreasing the concentration by
spreading it over a larger volume
p g g
Results in redistribution fractionation mixing
Results in redistribution, fractionation, mixing
and dilution

Types of Dispersion
1. Clastic dispersion: solid particles moved
physically
p y y
2. Hydromorphic dispersion: solid particles
dissolved in water & transported in
dissolved in water & transported in
chemical solution
3 Biogenic dispersion: solid particles moved
3. Biogenic dispersion: solid particles moved
by biological activities

Clastic dispersions

Dispersion
Dispersion
Dispersion
by wind
by wind

Subsurface
Dispersion by lateral groundwater
fan
p y g
flow- hydromorphic

Biogenic
Biogenic
El t di l d b d t i
Elements are dissolved by groundwater in
contact with the ore vein; and are
transported in solution in the direction of
groundwater flow. Plant roots take up
g p
nutrients from the water solution

Agents of Dispersion
Mechanical
• examples
Chemical-biochemical
• examples
p
– magma injection
– glacial transport
p
– dissolution/deposition
– melting/crystallization
– alluvial sorting
• results
• results
– fractionation common
– mixing
– fractionation not
common
– mixing
– dilution
common

Types of Dispersion
Deep-seated Surficial
Primary Diffusion into Precipitation
y
wallrock at
deposition
p
on seafloor
(VMS)
p ( )
Secondary Diffusion from
ore during
Weathering of
ore deposit
metamorphism

1. Primary dispersion
• Distribution of elts in unweathered rks
and minerals
• The main media of survey are rocks
• Not proved encouraging in mex b/c
• Not proved encouraging in mex b/c
• Rocks are not homogenous & hence
bl f t ti
problem of representativenes
• High cost of analysis
• Some deposits lack primary
dispersion
p

2 Secondary dispersion
2. Secondary dispersion
• Dispersion of elements during weathering
& diff i f lt f d i
& diffusion of elts from ore during
metamorphism
• Occur during weathering &
metamorphism of ore deposits
• Good for MEX b/c
• Dispersion covers larger/wider areas
Dispersion covers larger/wider areas
• Media of survey are soil, stream
sediments water air plants & animals
sediments, water, air, plants & animals

Dispersion Halos
Ore
Ore

Mobility
Mobility
• Characteristic of an element that allows it to
Characteristic of an element that allows it to
be selectively dispersed
• Ease with which the element is dispersed
l i h di i
relative to the surrounding matrix

Controls on Mobility
• Mechanical properties of medium/elt
– viscosity of the fluid
viscosity of the fluid
– size, shape and density of the grains
• Chemical differentiation
• Chemical differentiation
– relative stability in immobile solids
l ti t bilit i th bil fl id h
– relative stability in the mobile fluid phase

Simple Dissolution
+
⇒ =
+
4
2
4
a
a
SO
Pb
PbSO
=
+
=
4
4
2
PbSO
SO
Pb
a
a
a
K
+ =
2
4
PbSO
K
m
=
=
+
+
4
4
2
2
SO
SO
Pb
Pb
m
m
γ
γ

Selecting the Reaction
i l i
Simple reaction
=
+
+
⇒ )
(
3
2
)
(
)
(
3 aq
aq
solid CO
Zn
ZnCO )
(
)
(
)
( q
q
More realistic
−
+
+
+
⇒
+ )
(
3
2
)
(
)
(
)
(
3 aq
aq
aq
solid HCO
Zn
H
ZnCO
Trace Component
Trace Component
+
+
+
⇒
+ 2
)
(
)
(
3
2
)
(
)
(
3 aq
calcite
aq
calcite Zn
CaCO
Ca
ZnCO

Deep-seated Dispersion
Mechanisms
• Melt-solid equilibrium
Melt solid equilibrium
– size, charge, coordination, bonding
Melt fl id eq ilibri m
• Melt-fluid equilibrium
– complexing important
• Solid-fluid equilibrium

Surficial Dispersion
Surficial Dispersion
Dominated by aqueous fluid transport
• ionic potential (IP)=I.charge/I.radius
– IP<3 form ionic solutions & dispersion
p
dependent on pH; e.g alkalis, Ca, Ba, Sr, Fe2+,
Mn2+, Cu, Zn
– IP b/n 3 &10 less soluble & solubility
influenced by pH; e.g. Mg, Al, Fe3+, Mn4+
– IP>10 form soluble anion complexes; e.g. S, P,
N, S, As

• stability of solids
• aqueous speciation
• Co-precipitation and adsorption

Coefficient of Mobility
Empirically determined
water
drainage
C
R
M
K
⋅
=
100
rock
total C
R ⋅
M = elt concentration in water (mg/L)
M elt concentration in water (mg/L)
R= total mineral residue in the water (%)
C= element concentration in the rock (%)

Mobility of The Elements
1
1A
2
2A
3
3B
4
4B
5
5B
6
6B
7
7B
8
6
9
8B
10
8
11
1B
12
2B
13
3A
14
4A
15
5A
16
6A
17
7A
18
8A
H He
H He
Li Be B C N O F Ne
Na Mg Transition Metals Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra
Fr Ra
Lanthanides (Rare Earth Elements)
Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
y
Th Pa U
Actinides
K < 0 1 immobile 0 1 1 slight 1 10 moderate >10 high
K < 0.1 immobile 0.1 – 1 slight 1 – 10 moderate >10 high
Oxidizing pH < 4 Fe(II ) reducing uncertain? Radioactive acidic oxidizing or reducing

Indicator Elements
The elements measured in order to detect an
ore body
y
• Often the element of economic interest
• The ore element may not be suitable
• The ore element may not be suitable
– difficult to measure
– immobile
– difficult to interpret

Pathfinders
• Elements that are associated with the ore
elements in the target ore t pe and are sed
elements in the target ore type and are used
because of a more favorable geochemistry
t bilit
– greater mobility
– clearer patterns (greater contrast)
– easier to detect analytically
easier to detect analytically
• There is a direct correlation with the ore
element of interest
element of interest
• Examples include As for Au; Ni,Fe,Mg for
C V TiPt f F Pt C A A f C Ni
Cr; V,TiPt for Fe; Pt,Co,As,Au for Cu-Ni
etc

As as Pathfinder for Au
As as a pathfinder for Au
1200
600
800
1000
1200
pm
As
0
200
400
600
pp
Au
0
-600
-400
-200
0
200
400
Traverse position
p

As as Pathfinder for Au
As as a pathfinder for Au log scale
1000
10
100
1000
pm
As
0 1
1
10
pp
Au
0.1
-600
-400
-200
0
200
400
Traverse position
p

Element G roup A ssociations
Element
Associations
G roup A ssociations
General A ssociations K -Rb Ca-Sr A l-G a Si-G e
Zr-H f REE-La-Y PGM
Igneous R ocks
F l i Si K N
Felsic Si-K-Na
Alkaline A l-Na-Zr-Ti-N b-Ta-F-P-
REE
M afic Fe-M g-Ti-V
Ultram afic M g-Fe-Cr-N i-Co
Som e pegm atites Li-Be-B-Rb-Cs-REE-Nb-Ta-
U -Th
Som e Skarns M o-Sn-W
Som e Skarns M o-Sn-W
Potash Feldspars K -Ba-Pb
Other potash m inerals K -N a-Rb-Cs-Tl
Ferrom agnesian M in Fe-M g-M n-Cu-Zn-Co-N i
Sedim entary R ocks
Fe-oxides Fe-As-Co-N i-Se
M n-oxides M n-A s-Ba-Co-M o-Ni-V -Zn
Ph h it P A M Pb F U
Phosphorite P-A g-M o-Pb-F-U
Black Shales A l-A g-A s-A u-Bi-Cd-M o-Ni-
Pb-Sb-V -Zn