Ann: Reward Gold Mine Project Update, page-26

https://hotcopper.com.au/posts/78506449/single

Here’s a full summary and contextual analysis of the September 2024 Nature Geoscience paper by Voisey et al., alongside the Smithsonian Magazine coverage, with relevance to Hill End and its orogenic gold system.

Summary:

“Gold Nugget Formation from Earthquake-Induced Piezoelectricity in Quartz” (Voisey et al., Nature Geoscience, 2024)

1. The Core Hypothesis

The study proposes that gold nugget formation—particularly large, high-grade nuggets within quartz veins—can occur as a result of repeated earthquake activity. This mechanism operates via a piezoelectric process, where mechanical stress on quartz during seismic events generates electrical charge.

Quartz is a naturally piezoelectric mineral, meaning it can produce electric potential when stressed. The researchers found that in certain conditions:

·Quartz crystals embedded in fault-hosted veins undergo mechanical deformation during an earthquake.

·This deformation produces localized electric fields.

·These electric fields can electrochemically reduce gold from nearby hydrothermal fluids or finely disseminated gold in the rock.

·Over many repeated seismic events, gold accumulates at a specific point—forming a nugget that may grow incrementally over thousands to millions of years.

This process is described as the quartz acting like a natural battery, with gold acting as the cathode, gradually growing in place through micro-deposition events.

2. Supporting Observations

The study builds on observations that:

·Quartz veins frequently host gold in orogenic systems, yet large discrete nuggets are not evenly distributed.

·Many of the world’s largest nuggets (e.g. Welcome Stranger, 78 kg) come from tectonically active, quartz-rich orogenic belts (like Bendigo, Ballarat, Hill End).

·These nuggets are not always associated with extensive disseminated mineralization—implying a localized growth process, not bulk deposition.

Their experiments and modelling suggest that voltage differentials of ~1 volt/meter are sufficient to cause electrochemical reduction of gold complexes in solution, particularly in low-salinity, CO₂-rich metamorphic fluids—the exact fluid type found at Hill End, Fosterville, and Bendigo.

3. Timing: Post-Orogenic Nugget Formation

A key implication is that this mechanism may occur after the main phase of gold deposition, during post-orogenic stages when:

·Faults continue to reactivate under regional stress fields.

·Small, frequent earthquakes (microseismicity) persist over long periods.

·These repeated events continuously charge and discharge quartz veins, facilitating nugget growth after initial mineralization.

This aligns with observations from Hill End and Bendigo that high-grade shoots and nuggets often occur in structural intersections or fault bends, where strain is repeatedly localized.

4. Broader Significance: Explains Nugget Localization

Until now, models explaining gold nugget formation have leaned on:

·Supergene enrichment near surface (for secondary nuggets), or

·Bulk precipitation from hydrothermal fluids (for primary lode gold).

However, neither model convincingly explains why large nuggets form in very small zones while surrounding quartz is barren or low grade.

The electrochemical-piezoelectric mechanism accounts for:

·Localized nugget growth in otherwise barren quartz.

·Incremental accretion over time, not a single hydrothermal event.

·Occurrence of nuggets along reactivated faults and vein intersections.

5. Relevance to Hill End

Hill End’s geology shares key features that make it a prime candidate for this process:

	Feature	Presence at Hill End
	Quartz-rich bedding-parallel veins	Yes – extensive laminated quartz reefs (e.g. Paxton’s, Phillipson’s)
	Tectonically active setting	Yes – along the Lachlan Fold Belt and eastern margin of the Great Dividing Range
	Post-mineralization fault reactivation	Yes – late-stage reverse faults and fracture zones (Stage IV & V) with minimal new gold but structural remobilization
	Free gold and large nuggets	Yes – coarse visible gold common; Holtermann Nugget (~290 kg quartz-gold specimen) is a prime example
	Low-salinity, CO₂–CH₄ rich fluids	Yes – confirmed by fluid inclusions at ~293–340°C

The Holtermann Nugget, discovered in 1872 at ~50 m depth, could be a textbook example of this mechanism:

·Formed within quartz reef (Paxton’s vein).

·Located near a known fault intersection.

·Possibly grew over time through successive piezoelectrically induced precipitation events.

If correct, this would mean:

·Hill End’s large nugget zones may have formed after primary deposition, and

·Similar “battery zones” may exist at depth, where fault activity persisted post-orogeny.

Implications for Exploration

This hypothesis offers new targeting criteria:

1. Look for Piezoelectric “Battery Zones”:

·Quartz-rich fault intersections with a history of seismic reactivation.

·Structural traps that concentrate stress and fluid flow over time.

·Areas with brittle–ductile contrast that encourage slip during seismic loading.

2. Don’t Dismiss Barren Quartz Veins:

·Quartz veins with minimal sulfides but large nugget discoveries may reflect electrochemical growth, not bulk hydrothermal deposition.

·Drill targeting should consider stress concentration zones, not just geochemistry.

3. Reinterpret Old Workings:

·Historic mines with spotty but spectacular grades (like Hill End’s Reward shaft or Hawkins Hill) may reflect electrochemically upgraded shoots, not random nugget placement.

·Use geophysical methods (resistivity/IP) to locate analogous zones at depth.

Similar Systems – Comparative Context

Bendigo / Ballarat (VIC):

·Similar quartz-rich, tightly folded turbidite sequences.

·Frequent discovery of massive nuggets during the 1850s gold rush.

·Fold limbs and fault intersections show repeated structural reactivation—ideal for piezoelectric charging.

Beaconsfield (TAS):

·Hosted in a narrow quartz vein with sharply bounded ore shoots.

·Evidence of brittle reactivation in a carbonaceous slate package—possible electrochemical activity.

Western Australia (e.g., Widgiemooltha, Golden Mile):

Although more commonly supergene nuggets, some goldfields show quartz-hosted coarse nuggets near shear zones with microseismicity—possibly similar mechanisms in a different host.

Conclusion: Why This Hypothesis Matters for Hill End

1.Provides a physical mechanism for how large, discrete nuggets form in a quartz-hosted orogenic system—solving a long-standing mystery.

2.Aligns with known Hill End structural and fluid evolution, particularly in post-Stage IV–V fault zones.

3.Suggests a new phase of nugget growth after the main gold events (~343 Ma), driven by ongoing Great Dividing Range seismicity.

4.Introduces novel exploration criteria that could direct drilling and geophysics toward deep nugget-bearing zones.

This theory reframes Hill End as not just a historic coarse gold field, but a system where electrical and seismic forces may have shaped—and may still be shaping—its gold endowment. If proven, this opens the door to rediscovering Hill End through a new, science-led lens.