Tutor Lesson

The sandstone cliffs of the Grampians rise conspicuously above western Victoria, yet the region’s deeper geological puzzle lies mostly under the surrounding plains. The sandstone came from sediment deposited by ancient rivers and shallow seas, whereas the volcanic and metamorphic rocks beneath and around the plains are older and unrelated to it.

• What landscape existed before the Grampians?
• Through what processes did that earlier landscape develop?
• Why are volcanic rocks now exposed at separated locations across the plains?
• How far do the ancient rocks continue beneath younger sediment?

Sediment of the Murray and Otway basins forms a broad, subdued cover. Only isolated hills, boulders, and creek-bed exposures reveal small pieces of the older system, now called the Stavely Arc.

The Geological Survey of Victoria began during the 1852 gold rush to investigate geology, minerals, energy, groundwater, and information for infrastructure. In the nineteenth century, Norman Taylor and Ferdinand Krause travelled on foot and horseback, mapping rocks, streams, hills, and other features while searching for evidence relevant to gold and other deposits.

Their observation-based maps captured many broad components correctly and remain useful. They could distinguish the older volcanic and metamorphic rocks from Grampians sandstone, but plate tectonics was not yet available to explain their scattered occurrence.

• Geophysical measurement
• Deep seismic reflection
• Gravity surveying
• Magnetic surveying
• Stratigraphic drilling
• Three-dimensional geological modelling

Rock identification alone could not reconstruct the Stavely Arc. Scientists also had to determine the ages of its components and arrange its major events. The buried rocks were first assigned to the Ordovician, but later fossil evidence placed them in the older Cambrian, near 500 million years ago.

Tiny mineral grains in some rocks contain radioactive elements. Because those elements change at predictable rates, measuring the original material and its decay products can indicate when a mineral formed. Zircon preserves information from crystallisation and was used to date Stavely volcanic and intrusive rocks more precisely.

• Parts of the arc date to about 511–510 million years ago.
• Volcanic rocks farther east date to roughly 503–500 million years ago.
• Other evidence suggests activity may have started near 522 million years ago.
• The volcanic arc operated for more than 20 million years.
• Arc activity ended around 500 million years ago.

Ages connect rocks from different places and test whether they belong to one event. Two Stavely metamorphic belts had comparable ages despite recording different pressures and temperatures, linking them to the same tectonic episode in different positions.

The twentieth-century theory of plate tectonics supplied the framework missing from early surveys. It describes a crust and uppermost mantle divided into moving plates whose interactions produce mountains, volcanoes, ocean basins, earthquakes, deep trenches, metamorphic belts, and magmatic arcs.

Scientists compared the chemical fingerprints of Stavely volcanic rocks with young rocks in modern Pacific settings. Their similar chemistry indicated a comparable tectonic origin: Stavely belonged to a long, narrow magmatic arc rather than representing unrelated volcanoes.

1. An oceanic plate approaches continental crust.
2. The thinner, denser oceanic plate sinks beneath the lighter continent.
3. The descending plate induces melting deep in the mantle.
4. These conditions encourage partial mantle melting.
5. Magma moves upward through the continental crust.
6. Eruptions at the surface and intrusions below it create the arc.

Later erosion, burial, faulting, and deformation can separate or conceal the surface volcanic rocks and their underground intrusive counterparts. Because arcs can form in oceanic crust or beside continents, scientists mapped the surrounding rocks to distinguish the Stavely setting.

Mapping and geochemistry identified a continental-margin system comparable to the modern Andes. The setting is therefore described as an Andean-style volcanic arc or continental-margin subduction system.

Tight folds and numerous smaller structures show that the Stavely rocks underwent strong deformation. By mapping those details, structural geologists inferred larger forms concealed beneath the plains.

This work identified the Moyston Fault, which separates two fundamentally different packages.

• Ancient continental Australian rocks occur west of the fault.
• Rocks formed in or linked with the ancient Pacific Ocean occur east of it.

The fault marks the old boundary between continental and oceanic domains—the ancient Gondwanan margin. It connected Stavely to the edge of a continent, although evidence was still needed to decide whether the volcanoes formed there or originated farther offshore and arrived later.

At a crucial Mount Stavely outcrop, angular andesite fragments sit in volcanic debris. Its geological context supplied several connected observations.

• The breccia occurs with other volcanic sediments.
• Those sediments appear to have accumulated underwater.
• Some andesite clasts contain bubble-shaped cavities called vesicles.
• The vesicles record gas bubbles trapped as lava cooled.

Very deep water would have exerted enough pressure to restrict gas-bubble expansion. Vesicles therefore point to relatively shallow water. Together with the breccia, they support eruption in a shallow submarine setting near Gondwana rather than at a distant oceanic arc later pushed against the continent.

• Eastern rocks record comparatively high pressure and lower temperature.
• Western rocks record higher temperature and lower pressure; some became hot enough to begin melting.

Near a subduction zone, cold oceanic crust and sediment descend, so rocks can reach high pressure without extreme heating. Beneath the continental arc, rising magma creates strong heat at comparatively lower pressure.

• A cool, high-pressure belt develops beside the descending plate.
• A hot, lower-pressure belt develops near the magmatic arc.

Modern examples from South America to Japan helped interpret this arrangement. At Stavely, the shift from cool eastern rocks to hot western rocks shows that the slab dipped west beneath Gondwana and that the arc occupied Gondwana’s Pacific-facing eastern edge.

1. Stavely developed at a convergent plate boundary.
2. A magmatic arc should lie between the contrasting belts.
3. The subduction zone descended westward beneath Gondwana.

Surface and metamorphic evidence supported an arc interpretation, but its continuation into the deep crust still required a test. Deep seismic reflection supplied that test.

Survey trucks produced surface vibrations. Waves travelled downward, reflected from boundaries between rocks, and returned to sensitive instruments. Like an ultrasound on a far larger scale, the resulting profile displayed layers, faults, and major bodies in vertical cross-section.

The western Victorian survey imaged to about 60 kilometres and revealed a large reflective body in the middle to lower crust. Because it sits below exposed Stavely andesite, scientists interpreted it as the buried root system of the arc.

• The entire arc greatly exceeded the scale implied by scattered surface exposures.
• Surface volcanics connected to a substantial deep-crustal system.
• Younger basin sediment conceals much of that system.
• Western Victoria’s crust is more complicated than surface mapping alone suggests.

Seismic profiles give detailed depth information along selected lines. Gravity and magnetic surveys were used to trace the arc horizontally across the wider region.

Aircraft collected magnetic measurements along planned flight paths. Differences in magnetic minerals allowed buried volcanic and intrusive rocks to be distinguished from surrounding sediment, with useful information extending to roughly 30 kilometres depth.

Gravity changes slightly with rock density: denser material produces a somewhat stronger attraction. Measurements from broad regional station coverage and closely spaced traverses revealed subsurface density contrasts and reached deeper than the magnetic data.

By separating signals from Murray and Otway basin sediment from those of the basement, geophysicists effectively removed the younger cover in a digital interpretation. Nothing was physically stripped away; the processing simply clarified the older structures below.

The combined results revealed nineteen belts of volcanic and intrusive rocks. Scientists interpreted them as mostly buried remnants of the original Cambrian arc.

Maps provide an overhead view, while seismic profiles provide vertical slices. A full model combined both perspectives with several evidence types.

• Surface geological maps
• Interpretations of faults and folds
• Magnetic measurements
• Gravity measurements
• Seismic profiles
• Drill-hole records
• Rock ages and chemical compositions

An idea that looks plausible on a map may fail at depth; a supposed continuous fault, for example, may turn or end below the surface. Whenever an interpretation proved impossible in three dimensions, the team returned to structural observations and revised it. Their reasoning repeatedly followed the cycle of proposal, testing, and correction.

• Fault shapes
• Geometry of Cambrian volcanic belts
• Deep-crust architecture
• Connections among separate arc remnants

Bringing seismic information into the model became a turning point in assessing western Victoria’s copper potential.

The original arc was likely much simpler. Near 500 million years ago, its activity stopped while faulting and tilting divided volcanic and intrusive rocks into separate blocks. Later episodes added more folding, displacement, rotation, and tilting.

Large structures visible in the younger Grampians continue into the older basement. They therefore provided a guide to deformation where Cambrian rocks are poorly exposed.

• Around 440 million years ago, a deep ocean withdrew eastward.
• Sediment later forming the Grampians was laid over the Stavely Arc.
• Near 405 million years ago, Grampians strata were tilted and folded.
• That same event deformed the older Stavely rocks below.
• Visible Grampians structures became a template for interpreting the buried basement.

Undoing later folding, faulting, rotation, and tilting reconstructed the nineteen present belts as roughly three or four formerly continuous belts that were subsequently cut into fault slices. This showed why mapping every unit matters: although Grampians sandstone was not the exploration target, its structures explained the disruption of the older mineralised arc.

• Porphyry systems
• Epithermal systems
• Volcanic-hosted massive sulfide systems

VHMS formation resembles modern black-smoker activity. Heated acidic fluid passes through oceanic crust, carries dissolved metals upward through fractures, and meets cold seawater. Metals then precipitate around vents as chimneys and massive sulfide accumulations.

• Forearc areas near volcanic arcs
• Back-arc areas
• Mid-ocean ridge settings

As an intrusion cools, hot metal-bearing fluids move through fractures and can produce veins, hydrothermal alteration, and concentrations of copper and other metals.

• Deep magma supplies hot fluids carrying metals.
• Porphyry mineralisation develops around the deep intrusion.
• Fluids travel upward.
• Epithermal mineralisation forms at shallower levels near the volcanic surface.

Important Stavely mineralisation appears late in the arc’s life, near its shutdown around 500 million years ago. After earlier shortening and compression, release into extension produced a final magmatic pulse that carried metal-bearing granites and porphyry intrusions upward through fractured and tilted rocks.

• Long-lasting subduction
• Sustained magmatic activity
• A deformed and fractured crust
• A final change from compression to extension
• Metal-bearing intrusions and hydrothermal fluids

Drilling showed that mineralisation can also occupy sedimentary rocks between volcanic belts. Fluids can follow fractures and permeable layers into surrounding hosts, widening the exploration area.

• Mineralised zones
• Veins
• Hydrothermal alteration
• Potassic alteration
• K-feldspar and magnetite

Potassic alteration containing K-feldspar and magnetite is associated with hotter parts of a system and can indicate closeness to a productive magmatic-hydrothermal environment.

Private exploration had produced more than 1,800 company reports, over 30,000 surface geochemical samples, and almost 10,000 drill holes. Most work concentrated on exposed or shallow rocks, and many holes failed to obtain adequate samples of buried basement, leaving much of the project area untested.

• Prepare regional geological maps
• Run large geophysical surveys
• Bring historical exploration records together
• Construct regional tectonic models
• Recognise broad zones of mineral potential
• Release geological information to researchers and industry

This broad view is especially valuable when younger sediment hides the rocks with the greatest exploration interest.

• Field geology
• Structural geology
• Geochemistry
• Metamorphic geology
• Geophysics
• Seismic interpretation
• Geochronology
• Mineral-systems analysis
• Three-dimensional modelling
• Data management

Each specialty addressed a different part of the problem: mapping documented exposed rocks and structures; geochemistry identified tectonic setting; metamorphic study recovered pressure and temperature; seismic data imaged deep architecture; gravity and magnetics followed belts sideways; drilling collected covered rock; geochronology dated events; three-dimensional modelling tested geometry; and mineral-systems analysis evaluated exploration meaning. The interpretation was strongest where independent lines agreed.

Fourteen stratigraphic holes recovered concealed volcanic rocks, including andesite and dacite, as well as associated intrusions.

• The buried volcanic rocks belong to the Stavely Arc package.
• Their chemistry carries a subduction-related arc signature.
• Their compositions resemble exposed rocks of the Mount Stavely volcanic complex.
• Buried portions share the arc’s broad mineral potential.
• Both volcanic and sedimentary rocks can host mineralisation.

Drilling thus provided direct tests that reinforced the interpretation derived from surface geology and remote measurements.

• Gondwana continued for thousands of kilometres westward.
• The Pacific Ocean continued for thousands of kilometres eastward.
• Much of modern Victoria did not yet exist as its present landmass.
• Oceanic crust descended west beneath Gondwana.
• A volcanic chain followed Gondwana’s eastern margin.
• Magma entered continental crust below the volcanoes.
• Contrasting metamorphic belts developed across the system.
• Hydrothermal activity on the seafloor formed VHMS mineralisation.
• Later magmatism generated granites and porphyry systems.
• Shutdown was followed by faulting, tilting, and division into separate belts.

The reconstructed setting resembled today’s Andes: ocean beside a continental margin, a plate descending beneath the continental edge, and a long volcanic chain above rising magma.

After deformation, western Victoria became unusually stable. In many old systems, erosion removes shallow volcanic levels and leaves only deep roots exposed. Here, younger sediment covered and preserved upper parts of the Stavely Arc.

Many economically valuable deposits form in those upper volcanic and magmatic levels. Their survival means mineral systems erased elsewhere may remain in western Victoria.

1. The region records ancient plate-tectonic processes.
2. It preserves possible mineralisation in the upper levels of a continental volcanic arc.