Structural Analysis of Farley Lake Gold Deposit

STRUCTURAL ANALYSIS AND GOLD METALLOGENY OF THE
FARLEY LAKE GOLD DEPOSIT, LYNN LAKE GREENSTONE BELT (NTS 64C/16)

by C.J. Beaumont-Smith, D.R. Lentz1 and E.A. Tweed2

Beaumont-Smith, C.J., Lentz, D.R. and Tweed, E.A. 2000: Structural analysis and gold metallogeny of the Farley Lake gold deposit, Lynn Lake greenstone belt (NTS 64C/16); in Report of Activities 2000, Manitoba Industry, Trade and Mines, Manitoba Geological Survey,

p. 73-81.

SUMMARY

Preliminary structural and geochemical investigations of the Farley

Lake gold deposit have determined that the gold mineralization is the

result of emplacement of postdeformational, postplutonic quartz-carbonate-

sulphide veins into banded oxide- and silicate-facies iron-formation,

and the accompanying sulphidization of the iron-formation. The

high-grade quartz-carbonate-sulphide veins were emplaced along preexisting,

shallowly southwest-dipping joints and subvertical D2 faults

and shears. The thick sequence of iron-formation and interbedded

argillaceous sedimentary rocks that hosts the mineralization has undergone

significant D1 and D2 fold thickening.

INTRODUCTION

The Farley Lake gold deposit is located in the northern Lynn Lake

greenstone belt, approximately 45 km northeast of the town of Lynn

Lake (Fig. GS-15-1). The deposit consists of four zones of gold mineralization,

the Wendy, South, Southeast and East zones (Richardson and

Ostry, 1996), hosted by a thick sequence of banded oxide-, silicate- and

sulphide-facies iron-formation (Fedikow, 1995). Production from the

Wendy and East open pits between 1996 and 1999 totalled, based on

published reserves, 635 000 t of pyrrhotite-associated ore averaging 6.86

g/t Au and 363 000 t of pyrite-associated ore averaging 4.80 g/t Au,

respectively (Richardson and Ostry, 1996). The total preproduction

resource stood at 1.45 million t grading 3.90 g/t Au (Richardson and

Ostry, 1996).

The Farley Lake deposit is hosted by Wasekwan Group (Bateman,

1945) volcanic and sedimentary rocks that constitute the Agassiz

Metallotect (Fedikow and Gale, 1982), a stratigraphic succession

defined, from top to bottom, by high Mg-Cr basalt (picrite), rhyolitic to

dacitic volcanic rocks, argillaceous sedimentary rocks and exhalative

sedimentary rocks. The mine sequence is

tightly folded (F1) into an overall upright

anticlinal structure. A smaller synclinal inlier

of carbonaceous shale, plunging moderately

to the west (Peck et al., 1998), occurs in the roof of this larger F1

feature; the anticline is therefore cored by banded iron-formation. The

deposit is unique relative to other gold deposits in the Lynn Lake greenstone

belt in that gold mineralization (centimetre- to decimetre-scale

quartz-sulphide veins (QSV) hosted in banded iron-formation) is associated

with late faults and joints that are discordant with the earlier tectonic

fabrics. Other gold deposits, namely the Burnt Timber and

MacLellan deposits, are characterized by the emplacement of gold mineralization

and associated hydrothermal alteration early in the deformational

history, possibly prior to and coincident with fabric development

and peak amphibolite-grade regional metamorphism.

Investigations of the gold mineralization at the Farley Lake deposit

involved detailed structural analysis of the East pit, including geochemical

sampling of mineralization exposed in the pit and selected diamonddrill

core. Mining operations in the Wendy pit were suspended in 1998

and the pit is completely flooded, precluding it from structural analysis.

Production from the East pit concluded in October of 1999 and the pit is

in the process of flooding. Structural analysis of the East pit was initiated

in September 1999, with the mapping of the lower production benches.

Mapping and sampling of the upper benches and preliminary diamond-

drill core study and sampling were completed this summer.

DEPOSIT GEOLOGY

The Farley Lake gold deposit is hosted by a fold-thickened sedimentary

succession of thinly layered iron-formation and argillaceous

sedimentary rocks located within the core of an east-trending anticline

(Fig. GS-15-2; Peck et al., 1998). This sedimentary sequence is overlain

GS-15

1 Department of Geology, University of New Brunswick, P.O. Box 4400, Fredericton, New Brunswick E3B 5A3

2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3

Figure GS-15-1: General geology of the Lynn Lake greenstone belt and location of the Johnson Shear Zone (after Peck et al., 1998).

by dacitic (south) and rhyolitic (north) volcanic rocks and high-Mg-Cr

basalt (picrite). The southern margin of the deposit has been discordantly

intruded by a granodioritic to dioritic pluton of the Pool Lake intrusive

suite (Gilbert et al., 1980). The pluton is in direct contact with mineralized

iron-formation along the southern edge of the East pit.

The iron-formation unit is approximately 600 m wide and has a

strike length in excess of 6 km (Milligan, 1960). It is composed of, in

decreasing order of abundance, finely laminated magnetite-chert (oxide

facies), magnetite-grunerite-chert (silicate facies) and pyrrhotite-chert

(sulphide facies). The oxide facies consists of thin, alternating magnetite

and chert laminations of roughly equal proportions. The minor amounts

of interlayered carbonate within this facies appears to represent transposed

pre- to syn-D2 veins. The silicate facies is composed of finely laminated

chert and beige amphibole (grunerite-anthophyllite)-magnetite

layers. This facies probably resulted from an increased hemipelagic

component during deposition. The sulphide facies constituted a relatively

minor component in the East zone of the deposit, but represents a volumetrically

important constituent of the Wendy zone, largely hosting the

mineralization (Richardson and Ostry, 1996). Sulphide-facies ironformation

exposed in the lower portions of the East pit comprise thin

(1–2 m) interbedded units of finely laminated chert with thin pyrrhotite

laminae and abundant, coarsely disseminated pyrrhotite porphyroblasts.

The latter form 1 to 2 mm blebs and trains of discontinuously interconnected

blebs. Where well foliated, thin pyrrhotite stringers define the

foliation. Total pyrrhotite content approaches 40% by volume.

There appears to be a systematic distribution of oxide and silicate

iron-formation facies in the East pit. The southern portion of the pit is

dominated by oxide facies with minor interbeds of silicate- and sulphide-

facies iron-formation. The northern portion of the pit is underlain

by silicate-facies iron-formation with only minor oxide facies.

Interbedded with the iron-formation are thin, clastic sedimentary

units (approx. 1–2 m average thickness), which locally reach a thickness

of 50 m in the core of the regional antiform. These rocks comprise thinly

bedded sequences of argillite, chloritic argillite and minor siltstone.

Several 10 to 20 cm thick silicate-facies grunerite-garnet iron-formation

horizons are tightly fold-intercalated with the thick argillite unit exposed

in the west wall of the East pit.

Recrystallization associated with regional lower amphibolite facies

metamorphism has resulted in the general coarsening of the constituent

mineral phases, the growth of amphibole in the silicate-facies iron-formation,

and the development of calc-silicates in local calcareous ironformation

and premetamorphic carbonate veins. The timing of metamorphism

is broadly coeval with the third phase of regional deformation

(D3) and postdates the intrusion of the Pool Lake suite. The growth of

randomly oriented grunerite porphyroblasts overprints the main S2 foliation.

Locally, the porphyroblasts have a strong S3-parallel preferred orientation,

suggesting that the differences in degree of porphyroblast preferred

orientation reflect slight differences in the timing of grunerite

growth due to slight changes of bulk rock chemistry.

Intruding the Wasekwan supracrustal succession is a Pool Lake

suite granodioritic to dioritic pluton. The pluton is medium grained,

generally equigranular and locally well foliated, particularly near the

northern contact with the supracrustal sequence. Although commonly

referred to as granodioritic, the intrusion seems to have several phases

that were recognized during the original drilling program; the outermost

phases are tonalitic to dioritic in composition (see next section). Trace to

several per cent pyrrhotite and pyrite are commonly found with quartz

and carbonate veining associated with coincident chloritized, silicified

and carbonatized zones. Although locally mineralized with gold-bearing

sulphide-quartz (± carbonate±chlorite) veins, there are numerous late

fabrics with sulphide-quartz veins and alteration within each of the

intrusive phases that are devoid of gold mineralization. The alteration

zones, which are locally associated with anomalous gold abundances,

are intimately associated with ductile to brittle shearing that seems to

overprint a pre-existing ductile foliation defined by the preferred orientation

of biotite, amphibole and plagioclase (i.e. peak metamorphic

assemblages). In general, the fabric intensity (weak to intense) and alteration

increase toward the northern margin of the pluton. Chloritization

of the constituent biotite and hornblende, together with sericitization of

the feldspar, results in a distinct colour change that seems to coincide

with the introduction of quartz and carbonate, as well as pyrite and

pyrrhotite. It is uncertain whether there was considerable early-phase

Figure GS-15-2: Local geology of the Farley Lake deposit (after Peck et al., 1998).

syndeformational metasomatism, especially since the pluton seems to be

reversely zoned toward the margin. The relative timing of the granodioritic

and tonalitic phase to the dioritic phase is uncertain. The S2 foliation

seems most intense in the outermost zones of the intrusion; there is

minimal fabric within the granodioritic phase, even though there is a

high percentage of ferromagnesian silicates present (25%). Nonetheless,

the first-phase intrusions predate the main (F2) folding event, although

they are discordant with the folded stratigraphy. This indicates that subsequent

deformation, coincident with peak amphibolite-grade metamorphism,

followed their intrusion.

GRANODIORITIC TO DIORITIC INTRUSION

In order to ascertain whether the compositional change from the

granodiorite to the intensely foliated intrusive margin is associated with

a specific episode of hydrothermal alteration or is related to a primary

compositional effect, six drill-core samples were taken from available

core (diamond-drill hole DDH 341) that was drilled from south to north

into the southeastern Farley Lake zone (East pit area). The six-sample

profile has an increase in ferromagnesian phases (biotite±amphibole), in

part retrograded to hydrothermal biotite, then chlorite and carbonate,

from the weakly altered granodiorite into the diorite along the northern

contact. Petrographically, the intrusive phases obviously exhibit more

intense hydrothermal alteration of the lower temperature variety (Mgchlorite–

carbonate–sulphide) approaching the northern contact.

However, granodioritic phases exhibit considerable interstitial biotitic

alteration, which seems to have replaced an earlier hornblende- and

biotite-bearing granodioritic assemblage. The secondary biotite seems to

have a deeper reddish hue than the green igneous biotite. Titanite seems

to have been formed as a result of the selective ferromagnesian replacement

processes, although coupled with plagioclase alteration (anorthite

replacement). The gold-enriched sample (595 ppb Au at 152.4 m [500

ft.] in DDH 341) has intergrown Mg-rich chlorite and carbonate replacing

reddish biotite. Pyrrhotite with very minor chalcopyrite occurs in the

groundmass with the secondary assemblages, although it is unclear if it

is associated with chlorite-carbonate or the hydrothermal biotite. The

presence of chalcopyrite in the gold-bearing section is surprising, considering

the sample only contained 85 ppm Cu. Based on the original

drill log, the altered diorite has an apparent width of approximately 18.3

m (60 ft.) or a 5.2 m (17 ft.) true width. The most intensely altered sample

(103.9 m [675 ft.] in DDH 341) contained minor remnant clinopyroxene

replaced by an Fe-Mg amphibole (fine to coarse grained), which

was in turn partially replaced by and intergrown with Mg-chlorite. The

amphibole has a weak preferred orientation. Rutile is secondary after

ilmenite. There are several late carbonate veinlets in this sample.

These sample cores were crushed and pulverized in a soft-iron

swing mill by the Manitoba Geological Survey and analyzed for major

and trace elements, by x-ray fluorescence spectroscopy, instrumental

neutron activation analysis and inductively coupled plasma–mass and

optical emission spectroscopy, at Activation Laboratories Ltd., Ancaster,

Ontario (Table GS-15-1). For duplicate elements, preference was given

to techniques not involving dissolution of samples.

Figures GS-15-3 and -4 are variation diagrams illustrating the

major-element composition of samples across the profile. As can be seen

in Table GS-15-1, there is a consistent decrease in SiO2 from the core

toward the outer margin of the intrusion, consistent with petrographic

observations. The Al2O3 content decreases slightly from 15.76 to 15.35

wt % with decreasing SiO2, but then drops off to 8 wt % in the intensely

altered (amphibolitized and chloritized) zone near the intrusion margin.

If this is a diorite, it is possible to have lower Al2O3 contents with

higher Fe2O3 tot and MgO (32.1 and 9.3 wt %, respectively) at the margin

(Fig. GS-15-3b, -3c). The K2O, Na2O and CaO contents decrease

markedly toward the margin as well (Fig. GS-15-3d, -3e, -3f), which is

a function of both primary composition and alteration. The increase in

ferromagnesian components at the expense of alkali and alkali-earth elements,

including Rb, Ba and Sr, and decreased SiO2 content are consistent

with increasing amphibolitization (?cummingtonite) and chloritization

toward the outer margin. The original drill logs noted that these

rocks were granodioritic, tonalitic and dioritic in mineralogy, based on

textures, many of which were relict. Therefore it is difficult to ascertain

whether the more mafic looking rocks, which have greater fabric development

and more intense alteration, are actually tonalitic and dioritic in

composition. It is possible to test this using immobile elements and their

ratios.

Figure GS-15-4a illustrates consistently increasing TiO2 with

decreasing SiO2. This cannot be related to mass changes, as it is opposite

to what would be expected based on the decrease in Al2O3 (8 wt %).

Also, the TiO2 co-varies strongly with Sc and V, consistent with an iron

tholeiitic composition for the diorite, whereas the other granodioritic and

tonalitic samples have a more obvious calc-alkalic signature. There is a

decrease in P2O5, S, Ni (Fig. GS-15-4b, -4c, -4d), Co and Cr at the outer

margin relative to the less altered tonalitic and granodioritic phases of

the intrusion. The primitive-mantle–normalized spider diagram (Fig.

GS-15-5) for these samples shows that the profiles for the granodiorite

and tonalite are similar, although the dioritic sample is notably lower in

incompatible elements, except the heavy rare-earth elements (HREE),

and higher in TiO2 (Sc, V), as mentioned earlier. Therefore the light rareearth

element (LREE)/HREE profile of the diorite is notably flatter, typical

of a tholeiitic parentage, than for the calc-alkaline samples.

Similarly, immobile-element ratios (Fig. GS-15-6a) illustrate that the

dioritic sample has a typical tholeiitic signature (Zr/Y = 3), whereas the

other samples have values between tholeiitic and calc-alkalic compositions.

The Zr/TiO2 ratio is also consistent with a gabbroic to dioritic

composition. All samples plot in the I-type granitoid (volcanic-arc–related)

field using the Nb-Y discrimination diagram (Fig. GS-15-6b; Pearce

et al., 1984), consistent with their generally aluminous composition. The

Th-Zr-Nb discrimination diagram (Fig. GS-15-6c; Wood et al., 1979)

shows that the granodioritic and tonalitic samples are of volcanic-arc

affinity, whereas the dioritic sample has an enriched mid-ocean ridge

basalt (MORB) to within-plate association, which is consistent with the

Fe, Mg and TiO2 contents. The Zr versus Ga/Al contents of these samples

(Fig. GS-15-6d; Whalen et al., 1987) indicate an arc signature for

the granodiorite and tonalite, although the diorite falls into the anorogenic

field, which fits with the tholeiitic (proto-arc or rift-related) interpretation,

but not with the Nb/Y ratio, which is much less than 0.7 (see

Winchester and Floyd, 1977). Based on textural and compositional considerations,

it is probable that the diorite is an earlier intrusive phase that

was followed by the main volcanic-arc–associated tonalite and granodiorite.

It is also notable that the sample enriched in gold (596 ppb) is associated

with high sulphur, although it is evident from the logs and gold

assays that this is not always the case. Also, the one anomalous gold

sample has low As, Sb, Mo and W. Interestingly, Fedikow (1995) indicated

that the gold was not associated with some of the typical indicator

elements at the Wendy zone. The sample with the highest gold content is

not associated with the most altered (chloritic) sample.

STRUCTURAL ANALYSIS

A major objective of this study is to determine the timing of

mineralization relative to alteration, deformation, metamorphism and

plutonism in the area. Therefore, an understanding of the structural

history of the deposit is critical. Detailed structural analysis of the East

pit has delineated four generations of folds (F1–F4). The geometry and

distribution of the iron-formation appear to be largely controlled by F1

and F2 folds, with overprinting by the two younger folding events producing

minor redistribution of the older fabric elements. Mesoscopic F1

folds are rare and are generally recognized as isoclinal, commonly

intrafolial folds overprinted by S2 or refolded by upright, tight, F2

chevron folds (Fig. GS-15-7). The F2 folds plunge steeply to the east and

are dominantly Z-asymmetrical. A ubiquitous S2 axial-planar foliation

SiO2

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

TiO2

P2O5

LOI

Total

DDH341-100

(granodiorite)

DDH341-300

(granodiorite)

DDH341-500

(tonalite)

DDH341-515

(tonalite)

DDH341-580

(tonalite)

DDH341-675

(diorite)

56.51

15.76

5.93

0.102

3.39

5.86

4.93

1.51

0.525

0.34

3.73

98.59

45.91

15.35

10.61

0.18

8.8

4.81

3.79

0.31

0.819

0.26

8.69

99.52

40.16

32.1

0.361

9.3

2.83

0.13

0.49

1.786

0.09

3.9

99.14

55.87

15.88

6.69

0.093

4.05

5.99

4.48

1.89

0.588

0.38

2.85

98.76

53.92

15.46

6.97

0.117

4.03

6.58

3.83

1.65

0.598

0.42

5.15

98.72

50.34

15.14

7.8

0.104

5.56

6.39

4.31

2.09

0.766

0.5

6.06

99.07

Table GS-15-1: Analytical data for drillcore samples from the Pool Lake

suite granodioritic to dioritic pluton, southeastern Farley Lake zone.

Sample number (and rock type)

Major elements (wt %):

2.6

550

0.5

10.2

0.5

3.7

2.3

1.8

0.5

0.15

200

110

0.1

28.3

0.5

1.1

0.9

104

17.1

3.8

1.1

0.5

0.3

43.3

210

127

0.5

22.4

0.5

0.7

0.5

9.9

4.4

0.7

0.6

0.14

580

0.4

0.5

3.1

100

32.6

6.4

1.8

0.8

1.1

0.16

595

5.8

660

107

0.3

14.2

1.4

2.5

2.2

26.3

6.2

1.8

0.5

0.9

0.15

11.5

590

143

0.4

16.3

0.5

2.3

1.6

100

27.6

6.7

1.8

0.5

0.9

0.14

Trace elements (ppm):

127

983

608

120

285

300

128

1001

695

120

573

697

1790

101

781

693

Trace elements (ppm):

0.3

770

101

0.045

0.3

1230

103

124

204

0.081

0.3

1.6

2480

658

0.001

0.3

772

121

0.068

0.3

901

120

0.246

0.3

769

150

0.015

Trace elements by ICP-OES (ppm):

accompanied F2 development and represents the most penetrative secondary

fabric element.

Peck et al. (1998) interpreted the distribution of the iron-formation

within the core of a shallow west-plunging anticlinorium (see Fig. GS-

15-2). Key to this interpretation is the correlation of the dacite exposed

along the south margin of the East pit and the rhyolite north of the pit as

lateral facies equivalents. Consequently, the argillite exposed in the west

wall of the East pit was interpreted to occupy the core of a shallowly

west-plunging synform. Based on mesoscopic overprinting relationships,

this fold is most likely a macroscopic F1 fold. There is no systematic

change in F2 asymmetry across the argillite, suggesting that F2 is not

responsible for the distribution of the argillite and iron-formation.

The intensity of D2 fabric development, demonstrating the partitioning

of the deformation, is reflected in the large competency contrasts

between the units that form the deposit stratigraphy. The iron-formation

is tightly folded and moderately foliated. A steep D2 strain gradient

Figure GS-15-3: SiO2 versus a) Al2O3, b) Fe2O3 tot, c) MgO, d) K2O, e) Na2O, and f) CaO in the granitoid samples (see Fig. GS-15-5 for

symbols).

Figure GS-15-4: SiO2 versus a) TiO2, b) P2O5, c) S, and d) Ni in the granitoid samples (see Fig. GS-15-5 for symbols).

Figure GS-15-5: Primitive-mantle–normalized spider

diagram with the six samples illustrated (see Sun and

McDonough, 1989 for normalizing values). The bolded

elements are considered immobile under mostly

hydrothermal alteration conditions.

Figure GS-15-6: a) Y/TiO2 versus Zr/TiO2 diagram, with Zr/Y illustrated, b) Nb versus Y discrimination diagram (modified after Pearce

et al., 1984), c) Th-Zr-Nb discrimination diagram (modified after Wood, 1979); abbreviations: A, N-MORB affinity; B, E-MORB affinity;

C, within-plate affinity; and D, volcanic-arc affinity, d) Zr versus Ga/Al (see Whalen et al., 1987).

along the southern contact between the iron-formation and the granodioritic

intrusion is characterized by the development of protomylonite

and a narrow zone of mylonitic fabrics in the iron-formation along the

contact. The argillite interbedded with the iron-formation is intensely

folded and foliated, as evidenced by the development of phyllitic zones

and local fault displacement of F2 limbs. The margin of the intermediate

pluton exposed in the south wall of the East pit contains moderate S2

foliation development and preserves an older S1 slaty cleavage. An S3

foliation and F4 folds are also developed in the granodiorite.

Overprinting F2 are two generations of close to open folds. The F3

generation comprises northeast-trending, steeply plunging, Z-asymmetrical

folds. Although mesoscopic F3 folds are rare, a penetrative S3

crenulation cleavage overprints older fabric elements. The youngest generation

of folds comprises north- to northwest-trending, open chevron

folds. Folds of the F4 generation generally lack an axial-planar foliation,

but produced small-scale tight crenulations, which locally have differentiated

limbs that approximate a foliation.

Emplacement of the intermediate dykes postdates F2 folding. The

orientation of the dykes is subparallel to F2 axial planes and the dykes

do not outline any F2 folds or boudinage. The dykes predate F3 and generally

contain an S3-parallel spaced fracture cleavage. The intermediate

intrusion (granodiorite to diorite) of the Pool Lake suite seems to intrude

during late F2 or early F3.

Numerous premineralization faults transect the East pit. Many of

these occupy F2 limbs and appear to represent adjustments in response

to the tight F2 folding. Two major northwest-trending faults, the East and

Wendy faults, postdate the folding events, since they transect F2–F4 axial

traces. These parallel faults comprise steeply northeast dipping zones of

intense fracturing cored by decimetre-scale gouge zones characterized

by imbricated clasts, indicating a large component of normal movement.

GOLD METALLOGENY

Gold mineralization in the East pit is associated with two sets of

centimetre- to decimetre-scale, coarse-grained, quartz-sulphide and

quartz-dolomite-sulphide veins with minor chlorite (Fig. GS-15-8). The

veins have a wide variety of textures, including open space, vuggy textures;

laminated, crack-seal textures; and locally composite, zoned veins

consisting of crack-seal textures along one side and sulphide-matrix

quartz breccia along the other.

The timing of mineralization postdates both the folding and faulting

events. There are numerous examples of mineralized flats transecting

F4 fold axes, the latest folding event, and both the Wendy and East

faults (Fig. GS-15-9a, -9b). The shallow QSV (flats) were emplaced

along shallowly southwest-dipping faults and joints. These brittle-ductile

features record a sinistral offset of approximately 1 m. Narrow zones

of sinistral ductile bending of the bedding laminations indicate the initiation

of the joints as ductile structures followed by brittle failure due to

the elevation of the strain rate. The subvertical QSV (steeps) were

emplaced along moderate to steep south-southwest-dipping joints, along

subvertical S2 parallel faults, and within zones of intense S2 foliation

development (phyllitic zones). These mineralized veins are very high

grade (approx. 30 g/t Au; Peck et al., 1998) and are mantled by zones (up

Figure GS-15-7: Steeply east-plunging F2 fold in oxide-facies

iron-formation.

Figure GS-15-8: Shallowly southeast-dipping (flat)

quartz-carbonate-sulphide vein. Note the sulphidic

halo developed where the vein intersects iron–

formation, and the slight ductile bending of the bedding

lamellae adjacent to the vein.

to 60 cm wide) of intense sulphidization of the host iron-formation. In

the East pit, the sulphidization is manifested as replacement of the magnetite

lamellae by pyrite (and gold), preserving the primary laminations

and secondary foliations. The sulphidization halos are developed around

both the steeps and the flats. The link between emplacement of the mineralization

halos and sulphidization of magnetite is demonstrated by the

lack of halo development where the QSV cut the interbedded argillite or

intermediate dykes.

The low-temperature alteration assemblages in the later fabrics in

all phases of the intrusions indicate that gold mineralization entirely

postdates all intrusive events. As such, the gold is not associated with

magmatic-hydrothermal activity related to these intrusive phases. It is

possible that the late fabrics formed preferentially along the intrusion's

north margin, which is the southern margin of the iron-formation–hosted

gold mineralization and the focus of gold-mineralizing hydrothermalfluid

flow in the region; this could be due to the competency contrast

between the iron-formation and the intrusion. Although this is consistent

with the orientation of the southwest-dipping mineralized flats

(faults/shears) and S2 parallel steeps (phyllonite zones) in the area, the

intermittent gold mineralization along the intrusive contact is not entirely

consistent with this scenario, although the obvious control of sulphidization

processes by magnetite may explain the minimal mineralization

evident in the intermediate intrusion.

CONCLUSIONS

The intermediate Pool Lake suite intrusion is a zoned or composite

intrusion with a massive granodioritic core grading to a marginal phase

of dioritic composition. Although difficult to ascertain due to the variability

of alteration, the granodioritic and tonalitic phases have a metaluminous

volcanic-arc (I-type) affinity, whereas the diorite seems to

have an Fe-rich tholeiitic character, possibly related to a first-phase

extensional event associated with intrusion emplacement. The increase

in fabric development and at least two stages of hydrothermal alteration

(high T and low T) are recognized petrographically and seem to characterize

the zone. An earlier foliation (S2) in the outer margin, coupled

with the generally discordant nature of the intrusion, suggests that it was

emplaced syn- to post-F2 folding and fabric development, which is consistent

with the peak metamorphic mineralogy. Analogous to the late

controls on gold-sulphide-quartz veining in the iron-formation–hosted

sequence, the late chlorite-quartz-sulphide veins that locally contain

gold in the intrusion are associated with faults and shears that seem to

exploit the earlier S2 fabric and related structures. As outlined before and

based on fabric overprinting relationships, the gold mineralization is

controlled by late brittle shears/joints that parallel the steep S1-S2 fabrics

(steeps) and shallowly southwest-dipping brittle-ductile faults (flats) in

the iron-formation and argillite. It is possible, especially considering the

intensity of late shearing and low-temperature alteration along the intrusion-

supracrustal contact, that the faulting and shearing were controlled,

in part, by the competency contrast between the iron-formation and the

altered intrusion margin. Although it does not seem that the intrusion

played a direct role in the mineralizing process, it is probable that late

auriferous hydrothermal solutions were focused from depth by these

Figure GS-15-9: a) Emplacement of quartz-sulphide

vein (QSV) following F4 folding (arrow); b)

QSV 'flat' emplacement following faulting (arrow).

structures and enhanced fabrics. Due to the selective nature of the sulphidization

process (mainly of magnetite) that seems to be directly

linked to mineralization, the intermediate intrusion was a less favourable

host to the mineralization.

ACKNOWLEDGMENTS

This research is supported by the Manitoba Geological Survey, a

grant from the Geological Survey of Canada and a grant from the

University of New Brunswick. Additional in-kind support from Black

Hawk Mining Inc. and their staff was invaluable to our

research.Particular thanks are due to Paul Pawliw for his assistance. The

manuscript was kindly reviewed by Mark Fedikow (MGS).

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Town of Lynn Lake