Economic Geology Vol. 93, 1998, pp. 1369-1389

Multiple Deformations with Successive Subvertical and Subhorizontal Axial Planes in the Mount Isa Region: Their Impact on

Geometric Development and Significance for Mineralization and Exploration T H. BELL • AND K. A. HICKEY

Department of Earth Sciences, James Cook University, Townsville, Queensland 4811, Australia

Abstract

The first orogeny to affect most rocks exposed in the Mount Isa province caused significant repetition of stratigraphy through north-south shortening. Geological maps are dominated, however, by younger folds with subvertical, north-south-striking axial planes. These folds were produced by east-west shortening during a sec- ond orogeny. Both orogenies produced several foliations. However, some foliations have negligible geometric consequences, allowing the geometry of the region to be described generally in terms of just the three defor- mations, D1, D2, and Ds, which produced macroscopic folds, and the younger deformations, which produced brittle faults.

One of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization. That role has been identified fol- lowing advances in conceptual understanding of fold development and associated foliation. The deformation, which occurred between the D,z and D s events, produced geometric effects that were masked by overprinting of north-south-trending D2 folds by similarly trending D s folds. Significantly, these Da folds, which control the localization or upgrading of much mineralization in the Mount Isa province, did not form in rocks unaffected by this intermediate event because the near-parallelism of S,z and Ss in such rocks resulted in intensification of S,z rather than development of a new Sa.

The intermediate deformation event, called Da.5, locally produced highly asymmetric deflections in bedding and Sa, with subhorizontal axial planes that were subsequently overprinted by Ds folds and an Ss cleavage. No mesoscopic foliation was produced during Da.s, and its rotational effects on bedding were commonly subsumed within younger Ds structures. Thus D s disguises the presence of D,z.s, except in relatively rare locations where D.z folds were isoclinal, such as in the Tommy Creek block and on the northern end of the Snake Creek anti- cline southeast of Cloncurry, and northwest of Mount Isa in these three locations. In the latter locations, both limbs were affected, resulting in large-scale overturning of bedding on one limb prior to overprinting by Ds.

The asymmetric folds developed during Da.s created sites where bedding could be pulled apart during Da, resulting in implosion because of the high fluid pressure in the surrounding country rock, with brecciation, jostling, fluid access, and mineralization. Large-scale zones of mineralization form where D2.s was localized at an earlier formed heterogeneity, such as a thrust surface (e.g., at Mount Isa mine) or a granite-country rock contact. In these locations, early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during D s. This role of D2 s in the generation of D s allows recognition of regional zones for exploration that previously have received little attention.

Recognition of the role of D2.5 reveals a eyelieity in successive subvertical and subhorizontal events, not rec- ognized by most workers in this terrain, that may occur in other orogens around the world but has similarly gone unnoticed. After the first orogeny finished, a succession of three near-vertical and two subhorizontal events occurred during the east-west-shortening orogeny at Mount Isa. These were succeeded by younger kinks with east-west- to northeast-southwest-trending axial planes that appear to be the youngest ductile de- formation to have affected the Mount Isa province.

Introduction

SOME OROGENIC terrains are dominated by shallowly dipping foliations (e.g., the Scandinavian Caledonides, the European Alps, and the Otago Schists: Trouw, 1973; Dieterich and Casey, 1989; and Johnson, 1990; respectively), whereas others, such as the Mount Isa province (Fig. 1), are domi- nated by steeply dipping foliations. Yet, in terranes dominated by shalloMy dipping foliations, evidence for steeply dipping foliations is commonly preserved within porphyroblasts and in strain-shadow regions adjacent to deformed quartz veins (Johnson, 1990; Bell et al., 1992; Hayward, 1992; Aerden, 1994, 1995; Jones, 1994). Similar evidence for shallowly dip- ping foliations is commonly preserved in terranes dominated

Corresponding author: email, Tim. Bell@jcu.edu.au

by steeply dipping foliations (e.g., Steinhardt, 1989; Bell et al., 1992; Davis, 1993, 1994, 1995; Davis and Forde, 1994).

What are the geometric effects of the overprinting of suc- cessive orthogonal events at outcrop and map scale in areas dominated by vertical, or alternatively, horizontal foliations? Does such a succession resolve the problem of fold develop- ment in terrains where successive generations of folds have very similarly oriented axial planes? Resolution of these ques- tions is particularly significant for the Mount Isa province because this region contains a number of structurally con- trolled world-class orebodies of copper (Mount Isa: Perkins, 1984; Swager, 1985; van Dijk, 1991), Ag-Pb-Zn (Mount Isa: Perkins, 1997; Hilton: Myers et al., 1996; Dugald River: Xu, 1996, 1997) and gold (Eloise: Baker and Laing, 1998; Osborne: Adshead, 1995; Ernest Henry: Craske, 1995) that developed

0361-0128/98/2032/1369-21 $6.00 1369

1370 BELL AND HICKEY

50 km

Dugaid River Pb-Zn-Ag

Ernest Henry ß Cu-Au

ß Eloise Cu-Au

Fairmile

Pb-Zn-Ag

Western Fold Belt Eastern Fold Belt Wonga-Duchess Belt

Cannington Pb-Zn-Ag

Cu-Au ß Osborne

Tommy Creek region

Snake Creek Anticline

Fro. 1. Map showing location and broad geological division of the Mount Isa province, the Wonga-Duchess belt, the Tommy Creek region and the Snake Creek anticline region as well as several ore deposits.

during deformation and metamorphism (Fig. 1). The cur- rently exposed, near-surface geological relationships in this province do not necessarily reflect those developed at mine- able depths, as was demonstrated by Bell (1991) for the Leichhardt Valley, which contains both the Mount Isa and Hilton mines. This paper illustrates an even more dramatic example for an area immediately northwest of the Mount Isa mine, which has great implications for exploration both here and throughout the Mount Isa province. The increased un- derstanding of the structural development of this terrain con- siderably aids in the identification of zones where there is an increased potential for structurally controlled mineralization that has no surface expression but is at mineable depth.

The Mount Isa province of northern Australia contains greenschist to upper amphibolite grade rocks that underwent a complex deformation history from -1610 to 1510 Ma (Page and Bell, 1986), involving three main macroscopic deforma- tions (Swager, 1985; Page and Bell, 1986; Winsor, 1986; Loosveld, 1989; Valenta, 1994). D• produced north-south- directed thrusting in the western half of the fold belt (Bell, 1983, 1991; Loosveld and Schreurs, 1987) and possibly

south-north-directed thrusting in the portion to the east of the Wonga-Duchess belt at 1610 Ma (Fig. 1; Page and Bell, 1986; Loosveld, pers. commun., 1989; Bell et al., 1992; Lally, 1996, 1997). Ds produced east-west shortening and regional- scale folds with north-south-striking, near-vertical axial planes at 1544 Ma (Page and Bell, 1986). Do produced smaller-scale and more locally developed, approximately north-south- trending folds with near-vertical axial planes at 1510 Ma (Page and Bell, 1986), and appears to have been associated with much of the copper mineralization in the Mount Isa area (Perkins, 1984; Swager, 1985; Bell et al., 1988; van Dijk, 1991). Detailed microstructural work on these rocks by Bell et al. (1988), TH. Bell (1991, unpub. data), and Perkins (1997) has long revealed the presence of more foliations, par- ticularly subhorizontal ones, preserved within porphyroblasts and around heterogeneities, than generally can be observed in the field. This paper assesses the effect of one event, which produced structures with subhorizontal axial planes, on the geometric development of the predominantly steep-dipping structural grain of the Mount Isa province, and its implica- tions for mineral exploration.

The North-South-Shortening Orogeny D1 Evidence for successive orthogonal foliations, developed

during the early north-south-shortening orogeny commonly re- ferred to as Dr, is preserved in the vicinity of Mount Isa (Win- sor, 1986). The generally east-west-trending, near-vertical fo- liarion recognized by Winsor (1986) and foliation parallel to bedding with a north-south-trending (on average) mineral elongation lineation (Bell, 1991) are the most common ex- pressions of the D1 orogeny in the Mount Isa region. The lat- ter foliation and lineation are locally preserved to the north- west of Mount Isa as described in the rocks immediately west of the Mount Isa fault (Bell, 1991). A succession of orthogo- nal foliations formed during this D1 orogeny has also been preserved in the rocks adjacent to the Wonga-Duchess belt (Bell et al., 1992), as well as near Fairmile (location shown on Fig. 1; Mares, 1996, 1998) and west and southeast of Clon- curry (Lally, 1997), and is not dealt with further here.

The East-West-Shortening Orogeny This paper concentrates on the deformations and foliations

that developed during east-west shortening when two gener- ations of steeply dipping, roughly north-south-trending, re- gionally developed foliations and axial planes of folds were formed. These structures, which are generally referred to as resulting from Ds and Do, dominate the geometry of much of the Mount Isa inlier and control the overall grain of these rocks. Bell (1991) mentioned additional foliations that formed before, between, and after Ds and Do, both within the Mount Isa mine and in surrounding regions. He did not give these the status of distinct and separate deformation events be- cause they appeared to be developed microscopically only, with little geometric significance at a larger scale. However, the meso- and macroscopic expression of a deformation with a subhorizontal axial plane that formed between Ds and Do (Bell, 1991, p. 1619) has since become apparent from de- tailed mapping between Mount Isa mine and the Hilton mine to the north (Fig. 2: area labeled Fig. 3). Recent map- ping to the east in the Tommy Creek block (Lally, 1996,

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1371

(a) , + + + +:;••+ + 5km ,. ,+++++++.,

r• N,,+++++++++•l• ¾ ' ' N •-.,+++++++++% •,' ß

ß •'+ ++ + ++ ++ +'A \'' c •.+++++++++• \:- •-++++++++++% •'

•.++++++++++• •' • w•'•',.+ + + + + + •i •' • •++• .....

C d•.• HI ton mine •! I:ig o•"

Cainozoic Mica Creek Pegmatite

Mt Isa Group Sybella Granite

Haslingden Group

fault

S 2 trace D 3 fold

(b) o 2kin

Judenan Beds and Myally Subgroup

Eastern Creek Volcanics

Mount Guide Quartzite and

May Downs Gneiss

F•c. 2. a. Regional map of the eastern margin of the Sybella pluton to the west of Mount Isa showing the approximate trace of S2, determined from reconnaissance mapping, and the effects of D2.5 and Da. The location of susbequent more de- tailed maps is also shown. b. Detail from the regional map of Mount Isa showing the V-shaped bend of bedding adjacent to the Sybella Granite for the area mapped in detail in Figure 3.

1997), Snake Creek anticline (Holmes, 1992; New, 1993; Lally, 1997; K. Lewthwaite, pers. commun., 1997), Starra re- gion (Adshead-Bell, 1998), Monakoff, Marcnan, and Mount Freda (B.K. Davis, pers. commun., 1997), and Fairmile re- gion (Mares, 1996, 1998) has revealed a similar deformation succession.

D2

This deformation produced the bulk of the regional folds with steeply dipping north-south-trending axial planes that

dominate all geological maps of the Mount Isa province (e.g., the region between the Mount Gordon and Mount Isa faults; Fig. 4a) and occurred around 1544 Ma (Page and Bell, 1986). The axial plane foliation, S.2, which is the dominant regional schistosity, has a generally steeply south-pitching stretching lineation L• developed within it. Ds

This deformation generally produced more localized folds with steeply dipping approximately north-south-striking axial

1372 BELL AND HICKEY

+++++++++

++++++.

42

72;, •

TN

0 1 km

bedding upward and downward facing bedding

synform F 3 axial trace

Sybella Granite

fault Haslingden

faulted F 2 fold hinge Group Meerenurker Thrust

.• antiform -" F 2 .,. axial trace

f• Magazine Shales Mount Isa undifferentiated Group Moondarra Siltstone

•--• Myally Subgroup '• quartzites in Pickwick Metabasalt • Eastern Creek Volcanics ,,•-• Mount Guide Quartzite '• May Downs Gneiss

Ftc. 3a. Geological map showing bedding and younging measurements extending from just east of the Mount Isa fault to the Sybella Granite between Mount Isa and Hilton mines (location of map shown in Fig. 2).

planes (Fig. 3b). In the vicinity of Mount Isa these folds tend to have axial planes striking slightly west of north. However, they can vary considerably in orientation where they are close to curving granite margins because of the competency of these feldspar-rich igneous rocks (e.g., northwest portion of Fig.

3b). The axial plane foliation Ss is the only locally developed Ds strain commonly being accommodated by the reactiva- tion of the preexisting S,2 foliation. Where present, Ss can form a pervasive fabric containing a steeply south-pitching stretching lineation. This deformation occurred around

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1373

TN

o 1 km

S 2 .,. -- •' F 2 axial trace

S 2 with direction to synform ./. F 3 axial trace

interpreted bedding trace / fault antiform • faulted fold hinge

synform •• Ueerenurker Thrust F•c. 3b. Map of S2 measurements and direction to the synforms.

1510 Ma (Page and Bell, 1986). To the south of Cloncurry a fault/shear mosaic associated with mineralization at Stuart, Mount Dore and Kuridala is potentially of D3 age (W.P. Laing, pers. commun., 1997), but similar structures at Starra have been timed as D5 (Adshead-Bell, 1998).

D5

This deformation is generally very weak and only locally forms crenulations or a weakly developed crenulation cleavage. For example, only 11 measurements of S5 are

1374 BELL AND HICKEY

7715000 "•

TN

o lkm

6.? S3

S2,5

interpreted bedding trace

antiform

synform

F 2 axial trace

F 3 axial trace

fault faulted fold hinge

Meerenurker Thrust

FIe. 3c. Map of S3 and S•.• measure•nents.

shown on Figure 3d where this structure is oriented more northwest-southeast than north-south. Where the deforma-

tion is present in higher metamorphic-grade rocks west of the Mount Isa fault, it is associated with local retrogression. It has no large-scale geometric effects that we have been able to rec- ognize but is of interest because it extends the cycle of

successive, orthogonally overprinting deformations and is responsible for retrogression northwest of Mount Isa. D5 is a more significant deformation event to the east, however, where it is associated with Au mineralization at Starra (Ad- shead-Bell, 1998), andalusite growth in the Tommy Creek block (Lally, 1997), and staurolite and andalusite growth in

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT ISA REGION 1375

, ,,,/ //

TN

•. • 8 5 Meerenurker Thrust •. -- "' F 2 axial trace

L2 ø / ;'" ....... ' interpreted bedding trace • F 3 axial trace

•, L3 0 ...... • ..... antiform fault , •

•'% L• •....' synform ..•/•"• faulted fold hinge

FIG. 3d. Map of L•, L•, L•, and S5 measurements.

the Gilded Rose region north of the Snake Creek anticline (K. Lewthwaite, pers. commun., 1997).

The intervening horizontal deformations A deformation D2..5 that generated structures with a sub-

horizontal axial plane occurred between D2. and Da. The

effects of this event were first recognized in 1986 as subhori- zontal crenulations preserved on a microscopic scale within cordieritc porphyroblasts west of the Mount Isa fault (Fig. 4) and against zones of silicification and dolomitization within Mount Isa mine (Perkins, 1997). This deformation was sub- sequently observed in porphyroblast strain shadows (Fig. 5a,

1376 BELL AND HICKEY

A

zone of D2. 5 rotation of S O - S 2

!

Mount Isa

Group

Haslingden Group

4km

i

Eastern Creek Volcanics

Magazine Shales

undifferentiated

Moondarra Siltstone

quartzites within the Pickwick Metabasalt

bedding F 2 axial trace F 3 axial trace

fault faulted D 2 fold hinge Meerenurker Thrust

F]c. 3e. Cross section through A-A' on Figure 3a. Note the zone of strong rotation of S2 by D2.5.

b) in rocks from approximately 50 km southeast of Mount Isa (Whitelock, 1989), as well as from the Snake Creek anticline south-southeast of Cloncurry (Holmes, 1992; New, 1993), where the effects of younger Ds shear are low. The meso- and macroscopic effects of this deformation were first recognized in the area northwest of Mount Isa (Fig. 3a). However, we have since found mesoscopic (Fig. 5c) and, in places, macro- scopic evidence for this event within Mount Isa and Hilton mines (north end of drive to George Fisher mine), and be- tween them in an area to the north of Spear Creek (Fig. 6). Deformation was primarily taken up by rotation, reactivation,

and subsequent intensification of the existing S2 fabric (el. Bell, 1986; Davis and Forde, 1994; Davis, 1995). Actual foli- ations produced by this event, as distinct from rotated S2, have not been observed mesoscopically and are only pre- served microstructurally as shown in Figures 4 and 5. Large- scale folds generated by this deformation have been mapped recently to the east (Fig. 1) in the Snake Creek anticline (New, 1993) and the Tommy Creek block (Lally, 1996, 1997) and smaller-scale structures at Dugaid River (Xu, 1996, 1997), Fairmile (Mares, 1996, 1998), and east of Starra (Adshead-Bell, 1998). In this study we have termed the event

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1377

030/53o-----•1

FIG. 4. a. Sl inclusion trails in cordierite porphyroblasts from south west of Mount Isa on the northern edge of the Queen Elizabeth pluton. Boxed outlines mark areas enlarged. Plane polarized light. Width of base 9 mm. b. Sinistral rotation of Sl due to D2.5 (from moderate northeast to shallow southwest pitch at top of photo) plus rotation due to D3 on vertical por- phyroblast rim (more obvious in a). Width of base, 2.4 mm. c. Dextral D2 crennlation of S] preserved within porphyroblast. Width of base, 0.88 mm. d. Sinistral D•.5 crennlation of S]. This crennlation kinks the S• seam in c. Width of base, 2.12 mm.

1378 BELL AND HICKEY

1 090

,,,--. "' and .

..•

t ,;' •' ',,, •.- ',• ""... '" ' . (:-':... i...-: %,.:, .,.•.:- .... .. ,,?.,..., •,. ........ , ,,, -..;.- .-/I½.'. ',•, .... X•.- - ,',,, ,

i';:.t-:•i * ,, * :' ; • ..: •. ."-'-.'. . • .

.•:•.. ' . . .. ::.....,.. . ...... ::...*;..., ,,' .-:':.; ., -, ?. ,..,..,,-.....-*........,. ..... ..• ; .•,. .... .. . .....

FIC. 5. a. Garnet and andalusite porphyroblasts from Snake Creek anticline southwest of Cloncurry. Vertical thin section striking west-east. Horizontal and way up marked by single barbed arrow. S2, the main matrix foliation and S2.5, a differenti- ated crenulation cleavage preserved against the porphyroblasts are marked. Plane polarized light. Width of base is 6 min. b. Garnet porphyroblasts from the Snake Creek anticline showing steep S2 inclusion trails rotated clockwise by D•.5. Sa is pre- served in the matrix and developed during dextral shear. Section is vertical and marked same as a. Partially polarized light. Width of base is 7 min. c. Field photo of 2 D•.s folds and a Da fold. Scale shown by marker pen 13 cm long. Strike on rock face trends east-west. Photo taken looking south.

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1379

Spear Siltstone

87_j

0 200m

W

8•_.// bedding

Kennedy Siltstone

/ -/ uhart Shale

trace of bedding direction of younging

fault

.., / F2. 5 axial trace

,• representative subvertical S 3

/ trace of S 3

Kennedy .../,/, i / J

F[c. 6. Map and cross section of a macroscopic D•.5 st•cture l•ng approximately h•ay be•een Mount Isa and Hilton mines (location sho• in Fig. 2), to the noah of Spear Creek, •thin Upper Mount Isa Group rocks to the east of the Mount Isa fault.

D2.5 to emphasize its significance and to preserve the previous structural nomenclature used for the region. This deforma- tion and its significance for the geometric development of the Mount Isa province is described and discussed in detail below.

D2.s development west of the Mount Gordon fault

Figure 3a is a geological map of a region lying across the Mount Isa fault between Mount Isa and Hilton mines. The

western half of this map contains a large-scale structure that,

1380 BELL AND HICKEY

although folded by younger deformations, has a subhorizon- a) tal axial plane-form surface and was produced by Dzs. This structure has a large impact in cross section (Fig. 3e), yet on regional maps has simply resulted in a large-scale V-shaped bend in the bedding that looks only slightly anomalous (Fig. 2b); for example, it could easily be rationalized as resulting from bedding anastomosing around the Sybella Granite mar- gins. Resolution of the geometry of this fold has allowed us to understand the manner in which Ds folds develop both here and in other parts of the Mount Isa province.

Figure 3e reveals the dramatic rotation of the axial plane of a Ds syncline into a subhorizontal orientation by a large-scale Ds.s structure in the western half of the area. Bedding and crosscutting Ss on the western limb of this syncline were both rotated by the top-to-the-east shear effects of Dzs so that the beds were downward facing prior to being overprinted by This Dzs, overturned Ds syncline forms the eastern hinge of [9) a large-scale antieline-syndine fold couplet. D s refolding of the antielinal portion of the couplet after rotation by Ds.s is apparent in the northwest portion of the map shown in Fig- ure 3a. The progressive development of the cross-section geometry of this map, including the D s anticline to the west, is shown in Figure 7. Tight Ds folds with near-vertical axial planes were formed initially (Fig. 7a). These were then ro- tated to a shallow dip by the top-to-the-east shear operating in this location on the subhorizontal Ds. s axial plane (Fig. 7b). This provided a large expanse of shallowly dipping bedding and Ss that enabled the overprinting effects of the subsequent

steep Ds event to be revealed, as shown in Figure 7c. Where O) there was no rotation of Ss by Dzs, the effects of Ds cannot be distinguished from those of Ds, since Ss and Ss formed as sub- parallel foliations, such as in the area to the east. This was jux- taposed against the Dzs-affected region to the west by later faulting that uplifted the region of shallow-dipping Ss to the same erosional level (Fig. 7d).

Thus, the majority of mesoseopie and macroscopic folds to the west of the Mount Gordon fault in Figure 3a are Ds struc- tures. To the east of this fault, where the rocks lay at a higher structural level during Dzs time and were unaffected by the large-scale Fs.s strneture, the north-south-trending folds are Ds in age, commonly with prominent faults developed along their hinges (Fig. 3a, e). During Ds, these folds were gener- ally tightened without development of a separate Ss foliation. d) D2.5 development in a mineralized area east of the Mount Isa fault

Figure 2 shows the location of Figure 6 (between Mount Isa and Hilton mines) where a much smaller but still macro- scopic Dzs structure is present (Fig. 6). It also generated a V-shaped bend in the bedding on regional maps, yet is a very significant structure in cross section (Fig. 6). Bedding forms a tight asymmetric fold within an otherwise steeply west- dipping and west-younging sequence. Mirroring the geometry described in the previous section, bedding and Ss on the east- ern limb of a Ds syncline were rotated top-to-the-west by the effects of Dzs, so that the rocks were locally overturned prior to overprinting by Ds (the axial plane of the Ds.s fold, although folded by the effects of Ds, has a shallowly dipping form surface). Consequently, Ss in these regions is structurally downward facing on So and this is shown in Figure 8. This

S 2

I

I

•• 8ø S 2

S 3

Mt Gordon Fault Mt Isa Fault

FIG. 7. Schematic cross sections showing the progressive development of cross section Figure 3e. a. Bedding deformed by D• folds. b. The effects of a large-scale Dz5 structure. e. Ds folds overprinting the geometry shown in b. d. The effects of west side up displacement on the Mount Gordon and Mount Isa faults. Correlation of stratigraphy across the Mount Gordon and Mount Isa faults is complex due to D• thrusting, rather than the simple rela- tionship shown.

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1381

fold is thus a composite structure similar to that in Figure 3a and e, except that in this ease it initiated with top-to-the-west D2.5 shear on a near-horizontal axial plane (Fig. 8b).' This pro- dueed a zone of gently dipping bedding and rotated S 2 that was subsequently folded with an east-side-up shear sense during D 3 (Fig. Be). This particular geometry has enabled us to understand important aspects of the combined effects of D2.5 and D3 structures for mineralization, as discussed later.

Similarity of D2 and Ds folds It is significant that, in the regions depicted in Figures 3a

and 6, D2 and D, folds are geometrically similar, and it is the overturned So that reveals the presence of the intervening large-scale D•.5 folds and hence, allows these two events to be

(a)

/:////,/

(b)

(c)

,/

No Mineralization

at depth because S 3 parallel to S 2

FIG. 8. Schematic cross sections shmving the progressive development of the D•2.5, Ds structure between Mount Isa and Hilton mines to the north of Spear Creek. a. So, S2 relationship before D2 5. b. Top-to-the-west shear induced by D•.•. c. Overprinting effects of D s and east side up-shear.

distinguished (e.g., Fig. 3a, e). This similarity in geometry of D,2 and D• folds is why D•.• has gone undetected for so long in the Mount Isa province, compounded by the confinement of D•.• to narrow horizontal zones spaced some distance apart in a region of little topographic relief. The geometric similar- ity of D• and D• suggests that many D• folds may be present within the Mount Isa province but have incorrectly been in- terpreted as D2 structures. Because of the association of D3 with mineralization (Perkins, 1984, 1997; Swager, 1985; Adshead, 1995; W.P. Laing, pers. eommun., 1996; Xu, 1996, 1997; Baker and Laing, 1998), which has been substantiated by 4øAr-39Ar dating of alteration assemblages associated with the mineralization in the Cloneurry region (Perkins and Wyborn, 1996), this has particular implications for explo- ration, as discussed below.

Regional extent of the D,2.• structure to the west of the Mount Gordon fault

The D2.• fold structure shown in Figure 3e is quite large- scale, extending far to the north and south (Fig. 2). A regional reconnaissance of its distribution is shown in Figure 2, where an approximately continuous trace of S•, close to the eastern margin of the Sybella Granite and affected by D•.5, has been followed for some distance, particularly to the south. The map distribution in Figure 2 can be understood if it is exam- ined in conjunction with Figures 7 and 9, showing the pro- gressive effects of D•, D•.•, and Da. Initially vertical S2 (Fig. 9a) was rotated top to the east by D•.• (Fig. 9b) and then re- folded by Da (Fig. 9c). Heterogeneous strain during both D2.• and Da resulted in differential rotation of the S2-S•.• intersec- tion in a horizontal plane and the S•-Sa intersection in a ver- tical plane, respectively, creating the map pattern shown in Figure 2. This heterogeneous strain was localized against the granite-country rock contact because of the competency con- trast across the contact, and therefore is particularly apparent around the margins of the Sybella pluton.

The resulting interference pattern cannot be reconciled by the overprinting effects of D3 alone, given that Sa and S• ini- tiated with subparallel orientations and S3 is structurally downward facing on much of the bedding, as shown in Figure 10. Similar macroscale rotation of D• structures by D,2.5 has been described from the Snake Creek anticline (New, 1993; K. Lewthwaite, pers. commun., 1997) and the Tommy Creek block (Lally, 1996, 1997) within the Eastern fold belt of the Mount Isa province.

Shear sense during D,2. s To the west of the Mount Gordon fault (Fig. 3a) D•.• shear

displaced the overlying rocks to the east (i.e., dextral shear in cross section looking north). Within the Mount Isa and Hilton mines the shear sense during D2.• was also predominantly top to the east, whereas in the location shown in Figure 6 it was top to the west. Those locations, to the east of the Mount Isa fault, lay at a much higher crustal level at the time of D•.•, rel- ative to those west of the fault (both the Mount Gordon and Mount Isa faults in Fig. 3a lift the rocks to the west 4 to 6 kin; Bell, 1991). In the Tommy Creek block (Lally, 1996, 1997), at Dugaid River (Xu, 1996, 1997) and Fairmile (Mares, 1996, 1998), the shear during D•..• was top to the east. At the north- ern end of the Snake Creek anticline the shear during Da.5

1382 BELL AND HICKEY

a) 82 a)

..................... // •+++++++++++++++++

................. ?// ' // ++++++++++++++++ ++++++++++++++++ ++++++++++++++• /

-++++++++++ / +++++++++• / ++•+++++++ / ........ ;? / / I,,' // ß +++++++ /

...... :-? / / 4' //So '+++++• / I /I / /

/

b)

ß -•'-- D2. 5

c) D3

FIc. 9. Schematic diagram illustrating the effects of D2.5 and Ds on S2. a. Vertical S2 unaffected by D2.5. b. Top-to-the-east rotation of S• due to D•.•. c. Folding of the rotated portion of S• during Ds. In the third dimension these folds porpoise up and down.

was top to the west (New, 1993; K. Lewthwaite, pers. com- mun., 1997). So, although top-to-the-east shear dominates there is no consistent pattern, and overall, the D.•. 5 event may have been predominantly coaxial. We could expect this for a deformation that is interpreted to be associated with gravita- tional collapse of an overthickened orogen (e.g., Bell and Johnson, 1989; Aerden, 1994, 1995; Davis, 1994).

D4

D 4 occurred between Da and Ds, but is only manifest in the Mount Isa region as the flat-lying axial plane of kinks, making it easy to distinguish from D.•.s, which produces crenulations

b) s3 I w • E

--

/ /

S2 //

M! Isa Fault

FIG. 10. a. Sketch of our early explanation for the downward facing na- ture of bedding on S3 but not on S2, to the south of Mount Isa, close to the Sybella Granite. The granite is marked by + signs. b. Same as (a) but with a fault along the hinge and the younging direction of bedding shown. Note how So is upward facing on Sz but downward facing on S3 on the western limb of the syncline.

with variably folded axial planes. These rare kinks have been observed mainly to the west of the Mount Isa fault in the more schistose rock types. The shear sense on the kink is gen- erally top to the east. This event is very similar to the D• event in that it appears to have little or no geometric significance except that it continues the succession of orthogonally over- printing deformations. One possible consequence of D 4 could be the top-to-the-east shear observed on the Buck Quartz fault within the Mount Isa mine (Bell et al., 1988).

To the east, in the Tommy Creek block and the Snake Creek anticline, this deformation event is locally more intensely developed, forming crenulations rather than kinks with subhorizontal axial planes. Garnet, staurolite, and an- dalusite porphyroblast growth have occurred locally in both these regions during this event (Lally, 1996, 1997; New, 1993).

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT ISA REGION 1383

Significance for the Mount Isa Province

The origin of Ds folds Prior to recognition of the mesoscopic and macroscopic ef-

fects of D.2.s in the area shown in Figure 3 (where S.2 and Ss have different orientations) and the area shown in Figure 6 (where S.2 and Ss have similar orientations), the manner in which Ds folds developed in the Mount Isa province pre- sented a conceptual dilemma owing to the generally similar north-south-trending orientations and steep dips of both S,2 and Ss. The nature of the problem can best be illustrated by the D s Mount Isa fold at Mount Isa mine (Fig. 11a). In order to explain the origin of this east-verging fold, Bell et al. (1988) suggested that an early component of west side up-shear (Fig. 11c) overprinted the previously formed D,2 fold limb (Fig. lib) at a low angle (Fig. 11c). They argued that this was fol- lowed by bulk shortening accompanied by predominantly east side up-shear on the axial plane cleavage. The latter stage of deformation resulted in the dextral (looking north) reactiva- tion of bedding (Fig. lid; cf. Bell, 1986) that dominated the long limbs of the D s folds within Mount Isa mine (Bell et al., 1988).

Removal of this somewhat unsatisfactory requirement for initial west side up-shear on the S3 cleavage to produce the Mount Isa fold, which at a later stage during D s controlled the formation of much of the mineralization within the mine

due to west side up-shear on the bedding (Bell et al., 1988; Bell, 1991; Perkins, 1997), has been made possible by recog- nition of the role played by the intervening D,2.s deformation in rotating S,2 prior to the D s event. Indeed, as argued below, in any region where two successive deformations have very similar orientations, or where it has been inferred that suc- cessive deformations are coaxial and have similar primary fo- liation orientations, we suggest it is likely that an intervening event with a near orthogonal orientation was present.

We now interpret that D,2.5, although generally a relatively weak and spatially localized event, rotated steep S,2 and west- dipping So (Fig. 12a) into a shallower orientation by top-to- the-east shear (Fig. 12b), where they could be folded during subsequent D s horizontal shortening, thus forming the Mount Isa fold (Fig. 12c). Reactivation of So on the western limb and destruction of smaller-scale D s folds (Fig. 12d) re- suited in development of the 550 and 600 orebodies as well as the 1100, 1900 and 3000 orebodies on the greenstone base- ment contact as previously described by Bell et al. (1988). D2.5 in the mine produced microscopically observable S,2.• cleavage seams, but no mesoscopically visible foliation. It also produced mesoscopically observable fold geometries but these are only preserved in regions of Ag-Pb-Zn mineraliza- tion where So is folded by rare D,2 and more common D,2.5 and D s folds that interfere with one another (e.g., Perkins, 1997). Previously these somewhat complex interference geometries were regarded as resulting from disharmonic folding, com- plex folding (McClay, 1979) or attributed to slumping during sedimentation (McDonald, 1970).

Thus we argue that D s folds cannot be distinguished from D2 folds without the intervention of D,2.s because of the sub- parallelism of S,2 and S3. That is, throughout the Mount Isa province, where no D,2.5 is present, S,2 simply intensified dur- ing D s rather than a new S3 form. Since copper (Perkins,

a)

silica-dolomite

Paroo fault

I

Mt Isa Fold

" 0 500m

b)

c)

d)

Fro. 11. The progressive development of the Mount Isa fold as inferred by Bell et al. (1988). a. Cross section through Mount Isa mine showing the Mount Isa fold. b. Bedding and S2 prior to the effects of Ds. c. Generation of D3 fold short limb due to initial dextral shear in a localized zone. d. Re- activation of bedding due to subsequent bulk shortening.

1984; Swager, 1985; van Dijk, 1991), Ag-Pb-Zn (Perkins, 1997; Myers et al., 1996; Xu, 1996, 1997), and copper-gold mineralization (Baker and Laing, 1998; Adshead-Bell, 1998) within the Mount Isa province is generally associated with Do structures, D2.s becomes a key for future target generation and exploration of the deeper subsurface.

1384 BELL AND HICKEY

a)

i1• I I'1

So ii I I I I

,/'/,1/,/ .,7,7,'/'/,//

• I I S Z,/,//,/ " x,'/•/•//

, ,/ ,)/,• ,

b)

// // // // // // // //

// // // // // // // //

c)

d)

Fro. 12. The progressive development of the Mount Isa fold via the in- termediate effects of Ds 5. a. Post-Ds geometry of bedding dipping steeply west and S2 near vertical. b. Top-to-the-east shear in Ds 5 resulted in forma- tion of a monoclinal structure rotating bedding and Ss into a near-horizontal orientation. c. Horizontal shortening during the development of Da resulted in folding of both So and Ss and development of the Mount Isa fold. d. Re- activation of So on the western limb and destruction of smaller-scale Da folds.

Regionally scattered silica-dolomite mineralization Silica-dolomite alteration that formed during D3 occurs

over much of the western Mount Isa province without any significant associated copper mineralization (e.g., van Dijk, 1991). This problem has long been recognized by mineral ex- plorers using silica-dolomite as a guide for exploration (W.

Perkins, pers. commun., 1983). The area north of Spear Creek shown in Figure 6 is typical of this problem. Silica- dolomite alteration and some copper mineralization occur at the surface but no orebody is present at depth. The structural relationships present at this location provide a simple expla- nation for the presence of similar silica-dolomite alteration in other locations without an accompanying copper orebody, which has implications for exploration throughout the Mount Isa province (see immediately below).

The Ds.s fold between Mount Isa and Hilton mines Prior to collection of the younging data shown in Figure 6,

geologists from Mount Isa Mines Ltd. thought this structure was a Ds fold refolded during D3 (Fig. 13; W.G. Perkins, pers. commun., 1993). The combination of silica-dolomite alter- ation at the surface, some copper mineralization, and a Da fold made it look like a potential target for copper mineral- ization at depth, given that the close parallelism of Ss and Sa meant that gaping, implosion, and brecciation could occur over a considerable length of the axial plane of the refold dur- ing Da. Consequently, the structure was drilled from the west to intersect mineralization at depth. None was found because the fold was not a refold of a Ds fold at all, but rather the Ds.s structure described above overprinted by the effects of D3 as shown in Figure 8. The volume of rock that could gape, im- plode, brecciate, and mineralize during Da depended entirely on the width of the zone of S•2 and bedding rotated by Ds.s, and this was very narrow. Above and below this zone, as shown in Figure 8, D3 effects were no different from those of Ds, as the foliations were indistinguishable, and no mineral- ization occurred.

82

W • E

III I / \•

$o

$2

F•c. 13. Ds fold refolded by a Ds fold.

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT ISA REGION 1385

This discovery has considerable implications for future ex- ploration for copper, gold, and potentially, much of the Ag- Pb-Zn mineralization throughout the Mount Isa province. The restriction of the zone of mineralization to a confined

width in the area shown in Figure 6 suggests that exploring for zones of D.2.5 alone is not sufficient to find large-scale ore- bodies forming during D3, with the possible exception of an area containing gold mineralization, where a lot less metal is needed for the deposit to be economic. The generation of large-scale ore deposits during D3 appears to require a zone of D2.5 strain to overprint a macroscopic heterogeneity, such as the greenstone basement contact at Mount Isa mine, that can enlarge the scale plus aid the mineralization process. One such location is described and discussed below.

Can mineralization occur during Ds reuse and intensification of

D 3 reuse and intensification of S.2 occur in locations where there has been no intervening rotational effects of D.2.•. We consider it unlikely that very large-scale orebodies will form in such locations in country rocks where no previously devel- oped heterogeneities have formed. Deformation will simply tend to proceed as it did during D2, except possibly at a dif- ferent metamorphic grade, and intensify the S• cleavage. If, however, the metamorphic grade dropped or the strain rate locally increased during D3 so that the deformation was brit- tle rather than ductile, mineralization could still occur and form orebodies containing gold, such as those at Kuridala and Stuart (W.P. Laing, pers. commun., 1997).

Overturned bedding along the margin of the Queen Elizabeth phase of the Sybella Granite

Bedding on the western limb of the Mount Isa syncline to the south of Mount Isa is overturned, as shown in Figure 10, so that the structural facing on S.2 is the right way up but that on S3 is upside down. Prior to recognizing the large-scale na- ture of the D.2.• fold shown in Figure 3, we had attributed this effect to wrapping of S• around a primary granite contact that dipped moderately steeply to the west during the later stages of De, as shown in Figure 10a. However, a reconnaissance trace of S.2 southward (shown in Fig. 2) from the D•.• fold mapped in Figure 3a suggests the overturning of So and S.2 in such a way that S3 is structurally downward facing on So, and was produced by an extension of the same large-scale D.2.• structure to the south.

Consequently, we interpret that the shape of the northern (and southern) boundary of the Queen Elizabeth phase of the $ybella Granite pluton shown in Figure 2 results from the ef- fects on S2 of interference between this large-scale D2. 5 fold and D3. This relationship is shown in cross section in Figure 14, where the top of the Sybella Granite (Fig. 14a) is sheared to the east by the D2. 5 structure (Fig. 14b), and then over- printed by the effects of Ds (Fig. 14e). The action created an antiform through the eastern edge of the pluton cored by coun- try rock, which, because of heterogeneous strain across the contact between them, resulted in dramatically doubly plung- ing F• folds. Porpoising of F• folds produced the map pattern of granite and adjacent country rock shown in Figure 2. This interpretation suggests that whereas the base of the Queen

Elizabeth phase now has an overall subhorizontal attitude (see below) folded by D• folds, it was originally steeply dipping.

If the granite intruded Haslingden Group rocks as shown in Figure 14a, then, after rotation of this contact into a shallow orientation in D.2.s, bedding would have lain at a high angle to the granite contact (Fig. 14b). That would be an ideal orien- tation to promote gaping of the granite-country rock contact during D•, eausing mineralization similar to that which ere- ated the 1100 orebody at Mount Isa, provided the thermo- chemical conditions were appropriate (Fig. 14e). Indeed, recognition of such a role for D2.s suggests considerable po- tential for mineralization below the Queen Elizabeth phase of the Sybella Granite pluton on its contact with the country rock shown in Figure 14d. It is noteworthy that the interpre- tation of this D.2.• structure at the base of the Sybella Granite has been independently confirmed by the Australian Geolog- ical Survey Organisation (AGSO) seismic profile (see this vol- ume). A similar geometry should exist below the easterly pro- trusion of Sybella Granite to the west of Hilton mine, which is shown in the northern portion of Figure 2. During D a, dose to this northern portion of the Sybella pluton, the meta- morphic grade is lower than that found around the Queen

(a)

(b)

(c)

present erosion

++++Wi\,•/---......._•/i • ++++++I&•-/N ++++++W'\ •1

FIG. 14. Schematic diagrams showing the effects of west-east shortening on the Sybella Granite and its adjacent country rock. a. The country rock- granite relationships prior to D2 • (omitting D2 effects for simplicity). b. Top- to-the-east shear due to D2 •. c. The effects of Da on the section in b.

1386 BELL AND HICKEY

Elizabeth phase to the south (M.J. Rubenach, pers. commun., 1996). Consequently, the base of this granite may be a better target than the southerly phase to explore for Mount Isa-style copper mineralization at depth.

D2.s rather than D1 generation of east-west-trending zones of macroscopic folds and mineralization potential

The curvature of fold axial planes from north-south to east- west trending on the immediate northern margin of the Snake Creek anticline to the southeast of Cloncurry (New, 1993; Lewthwaite, pers. commun., 1997) and the east-west- trending folds in the Tommy Creek block on the northern margin of the Duck Creek anticline to the west of Cloncurry (Lally, 1996, 1997) were produced by D.2.5 rotation of large- scale D.2 folds and subsequent overprinting by D3. This is completely contrary to previous interpretations and suggests that these regions are highly prospective for copper and gold mineralization.

Mineralization potential of regional zones affected by D2.s No significant larger-scale zones of mineralization have so

far been discovered in the D.2.5 affected portions of the Tommy Creek block, the northern end of the Snake Creek anticline, and the region between the eastward protrusions of Sybella Granite at the top and bottom of Figure 2, in spite of the presence of large-scale D.2.• and D3 structures. This may indicate, as suggested above, a lack of major structural het- erogeneities on which D.2.• could localize, that could then aid gaping, implosion, and hence, mineralization during D•. Al- ternatively, such heterogeneities may be present but their dis- covery requires detailed mapping and subsurface exploration. Perhaps the thermochemical conditions were not right for the formation of extensive zones of copper and gold mineraliza- tion. However, these three regions all contain small-scale zones of copper mineralization (some of which are not plotted on re- gional maps) with gold locally present at Tommy Creek and north of the Snake Creek anticline. Some uranium mineraliza-

tion is also present in an area between the eastward protrusions of Sybella Granite as well as at Tommy Creek, suggesting they do have potential for large-scale orebodies to form.

During D.2.• and D•, the area west of Mount Isa and the east-west-trending portion of the Tommy Creek block were affected by amphibolite facies metamorphism and the north- em end of the Snake Creek anticline spanned the boundary between upper greenschist and lower amphibolite facies (Fig. 4; Lally, 1997; Lewthwaite, pers. commun., 1997). Metamor- phic grade at the time of mineralization may be a significant control on the type and scale of orebody that forms, although ore deposits in the Mount Isa province occur in rocks ranging from lower greenschist to upper amphibolite facies prior to or during mineralization (Perkins, 1984, 1997; Adshead, 1995; Xu, 1996, 1997; Adshead-Bell, 1998; Baker and Laing, 1998). Prior to erosion, the area in Figure 2 between the eastward protrusions of Sybella Granite may have lain just below this granite which, through the porpoising of F • folds to a higher crustal level, was eroded and removed. All three regions should be targeted for exploration, if only because of the scale of D.2.• and, in the future, the base of the northern and south- ern protrusions of the Sybella Granite should be examined with care.

As more detailed structural work is done throughout the Mount Isa province, other locations where large-scale rota- tion of S,2 by D,2.s occurred may be revealed. The opportuni- ties for exploration for deeper zones of mineralization that have no geochemical or geophysical expression will increase dramatically as this occurs, as in the case of the regions de- scribed above.

General Significance Structural mapping in regions dominated by steeply dipping foliations

Mapping in regions dominated by steeply dipping foliations that lack significant relief can be conducted for many years without the recognition of weaker deformations associated with spatially localized subhorizontal axial planes. Indeed, it has taken 50 years of locally detailed as well as regional map- ping within the Mount Isa province to recognize the presence of these intervening weak deformations, and even longer to realize their significance. Yet i km of three-dimensional ex- posure at Mount Isa mine has been available for some 30 of these 50 years. Two main reasons for this lack of recognition were the absence of an axial cleavage associated with defor- mations with subhorizontal axial planes, and the general sub- suming of the rotational effects on bedding of the first of these deformations within the apparently normal develop- ment of younger, upright folds, as described above.

Similar deformations are predicted to exist in many other orogens dominated by steeply dipping foliations. The east- west-shortening orogeny of the Mount Isa province produced a succession of at least five deformations with orthogonal axial planes, three of which are generally very weak. Successions of weak deformation events overprinting stronger orthogonal phases have recently been recognized within the Tasman and Kimberley orogens and the Adelaide fold belt. These orogens are dominated by subvertical foliations and have low topo- graphic relief (in North Queensland, Davis, 1993, 1994; Davis and Forde, 1994; in Victoria, Forde and Bell, 1994; in the Kim- berleys, Mares, 1986; in the Adelaide fold belt, Steinhardt, 1989). One should be aware of this possibility when mapping in regions with low relief, and look for evidence of the re- mains of shallow dipping structures preserved in the strain shadows of competent portions of rock.

Coaxial deformations and development of orthogonal foliations

Many examples of terranes exist that appear to have under- gone a series of coaxial (colinear) deformations with similarly oriented fold axes and formation of two or more subparallel foliations (e.g., Mukhopadhyay, 1965; Watkinson and Cob- bold, 1981). Recent work using inclusion trails in porphyrob- lasts has revealed a history of overprinting of near-orthogonal foliations. Regions dominated by shallow foliations where such a history has been documented include the Otago Schists of New Zealand (Johnson, 1990), the Chester dome region of Vermont (Hayward, 1992; Bell and Hickey, 1997), and the Alps around Lukmanier Pass (Bell and Hayward, 1991). Regions dominated by steep foliations include the Kimberleys of Western Australia (Mares, 1996, 1998a), the Archean around Cue in Western Australia (Wilkins, 1993),

DEFORMATIONS WITH SUBVERTICAL AND SUBHORIZONTAL AXIAL PLANES IN THE MT. ISA REGION 1387

and the eastern Mount Isa province southeast of Cloncurry a (Mares, 1996, 1998a). Some regions preserve a series of both shallow and steeply dipping foliations as a series of flat and steep belts including the Grampians of Scotland (Thomas, 1979; Treagus, 1987) and the Lepontine dome of the Alps (Merle et al., 1989).

A model for orogenesis that attempts to explain the gener- ation of such orthogonal foliations was proposed by Bell and Johnson (1989). They developed the model to explain sub- horizontal and subvertical axial planes of crenulations, and fo- liations defined by truncations preserved in porphyroblast cores and rims (e.g., Bell and Hayward, 1991; Hayward, 1992; Jones, 1994; Spiess and Bell, 1996; Mares, 1996, 1998). Bell and Johnson (1992) argued that this orthogonal relationship 19 of foliations subparallel and subperpendicular to the Earth's surface is generally not preserved in the rock matrix because of the rotational or reactivational effects of younger deforma- tions on earlier foliations (e.g., Bell, 1986; Davis and Forde, 1994; Davis, 1995). However, the Mount Isa province pro- vides an example where one can see this history preserved be- cause the deformation has not been as extensive as in other

belts. The preservation of successive vertical and horizontal foliations within the porphyroblasts can be explained by lack of rotation of the porphyroblasts during subsequent non- coaxial ductile deformation, and this phenomenon has been documented around individual folds (Bell and Forde, 1995; Bell and Hickey, 1997), regionally (Bell et al., 1998), and around large-scale oroclines (Mares, 1996).

Fold development Many parasitic folds that appear to result from just one

phase of deformation may actually result from two phases. Bell and Johnson (1992; fig. 14) recognized this phenomenon in schists in which they found that the shear sense on the axial plane cleavage did not change across the hinge of the fold. The two-deformation-stage process required to form the folds shown in Figures 9 and 15 seems to have been respon- sible for many, if not all, of the D a folds in the Mount Isa province. The near-parallelism of S•2 and Ss makes D.• and Ds folds indistinguishable except where the rotation of S•2 by D2.5 has occurred (Fig. 9); S•2 simply intensified instead.

Overprinting of orthogonal events in terrains dominated by steeply or shallowly dipping foliations might commonly in- volve the sequence shown in Figures 9 and 15 and not be rec- ognized because the macroscopic and even mesoscopic folds appear to be attributable to just one event. Complications in small-scale fold geometry, such as those seen in the Pb-Zn ores at Mount Isa, might be attributed to polyclinal folding or local, unexplained heterogeneities.

Recent microstructural work plus the detailed fieldwork re- ported herein on mesoscale and macroscale geometries suggest that if a foliation formed with a subvertical orientation and

locally dips east, then outcrops and thin sections should be ex- amined for top-to-the-west asymmetric deflections in the out- crop around heterogeneities such as veins and porphyrob- lasts. Strain shadows around such heterogeneities can protect portions of the rock from the effects of subsequent stronger events and allow one to determine whether there is evidence for a shallow event.

S2

Sz.5

I I

I I

I I

I

S3

Fro. 15. The development of Ds folds, taking account of the effects of D2 5. a. After D 2. b. After D2.s.e. After D 3.

Acknowledgments We •vould like to aeknoMedge the permission of Mount Isa

Mines for fieldwork on their leases. We also aeknoMedge many constructive arguments and discussions with Bill Perkins and Mike Rubenaeh, over the years, about the struc- tural and metamorphic interpretation of the rocks of this re- gion. Their intimate knowledge of the structure of the mine and the metamorphism of the region, respectively, was of great assistance. We thank Bill Laing, Evan Leiteh, Rick Valenta, and Pat Williams for their constructive reviews. We would also like to acknowledge funding for this research from the ARC.

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1388 BELL AND HICKEY

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