Relationship of hydrothermal alteration to structure and stratigraphy at the Coniaurum gold mine, northern Ontario

DARWIN W. PIROSHCO Ontario Geological Survey, 77 Grenville St., 9th floor, Toronto, Ont., Canada M7A 1 W4

AND

C. JAY HODGSON Department of Geological Science, Queen's University, Kingston, Ont., Canada K7L 3N6

Received November 24, 1987

Revision accepted April 14, 1988

The gold mineralized zones of the Coniaurum mine, Porcupine camp, northeastern Ontario, are on the eastern end of the northeast-trending Hollinger -McIntyre ore system. The ore zones are quartz -ankerite (plus accessories) veins and vein sys- tems and associated pyritic wall rock, hosted by a sequence of mafic volcanic rocks and discordant quartz- feldspar porphyry stocks of Archean age.

A least altered facies and three alteration facies can be distinguished within the mafic volcanic rocks: a chlorite facies, an ankerite facies, and a vein envelope facies. The chlorite facies is widespread, overprints the least altered facies (i.e., chlorite replaces actinolite), and hosts barren and locally mineralized quartz veins bordered by vein envelope facies alteration. The ankerite facies is coextensive with subparallel shear zones, which crosscut the axial trace of the Coniaurum anticline, and hosts most of the mineralized vein systems. Addition mineralization occurs within graphitic sediments in the crest area of the Coniaurum anticline.

On the basis of the above relationships, the shear zones, hydrothermal alteration, and mineralization are interpreted to be late (i.e., syn- to post-development of the Coniaurum anticline).

The mineral assemblages of the chlorite and ankerite alteration facies are interpreted as resulting from lateral gradients in Xco2. Replacement textures between minerals at the alteration facies boundaries indicate the hydrothermal system first grew outwards but later collapsed inwards and the vein envelope facies is superimposed on the more widespread ankerite and chlorite facies.

Les zones aurifbres de la mine Coniaurum, dans le camp minier de Porcupine, au Ontario nord-ouest, sont 1ocalisCes B I'extrCmitC orientale de la bande de minerai Hollinger -McIntyre, de direction nord-est. Les zones minCralisCes incluent les filons et les systkmes filoniens de quartz-ankirite (plus mineraux accessoires) a Cpontes pyritiskes, recoupant les roches encaissantes formCes de volcanites mafiques et les petits massifs intrusifs discordants d'ige archken.

Dans les roches volcaniques mafiques, on reconnait un facibs faiblement altCrt et trois facibs d'altkration : un faciks a chlo- rite, un facies B ankCrite et un facibs enveloppant les filons. Le fac2s B chlorite est rCpandue, et il surimpressionne le facibs de faible altCration (c.-a-d., la chlorite remplace l'actinote); il inclut les filons de quartz stCriles ou localement minCralists bordCs par le faciks enveloppant les filons. Le faciks a ankCrite suit les zones de cisaillements subparallkles qui recoupent la trace de l'axe en surface de l'anticlinal de Coniaurum, et il renferme a peu prbs tous les systbmes filoniens minCralisCs. Une minkralisation plus abondante apparait dans les stdiments graphitiques de la charnibre de l'anticlinal de Coniaurum.

Ces relations gCologiques revklent que les zones de cisaillement, I'altCration hydrothermale, ainsi que la minkralisation sont des Cvenements tardifs (c.-a-d. synchrone ou postCrieur au dCveloppement de l'anticlinal de Coniaurum).

Les assemblages minCralogiques des facibs d'altCration chlorite et a ankCrite semblent rksulter de gradients IatCraux en Xco2. En bordure des faciks d'altkration, les textures de remplacement entre les minCraux indiquent que le systbme hydrothermal croissait au dCbut vers I'extCrieur, mais plus tard il s'attknua vers llintCrieur, et le facibs enveloppant les filons s'est supperposke sur les facibs B ankCrite et a chlorite qui occupent des aires plus grandes.

[Traduit par la revue]

Can. J. Earth Sci. 25, 2028-2040 (1988)

Introduction Although a close relationship between gold-bearing quartz

lodes and rusty weathering, carbonate-altered rock has long been recognized in the Porcupine camp of northeastern Ontario (Fig. I), the relationship of the alteration to the structural geol- ogy and stratigraphy has not been well documented. At the Coniaurum mine, which includes the northeast extension of the Hollinger - McIntyre mineralized zone, gold-bearing quartz lodes, occurring mainly within zones of carbonate-altered rock, are well exposed at the surface.

During 1983, a grid was established and detailed geological mapping was carried out by the senior author on the Coniaurum property (Fig. 1) to determine the timing and structural- stratigraphic controls of the carbonate-altered zones. Petro- graphic studies were later carried out on approximately 100 Printed in Canada / Imprim6 au Canada

rock samples to determine the relationship of the carbonate alteration to penetrative structural fabrics. This paper is based on both the fieldwork and petrographic studies.

The Coniaurum mine produced 1 053 544 oz (477 179 kg) of gold and 186714 oz (84 368 kg) of silver between 1928 and 1962. It is now inactive.

Regional geology Stratigraphy

The regional geology of the Timmins area is described by Pyke (1982) and Ferguson et al. (1968) and has recently been reviewed by Hodgson (1983). Based on the structural and stratigraphic setting, four lithological assemblages are recog- nized; the Tisdale Group, the Deloro Group, the Older Sedi- ments, and the Younger Sediments (Hodgson 1983). The

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

PIROSHCO AND HODGSON 2029

Y Y Y * V V V I "

- v * * m e w - ~ w e - v v v ~ v ~

I.'.'] PORPHYRY INTRUSIONS YOUNGER SEDIMENTS

OLDER SEDIMENTS P-El F Porcupine-Destor fault

TISDALE GROUP f imiskaming unconformlty Calc-alkaline volcanics

1:::::I Tholeiitic volcanics

Kornatiitic volcanlcs - Fault DELORO GROUP . . . . . . . . . Unconformity

Calc-alkaline V O ~ C ~ ~ ~ C S

FIG. 1. Location and geology of the Porcupine camp.

interpreted structural relationships of these assemblages are shown in Figs. 1 and 2.

In this study, the traditional lithostratigraphic terminology of the sedimentary rocks described by Ferguson et al. (1968) is used (Figs. 1, 2) because of possible problems with strati- graphic correlation of Lorsong's (1975) four formations of the Porcupine Group, and the major problem, raised by Lorsong (1975) and Pyke (1982), of including units above and below the Timiskaming Unconformity in one lithostratigraphic group. The Older Sediments (Figs. 1 and 2) are equivalent to the Keewatin Series sediments (Ferguson et al. 1968), which are intercalated with and conformably overlie the Deloro and Tisdale Group volcanic rocks. The Younger Sediments are equivalent to the Timiskaming Series sediments (Ferguson et al. 1968), which unconformably overlie the Older Sedi- ments and volcanic rocks.

Intrusive into the Tisdale Group and sedimentary rocks, and showing a close spatial relationship to gold, are felsic quartz porphyry stocks, dikes, and irregular bodies. Most of these appear to have been emplaced into deformation zones that

transect, or are associated with, folds affecting all of the strati- form rocks in the area (Fig. 1).

Structural geology Folds of two ages can be recognized in the Tirnrnins area

based on interference patterns and the relationship of folds to the dominant foliation. The younger folds are asymmetric structures with axial surfaces parallel to the main east- to east- southeast-striking foliation (Hodgson 1983). The major fold structure in the area, the Porcupine syncline, has been inter- preted to predate the main foliation and the asymmetrical folds on the basis of the foliation transecting the axial surface of the syncline (Roberts 1981). However, the axial surface trace is not well defined owing to poor exposure.

Property geology Stratigraphy

The Coniaurum property is underlain predominantly by mafic flows of the Tisdale Group (Fig. 3) that strike northeast, dip steeply southeast, and face southeast. In the area of the

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

Hodgson (1983)

CAN. J. EARTH SCI. VOL. 25. 1988

GOLD CENTRE FORMATION

VI POND FORMATION

\

I F-I CENTRAL \ FORMATION

Pyke (1982)

Carter (1948)

Metasedimentary rocks M Unconformity

FIG. 2. Relationship of the Coniaurum mine stratigraphy, established 1 by Pyke (1982) and Hodgson (1983).

Coniaurum mine, the mafic flows have been subdivided by Carter (1948) into three formations termed, from oldest to youngest, the Central Formation, the Vipond Formation, and the Gold Centre Formation. Each of these formations was sub- divided into a series of flow units, as summarized by Ferguson et al. (1968). The distribution of the three formations and flow units on the Coniaurum property is shown in Fig. 3. The rela- tionship between Hodgson's (1983), Pyke's (1982), and Carter's (1948) stratigraphic subdivisions is shown in Fig. 2.

The Krist fragmental unit of the Tisdale Group, which con- sists entirely of felsic pyroclastic breccia, is exposed in the far southeastern portion of the property (Fig. 3) and has been described in detail by Pyke (1982).

Intrusive rocks Three types of intrusive rocks are exposed on the Coni-

aurum property: a quartz-feldspar porphyry stock (the Coni- aurum porphyry; subunit 6a, Fig. 3), heterolithic breccia dikes (subunit 6b, Fig. 3), and diabase dikes (unit 7, Fig. 3). The relationship of the intrusive units to the volcanic and sedi- mentary rocks is shown in Fig. 3.

Quartz - feldspar porphyry The Coniaurum porphyry is a stock of quartz -feldspar por-

phyry plunging 60" east and intruding the lower massive to pillowed flow units (C14 and C15 flows, Fig. 3). It consists of quartz or rare albite porphyroclasts in an intensely foliated sericite- and quartz-rich groundmass. Contacts with adjacent wall rock are sharp to gradational and highly irregular, but clearly crosscut the host volcanic rocks. The porphyry merges at depth with the main mass of the Pearl Lake porphyry, which crops out on the McIntyre property to the west (Ferguson et al. 1968).

Heterolithic breccia dikes Heterolithic breccia dikes are exposed at two locations on

the property. At one location, immediately south of the Coniaurum porphyry (Fig. 3), a 25 cm wide dike is exposed in one outcrop. It strikes parallel to nearby volcanic units

by Carter (1948), with the regional stratigraphic framework established

(065") but dips northwest (60°), whereas the volcanic rocks dip southeast. The rock consists of 10- 15 % subangular mafic volcanic fragments up to 5 cm in diameter, 5- 10% sub- rounded to subangular, hematitic quartz-feldspar porphyry fragments up to 5 cm in diameter, and rare chert and green mica-rich fragments up to 1 cm in diameter (Fig. 4a), all in a weakly to moderately foliated chlorite + sericite + quartz + albite-bearing matrix.

The second breccia dike, located near the power line (Fig. 3), is exposed for a strike length of 200 m, strikes normal to volcanic units (300') and dips vertically. Contacts with the adjacent country rock are sharp but irregular, with local apo- physes extending outwards from the main dike for distances of at least 10 m.

This dike is similar in composition to the dike exposed to the north except the porphyry fragments are not hematite bearing (Fig. 4b). These fragments are grey and subrounded to sub- angular (Fig. 4c).

In addition to the two breccia dikes described above, a nar- row (1 -3 cm wide), northwest-stirking breccia dikelet cross- cuts the C 15 flow in the vicinity of the no. 12 vein (Fig. 4d). This dikelet does not contain porphyry or mafic volcanic frag- ments but consists of foliated chlorite, quartz, and calcite.

Diabase dikes North-striking Matachewan diabase dikes (Pyke 1982) up to

15 m wide occur on the property and characteristically display plagioclase glomeroporphyritic textures.

Structural geology The rocks on the Coniaurum property, excluding the diabase

dikes, contain weak to intensely developed foliation and a well-developed east-plunging (40 - 55 O) stretching lineation (Fig. 3). The predominant foliation and lineation are parallel with the axial surface of a major fold structure on the property, the Coniaurum anticline. The Coniaurum anticline is inter- preted as an open, upright, S-shaped structure with a subsidi- ary fold present on the south limb (Figs, 3, 5).

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

m~

ua

rt

z-

~e

c

arb

on

ate

- s

erl

cit

e s

ch

ist

m~

ua

rt

z-

~e

c

arb

on

ate

-se

ric

ite

altera

tion

INT

RU

SIV

E R

OC

KS

7

Dia

ba

se

dik

e

6

a) q

ua

rtz-

feld

spa

r p

orp

hyr

y (q

ua

rtz-

seri

cite

a

chls

t)

b)

he

tero

llth

ic b

recc

ia d

ikes

FE

LS

IC V

OLC

AN

IC

RO

CK

S

5

Krl

st

Fra

gm

en

tal

Un

it a)

fe

lsic

he

tero

lith

ic p

yro

cla

sti

c b

rec

cia

S

ED

IME

NT

AR

Y R

OC

KS

4

a)

inte

rflo

w t

uff

ac

eo

ua

sedim

ent

MA

FlC

VO

LC

AN

IC R

OC

KS

3

Go

ld C

en

tre

Fo

tma

tlo

n

a)

nm

ygd

alo

ida

l b

) p

illo

we

d (

un

diffe

ren

tia

ted

) C

) h

ya

locla

atl

te

2 V

lpo

nd

Fo

rma

tio

n

e)

va

rio

litic

b

rec

cia

ted

(V

10

f)

hya

locla

sti

te

\

Ge

olo

gic

al

co

nta

ct

1

Ce

ntr

al

Fo

rma

tio

n

Pill

ow

s

a)

ma

ssiv

e (

C1

4)

b)

am

ygdald

dal

pill

ow

ed

(C

15

) F

au

lt

C)

hya

locl

ast

ite

-45

Conia

uru

m A

nti

clin

e

0

100

20

0

30

0

me

tre

s

FIG

. 3.

Surf

ace

geol

ogy

of t

he C

onia

urum

min

e.

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

2032 CAN. J. EARTH SCI. VOL. 25, 1988

FIG. 4. (a) Sample of the breccia dike exposed immediately south of the Coniaurum porphyry showing mafic volcanic fragments (dark grey) and lesser hematitic porphyry fragments (light grey) within a quartz + chlorite + calcite matrix. (b) Sample of the breccia dike exposed along the power line (Fig. 3) showing roughly equal proportions of chlorite + calcite-rich mafic fragments and quartz porphyry fragments (light grey). (c) Sample of a porphyry fragment from the breccia dike exposed along the power line (Fig. 3), showing relatively good preservation of porphyritic textures. (d) Outcrop of a breccia dikelet crosscutting the C15 flow in the vicinity of the No. 12 vein.

Foliation is most intensely developed within several north- east-striking zones (25-100 m wide) on the Coniaurum property that are coincident with zones of rusty weathered car- bonate alteration and quartz veining. The two most extensive schistose zones, shown in Fig. 3, are planar, generally strata- bound structures. One corresponds to the upper part of unit l (the C14-C15 flow units) and the lower part of unit 2 (the 99 flow unit), and the other corresponds to the top of unit 2 (the VlOA-V1OB flow units). Several other narrow (less than 25 m wide), discontinuous zones occur within individual flow units or along flow contacts.

The two main schistose zones have the geometric charac- teristics of left-handed shear zones (Ramsay 1980). Within them, an east-striking foliation is inclined to the zone bound- aries, but is parallel to the axial surface of the Coniaurum anticline. A second, northeast-striking foliation, which is restricted to these zones, is parallel with the zones and is inter- preted as a shear-related foliation or C-fabric. The crosscut- ting relationship between the northeast- and east-striking foliations, however, could not be established. Outside of the shear zones, the east-striking foliation predominates (Fig. 3) and may represent either a flattening fabric (or S-fabric) related to the deformation that formed the shear zones or could

represent an earlier deformation event (Dl) unrelated to the shear zones (Dz). A detailed study of foliation relationships in the Timmins area is needed to resolve this problem.

The shears are limb thrusts, in relation to the Coniaurum anticline, as they are stratabound on the fold limb, but crosscut the stratigraphy in the hinge area (Fig. 3).

In addition to the shear zones, a northwest-striking fault is interpreted to occur on the property (Fig. 3) on the basis of the mapped offset of lithological units. This fault is parallel to local, narrow domains of northwest-striking foliation on the property. The relationship of this foliation to the foliations described above could not be determined. The latest structural features observed are locally developed, north-northwest- striking kink folds that deform the main east-striking foliation.

Quartz veins and mineralization Ore zones at the Coniaurum mine are auriferous quartz +

ankerite + sericite + tourmaline + pyrite vein systems. A vein system consists of one or more parallel to subparallel veins and associated auriferous pyritic wall rock alteration haloes. The auriferous veins mapped on surface (no. 9, 12, and 40 veins, Fig. 3) occur within the two shear zones described above. Cross sections of underground workings

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

PIROSHCO AND HODGSON 2033

FIG. 5. North-south cross section through the Coniaurum mine near the western property boundary showing the general relationships of quartz vein systems (stopes) to structure, lithologic units, and quartz-feldspar porphyry intrusions. Modified after Ferguson et al. (1968). The location of the cross section is shown on Map 2075 of Ferguson et al. (1968).

b

N 2a I S 4

-, Coniaurum Anticline

I

/-

----- \

near the western boundary of the Coniaurum mine show that most economic veins on the property occur within a northeast- trending subvertical zone that successively cuts the massive 99 flow, the C14 and C15 flows, and the Pear1 Lake porphyry with depth (Fig. 5). In cross section, the individual veins show clear discordant relationships with lithological units. Other veins of economic importance are in narrow, subparallel shear zones within and adjacent ot the Coniaurum porphyry and the C14 and C15 flows, within the VlOA and VlOB flow units, and along a number of flow contacts. Graphitic interflow sedi- ments, which are important host rocks for gold mineralization at the adjacent McIntyre mine (Griffis 1962), host significant gold mineralization on the crest of the Coniaurum anticline in units overlying and underlying the 99 flow. Auriferous quartz veins in the V8 flow are rare and when present, are narrow and discontinuous.

At least four crosscutting vein types are recognized at the Coniaurum mine. Vein emplacement and deformation over- lapped in time since the earliest formed veins are the most

1

intensely deformed, whereas the latest veins are the least deformed. The four vein types, in order of their time of emplace- ment, are (1) nonauriferous ankerite veins, (2) quartz + ankerite + tourmaline + sericite + pyrite veins containing native gold and with auriferous pyrite alteration haloes, (3) quartz + ankerite + pyrite ladder veins (Jones 1948) containing native gold, and (4) nonauriferous, late-stage, quartz + calcite veins. The second type are dominant in the vein systems that were mined.

The quartz veins typically strike east, oblique to the shear zones, or northeast, parallel to the shear zones, and dip sub- vertically to moderately north. Several veins dip south, parallel or subparallel to the flow contacts. The vein systems plunge 45 - 55" east.

Quartz porphyry -- Fault

-cii Inferred shear zone 2a Id Flow units

Quartz vein (stope) -- X Anticline 'I- Geological contact Syncline

Hydrothermal alteration

A detailed study of the carbonate mineralogy within car- bonate alteration zones in the Porcupine camp has not been

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

2034 CAN. I. EARTH SCI. VOL. 25, 1988

pyrite ~- F I G . ~ . Volume percent of mineral phase verses distance from

mineralized quartz + ankerite + tourmaline + sericite vein system.

undertaken. Burrows (191 1) has described rusty weathered carbonate from quartz veins and other carbonate-bearing rock in the Porcupine camp as ankerite (Mg:Fe < 4: 1) based on the chemical analysis of seven carbonate separates. Microprobe analysis of carbonate from the Dome mine by Fryer et al. (1979) shows that the predominant carbonate from mineral- ized structures is ferroan dolomite (Mg:Fe > 4: 1) and that from highly altered carbonate-rich rock is ferroan dolomite and iron-rich magnesite. A quantitative study of the carbonate at the Coniaurum mine was beyond the scope of this study, and the rusty weathered carbonate is referred to as ankerite so as to be consistent with the regional study of Burrows (1911).

The Early Precambrian volcanic and sedimentary rocks in the Timmins area have undergone regional metamorphism to the lower and middle greenschist facies (Pyke 1982). In this study, rocks that have undergone greenschist facies meta- morphism and have not been affected by hydrothermal altera- tion (i.e., carbonate alteration) are referred to as the least altered facies.

Three main alteration facies have been identified at the Por- cupine camp (Fyon and Crocket 1983), the Mclntyre mine (Smith and Kesler 1985; Burrows and Spooner 1986), and the Coniaurum property. They are (1) a least altered facies, con- sisting of the regional greenschist metamorphic assemblage; (2) a chlorite facies, hosting barren and locally auriferous quartz -carbonate veins and forming the outer alteration zone; and (3) an ankerite facies, hosting auriferous vein systems and forming the inner alteration zone (Fig. 6). In addition, a fourth alteration facies is recognized in this study, occurring as envelopes less than 2 m wide around quartz + carbonate veins. This is termed the "vein envelope facies" and has been described at the Hollinger mine by Jones (1948) and at the McIntyre and Hollinger mines by Smith and Kesler (1985).

In outcrop, the boundary between the chlorite and ankerite alteration facies is marked by a change in colour in the

weathered rock from green to rusty brown, and in fresh rock from green to bleached in appearance. The boundary between the least altered and chlorite alteration zones does not occur in the property but has been mapped at a reconnaissance scale in the Hollinger - McIntyre - Coniaurum complex by Smith and Kesler (1985). The matrix and fragments of heterolithic brec- cia dikes are altered to the alteration assemblage of the host volcanics.

Least altered facies Distribution and morphology An isolated outcrop of least altered rock was observed in the

99 flow. The extent of the least altered zone could not be determined.

Petrography The least altered rock consists of 65 % subhedral actinolite

grains up to 0.5 mm in length, 12% subhedral and anhedral albite grains up to 0.3 mm in size, 8% anhedral clinozoisite grains up to 0.3 mm in size, 5 % chlorite, 5 % anhedral quartz grains up to 0.2 mm in size, and 5% interstitial calcite.

Chlorite alteration facies Distribution and morphology Rocks on the property are predominantly altered to the

chlorite alteration facies. The width of the zone on the Coni- aurum property, inferred from mapping, is greater than 100 m, but on the southern and southeastern sides of the property the zone probably extends 500 m outwards from the mineralization to the contact of the mafic volcanic sequence with the Krist fragmental unit.

Petrography A feature common to all rocks in the chlorite alteration

facies that have not been subject to penetrative deformation is the preservation of the original textures of the least altered rock. The first mineralogical change observed in the rocks on the margins of the chlorite facies is the replacement of the fringes, and of zones parallel to cleavage planes at the cores of amphibole, by chlorite. Within the zone of chlorite altera- tion facies, amphibole is completely replaced by chlorite and clinozoisite, and albite is partially to completely replaced by calcite + quartz + chlorite. Chlorite, also present as amyg- dules with quartz and calcite, forms up to 70% of the rock, whereas calcite seldom forms greater than 20%. Quartz typi- cally forms up to 5 % . The disappearance of clinozoisite coin- cides with the first appearance of a potassic phase, sericite, within the chlorite facies, but sericite rarely exceeds 5 % . Seri- cite, along with quartz and calcite, commonly replaces albite or exists as scattered, discrete subhedra up to 0.5 mm in diameter within the chlorite- and calcite-bearing groundmass.

Deformation superimposed on the chlorite-calcite assem- blage results in a chlorite schist with an intense foliation defined by aligned chlorite, parallel seams of opaque residue, and elongate patches of calcite.

Ankerite alteration facies Distribution and morphology The ankerite alteration facies is best developed within two

distinct stratigraphic zones coextensive with the two defor- mation zones described above in the section on structural geology.

In cases where the zones are discordant to stratigraphic units or are developed as narrow zones within a relatively thick flow

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

PIROSHCO AND HODGSON 2035

boudinaged rotated

ankerite v e

ankerite- quartz- sericite

FIG. 7. (a) Sample showing an ankerite vein from the ankerite alteration facies, boudinaged and rotated by an intensely developed foliation defined by aligned sericite + quartz + ankerite. (b) Sketch of sample (Fig. 7a) showing mineralogy. (c) Photomicrograph showing an ankerite vein crosscut by a foliated altered zone of ankerite + sericite + quartz, overgrown by a postkinematic pyrite porphyroblast (crossed polars; scale bar = 1 mm). (d) Photomicrograph showing euhedral ankerite porphyroblasts containing inclusion trails of quartz + sericite. Kink bands in the cleavage are defined by the domains of reoriented sericite and the rotation of the ankerite porphyroblasts (crossed polars; scale bar = 0.2 rnm). (e) Photomicrograph showing fibrous calcite overgrowing a postkinematic pyrite porphyroblast (crossed polars; scale bar = 0.2 mm).

unit, or along flow contacts, the contact between the chlorite Coniaurum anticline, the trend of the contact between the and ankerite alteration facies is commonly gradational over ankerite and chlorite facies is northeast, discordant to the distances of up to 1 m. In cases where the zones are per- stratigraphy and the trace of the axial plane of the Coniaurum vasively developed in a particular flow unit, the contact occurs anticline. at the rock unit contact and is sharp. Near the crest area of the Narrow ankerite veinlets and auriferous quartz veins and

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

2036 CAN. J . EARTH SCI. VOL. 25, 1988

veinlets are ubiquitous to the zones of ankerite facies. The quartz veins, where exposed on surface (no. 9, 12, and 40 veins, Fig. 3), are typically less than 50 cm wide and less than 50 m in strike length.

Deformation superimposed on the zones of ankerite altera- tion facies produces an ankerite + sericite + quartz schist with an intensely developed foliation defined by aligned seri- cite and quartz. The ankerite veinlets are typically boudinaged and rotated (Figs. 7a-7c) in the schistose domains, and the necks of the boudins are commonly crosscut by quartz ladder veins with associated vein envelope facies alteration. Breccia dikes crosscutting the schistose domains of the ankerite altera- tion facies show extreme elongation of rock fragments, have an intense planar foliation, and are typically altered to an ankerite + sericite + quartz assemblage.

Petrography The colour change observed at the boundary between the

chlorite and ankerite alteration facies corresponds to the for- mation of ankerite, the disappearance of calcite, the progres- sive destruction of chlorite, and increases in the amounts of sericite and quartz. Calcite coexisting with ankerite or replace- ment textures between calcite and ankerite are rarely observed. However, coexisting ankerite and calcite were observed in one thin section, and in several cases calcite was observed as inclusions in ankerite.

The texture of the least altered rocks is invariably obliter- ated in the ankerite alteration facies. Mesoscopic, primary vol- canic structures (pillows, amygdules, varioles), however, are preserved in the altered zones.

Rocks of the ankerite alteration facies consists of either a medium-grained, crystalline mosaic of ankerite grains (up to 50%) intermixed with quartz and sericite, or scattered sub- hedra of ankerite (up to 50%) up to 1 mm in size, within a finely crystalline sericite- and quartz-rich groundmass. In thin section, ankerite is recognized by its relatively high relief, brown oxidation stain and deep blue colour when stained by potassium ferricyanide solution. Less than 50% sericite and quartz may occur as felted to foliated mats, as discrete grains up to 0.3 mm in size intermixed with ankerite, or as amyg- dules. Within intensely schistose domains, ankerite por- phyroblasts typically contain inclusion trials of sericite and quartz (Fig. 7d).

Leucoxene is stable in both the chlorite and ankerite altera- tion facies and occurs as local discrete, irregular to skeletal grains within the rock groundmass.

Vein envelope facies Distribution and morphology A relatively intense alteration facies is common in bleached

haloes adjacent to mineralized quartz + ankerite + sericite + tourmaline veins at the Coniaurum mine. The rocks in this facies are characterized by varying amounts of quartz, seri- cite, ankerite (or calcite), and pyrite. Such veins and alteration typically occur within the ankerite alteration facies but in rare occasions are developed in the zones of chlorite alteration (Jones 1948). Within the chlorite alteration facies the veins contain calcite rather than ankerite and are only weakly auri- ferous.

Quartz veins bordered by the vein envelope facies, which crosscut schistose domains, are typically boudinaged and (or) rotated.

Petrography Where penetrative deformation has not affected the altered

rocks, up to 50% quartz occurs as very finely crystalline aggregates. Scattered anhedral grains and grain aggregates of ankerite and sericite (0.1 mm in size) occur intermixed with the quartz. Within the finely crystalline quartz, coarser grained patches (up to 0.3 mm in diameter) of quartz are observed. These exhibit sutured grain boundaries, mortar tex- ture, and undulatory extinction. Ankerite occurs as scattered anhedral porphyroblasts within a finely crystalline ground- mass of sericite and quartz and was also commonly observed within foliated zones up to 2 mm in width or in veins that crosscut Fe-rich ankerite veins and subhedra (Fig. 7c). In I thin section, the ankerite of the vein envelope facies is dis- tinguished from the ankerite in the broader zones of ankerite alteration facies by its lower relief, its lack of an oxidation stain, and its lighter blue colour when stained by a potassium ferricyanide solution. Although not a quantitative method for determining Fe content in ankerite, the intensity of the blue stain obtained by using the above solution is proportional to the Fe content of ankerite, suggesting that the ankerite of the vein envelope facies has a higher Mg/Fe ratio than that in the

I ankerite alteration facies. The low relief and lack of oxidation stain in the ankerite of the envelope facies are also consistent with low Fe content.

Sericite increases in amount within the vein envelope facies (15 -20%), relative to the ankerite facies, and occurs pre- dominantly in intensely foliated, discontinuous domains up to 2 mm in width, which typically crosscut the early ankerite veins (Figs. 7a, 7b). Sericite also occurs as inclusions in ankerite or pyrite.

Chlorite decreases markedly in volume percent in the enve- lope alteration facies, relative to the ankerite and chlorite alter- ation facies, and is commonly totally replaced by sericite and quartz in a zone approximately 0.5 -2 m wide around miner- alized vein systems. The loss of chlorite adjacent to vein mar- gins is responsible for the intensely bleached appearance of the vein envelope alteration facies.

Pyrite in the envelope facies is concentrated within and adja- cent to ( < 2 m) the mineralized quartz + ankerite veins, although, locally, disseminated pyrite unrelated to veins is present. Within zones lacking a strong penetrative foliation, sericite, quartz, and ankerite of the ankerite alteration facies commonly occur as minute inclusions within the pyrite sub- hedra. Within foliated zones, pyrite occurs as both pre- and syn-kinematic subhedra, elongate parallel to the foliation, and as postkinematic subhedra, overgrowing the foliation (Fig. 7c). Pyrite subhedra also locally have fibrous overgrowths and fracture fillings of calcite (Fig. 7e).

Summary of geological relationships The following are the main features established in this study

of the geology of the Coniaurum property. These features must be explained by any model of the geological development and the gold mineralization of the area.

(1) Heterolithic breccia dikes strike normal and parallel (but discordant along dip) to the volcanic units.

(2) The dominant foliation strikes east, dips steeply, is axial planar to the Coniaurum anticline, and is superimposed on all of the rocks, including the heterolithic breccia dikes, except the late diabase dikes.

(3) Shear zones, characterized by intensely developed northeast-striking foliation and rusty weathered carbonate alteration, are up to 50 m wide and are generally stratabound,

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

PIROSHCO P rND HODGSON 2037

but crosscut the axial trace of the Coniaurum anticline. Their geometry and kinematic sense are consistent with their development as limb thrusts in relation to the Coniaurum anticline.

(4) The dominant quartz vein system strikes 065-075", parallel to the strike of the stratigraphic units on the south limb of the Coniaurum anticline, and the shear zone boundaries, but dips steeply north, discordant to the south dip of the volcanic units and the Coniaurum porphyry. Individual quartz veins strike predominantly east and northeast.

(5) Early quartz and ankerite veins are highly deformed and late quartz ladder veins are relatively undeformed.

(6) A least altered facies and three mineralogically distinct alteration facies are identified. Three of these are widespread in their distribution, and were previously recognized by Fyon and Crocket (1983). They are (a) a least altered facies, consist- ing of the greenschist metamorphic mineral assemblage of actinolite, albite, clinozoisite, chlorite, calcite, and quartz that does not host quartz + carbonate veins; (b) a chlorite facies, characterized by the assemblage of chlorite + calcite + albite + quartz, hosting both barren and mineralized quartz + carbonate veins with associated vein envelope alteration; (c) an ankerite facies, characterized by the assemblage ankerite + sericite + chlorite + quartz and hosting most, but not all, of the mineralized quartz + carbonate vein systems; and (d) a vein envelope facies, characterized by the assem- blage ankerite, sericite, and pyrite but without chlorite, which occurs as envelopes to quartz-carbonate veins within the ankerite facies and to a lesser extent within the chlorite facies.

(7) The ankerite alteration facies occurs as (a) discontinuous zones along and in part coextensive with the shear zones and (b) discontinuous zones along flow contacts, which locally crosscut the flow contacts.

(8) At the boundary of the least altered facies and chlorite alteration facies, amphibole, clinozoisite, and albite are pro- gressively destroyed in that order, coincident with increases in chlorite and, to a lesser extent, calcite and quartz.

(9) At the boundary of the chlorite and ankerite alteration facies, calcite and chlorite are progressively destroyed coinci- dent with increases in ankerite, sericite, and quartz.

(10) The ankerite and chlorite alteration facies are superim- posed on the heterolithic breccia dikes.

(11) At the outer contact of the vein envelope subfacies, ankerite of the ankerite alteration facies is progressively replaced by a later, possibly more magnesian ankerite, chlorite is destroyed, and the amount of sericite, quartz, and pyrite increases.

(12) Within the vein envelope facies, pyrite subhedra com- monly have fibrous overgrowths and fracture fillings of cal- cite, quartz, and chlorite.

Discussion and interpretations

Structural interpretation Although the orientation of the fabric elements, shear zones,

and diabase dikes are consistent with a conjugate ductile shear zone model (Rarnsay 1980; Fig. 8), there is little kinematic evidence to show that rotation (i.e., faulting) has occurred within the shear zones. Stretching lineations, which are well developed within relatively undeformed lithologies on the Coniaurum property, are difficult to recognize within the shear zones because of the intense overprinting of deformation and carbonate alteration. Rotation or reorientation of linear

fabrics within the shear zones, therefore, could not be resolved. The orientation of the quartz veins within the shear zones

(Fig. 3) is, however, consistent with shear fractures formed within a Riedel shear system (P and D shear fractures of Reidel 1929) and suggests a rotational component to the shear zones. Detailed work documenting the relationship between the quartz veins and planar fabrics on a regional scale is needed to verify this interpretation.

The crosscutting relationship between the northeast- and east-trending planar fabrics is unresolved, but the domainal nature of the northeast fabric within northeast-trending zones suggests that it is relatively late. In the case of the shear zone model stated above, the east- and northeast-striking fabrics correspond to flattening and shear-related fabrics, respectively (Fig. 8), formed during progressive deformation. Alterna- tively, the east- and northeast-striking fabrics could have formed during separate periods of deformation (i.e., Dl and D2).

The northwest-striking, vertically dipping and northeast- striking, northwest-dipping breccia dikes are interpreted to be related to the intrusion of the Coniaurum porphyry.

Ankerite and chlorite alteration facies and penetrative defor- mation

Northeast-striking, north-dipping shear zones were the loci for the development of the zones of ankerite alteration facies, which are flanked by zones of chlorite alteration facies. This spatial relationship of alteration and shear zones indicates that the alteration was controlled by the shear zones, or precursor fracture zones, which were coextensive with the shears and clearly crosscut the axial plane of the Coniaurum anticline. The planar fabrics and the alteration also clearly crosscut the heterolithic breccia dikes and therefore must postdate them as well (Fig. 9).

The mineral assemblages of the various alteration facies can all be considered as greenschist mineral assemblages (Turner 1981), but they are indicative of different Xco, in the attend- ing fluid phase (Clark et al. 1986). The assemblage of the least altered facies is indicative of the lowest Xco2, the assemblage of the chlorite facies and the ankerite alteration facies are indicative of successively higher Xco, Therefore, the zonal distribution of these alteration facies is interpreted as a result of lateral gradients in Xco,

The replacement relationships at the boundaries of the vari- ous alteration facies are consistent with their being formed synchronously, with the Xco, fluid regimes of both the ankerite and chlorite alteration facies growing outwards as alteration proceeded.

Penetrative deformation and quartz veins with alteration haloes

Since some quartz veins are deformed (boudinaged and folded) and plunge 45" east, whereas others are undeformed, the quartz veining was partly synchronous with, and partly postdated, the penetrative deformation.

Figure 10 summarizes the progressive sequence of vein and alteration development within the northern shear zone at the Coniaurum mine, based on petrographic and field observa- tions. The textures show that the mineralized quartz -ankerite veins and vein envelope alteration facies are superimposed on the ankerite alteration facies and, locally, the chlorite altera- tion facies. Inclusions of alteration facies minerals is pyrite, and the crosscutting relationship between the relatively Fe-poor

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

2038 CAN. 1. EARTH SCI. VOL. 25, 1988

1

minor cleavage

T diabase dikes

I r , T o - . I

- -

FIG. 8. Structural interpretation of the Coniaurum mine property based on a conjugate ductile shear zone model (Ramsay 1980).

ankerite of the vein envelope alteration facies and the Fe-rich ankerite of the ankerite alteration facies, are particularly com- pelling evidence for the superposition relationship.

The carbonate mineralogy of the vein envelope and ankerite facies is consistent with their being derived from the same fluid source. However, the relative abundance of sericite and pyrite in the vein envelope facies indicates it possibly devel- oped in a chemical environment relatively rich in K + , H f , and S- .

The presence of calcite filling fractures in postkinematic pyrite euhedra, quartz - chlorite - calcite pressure shadows on pyrite, and late quartz - calcite veins within the alteration facies suggests that late in the history of alteration, after most of the penetrative deformation, the hydrothermal system col- lapsed inwards. The collapse caused a superposition of the low-Xco2 facies on previously formed high-Xco, facies. However, it is not clear whether the superimposed low-Xco, facies represents fluids evolved from the fluids that formed the

Coniaururn Porphyry

Heterolithic Breccia Coniaurum Anticline

Penetrative Deformation Ankerite Facies

Chlorite Facies

Vein Envelope Facies

Quartz Veins

Buckle Folds

FIG. 9. Relative timing of geological events observed at the Coniaurum mine property.

placements of less than 30 m, and the formation of the small- scale buckle folds within the shear zones postdated the bulk of the penetrative strain and most of the mineralization and alter- ation in the system.

ankeriteand vein envelope facies alteration or the facies was derived from a different fluid source. Diabase dikes

The north-striking Matachewan diabase dikes clearly cross- - Faults cut the Coniaurum porphyry, carbonate alteration zones, and

The development of a complex system of faults, with dis- penetrative fabrics.

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

PIROSHCO AND HODGSON

LEGEND

Ankerite vein

Ankerite-sericite-quartz- chlorite alteration

Quartz-ankerite fl (PIUS accessories, vein

Quartz-ankerite-sericite alteration

I Pyrite

Foliation - Quartz-pyrite vein

4 - cren,ulation cleavage and rotation of earlier formed minerals

5 - quartz-calcite veins

FIG. 10. Paragenetic sequence showing the development of planar fabric, veins, and alteration minerals within the ankerite alteration facies based on petrographic and field relationships.

Conclusions The following are the main conclusions from this study: (1) At the Coniaurum mine, the shear zones, the main

schistosity, and the diabase dikes correspond in their orienta- tion to a ductile shear zone model with a maximum bulk strain axis (direction normal to the plane of flattening) oriented north-south. However, since the regional relationships between the two sets of planar fabrics and linear fabrics observed on the property are unresolved, an alternative inter- pretation is that the northeast- and predominant east-trending fabrics represent two deformation events.

(2) The shear zones, quartz veins, and alteration show a clear discordancy with lithologies at the crest of the Coni- aurum anticline.

(3) When the shear zones are oriented parallel to the strike of lithological units, the lithologies play an important role in controlling the distribution of alteration by providing aniso- trophies along which shearing and alteration are localized.

(4) The ankerite and chlorite alteration facies were devel- oped synchronously along shear zones.

(5) Quartz veins and the vein envelope alteration facies are superimposed on the ankerite and chlorite alteration facies.

(6) Chlorite from the chlorite alteration facies postdates the actinolite from the least altered facies.

(7) The concentration of mineralization in graphitic inter- flow sediments at the crest of the Coniaurum anticline, and the relationship of quartz veins to the anticline, suggests that mineralization was syn- to post-development of the anticline.

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.

2040 CAN. 1. EARTH SCI. VOL. 25, 1988

(8) The zonation of ankerite and chlorite alteration facies is the result of lateral gradients in Xco,, rather than gradients in temperature.

(9) The similar mineralogy of the vein envelope and ankerite facies indicates they may have evolved from a single fluid source, but the difference in relative abundance of minerals (sericite-pyrite) in the facies suggests they formed in different chemical environments.

(10) The superimposed low-Xo2 facies (quartz -chlorite- calcite) represents possibly a second fluid source, which became predominant when the C02-r ich hydrothermal system (quartz - ankerite - sericite) collapsed.

Acknowledgments

This study was funded and supported by the Ontario Geo- logical Survey and Natural Sciences and Engineering Research Council of Canada operating grant t o C . J. Hodgson. W e would like to thank Phil Walford, formerly of Pamour Porcu- pine Mines Ltd., and John Kirwan for their support, encour- agement, and hospitality during the fieldwork for this project. The paper benefited greatly from criticisms by Dr. J. H. Crocket, Dr. J. A. Fyon, and a n anonymous reviewer. This paper is published by permission of the Ontario Geological Survey.

BURROWS, A. G. 191 1. The Porcupine gold area. Ontario Bureau of Mines, Geological Report 20, Part 2.

BURROWS, D. R., and SPOONER, E. T. C. 1986. The McIntyre Cu-Au deposit, Timmins, Ontario, Canada. In Proceedings of Gold '86, an International Symposium on the Geology of Gold, Toronto, pp. 23 - 39.

CARTER, 0. F. 1948. Coniaurum mine. In Structural geology of Cana- dian ore deposits. Canadian Institute of Mining and Metallurgy, Montreal, Que., Vol. 1, pp. 497-503.

CLARK, M. E., ARCHIBALD, N. J., and HODGSON, C. J. 1986. Struc- tural and metamorphic setting of the Victory gold mine, Kambalda, western Australia. In proceedings of Gold '86, an International symposium on the Geology of Gold, Toronto, pp. 40-55.

FERGUSON, S. A,, BUFFAM, B. S. W., CARTER, 0. F., GRIFFIS, A. T.,

HOLMES, T. C., HURST, M. E., JONES, W. A., LANE, H. C., and LONGLEY, C. S. 1968. Geology and ore deposits of Tisdale Town- ship, District of Cochrane. Ontario Department of Mines, Geologi- cal Report 58.

FRYER, B. J., KERRICH, R., HUTCHINSON, R. W., PEIRCE, M. G., and ROGERS, D. S. 1979. Archaean precious-metal hydrothermal sys- tems, Dome Mine, Abitibi Greenstone Belt. I. Patterns of alteration and metal distribution. Canadian Journal of Earth Sciences, 16: 421 -439.

FYON, J. A,, and CROCKET, J. H. 1983. Gold exploration in the Tim- mins area using field and lithogeochemical characteristics of car- bonate alteration zones. Ontario Geological Survey, Study 26.

GRIFFIS, A. T. 1962. A geological study of the McIntyre mine. Transactions-the Canadian Institute of Mining and Metallurgy and the Mining Society of Nova Scotia, 65: 47-54.

HODGSON, C. J. 1983. The structure and geological development of the Porcupine camp-a re-evaluation. Ontario Geological Survey, Miscellaneous Paper 1 10, pp. 2 1 1 -225.

JONES, W. A. 1948. Hollinger mine. In Structural geology of Cana- dian ore deposits. Canadian Institute of Mining and Metallurgy, Montrkal, Que., Vol. 1, pp. 464-481.

LORSONG, J. 1975. Stratigraphy and sedimentology of the Porcupine Group (Early Precambrian), northern Ontario. B.Sc. thesis, Uni- versity of Toronto, Toronto, Ont.

PYKE, D. R. 1982. Geology of the Timmins area, District of Cochrane. Ontario Geological Survey, Report 219.

RAMSAY, J. G. 1980. Shear zone geometry: a review. Journal of Struc- tural Geology, 2: 83 -89.

REIDEL, W. 1929. Zur Mechanik geologoischer Brucherscheinungen. Zentralblatt fiir Mineralogie, Geologie und Palaontologie, Sect. B, 1929B: 354-368.

ROBERTS, R. G. 1981. The volcanic-tectonic setting of gold deposits in the Timmins area. In Genesis of Archean, volcanic-hosted gold deposits. Ontario Geological Survey, Miscellaneous Paper 97, pp. 16-28.

SMITH, T. J., and KESLER, S. E. 1985. Relation of fluid inclusion geochemistry to wall rock alteration and lithogeochemical zonation at the Hollinger-McIntyre gold deposit, Timmins, Ontario. CIM Bulletin, 78: 35-46.

TURNER, F. J. 1981. Metamorphic petrology -mineralogy, field and tectonic aspects. McGraw-Hill, New York, NY.

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IV N

CC

HA

PEL

HIL

L o

n 11

/10/

14Fo

r pe

rson

al u

se o

nly.