Forde, A. and Bell, T.H., 1994. Late structural control of mesothermal vein-hosted gold deposits in...

ELSEVIER Ore Geology Reviews 9 (1994) 33-59

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Late structural control of mesothermal vein-hosted gold deposits in Central Victoria, Australia: Mineralization mechanisms and

exploration potential

A. Forde, T.H. Bell Department of Geology, James Cook University, Townsville, Qld. 4811, Australia

(Received January 12, 1993; revised version accepted August 19, 1993 )

Abstract

Comparison of the structural relationships between five gold deposits in Ordovician rocks of the central Victo- rian slate belt reveals a similar history and structural control of gold mineralization. Large quartz veins, spatially associated with gold mineralization, such as the famous saddle reefs of Bendigo, formed during the second deformation, earlier than the gold mineralization itself. Where they were overprinted by the effects of the fourth deformation, they provided structural sites for gold deposition on their boundaries with country rock where brecciation, jostling and fracture were favoured. Access of fluids resulting from metamorphism and deformation to the brecciated, fractured and jostled blocks of country rock created conditions suitable for the precipitation of gold through a sequence of alteration, replacement and infill. The dominant structural relationships that resulted in the generation of these sites were (a), the high angle of the axial plane of the fourth deformation to earlier structures and (b), the presence, during mineralization of large, pre-existing quartz veins that acted as structural discontinuities on which brecciation could occur. These factors allow a predictive capacity for exploration in the region.

I. Introduction

Around the world, gold production from vein systems hosted in metamorphic terrains (Hutch- inson, 1987) has dominantly come from greenstone sequences such as those of the Yilgarn in Western Australia (Mueller et al., 1988; Groves et al., 1989), the Superior and Slave Provinces of Canada (Roberts, 1987) and Zimbabwe (Foster, 1989). Significant production has also come from vein systems hosted by low metamorphic-grade Palaeozoic and younger turbidite sequences including the Meguma Terrane in Nova Scotia (Mawer, 1987), the North American Cordillera (Bonham, 1989) and the central Vic-

torian Goldfields of Australia (Sandiford and Keays, 1986; Ramsay and Willman, 1988). All of these deposits are structurally controlled and have recently been demonstrated to have developed late in the deformation/metamorphic history (e.g., Craw, 1989; Vearncombe et al., 1989; Kontak et al., 1990; Robert, 1990; Forde, 1991; Wilkins, 1993 ). This, combined with progress in understanding the processes by which brecciation can be localized during ductile deformation (e.g., Bell et al., 1988; De Roo, 1989; Aerden, 1991, 1993; Bell, 1991, Hayward, 1992), as well as the role of deformation partitioning in dissolution and the transfer of material and control- ling metamorphic reactions (Bell and Cuff, 1989;

0169-1368/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0169-1368(93) E0022-2

34 A. Forde, T,H. Bell / Ore Geology Reviews 9 (1994) 33-59

Bell and Hayward, 1991 ), provides new ways of exploring such areas, particularly for blind targets.

This new approach can be illustrated using the deposits of the central Victorian Goldfields, including Bendigo, St Arnaud, Ballarat, Inglewood and Dunolly (Fig. 1 ). Over 2000 tonnes of gold has been won from mining of alluvial and vein deposits in this region (Bowen and Whiting, 1976) and those at Bendigo have been used in textbooks for many years as classic examples of gold veins formed as saddle reefs during fol~ng (e.g., Park and MacDiarmid, 1975; Boyle, 1979; Guilbert and Park, 1986). However, detailed microstructural work on the timing and location of the gold mineralization by Forde ( 1991 ) has shown that these gold deposits did not form during the obvious folding event that produced the saddle reefs (see similar findings for gold deposits in thc Mcguma Tcrrane, Kontak et al., 1990 ). Instead, they formed during fracture and brec-

ciation of the contacts of these earlier developed structures with the adjacent country rocks during a younger and much weaker deformation, that previously had been regarded as so minor and local that it was dismissed as insignificant or related to faults.

Weak deformation events like this have significance only for mineralization involving a very valuable resource such as gold. This is because structural traps associated with such events generally tend to be relatively small in scale. How- ever, recognition of the timing and orientation of structures developed during these weak events has considerable potential for targetting exploration on a regional scale. This potential results from the fact that the local development of such weak deformations may produce some deflection of the macro-structural relationships seen on regional maps or on aeromagnetic maps; these can then be selected for detailed work. That is. weak deformations develop structures that are

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A. Forde, T.H. Bell / Ore Geology Reviews 9 (1994) 33-59 35

commonly very coarsely parti t ioned in space. Consequently, they only affect narrow zones and leave large portions of country rock totally undisturbed.

2. Deformation history

The foliation produced by the first deformation in the central Victorian goldfields has been intensely overprinted by the second event. How- ever, in each area, this first event can be detected by the differentiated character of the crenulations associated with the second deformation event such as shown in Fig. 2 (see also fig. 6 in Forde, 1991 ), and, where carbonate porphyrob- lasts are present, by the horizontal foliation preserved as inclusion trails within these porphy- roblasts (see Fig. 4 in Forde, 1991 ). The foliation preserved in the hinges of crenulations is commonly very shallowly oriented also, suggesting that this first deformation was associated with thrusting (cf., Bell and Johnson, 1989, 1992).

The second deformation produced regional folds that contain the classic quartz saddle reefs (e.g., fig. 4-12 in Park and MacDiarmid, 1975; fig. 44 in Boyle, 1979 ; fig. 3-21 a in Guilbert and Park, 1986 ) and the dominant regional cleavage $2. These folds (F2°; terminology after Bell and

Duncan, 1978) are approximately N-S-trending, tight and upright with a 10-1000 m wave length and a cleavage ($2) lying parallel to their axial planes (Baragwanath, 1923; Beavis, 1967 ). This cleavage is locally preserved as a differentiated crenulation cleavage at stages three and four (Bell and Rubenach, 1983 ) as shown in Fig. 2, but is commonly further developed to stages five and six where the hinges of crenulations seen in this figure have been destroyed.

The third deformation produced a local crenulation cleavage $3 (Fig. 3), which is sub-horizontal in all deposits except Bendigo where it was not observed. $3 varies in intensity from a locally penetrative crenulation cleavage to isolated kinks and is generally only visible in thin section.

The very weak fourth deformation produced $4, a crenulation cleavage that is vertical, striking predominantly E-W to NE-SW in all the deposits (Figs. 4 and 5 ). The variation in strike recorded in Fig. 5 is a direct result of the coarse (several kilometres) scale of deformation partitioning, and hence excessive anastomosing (Bell, 1981 ) associated with this weak deformation. $4 is possibly more penetratively developed at con- siderably deeper crustal levels. However, this foliation is commonly not developed where there are no gold deposits in the immediate vicinity. The L42 crenulation axes formed during this de-

Fig. 2. $1/$2 relationships in Ballarat. S1 is the crenulated foliation trending top left to bottom right, $2 is the spaced vertical foliation. Crossed polars, vertical thin section, base of photograph is horizontal, width of base is 12 mm, looking south.

Fig. 3. Near-horizontal $3 in Ballarat. Both $2 (vertical) and D2 carbonate veins (dark ellipsoidal zones parallel to $2 ) are deformed by $3. Section is vertical, base is horizontal, view is to the south, plane polarized light, base of photo is 10 mm.

36 A. Forde, T.H. Bell / Ore Geology Reviews 9 (1994) 33-59

4 ), kinks and shear bands, and where present is commonly visible in core and outcrop. Va~ing shear senses (Bell and Johnson, 1992 ) on $4 sug- gest this deformation was overall coaxial.

Previous workers in the Victorian Goldfields have described only one regional deformation (Gray, 1988; Cox et al., 1991 ). This deformation is the second event described hereto

Fig. 4. Horizontal section from Bendigo showing $2 (top left to bottom right) deformed by $4 (top right to bottom left). North is towards the top of the page. Partially crossed polar- izers, bottom of photograph is 3 ram.

St Arnoud • Bendigo u Dunolly

3. Quartz vein types

Five main groups of quartz veins are present in the five deposits described herein:

(a) Subhorizontal (Baragwanath, t 923: Chace, 1949) crack-seal (Ramsay, 1980) veins formed due to vertical extension (e.g., Fig. 6a). Outside the gold deposits, this type of vein is not associated with gold mineralization. They were called spurs by miners and geologists and up to half the gold mined was obtained from some of them (StiUwell, 1917, 1918a, 1919) although many spur veins were barren (Stillwell, 1918b) and gold mineralization is not genetically linked with their formation.

(b) Fault reefs that occur as planar quartz bodies on zones of movement in the rock. In- cluded in this type are the "leather jacket formations" of Ballarat East, the "legs" and "backs"

Fig. 5. Poles to $4 for all five deposits from dam measured in surface exposures and oriented drillcore. The amount of $4 visible in outcrop and core is limited at each location although it was always recognized. $4 was much more readily observed in thin sections at each of these locations.

formation are steeply plunging as $2, the foliation that is crenulated, is steeply dipping. $4 varies in character from an almost penetrative crenulation cleavage to spaced crenulations (Fig.

Fig. 6(a). A fibrous sub-horizontal crack-seal vein from Bal- Iarat with crosscutting D4 quartz-chlorite alteration at the left and bottom of the photograph. $2 is vertical. Crossed polars, section is vertical, base of photograph is horizontal, looking north, width of base is 10 mm.


of Bendigo, the east-dipping reefs at Inglewood and across-strike faults such as the Collmann and Tacchi Crosscourse of Bendigo (see figs. 12A, B, 16B of Dunn 1896, and figs. 6, 7 and l0 of Chace 1949).

(c) Saddle/trough reefs formed across the hinges of F ° anticlines/synclines (Fig. 6b). They are, in places, repeated vertically over large distances along the axial plane of the folds. Photo- graphs of these reefs are presented in Dunn ( 1896, plates 7B and 8), Chace ( 1949, figs. 4 and 5) and Thomas ( 1953a, fig. 3).

(d) Laminated veins, including ribbon quartz and book structure (McKinstry and Ohle, 1949), occurring as thin (1-50 cm), areally extensive veins that lie on bedding (e.g., Fig. 6c). Dunn (1896) showed that some laminated veins in Bendigo could be traced over more than one F ° fold pair (also Wilkinson, 1988b) and are not confined to hinge regions of folds. Bands of sheared-wallrock inclusions parallel to the vein walls give these veins their laminated textures.

Fig. 6 (c). Laminated vein and wallrock from Inglewood. Note wallrock inclusions on vein edge and deformed character of the quartz. Slight compositional layering parallel to the veto wall is bedding. Crossed polars, section is vertical, base of photograph is horizontal, width of base is 15 ram.

Fig. 6 (b) . Sketch from photograph of saddle reef from No. 4 level, New Red White and Blue Mine at Bendigo. The brecciated fragments of slate (black) are rotated by large amounts relative to one another. (Redrawn fron plate 7A, Dunn 1896; approximately 3-5 m across the base.) White=quartz, black= slate, dots= sandstone.

Fig. 6(d) . Breccia vein from Ballarat. Wallrock with S 2 is at the bottom of the photograph. Note the rotation of $2 in the clasts of up to 90 ° relative to $2 in the wall rocks as well as one another, yet lack of rounding, strongly implying an implosive character the genesis of such veins (see text). Plane polarized light, width of base is 17 ram.

This type of vein occurs in all deposits with the gold lying along the carbonaceous laminae. This type of vein also occurs in the turbidite-hosted gold deposits of the Meguma Terrane in Nova Scotia (e.g., Haynes, 1986; Mawer, 1987; Kon- tak et al., 1990)

(e) Breccia veins (Perkins, 1984; Swager, 1985; Bell et al., 1988; Aerden, 1991, 1993; Forde, 1991; Van Dijk, 1991; Hayward, 1992) developed on discontinuities in the rockmass, such as the margins of pre-existing veins with


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Fig. 7 (a). Level 2 of the Central Deborah mine, Bendigo, showing measured orientations of bedding, $2 and $4. A folded laminated vein (F) is present in the No. 2 West Crosscut and is shown in fig. 2 of Forde ( 1992; Modified after Hinde, 1988 ). Line A-B shows the location of the cross-section drawn in Fig. 7b. The inset map shows the location of the anticlinal F2 hinges in Ballarat, which form the lines of lode, and the location of zones of mineralization along them (thicker black spindles). It also shows the location of the Central Deborah Mine.

country rock, bedding planes and faults (e.g., Fig. 6b, d). These discontinuities have gaped during deformation, with implosion of some of the wallrocks, generating rotated clasts of brecciated

waUrock within their cores and jostled and fractured blocks on their m a i n s (Fig. 6d), This process does not involve hydraulic fracturing, and has considerable significance for mineral-


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Fig. 7 (b) . Cross-section through the Central Deborah Mine along the line A-B in Fig. 7a.

ization as discussed below. These veins contain a distinctive alteration system and gold (Forde, 1991).

4. Structural relationships within each deposit

4.1. Bendigo

Country rock: The Bendigo Goldfield, located 20 km north of the Harcourt Batholith in north- central Victoria (Fig. 1 ), lies in a NNE-trending zone of lower to middle Ordovician turbidites bounded by the N-S trending Whitelaw Fault to the east and the Sebastian Fault to the west (Wilkinson, 1988a). The hinges o fF ° folds (terminology after Bell and Duncan, 1978 ) undulate within $2 producing a series of domes and bas- ins. In general, they plunge to the south at the southern end of the gold field and to the north at the northern end (Stone, 1937). Plunges locally reach 60 ° (Dunn, 1896). $4 is weak and best seen in thin section or as a crenulation lineation, L42 , in outcrop. It is relatively intense where the New Chum line of reef outcrops at Victoria Hill in Bendigo, dipping 70 ° to the NNW and striking 060 °. In the Central Deborah mine (Figs. 7a and 7b), the orientation of $4 is vertical and strikes 040 °. Dunn (1896, p. 34) recorded that the cleavage in the Confidence Tribute Mine was vertical and perpendicular to the general cleavage trend. In the same mine, he noted examples of "double cleavages", suggesting that a late, steep, high angle foliation was prominent in this mine.

Much of the F ° plunge variation will have resulted from heterogenous strain in the D2 fold- forming event (e.g., Sanderson, 1973). How- ever, some could be due to the overprinting effects of D4; e.g., the NNE-SSW-trending "up- thrust" of Whitelaw ( 1914), the crossfolding of Hinde (1988), and the NE-SW-striking faults (crosscourses) that offset D2 structures. The Collmann and Tacchi Crosscourse, the largest known in the field, is mineralized and was prof- itably worked in several places (Wilkinson, 1988a).

Gold-bearing ore: Only approximately a quarter of the gold obtained from quartz reefs in Bendigo came from the classic saddle reefs. An- other quarter came from fault reefs. However, almost half the production was from spur veins


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Fig. 8 (a). Foliation orientations on a portion of the Nelson Line of Reef, St Arnaud shown in the inset. Although outcrop, apart from portions of the quartz reefs, is very poor throughout the region, $4 lying at quite a high angle to So and $2 can be readily seen. A-B marks the location of the cross-section shown in Fig. 8b and STA 4 and STA 7 mark the location of the diamond drill holes used to construct the crosssection in Fig. 8b.

(Stillwell, 1917, 1918a, 1918b, 1919). Detailed geological work was done during this period of mining at Bendigo because of the importance of this mineral field and many of these geologists commented on the lack of understanding of shoot controls (e.g., Dunn, 1896; Whitelaw, 1914; Chace, 1949 ). Stillwell ( 1918a) noted that variation of the gold content in the quartz can occur over very short distances. Whitelaw ( 1914) stated that gold in saddle reefs may be uniformly disseminated through the cap but that gold elsewhere is contained in high-grade shoots, whereas Chace (1949) was of the opinion that the ore shoot problem could probably be solved by ex- amining the orientation and distribution of late fracturing and brecciation.

The gold shoots in Bendigo take two forms. In the saddle reefs, the high-grade material oc-

curred parallel to the hinge line on or close to the crest of the fold (Whitelaw, 1914), whereas in the leg reefs (mostly the east legs) the shoots generally pitched north. In the Hustler's Line of reef, drop in grade in a saddle going northwards from a dome was commonly compensated for by the appearance of a shoot in the east leg (Whi- telaw, 1914). This could be explained by gaping and brecciation during D4 (the mechanism for this is described later). Indeed, it was realized early this century that in many cases "the payable portions of reefs are to be found in those channels intersected by diagonal vertical zones" (Stillwell, 1918a). These may have resulted from NNE to NE striking zones of near-vertical $4 (see below), causing good mines at a shallow level to be payable at deeper levels whereas adjoining mines were commonly poor throughout all levels


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Fig. 8 (b). Cross-section through St Arnaud parallel to $4 along line A-B in Fig. 8a. Cross-section shows that the zones of mineralization are localized on or close to D2 fold hinges.

(e.g., Stone, 1937). For example, at the Grea t Extended Hustlers Mine, the saddles conta ined insignificant gold 1000 m north of the F ° dome, but fur ther nor th at the Uni ted Hustlers and Re- dan Mines were rich lodes with most of the gold coming f rom reefs in the eastern beds (Wilkin- son, 1988b).

4.2. S t A r n a u d

Country rock: This deposit is located on the western side of the Avoca Fault Zone, 110 km northwest o f Ballarat (Fig. 1 ). The host rocks are unfossiliferous upper Cambr ian to lower Ordov- ician quartz turbidites known as the St Arnaud

42 A. Forde, T.HI Belt / Ore Geology Reviews 9 (1994) 33-59

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Fig. 9 (a), Map of portion of the Ballarat Goldfield. Much of the Goldfield is now a suburb of the City of Ballarat. Due to poor outcrop, this map was essentially constructed by Bara- gawanath (1953) from the numerous mines along the various lines of lode. A-B shows location of cross-section in Fig. 9b.

Beds (Geol. Surv. Vic. 1:250,000 series; St Ar- naud sheet, 1976 ). No large faults outcrop in the St Arnaud area, but the along-strike extension of the Avoca Fault Zone (a NE-trending, major fault system) may occur several km to the SW (Gray, 1988).

The regional orientation of the main foliation, $2, is steeply dipping to the SW. However, near the old Lord Nelson Mine close to the township of St Arnaud, the structure is more complex (Fig. 8a). Two intense crenulation cleavages ($2 and $4) are visible in outcrop of the host rocks of the deposit. $2 dips steeply to the SW, whereas $4 is usually subvertical and strikes NE. No major folds are present in the mine sequence but some minor F ° folds (Fig. 8b), detected by detailed

structural logging of drillcore, verge to the NE as does the bedding/S2 relationship. L ° plunges gently to the SE. $3 is a spaced crenulation cleavage, and measurements from outcrop, oriented drillcore and oriented thin sections (fig. 2 in Forde, 1991 ) show that it has a subhorizontal orientation.

Gold-bearing ore: The gold-bearing structures contain clasts of waUrock within which $2 and a later foliation (S3/S 4 ) are rotated relative to their orientations in the surrounding rock (Forde, 1991). They are, therefore, considered to be D4 breccia veins and have the following characteristics: (a) They occur adjacent to F ° minor fold hinges; (b) they are sub-parallel to bedding; (c) Microstructural timing shows that the veins formed during D4 (Forde, 1991 ); and (d) Laminated veins occur adjacent to all the St Arnaud breccia veins observed.

Quartz, carbonate, sulfides (pyrite and arse- nopyrite) and chlorite alteration of the country rocks is intimately associated with these veins.

4.3. Ballarat

Country rock: The BaUarat Goldfield occurs about 100 km NE of Melbourne (Fig. 1 )° The area is divided into three smaller fields; Ballarat East, Ballarat West and Nerrina which occur in an en echelon arrangement with a long axis aligned NE-SW (Baragwanath, 1953t. This study was primarily concerned with the Ballarat East field where the host rocks are tightly folded on N-S axes with vertical east limbs and a steep crenulation cleavage ($2) parallel to the axial planes (Fig. 9a). The Ballarat West folds are upright; no folds have been recognized in the Ner- rina field nor for a considerable distance west and east of the Ballarat field (Baragwanath, 1953). The block of rocks hosting the Baltarat deposits is bounded by the Avoca Fault Zone to the west and probably the Campbelltown and Muckle- ford faults to the east (Wilkinson, 1988a). Late strike-slip crosscourses trend NE-SW across the field and are more common in the Ballarat East field than in the Baltarat West or Nerrina fields. Thin sections reveal the presence of a flat-lying

A. Forde, T.H. Bell/Ore Geology Reviews 9 (1994) 33-59 43

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Fig. 9(b). Cross-section looking north along line A-B in Fig. 9a. F ° axial planes dip steeply west. Leather Jacket faults and associated horizontal spurs occur on the vertical east limb of the major First Chance anticline. (Modified from Baragwanath, 1953, and D'Auvergne, 1988. )

S 3 that deforms $2 (figs 5 and 6, Forde, 1991 ). The form of $3 varies from a kink-like crenulation cleavage to very spaced and differentiated microshears.

S 4 is also a non-penetrative foliation. It is visible in core and in outcrop and, in thin section, overprints $3 (Forde, 1991, fig. 2 ). Twenty measurements of $4 from both oriented drillcore and


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outcrop show that $4 strikes E-W and dips steeply (Fig. 5 ). Lack of a consistent sense of bulk movement, suggests that the strain during D4 was coaxial.

Gald-I~ming ore: The gold-bearing structures in BaUarat East are breccia veins that formed in D4 (Forde, 1991 ). Three types of quartz bodies contain gold.

( 1 ) Leather Jacket Formations are large quartz bodies located on the leather jacket faults (large reverse faults named for the miner's term for the leathery nature of the sheared slates on the fault). These faults lie parallel to bedding on the more gently dipping west limbs of F ° anticlines and cut across the vertical east limbs, having a lower dip where this happens (Fig. 9b). The leather jacket formations occur in these areas of lower fault dip, and are lenticular bodies of quartz commonly ar- ranged in steeply pitching shoots on the faults

(Baragwanath, 1953). Baragwanath ~1923) noted that; (a) "The quartz forming the leather jacket formations is never laminated" in contrast to the fault reefs. (b) "The quartz development is largely re- stricted on these leather jacket faults to the fis- sured beds". This observation is not clearly explained, but probably means that the largest occurrences of quartz in the faults are where the fault intersects beds carrying more D2 spurs than usual.

(2) Spurs or spur veins are horizontal (Bar- agwanath, 1953 ) crack-seal veins (Fig. 6a, figs. 5 and 8 of Forde, 1991 ) lying close to leather jacket faults. Baragwanath (1953) described these spurs as a series of branches that extended outwards more or less horizontally from the fault planes in a pinnate arrangement (Fig. 9b), and they can be regarded as filling horizontal tension

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Fig. 1 1 ( b ) . D e t a i l ( 1 : 1 0 0 0 ) of pits along a steep F ° short limb. This limb is locally overturned as indicated by bedding being steeper than $2. Map symbols same as shown in legend for Fig. 1 l a except pits are solid black and fold hinges are dashed with stemless arrows.

gashes related to the reverse faults. In the Brit- tania United mine in Ballarat East, horizontal spur veins were richly mineralized but were barren where vertical (Gregory, 1907). This suggests that the mineralization was emplaced subsequent to vein formation and is discussed below.

(3) The "lodes" of Ballarat East (Dunn's Lode and Robert's Lode) were saddle reefs (Barag- wanath, 1923; see Fig. 9b). Little information on the distribution of the gold within these veins is available.

4.4. Inglewood

Country rock: The Inglewood deposit is located about 40 km NW of Bendigo (Fig. 1 ). A lower Devonian granite outcrops less than 5 km to the west; no major faults have been recognized in the Inglewood area (Geol. Surv. Vic 1:250,000 series; St Arnaud sheet, 1976 ). In this area, $2 dips steeply to the E (Fig. 10a) and numerous asymmetric macroscopic F ° folds have- near vertical W-dipping limbs, whereas E-dipping limbs dip at 60 °. Their wavelengths are 100- 200 m and interlimb angles are 30 ° -40 °.

A weak horizontal $3 at Inglewood commonly occurs as the axial planes of 5-10-mm-wide kinks of $2, but may also be present as a seamy anastomosing foliation. $4 is similar in morphology to $3 but is everywhere steep, striking NE-SW to E-W. It is most intense in mullock dump samples from the now abandoned mines on the Co- lumbian reef. This was the largest and richest orebody in the Inglewood district (Cundy, 1911).

Gold-bearing ore: Mineralization at Inglewood is developed in early laminated veins that lie parallel to bedding and subhorizontal crack-seal quartz veins formed in D2 that have been overprinted by D4 breccia veins containing the gold (Forde, 1991 ). The D2 veins probably formed on reverse faults that are bedding-parallel on the east limbs of the F ° folds, but which cut across the bedding in the steeper west limbs (Fig. 10b ). This geometry is directly analogous to the leather jacket faults in Ballarat. The reefs in Inglewood are shown in mine sections as planar bodies (Fig. 10c) occurring in parallel N-S-striking sets. They

48 A. Forde. "I~H. Bell / Ore Geology Reviews 9 (1994) 33-59

consist of a mixture of quartz fill on dilational jogs of the reverse fault and subhorizontal tension gash veins overprinted by the D4 breccia veins which contain the gold.

4.5. Dunolly

Country rock: The Dunolly goldfield (Figs. l and 1 la) forms one corner of the "Golden Tri- angle" in central Victoria, and was the site of alluvial and quartz mining in the late 19th century (Caldwell, 1935 ). The host rocks are deformed, lower Ordovician, low-grade metasedimentary rocks (Geol. Surv. Vic. 1:250,000 series) and contain a group of gold-bearing quartz reefs, including the old Sydenham mine site, south of DunoUy township. In this region, strongly cleaved pelites and sandstones are steeply east,dipping and striking at 350°-360 °, but elsewhere bedding (and cleavage) strike consistently NE-SW (Fig. l la) , suggesting that this is an area of anomalous bedding and cleavage strike.

Detailed mapping shows $2 to be steeper than, but close in strike to, bedding and to lie parallel to the axial planes of asymmetric F ° folds that rapidly die out along strike (e.g., Figs 1 l a,b). The W-dipping short limbs are mostly steeper than the E-dipping long limbs and are locally overturned as revealed by bedding being steeper than cleavage in outcrop at these locations. The overall vergence is to the west; that is, the area is on the eastern limb of a large anticline, but the core of this fold does not lie in the mapped area. The F ° plunge varies within $2 such that the folds are doubly plunging. The strike of bedding, cleavage, and fold axial planes varies markedly from north to south (see Fig. 1 la) , in places producing a sinuous fold trace at map scale.

$3 is defined by the axial planes of very fine kinks of $2 that produce horizontal lineations on $2 surfaces where they intersect it. This foliation only occurs in pelitic units. A younger foliation, denoted here as $4, consisting of kink bands that deform cleavage and bedding, varies consider- ably in strike and is only developed in outcrop in the area around the Sydenham Hill roadcut. However, this youngest set of kinks was common in specimens from mullock heaps around

the old Sydenham mine. The range in orientation suggests different ages or a conjugate set of kinks, but there is no evidence to establish a relative timing.

Gold-bearing ore: Quartz mineralization in mullock from the nearby Sydenham mine shows an alteration sequence similar to that associated with gold mineralization in other central Victo- rian gold deposits. The paragenesis involved sip icification of the country rock followed by development of sparry carbonate and chlorite and the rocks show development of D4 kinks and locally an $4 foliation has formed. However, detailed microstructural work to determine with absolute certainty that the gold mineralization was syn- D4 was not carried out at Dunolly.

At Dunolly, the reefs occur on the western or eastern limbs of anticlines but not in their hinges. This is shown very clearly in Fig. 1 l b where the pits are almost entirely confined to the western limb of the folds mapped. Other large reefs, such as the Sydenham and Quaker reefs, occur on the east limbs of antiforms where the bedding dips east and the reef itself dips west (Caldwell, 1935). These are very similar in geometry to those at Ballarat (Fig. 9b).

5. Origin and structural models of mineralization

Mineralization commonly involves brecciation, alteration and replacement of the country rock as well as some infill under open system conditions (e.g., Phillips, 1972, 1986). During ore formation synchronous with deformation, the main processes operating are access of the mineralizing fluid to the margins of the about to form orezone and the ability of ions and mole- cules to diffuse in and out of the material being altered and replaced. Since the latter process is probably slow relative to the former, the jostling and brecciation associated with mineralization are probably extremely impoi'tant processes in maintaining a locally open system, as they prevent the resealing that occurs during each incre- ment of crack-seal veining (see following section). The manner in which this mineralizing fluid gets to the site of ore formation during duc-


tile deformation, that is whether by fluid flow or diffusion along cleavage seams, is debatable (compare Etheridge et al., 1984 with Bell and Cuff, 1989). Significantly, the mechanism proposed by Bell and Cuff ( 1989; see discussion on pp. 634-637 in Bell and Hayward, 1991 ) would allow an orebody to form without requiring the pathway for the material through the crust to become the massive zone of alteration that would result from the Etheridge et al. (1984) mechanism.

Vein formation, on a particular heterogeneity within a rockmass, requires the combined deformation-induced extension (resolved on the heterogeneity) and fluid pressure to exceed the combined tensile strength of the surface and the lithostatic load resolved on it (e.g., Etheridge et al., 1984). Two quite different processes have been advanced to explain two distinctive types of vein common to zones of mineralization; that is, crack-seal veins and breccia veins. Both of these processes have been proposed for the generation of gold mineralization in the central Vic- torian gold deposits, as shown in Table 1.

5.1. Crack-seal veins

The first of these types are fibrous crack-seal veins with inclusion trails of wallrock (Ramsay,

1980). Etheridge et al. (1984) inferred that this type of vein forms by an increase in fluid pressure to greater than lithostatic, which, aided by concurrent deformation, causes the rock to fracture. They argued that, on fracturing, the fluid pressure is immediately lowered, causing the fracture to shut and be sealed by a 20-40 ~m in- crement of vein material. Such a process should allow little or no interaction of the fluid bleeding into the fracture with the surrounding country rock and hence generate little or no mineralization. This has been confirmed by detailed microstructural investigations of many different structurally controlled orebodies by Perkins (1984), Swager (1985), Bell et al. (1988), De Roo (1989), Aerden (1991, 1993), Forde ( 1991 ), Van Dijk ( 1991 ) and Hayward ( 1992 ). Crack-seal veins in these deposits are not mineralized where there has been no overprinting by breccia veins, the second type mentioned above.

5.2. Breccia veins

Breccia veins are distinctively different from crack-seal veins as the latter veins generally re- flect the mineralogy of the host rock, whereas the former always show alteration presumably due to open system conditions. They locally contain clasts which may bear cleavages rotated relative

Table 1 Summary comparison of the models for mineralization in the Central Victorian Gold Fields with examples of the proponents of each model

MINERALIZATION MODEL EXAMPLES

Discrete deformation stages not recognized. Main quartz bodies are syntectonic and due to breccia veining/replacement. Gold deposition during brecciation/veining later than the main quartz veins.

Discrete deformation stages not recognized. Folding, cleavage, all types of veins are syndeformaiton and broadly contemporaneous. Quartz veins due to crack-seal processes with no involvement of breccia veining or replacement. Gold deposition as pan of vein formation during deformation.

Recognition of existence of at least four discrete deformation events. Bedding-parallel veins form pre folding possibly during D~ or very early in D2. Folding, main cleavage, saddle reefs, fault reefs and spur veins are D2. Later breccia veins and gold are D4. Mineralization mechanism involves gaping, breccia veining and gold mineralization on earlier-formed structural discontinuities during D4.

Stillwell 1918a,b, Stone 1937, Chace 1949

Cox et al. 1987, Cox et al. 1990

Forde 1991, Forde and Bell, this paper.


to the same cleavages in the surrounding country rock, as well as in adjacent clasts, by angles as great as 90 ° (e.g., Perkins, 1984; Swager, 1985; De Roo, 1989; Forde, 1991;Aerden, 1991, 1993; Van Dijk, 1991; Hayward, 1992 ).

The cleavages may be less developed in the rotated clasts than the same cleavage that pervades the surrounding country rock (e.g., fig. 4 in Swa- ger, 1985 ) indicating that the breccia vein formed during regional ductile deformation rather than faulting (cf., Phillips, 1972, 1986; Sibson et al., 1988; Sibson, 1992--Murphy, 1984 recognized breccia veining due to ductile deformation but attributed it to shear failure on a major fault at depth). This is strongly supported by the presence of pods of breccia vein located in fold hinges or against earlier formed veins that are quite isolated from faults such as the pod shown by Per- kins (1984, figs. 30 and 38), the 500 and 650 copper orebodies at Mount Isa Mine (Bell et al., 1988), the main orebody at Elura (De Roo, 1989) and the orebodies at Hercules (Aerden, 1993). Breccia vein systems may also form on earlier developed faults during subsequent regional ductile deformation rather than during faulting (cf., Sibson et al., 1988 ); for example the 1100 and 1900 copper orebodies at Mount Isa Mine (figs. 7, 14, 15 and 16 in Bell et al., 1988). The breccia veins described by Robert and Brown (1986a,b) may be transitional between the types described herein and those described by Phillips (1972, 1986), Murphy ( 1 9 8 4 ) a n d Sibson (1992).

Breccia veins are commonly located on a variety of heterogeneities including portions of the contacts of previously formed veins with their wallrocks, whether these veins be crack-seal or some other type. Mineralization within the copper deposits of Mt. Isa (Perkins, 1984; Swager, 1985; Bell et al., 1988) and surrounding regions (Van Dijk, 1991 ), the Pb-Zn deposits of Elura (De Roo, 1989), Hercules and Rosebery (Aer- den, 1991, 1993) and the gold deposits of the Victorian goldfields (Forde, 199I ) and Haile (Hayward, 1992) is always in and immediately adjacent to breccia veins of this type.

5.3. Breccia vein formation

Breccia veins cannot develop by incremental growth in the manner of crack-seal veins, even though they can form during the same deformation event and are commonly juxtaposed (Per- kins, 1984; Swager, 1985; Bell et al., 1988; De Roo, 1989; Aerden, 1991, 1992; Forde, 1991 : Van Dijk, 1991 ). We argue, in contrast to the origin inferred by Etheridge et al. (1984) for crack-seal veins, that an increase in fluid pressure is not required for breccia vein formation; instead the fluid pressure may remain around lithostatic, and the lithostatic load probably remains relatively constant as breccia veins form.

We contend that the parameters most critical for veins of this type to form are deformation- induced extension across, and the tensile strength of, the heterogeneity. Veins not only form along heterogeneities with the least tensile strength (such as the margins of older veins, foliation and faults), but also across other surfaces where the pattern of deformation partitioning and manner of deformation (axial plane shear versus reactivation of pre-existing foliations) can change (e.g., Bell et al., 1988).

We argue that instead of the rock being jacked open by fluid pressure exceeding load pressure during concomitant deformation, followed by collapse and sealing of the fracture, breccia veining involves the rock being pulled apart by opposing shears across structural heterogenities while the fluid pressure is close to tithostatic. This process, is shown schematically in Fig. 12. Syn- thetic shear parallel to the axial plane cleavage S~ (which would be forming N-S in Fig. 12) ro- tates both bedding and the vein anticlockwise until bedding reaches the orientation shown in Fig. 12a where it can begin to reactivate. The phenomenon of reactivation has been described in some detail by Bell (1986 } and involves antithetic shear along the bedding planes as shown in Fig. 12b. However, the vein is in an orientation where reactivation is impossible and continues to deform by synthetic shear parallel to the axial plane.

A. Forde, T.H. Bell I Ore Geology" Reviews 9 (1994) 33-59 51

[ / / / / / / / / A v / / / / ~ / / / A v , , , . •

~ V / / A I l I A

azZA

I l I A Y I / I I / I /

V / / A ~ / / / A I l i a

I [a/ / ///i/ /A

Fig. 12. Sketch showing mechanism of breccia vein formation from a pre-existing crack-seal vein. (a) . Cross-section through bedding (diagonal lines) cross-cut by an earlier formed crack-seal vein (runs E-W). The synthetic shear sense on the vertical axial plane cleavage (cleavage not drawn in but shear sense is shown) is sinistral with an antiform forming out to the right. (b) . Same as (a) except that bedding has started to reactivate as described in detail by Bell ( 1986 ). The sense of shear on the bedding during reactivation is antithetic to the antiform forming out to the right. However, the vein is at too high an angle to the cleavage to reactivate and continues to deform by synthetic shear along the axial plane cleavage ( See Bell, 1986 for details of this process ). (c). The opposing shear senses across the vein/country rock contact have resulted in gaping (black shaded region) through the country rock being pulled off one or both margins of the vein. This can occur off both sides but is most common off only one margin of the pre-existing vein. (d) . The result of the implosion of the country rock and vein into the space created along the gape in (c) due to the high fluid pressure in the wall rocks. The resulting brecciation and jostling prevents boundaries of blocks resealing, allowing access of fluid and material for reaction and tranformation of the pre-existing rock by replacement.

The resulting opposing shears across the vein/ country rock contacts create a gape as shown in Fig. 12c and generate a drastic reduction in fluid pressure on the pullapart. This causes a large im- balance in fluid pressure between the waUrock and the adjacent void, and results in implosive brecciation of the wallrocks. Brecciation fills the gape with jostled or rotated wallrock clasts that do not fit together (Fig. 12d; superb examples of this process have been documented by Swager, 1985, and for the deposits described herein by Forde, 1991 ). The margins of this zone also contain fractured and jostled wallrock that cannot close precisely. This mismatch results in the ac- cumulation and maintenance of a fluid phase within the breccia zone and its jostled margins, which in some environments has a considerable capacity for mineralization by alteration and replacement of the brecciated and fractured wallrock (e.g., Perkins, 1984; Swager, 1985; Bell et al., 1988; De Roo, 1989; Aerden, 1991, 1993; Van Dijk, 1991; Hayward, 1992 ).

The role of ongoing deformation in relation to

such heterogeneous structures is, therefore, critical in the localization of breccia vein formation in time and space. This is especially true of weak deformations that occur late in the deformation history because they are coarsely partitioned in space and have a wide range of pre-existing heterogeneities on which to nucleate (e.g., fig. 17 in Bell et al., 1988). Within the Victorian Gold- fields, gaping occurred on bedding, early laminated veins, D2 crack-seal veins and D2 breccia veins during D 4. The exact mechanism of breccia vein formation during D 4 varied from deposit to deposit depending mainly on the geometry of the pre-existing F2 ° folds, D2 reverse faults, and veins in the area. Consequently, although the models of breccia vein generation for each of the five deposits differ slightly in emphasis, all are variations on the common theme of development of local contrasting shear senses during D4

that resulted in gaping along and brecciation adjacent to pre-existing structural discontinuities. They are described in some detail below.


5.4. Structural setting for breccia veining in the Victorian Goldfields

5.4.1. Breccia vein development through gaping on saddle veins

Since both S 2 and $4 are steeply dipping foliations, their intersection, the L 2 crenulation axes, are steeply pitching within $2 in each deposit. No L 4 stretching lineations were observed on $4 planes. However, extension during D 4 occurs on horizontal veins (such as the spur veins) rather than vertical ones. Consequently, L 4 was probably steeply pitching within $4.

For a zone of mineralization to become an economically viable ore deposit, a large-scale zone of breccia veining is needed. Consequently, to successfully predict the location of blind targets for drilling, large-scale structures that can gape are absolutely essential. Such structures in- clude pre-existing faults and folds.

Analysis of all five deposits examined herein in this light has revealed two basic structural styles of mineralization; ( 1 ) breccia vein development through gaping on saddle veins, (2) breccia vein development through gaping on structures oblique to bedding. The generation of each of these modes of mineralization depended on very similar processes at each deposit that differed only in the geometry of the pre-existing heterogeneity that was overprinted by D6 (see below). At Bendigo, the site of a large volume of mineralization, both basic types of structures were present on which gaping could occur. How- ever, there was a large range of structures present of the second type on which gaping and hence breccia veining could occur. D4 was also relatively more intensely developed than at some of the other deposits. The much decreased scale of mineralization at Inglewood probably resulted from a lower degree of development of D4.

It is noteworthy that the orthogonal relationship between $2 and $4 at Ballarat eliminated one major structural environment for mineraliza- t i o n u t h e saddle reefs. However, the strong development of veins on leather jacket faults, combined with the intensity of Da, provided a large volume of sites for gaping. Consequently, mineralization, near identical to that at Inglewood, was developed in much greater abundance (see below).

At Bendigo and St Arnaud, the average strike of 8 4 is NE, at a moderate angle to the more N- S-trending D2 folds. Gaping and hence breccia veining can be produced wherever a zone of D4 deformation cross cuts a D2 anticline. This is shown in Fig. 13 for both a coaxial and non- coaxial overprint by the D4 event. In the coaxial case (Fig. 13b), the weak D4 event causes bedding on both limbs of D2 folds to antithetically reactivate relative to developing $4 from the commencement of deformation, but the hinges of two D2 folds can only deform by synthetic shear on $4 (e.g., Bell, 1986). The two modes of deformation interact near the D2 hinges, tending to pull the country rock away from the more competent pre-existing quartz veins. This results in gaping along the D2 hinge, especially in situa- tions where reactivation is strongly localized along the bedding/early vein contacts, causing breccia veins to develop on the margins of these older veins (Fig. 12 ). Alternatively, if D4 is non- coaxial with a dextral shear sense looking NE. gaping of the quartz vein-country rock bound- ary will tend to occur on the eastern side of the antiform as shown in Fig. 13c. Thus, breccia veins can develop wherever kilometre-scale partitioning during D4 has resulted in the local presence of this weak event (e.g., the Hustlers Line of reefs; cf., fig. 14 in Bell and Hayward. 1991 ).

Some locations offset to the east from north- plunging D2 culminations have become mineralized (Whitelaw, 1914). The cross-sectional geometry along the line C-D in Fig. 13a is very similar to that across the fold hingealong the line A-B and shown in Fig. 13b. If earlier developed quartz veins are intersected twice along such a section, then gaping and mineralization could develop in an identical manner to that shown in Fig. 13b for the cross-section along the line A-B. However, no zones offset to the west from south- plunging D2 culminations are mineralized and yet identical cross-sectional relationships are possible in these locations. This suggests that the L~ stretching lineation was steeply south pitching within $4, rather than vertical, as this geom-


I

I

I ] Ic Fig. 13 ( a ). Shows a north plunging D 2 anticline. The narrow stippled layer is either a pre-D2 vein or a sy n -D 2 saddle reef. A NE- trending zone of deformation generated during D4 occurs between the dashed lines running across the fold. (b) . Cross-section along line A-B in (a) showing the effects of overprinting the D2 fold during D 4 with a zone of coaxial shortening on the scale of the pre-existing D2 fold between the two dashed lines which in this figure have an apparent dip parallel to $4 as shown. Reacti- vation of bedding will occur on the limbs of the earlier fold but cannot occur in the hinge. Here, synthetic shear (parallel and relative to the fold axial plane ) will tend to develop a new cross cutting cleavage $4. However, gaping (shown by black shading) will tend to occur where the shears must switch sense from antithetic to synthetic near the hinge (shear cannot be accommodated along bedding lying perpendicular to $4 during D4) pulling the bedding off the vein because of the strong competency contrast that it creates. This will result in implosion of the adjacent wall rock of predominantly the weaker bedded sediment into the gape resulting in the generation of breccia veins on the margin of the older vein. A somewhat similar cross-section in appearance will also occur along the line C-D and could result in gaping and brecciation down the NE side of an anticlinal hinge as locally observed at Bendigo. (c). Cross-section along line A-B in (a) showing the effects of overprinting the D2 fold during D 4 with a zone of consistently dextral shear on the scale of the pre-existing D2 fold between the two dashed lines. Such non-coaxial deformation can only result in reactivation on the eastern limb of the earlier developed D2 fold. This will lead to gaping (shown by black shading) against the quartz vein and hence implosion and brecciation/jostling and mineralization on the eastern side of the D2 antiform.

etry would result in no gaping in the latter locations. An alternative way in which these NE- plunging zones of mineralization on limbs of D2 folds could have formed is if near-horizontal spur veins were present on the limbs within the zone through which the most intense deformation partitioned during D4 as described below.

5.4.2. Breccia vein development through gaping on structures oblique to bedding

The most simple example of how opposing shear senses can develop across heterogeneities d u r i n g D4 and result in gaping and breccia veining has been shown in Fig. 12 where $4 cuts across bedded rocks containing an earlier formed crack- seal vein. The sense of shear operating during cleavage formation is opposite to that occurring when bedding becomes reactivated. However, if

the vein at this stage lies at a high angle to the developing foliation, no reactivation will take place along the vein walls or along structures within the vein lying near parallel to the vein walls (which may be present where the vein is a laminated one). Consequently, opposing shear senses are established across the vein wall-bedding contact resulting in gaping as shown in Fig. 12. If fluid pressure equals load pressure then, on gaping, the reduction in pressure at the gape causes explosive brecciation of the wallrocks into the space provided on the gape with jostling and fracturing of country rock in more distal portions of the breccia zone (e.g., Phillips, 1972, 1986; Murphy, 1984; Swager, 1985; Bell et al., 1988; Aerden, 1991; Van Dijk, 1991; Forde, 1991; Sibson, 1992).

This style of breccia vein formation occurs in all of the deposits and the most important styles


can be essentially divided into two subsets. Gaping on spur veins: Spur veins were present

in probably all of these deposits but, at Bendigo, breccia veins forming on spur veins (Forde, 1991 ) contained 50% of the gold mined (Still- well, 1917, 1918a,b, 1919 ) and, consequently, were a far more significant resource than the more famous saddle reefs. Spur veins were also a significant resource at Ballarat (Fig. 9b). The spur veins formed initially during D2 as sub-horizontal crack-seal veins at a high angle to bedding. During D4 they gaped against bedding in an identical manner to that shown in Fig. 12, resulting in brecciation and jostling of the adjacent country rock. The influx of fluid resulted in reaction, replacement, alteration and mineralization of both the rotated and jostled blocks within and against their margins, respectively.

Gaping on fault veins oblique to bedding: This type of mineralization occurs in all deposits but is the predominant type at Ballarat, Inglewood and Dunolly. This is probably because $4 is near

orthogonal t o S 2 in each of these deposits which tends to prevent gaping in D2 fold hinges: for example, saddle reefs were not a conspicuous fea- ture (Baragwanath, 1923 ) at any of these deposits. It is noteworthy that the "saddle reefs" of the Duma's and Robert's Lodes at Ballarat. which both lay on the First Chance Anticline immediately above and in contact with the Number 2 leather jacket (Fig. 9b), were not saddle reefs. Baragwanath's ( 1953 ) cross-section through the Llanberris No. ! Mine does not show a saddle reef in the location of Roberts Lode and this was confirmed by recent drilling between these lodes by Ballarat Goldfields (Fig. 9b). Instead, as shown by Baragwanath (1953), it consists of a zone of mineralization trending along or very close to the axial plane of the Last Chance Anticline.

The leather jacket faults at Ballarat, and the vein-filled faults at Inglewood, predate D4. Much of the vein quartz along both of these fault systems could have been emplaced as quartz fill of

II v / /~

A

by D¢ X Z B

J

M Z c

Fig. 14(a). Typical cross-section perpendicular to $2 at Ballarat. Cross-sections at Inglewood and Dunolly are near-identical except that the veins along faults are oblique to bedding on the west limb of the antiform (e.g., Figs. 10b and 1 lb respectively). X Y and Y Z mark the west and east limbs of the antiform sketched in (b). M N marks a vein along a fault on the YZ limb as sketched in 3-D in (c). The shear sense during reactivation of the folded bedding during D4 is shown on the YZ limb. (b). Zone of deformation due to D4 that is orthogonal to a pre-existing D~ fold. D4 causes a new fold to overprint the earlier fold as shown. The overprinting fold is hidden on the Y Z limb and has been drawn in (c) with the X Y limb removed. (c). Three-dimensional diagram of the Y Z limb of the D2 antiform, where Cut by a vein along a fault (MN) as shown in (a), showing the overprinting effects of a fold produced in D4. The solid (in front) and dashed arrows (behind) on the KL limb in the vein parallel to a fault show the synthetic shear sense operating on the $4 cleavage during D4. The solid and dashed arrows on the K'L ' limb in the bedding show the antithetic reactivation shear sense (Bell, t 986 ) operating on the bedding from essentially the commencement of D4 and also shown on the section in (a). These shear senses are opposed and result in gaping of the country rock/vein contact, in a manner very similar to that shown in Fig. 12(c) but not drawn on this diagram for clarity, leading to implosion and the generation of breccia veins that contain the gold mineralization.


a dilational jog on a D2 reverse fault (e.g., as described by Cox et al., 1987, 1990, for their single deformation event which correlates with our D2 ). However, Forde ( 1991 ) has determined that although quartz vein material was deposited during D2, in both the faults and spur veins in adjacent tension gashes, mineralization formed predominantly by breccia veining during D4.

This occurred by gaping on the margins of these earlier developed veins and the country rock during reactivation of the bedding and a mechanism for this is shown in Fig. 14. At Ballarat, only the contacts of east-dipping bedding with leather jackets were mineralized (cf., Baragawanath, 1923, 1953) because reactivation of bedding from the commencement of D4, without reactivation of the leather jackets, was only possible on this limb. That is, the veins only lay at a high angle to So on the eastern limbs of antiforms (as shown in Fig. 14a), and hence gaping, breccia veining and gold mineralization could only be created on these limbs as shown in Fig. 14c and described in the figure caption. An identical style of mineralization developed at Inglewood except that it is located on the west limbs of anticlines. As at Ballarat, earlier developed faults and associated veins lay at a high angle to bedding (e.g., Fig. 10b, along the line of the Columbian and Maxwell's Shafts) compared to near parallel to bedding on the other limb. This caused gaping to develop during D4 with a mirror image to that shown in Fig. 14C. At Dunolly, mineralization geometries near identical to those at Ballarat as well as Inglewood occurred on the eastern and western limbs ofantiforms respectively (e.g., the Sydenham and Quakers Reefs and the pits mapped in Fig. 11 b).

6. Discussion

6.1. The role of the saddle reefs

The spectacular geometry and multiplicity of the saddle reefs at Bendigo has tended to domi- nate work and concepts on the origin of gold in the Central Victorian Goldfields (e.g., Nicholas, 1881; Dunn, 1896; Bateman, 1918; Stillwell,

1918b; Taber, 1918; Lindgren, 1920; Hulin, 1929; Mckinstry, 1935; Chace, 1949; Cox et al., 1987). Although the bulk of mines at Bendigo are no longer accessible, the breccia vein character of the mineralized portions of saddle reefs is very obvious in the numerous photographs taken of these veins by Dunn ( 1896; e.g. Fig. 6b) and is very similar to that of the saddle reefs accessible in the Central Deborah Mine. Forde (1989, 1991) demonstrated that these latter saddle reefs formed late during D2 as breccia veins. They developed due to reactivation of bedding on F ° limbs combined with the maxi- mum vertical extension during D2 being at the hinges of these folds, as shown in Fig. 13; this caused gaping on the bedding at the anticlinal axis and in particular was localized on the laminated veins which pre-date the F ° folds (see also plates 9, 10 and 14 of Dunn, 1896). However, these saddle reefs predate the gold mineralization, which occurred by another younger breccia veining event in D 4 (Forde, 1991) superim- posed over the original bedding parallel veins in the fold hinges. Indeed, as described above, mineralized saddle reefs contain only a small portion of the total bulk of gold mined throughout the Central Victorian Goldfields, including even Bendigo itself.

Although the breccia vein and/or replacive character of the mineralized portions was recognized by several workers (Bateman, 1918; Still- well, 1917, 1918b, 1919; McKinstry, 1935; Chace, 1949), only a few realized that it was a later phase than the bulk of the vein material. However, it was recognized by some, including (1) Hulin (1929), who proposed that the ore shoots in Bendigo conformed to a pattern he observed in many deposits in that they formed by brecciation of earlier-formed host veins, (2) Stone (1937, p. 890), who stated that "veining by later quartz is characteristic of gold-bearing quartz" (p. 890) and "that gold and rarer sulphides are associated with later generations of quartz" (p. 891), and (3) Chace (1949), who wrote "it may be considered that interminerali- zation fracturing and brecciation, and deposition of late quartz, sulphides and gold, were su- perimposed on the quartz bodies that formed by

56 A. Forde. T.H. Bell / Ore Geology Reviews 9 (1994) 33-59

replacement and open space filling". "It is likely that vein reopening and brecciation took place after the main quartz bodies had formed. The ef- fect ..... was to maintain permeability so that late solutions carrying some silica, sulphides and gold gained access to the otherwise solid quartz bodies".

6.2. Implications for exploration

The distinction made herein between the five vein types has enabled derivation of models with a predictive capability that allow structural sites for the potential mineralization of the various pre-existing vein types to be investigated. In particular, the consistent timing of the gold mineralization during D4 (Forde, 1991 ) and its strong genetic link with zones of $4 development provides a tool that may be used in exploration for blind deposits which differs markedly from the high gold values at the surface required by prospectors before any mining was undertaken. In- deed, gold mineralization has to occur at or near the surface to be detected by both simple pros- pecting as well as sophisticated geochemical techniques. Clearly, these methods do not have the capacity to find blind gold deposits in Victo- ria. In addition, vein gold deposits are noto- riously heterogenous in terms of the distribution of ore grade.

To locate zones for taking out exploration ten- ements along which potentially large blind orebodies may be present it is necessary to delineate zones of higher strain in D4. This can be done by examination of regional geology or geophysical maps for deflections in bedding trends whose axial planes have trends in the E-W to NE-SW range covered by the bulk of $4 data in Fig. 5. These regions can then be examined on the ground or with oriented samples cored by diamond drilling from the bot tom of percussion holes if no outcrop is present, for the presence of $4. Once zones have been delineated that have been affected by the macroscopically coarsely partitioned D4, for the tatgetting of drill holes it is necessary to find the intersections of these zones with areas that contain pre-existing structures on which gaping could have occurred in D 4.

Subsequent drilling should be oriented along the trend of $4 in the immediate vicinity to take ad- vantage of the fact that a variety of earlier structures can be mineralized. Suitable earlier structures are:

( 1 ) Regions of large-scale F ° folding which have been overprinted by zones D4. Where D4 is weaker, zones where $2 lies non-orthogonal to $4, as at Bendigo and St Arnaud, are best, as these have a greater range of geometries that can be mineralized. However, zones where D4 is more intense and $4 lies near-orthogonal to $2 are also prospective, even though they contain less types of potential structural traps, as exemplified by Ballarat, Inglewood and Dunolly.

(2) Large reverse faults containing vein mineralization that lie at a high angle to bedding on one or both limbs of a fold (e.g., leather jackets ).

(3) Zones of well developed spur veins or crack-seal veins with shallow orientations.

Of these structures, the first is the easiest to locate because these regional D2 folds, have al- ready been mapped throughout the area. Conse- quently, zones where $2 has been deflected about the NE-SW- to E-W-striking axial plane of the D4 event, can be determined from regional geological and/or geophysical maps. Alluvial covered areas containing D4 deflections revealed by aeromagnetic maps would be highly prospective because the oldtime prospectors, who only had gold pans to explore with, would have missed them. Without outcrop or drill hole information, prediction of whether quartz veins are present, that can get mineralized in zones of D4, is not possible. However, within macroscopic zones affected by D4, D2 hinges can be defined by ex- tending the mapped trends of macroscopic anticlines and synclines to the north and south through the deflection as revealed by geologic or aeromagnetic maps. The portions of these D2 folds which are most likely to be mineralized (as desribed herein ) can then be targetted and drilled to determine whether suitable early quartz veins and gold mineralization are present. These drill holes will also provide structural information on the orientation of bedding and whether earlier fault veins and saddle veins, and spurs are present, that will enable further more precisely con-


trolled structural targetting of drill holes. In regions where S 4 lies perpendicular to the

axial plane of D2 folds, the only structures with a strong predictive capacity, since significant mineralization will generally not occur in Dz fold hinges, are the large reverse faults (numbered 2 above ). Since these faults may not have a significant geometric expression at the surface, drilling should be specifically targetted to intersect those limbs of anticlines where the bedding lies at a high angle to any reverse fault known from outcrop, old workings or short drillholes in the immediate surroundings.

The third type of structures mentioned above is virtually impossible to predict at depth with any certainty. Consequently, they are the least useful at the initial stages of target generation. However, spur veins were developed quite commonly during D2 adjacent to structures such as those just described, and consequently, any drilling for one of the other style of targets in zones of more intense D4 may intersect zones of such veins that have been mineralized.

Structures with unrealized potential for large- scale mineralization in the Victorian Goldfields, and which can be traced through D4 deflections, are early developed thrusts (e.g., Gray, 1988 ) in locations where they juxtapose bedding at high angles, such as on ramps (cf., Bell et al., 1988; Bell, 1991 ).

Acknowledgements

Ballarat Goldfields Ltd., Bendigo Mining NL, BHP-Utah Minerals International, Goldtech NL and Compass Resources kindly provided access to the deposits, drillcore and internal company reports. BHP-Utah Minerals International are acknowledged for providing scholarship funds for Forde. Frank Bunting is gratefully acknowledged for useful information and discussion. Mark Hinman and Chris Mawer critically read the manuscript and we thank them for their sugges- tions for improving it. Critical comments by David Groves and Rob Kerrich greatly helped improve the text.

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