The Victorian Geological Opportunity

A New Search Space - The Opportunity

How did Victoria's gold deposits form ? How many different gold mineralization styles are there? Which style should we target? Where does the geological opportunity lie? And how do we explore for it?

  • Victoria hosts one of the giant orogenic gold ore fields in the world;
  • There are two distinct sub-types of orogenic gold mineralization in Victoria, formed during different metallogenic events at approximately 445 Ma and 370-380 Ma and occurring in distinct regional domains;
  • Spatial data analysis of gold deposit patterns reveals important properties of the spatial distribution of orogenic gold mineralization. These can be explained by the deep crustal architecture of Victoria;
  • A majority of gold in Victoria has been produced from mesozonal gold systems, targeted by the old-timers in the Bendigo and Stawell zones;
  • The Melbourne and Eastern Bendigo Zones host almost exclusively all historic epizonal gold deposits (and therefore all the new opportunities too) because they are entirely underlain by older Proterozoic to Cambrian continental crust of the Selwyn Block, which has been the fluid source for these deposits;
  • Fosterville has rewritten the Victorian geological opportunity for epizonal gold deposits. We now understand that epizonal systems can develop extremely high-grade, free gold deposits.
    • This has provided insight into a never-explored search space for high grades at depth below historic mines and oxide epizonal gold targets (just like Fosterville) that only exist in the Melbourne and eastern Bendigo Zones.
  • Mawson owns or is joint venturing into three high-grade, epizonal gold exploration projects in the heart of the new search space, with numerous historic mines that lack drill testing beyond 50 metres depth. The Company also has the right of first refusal over the largest contiguous land package in the State of Victoria with 3,600sq km of high priority exploration ground.

Figure: Schematic world map showing interpreted age of basement rocks and distribution of giant orogenic gold, Carlin-type, IRGD (intrusive related gold deposits) and IOCG (iron oxide copper gold)

Modified from: https://reader.elsevier.com/reader/sd/pii/S1674987115000808?token=927DCC2EAF8EB5A3F63D462B4E0E0F4BA65EAA7FB4732F8C11B716D72D7CB58A20E359D983EBBEBFA29E4BA6BD2ECC51

A Brief Tectonic History of Victoria

To understand the opportunity we have to invoke some big picture geology:

  • Victoria (and the Western Lachlan Fold belt) have experienced three major regional deformational events (orogenies):
    • Delamerian (520 to 490 Ma) - documented along the western margin of the Stawell Zone but may have affected the entire Stawell Zone.
    • Benambran (455 to 440 Ma) - the main deformational event in the Bendigo Zone which also affected the Stawell Zone.
    • Tabberabberan (~390) - the main deformational event in the Melbourne Zone which also affected the eastern Bendigo Zone (ie Fosterville) and reactivated some pre-existing structures further west.
  • The bulk of the exposed volcano-sedimentary rocks in Victoria are characterised by regional metamorphism of the greenschist facies, with the amphibolite facies present along the western margin of the Stawell Zone and the subgreenschist facies prevalent in the eastern Bendigo and Melbourne zones
  • The geodynamic history of Victoria
    • Origin as an oceanic basin in a forearc setting in the Cambrian, evolving into a sediment-filled back-arc basin, inverted and cratonised by the Benambran Orogeny.
    • Convergence between the Cambrian Delamerian Orogen in the west and the Selwyn Block in the east in a back-arc setting in the Ordovician, culminating in their collision and deformation of the Stawell and Bendigo zones during the Benambran Orogeny.
  • A major feature of this collisional event was that the Cambrian oceanic crust of the Bendigo Zone was not consumed by subduction but tectonically thickened by shallow thrusting.
  • The Melbourne Zone, deposited on top of the micro-continental Selwyn Block, was not deformed during the Benambran Orogeny, protected by the underlying rigid crustal block. It was inverted and tectonically thickened by the Tabberrabberan Orogeny.

Deep Crustal Architecture Controls Gold

  • Understanding of the overall crustal architecture and geodynamic evolution of Victoria has greatly improved over the last decade, particularly following the interpretation of a series of deep crustal seismic surveys transecting the Western Lachlan orogen
  • The scale and broad regional distribution of gold endowment in Victoria is largely controlled by the volume of originally hydrous crustal source rocks – limited by crustal architecture, which was in turn has been defined by the tectonic evolution.
    • The most critical structural mineral system controls are related to crustal block boundaries in the middle to lower crust, this being the western margin of the Selwyn Block in Victoria.
  • Spatial data analysis of gold deposit patterns reveals important properties of the spatial distribution of orogenic gold mineralization. These can be explained by the deep crustal architecture of Victoria.
  • Seismic data indicate that both the Stawell and Bendigo zones are composed of Cambrian oceanic and arc-related metavolcanics and interbedded metasedimentary rocks, overlain by thick Cambrian to Ordovician metaturbidites.
    • The structurally thickened older Cambrian metavolcanics and associated metasedimentary rocks form a contiguous lower crustal region below a depth of approximately 6 km in the Stawell Zone and approximately 15 km in the Bendigo Zone.
  • The Melbourne Zone has a completely different composition of the middle and lower crust.
    • It is entirely underlain by older Proterozoic to Cambrian continental crust of the Selwyn Block which also underlies the eastern Bendigo Zone (ie Fosterville).
    • The Selwyn Block is overlain by up to 15 km of tectonically thickened Palaeozoic turbidites in the Melbourne Zone, thinning out under the eastern Bendigo Zone where its outer margin underlies 35 km of Cambrian and Ordovician volcanic and sedimentary rocks.

Two Distinct Sub-types of Orogenic Gold

  • There are two distinct sub-types of orogenic gold mineralization in the Victoria, mostly formed during different metallogenic events and occurring in distinct regional domains at approximately 445 Ma and 370-380 Ma.
  • Both styles commonly occur in clusters, sometimes >10 km along the dominant structural trend, spatially grouped and described as gold ore fields.
    • The 445 Ma event was the main phase of gold mineralization in the region, forming almost all significant gold deposits in the Bendigo and Stawell zones (mesozonal gold). It coincided with the late stages of the Benambran Orogeny which was not accompanied or closely followed by any documented magmatism.
    • The 375 Ma event formed all the epizonal gold deposits in the Melbourne Zone and the eastern Bendigo Zone, including Fosterville. This gold event post-dated the main phase of the Tabberabberan Orogeny and was broadly synchronous with the emplacement of post-tectonic granites and dykes.
  • At a regional scale, both between and within individual structural zones, the spatial distribution of ore fields is strongly heterogeneous.
    • The Bendigo Zone is by far the most richly endowed region, with >70 Moz of total gold production (60% of which from alluvial deposits),
    • Compared to 7 Moz produced from the Stawell Zone and
    • >4 Moz from the Melbourne Zone - considered to be underexplored, not targeted by the old-timers and the new search space

The Melbourne Zone Opportunity (and Eastern Bendigo Zone)

And the geological reasons for this heterogeneous distribution can be described in one picture as below:

Image modified from www.mandalayresources.com/wp-content/uploads/2019/12/2019-12-01-MND-IR-PPT_final-1.pdf and https://api.research-repository.uwa.edu.au/portalfiles/portal/5263287/Lysytsyn_Volodymyr_2015.pdf