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Ore-inspiring structures
- some numerical modelling
perspectives on orogenic
architectures favourable for
formation and preservation
of mineral deposits 1
Peter Sorjonen-Ward, Paul Gow ,
Phaedra Upton2 Yanhua Zhang
CSIRO Exploration and Mining
www.dem.csiro.au
Current addresses 1 pagow@mim.com.au
2 phaedra.upton@stonebow.otago.ac.nz
Purpose of presentation
• Consider orogenic architecture that
favours both formation and preservation
of deposits
• Review concept through coupled
numerical models of deformation and
flow based on
– Archean Yilgarn craton
– Modern PNG collisional zone
• Smaller scale aspects not discussed
Do mineral systems represent
this?
Or do they change with scale like
this?
Butterflies by M C Escher, 1950
Requirements for the formation
and preservation of ore deposits
• Critical architectures that efficiently transport
and focus mineralizing fluids
• Faults as episodic channels or seals –
feedback between
– strain softening or hardening
– rupture, dilation and precipitation of minerals
• Pervasive versus partitioned flow and access
to rock
• Geodynamic settings that favour preservation
of deposits
– Porphyry and epithermal systems dominant in
young mountainous terrain
Generating sufficient fluids
in the right place at the right time
“structural control of ore deposits only takes place on
faults that were active at the time that the hydrothermal
system was active” Mike Etheridge, 2000
Hence, active coupling between fluids and deformation
• In some terrains where architecture is potentially
favourable, fluid production is ill-timed with respect to
thermal peak
• In some terrains, architecture is inappropriate – faults
do not form connected network for accessing fluids
• In some terrains, fluid supply is the limiting factor
networks
Generating sufficient fluids
in the right place at the right time
What processes and sources generate an adequate fluid
supply?
– Granulitic lower crust inappropriate since already
dehydrated
– Fluids exsolved during crystallization of volatile-rich
granites
– Local metamorphic devolatilization
– Rapidly formed accretionary prism could provide a
more steady supply of fluid, but in many cases
mineralization is late
– Orogenically derived meteoric fluids if downdraw is
feasible
– Basinal fluids in submergent foreland basin or
Modelling orogenic
architecrture
• Thermomechanical modelling at
orogenic scale well advanced
• FLAC3D coupling of deformation and
fluid flow
– Darcy fluid flow in porous rock
– Mohr-Coulomb elastic-plastic rheology
– Feedback between fluid pressure and rock
failure
– No temperature dependance
– No time dependance
Mechanisms for enabling fluid flow through
low permeability environments
Lithostatically overpressured system –
requires sustained fluid supply
Critical orogenic architecture for
generating ideal depositional
sites
• Dilational jogs in strike-slip systems are
commonly invoked, based on earthquake
research
– Regional analysis often suggests this, but detailed
studies often show more complex features
• Importance of thrust-related subhorizontal
systems
– Yilgarn, PNG, central Asia (Muruntau)
• Interaction between thrusts and reactivated
transfer structures also considered important
• Need to compromise between flow network
that maximizes fluid-rock or fluid-fluid
Regional impression
Left-stepping sinistral
dilational jog
Local environment
Back rotation within
contractional oblique-slip
duplex
Pampalo deposit,
Finland
Deposits in hanging-
wall
of thrust systems:
Porphyry Cu/Au
deposits
in PNG fold belt
Id
en + + +
be
rg GRASBERG 3000m
Fubilan Monzonite
# 1F
au + Intrusive Porphyry
lt Complex
Darai Fm +
Id
+ + +
en +
be + +
rg
#2
+ Parrots Beak
Fa 0m + Thrust
ul Ieru Fm
t
Kucing Liar
1000m Mineralisation
-3000m
Grasberg Deposit (plan view) Ok Tedi Deposit (cross-section)
from Widodo et al.,1999 from Mason (1994)
From Mason (1994)
Fluid flow in thrust terrain controlled
by hydraulic head, deformation and
permeability
Homogeneous permeability Highly permeable thrust
High permeability in basal Fluid sources related to
thrust and footwall stratigraphy melting and metamorphism
Some regional numerical models
relating to mineralization during
convergence
• Interaction between thrusts and oblique
convergence in PNG
– Correlation between mineralization, uplift
rate and reactivation potential of transfer
faults
• Divergent compressive structures in
Yilgarn
– Promoting lateral fluid flow and variable
uplift to maximize potential for thermal and
pressure gradients and mixing of diverse
PNG tectonic setting
133º E 148º E
Caroline Plate Mussau
New
Guin Manus Trench Trench
ea T Pacific Plate
r e nc moving wes t
h
2º S 2º S
Ac
c re
Mo t ed
bi le Arc
Grasberg Fo ld Be Te r
lt Bism arck Sea
an d Frieda
r an Plate
e
Ok Tedi Thr
us t Porge ra
Fly
Pla Be
t for lt
m Solomon Sea
Plate
Australian
Plate
approximate limit of
Indo-Austra lian Coral Sea sp reading
P late moving n orth
0 200 400
kilometres
PNG FLAC3D model geometry
Architectural elements
•Terranes of different strength
Mo
bile
Be
•Contrast in platform strength
F ol lt
dB
elt •Arc-normal inherited transfer faults
Fly
Platform •Shelf-edge extensional fault
Dynamic Elements
0 250 500 •Oblique sinistral collision
Port
Kilometres Moresby
•Convergence angle at15º and 45º
inden
•1-2% shortening
mobi tor
exte le be 1000 km
nsi lt
featu onal al c)
re m r
or
stron c -n nsfe fold
platf g ar tra belt
orm 600 km
wea
k pla
tform
Modelling volumetric strain
Collision Obliquity: 45° - Greater
volumetric strain at
higher collision
angles
- associated with
vertical extension
- most pronounced
Collision Obliquity: 15°
where weak 1.0 %
structures cut fold
belt
0.5
0
Crustal uplift rates in PNG collision
(a) zone
Weak/Strong Australian Crust
45° (b)
Weak Arc-Normal Structures
45°
Effect of varying
max zd = 2e3 max zd = 2e3
strength of
crustal units and
15° 15°
transfer faults
max zd = 5e2 max zd = 5e2
inden
mobil tor
exte e belt
nsio
featu nal al
re m
Weak Extensional Feature and nor fer
stro c- ns fold
Weak Extensional feature Weak Arc-Normal Structures platf ng ar tra belt
orm
(c) (d) wea
k
45° 45° platf
orm
max zd = 3.5e3 max zd = 3.5e3
15° 15°
Contours of
vertical
displacement
max zd = 1.4e2 max zd = 1.4e2
dark = higher
Modelling vertical displacement
4500
south north
4000 Current day topography
3500
Elevation (m)
3000 PNG Elevation
2500 Irian Jaya Elevation
2000
1500
1000
500
0
300 350 400 450 500 550 600 650 700 750 800
distance (km)
Reactivat
Greater uplift
ed
against
extension
strong
al
Australian
structure
crust
adds peak
Incipient development of “pop-up” in uplifted
Southeast Northwest
region
(a) inden
mobil tor
Mesh exte
nsio
e belt
featu nal al
re m
nor fer
stro c- ns fold
platf ng ar tra belt
Fold belt Indentor orm
wea
k platf
orm
-13 -15
10 10
(b) -18 Initial Intrinsic
-17 10
10 Permeability (m )
2
(c) Cumulative
Volumetric
High volume str ain at the Strain
northern end of the fold belt
(d) Cumulative
fluid flux Fluid Flux
maxima
Transfer of deformation within
orogen
from thrust wedge to interior
Thrusting
velocities
Incremental shear strain
low
Potential backthrust
formation where shear
hig
strain is localizing
h
Some regional numerical models
relating to mineralization during
convergence
• Interaction between thrusts and oblique
convergence in PNG
– Correlation between mineralization, uplift
rate and reactivation potential of transfer
faults
• Divergent compressive structures in
Yilgarn
– Promoting lateral fluid flow and variable
uplift to maximize potential for thermal and
pressure gradients and mixing of diverse
Eastern Goldfields Province
Yilgarn structural domains Southern Cross Province
Symmetry and asymmetry
Tectonic wedging architecture
• Allows uplift with preservation of seal
• Lateral variations in thermal structure
• Lateral fluid flow
• Role of footwall rheology
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
Bardoc shear
not dilating
at depth
Fluid focussing in tectonic wedges
Fluid source beneath “Kalgoorlie region”
- Bardoc shear still not active conduit
Fluid flow streamlines - Pluton P3 active
20 km
Fluid flow streamlines Pluton P1 0 km
20 km
-20 km
0 km
-40 km
-20 km -5 2 -1
x 10 m s
0.6 0.4 0.2 0 -0.2
-40 km
-5 2 -1
x 10 m s
0.4 0 -0.4 -0.8 -1.2 -1.6
Fluid flow streamlines Pluton P4 active
20 km
Fluid flow streamlines Pluton P2 active 0 km
20 km
-20 km
0 km
-40 km
-20 km
-5 2 -1
x 10 m s
-40 km 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1
-5 2 -1
x 10 m s
0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0
Hydrostatic pressure gradient – thermal effect of pluton
location
Blue = anticlockwise flow, red = clockwise flow
Yilgarn 2D FIDAP thermal
convective chemical model
Dissolution
regions (red)
Precipitation of Au (blue)
Maximum precipitation rate: 10.6 ppm per million years
Geometry and permeability structures
control temperature distributions and
fluid mixing which in turn control the
locations of gold precipitation
Yilgarn numerical models
- principal conclusions”on tectonic
wedging
• 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 Scotia-
Kanowna and Goongarrie–Mount
Pleasant Antiforms
General implications of tectonic
wedging architecture
- Potential to create fault-bounded domains of
differential uplift and overpressuring beneath
relatively impermeable units
- Generates opportunities for mixing of
separate fluids or destabilization through
rapid changes in pressure and temperature
- May also contribute to the formation and
preservation of greenschist facies deposits,
in contrast to the lower long term
preservation potential for deposits formed in
elevated foreland fold and thrust belts.
General implications and
•
speculation
Reinforces the dynamic feedback between
deformation, magmatism and fluid production and
migration
• Requires that magmatic and metamorphic fluid
generation is precisely timed with respect to
deformation
• Alternative fluid – and possibly heat - sources
required if lower crust is already anhydrous
• Importance of post-collisional subsidence and waning
volcanism
– Skellefte district, Sweden
– Tasmanian Cambrian
– Yilgarn
• Need to study orogenic systems to identify wedging
architectures, potentially through
Implications for (future) PNG
mineralization
• What will prevent loss of deposits formed at
high crustal levels in areas of rapid uplift?
• Could deposits also be forming at depth
equivalent to greenschist or amphibolite
facies?
• If this is the case, then would greenschist
facies gold deposits be exhumed within
sinistral strike-slip systems orthogonal to
recent granite-related transfer trend?
• Changes in convergence vector expressed as
– variations in uplift rate and hence lateral
variations in metamorphic grade
– systematic changes in simple shear kinematic
strain is localizing h
formation where shear hig
Potential backthrust
low
Incremental shear strain
E º 84 1 E º 33 1
u as suM et alP e n ilora C
h cne rT eN
westwards?
et al P cificaP h cne rT s unaM uG w
aeni
t se w gn ivom e rT
hcn
S º2 S º2
translated
cA
er c
de t oM
crA el ib gre bsarG
ae S kcra msiB T
PNG fold belt is
etal P rr e t leB dl oF
adeir F
ena d na
ar egro P ideT kO
rhT
t su
to be exhumed as
ylF
tleB lP
aeS nomol oS of ta
mr
et alP
nail artsuA
hosted gold deposits
et al P
Future shear zone-
fo ti mil et amix orpp a
gni da er ps a eS l ar oC n ail art suA-od nI
htro n g nivom e tal P
004 002 0
serte molik
Orogenic processes, mineralization
and preservation potential
• Rifting and subsidence of arc maybe critical
• PNG deposits related to rapid uplift of elevated
terrain during ongoing plate convergence driven uplift
of elevated terrain
• Tectonic wedging
– Provides potential for seal and lateral gradients in
fluid pressure and supply
– Potential for preservation compared to mineral
systems formed in elevated terrain, if isostatic and
thermal history appropriate
– Local extensional domains but essentially
compressive yet with decompression
Unfavourable orogenic
architectures?
• Orthogonal collision with aborted subduction of
buoyant cratonic foreland
• Rapid isostatic response and limited magmatism
• Examples include:
– Helvetic – Penninic nappes in Alpine system
– Caledonian in Norway
– 1.93-1.90 Ga stage of Svecofennian Orogeny
• “Steady-state” orthogonal subduction beneath
continental margin
• Examples include:
– Cretaceous Shimanto accretionary complex,
despite sediment supply and postulated ridge
subduction as anomalous thermal source
The end
Yes, it really is
Effect of pluton location on fluid flow patterns
Blue = anticlockwise flow, red = clockwise flow
Pluton P3
Pluton P1
Pluton P4
Pluton P2
