Download Recognizing optimum Banded-Iron Formation

Recognizing optimum
Banded-Iron Formation-hosted gold environments:
Preliminary results from the Meadowbank deposit
S. Castonguay(1), V. Janvier(2), P. Mercier-Langevin(1), B.
Dubé(1), V. McNicoll(1), M. Malo(2), S. Pehrsson, S.(1)
(1) Geological Survey of Canada
(2) Institut National de la Recherche
Targeted Geoscience Initiative-4:
Lode Gold project
 Main goals :
 1) Provide key descriptive
and genetic parameters and
improved geological and
exploration models to
detect new (deep/buried
≥200m) deposits;
Distal and proximal hydrothermal
signature footprints (geological setting)
2) Develop more robust exploration vectors (signatures-footprints)
and means to identify fertile mineral systems (PROCESSES) to
reduce exploration risk
3) Train and mentor students to increase the HQP available to the mineral
industry.
Canadian Gold Production in 2011 (98 t)
Canadian Gold Production in 2011
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MAC, Facts and Figures 2012
Lode Gold project: Main Themes
 1) Intrusion-related and stockwork-disseminated deposits (Can. Malartic).
 2) Gold fertility of major fault zones (e.g. Timmins area)
 3) BIF-hosted gold deposits.
Gold deposits vs BIF-hosted gold deposits (≥1Moz)
BIF-hosted Au deposits are not common but have the potential for world-class size
Homestake 40 Moz Au
Gold deposit ≥1Moz
BIF ≥1 Moz
Dubé , Gosselin and Bécu, 2010
BIF-hosted gold deposits
Two types of BIF-hosted mineralization :
 Entirely hosted within BIF: form commonly one large «isolated» deposit
(Musselwhite; Meadowbank)
 Partly hosted in BIF and present within a district (Meliadine, Geraldton):
associated with:
 Major fault zones
 Abundant quartz veins
Dubé , Gosselin and Bécu, 2010
Six gold deposits have been found at the Meliadine
project: Gold mineralization is mostly in laminated
quartz-vein, stockwork and sulphidized BIF.
BIF-hosted gold deposits: key scientific hypotheses
1.
BIF–hosted gold deposits in the Churchill Province are genetically
related and structurally controlled by Paleoproterozoic (1.83 Ga)
tectonometamorphic events superimposed on Archean rocks.
BIF–hosted gold deposits commonly form large isolated entities
without an associated district.
The first-order exploration criteria and main geological controls on
gold concentration have to include:
2.
3.




4.
major shear zone
macroscopic isoclinal folds
unconformity
presence of black shales or ultramafic rocks
The footprint of BIF-hosted gold deposit can lead to better
exploration vectors, if one can differentiate:
1.
2.
The hydrothermal alteration from regional or contact metamorphism.
AND
the nature and timing of distal vs. proximal hydrothermal alterations.
Meadowbank integrated activities

Deposit scale: “Geology, structure,
geochemical and hydrothermal
footprints and genesis of the worldclass Meadowbank gold mine”


Sub-regional or “district scale”:
Bridging the gap between the
regional geological setting and the
deposit scale knowledge
Looking “outside the box”

GEM Compilations and synthesis

Collaboration GSC, INRS with
Agnico-Eagle Mines
Geological setting

Meadowbank mine area lies within
the Rae Domain of the western
Churchill Province.

The area is underlain by
Neoarchean supracrustal rocks,
surrounded by granitoid and
gneissic complexes and regionally
overlain by pre- to post Hudsonian
Proterozoic basins and 1.83-1.75
granitic plutons

The Woodburn Lake and Prince
Albert gps form a ca. 1500 km long
greenstone belt

The mine area is underlain by the
Archean Woodburn Lake (2.71
Ga) and the Paleoproterozoic
Ketyet River gps.
Davies et al., 2010
Woodburn Lake Gp
and the mine area

East-facing Neoarchean sequence
comprising:







2.62 Ga
Felsic to intermediate
volcanoclastic rocks
Komatiitic basalt
Clastic and chemical
sedimentary rocks
Volcanic cycles are separated by
conglomerates or/and faults
A disconformity separates the
volcanic sequence from the
Amarulik wacke
The paleoproterozoic Ketyet
River Gp.: siliciclastic sedimentary
and mafic volcanic-intrusive rocks
2711 Ma
2612 Ma
Mine area
2612 Ma
Greenschist (N) to amphibolitefacies (S) reg. metamorphism
BIF’s and deposit area
 several BIF horizons (East, Central,
West);
 Meadowbank deposit in the Central
BIF; so far the only with
economical resources
Pehrsson et
al., unpubl.
Structural Geology
Up to 7 phases of regional deformation are
documented (Pehrsson et al.)
p
•First 2 phases are restricted to the Archean
Woodburn Lake Gp.;
•Mostly micro/mesoscopic evidence
•Hard to differentiate in area of strong
overprint.
p
Basal Ketyet River Gp
Archean fabrics
•DA1: foliation within in conglomeratic clasts
(pre-2.71 Ga)
•DA2: axial-planar foliation to micro/mesoscopic
isoclinal folds (larger scale structures
suspected),
SA1-2
FP1
•reworked by Proterozoic fabrics (Dp1-2)
Volcanoclatic Woodburn Lake Gp
th
ru
st
Proterozoic deformation
Dp
2
•DP1: Greenschist-facies
•SP1, schistosity largely subparallel to
bedding/layering
2.62 Ga
•commonly transposed
•Isoclinal folds (FP1)
•DP2: Mid-greenschist to amphibolite
• Dominant regional-scale
tectonometamorphic phase
•N-trending crenulation to penetrative
foliation (often a composite SP1-2)
2612 Ma
2612 Ma
•close to isoclinal shallowly doublyplunging, NW-vergent folds
•NW-directed thrusts (Third Portage
Thrust)
•The timing of DP2 1.91-1.83 Ga
Pehrsson et
al., unpubl.
Proterozoic deformation
Fp2 fold affecting strong Sp1
foliation
Composite Sp1-2 fabric in BIF
Pehrsson et
al., unpubl.
Proterozoic deformation
•DP3: Greenschist-facies
Fp4
Fp4
•chevron to similar-style folds; subhorizontal plunges; shallowmoderately-inclined axial surface
•SP3 crenulation only locally developed
•No apparent maps-scale influence
•DP4:
•Map- to outcrop-scale folds
•local S4 crenulation cleavage and
lineation.
•Fp4 form prominent kilometre-scale,
open to tight, NNE-trending upright,
moderately to shallowly-plunging folds
•Form an S-shaped asymmetric fold
pair in the mine area
Pehrsson et
al., unpubl.
Proterozoic deformation
Shallow-inclined,
sub-horizontal FP3 East BIF
SP4 crenulation of SP2
Tern Porphyry
Pehrsson et al.,
unpubl.
Deposit Sequence
Mostly west-dipping package, from East to West:
- Intermediate to felsic volcanoclastic rock (green)
(from Sherlock et al., 2001)
Deposit Sequence
•Banded iron Formation (red)
•Ultramafic rocks, talc schist, strongly sheared
near contacts (mauve)
(from Sherlock et al., 2001)
Deposit Sequence
- Basal conglomerate, with Py-bearing IF fragments
- Quartzite, with fuschite and BIF fragments
(yellow)
(from Sherlock et al., 2001)
Mineralization
• Two main orebodies lie along the same horizon:
• Portage (N) and Goose (S) pits
• The ore zones are typically 6-7 m wide
• 5 up to 40 g/t Au
• hosted in the BIF, predominantly along
the contacts with ultramafic rocks.
(from Sherlock et al., 2001)
Mineralization
•The predominant gold mineralization is
associated with pyrrhotite and pyrite
replacing magnetite sub-parallel to BIF
layering
Mineralization
• or narrow stringers of disseminated sulphides.
Mineralization
Gold also is found in:
•Py-Po veins
•high-grade quartz
veins (up to 300 g/t)
crosscutting the BIF
and footwall volcanoclastic rocks.
•Suggest a component
of remobilization
Published and actual mine models
B
B’
Geometry and structural controls:
 fold interference pattern Fp1-2/4
 Mineralization in strained lower
limb, within BIF
Fp4
B
UM
B’
Fp4
Sherlock et al., 2004
UM
Fp1-2
Sherlock et al., 2001
Mine model
New detailed mapping in Portage Pit
Portage south ramp (West face)
UM
quartzite
BIF
IV
Fold model
 Mapping suggest fault imbrication by two Dp2 N-S shear zones
preferentially developed within the ultramafic units.
 Role of faulting is underestimated
New detailed mapping in Portage Pit
Portage south ramp (West face)
UM
quartzite
BIF
IV
Next slide
 These shear zones appear to truncate the ore zones, and
possibly control their distribution.
New detailed mapping in Portage Pit
IV
IF
UM
IV
Portage south ramp (West face)
 Eastern shear zone cut S0 and S1-(2?) in the footwall sequence
 Timing syn- to late- Dp2
New detailed mapping in Portage Pit
Portage centre
(looking North)
1m
 Ultramafic –hosted shear zones cut mine sequence;
New detailed mapping in Portage Pit
UM BIF
quartzite
MV
IV
Next slide
Portage North:
 Two quartzite horizons: suggesting Dp1-2
fold or fault imbrication
 Megascopic Fp4 fold folding mine
sequence and shear zones
 Marks the end of the Portage orebody
FV
Portage North (looking West)
New detailed mapping in Portage Pit
Portage North
(looking West)
1m
 Portage North: megascopic Fp4 fold; Fp3-4 fold interference
Level mapping using drill sections; AEM database
2
Some key observations:
•Early isoclinal folding (Fp1-2)
•Fault imbrication of mine sequence
•Late sinistral faults (Dp5)
1
1
2
3
3
Portage
Portage South section (N6100)
Fp1-2
N6100
Level 5090
Fp4
1
2
3
Portage
Goose
N6100
Level 5060
1
Goose
3
Portage
Dp5 fault
New study of drill sections: detailed description and
geochemical analyses
Portage South (section N6175)
•5 sections selected along the deposit
•3 of 5 sections studied; 15 drill holes; 400
geochem samples (data interpretation ongoing)
•Possible zonation of grunerite and chlorite
•Different UM units; amphibole porphyroblasts
only in eastern unit
•Core angle analysis suggests asymmetric folding
Preliminary implications and outstanding questions
•Role and timing of faulting vs. mineralization
Third Portage Trust
•Third Portage Thrust southern extension ?
•Fault zones and UM not mineralized:
• reactivated early fertile faults ?
•Role of pre-DP2 structure:
• “mineralizing events” or « ground
preparation »
Mine area
Pehrsson et al., unpubl.
2013-2014 objectives: wish list


Continuing detail mapping of Goose pit
Improve the tectonostratigraphic framework with geochronology:
 key sedimentary units: characterize unconformities or document structural repetition
 Key volcanic or intrusive phases

Structural geology:
 Role of pre-DP2 structural history: “mineralizing events” or « ground preparation »
 Characterize the main DP2 “Third Portage” fault zone and its probable splays; Relationship
with the mineralization, unconformities and volcanic cycles (repetition?)
 Investigate if post-mineralization deformation contributed to structural controls or
secondary ore enrichment.

Characterize the hydrothermal signature of mineralization

Integration of deposit-scale data with regional knowledge to produce the deposit
footprints

Ultimately, extract « generic » exploration vectors to test terrane fertility for BIFhosted gold mineralization and reduce exploration risk in the Churchill Province and
beyond …
Acknowledgments:
Agnico-Eagle Mines:
Meadowbank and exploration
divisions
CNGO
Nunavut Research Institute