Economic Geology Vol. 83, 1988, pp. 69-85

Structural Controls on Development and Localization of Syntectonic Copper Mineralization at Mount Isa, Queensland

T. H. BELL, W. G. PERKINS,* AND C. P. SWAGER**

Department of Geology, James Cook University, Townsville, Queensland 4811, Australia

Abstract

Three major deformation events were responsible for the structural relationships in the Mount Isa mine where steeply west-dipping siliceous and dolomitic sediments of the Mount Isa Group, which host the Ag-Pb-Zn ores as well as the epigenetic Cu orebodies, are truncated against the underlying greenschists. The Cu orebodies and their alteration enve- lope, the silica dolomite, developed during the third deformation (D3) within the dolomitic Urquhart Shale, adjacent to this greenschist contact.

This contact is a fault that formed during thrusting in D• and was rotated and overturned by the combined effects of D1 and D2 into a shallowly dipping attitude on the western limb of a regional-scale upright D2 antiform. Consequently it lay at a high angle to the steeply plunging stretching direction established during the third deformation (D3). This fault was a plane of considerable anisotropy and competency contrast because the rocks above were dolomitic siltstones whereas those below were greenschists, with bedding respectively at a high angle and subparallel to the fault.

Folding during D3 generated opposed senses of shear and hence differential movement across the fault, especially at fold hinges. This, in combination with the differential anisot- ropy and competency contrast across the fault, and extension at a high angle to it, resulted in localized fracture and opening of the contact and consequently a rapid drop in fluid pres- sure, possibly on several occasions in different locations along the greenschist contact. The large imbalance in fluid pressure across the walls of these localized openings caused explo- sive fracturing and local brecciation, especially of the dolomitic rocks on the fault walls into the space created. This provided access for the syndeformation hydrothermal fluid to all faces of the jostled blocks of dolomitic shales enabling their alteration to form the silica dolomite and the Cu ores.

Introduction

THE Cu and Ag-Pb-Zn orebodies at Mount Isa in northeastern Australia (Figs. i and 2) occupy a spe- cial position among sediment-hosted massive sulfide deposits (cf. Gustafson and Williams, 1981) in that they occur within the same formation (the Urquhart Shale) but show a consistent spatial separation and different host-rock characteristics (Mathias and Clark, 1975).

The Pb-Zn orebodies are arranged en echelon (Fig. 3) and consist of stratiform galena and sphaler- itc layers within dolomitic shale. Neudert (1983, 1984) suggested, on the basis of a sedimentological analysis, that both galena and sphalerite were em- placed within the sedimentary pile rather than de- posited at the sediment-water interface (cf. Mathias and Clark, 1975; Finlow-Bates and Stumpfi, 1979). Detailed microstructural work to determine the

timing of mineralization in the Ag-Pb-Zn orebodies

* Present address: Mount Isa Mines, Ltd., Mount Isa, Queens- land 4825, Australia.

** Present address: Geological Survey of Western Australia, Kalgoorlie, Western Australia, Australia.

relative to the deformation history has not yet been done.

The Urquhart Shale host to these orebodies is part of the Mount Isa Group (Fig. 2), a sequence of middle Proterozoic sediments (Page, 1981) located in the Leichhardt River fault trough in northwest Queensland (Plumb et al., 1980; Derrick, 1982; Fig. 1). The Mount Isa Group is dominated by partly carbonaceous, siliceous to dolomitic siltstones of a shallow-water facies (Neudert and Russell, 1981); it unconformably overlies the Haslingden Group which consists of metabasalts, sandstones, and quartzites.

The Cu ore and its silica dolomite envelope occurs within the Urquhart Shale, mainly immedi- ately above the greenschist basement contact. It protrudes upward as often bedding-parallel lobes (Fig. 3). The silica dolomite developed by complex silicification and dolomitization processes from the originally dolomitic siltstone as well as locally from the Pb-Zn orebodies. The chalcopyrite (+ Fe sulfide q- quartz) ore deposition took place during D3 and represents the last stage in the overall replacement history (Perkins, 1984; Swager, 1985).

The various structural elements that formed the

0361-0128/88/771/69-1752.50 69

70 BELL, PERKINS, AND SWAGER

FIG. 1. Location of Mount Isa, in the Proterozoic of north- western Queensland.

framework for the relative timing of the replace- ment processes were only briefly described by these authors. This paper documents in more detail the geometry and deformation history of the mine

N

3 k•!

X Mr. Isa Mine

um Upper Mr. Isa Group us Urquhart Shale nb Native Bee Siltstone

bs Breakaway Shale ms Moondarra Siltstone

vv Haslingden Group Surprise Creek Fro.

bedding D 3 fold with plunge geological boundary fault

FIG. 2. Geologic map of the Mount Isa mine area.

w

f ¾/00•" / / UROU- I HART SHALE //

? I I

I Pb-Zn orebodies v v• ' :•.:• Silica dolomite & Cu orebodies

v• Greenstone

FIG. 3. Schematic cross section through the southern part of the Mount Isa mine (after Mathias and Clark, 1975).

area. We have drawn on' structural mapping and microstructural studies in and around the mine area

and have incorporated results from regional struc- tural studies (Bell, 1983).

The fold, cleavage, and vein structures in the do- lomitic shale hosting the ore, and the geometry and movement history of the major fault separating the shale sequence from the underlying greenschists, are described. Previously this complex fault has been interpreted as a subhorizontal basement across the Mount Isa valley (e.g., Smith, 1969; Fin- low-Bates and Stumpfi, 1979; Young, 1984) and has been explained as a growth fault (Smith, 1969) or a curved normal fault (Dunnet, 1976). Here the sub- horizontal extent of the fault is interpreted to be much more restricted, and its origin is explained in terms of the recently proposed thrust hypothesis for the Mount Isa block (Bell, 1983). Finally we discuss the structural effects of the different bedding orien- tations across the fault in the localization of the Cu

mineralization during post-thrusting deformation.

Deformation History

Macroscopic structural relationships

Recent structural analyses (Perkins, 1979; Win- sor, 1982, 1986; Bell, 1983) have led to a reinter- pretation of the regional deformation history of the Leichhardt River fault trough. Three major phases of deformation have been identified: D• formed a large-scale north-south-directed thrust system, with local folds having east-west-striking axial planes (Bell, 1983; Winsor, 1986); D2 developed macro- scopic regional upright folds with north-south- striking axial planes; and D3 locally formed north- northwest to south-southeast-striking fold zones. Winsor (1982, 1986) reviewed the earlier proposed deformation schemes and correlated these with the

present one. At the Mount Isa mine, the Mount Isa Group lies

on the western limb of a large-scale D2 anticline

STRUCTURAL RELATIONS, MT. ISA, AUSTRALIA 71

(Winsor, 1986). The mine sequence is truncated by a variably east-dipping fault contact with underly- ing greenschists and quartzites of the Eastern Creek Volcanics (Fig. 3). This greenschist contact curves from a large angle at depth toward parallellism with bedding at higher stratigraphic levels within the Mount Isa Group; at the surface it is called the Paroo fault (Figs. 2 and 3).

The Mount Isa Group sequence is traversed by several D3 fold zones with subvertical, north-north- west-south-southeast-striking axial planes (S3) and steeply south-plunging stretching lineations (Per- kins, 1979; Winsor, 1982, 1986; Swager, 1985). The largest D3 fold in the mine is the Mount Isa fold (Fig. 2) which has a wavelength of 200 to 400 m and a near-vertical short limb. Other fold zones are

usually not wider than 10 to 20 m. Effects of post- D3 deformation are most commonly observed along the greenschist contact.

The west-dipping Mount Isa fault, to the west of the Paroo fault, is a steep reverse fault that trun- cates the Mount Isa fold and separates rocks in the mine area from equivalents of higher metamorphic grade to the west.

Structures in the Urquhart Shale

The fine-grained (5-80 t•) dolomitic rock types in the Urquhart Shale are divided into three groups on the basis of the ratio of dolomite to mica + carbona-

ceous material: (1) black shale with less than 25 percent dolomite, (2) dolomitic shale with 25 to 75 percent dolomite, and (3) dolomitic siltstone with more than 75 percent dolomite. Quartz and K-feld- spar are minor constituents. Pyrite is present as a minor phase in the shales but also occurs as nearly massive beds. The cleavage seams in the shales con- sist of concentrations of white mica and carbona- ceous material and have resulted from extensive

dissolution of dolomite and quartz in zones of pro- gressive shearing strain (e.g., Bell et al., 1986).

The presence of highly ductile galena _+ sphaler- ite beds within the shale sequence strongly in-

fiuenced its overall competency and tight to iso- clinal folding of the shale layers was only observed in sequences with interbedded sulfide. Textural de- velopments in the galena +_ sphalerite beds have been the subject of several studies (e.g., McDonald, 1970; McClay, 1978).

The most common (micro)structural elements in the Urquhart Shale are summarized in Table 1. D• deformation features, such as S1 micaceous seams (Fig. 4) and small-scale thrust imbrications (Fig. 5), were only rarely observed. D2 resulted in the devel- opment of cleavage seams (S2) at a low angle or parallel to bedding and the formation of dolomite veins subperpendicular to bedding. These veins are characterized by a constant width (except where later dissolution occurred), a monomineralic com- position, and commonly abrupt terminations at lith- ological boundaries (e.g., Fig. 4). The vein fibers (sub)parallel to S• are interpreted to have formed by a crack-seal mechanism (Ramsay, 1980). How- ever, the fibrous texture is commonly destroyed by recrystallization resulting in the formation of a me- dium-grained (up to 1 mm) polygonal granoblastic aggregate (Fig. 4A). This took place before or early during D3 because the recrystallized veins show the effects of D3, including strong undulose extinction and incipient boudinage (necking) in F3 ø fold hinges (Fig. 4A and C; terminology of Bell and Duncan, 1978, i.e., "For the nth deformation, the terminol- ogy becomes Dn, Sn, F•, L• where the superscript represents the S-surface affected by Dd' (p. T1).). The D2 veins commonly show a mirror image cur- vature across the axial plane of F3 ø folds: they are subperpendicular to bedding in dolomitic siltstone and are rotated toward S3 in the shales, with sharp changes in vein orientation across the layering (Fig. 4A). This apparent vein rotation increases in zones of high D3 strain (Fig. 4B). The consistently oppo- site sense of rotation of D• veins on the F3 limbs and the correlation between strong rotation and high D3 strain suggest that the D• veins rotated during D3, due to reactivation of So and then S2 (Swager, 1983;

T•,BLE 1. Summary of (micro)Structural Elements in the Urquhart Shale (from Swager, 1985a)

Folds Foliations Veins (composition, texture, orientation)

D•, not found

D•, not recognized on mine scale

D3, fold zones, 10 to 20 m wide, locally 100 m

S• seams in siltstone; widely spaced, subperpendicular to So

S•, pervasive slaty cleavage in dolomitic shale; layering in black shale; (sub)parallel to So

S3 slaty cleavage in dolomitic shale; S3 crenulated cleavage in black shale

D• extension veins, dolomitic, polygonal; two sets(?): (1) small angle to So, (2) high angle to So

D• extension veins, dolomitic, fibrous or polygonal; fibers _ parallel S•; very high angle to So

(1) breccia veins, dolomitic, polygonal, crosscutting and/or parallel to So, (2) Da extension veins: dolomite _ quartz, fibers parallel Sa

D4, very local S4 crenulation (cleavage) in black shale Not observed

72 BELL, PERKINS, AND SWAGER

FIBROUS

RECRYSTALLIZED

D 2 VEIN ------'---__ DOLOMITIC SHALE

SI SEAM DOLOMITICSILTSTONE

STRUCTUI1AL RELATIONS, MT. ISA, AUSTRALIA 73

FIG. 5. Photograph of a small-scale imbrication within the mine. Photograph taken looking south, hence movement possible is anywhere between east over west through to north over south or perhaps even south over north. Note the magnificent preserva- tion of flat and ramp geometries and how the bed above the flat is the same in each case with the transition to hanging-wall ramps beginning immediately above locations 1, 2, and 3. Maximum width 20 cm.

Bell, 1986), rather than that vein refraction oc- curred during D•. No mesoscopic D• folds have been observed in the mine, but a good example of S• oblique to bedding and verging eastward, but folded around a D3 fold, is preserved in the railway underpass north of the mine.

At higher levels in the mine, Da resulted in a well-developed Sa cleavage only within the Fs ø fold zones. At lower levels it formed with variable in-

tensity throughout the sequence but is best devel- oped in zones outside the silica dolomite. Fibrous extension veins formed during Da are rare and re- stricted to highly deformed fold limbs. Breccia veins, locally containing displaced and rotated shale fragments in a coarse (up to several mm) dolomite matrix (Fig. 4D), commonly crosscut bedding at a high angle, with bedding-parallel branches. These breccia veins •are interpreted to form during Da be- cause irregularly rotated fragments within them contain variously oriented Sa seams (Perkins, 1984, p. 629). Furthermore, the seams are less strongly developed, and the bedding-cleavage angle is con- sistently higher, in all fragments. Generally, the veins show evidence of Da effects including strong

Sa development against their margins, pinch-and- swell structures, and dissolution of their walls. They are also crosscut by subhorizontal extension veins formed at later stages during Da.

Fs ø folds generally range from open to tight in the shales and the silica dolomite but vary from tight to isoclinal in sequences with interbedded galena- sphalerite layers (Fig. 6A). Figure 6B shows a typi- cal zone with disharmonic F•s folds of variable in- tensity. The open folds plunge approximately 10 ø north-northwest whereas the close to very tight folds plunge 15 ø and 25 ø to 35 ø south, respectively. Perkins (1979, 1984) demonstrated an overall F•s plunge reversal from south-southeast in the south- ern to north-northeast in the northern mine area.

These plunge reversals are the result of inhomo- geneous strain during Da (cf. Sanderson, 1973). However, there could be a component of Da folding that has been obscured by the effects of reactivation of So and S• during Da, because the D• and Da de- formation phases have similar orientations.

Small-scale Da elements such as L• and bedding- Da vein intersections show a wider spread than F•s axes, but they have a clear north-south trend with a shallow plunge to the north or south (Fig. 6A). The Fs ø folds, with north-northwest-striking subvertical axial planes and mainly north-northwest-plunging fold axes, display a constant asymmetry in cross sec- tion which indicates an antiform to the east. The

overall D• to Da fold geometry in the mine area implies that Sa and Sa cleavages have the same ver- genee on the long west-dipping limbs of the l•s folds and the opposite vergenee on their short limbs. However, the extensive reactivation of Sa on the limbs of Da folds has often made the Sa vergenee difficult to establish because of the extremely small angle between Sa and bedding. In fact it is likely that the vergence has been switched on one limb because of the degree of reactivation that has taken place (e.g., Bell, 1986, fig. 18).

Post-Da structures, including 04 crenulations, are largely restricted to domains along the greenschist basement contact, although kinking of Sa in post-Da shear zones is observed at higher structural levels. Several major transverse faults (Mathias and Clark, 1975), which crosscut and offset both the green- schist contact and the silica dolomite, also postdate Da (see below).

FIG. 4. Cleavage and vein structures. A. Schematic Ds fold drawn looking north with common microstructural elements in an idealized sequence of black shale, do!omitic shale, do!omitic siltstone, and pyrite bed. B. Negative print of shale-siltstone sequence in cross section (drawn looking south) on the vertical short limb of an I•3 fold. Fine elongate Ds microveins (black arrows) occur subperpendicular to bedding in the siltstone (dark); shale (light) shows strong S3 and dextral rotation of D2 veins (black). Plane polarized light. C. Dissolution along AB of fibrous D2 vein dolomite, and possibly some matrix material, in I•3 fold hinge. Bedding (So) is indicated at a high angle to the vein. Plane polarized light. D. Breccia vein (in I•3 hinge) containing a rotated pyritic shale fragment. Strong S3 development adjacent to the vein, possibly with minor dissolution of the vein dolomite. Plane polarized light.

74 BELL, PERKINS, AND S WAGER

A N

w E

ß

ß

•O CN

DOLONITIC SHALE WITH D 2 VEIN ATTITUDE

ß/?S O

ß F30 ß D I VEIN/S O 0 L20 . D 2 VEIN/S O

,OPEN

'TIGHT

Fig. 6. F• fold structuresß A. Tight to isoclinal Fs ø folds in a sequence of alternating shale and sulfide-rich beds, and stereo- grams with structural data measured on long limbs from two lo- calities in the southern and northern part of the mine. Note the steeper plunge of Fs ø folds in the northern part. B. Series of tight Fs ø folds with short wavelengths on the western edge of the open folded zone. The folds show a plunge reversal from northwest to south and a steepening of the plunge with increasingly tighter foldsß From the southern part of the mine.

Structure and character of the greenschist contact The greenschist contact is a major zone of shear-

ing of variable width and complex history, separat- ing the greenschists and quartzites of the Eastern Creek Volcanics from the shales of the Mount Isa

Group. The overall shape of the contact is smooth and curved wherever post-D3 shearing or trans- verse faults are absent, although in detail it is highly complex. Broad subhorizontal plateaus--crosscut

by post-D3 faults--are bounded by steep slopes on the eastern and western boundaries (Fig. 7A-E). Hinges of curvatures in the subhorizontal domains lie within the Sa plane, indicating folding of the contact during Da. The surface expression of the greenschist contact, the Paroo fault, shows a well- developed Da fold (B in Fig. 2) with the same ver- gence as the Mount Isa fold.

Bedding in the shale sequence is generally con- stant in orientation, although toward the contact it rotates from steep to shallower west-dipping atti- tudes. This effect is particularly obvious in the southern mine area. Bedding has only been found in a few locations in the strongly deformed and altered greenschists. It had orientations parallel to the sub- vertical portion of the greenschist contact to the west, and east-dipping attitudes near the transition in orientation of this contact from subhorizontal to steeply east-dipping to the east (Fig. 7A and D). However, recent mine development has exposed a local steeply west-dipping and younging portion of quartzite within the greenschists (D. Larsen, pers. commun., 1987).

Several distinctive rock types are developed at different localities along the contact, particularly in the southern mine area where irregularly distrib- uted carbonaceous slaty shale (mine term is "car- bonaceous mylonite") bodies (cf. Mathias and Clark, 1975) occur between the greenschists and the copper ores. These bodies are either continuous with highly deformed shales to the footwall of the silica dolomite (Fig. 7A) or are discrete and fault bounded (Fig. 7B) but lithologically similar to the strongly cleaved Urquhart Shale. The continuous bodies are largely the result of post-D• rotation on the "buck quartz" fault (Figs. 7A, B, E, and 8); the fault-bounded blocks were most likely eraplaced during the formation of the greenschist contact (see Discussion). Sharp contacts between greenschist and strongly cleaved shale do occur (Fig. 9), com- monly with thin sericitic layers (up to several cm) parallel to them (J. Knights, pers. commun., 1984). These contacts are folded about S3 (Fig. 9). S] lies parallel to the layering in the sericite layers and is similarly overprinted by Sa.

Younger structures in the greenschist contact zone

The Sa cleavage generally intensifies within a zone above the greenschist. In this zone both bed- ding and Sa curve toward the contact (Fig. 7A, B), and S• is overprinted by a weak northwest-striking crenulation cleavage (S4). The carbonaceous slaty shale boundary commonly corresponds to the first appearance of S4. In most but not all domains, S4 is also curved toward the greenschist contact. This suggests that reactivation of S3 and perhaps S4 has occurred (Bell, 1986) due to post-Da and 94 shear

STRUCTURAL RELATIONS, MT. ISA, AUSTRALIA 75

v I I I

v I I

¾ • ,'"'" ¾ ¾ • v

¾ ¾

.' ' Cu ORE

• BEDDING

500 m

ß ---" FAULT

k k BUCK QUARTZ FAULT ZONE

j'] DOLOMITIC BRECCIA AND 1 v v CHLORITE SCHISTS AND QUARTZITES RECRYSTALLISED SHALE • SILICA - DOLOMITE • ) CARBONACEOUS SCHISTS AND MYLONITES •/ SILICEOUS BRECCIA J

FIG. 7. A. A cross section of the 4500mN section, showing ore distribution and bedding (drawn looking north). A is modified from Perkins, 1984.

zones or faults localized about the greenschist con- tact zone.

These post-Da and 04 structures include quartz- filled, smoothly curved shear zones, irregular brec- cia zones that truncate or rotate D4 crenulations, and complex kink folds with curved subhorizontal axial planes. One important example of these struc- tures is the buck quartz fault which follows the sub- horizontal contact between copper ore and the greenschists over a large area in the southern part of the mine (Fig. 7A). It consists of a body of white quartz, up to 15 m thick, with intercalated slices of carbonaceous slaty shale. Toward the east, it cuts upward through the Mount Isa Group, and at the surface it appears to join a moderately west-dipping

fault. Toward the west and north, it cuts down into the underlying Eastern Creek Volcanics (Fig. 8), where it loses the thick quartz filling. The relative displacement of the upper block is eastward with a net shift of a few hundred meters.

Another similar fault is the S48 fault (Fig. 8) which has a strike of 020 ø to 040 ø and a net hori- zontal shift of the northern block 200 m to the northeast. North of the S48 fault, the buck quartz fault appears to have a displacement of only 70 m (M. Klemm, pets. commun., 1984). A tear fault re- lationship across the S48 fault accounts for this change. The suggested fault block relationships are shown diagrammatically in Figure 8. These fault re- lationships postdate the ore.

76 BELL, PERKINS, AND SWAGER

/

/

500m

/ I

I /

•/• •/•

•//• 9/L

/ /

/ /

/ /

ß /,,.//'.• • ...'• I I • ./ "• ':'**'-,•;' ':'" / ,/ d"' /" 't' .M I..' /;7' ,x •/•,H,

,,:,' , ,: /•' I•11 ? x;.x/ I .4.' •/ x' • •.' ß/. 'Yt• I I

ß • ? I

LEGEND

•--- '-• SILICA DOLOMITE

------ S 2 CLEAVAGE

• S 3 CLEAVAGE AND SCHISTOSITY • GEOLOGICAL BOUNDARIES, FAULTS

BUCK QUARTZ FAULT ZONE

CHLORITE SCHISTS AND QUARTZITES

CARBONACEOUS SCHISTS AND MYLONITES

FIG. 7. (Cont.) (B). A cross section of the 4500mN section showing S2, S3, and their rotation, plus a D3 fold in the greenschist contact.

Origin of the Greenschist Contact

Any interpretation of the origin of the greenschist contact should be compatible with the following observations:

1. The contact is sigmoidal in shape, and where horizontally oriented, it truncates bedding in the shale sequence at a high angle; but where it steep- ens, it curves toward parallelism with the bedding (Fig. 7). This is demonstrated very well by the west- ern steep contact where the older Eastern Creek Volcanics overlie the younger Mount Isa Group sed- iments. Toward the east, drill core information shows that the contact curves back to a steeply east-dipping orientation. Deep drilling to the south of the mine also suggests that the contact steepens at depth.

2. The Mount Isa Group sediments dip uniformly toward the contact, independent of its orientation apart from the minor rotational effects of D3. Bed- ding in the greenschists is parallel or at a low angle to the contact. Between the Paroo fault and the

Mount Isa fault the greenstone sequence youngs consistently westward, i.e., in the same direction as the underlying Mount Isa Group.

3. The sediments are uniformly fine grained right up to the contact itself, and no conglomerates or synsedimentary breccias have been found.

4. Greenschist shale contacts and an early seri- citic foliation parallel to the contact are folded by D3. The sericitic bands are interpreted as ductile shear zones that have been extensively metasoma- tized (J. Knights, pers. commun., 1984). They lie at a high angle to S2 and suggest that ductile shearing

STRUCTUBAL RELATIONS, MT. ISA, AUSTRALIA 77

I vi I I

v I

v\ \

500m

ROCK OPEN CUT

LEGEND

Cu ORE V V Pb/Zn ORE BEDDING - POSITION ACCURATE

V BEODING - POSITION INFERRED

DOLOMITIC BRECCIA AND 1 v RECRYSTALLISED SHALE •, SILICA DOLOMITE s,.,CEOUS B.ECC,A j CHLORITE SCHISTS AND QUARTZITES

BASEMENT CONTACT SHEAR ZONE

•"• / / //

//

FIG. 7. (Cont.) C. The 5719mN section.

4/L

9/L

13/L

15/L

19/L

took place before 92, between two lithified se- quences.

The greenschist contact has been explained in the literature as a growth fault (Smith, 1969; Mathias and Clark, 1975) and as a spoon-shaped normal fault (Dunnet, 1976). If the contact developed as a growth fault with a vertical displacement of several kilometers in order to juxtapose Eastern Creek Vol- canics and Urquhart Shales, we would expect evi- dence for penecontemporaneous faulting within the Mount Isa Group sediments. The absence of such sedimentological evidence is a strong argument against growth faulting. Spoon-shaped normal faults, or growth faults, cannot explain the overall sigmoidal shape of the contact, unless complex later faults are invoked (Smith, 1969).

An alternative explanation for the greenschist contact, which accommodates the presently avail- able geometrical constraints, has been proposed based on the recently developed D• thrust hypoth- esis for the Mount Isa fold belt (Bell, 1983). Figure

10 shows a pre-D2, -93, and post-D3 faulting sche- matic east-west cross section, drawn normal to the direction of thrusting. This cross section was ob- tained by extending the thrust interpretation of Bell (1983) southward to Mount Isa. Figure i in Bell (1983) shows that the western lateral ramp to the duplex changes orientation in the southwestern corner of the map (i.e., 50 km north-northeast of Mount Isa) from parallel to oblique to the north- south direction of thrusting required to form the duplex. This obliquity would result in overturning of the lateral ramp as more horses formed within the duplex southward toward Mount Isa (Fig. 10).

The greenschist contact within the mine is inter- preted to equate directly with this overturned por- tion of the lateral ramp. On a macroscopic scale bedding in the overriding thrust would lie parallel to both the fiats and the lateral ramp as shown in Figure 10. Bedding in the greenschist sequence is parallel to the contact with Mount Isa Group rocks on the western side of the mine (Fig. 7A). However, on the eastern side, east-dippi'ng bedding in the

78

/ /

/

BELL, PERKINS, AND SWAGER

500m

t

MOUNT ISA FOLD

4/L

/ /

LEGEND

Cu ORE Pb/Zn ORE V BEODING - POSITION ACCURATE (ALSO SHOWN BY Pb/Zn)

BEDOING - POSITION INFERRED

DOLOMITIC BRECCIA AND 1

RECRYSTALLISED SHALL I SILICA DOLOMITE' SILICEOUS BRECCIA FAULTS ANO SHEAR ZONES

CHLORITE SCHISTS AND QUARTZITES

BASEMENT CONTACT SHEAR ZONE

9/L

I /

/ /

/ /

/

/•' /

/ /

/ /

13/L

15/L

19/L

/

/ / /•/ / \\ / / / /

/ ? /-,

/ ! /

!/

greenschists locally makes a 45 ø angle with the ramp (Fig. 7A) and recent development immedi- ately below the 1100 orebody has passed through silicified quartzite that dips 70 ø W and youngs westward (Fig. 7A, P. Larsen, pers. commun., 1987). The detailed distribution of bedding is un-

certain in the greenschist sequence below the mine since these rocks either never contained well-devel- oped bedding or have been too highly altered (e.g., silicified in the case of quartzites). The local 45 ø E and 70 ø W dips shown in Figure 7A could be the result of boudinage of the uppermost thrust sheet as

STRUCTURAL RELATIONS, MT. ISA, AUSTRALIA 79

FIG. 9. Photo showing the greenschist contact, with sericitic layering between chlorite schists (Haslingden Group) and slaty shales (Urquhart Shale-Mount Isa Group) folded by D3 with an S3 cleavage axial plane. Photo courtesy of J. Knights.

Accurately known surface

Inferred surface from model

Underlying surface I Aclual movemenl

Apparent moveme• FIG. 7. (Cont.) E. A three-dimensional sketch looking south

over 3,500 m of the mine. It shows the known (full lines) and interpreted (dashed lines) shape of the greenschist contact, the S48 fault, and a part of the buck quartz fault.

the imbricate formed below or it could be a product of the variable effects of folding during Da in the rocks on either side of the greenschist contact (see

FIG. 8. Simplified block diagram showing the postulated rela- tionship between the buck quartz andS48 faults. The movement direction is parallel to the fault intersection. In the northern do- main, most of the movement along the buck quartz is transferred to the S48 fault which acted as a tear fault.

below). Alternatively, two small hanging-wall lat- eral ramp irregularities were present in the overthrust greenschists.

Maeroseopieally, the movement directions that are possible during thrusting, considering only the local geology in the immediate vicinity of Mount Isa, range from north to south along a lateral ramp, to west to east over a frontal ramp. However, small- scale imbricate geometries seen within the mine preclude west to east thrusting (e.g., Fig. 5) and can only be interpreted in terms of north to south through to east to west thrusting. Hence, we inter- pret the thrusting to be north to south in accor- dance with Bell's (1983) interpretation of the large-scale duplex to the north of which Mount Isa is simply a southern extension. The detailed contact shape indicates that the ramp is not a planar feature but is slightly undulating due to the doubly plung- ing effects of Da folds and post-Da faults.

Thus, after Bell (1983), we interpret the [•ult contact between basic voltanits (greenschist) and Mount Isa Group sediments to be formed as a lateral ramp during D• and to have essentially reached present-day configuration after D•. The relation- ship of this ramp to the regional thrust geometry is discussed in detail by Bell (1983).

FIG. 10. Schematic cross section looking north showing the post-D• geometry through the eventual mine site which forms at the location marked with a "+."

8 0 BELL, PERKINS, AND SWAGER

Structural Controls on Syn-Da Copper Mineralization

Silica dolomite distribution

The silica dolomite distribution shows a north- northwest-south-southeast-striking, elongate pat- tern (Fig. 11) that is partially controlled by the sub- horizontal or shallowly dipping portion of the greenschist contact (Fig. 7E). Totally silicified shales within the major copper orebodies form the silica- and Cu-rich core of the silica dolomite along and close to the contact. Away and updip from it, zones of less silicified, less brecciated, and more dolomitic rock types occur, and the silica dolomite splits into a number of lobes more or less concor- dant with the shales.

The major body of silica dolomite along the steep east-dipping contact in the north (Fig. 7D) is a con- tinuation of a similar body to the south-southeast that developed along the subhorizontal contact. This northern silica dolomite lobe which contains the 650 Cu orebody, developed in a Da fold zone (Fig. 7D; Perkins, 1984). The localization of miner- alization and alteration within this fold zone pro- vides an explanation for the attitude of this silica dolomite body which is steeper than the transgres- sive to bedding. Subsidiary preferential control by Da short limbs of folds is also common within the 1100 and 1900 orebodies. Adjacent to the east on

the shallowly dipping portion of the greenschist contact (Fig. 7D), another silica dolomite body has formed.

Silica dolomite formation with chalcopyrite depo- sition took place during Da (Perkins, 1984; Swager, 1985), and thus postdated D1 and D2 rotation of the greenschist contact into its present position. This timing and the above spatial relationships suggest a structural control on the localization of the alter-

ation and mineralization processes.

Silica dolomite distribution and fluid flow Perkins (1984) and Swager (1985) interpreted

the highly siliceous core as the most intensely al- tered domain, implying that the major channels for the introduction of the hydrothermal fluids were along and directly above the greenschist contact. They argued that the more dolomitic rock types away from the contact and in the northern mine area represent replacement products from less sili- ceous fluids which had already interacted with un- derlying shales. This suggests that the fluid flowed from the south-southeast upward to the north- northeast (Perkins, 1984, p. 634).

Recent Pb isotope studies (Gulson et al., 1983) indicate that Cu sources apart from the Eastern Creek Volcanics (cf. Smith and Walker, 1971) were involved in providing copper ions for the deposit. This required large-scale fluid flow involving meta-

•• • •• • • •.s• -•>

• • 2ooo mE

LEGEND •u ORE •'• SILI• DOLOMITE b b b •RBON•EDUS MYLONITE P•/Zn ORE • • •EOLO•I•L BOUNDARY • BEDDIN• '•L• SNO•N BY PD/Zn ORE FAULT v • • BASEMENT •REENS•HIST • PLUNGE OF FOLD

FIG. 11. Plan of the 17 level showing how the Mount Isa fold and the silica dolomite and copper orebodies trend north-northeast. The plan also shows how a Ds fold in the greenschist contact is folded yet bedding interdigitating with the adjacent 1100 orebody is apparently not folded apart from the small-scale intrafolial Ds folds.

STRUCTURAL RELATIONS, MT. ISA, AUSTRALIA 81

morphic fluids (Perkins, 1984; Swager, 1985). Such fluids could have caused alteration of the Eastern Creek Volcanics below the contact at the same time

as the first replacement occurred in the dolomitic shales above the contact (Swager, unpub. data).

The greenschists below the contact are strongly altered (i.e., silicified) equivalents of the metaba- salts in the Eastern Creek Volcanics (Smith and Walker, 1971). Their metamorphic alteration and deformation history has been determined by Swager (unpub. data). He showed that they had al- tered during Dz and hence could not have acted as a source rock for copper deposited within the silica dolomite (cf. Smith and Walker, 1971; Gulson et al., 1983).

Interpretation of the role of deformation in fluid access and silica dolomite formation

Role of pre-Da deformations: Extension of the thrust interpretation of Bell (1983) southward to Mount Isa suggests that a lateral ramp to a major duplex formed during D]. This resulted in bedding in the Mount Isa Group rocks above the greenschist contact dipping 60 ø to 70 ø west whereas the gross distribution of bedding in the Haslingden Group rocks below dipped 40 ø to 50 ø eastward (Figs. 10 and 12). Dz produced an anticline to the east of the Mount Isa mine and a syncline to the west and hence the bulk shear couple that operated at the mine site was sinistral (looking north; Fig. 12). Therefore, when Dz commenced, the Sz cleavage formed at a low angle to bedding in Mount Isa Group rocks but at a high angle to bedding in Has- lingden Group rocks as shown in Figure 13A.

Bedding in the Mount Isa Group rocks above the greenschist contact had an orientation where reac- tivation could readily occur during D2 whereas this

FIG. 12. Schematic cross section looking north and showing the post-D2 geometry and thus the bulk rotation of the green- schist contact to west dipping that occurred at the eventual mine site (marked with a "+") during D2.

FIG. 13. A. Schematic cross section through the mine (looking north) showing the detailed bedding relationships across the greenschist contact at the start olD= time and the role of develop- ment of an axial plane S=. B. Same as A except that it shows the effects of localized reactivation of So. Reactivation causes rotation of S= and then the formation of extension veins perpendicular to bedding during D=. The opposed senses of shear across the green- schist contact may aid the formation of the extension veins in the shales above the contact.

was not the case in the rocks below. Hence, in some beds in the rocks above, reactivation by antithetic shearing on the bedding commenced as shown in Figure 13B. This caused rotation of S2 toward the bedding in those layers which had reactivated. It also generated a discordance in deformation parti- tioning across the greenschist contact which re- sulted in extension veins forming in the Mount Isa Group rocks above at a high angle to So and S2. Some of the fractures that contain these veins nu-

cleated from S] cleavage seams. However, as men- tioned above, the bulk movement of the rock during D2 folding involved a sinistral shear couple and as a result the greenstone contact was rotated into a shallow west-dipping orientation as shown in Fig- ure 12.

In summary, the combined effected D] and Dz resulted in a pre-Da geometry at the base of the mine whereby steeply west-dipping So in the dolo- mitic shales abutted shallowly west-dipping So across a shallow west-dipping fault as shown in Fig- ure 12 and 14A. The totally different bedding ori- entations on either side of this shallow-dipping por- tion of the fault (the greenschist contact) and the ductility contrast between greenschists and dolo- mitic Urquhart Shales meant that the greenschist contact formed a major structural discordance, and thus, a plane of weakness along which fracturing and fluid access could occur during Da.

Role ofDa: In the mine vicinity, a number of fold zones (the largest of which is called the Mount Isa fold) were produced during Da on the limb between the large-scale Dz anticline to the east and the syn- cline to the west. The bulk movement involved in the initiation of each zone was dextral shear in cross

sections drawn looking north (Fig. 14B), but it was soon dominated by bulk inhomogeneous shortening and Sa cleavage formed axial planar to the Da folds on the long as well as the short limbs. As deforma-

8 9, BELL, PERKINS, AND SWAGER

FIG. 14. A series of schematic diagrams showing the cross- sectional geometry looking north through the mine from pre- to post-Ds. A. The post-D2 geometry. Compare it with Figure 12. B. The early stages of Ds when dextral shear produces the short limbs of the Ds folds in the mine. C. The effects of the commence- ment of reactivation of bedding and unfolding of the D3 folds in some locations and the tightening in others during Da. D. The formation of intrafolial folds late in D3 as reactivation of bedding continues, and the commencement of reactivation of the short limbs of Ds folds which sets up opposing senses of shear across the greenschist contact at some Ds hinges. Note how the greenschist contact has been progressively rotated clockwise from A to D.

tion continued, the pattern of deformation parti- tioning shifted such that reactivation of the bedding and hence antithetic shear commenced on the long limb (Fig. 14C). Below the greenschist contact shearing remained synthetic relative to the devel- oping Ds fold. However, as reactivation of the bed- ding continued, unfolding of the Ds folds occurred in zones of reactivation (Fig. 14C). Reactivation of the bedding commenced on the short limbs of Ds folds, both above and below the greenschist con- tact, when these had been rotated sufficiently (Fig. 14D), even though in zones of reactivation of the long limb these folds were completely unfolded (Fig. 14D).

Unfolding due to reactivation (Bell, 1986) thus formed intrafolial (i.e., the folds die out along their axial planes) Ds folds on a variety of scales in the Mount Isa Group rocks (Figs. 9,, 11, and 14). The major mass of the 1100 orebody lies adjacent to a large Ds fold in the greenschist contact which shows up in plan view (Fig. 11, location 5000mN 1700mE) but not in section (Fig. 7A) although it is readily visible 1,9,19 m to the north (Fig. 7C). Bed- ding in Mount Isa Group rocks, which interdigitates with the 1100 orebody immediately adjacent to this fold in the greenschist contact, is unfolded in both plan (Fig. 11) and section (Fig. 7A and C; see also fig. 18 in Perkins, 1984) except for small-scale in- trafolial Ds folds with subhorizontal to shallow east- dipping short limbs and wavelengths of generally less than 5 m (Fig. 11). Figure 15 shows the pro-

gressive effects of reactivation during Ds that ex- plain these relationships. The preservation of a dis- tinct Ds fold in plan view is due to the pre-Ds change in orientation of the greenschist contact from shallow to steep at this location. The 3000 and 3500 orebodies can be related to Ds fold effects in a similar manner. The 9,00, 500, 650, and 1900 ore- bodies are located on and extend away from Ds folds (Fig. 7C and D) and hence all the major silica dolomite-hosted orebodies have a spatial relation- ship to Ds structures.

We infer that reactivation of the bedding above, but not below, the greenschist contact on one limb, combined with the competency changes and Ds ex- tension at a high angle to it, led to brittle failure and localized opening of the greenschist contact; this caused rapid reductions in fluid pressure from PF-PI• (Fig. 16A). Locally, opening of the greenschist con- tact promoted opening of folded bedding close by that was in the right orientation to gape during reactivation (Fig. 16A). We do not know the di- mensions of these openings. If after failure the de- gree of replacement of the shales during the forma- tion of the silica dolomite had not been so great, then the space created could potentially have been calculated from void space associated with breccia- tion. However, Perkins (1984) has shown by using marker beds that there is no overall stratigraphic increase in the thickness perpendicular to bedding in domains of silica dolomite formation. Hence, we would expect that only a relatively small amount of void space was created during the fracturing event which allowed the silica dolomite to form.

We interpret that opening of the greenschist con- tact rapidly dropped the fluid pressure from PF-Pi•

FIG. 15. A series of schematic cross sections looking north through the eventual site of the 1100 orebody. The series shows the progressive role of reactivation during Ds in generating the cross-sectional geometry shown in Figure 7A in relationship to the plan view geometry shown in Figure 11.

STRUCTURAL RELATIONS, MT. ISA, AUSTRALIA 83

/ L/ / L/ //•

FIG. 16. A series of schematic cross sections of a portion of Figure 14. The cross sections show how gaping on the greenschist contact occurs due to the combined effects during Da of the competency contrast across it, the stretching direction perpen- dicular to it, and the antithetic shearing locally above but not below it. A. Cross section showing the opposed senses of shear resulting from the reactivation of the short and long limbs of D3 folds above, but only of the short limb below, and the resulting opening of the greenschist contact. B. Cross section showing that fractures form preferentially in the dolomitic shales above the greenschist contact because of the orientation of bedding in these rocks relative to the rocks below. C. Cross section showing that explosion of the fractured rock into the openings on the green- schist contact prevented closure of the space created. This allows the fluids to cause alteration and replacement of the country rock.

and resulted in fracturing and local brecciation of the dolomitic rocks back onto the contact as the

high fluid pressure in the wall rocks caused fluid to vent into the space created (Fig. 16B). The degree of fracturing in the greenschists below the contact was only minor because bedding development in these rocks is very weak and at an orientation that did not allow ready propagation of fractures away from the contact. However, they were silicified during D3, as well as during D2, up to some 30 m from the greenschist contact (Swager, unpub. data; D. Larsen, pers. commun., 1987). This fracturing would have abruptly increased the local permeabil- ity in the rocks above the contact, allowing further smaller scale rapid fluid flux and associated breccia- tion, until bulk disequilibrium was sufficiently re- duced.

The scale of fracturing decreased rapidly away from the greenschist contact, as the space created on the contact was filled by fluid, jostled blocks of dolomitic siltstone and breccia, and was accommo- dated in the fault surroundings as smaller voids be- tween fragments. This generated an upward closing zone of fractured rock (Fig. 16C). The void space created on jostled block boundaries could not be closed by ductile deformation because this would have required impossibly high strain rates. Hence the void space remained open to fluid flow even after the pressure had reequilibrated. This provided north-northwest-south-southeast-striking channels, adjacent to the greenschist contact, for the rapid influx of the fluids into the chemically reactive do- lomitic shales.

The commonly strata-bound nature of the silica dolomite lobes indicates that bedding was a struc-

tural anisotropy along which, but not across which, fluid flow could easily proceed. The silica dolomite lobes interdigitating with the Pb-Zn beds are an il- lustration of this. The shales in between the galena +_ sphalerite layers were widely fractured and veined after the first two deformation events.

Joints, veins, and bedding-parallel fractures in the shale layers provided effective pathways for the fluids, with the ductile sulfide beds acting as imper- meable barriers. For example, D2 veins on Da fold limbs also lay at a high angle to La a, and were thus in a favorable position for reopening during the syn-Da alteration process (Swager, 1985).

In summary, we suggest that during Da the sub- horizontal portions of the greenschist contact were domains of structural weakness lying at a high angle to the maximum extension direction and that they played a crucial role in the initial localization of the interaction of the hydrothermal fluids with the do- lomitic shales. The overall bedding anisotropy, and other preexisting structures such as the north- south-striking Dg veins, would favor fluid migration along the bedding-vein intersections and thus in a north-south direction. Fluid migration trends across the layering are mainly controlled by the position of Da fold zones but are also affected by fracturing along the boundaries of the north-south-striking Dg veins as well as by variations in fracture and vein intensity.

Of special significance to the genesis of the cop- per orebodies at Mount Isa and hence exploration for other large copper deposits is the variation in deformation intensity of Da throughout the western Mount Isa inlier. The spacing of zones of intense Da is on the order of 10 kms or more. The significance of this is shown in Figure 17. If a regional deforma- tion is manifested as local zones of intense deforma-

tion at the earth's surface, it is likely that it will be weaker but more penetratively distributed at depth because the higher temperatures and pressures deeper in the crust would enhance ductility. Defor- mation partitioning would occur on a finer scale as a result of this increased ductility generating more penetratively distributed zones of shearing strain and hence dissolution and cleavage formation (Bell et al., 1986). At higher levels in the crust, decreas- ing ductility results in coarsening of the scale of deformation partitioning and the penetrative distri- bution of deformation at depth changes upward into zones of deformation partitioning of ever-increasing scale as shown in Figure 17. The net result is chan- neling of fluids from throughout the crust at depth into narrow zones of intense deformation as they move toward the earth's surface. The volume of

fluid passing through a narrow zone of intense de- formation nearer the earth's surface becomes a function of the width between the zones.

84 BELL, PERKINS, AND SWAGER

0 5km

I ," ,',, // r ,, ,, , ,,

•IO. ]2. Schematic cross sectio• o[ t•e crust ]oo•i•E •ort• throuE• t•e mi•e s•owi•E t•e Eeometry prior to D3 but wit• t•e patter• o[ ae[ormatio• partitio•i•E auri•E D• superimposed o• it. T•e s[etc• fiJustrates t•e importance oltre JocaJizea o[i•te•se zones o[D• at t•e surGce [or [ocusi•E a JarEe voJume •uia (arrows) [ormea i• more pe•etrativeJy but less roc[s at aept• t•rouEh t•e structuraJ trap created o• t•e sc•ist co•tact.

Continuing research at Mount Isa is aimed at studying the origin of the fluids, their pathways, and their interaction with both Haslingden and Mount Isa Group rocks.

Conclusions

1. The three major regional deformation events recognized in the Mount Isa district (Winsor, 1982, 1983, 1986; Bell, 1983) are also distinguished in the Mount Isa mine area. The greenschist contact with the Mount Isa Group crops out as the Paroo fault and is interpreted after Bell (1983) as a lateral ramp formed by southward thrust imbrication dur- ing D1. D• was responsible for most of the tilting of the sequence into a steeply west-dipping orienta- tion. Da resulted in a number of north-northwest- striking fold zones. Later events, which do not influ- ence the overall rock distribution, are concentrated along the greenschist contact.

2. The greenschist contact nearly reached its present position during D•, and consequently, lay at a very high angle to the direction of maximum ex- tension during Da. The resulting tensional forces across the greenschist-shale contact favored tensile fracturing along the subhorizontal contact. Tensile fracturing also occurred preferentially within the steeply dipping shales due to the presence of veins that formed during D•, perpendicular to bedding.

The greenschist contact zone thus exerted a domi- nant structural control on the initial pathways for the hydrothermal fluids which then formed--dur- ing Da--the silica dolomite and its copper ore- bodies.

3. If metamorphic fluids and structural controls on the migration of these fluids played the impor- tant role suggested by us, then structural analyses are the first requirement for the exploration for other (concealed) copper deposits in the Mount Isa district.

Acknowledgments We would like to thank Mount Isa Mines, Ltd.,

and James Cook University for support during the course of our studies. In particular, we are grateful for the assistance and contributions of Rennie Blair, John Knights, Maree Klemm, Ken Mills, Adelheid Weniger, and Colin Winsor. Mine plans and sec- tions were prepared by the Mount Isa Mines geol- ogy drafting department. Large-scale interpreta- tions are based on many years of mapping and core logging by past and present mine geologists. An early review by Jacob de Roo was greatly appre- ciated.

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