2. 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
5. 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
6. 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
8. 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
9. 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
14. 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
15. 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
16. 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
17. 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
18. 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
20. 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
21. 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
23. 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
24. 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
25. 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
26. • stability of solids
• aqueous speciation
• Co-precipitation and adsorption
27. 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 (%)
28. 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
29. 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
30. 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
31. 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
32. 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
33. 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
