Vein geometry in the Hilton area, Mount Isa, Queensland: implications for fluid behaviour during deformation

158 (1989) 191-207

Tectonophysics,

191

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Vein geometry in the Hilton area, Mount Isa, Queensland: implications for fluid behaviour during deformation R.K. VALENTA Monash University, Department

of Earth Sciences. Clayton, Vie. 3168 (Australia)

(Received May 5, 1987; accepted August 3, 1987)

Abstract Valenta, R.K., 1989. Vein geometry in the Hilton area, Mount Isa, Queensland: implications for fluid behaviour during deformation.

In: A. Ord (Editor), Deformation

of Crustal Rocks. Tectonophysics,

158:191-207.

Folding and shear zone movement occurred synchronously in rocks of the Mid-Proterozoic Hilton area, Northwest Queensland. Syndeformational with shear zone intersections. (1)Fibrous

Mount Isa Group in the

veins developed in shear zones and in sets spatially associated

Veins can be divided into the following two types:

and non-fibrous quartz-dolomite

veins which formed in orientations consistent with the regional E-W

subhorizontal shortening direction. (2)Fibrous and non-fibrous quartz-dolomite indicating local syndeformational

veins which formed parallel to shear planes, S, cleavage, and bedding,

extension parallel

to the regional shortening direction.

The complex geometry of the shear zones suggests that shear parallel veins formed as a result of movement over asperities during shear zone movement. &-parallel

veins, on the other hand, appear to have formed during restricted

episodes of fluid overpressuring during post-failure stress drops. Shear zones are divisible into highly graphitic and weakly graphitic end member types. Highly graphitic shear zones show abundant dissolution

and little veining, while weakly graphitic shear zones show abundant veining and little

dissolution.

that highly graphitic

This indicates

graphitic shear zones were syndeformational

shear zones were syndeformational

Introduction

1983). The style and geometry during

The interaction

between

veining and deforma-

tion has been a subject of interest (Hancock, Beach,

1972;

1975,

Durney

1977;

and

Ramsay,

for many years Ramsay,

1980).

inconsistent

with regional

inconsistencies

changes

in stress

during

deformation

Swager, during

1983) shear

are

and

usually

the

asso-

geometry,

(Kerrich

0040-1951/89/$03.50

movement

(e.g.

by

conditions

and Allison,

or by the influence zone

directions.

explained

fluid pressure

and vein characteristics

com-

in orientations

1978;

from

brittle-plastic

transition.

internal

veins relative

re-

cators of the interaction

interaction

Factors

characteristics,

to deformation

as

and timing

of

are sensitive

rocks

of folds located

indi-

between stress, fluid pres-

This study relates geometry to the framework

at

such

sure, and strain rate in the brittle-plastic

low-grade

Guha

gional fault zone. An attempt

0 1989 Elsevier Science Publishers B.V.

to plastic

can give vital information

aspects of fluid-rock

of asperities et al.,

of veining changes

brittle

gimes (e.g. Murphy, 1984; Gibson and Gray, 1985) on mechanical

Veins

compression

the transition

1973;

monly develop within and are systematically ciated with shear zones, sometimes These

fluid pathways, while weakly

fluid sinks.

next

regime.

and timing of veins and

shear

zones

to a long-lived

in re-

is made to explain

192

syndeformational tening direction

extension

to the shor-

Regional geology

in terms of fluid pressure (P,)

and stress conditions Classification

parallel

during brittle deformation.

of shear zones based on composi-

tion has made it possible to correlate mechanical behaviour and fluid behaviour.

Shear zone char-

acteristics are used to distinguish between regimes of continuous rock-water

interaction and regimes

of episodic

rock-water

interaction.

This paper

concentrates

on features developed in unmineral-

The Hilton deposit is located within the MidProterozoic

Mount

Queensland.

It is a stratiform

Isa

Inlier

in

Northwest

Pb-Zn

deposit

hosted by impure dolomitic sediments of the Mt. Isa Group (Mathias and Clarke, 1975). The Hilton area occurs immediately to the east of a major (100 km ) N-S

trending high strain

zone which includes the Paroo Fault Zone (PFZ)

ized rock. Structures within the mineralized zones

(Fig. l), the Mount Isa Fault Zone(MIFZ)

are similar in many ways, and have been dealt with in Valenta (1988).

l), and other N-S faults further to the west (Wilson, 1975). Rocks in the mine strike N-S and are

(Fig.

BARKLY SHEAR ZONE

EASTERN CK.

2’

BEDDING

Fig. 1. Map of the Hilton

area, with a cross section through

the Hilton mine.

193

a

Fig. 2. Hilton orientation data. a. F, and 4

folding: squares indicate fold axes; dots indicate F, axial surfaces, crosses indicate F3

axial surfaces. b Shear zones: stars indicate shear zones with predominantly sinistral strike-slip movement: crosses indicate shear zones with predominantly dextral strike-slip movement; and triangles indicate shear zones with predominantly high-angle reverse movement.

bounded on the west by the Paroo Fault Zone (Fig. l), across which rocks gradually increase

zones are common, both types intensifying toward the PFZ.

from very low metamorphic

The PFZ and MIFZ are large scale features which are traceable over distances of over 50 km.

grade (Saxby et al.,

1978) up to amphibolite facies (Wilson, 1975). Recent structural work (Winsor, 1983, 1985, 1986; Swager, 1983; Perkins, 1984) has shown that there are three deformation episodes in the Mt. Isa area. D, produced E-W folds and slaty cleavage, with associated late-D,, pre-D,, E-W faults (Winsor, 1986). D, produced open regional N-S folds and cleavage, dilation on pre-existing faults

(Winsor,

1986),

and late D,

E-W

to post-D,

faults in various orientations (Winsor, 1986). D, resulted in the local development of NNW-SSE cleavage and minor folds, with late faults at a low angle to S, (Winsor, 1986). S, and S, occur together in the Lake Moondara area (Winsor, 1983) and underground at Mt. Isa (Swager, 1983). The timing relationship between S, and S, in the Mt. Isa area is based largely on orientations and overprinting relationships of syntectonic veins (Winsor, 1983). Most small-scale features in the Hilton mine show orientations consistent with the D, and D, deformation episodes (Fig. 2). The mine area lies on the west limb of a large-scale anticline, interpreted by Winsor (1986) as a compound F,/F, anticline. Bedding parallel and cross-cutting shear

Structural relationships with D, and D, features in the Hilton and Mt. Isa (Perkins, 1984) would indicate that movement on the combined MIFZ-PFZ began before D, and continued well into post-D, time. Structural features Shear zones Shear zones occur in three main orientations: bedding parallel, NE-SW, and NW-SE (Fig. 2b). The first two types are most common in the mine area. Bedding parallel shear zones occur throughout the mine area, and all shear zone types become thicker and more closely spaced toward the PFZ. Slickenside data are summarized in Fig. 2b. NE trending shear zones show predominantly strike-slip movement, while both subhorizontal and subvertical movements are indicated by striae on NW trending and bedding parallel shear zones. Shear zone orientations and block movements are summarized in Fig. 3. Movement sense has been determined by the following methods: (1) offset on bedding, (2) fold asymmetry, (3) minor offsets

on earlier veins, and (4) rotation and offset of S, and S, cleavage planes. These methods confirm an EARING

overall west-block-up movement for bedding parallel shear zones, north block west and down for NE-SW shear zones, and north block east and up for NW-SE shear zones. This movement geometry

suggests E-W

subhorizontal

compres-

sion during shear zone movement. Figure 4 summarizes variations characteristics Shear

in shear zone

as a function of graphite content.

zones have been

subdivided

into

highly

graphitic and weakly graphitic end member types Fig. 3. Schematic block diagram showing relative movements of shear zones. Shaded area represents the zone of intense shear zone deformation and vein system development. Broken lines indicate movement direction on high-angle reverse faults.

(Valenta, 1988). Weakly graphitic shear zones are large areas of polyclinal and disharmonic folds separated by discrete, commonly curved, shear planes (Fig. 4b). Highly graphitic shear zones are

a

d

Fig. 4. Shear zone internal structures and vein orientations. a. Vein orientations: crosses indicate shear parallel veins; diamonds indicate minor shear parallel veins; circles and triangles indicate bedding perpendicular

and fold axis perpendicular

veins

respectively. b. Structures developed in weakly graphitic shear zones. c. Structures developed in moderately graphitic shear zones. d. Structures developed in highly graphitic shear zones. Veins are shown in black.

195

narrow

areas

which

veining

common

containing and

up to 10% graphite

disharmonic

folding

in

are less

angles show a large variation in orientation (Fig. 4b, d), and curved axial surfaces and fold axes are common

(Fig. 4d).

(Fig. 4b). Folds

within

shear

zones

are

The three shear zone types shown in Fig. 4 commonly occur together within large shear zones. This suggests that the change in graphite content

roughly coaxial with folds outside shear zones, and are east verging. S, is variably developed in folds of this type. In some cases, S, is parallel to

occurs during

the axial plane

progressive

deformation

within large

of shear-associated

other cases it is clearly

shear zones.

associated Folds

Folding Deformation accommodated

in

the

Hilton

mine

largely by mesoscopic

shear zone movement. has had a strong

The presence

has

been

folding

and

of shear zones

effect on the style of folding

and

by later shear-

folding. within

shear

complex

interaction

of open

folds

zones

commonly

vertical,

minor

offset in a vertical within

show

a

with minor shear zones. Limbs

are cut by shallow

more commonly extensional Bedding

folds, while in

overprinted

fold

zones

W-dipping,

or

shear zones with direction

generally

(Fig. 4~). shows

en-

intensity of deformation. The description of folding will therefore deal separately with folds devel-

hanced fissility with striations on bedding planes, indicating slip on bedding planes during folding.

oped within

East-dipping veins, bedding parallel veins, and shear parallel veins all develop, and will be discussed in more detail later.

and outside

shear zones.

Folds outside shear zones occur in barren dolomitic siltstones, and to a lesser extent within ore zones. Mesoscopic folds are tight to open, and occasionally isoclinal within ore zones. Folds generally display east vergences, and there is a large variation in axial surface dip. Curved axial surfaces are common. Two coaxial phases of folding are recognized. The first phase produced upright E-

Relationship

between folds/shear

zones und timing

There is a strong spatial relationship between folding, cleavage, and shear zones in the Hilton mine

(Fig. 4b, d). S, cleavage

intensifies

toward

verging folds (regional F,) with variably developed axial plane cleavage (S,). These are locally

the contact with shear zones, although the latest shear zone movement generally overprints S,. Al-

overprinted

bitized

by rare neutral

folds ( F3) with gently

to E-verging

E-dipping

reclined

axial plane cleav-

age (S,). Polyclinal and disharmonic occur near the contacts with major

folds usually shear zones.

Fold axes in the mine plunge shallow NNW or SSE (Fig. 2a), and commonly show a large variation within small distances. Cleavages defined by dark seams of opaque material commonly develop in hinges and next to shear zones. Long limbs display boudinage or sets of inclined faults subparallel to S, with extension in a vertical direction. Early bedding parallel veins are folded by Fz and F3, and bedding perpendicular and saddle reef veins develop in hinge zones. All F, folds face upward as indicated by younging structures such as graded bedding, sole marks, and cross bedding. Mesoscopic folds within shear zones are closely associated with minor shear planes in variable orientations, and are strongly polyclinal and disharmonic (Fig. 4b). Axial surfaces and interlimb

dykes

intrude

commonly contain quently deformed. coincident

NE-SW

shear

zones

and

an S, cleavage which is subseZones of intense folding are

with shear zones,

and bedding

parallel

shear zones commonly act as detachments for F, and F3 folds. In general, however, the latest shear zone movement appears to cross-cut D, and D, features, suggesting that shear zones continued to form subsequent to D,. Veining

Syntectonic veins occur in a number of settings in the Hilton mine. In general, veins occur within shear zones (Fig. 4) in structural positions controlled by shear zone geometry (Fig. 6), and to a lesser extent elsewhere in the mine (Fig. 5). Vein characteristics are summarized in Table 1. For the purpose of description, vein occurrences have been divided into the following types (p. 197ff).

Fig. 5. General

vein sets. Veins shown in black. Vein orientations: bedding

parallel

veins; crosses indicate

dots indicate

S’, parallel

fold axis perpendicular

veins; diamonds

indicate

veins; triangles

perpendicular

indicate

early

veins.

W . BEDDING r s*

DYKE TRACi FAULT ZONE

\

f--J

EVl

a

BVl

m

EVZ

fe3

BV2

a

BV3

1 M \

Fig. 6. Vein orientations

and cross-cutting

relationships

(BP’ 1, 2, and 3 on drawing);

within

circles indicate

vein systems: NE-dipping

crosses

indicate

&-parallel

and bedding-parallel

vein sets (EV I and 2 on drawing).

veins

197

as simple veins or groups of variable density (Fig.

General vein sets

5). They are commonly confined to one layer or a Figure 5 summarizes the orientations of various extension veins occurring away from shear zones. A

typical example of veining within a fold hinge is

shown in Fig. 7b. Four major sets are recognized:

group

of

layers,

with

vein

ends

terminating

abruptly on bedding planes (Fig. 5). En echelon veins are rare, and occur in both NW-SE NE-SW

arrays. Occasionally

from narrow

fibrous

fringes

and

there is a gradation to equant

centres

(Fig. 8a), in which linkage of en echelon cracks is Fold axis perpendicular

indicated

Veins of this type develop as simple planar

consistent small angle to the main vein (Fig. 8a,b).

by the presence of minor cracks at a

features or enechelon arrays (Fig. 5). They occur

Larger veins of this type commonly contain rotated

Fig. 7. a. Late bedding parallel veins, forming due to mechanical effects. b. Veins developed in fold hinges. Bedding perpendicular veins are well developed in pyritic layers and poorly developed in shale layers. A saddle reef is developed in the hinge. c. Typical breccia vein occurring in vein systems.

198

199

TABLE

1

Vein characteristics Max width

Max. length

(cm)

(m)

General veins

Mineralogy

Timing

Internal textures

(rel. to D2)

F perpendicular

10

0.3

qtz-dol-cal

fib-blocky

pre-post

S, perpendicular

5

0.5

qtz-dol-cal

fib-blocky

pre-post

Early S, parallel

5

qtz-dol-ank

fib

pre-

Late S, parallel

5

0.3

qtz-dol-cal

blocky

pre-post

10

2

qtz-dol-cal

fib-blocky

syn-post

15

4

qtz-dol-cat

blocky

pre-post

0.3

qtz-dol-cal

blocky

pre-post

.S, parallel

10

Shear zones

Shear parallel

5

Other orientations Vein system

NE-dipping set

15

3

qtz-dol-py

fib-blocky

pre-post

S, and S,, parallel

50

2

qtz-dol-py

fib-blocky

syn-post

breccia fragments which retain the early fibrous fringes (Fig. 8~). Bedding perpendicular east dipping Veins of this type are abundant throughout the

(Table 1). throughout

Early bedding . parallel veins occur the Hilton area, mainly within the

Upper Mount Isa Group. Cu and Pb-Zn Sulphides and their alteration minerals occasionally replace original

quartz-dolomite

vein assemblages.

The

mine. En echelon arrays in this orientation can be up to 5 m in length, and their asymmetry indicates

veins occur in anastomosing

west-block-up slip under subhorizontal E-W com-

Fibres are usually oriented subperpendicular to the vein walls, though oblique and curved fibres

pression (Fig. 5). Groups of veins are often concentrated in individual beds or groups of beds bounded by minor shear planes (Fig. 5). Veins terminate abruptly on shear planes or seemingly undeformed bedding planes (Fig. 7b). Vein groups of this nature are internally deformed during later folding and shear zone movement. Veins are generally fibrous, though in some cases they consist of an early fine grained fringe and a later equant undeformed fill (Fig. 8a,c). Later recrystallization and dissolution

groups separated by

thin laminae of country rock.

can occur. In larger veins it is possible to see variations in composition subparallel to the vein walls. Curved fibres with undulose extinction are common, and recrystallization to equant habits occurs in fold hinges. In some cases veins of this type occur as saddle reefs in gentle F, and F3 folds, indicating that they are syn- to post-folding. The general case, however, is that veins of this type are recrystallized and partially dissolved in

is common.

zones of F, folding, indicating folding.

that they predate

Bedding parallel Bedding parallel veins show a wide variation in composition, occurrence, and internal textures

Late bedding parallel veins are distinguishable from early bedding parallel veins in a number of ways. They contain large amounts of calcite in addition to quartz and dolomite, and grains are

Fig. 8. a. Fold axis perpendicular vein showing early fibrous quartz fringe (white) and later blocky dolomite fiil (gray). Asymmetrical cracks visible at vein margins. b. Separated intercrack counter rock segments. c. Rotated breccia fragments with fine-grained fibrous quartz fringes in blocky dolomite. d. S, parallel veins (horizontal) parallel vein with internal quartz-dolomite

S,

seams and preferred dimensional

veins. g. Multistage

with intensification orientation

of S, (horizontal)

of dolomite.

veining within vein systems. h. Undeformed

f. Fibrous

at vein margins. e. S,

quartz vein cutting earlier

blocky dolomite vein cross-cutting

&-parallel vein. Scale bar in each case is 1 mm.

deformed

200

commonly coarse and equant (Fig. 7a), with varying amounts of later deformation. type occur almost exclusively

Veins of this

in areas of small

scale F, and F3 folding, as saddle reefs (Fig. 7b) or discontinuous angular veins which are roughly bedding parallel

cross-cutting relationships (Fig. 7b). However, early bedding parallel veins are clearly cross cut by bedding perpendicular, fold axis perpendicular, and early bedding parallel veins.

but cut up and down through

layers. Where they occur together with bedding

S, parallel

veins,

Veins of this type develop mainly in zones of intense S, cleavage. Narrow planar S, parallel

the three vein types appear to show ambiguous

veins tend to show fibrous textures, with fibres

perpendicular

and fold axis perpendicular

(a)

(b)

B

(d)

-

cI----l

Fig. 9. Vein-S, relationships. a. Pre-S, vein, recrystallized with S, intensification (vertical) at vein margins. b. Post-S, vein opening parallel to S,. c. Syn-S, vein, opening parallel to S, (vertical) and showing intensification of S, at vein margins. d. Syn-S, vein, opening parallel to S, and showing internal development of S, in the form of seams and preferred dimensional orientation of dolomite.

oriented subperpendicular

to the vein walls (Fig.

veins, longer veins with asymmetric breccia frag-

Sf). Larger S, parallel breccia veins generally show

ments,

either coarse-grained

slivers and anastomosing

or compositional

subhedral internal textures,

lamination

of

equant

grains

or compound

arrays with shear-parallel layers of country rock

(Fig. 4b, c, d).

are angular (Fig. 7c), and vary in size from less

Pre-existing structures within shear zones have clearly controlled the geometry of the veins. Thin

than 1 mm up to 30 cm. Breccia

graphitic partings are observed within some veins,

sub-parallel

to the vein walls. Breccia

fragments

fragments are

generally unaltered, though minor silicification and

and the margins are commonly

dolomitization

complex cross-cutting

occur in some cases. Large frag-

ments show little or no rotation,

while smaller

narrow zones of

breccia veining.

Internal deformation is highly variable. In some

fragments can show rotations of greater than 90 O.

cases, veins consist

In some cases individual veins show clearly defined breccia fragment-rich and breccia fragment-

subhedral grains, while elsewhere veins show strong internal recrystallization and dissolution. A

poor areas (Fig. 7~). Larger breccia fragments often contain smaller, slightly rotated, breccia

number of features indicate that shear parallel veins formed at various stages during the shearing

veins at a small angle to the main vein. Clasts

history.

appear to show a perfect fit in cases where there

(1) The occurrence of compound veins with sharply defined zones of high and low internal

has been minor brecciation

and rotation. The rare

of very coarse

occurrence of altered fragments suggests, however,

deformation,

that dissolution and replacement make it difficult to reconstruct zones of more intense brecciation and rotation. Breccia fragments show transport of up to 5 cm within veins (Fig. 7~). These features indicate a complex history of veining in which opening on planar features occurred simultaneously with brecciation. Figure 9 summarizes the various relationships observed between S, and S, parallel veins. A large proportion

cated by minor compositional zones.

of S, parallel veins appear to have had some effect on S,. This may take the form of intensification

of S,

at vein walls (darker

areas which

displace veins in Fig. 8d) in which cleavage intensity is strongly controlled by vein margin irregularities, S, parallel dissolution zones within veins (Fig. 8e,h), or breccia veins with rotated S, in the fragments. Similar textures occur in breccia veins in copper areas of the Mt. Isa mine (Swager, 1983). This indicates that opening occurred on S, planes prior to the end of S, cleavage formation. Veins within shear zones Veins in this setting develop in four main orientations (Fig. 4a), though each set shows considerable spread due to the complex nature of shear zone internal structures. Shear parallel veins Veins in this orientation are found in most shear zones. They occur variously as single lens-like

undeformed

in which repeated opening is indichanges in different

(2) Shear parallel veins which are deformed and cross-cut by minor shear planes (Fig. 4b). (3) The common occurrence of relatively undeformed shear parallel veins at shear zone margins, grading into strongly dissolved, folded, and boudinaged veins in the internal portions of shear zones (Fig. 4d). Other veins East dipping and subhorizontal veins form parallel to the common minor shear plane orientations (Fig. 4b, c), while E-W veins occur bounded groups which are perpendicular shear planes (Fig. 4b, c). They are similar shear parallel veins in composition and

as shear to major to major internal

textures, but are generally smaller (Table 1). In most cases, veins of this type occur as layer bounded groups, in a manner similar to bedding perpendicular and fold axis perpendicular veins (Fig. 4b, c). Vein systems Zones of intense veining occur where large scale shear zones intersect (Figs. 1 and 3). In these zones, vein sets of different orientations can occur over large areas in spacings as small as 10 cm.

202

from one such zone are sum-

Vein orientations

Summaty

of vein timing

marized in Fig. 6. Veins in this zone occur in two and

Table 1 summarizes the timing of veins in the

shallow NE dipping (Fig. 6a). However,

Hilton area. In the general case, all veins except

major orientation NW-SE

composition,

groups;

N-S

subvertical,

internal texture, and vein timing are

for the early bedding parallel systematically

variable within each group.

framework.

related S,

to

the

set appear to be D,

deformation

parallel veins show the most re-

stricted timing, occurring late syn- to post-S,. N W-SE

the other hand, fold axis perpendicular,

veins

These are the most common veins within vein

perpendicular,

and NE-dipping

On

bedding

veins show the

systems. They occur as single veins or anastomos-

widest spread in timing relative to D,. Veins within

ing groups spaced at intervals of 1 cm or more, and are equivalent in timing and composition to the previously described bedding perpendicular

shear zones are difficult to fit into this framework. They appear to have formed throughout the defor-

veins. In the core of the vein system they occur as dense arrays in which veins are interconnected. There is commonly a large variation in degree of

mation history of the shear zones, which themselves have been shown to span a time from pre-D, to post-D,.

vein deformation within the core (Fig. 8g). Early veins in this orientation consist dominantly of

Discussion

equant

coarse-grained

Origin of early veins

consist

mainly

quartz,

of fibrous

while later veins

quartz

and dolomite

(Fig. 8f). In some cases, vein fill consists of a fine grained fibrous quartz fringe with a coarser grained fibrous dolomite centre.

Early bedding parallel veins do not show a systematic spatial correlation with deformation features in the Hilton area. It is possible that these veins are features caused by periodic overpressuring during burial dewatering (Fyfe et al., 1978).

N-S vertical These veins are similar to the previously described S, parallel vein set. Breccia, and fibrous veins occur in this orientation, in spacings much closer than those observed in general S, parallel

Localization

of vein systems

Figure 1 shows the area in which a large scale vein system is developed within the Hilton mine.

veins. The only demonstrably syn-S, (Fig. 8e, h) veins are dolomite rich and recrystallized to subequant textures. They contain irregular seams

On the scale of the area, this would correspond to the zone of intense shear zone deformation shown in Fig. 3. It is important to note that the area of

which are oriented sub parallel to S, (Fig. 8e, h). Dolomites show sutured boundaries, and there is a strong preferred dimensional orientation of

vein system localization is an area where space problems would arise during continued movement

dolomite parallel to S, (Fig. 8e, h). Veins of this type are cut at oblique angles by veins (Fig. 8h) composed of equant dolomite which coarsens into the vein centre. The veins contain breccia fragments of the earlier S, parallel vein set. In these occurrences there is a strong rotated S, in the breccia fragments, but the equant vein material is virtually undeformed. Later breccia veins can be either S, parallel or NE-dipping, and individual veins often bend from one orientation to the other (Fig. 6).

on the shear zones (Fig. 3). Thus it would appear that shear zone geometry is the main control on localization of vein zones. Evidence for abnormal syndeformational

extension

The movement geometry of shear zones in the Hilton area (Figs. 2b and 3) suggests E-W subhorizontal compression during D, and D,. However, the following points suggest local syn-D, extension parallel to the regional compression direction:

203

(1) Veins which are parallel strong dissolution

to S,

and recrystallization,

and show resulting

in shape preferred orientations and solution seams

in turn cause heterogeneous fluid flow patterns and irregularities in shear-parallel veins. (2) On a small scale, shear planes commonly

parallel to S, (Fig. 8e). This indicates that the veins formed after the start of cleavage develop-

parallel a single layer for a considerable distance, and then abruptly cut across layering at a high

ment, and were internally

angle,

deformed during con-

thus

transferring

movement

to another

tinued cleavage development. Opening in a direc-

horizon. Veins are commonly thickest in the areas

tion perpendicular

where the shear planes bend.

to S, is indicated by $-per-

(3) Gentle

pendicular fibres.

folds with axial surfaces

subper-

(2) Veins which are parallel to S,, contain &-perpendicular fibres, and show an intensifica-

pendicular to shear zone walls occur in close as-

tion of S,

extensional

in the country

rock as the vein is

sociation with conjugate minor shear planes with movement.

The

former

indicates

approached (Fig. 8d). This indicates that the veins

shortening subparallel to the shear zone, while the

formed after the start of cleavage development, and vein margin irregularities caused variations in

latter indicates extension subparallel to the shear zone. These seemingly conflicting features coexist

cleavage intensity during subsequent cleavage development. (3) Individual shear-parallel veins which show distinct internal boundaries between zones of

as a result of periodic fluctuation of the shear plane between the shortening and extension fields.

varying dissolution

and recrystallization,

indicat-

Mechanical models generally consider the effect of movement over asperities (e.g. Ladanyi and Archambault, 1970; Guha et al., 1983) or the

ing periodic reactivation of pre-existing veins during continued deformation. This indicates re-

effect of movement terminations (e.g. Muraoka and Kamata, 1983). Geological asperity models in

peated opening at a high angle to the shortening direction.

effect divide the zone of movement into high-strain

Given that in normal circumstances

it is dif-

ficult to have extension parallel to the shortening direction, and assuming that the regional shortening direction now observed is similar in orientation to the maximum principal stress (a,) during D, and D,, it is obvious that during vein formation an abnormal local stress framework must have been in effect. The main factors reponsible for abnormal stress fields are mechanical irregularities and abnormal fluid pressures. The following sections will discuss the possible influence of these two variables. Mechanical

irregularities

A number of features within shear zones indicate that mechanical irregularities may have been important in the formation of shear-parallel veins: (1) Shear zones are strongly non-planar in plan and section, commonly bending into parallelism with other shear zones or bedding (Fig. 4b, c). Movement between two irregular surfaces results in variations in aperture at various scales (Guha et al., 1983; Brown, 1987). These aperture variations

and low-strain zones. The position and effectiveness of these zones is determined by factors such as height and wavelength of the asperities (Guha et al., 1983) and conditions during deformation (Knipe, 1985). In terms of observed veining at Hilton, one would expect to see shear veining in zones which have opened as a the overriding of asperities, and shear extensional features where major bends

parallel result of parallel in shear

zones have caused increased shortening perpendicular to the shear zones (e.g. Guha et al., 1983, fig. 7c). Muraoka

and Kamata

(1983)

considered

the

effect of shear zone terminations and competency contrast on strain patterns. Their results can be applied generally if the assumption is made that movement on a shear zone occurs in a finite zone at any one time, rather than assuming simultaneous movement all along the shear zone. In this way, each movement episode produces zones of compression and extension (Muraoka and Kamata, 1983, fig. 10). The theoretical strain heterogeneity produced by this method is consistent with the observed occurrence of apparently simultaneous shear parallel shortening and extension, and would

204

allow the formation of veins in anomalous orien-

and Mt. Isa (Swager, 1983), there is evidence

ton

tations.

that tensile failure occurred during deformation, with extension

in a direction

parallel

to the re-

gional direction of maximum compressive

Fluid pressure (P,) during veining

stress.

In order to produce this, the following Pf condiS, parallel veins form within vein systems in

tions are required (Fig. 10a) (Secor, 1965; Sibson

areas where the only evidence of brittle deforma-

et al., 1975; Etheridge, 1983):

tion is the presence of extension veins. It would be

(1) Pf approaches lithostatic values.

difficult, therefore, to assert that S, parallel veins

(2) Pf r + + T’.

in this setting form as a result of the influence of

(3) ul--u3 5 T.

small

scale

mechanical

irregularities.

Thus,

it

T’ in this case is the tensile strength of the rock

would appear that the important influence in this case must be fluid pressure (P,).

including the effect of planar elements such as bedding and cleavage. The differential stress com-

In order to form veins, Pf must be high enough to shift stresses at least partially into the tensile field (Etheridge et al., 1984). The expression Pf 2 u3 + T (where u3 is the minimum principal stress and T is the tensile strength of the rock) explains the general conditions required for tensile failure

monly

shows a considerable

brittle

(1977) reports a typical range in stress drops of 1 to 10 MPa for examples from Southern California, while stress drops as high as 35.5 MPa have been reported (Brune and Allen, 1967).

(Etheridge, 1983). Failure of this nature should produce veins subparallel to the direction of maximum compressive stress (Ramsay, 1980). At Hil-

Sibson et al. (1975) have outlined the stress, fluid pressure, and fluid flow conditions before

a

b ,BULK

drop after

failure on large faults (Chinnery, 1964; Brune and Allen, 1967; Hanks, 1977; Sibson 1977). Hanks

FAILURE

1

ROCK

ANISOTROPY

Pf

FLUID

FLOW

AROUND DILATANT ZONE

TIME Fig. 10. Differential stress, fluid pressure and fluid flow up to and after a failure episode. a. Mohr diagram showing conditions before (i) and after (ii) failure. Stress drop allows dilation of anisotropies oriented at a high angle to q. b. Changes in differential stress, fluid pressure and fluid flow with time (after Sibson et al., 1975). The period of time in which differential stress is low and fluid pressure is high is shaded.

and after brittle failure on a fault which contains a

of

dilatant zone (Fig. lob). As stresses build up prior

reached similar conclusions

to failure, fluid flows into the dilatant zone. Im-

volumes in a study of volume loss in calcite-rich

mediately after failure, the differential stress drops,

rocks of the Apennines.

resulting in collapse of the dilatant zone. At this

dence of fluid presence in weakly graphitic shear zones is largely mechanical, taking the form of

point, differential

stress is at a minimum, and Pr

is at its highest value (Fig. lob). If the stress drop is sufficient

to satisfy condition

will be a situation

(1) above, there

in which tensile

failure

on

the original

rock

volume.

Engelder

(1984)

with respect to fluid

On the other hand, evi-

tensile veins in various orientations. Tensile veining generally occurs in regimes of transient high P, (Etheridge et al., 1984)

suggest-

to what was initially u1 is

ing that large volumes of fluid were present within

possible (Fig. 10a) while fluid is flowing out of the

weakly graphitic shear zones only for short peri-

dilatant zone (Fig. lob).

ods up to and after failure. On the other hand, the

planes perpendicualr

In the previously described vein systems (Figs.

intense dissolution in highly graphitic shear zones

6, 7c, 8e-h), two general types of vein develop. The most common is the set which is shallow

would suggest the relatively constant presence of large volumes of fluid during deformation. A pos-

NE-dipping (Fig. 6, EV’s (extension veins) 1 and 2). Veins of this type appear to have formed continuously during the movement history of the shear zones. The $-parallel veins (Fig. 6, BV ‘s (breccia veins) I, 2, and 3) on the other hand, are rarer and are bracketed in time by the NE-dipping set. This would suggest that the NE-dipping veins

sible explanation

for this is the nature of permea-

bility in the two different shear zone types. Highly graphitic shear zones are roughly planar zones of penetrative microfracturing in which the permeability was probably high regardless of the surrounding stress conditions. Weakly graphitic shear zones are irregular bodies with a large proportion

in this case formed under general tensile failure conditions (Fig. lOa). They represent veins which formed perpendicular to u3 in the dilatant zone prior to failure (Fig. lob). S, parallel veins, on the

of undeformed and relatively impermeable rock. Permeability in weakly graphitic shear zones would therefore be highly variable and strongly dependent on the geometry of individual fracture

other hand, represent veins which formed under the restricted set of stress and fluid pressure conditions which were satisfied for a short period of

surfaces and the differential stress and fluid pressure conditions at a given time. The relative scarcity of veining within highly graphitic shear zones

time after failure on the shear zone (Fig. (Sibson et al., 1975).

suggests that deformation within those zones occurred by slow creep processes with only minor

lob)

dilation, Fluid behaviour acteristics

as a function

of shear zone char-

The fact that highly graphitic shear zones show dissolution and little veining, and weakly graphitic shear zones show weak dissolution and abundant veining, suggests that fluid behaviour in the two end member types is fundamentally different. Evidence for the presence of large volumes of fluid within highly graphitic shear zones is based on estimates of fluid volumes required in order to passively concentrate graphite by dissolution of carbonates. Preliminary mass balance calculations (Valenta, 1988) have shown some shear zones display volume factors as low as 0.4, requiring the circulation of enough fluid to have dissolved 60%

while brittle

deformation

process

failure was the dominant in weakly

graphitic

shear

zones. As a consequence of this, fluid pressure and permeability variations would be largest in weakly graphitic shear zones (Fig. lob), and failure episodes within this shear zone type would exert a strong influence on fluid movement during deformation (Sibson et al., 1975). Conclusions Veins in the Hilton area show a large variation in orientations and characteristics. Vein orientations are commonly influenced by small scale features such as folds, bedding, cleavage, and shear planes, indicating localized reversals in the stress field in response to deformation.

206

Vein systems developed as dilatant zones associated

with stress buildup and failure on large

scale shear zones. Large scale shear zone geometry controlled the localization

of vein systems. In the

general case, veins within these systems developed perpendicular

to u3. Veins formed perpendicular

to what was originally

u, in the restricted

time

after failure episodes in which Pf was high and differential

stress was low.

Highly graphitic shear zones deformed by slow creep processes and acted as fluid pathways, while weakly graphitic shear zones deformed by brittle processes and acted alternately as fluid sinks during stress buildups and fluid sources after failure (Sibson et al., 1975).

Engelder. T., 1984. The role of pore water circulation the deformation

during

of foreland fold and thrust belts. J. Geo-

phys. Res., 89: 4319-4325. Etheridge,

M.A., 1983. Differential

regional

deformation

stress magnitudes during

and meatmorphism:

upper bound

imposed by tensile fracturing. Geology, 11: 231-234. Etheridge, M.A., Wall, V.J. and Cox, SF.,

1984. High fluid

pressures during regional metamorphism implications

for mass transport

and deformation:

and deformation

mecha-

nisms. J. Geophys. Res., 89: 4344-4358. Fyfe, W.S., Price, N.J. and Thompson,

A.B., 1978. Fluids in

the Earth’s Crust. Elsevier, Amsterdam, 383 pp. Gibson,

R.G. and Gray, D.R.,

tion during thrust

1985. Ductile-to-brittle

sheet emplacement,

transi-

Southern

Appa-

lachian thrust belt. J. Struct. Geol., 7: 513-525. Guha, J., Archambault,

G. and Leroy, J., 1983. A correlation

between the evolution of mineralizing fluids and the geomechanical development of a shear zone as illustrated by the Henderson 2 mine, Quebec. Econ. Geol., 78: 1605-1618.

Acknowledgements

Hancock, P.L., 1972. The analysis of en-echelon veins. Geol.

I would like to thank Bruce Hobbs, Paul Williams, Dave Gray, and Nick Oliver for ideas,

Hanks, T.C., 1977. Earthquake stress drops, ambient tectonic

Mag., 109: 269-276.

discussions, and constructive criticism of an early draft of this paper; Vie Wall, Chris Waring, and Cees Swager for ideas and discussions; Draga Gelt for drafting assistance; and Steve Moreton for photography help. Field support from Mount Isa Mines Ltd. is gratefully acknowledged. I would like to thank Brice Mutton, Bill Perkins and Rod Johnson of MIM for useful ideas and discussions. This work was carried out during tenure of a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship.

stresses and stresses that drive plate motions. Pure Appl. Geophys., 115: 44-458. Kerrich, R. and Allison, I., 1978. Vein geometry and hydrostatics during Yellowknife

mineralization.

Can. J. Earth Sci.,

15: 1653-1660. Knipe,

R.J.,

1985. Footwall

geometry

and the rheology of

thrust sheets. J. Struct. Geol., 7: l-10. Ladanyi, B. and Archambault, behaviour

of a jointed

G., 1970. Simulation

rock mass. In:

(Editor), Rock Mechanics-Theory Mining Engineers-American

W.H.

and Practice. Society of

Institute Mining Metall. Pet-

roleum Engineers, New York, N.Y., pp. 105-125. Mathias,

B.V. and Clark, G.J.,

1975. Mount Isa copper and

lead-zinc ore bodies-Isa and Hilton mines. In: C.L. Knight (Editor), Economic Geology of Australia and New Guinea. 1. Metal.

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