applied structural geology - srk consulting
TRANSCRIPT
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The role of pre-existing
veining in the localization of
auriferous vein systems:
Examples from the Canadian
Shield.
James P. Siddorn, P.Geo.
Principal Consultant
SRK Consulting (Canada) Inc.Email: [email protected]
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Outline
• Typical structural setting of shear zone hosted gold
deposit:
• Association with crustal scale shear zones;
• Vein geometry in shear zones;
• Controls on ore plunge;
• Competence contrasts in gold deposits:
• Vein control on later vein deposition;
• Needed for world-class deposit?
• Potential for localizing late stage gold enrichment;
• Influence on ore plunge.
• Controlling geometries; and
• Examples:
• Detour Lake gold deposit, Ontario;
• Seabee gold deposit, Saskatchewan; and
• Con gold deposit, Northwest Territories.
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Crustal Scale Faults
Figures modified from Robert et al., 2005
Deposit/district sizes from Gosselin and Dube GSC Open File 4894 (2005)
Gold camps and deposits are
spatially associated with second-
order structures in close vicinity to
crustal-scale structures.
Distribution of gold deposits in the Timmins camp
35
9
18
2
20 District/deposit size in Moz of gold
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• Veins form in or adjacent to both brittle
and ductile fault zones;
• Veins generally form oblique to their
related fault, and the sense of obliquity
is related to fault movement
direction/sense; and
• With continued deformation the veins
will fracture, fold and boudinage.
Veins in fault zones
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Ore Plunge Control
From Poulsen and Robert (1989)
In a low strain, shear
zone hosted, deposit
the ore plunge will be
aligned with the
intersection of the
foliation with
extensional veining,
normal to the
stretching lineation
(transport direction).
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Ore Plunge Control
With increased
progressive strain the
ore plunge will rotate
as fold and boudin
axes in auriferous
veins rotate parallel to
the stretching lineation
(transport direction).
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• Formation of a quartz-carbonate
vein in a schistose fault zone
(e.g. biotite, chlorite, sericite)
creates a large competence
contrast between the strong vein
and the surrounding weak
schist;
• This creates a positive feedback
mechanism where during
subsequent deformation the vein
will fold/boudinage/fracture
creating low stress sites that will
focus the deposition of
subsequent hydrothermal fluids.
Competence contrasts in ductile fault zones
Quartz vein in graphitic schist, Obuasi,
Ghana
Pyrrhotite infilling boudin necks in a
quartz vein, Detour Lake, Ontario
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Veins exploit pre-existing veins
Quartz-tourmaline vein xc boudinaged ankerite
vein, Red Lake
Quartz-tourmaline vein xc boudinaged ankerite vein,
Dome, Timmins
Quartz-amphibole vein infilling boudin necks in
ankerite vein, Red Lake
Quartz vein xc quartz vein, Con, Yellowknife
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Positive feedback
Deform ductile
shear zone
Deposit vein
system
(barren or
auriferous)
Deform vein
system
Deposit veins
localized on
1st/2nd/3rd vein
system
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Significant gold enrichment?
Con, Yellowknife
• Related to orders of magnitude variation in gold
grade (~10 g/t to 1500 g/t); and
• Often associated with chalcopyrite, sphalerite
and galena.
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Significant gold enrichment?
Detour Lake, Ontario
2.2 g/t 91.5 g/t
• Chalcopyrite-pyrrhotite-pyrite infilling
extensional fractures and boudin necks in
deformed quartz veins; and
• Related to orders of magnitude variation in gold
grade.
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Significant gold enrichment?
Seabee, Saskatchewan
Flat-lying cpy-py veins x-c quartz vein.
Flat-lying cpy-py veins x-c quartz vein.
• Chalcopyrite-pyrrhotite-pyrite infilling
extensional fractures in deformed quartz veins;
• Related to orders of magnitude variation in gold
grade.
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Timing and gold endowment
• It is important to understand
the timing relative to
deformation of the vein
generations, and the
controlling kinematics of the
deformation at that time.
• It is also important to
understand the relative gold
endowment of the different
vein generations; e.g.
barren-auriferous,
auriferous-auriferous.
Folded gold with axial planar cleavage, Rainy River gold
deposit, Ontario
Gold in cross-cutting fracture, Rainy River gold deposit,
Ontario
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Controlling Geometry? – Red Lake
Folded ankerite vein, CARZ zone, Phoenix Island, Red Lake
(Rubicon Minerals Corp. exploration property)
Conjugate quartz-amphibole veins
within ankerite vein
Folded ankerite vein crosscut by quartz-
tourmaline veins
Overall geometry of later vein system can be
strongly controlled by geometry of earlier
deformed (folded/boudinaged) vein system
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Ore Plunge Control
• It is important to
document the strain
state of the different vein
generations, in particular
the competent veins that
host the later vein
generations; and
• As the competent veins
deform with increased
progressive strain their
fold and boudin axes will
rotate parallel to the
stretching lineation.
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Low strain setting –
Detour Lake, Ontario
US$775 pit outline
Sunday Lake
Deformation Zone
400m
Gold mineralization occurs in a vein field that extends at least across a strike length of 3 km’s, a width of 200-350 m and 1 km below surface.
Deposit scale geological map, Detour Lake
Placer Dome open pit, Detour Lake
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Detour Lake – Gold mineralization
• Broad stockwork of overall east-west striking quartz-carbonate veins developed along sub-parallel biotite shear zones in hanging wall of the Sunday Lake Deformation Zone.
17
Quartz-carbonate and sulphide veins, Detour Lake
Su
nd
ay L
ak
e
Defo
rma
tio
n Z
on
e
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Detour Lake – Deformation
18
• Steep plunging mineral stretching lineation, asymmetric pressure shadows and sub-horizontal extensional veins indicate reverse north-over-south kinematics.
Steep mineral
stretching lineation
Asymmetric pressure
shadow around
plagioclase phenocryst
Flat pyrite-pyrrhotite vein
Flat extensional
quartz vein
Boudinaged quartz vein
sub-parallel to steep dipping
E-W striking foliation
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Detour Lake – Gold enrichment?
Pyrrhotite infilling a boudin neck in quartz vein
Pyrrhotite-pyrite-chalcopyrite extensional vein array
Structural Framework in low strain-late extension setting
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Average
Foliation:
109/82
Average Vein
289/89
20
Detour Lake – Ore Plunge
$775 Pit
Intersection 33 283
Boudin and fold axes of auriferous quartz veins, and quartz vein intersections plunge moderate to the west corresponding to the ore plunge, which is orthogonal to the stretching lineation.
Stretching lineation
Leapfrog gold grade shells:
Grey = 1 g/t
Red = 3 g/t
W E
Ore plunge
Long Section showing Leapfrog gold grade shells
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N S N W EBK
From Corrigan et al. (2007)
High strain setting –
Seabee, Saskatchewan
Seabee
Seabee
Saskatchewan Geological Survey Map 240; Delaney et al. (2001)
Seabee
Porky West
Porky Main
Santoy 7
Santoy 8
• Laonil Lake intrusion is a multiphase
intrusive complex (1889 ± 9 Ma)
composed of gabbro, quartz diorite and
diorite hosting the Seabee deposit.
• Eyahpaise intrusion is composed of
granodiorite and tonalite (1859 ± 5
Ma).
• Pine Lake greenstone belt: 1889-1846
Ma mafic to intermediate metavolcanic
and volcaniclastic rocks; and
• Porky Lake metasediments: >1845Ma
arenites, wackes, conglomerates and
biotite schist.
N
N
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Seabee – Geology
• ENE striking, steeply south dipping anastomosing biotite-actinolite-
chlorite shear zones containing quartz veins (blue) crosscutting Laonil
Lake intrusive complex (“LLIC”, 1889 ± 9 Ma).
• LLIC is a gabbro, quartz diorite and diorite composite intrusion.
100 m
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Seabee – Deformation
• Steep down-dip stretching lineation, horizontal extension veins, and
asymmetric boudins indicating reverse, south over north displacement.
Hornblende-biotite-plagioclase stretching
lineation along foliation plane.
E W
Horizontal extensional quartz veins Asymmetric quartz boudins
indicating south over north shear
sense.
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Isoclinally folded and
transposed quartz vein.
Seabee – Vein system
Boudinaged and isoclinally folded
quartz vein.
Boudinaged and isoclinally folded
quartz vein .
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Eastward plunge of folded quartz vein (XS in inset)
N S
E W
Seabee – Ore plunge
E W
Eastward plunge of boudin in quartz vein
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Seabee – Gold enrichment?
Seabee, Saskatchewan
Flat-lying cpy-py veins x-c quartz vein.
Flat-lying cpy-py veins x-c quartz vein.
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Seabee – Ore PlungeW E
200 m
Structural Framework in high strain-late extension setting
Long Section showing mined stopes and resource blocks
Stretching lineation
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High strain setting –
Con, Yellowknife
Slave Province
Yellowknife Greenstone Belt
Yellowknife location
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Con – Geology
• Produced 5.5 million oz’s Au.
• Strike length: 10,000 ft.
• Depth: 6500 ft.
• Refractory gold in arsenopyrite and free-milling gold.Geological Map
Geological Cross-section
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Con – Deformation
3 deformation phases:
• D1 Early extension;
• D2 Reverse-dextral shearing; and
• D3 Proterozoic brittle faulting.
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Boudinaged “fishhook” fold in quartz vein
Con – Vein system
Boudinaged and isoclinally folded quartz vein.
Transposed and boudinaged quartz vein Transposed and boudinaged quartz vein
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Con – Gold enrichment?
Con, Yellowknife
• Three phases of auriferous quartz-
carbonate veining associated with
free-milling gold.
• Often located in extensional settings
created during deformation of pre-
existing veins.
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Con – Orientation of Fold & Boudin axes
103
202
High strain-late extension
setting
Long Section showing mined stopes and resource blocks
56
W
E
Stretching lineation
Equal angle stereonets display fold/boudin
plunge in auriferous veins
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End MembersLow strain-
late extensionHigh strain-
late extension
High strain-
early extension
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Conclusions• Competence contrast between veining and schist
creates a positive feedback where during subsequent
deformation the vein will fold/boudinage/fracture
creating low stress sites that will focus the deposition of
subsequent veining;
• Later gold rich hydrothermal fluids are often associated
with an abundant sulphide (cpy-po-py) phase;
• The influence on ore plunge is controlled by the gold
endowment of each phase of mineralization and the strain
state of the vein systems; and
• For which there are 3 principal end members:
• Low strain-late extension (ore plunge parallel to the intersection of
extension and shear veins, orthogonal to the stretching lineation)
• High strain-late extension (deposit scale ore plunge parallel to the
boudin&fold plunge/stretching lineation; stope scale ore plunge can
be parallel to the intersection of extension veins and boudinaged
veins); and
• High strain-early extension (ore plunge parallel to the boudin&fold
plunge/stretching lineation).
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Acknowledgements
Claude Resources Inc.Brian Skanderbeg
Detour Gold Corp.Gerald Panneton, Pat Donovan, Laurie Gaborit, Roger Aubertin
Rubicon Minerals Corp.Ian Russell, Terry Bursey, Matt Wunder, David Adamson
Rainy River Resources Ltd.Paul Geddes, Wally Rainer
SRK Consulting (Canada) Inc.Jean-Francois Couture, Ivo Vos, Blair Hrabi