Bendigo Goldfield – Geology
Bendigo lies within the Bendigo-Ballarat zone of the Palaeozoic Lachlan Fold Belt of eastern Australia. This major fold belt has an orogenic history stretching from the Cambrian to the Carboniferous recording arc-continent collision, marine and non-marine sedimentation, folding and faulting, volcanism and igneous intrusion and regional metamorphism. The Bendigo Goldfield is the largest “slate belt” goldfield in the world.
Cambrian volcanism and sedimentation are regionally significant in that they are the oldest rocks within the Bendigo-Ballarat zone but do not outcrop in the Bendigo region. Their presence at depth is significant in the evolution of the Bendigo Goldfield.
Lower Ordovician marine sediments and metasediments comprise the main part of the Palaeozoic bedrock at Bendigo. These sediments belong to a sequence of turbiditic sandstones, siltstones and mudstones known as the Castlemaine Supergroup. The provenance of the turbidites is entirely continental, consisting of a mixture of quartz, mica, rutile, tourmaline and lithic fragments. Due to the lack of distinctive lithological units, regional subdivision of the Ordovician is based on unique graptolite assemblages. The total thickness of the Castlemaine Supergroup is estimated to be in excess of 5 km.
In the Late Ordovician to Late Devonian the Bendigo-Ballarat zone underwent a period of crustal shortening (regional compression) resulting in extensive folding and reverse faulting which have significantly increased the apparent thickness of the Castlemaine Supergroup (Figure 1).
The Harcourt Granodiorite, a post-tectonic Upper Devonian granitoid, intruded the Ordovician sediments to the south of Bendigo (Figure 2).
In the Mesozoic (Jurassic) lamprophyre dykes intruded the Ordovician sediments, and evidence of at least one diatreme eruption has been preserved.
The Bendigo Goldfield lies in a 9 km wide NNW trending block of Lower Ordovician turbidites bounded by the regional Whitelaw and Sebastian Faults (thrust faults). The productive portion of the goldfield lies in a zone 15 km long by 5 km wide central to the fault-bounded block.
- StratigraphyThe bedrock of the Bendigo area is composed of a very uniform sequence of turbiditic sandstones, siltstones and mudstones interbedded with hemipelagic mudstone and minor ‘cone-in-cone’ limestones. Sedimentary structures are perfectly preserved and are only locally modified by deformation. Graded bedding, cross bedding and other sedimentary structures provide evidence for the younging direction of beds. Graptolites mainly within black mudstones define the age of the sediments as Lower Ordovician (Lancefieldian to Castlemainian stages).Whilst it is difficult to identify mappable lithological units, approximate stratigraphic position can be determined from weathering colours, sand to mud ratios, and the overall upward fining of the sequence. Higher stratigraphic levels are characterised by more complex bed thickness variations often forming recognisable cycles 15 to 50 m thick.
Sandstone beds are often grouped in 10 to 20 m thick packages able to be traced continuously along strike for up to several kilometres. However rapid facies changes and lensing of units over short distances along strike is also common making correlation across anticlines and even between adjacent mine levels uncertain.
- StructureThe generalised structure of the Lower Ordovician sediments within the Bendigo Goldfield is that of regular, close spaced folding with extensive local reverse faulting related to the compression event generating the folding. Within the confines of the Bendigo Goldfield, mineralisation is known to occur through at least 3.2 km of the stratigraphic succession.The main characteristics of folds at Bendigo are their regular frequency, continuity along strike, and their strong structural control of gold-quartz mineralisation. Some folds are continuous for up to 25 km along strike and mining has traced individual anticlines to as deep as 1.6 km. Wavelengths vary from up to 300 m whilst amplitudes vary from 250 m to 600 m.
Folds plunge 5o-10o south at the southern end of the field and 5o-15o north at the northern end producing a broad domal structure in the centre of the goldfield. Local variations in fold plunge give rise to smaller dome and basin structures along the strike of individual anticlines.
Chevron folding (interlimb angles of < 40o) is apparently restricted to the mineralised portion of the Bendigo Goldfield with folding to the east and west more open in character. This variation is important in developing and understanding the mineralisation controls.
Several classes of faults occur in the area, most of which, with the exception of the Whitelaw and Sebastian faults, have small displacements. The Whitelaw and Sebastian Faults are steeply west-dipping reverse faults with a strike fault component and a displacement of at least 1,500 m.
The three main types of minor faults are bedding-plane faults (backs), break faults and oblique-faults (crosscourses) all of which may host quartz-gold mineralisation.
Bedding-plane faults are reverse dip-slip faults confined to single bedding planes. They are characterised by the presence of 1 cm to 200 cm of laminated quartz associated with black carbonaceous pug. The laminated quartz is so characteristic that the term ‘bedded vein’ is usually synonymous with the term ‘bedding-plane fault’. Some bedded veins, however, are not accompanied by pug and show only small displacements parallel to bedding.
Break fault is the general term used to describe reverse faults striking sub-parallel to bedding but truncate bedding when viewed in cross section.
Break faults form as limb thrusts by reactivation of bedding plane faults which then truncate the anticlinal hinge zone. The faults have limited vertical continuity but may extend along strike for several kilometres. The faults may be either east or west dipping, repeat at regular intervals with depth and are the major structural control to mineralisation in the Bendigo Goldfield.
Oblique faults cut across the regional structural trend and were termed ‘crosscourses’ by miners because the faults crossed the reefs displacing them horizontally..
A well-developed cleavage (S1) has developed during folding and is most strongly developed in fold hinges. Cleavage occurs in two main forms dependent on host lithology. In pelitic rocks the closely spaced (sub-millimetre scale) slaty cleavage is parallel to the fold axial surface.
Cleavage in sandstones develops as a more widely spaced differentiated solution cleavage and is best developed at fold hinges as convergent fans. Cleavage lamellae are separated by up to several centimetres and are commonly refracted across bedding contacts between sandstone and shales.
- Gold MineralisationGold mineralisation at Bendigo is believed to be synchronous with a major period of regional compression (crustal shortening) during which the Ordovician sediments detached from the underlying Cambrian oceanic crust. The increase in crustal thickness of the overlying Ordovician sequences through folding and thrust faulting is thought to have caused prograde regional metamorphism of the underlying Cambrian stratigraphy resulting in the formation and release of auriferous hydrothermal fluids. Fluids migrated up fault splays associated with major regional structures to be precipitated in nearby favourable low pressure dilation zones (e.g. faults, bedding planes, fold axes) created when the hydraulic pressure of trapped fluids exceeded lithostatic pressure during local seismic events.Known gold mineralisation is exclusively associated with quartz veining. Gold occurs as:
- Free gold in quartz
- In association with sulphides in quartz
- In association with fragments or laminae of wall rock in quartz
- Associated with sulphides in wall rock adjacent to quartz veins.
Surrounding the mineralised quartz reefs is a broad halo of weak hydrothermal alteration consisting of sericite, chlorite, and carbonate and disseminated pyrite. Weak pervasive silicification is also present but is restricted to sandstones.
The intensity of alteration increases as the mineralised quartz reef is approached and large crystals of arsenopyrite are commonly present within a few metres of the mineralised reef.
The reefs are composed of two main quartz types, laminated and bucky quartz.
- Laminated quartz consists of multiple laminae or veinlets of quartz, separated by millimetre thick slivers of wall rock and/or sulphides typically developed in bedding parallel veins. The gold may be either fine or coarse grained, and is typically restricted to only one or a few laminae.
- Bucky quartz describes the coarsely crystallised, non-laminated and sometimes-vuggy quartz in non-concordant veins that transect bedding. The gold, when present, is typically free and coarse grained (0.25 to 2.5 mm), except when associated with sulphide.
In addition to laminated and/or bucky quartz, veins may contain a dolomitic carbonate (typically ankerite), albite and from 0.5 to 2.5% sulphides (pyrite, lesser arsenopyrite, galena, sphalerite, minor chalcopyrite and rare pyrrhotite). The gold is typically free and coarse grained (usually 100 µm to 2 mm). Approximately 5% of reef gold production has come from roasted sulphide concentrates. Visible gold has often been reported historically in association with coarse grained galena and sphalerite.
Individual quartz reefs occur in a wide variety of structural settings and may display relatively complex cross sectional shapes but can be classified into 5 broad categories (Figure 3).
- Saddle Reefs and Bedded Leg Reefs: conventional saddle reefs occupy the dilational void of the fold hinge. These are usually small in sectional area but extensive along strike. They form in areas of high rock competency contrast. The bedding parallel legs consist of laminated quartz produced by multiple crack seal events.
- False Saddle and Neck Reefs: modified saddle reefs with extended necks either on one conjugate thrust fault or axial plane shear.
- Fault Reefs: strike parallel fault mineralisation as fissure veining, breccia, spurs, and massive quartz in discordant fault to bedding setting. Mineralisation may sit away from the anticline zone depending on the penetrative power of the fault in the discordant limb. The west or east dipping thrust faults will also significantly displace the anticline and may produce false saddles.
- Spur Reefs: stockworking or spur veins occur as stratabound zones that close against bedded or discordant faults. A brittle medium provides the best host where veining is often orientated normal to bedding and thins away from the plane of movement. Mineralisation of this style can be caused by a combination of bedding plane slip and flexure in a sandy medium.
- Cross-course Reefs: oblique fault mineralisation occurs as fissure veining, breccia, spurs, and massive quartz. Mineralisation may form long, steeply plunging shoots, which may extend well out into the limbs of the folds. The steep plunge reflects the line of intersection of the bedding oblique fault with either a receptive lithological unit or bedding parallel structure.
Cross-course reefs are relatively rare in the Bendigo Goldfield and have been responsible for only a small percentage of the total gold production. The first four categories represent the full array of typical “Bendigo style” reefs and while they display differing geometry and structural controls they are all part of a single group of reefs which display many similarities critical to the successful exploration and redevelopment of the goldfield. The most important reef similarities are:
- Occur in close proximity to the anticline axis.
- Display small cross sectional area and limited depth extent.
- Occur in groups or “clusters” suggesting interrelated structural controls.
- One long dimension which in each case is parallel to the crest of the associated anticline (i.e. sub horizontal).