Transcript SorjonenWard-et-al-2..

Numerical modelling of fluid
and heat transport during
deformation in the late
Archean Yilgarn craton
and its relevance to
late orogenic gold
mineralization
Peter Sorjonen-Ward, Bruce Hobbs,
Alison Ord, Yanhua Zhang and
Chongbin Zhao
CSIRO Exploration and Mining
Exploration Geodynamics Chapman Conferemce
Numerical modelling applications to
orogenic gold mineralization in the
Yilgarn
Scope of presentation
• Yilgarn architecture and boundary
conditions
• Coupled fluid flow and deformation
• Coupled thermal and fluid flow models
Modelling here is addressing potential
viability of fluid pathways, not
constrained by mass balance or time
Generating and sustaining a
mineral system requires
• An architecture that enhances fluid flow
with
– efficient fluid-rock interaction in the source
region
– efficient focussing into depositional site
• Mechanisms for timely fluid production
• P-T conditions and fluid chemistries that
optimize extraction and depositional
efficiency
Models for Yilgarn fluids and gold
- provenance and pathways
• Deposits formed across a range of metamorphic
grades over a similar time – crustal continuum model
• Many deposits formed relatively late with respect to
metamorphic peak
• Some areas, such as Coolgardie region have mineral
parageneses recording temperature gradients away
from plutons (Witt-Knight-Mikucki model)
• Isotopic and geochemical alteration attributes
suggest fluid derivation and prolonged interaction
with radiogenically evolved regional scale crustal
reservoir
Implications:
– Fluid flow across lateral as well as vertical
temperature gradients
Yilgarn geology and
magnetics
Low-pass filtering by Paul Gow
Eastern Goldfields Province
Southern Cross Province
Yilgarn structural domains
Magmas and fluids –
regional scale
• Large scale magnetic anomalies relate
to monzogranites emplaced to present
level within 10 Ma of mineralization
• Dominant gold mineralizing fluids are
weakly reducing, weakly acidic and of
low salinity
• Evolved isotopic signatures suggest
interaction with – though not necessarily
derivation from granitic lower and
middle crust
Yilgarn mineralization broadly synchronous
across range of metamorphic grades?
South Polaris deposit in
Southern Cross Province
Gold deposited in equilbrium
with diopside and K-feldspar
Racetrack deposit in
Ora banda domain
Sub-greenschist facies
gold deposition
Critical structural elements and
requirements for the Yilgarn
– Structural studies of mineralized veins indicate
compressive deformation during regional uplift and
decompression
– Limited strain at site of deposition but coeval high
strains at depth require major decoupling in middle
crust – coincident with granitic sheets?
– Generation of large volume of fluids, within large
lower crustal reservoir, relatively late, in order to
satisfy geochemical mass balance and isotopic
constraints
– Seismic data indicate reflectors of opposing dip,
which suggest domains of tectonic wedging,
backthrusting and “pop-up” structures
– Favourable architecture for formation of
overpressured seals and rapid uplift of deeper
Models designed to
investigate
1. Architectures and mechanisms that promote
both lateral and upwards fluid flow during
contractional deformation
2. Potential for convective flow systems
3. Thermal impact of plutons embedded in
regional metamorphic regime
4. Consequences for fluid flow and
mineralization patterns triggered by fluid
mixing
Models designed to
investigate
1. Architectures and mechanisms that
promote both lateral and upwards fluid
flow during contractional deformation
2. Potential for convective flow systems
3. Thermal impact of plutons embedded in
regional metamorphic regime
4. Consequences for fluid flow and
mineralization patterns triggered by fluid
mixing
Generating sufficient fluids
in the right place at the right time
• Granulitic lower crust inappropriate since already
dehydrated?
• Fluids from melting in lower crust sequestered again
during crystallization of hydrous phases (where not
restitic)?
• Fluids exsolved form crystallizing granites insufficient?
• Local metamorphic devolatilization insufficient?
• Rapidly formed accretionary prism could provide a more
steady supply of fluid, but in many cases mineralization
is late and evidence for accretionary prism is lacking
• Orogenically derived meteoric fluids if downdraw is
feasible (and isotopic characteristics are appropriate)
• Basinal fluids in submergent foreland basin or extending
arc terrain (if salinity and isotopic attributes of
mineralizing fluids is consistent)
Intrusive sheets in
basal part of
Karakoram Batholith
Deformed
amphibolites and
intrusive sheets at
base of Karakoram
Batholith
Lithostatically overpressured system
– requires sustained fluid supply
Symmetry and asymmetry
Interpreting the seismic
W-directed middle crustal duplexes could
represent:
• Imbricated basement substrate, which implies
foreland to west – difficult to understand given
higher grade and granite abundance in this
region
• Inherited seismic fabric from earlier event –
unlikely given volume of melting and
reworking at 2.7-2.6 Ga
• Deformation controlling melt migration from
Tectonic wedging architecture
FLAC3D models coupling
deformation and fluid flow
•
•
•
•
Darcy fluid flow in porous rock
Mohr-Coulomb elastic-plastic rheology
No temperature dependance
No time dependance
Transfer of deformation within
orogen
from thrust wedge to interior
Thrusting
velocities
Incremental shear strain
low
hig
h
Potential backthrust
formation where shear
strain is localizing
FLAC3D model of Yilgarn
section
Why topographic elevation in
the west?
• Pressures greater in west,
not merely higher
temperatures
• Envisage that system is
about to collapse,
removing relief and
exhuming higher grade
rocks by extensional shear
along east-dipping
Kunanalling and Ida faults
• Alternative modified model
Simulating the generation of fluid
sources during contractional
deformation
• Scenario 1: Fluid production
in lower crust through
dehydration and partial
melting during crustal
thickening
• Scenario 2: Fluid production
through uplift and
decompression melting
during ongoing compressive
deformation
Fluid source beneath overthrust terrain
Fluid source beneath “Kalgoorlie region”
No topography or fluid source
Lateral flow less prominent, but oblique flow in
faults
and zones of deformation-induced dilatancy
(brown)
Fluid source and
topography
No fluid source
or topography
Deformation and fluid flow
modelling
- principal conclusions
• Hydraulic head due to topographic elevation
during contractional deformation is critical to
lateral fluid flow
• Precise depth and location of fluid source is
less important though obviously critical as
potential reservoir supply
• Downwards fluid flow is possible during
compressive deformation given appropriate
fluid pressure gradients
Models designed to
investigate
1. Architectures and mechanisms that promote
both lateral and upwards fluid flow during
contractional deformation
2. Thermal evolution and potential for
convective flow systems
3. Thermal impact of plutons embedded in
regional metamorphic regime
4. Consequences for fluid flow and
mineralization patterns triggered by fluid
mixing
THERMAL PROCESSES
MODELLED SO FAR
• Conductive delay due to plume impact,
and critical temperature thresholds for
devolatilizing reactions in middle and
lower crust
• Full-crustal circulation to simulate
regional metamorphic pattern and Hall
model
• Effect of smaller scale convective
processes and embedded plutons to
simulate lateral fluid flow models
Rate of thermal evolution with
respect to external factors
• Conductive heat transfer from plume
impingement
• Radiogenic heat production
• Advection through magma emplacement
• Erosion plus uplift during thrusting leads to
higher geothermal gradient near surface early
during orogenesis
• Sedimentation, burial and radiogenic heat
production lead to higher gradients in middle
crust later during orogenesis
2660 Ma
thermal peak in
lower and middle
crust
2680 Ma
dolerites and
initial rifting
phase recorded
by Black Flags
2705 Ma
komatiites
Conductive thermal evolution in the Yilgarn, as a
consequence of plume related to komatiites at 2705 Ma,
showing the time at which metamorphic and melt
generation thresholds are attained at particular crustal
levels (granite data courtesy of L. Wyborn)
Rate of thermal evolution with
respect to deformation
• Influence on geothermal gradient
• Influence on rheology and deformation mechanisms
• Influence on timing of fluid production in hydrous
sequences
EGF-01 Yilgarn profile
Active Honshu arc compared to post-orogenic Yilgarn
Model geometry for coupled
fluid flow, heat flow and
fluid-fluid chemical reactions
Hydrostatic pressure gradient
Pressure gradient near lithostatic
Model results - some caveats
• These models simulate fluid-fluid
reactions, not fluid-wallrock reactions
• Results are highly dependent on
permeabilities assigned to crustal units
and structures
• Sensitive to (lack of) thermodynamic
constraints!
Lithostatic pore pressure gradient
with no plutons active
Fluid flow streamlines
20 km
0 km
-20 km
-40 km
X 10
0.2
0
-0.2
-5
m2 s-1
-0.4
Blue = anticlockwise flow, red = clockwise flow
Fluid flow streamlines Pluton P1
20 km
Fluid flow streamlines - Pluton P3 active
0 km
20 km
-20 km
0 km
-40 km
2 -1
x 10 -5 m-20s km
0.4
-0.4
0
-0.8
-1.6
-1.2
-40 km
2 -1
x 10 -5 m s
0.6
0.4
0
0.2
-0.2
Fluid flow streamlines Pluton P2 active
20 km
0 km
Fluid flow streamlines Pluton P4 active
-20 km
20 km
-40 km
0 km
2 -1
x 10 -5 m s
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1.0
-20 km
-40 km
2 -1
x 10 -5 m s
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
Hydrostatic pressure gradient – thermal effect of pluton
location
Blue = anticlockwise flow, red = clockwise flow
Effect of pluton location on fluid flow patterns
Blue = anticlockwise flow, red = clockwise flow
Pluton P3
Pluton P1
Pluton P4
Pluton P2
Effect of pluton location and pressure
gradient on convective streamline
patterns
Fluid flow streamlines
Fluid flow streamlines
with no pluton
Fluid flowlithostatic
streamlines
20 km
20 km
0 km
0 km
-20 km
-20 km
-40 km
-40 km
m2 s-1
0.8
0.4
0.6
0
0.2
-0.2
X 10
-0.4
0.2
Fluid flow streamlines Pluton P1
-0.2
0
-5
m2s-1
-0.4
Fluid flow streamline
with Pluton P1 actives
Fluid Lithostatic
flow streamlines
20 km
20 km
0 km
0 km
-20 km
-20 km
-40 km
-40 km
2 -1
x 10 -5 m s
0.4
-0.4
0
-0.8
2 -1
x 10 -5 m s
-1.6
-1.2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
-0.2
Fluid flow streamlines Lithosatic with Pluton P2 active
Fluid flow streamlines Pluton P2 active
20 km
20 km
0 km
0 km
-20 km
-20 km
-40 km
-40 km
2 -1
x 10 -5 m s
2 -1
x 10 -5 m s
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1.0
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
-0.2
Ongoing evaluation of models
against field-based constraints
• Isotopic and geochemical evidence for prograde
or retrograde alteration in specific shear zones,
such as
-
down-temperature alteration during upflow (K metasomatism)
up-temperature alteration during downflow (Na metasomatism)
• Compare P-T conditions from metamorphic
assemblages with temperature distribution
predicted by model convection
• Confirm presence of K-feldspar or muscovite or
aluminosilicate stability in alteration
assemblages predicted by pH distribution for
models that couple fluid chemistry
Coupled thermal and fluid flow models
- principal conclusions
• Thermal effect of small plutons
emplaced ahead of a prograding
metamorphic front can have a
significant impact on the pattern and
intensity of fluid transport and
convection:
- at distances considerably greater than pluton diameter
- with focussing into adjacent more permeable layers
- promote lateral thermal gradients
Yilgarn numerical models
- principal conclusions
• Indicate generic
structural sites that are
favourable for fluid
mixing and gold
precipitation
-
-
footwall environments related to
major shear zones, such as the
Bardoc Shear
at rheological boundaries within
broad antiforms such as the ScotiaKanowna and Goongarrie–Mount
Pleasant Antiforms