T. J. Rawling

Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

A ∼400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.

Factors controlling the location of gold mineralisation around basalt domes in the stawell corridor: insights from coupled 3D deformation – fluid-flow numerical models

We present coupled 3D deformation – fluid-flow models which place constraints on the importance of basalt dome shape and interpreted synmineralising shortening direction in localising gold mineralisation around basalt domes in the Stawell corridor, western Victoria. Gold mineralisation in the Magdala orebody at the Stawell mine occurs predominantly within a thin metasomatised unit named the Stawell Facies which blankets the basalt domes and also occurs close to parasitic fold-like basalt lobes on the basalt domes. In dome-scale models that do not contain basalt lobes, areas with the maximum fluid-flow rates occur on the tops of the flanks of the domes where there is a dramatic change in dip of the basalt, and a change from contraction to dilation which creates a significant pore-pressure gradient. In models that contain basalt lobes, the location of high fluid-flow rates is strongly controlled by the presence of these lobes. High fluid-pressure gradients are created between the contracting Stawell Facies in the area between the lobe and the main domes and those areas dilating above. Areas of significant dilation occur on the shallow-dipping portion at the top of the dome and cause fluid to flow towards them. Areas that have significant dilation are also areas of tensile failure in some cases and are coincident with areas of known quartz vein-associated mineralisation. In the Magdala Dome models, only the east-northeast – west-southwest- and east – west-shortened models record high fluid-flow rates in areas of known mineralisation, which is consistent with the interpreted synmineralisation-shortening directions. Therefore in this situation, fluid-flow rates during east-northeast – west-southwest- and east – west-shortening can be used to indicate the potential location of gold mineralisation. In numerical models of the Kewell Dome (a prospect to the north), the position of areas of high fluid-flow rate when shortened in the east-northeast – west-southwest and east – west direction, combined with information from limited drilling, indicated the potential for gold mineralisation at the southwest end of the dome. Diamond drillholes in this area yielded significant gold values.

3D structural modelling and implications for targeting gold mineralisation in western Victoria

A 150×150 km area of western Victoria has been modelled in three dimensions to a depth of ∼20 km. This was constructed through integrated analysis and serial cross-sections of geological and geophysical datasets, utilising mapped positions of major faults, intrusive bodies and lithostratigraphic packages as primary inputs. These are extrapolated to depth and under cover through interpretation of upward continued multiscale wavelet edges of aeromagnetic and gravity data, inversion of the gravity field and the positions of acoustic boundaries. The objective was to develop an understanding of crustal structure in the context of gold mineralisation potential. The upper crustal structure is modelled as comprising a planar array of northwest-trending, steep to moderate inclined mainly east-dipping faults (Moyston, Pleasant Creek). These are interpreted to merge with a basal detachment (Western Fault). This elongate dome-shaped detachment overlies a buried wedge of inferred Proterozoic basement. We suggest that gold distribution in the upper crust may be influenced by the position in the mid-crust of the leading edge of the wedge and its interface with the mafic substrate that largely encloses it. The Coongee Fault appears to be a first-order regional-scale control on the localisation of gold deposits adjacent to basaltic dome prospects in the Stawell corridor. It represents a backthrust, with superimposed sinistral transpression, and is interpreted to have developed above the mid-crustal ramp detachment. Cross-faults that intersect the Coongee Fault may have high exploration potential for localising both orogenic- and intrusion-related gold.

Application of 3D models and numerical simulations as a predictive exploration tool in western Victoria

3D models and computer-based numerical simulations have been used in the exploration industry for some time to visualise the geometry and mechanisms resulting in the formation of orebodies. However, due in part to computational limitations, few numerical simulations have been run on complex (real) geometries in order to predict the location of new ore systems. Presented here are the results of an exploration program developed by the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) and MPI (now Leviathan Resources) in the orogenic-gold system of western Victoria that utilised 3D modelling and numerical finite-element simulations to successfully target several new orebodies and predict their geometries and extent. Existing drillcore databases were utilised to constrain the geometries of known deposits and associated mafic domes, the effects of known post-mineralisation faulting was systematically removed and syndeformation fluid flow was then modelled within the system. The results of these simulations were compared with the known geometry of the mineralised systems about these deposits in order to test the simulation parameters and accuracy. 3D models were also developed of poorly constrained target domes in regions with no outcrop utilising potential-field datasets and limited drilling data. Simulations were then run on these model geometries using the tested parameters in order to predict the likelihood of mineralisation in these systems, its geometry and (most importantly) its location. These targets were then drilled resulting in the discovery of previously unknown gold deposits associated with the Kewell Dome northwest of Stawell.

Numerical modelling of an evolving gold-lode system: structural and lithological controls on ore-shoot formation in the Magdala goldmine, western Victoria

Variable distribution of elevated gold grades in the Magdala Central Lode system is controlled by preferential and localised reactivation of pre-existing faults in a progressively rotating stress field. Populations of slickenline lineations on fault surfaces in combination with extension vein arrays associated with the faults indicate Central Lode formed initially during southwest – northeast compression, but was subsequently locally reactivated during east – west compression on lower angle fault segments to produce a series of ore-shoots within the overall lode system. A series of 2D cross-section numerical models have been used to examine how deformation and fluid flow are portioned at different stages of the lode evolution. Models have been constructed in the plane estimated to contain maximum and minimum principal stresses during the events leading up to, and during, mineralisation as indicated by the lineation and extension vein data. Results of the models show that during southwest – northeast compression, a major shear zone forms along the boundary between the Stawell Facies and the overlying Albion Formation consistent with the extent and location of the Central Lode Shear Zone. In a second series of models it is shown that during east – west compression, shear failure is considerably more localised to regions overlying lower angle lithological contacts. This localisation of shear failure in these east – west compression models shows strong correlation with the distribution of gold in grade shell models, confirming the structural and lithological factors identified as controlling the mineralisation.

Influence of volcano-sedimentary facies architecture on strain partitioning during the evolution of an orogenic-gold lode system, Stawell, western Victoria

The localised distribution of gold in a series of ore shoots adjacent to the western flank of the Cambrian Magdala Basalt is controlled by formation and partial reactivation of faults and shear zones that closely follow the geometry of the basalt. The recognition of the controls on the geometry of the basalt and overlying metasedimentary units is therefore vital for exploration in the region. Detailed structural mapping around one of the steep, west-dipping basalt sheets that comprise the Magdala Basalt, as well as the overlying metasedimentary units, has been used to compare the recorded structural history in the basalt relative to that in the metasediments to determine how strain has partitioned during the evolution of the deposit, and if the geometry is a product of primary volcanic processes or early ductile deformation. The results of this work show that much of the deformation history at the Magdala mine is recorded within the metasediments, while the basalt itself appears only affected by the late brittle deformation. The lack of evidence for early ductile deformation in the basalt and the partitioning of strain into the Albion Formation early in the deformation process suggests that the variable geometry of the basalt is a product of primary volcanic processes, rather than ductile deformation. The irregular geometry of basalt flows along the southwestern flank of the Magdala Basalt may be related to the dominance of distal volcanic facies, primarily pillow basalt, the variability of which influences localised deformation and gold mineralisation during an episode of low displacement east – west compression.

Structural and lithological controls on the high-grade Hangingwall Reef quartz – gold veins, Stawell, Victoria

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.

Thermal insulation and geothermal targeting, with specific reference to coal-bearing basins

Coal-bearing basins have increased economic potential for enhanced geothermal systems owing to thermal insulation provided by the coal and associated organic-rich sediments. In such insulation-dominated prospects, heat refraction effects associated with buried insulators can produce negative surface heat-flow anomalies in the most prospective areas. The Latrobe Valley in Victoria, Australia, is an archetypal coal basin. Using numerical simulations incorporating the coal geometries, we show that the Latrobe Valley coals might elevate the temperature of the rocks at 4 km depth by some 30–35°C relative to a ‘base’ condition with no coal. This effectively boosts the average geothermal gradient by about 30%, with a corresponding improvement in the economic case for geothermal energy in the Latrobe Valley.

Structural transect and forward modelling of geophysical data across the St Arnaud Group, Victoria

The integration of detailed field mapping with 2.75D forward modelling and worming of potential-field datasets has constrained the 3D geometry of regionally significant faults and allowed a detailed understanding of the crustal-scale architecture of the St Arnaud Group in the Stawell Zone to be developed. The forward modelling has also highlighted the existence of a major west-dipping, non-outcropping fault, the Burrom Fault, between the Concongella and Landsborough Faults. The structure of the upper crust is characterised by major reverse faults. These faults originate from a basal décollement that is located within mafic volcanics underlying the St Arnaud Group. Deformation has resulted in slices of these mafic volcanics being ramped up along the traces of the major faults.

Redefined crustal structure of the Buchan Rift, northeast Victoria: evidence from potential field modelling of newly acquired land-based gravity data

ABSTRACT The Buchan Rift, in northeastern Victoria, is a north–south-trending basin, which formed in response to east–west crustal extension in the Early Devonian. The rift is filled mostly with Lower Devonian volcanic and volcaniclastic rock of the Snowy River Volcanics. Although the structure and geometry of the Buchan Rift and its major bounding faults are well mapped at the surface, a discrepancy exists between the surface distribution of the thickest rift fill and its expected potential field response. To investigate this variation, two new detailed land-based gravity surveys, which span the rift and surrounding basement rocks in an east–west orientation, have been acquired and integrated with pre-existing government data. Qualitative interpretation of the observed magnetic data suggests the highly magnetic rocks of the Snowy River Volcanics have a wider extent at depth than can be mapped at the surface. Forward modelling of both land-based gravity data and aeromagnetic data supports this interpretation. With the Snowy River Volcanics largely confined within the Buchan Rift, resolved geometries also allow for the interpretation of rift boundaries that are wider at depth. These geometries are unusual. Unlike typical basin inversions that involve reactivation of rift-dipping faults, the bounding faults of the Buchan Rift dip away from the rift axis and thus appear unrelated to the preceding rifting episode. Limited inversion of previous extensional rift faults to deform the rift-fill sequences (e.g. Buchan Synclinorium) appears to have been followed by the initiation of new reverse faults in outboard positions, possibly because the relatively strong igneous rift fill began to act as a rigid basement ramp during continued E–W crustal shortening in the Middle Devonian Tabberabberan Orogeny. Overthrusting of the rift margins by older sediments and granite intrusions of the adjacent Tabberabbera and Kuark zones narrowed the exposed rift width at surface. This scenario may help explain the steep-sided geometries and geophysical expressions of other rift basins in the Tasmanides and elsewhere, particularly where relatively mechanically strong basin fill is known or suspected.