Transcript Fluid Sources – solubility sensitivity

Exploration Science
Linking fundamental controls on
ore deposition with the
exploration process
Andy Barnicoat
AESC Perth July 2008
Outline
Mineral Systems as a framework for
understanding ore bodies
Fundamental controls on ore formation
Exploration science
Some system perspectives
Using Mineral Systems’ concepts in targeting
Acknowledgements
All in the pmd*CRC, especially Bruce Hobbs, John
Walshe, Heather Sheldon & Alison Ord for
conversations
ERC members, particularly Nick Archibald, Greg Hall,
Jon Hronsky & Dugi Wilson for encouragement
Bob Haydon & Russell Korsch for support
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Mineral Systems
‘All geological factors that control the
generation and preservation of mineral
deposits…’
‘…stressing the processes that are involved in
mobilising ore components from a source,
transporting and accumulating them…’
From ‘Australian Proterozoic Mineral Systems: Essential
Ingredients and Mappable Criteria’
Wyborn, Heinrich & Jaques, 1994
Mineral Systems Science is a part of Earth Systems
Science, which encompasses the interactions between the
biosphere, atmosphere, hydrosphere and ‘lithosphere’
(read solid Earth)
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Why Mineral Systems?
Predictive capacity going beyond pattern
matching
Enables exploration undercover
Mineral Systems form a basis on which to drive
integration of research efforts and ensure a focus on
exploration outcomes
• a means to link geology to physical and chemical controls
on ore formation
• a way to link to exploration
• a cross-scale view
• a framework within which to place projects and individual’s
work
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Mineral Systems work flow
The Why Question
Why is the ore body there?
5 Questions
1. What are the geodynamic and PT
histories?
Inputs from:
2. What is the architecture of the system?
Data Compilation
3. What are the fluid sources/reservoirs?
Data Collection
4. What are the fuid flow drivers &
pathways?
Simulation
5. What are the metal and sulphur
transport & depositional processes?
The Where Question
Where is the next ore body?
Developed from inspiration provided by Tom Loutit by John Walshe,
Alison Ord, Bruce Hobbs & Greg Hall in the AGCRC 1997-98
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Mineral systems and
deposit types
Mineral systems are broad, and the concept
embraces explicitly factors across all scales
Differences in depositional environment will
lead to distinctive accumulations controlled by
variations in
• lithology
• structure
Deposit types represent the expressions of
these relatively local variations
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An analogue for Mineral Systems
VW Golf platform
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An analogue for Mineral Systems
A ‘Common platform’ underpins the obvious
differences in surface expression
VW Golf
Skoda Octavia
Audi TT
VW Beetle
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Why do ore deposits form?
...because a lot of the appropriate mineral(s) have been
deposited
Zinc - Century, Queensland
Gold - Goldstrike, Nevada
So – what controls mineral deposition?
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Why deposition of minerals occurs
Rate of
deposition
Velocity
Gradient
of
in
=
.
transport
carrying
medium
capacity
Examples:
1.
Heavy mineral deposition controlled by flow rate and
entrainment capacity (proportional to velocity2)
2.
Magmatic deposits controlled by magma supply rate
and changes in temperature, magma composition, etc.
causing deposition
3.
Residual deposits (e.g. bauxites) where dissolution
and removal of gangue leads to ore formation
4.
Hydrothermal systems, where fluid flow rate and
changes in P, T or chemistry lead to deposition
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Fundamental relationship
Rate of
mineral
deposition
=
Fluid
velocity
•S
Rate of
change of
solubility
with P, T, C
.
Gradient
of P, T, C
Maximise…
Focused
fluid flow
P, T & C at
values where
largest
solubility
changes occur
Temperature,
pressure,
compositional
gradients
After Phillips, 1990, 1991 and Hobbs & Ord, 1997
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Exploration science
Geology
Key Parameter
is reflected in
A. Gradient in
hydraulic
potential
B. Porosity
C. Permeability
D. Solubility
sensitivity to
P, T, C
E. Spatial
gradient of
P, T, C
Exploration
scale-dependent
translation
5 Questions
1. Geodynamics
2. Architecture
3. Fluid
reservoirs
4. Flow drivers &
pathways
5. Deposition
Terrain
Selection
Area
Selection
Drill
Targeting
F. Time
(duration)
Why?
What?
‘Practical Proxies’
Where?
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Olympic Dam Cu-U-Au --------------------------------- 29b
In summary…..
Sandstone-hosted Pb-Zn ------------------------------- 30a
Sediment-hosted Cu ----------------------------------- 30b
Sandstone U ----------------------------------------- 30c
Sedimentary exhalative Zn-Pb ---------------------------- 31a
Bedded barite ----------------------------------------- 31b
Amount
krg Hydraulic
Solubility
of
Emerald
veins
----------------------------------------31c . Spatial
.
=
. dt
•
Gradient
m
sensitivities Gradients
mineral
∫

)
S(
Southeast Missouri Pb-Zn ------------------------------- 32a
Appalachian Zn --------------------------------------- 32b
Critical
factors
for forming a (giant)
Kipushi
Cu-Pb-Zn
------------------------------------32c
ore
system:
Low-sulfide
Au-quartz vein --------------------------------- 36a
Homestake
Au ------------------------------------------36b
Gradient
in hydraulic potential
Driver
Unconformity
U-Au ------------------------------------37a
Geodynamics; Architecture;
Deposition
Porosity
Gold on flat faults ----------------------------------------37b
Architecture;
Pathway
Permeability
Solubility Sensitivity *
Spatial gradient
of P, T,the
X process…..
Architecture; Deposition
1 equation
describing
Geodynamics
Maximum
duration
65 USGS
deposit
types!
*This will
be zero
if anfor
appropriate
fluid…
is not present!
Thanks
to John
Walshe
the inspiration
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Geodynamics: Solubility Sensitivity
Crustal growth and gold
200
Gold
Resource
Moz
Vol %
Crustal
growth
150
100
50
14
12
10
8
6
4
2
3
2
Ga
1
0.5
From Groves et al., 2005
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Geodynamics: Solubility Sensitivity
Mantle Convection
Two layer convection in early Earth
Periodic whole-mantle overturns
Facilitated by spinel-perovskite transition (670 km)
From Davies (http://rses.anu.edu.au/gfd/davies/pages/episodtect.html)
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Geodynamics: Solubility Sensitivity
Evolution of mantle convection
Mantle temperature
undergoes episodic
temperature surges due
to convective overturns
Whole mantle
convection
Intermittent penetration
by plates
Internal boundary layer
instability
Davies, 1995
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Geodynamics: Solubility Sensitivity
Yilgarn crustal growth
Compiled by Karol Czarnota & Richard Blewett
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Geodynamics & Architecture: Porosity,
Permeability and Solubility Sensitivity
Great Basin facies distribution 1
Cook & Corboy, 2004
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Geodynamics & Architecture: Porosity ,
Permeability and Solubility Sensitivity
Great Basin facies distribution 2
Deep Structures
seen in Geophysics
as well as isotope
data etc.
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Geodynamics & Architecture: Porosity ,
Permeability and Solubility Sensitivity
Great Basin geophysics
Grauch et al., 2003
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Geodynamics & Architecture: Porosity,
Permeability and Solubility Sensitivity
Great Basin geodynamics
118°
42°
116°
Oceanic crust
Transitional
crust
Continental
crust
Dense crust
40°
Tertiary
Magmatism
Acid
IntermediateMafic
Carlin-type
deposits
After Grauch et al., 2003
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Fluid sources: solubility sensitivity
What about ore-forming fluids?
Ore-forming fluids are chemically similar to
other crustal fluids
Controlled by reactions with (crustal) rocks
Yardley, 2006
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Fluid sources: solubility sensitivity
Potential fluid sources
Meteoric
water
Bittern
Evaporites
Crust
Sea water
Basinal
fluid
Magmati
c fluids
Metamorphic fluid
Mantle
Note: all paths
schematic
Mantle
fluid
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Fluid sources: solubility sensitivity
Salinity in crustal fluids
Fluids from shallow marine
& continental settings
Temperature °C
Fluids from accretionary/
oceanic settings
From Yardley & Graham, 2002
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Fluid sources: solubility sensitivity
Meteoric Fluids
Sourced from the surface
• Needs rainfall & sunaerial topography
Wide range of chemistries
• Acetate especially
important around oil
window (80-120°C)
Size of meteoric systems may
be huge
Transit time for fluid ~2 Myr
Don’t in general carry a lot of
metal (uranium?) but can be
• directly important dilution
• Indirectly important –
interact with rocks and
other fluids and evolve to
more saline solution
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Radke et al., 2000
Fluid sources: solubility sensitivity
Basinal Fluids
Formation water
• water present in pores and fractures
immediately prior to drilling
Connate water
• water trapped with the sediment and
subsequently unmodified.
Sedimentary basin waters have a range of
origins
• connate,
• (modified) meteoric water
• recent
• in the geological past
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Fluid Sources – solubility sensitivity
Basinal fluids - origins
Most basinal fluids (formation waters) are of
recent meteoric origin (c.f. Great Artesian
Basin)
Kharaka & Hanor, 2005
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Fluid Sources – solubility sensitivity
Metamorphic fluids
‘Metamorphic’ fluids have mixed origin
• Devolatilisation
• External fluids reacting with metamorphic
rocks
Devolatilisation
• During heating, volatiles released
• Micas, chlorite, amphiboles yield water
• Carbonates
react (eg to
calc-silicates)
and CO2 and
H2O may be
formed
• Constant temperature
leads to no fluid
generation
• Decrease in temperature
leads to resorption of
residual fluid by
retrogression
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Fluid Sources – solubility sensitivity
Metamorphic fluid production
Key results:
• Maximum fluid flux ~10-11
m3/m2/s (very small)
15km
Basalt
T = 20 °C
Granite
magma
e.g. voluminous
High-Ca granite
magma (M2)
Sheldon, 2008
• Duration << 1 million
years
• Time-integrated flux is
~1000 times smaller than
flux required for
mineralisation (Cox, 1999)
 Metamorphic fluid
must be FOCUSED for
mineralisation
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Geodynamics – solubility sensitivity
Magmatic fluids – components
CO2 facilitates formation of a fluid phase/phases
• primary source of CO2?
CH4-rich fluids occur in mid-ocean ridge settings
• high-T re-speciation of initially CO2-rich fluids
produced methane ; may co-exist with NaCl-rich
fluids
S levels are elevated in arc-related magmas
• S isotope data shows ‘extra’ sulphur to be
derived from subducted material
Cl is also sourced from slab-derived fluids in arc
settings
• Additional (lower crustal) sources may also exist
Data from Kelly & Früh-Green, 2001; de Hoog et al 2001; Kent et al., 2002
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Geodynamics: solubility sensitivity
Cl and magmas
Cl-rich fluids will form due to changes in melt
composition
6
4
basalt
• mafic melt cooling,
crystallising and
differentiating can
develop brine ±
vapour phase
insoluble in felsic
magma
8
wt% H2O in melt
• Cl solubility
decreases
dramatically
from mafic
to felsic melts
2
0
0
1
2
3
wt% Cl in melt
Webster, 2004
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Geodynamics: solubility sensitivity
Role of mafic magmas
Carriers of most of the CO2, sulphur and Cl seen in
magmatic systems and their fluids
Injection of mafic magmas into felsic magma chambers
critical
• add volatiles
• add energy
• add volume (magma
plus separating
fluid phase)
Bimodal magmatic
systems likely to
be an important sign
• Williams suite in Isa
• mafic to low-Ca
suite in Yilgarn
• Hiltaba suite in
the Gawler
Hollocher; www.union.edu 33
Fluid flow drivers: hydraulic potential
Fluid flow rates
log fluid flux (m/yr)
-8
-6
-4
-2
0
Deformation
Convection
Topographic/meteoric
Metamorphism
Compaction
Intrusions
There is considerable overlap in flow rates, making it
difficult to predict which one will dominate.
From Heather Sheldon 2008 presentation
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Fluid sources and drivers, geodynamics:
hydraulic potential, solubility sensitivity
What’s really important?
Two types of fluid
source are likely
dominate the fluid
budget
• meteoric
• magmatic
Understanding
palaeogeography
• brine sources
• sub-aerial
topography
Magmas
• bimodal suites
• evidence of mantle
input (cf Olympic
Province in the
Gawler)
Karakoram Mountains
Yellowstone
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Deposition: porosity, solubility
sensitivity
Base metal transport
ZnCl+ + H2S(aq) =
8
ZnS + 2H+ + ClH2S, H+ and Cl- influence
zinc solubility
2H+
3KAlSi3O8 +
=
KAl3Si3O10(OH)2 + 6SiO2 + 2K+
CaCO3 + H+ = Ca2+ + HCO3-
total Cl m
Acidity can be absorbed
by reacting with
feldspars, carbonates,
etc.
6
4
total Zn ppm
2
100
200
300
400
500
1000
500
400
300
200
100
50
10
1
0.1
0.01
T(C)
Zinc solubility at 1 kbar from sphalerite
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Deposition: porosity, solubility sensitivity
Base metal precipitation
To be viable, base metal grades need to be
>10%
That means space is required
Muscovitisation of K-feldspar reaction has a
solid volume decrease of 15%
Calcite dissolution shows a solid volume
decrease of 100%
• Best host rocks for base metals
are carbonates and arkosic
sandstones
• Clean sandstones will not be
effective
• Other potentially good hosts
include fractured felsic igneous
rocks
Image by Graham Phillips, RDR
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Using Mineral Systems
Conceptual
Mineral System Understanding
Geodynamic Episodes
Pre-ore
endowment
(<2665 Ma)
Lithospheric
extension Au
Geochemical
Gradients
Inversion Au
(≥2660 Ma)
(~2665-2655 Ma)
Identification of mappable mineral system process proxies for each subdivision
Practical
• Crustal
endowment
Pre-ore
prospectivity
• Extensional SZ
• Metasomatised
mantle melts
• Deep pathways
D3
prospectivity
• Domes
• Major faults
• Upper plate
D4-D5
prospectivity
• Redox gradients
• Hydrothermal
system indicator
Geochemical
prospectivity
Au mineral camp (60x60 km area) selection
Slightly modified after Karol Czarnota et al., 2008
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Camp-scale Au Targets
Large gold
deposits reflect
long lived
mineral systems
affected by
multiple gold
events
A combination of
the prospectivity
maps related to
various
processes
identifies
prospectivity
based on Mineral
Systems
100 km
From Czarnota
et al., 2008
39
Summary
Exploration Science is an integrated view of
ore-forming processes that explicitly ranges
across scale
Exploration science connects
• fundamental physico-chemical controls on
deposition to
• observable view of the Mineral Systems (the
‘practical proxies’) and
• the exploration process
Understanding the connections will ensure
that ‘practical proxies’ are relevant to
understanding ore formation
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