Significance of Radiometric Dating and Fluid Chemistry at Hill...

Significance of Radiometric Dating and Fluid Chemistry at Hill End

The combination of radiometric dating of gold-associated sulfides and fluid inclusion analysis from quartz veins provides critical insights into the timing, source, and mechanisms of gold deposition at Hill End. These findings enhance our understanding of the paragenesis (sequence of mineral formation) and the factors controlling gold mineralization.

1. Timing of Gold Mineralization – Late-Stage Deposition Post-Peak Metamorphism

Radiometric Dating of Arsenopyrite and Pyrrhotite

• Key Dates:

o The first pulse of gold deposition occurred at ~356 Ma, corresponding to Stage III of the paragenesis.

o The second pulse occurred at ~343 Ma, corresponding to Stage IV of the paragenesis.

o These dates are well after the peak of regional metamorphism during the Tabberabberan Orogeny, which peaked around 370–365 Ma.

Significance:

• Post-Metamorphic Timing:

o The gold pulses occurred after peak deformation and metamorphism, meaning that mineralization was related to late-stage orogenic processes when the crust was decompressing, allowing fluids to migrate through existing fold and fault structures.

o This late-stage gold event is typical of orogenic gold systems, where peak metamorphism drives fluid production but the actual deposition of gold occurs during retrograde cooling as pressure and temperature drop.

• Syntectonic Fluid Flow and Structural Control:

o The dating indicates that gold-bearing fluids infiltrated re-activated structures (bedding-parallel veins, reverse faults, and anticline limbs) after the major deformation that formed the Hill End Anticline.

o This timing suggests that mineralization was associated with late-stage folding and fault reactivation, when previously tight folds were being incrementally stretched and fractured, creating the necessary dilation zones for fluid flow and gold precipitation.

2. Nature and Source of Gold-Bearing Fluids – Mesothermal Characteristics

Fluid Inclusion Data from Quartz Veins

• Low-Salinity, CO₂-Rich Fluids:

o Fluid inclusions in quartz veins associated with gold at Hill End reveal that the mineralizing fluids were:

§ Low salinity (1–4 wt% NaCl equivalent) – indicating a low amount of dissolved salts, typical of metamorphic fluids derived from devolatilization of sediments.

§ Moderate temperatures (293–340°C) – characteristic of mesothermal (orogenic) gold systems formed at mid-crustal depths (~5–10 km).

§ Dominated by H₂O–CO₂–CH₄ gases – suggesting the presence of reduced fluids that carried gold in solution, typical of deep-seated metamorphic sources.

• Significance:

o Fluid Source:

§ The fluid chemistry points to a deep metamorphic origin, likely derived from devolatilization of carbonaceous sediments (slates and siltstones) during peak metamorphism.

§ The presence of CO₂ and CH₄ suggests that these fluids were generated by breakdown of organic matter in the host rocks or by interaction with carbonates and graphite-rich slates at depth.

§ These fluids migrated upward through fractures and faults, transporting gold as bisulfide complexes (Au(HS)₂⁻) or as gold-chloride complexes.

o Mesothermal/Orogenic Signature:

§ The low salinity and moderate temperature align perfectly with the characteristics of mesothermal orogenic gold systems.

§ These systems typically form at 5–12 km depth in compressional tectonic regimes, where metamorphic fluids ascend through reactivated structures during the waning stages of orogeny.

§ H₂O–CO₂–CH₄-rich fluids are common in these systems and are capable of transporting high concentrations of gold, precipitating it rapidly when pressure and temperature drop or when the fluid encounters a reducing environment (e.g., carbonaceous slates or arsenopyrite-rich rocks).

3. Implications for Gold Precipitation – Sulfidation and Structural Traps

Sulfidation as the Primary Mechanism for Gold Deposition

• The interaction of gold-bearing fluids with Fe-rich sediments and pre-existing sulfides (pyrrhotite, pyrite, and arsenopyrite) caused sulfidation reactions that destabilized gold complexes and resulted in gold precipitation.

• Reaction Pathway:

o Fluid interaction with pyrrhotite and Fe-rich host rocks led to sulfidation and the formation of arsenopyrite and pyrrhotite, releasing gold from solution.

o The addition of sulfur to the fluid reduced its gold-carrying capacity, causing gold to precipitate as visible gold in the quartz veins.

• Structural Focus for Gold:

o The intersection of bedding-parallel veins with steep reverse faults created zones of dilation, where the fluids slowed and deposited their gold load.

o Gold precipitation was most effective in lower-pressure, structurally favourable zones such as fold hinges, limb fractures, and fault intersections.

4. Two Distinct Gold Pulses – Progressive Enrichment Over Time

Stage III (~356 Ma): Initial Major Gold Pulse

• Gold-bearing fluids infiltrated bedding-plane veins and fault intersections, forming the earliest high-grade zones.

• The initial deposition enriched the main quartz–sulfide veins, forming the high-grade pockets that characterized early mining at Hill End (including the famous Holtermann Nugget zone).

• This pulse generated coarse free gold in laminated quartz reefs on the eastern limb of the anticline.

Stage IV (~343 Ma): Secondary Gold Pulse and Remobilization

• A second pulse of slightly hotter fluids (~300–340°C) enriched earlier structures and remobilized gold along fault planes.

• Vertical Ore Shoots:

o This pulse formed vertical high-grade ore shoots where reactivated structures (e.g., Paxton’s and Phillipson’s veins) intersected the main bedding-parallel vein sets.

o The additional gold contributed to the development of steeply plunging ore shoots that extended down-plunge well beyond the original workings.

5. Minimal Gold in Late-Stage Fluids (Stage V) – Focus on Base Metals

Late Quartz–Carbonate Veins with Minimal Gold

• The final stage of fluid activity (Stage V, post-343 Ma) introduced hypersaline brines with Pb, Sb, and base metals, but very little additional gold.

• Hawkins Hill:

o These late-stage fluids deposited stibnite (Sb) and galena (PbS) in some parts of the Hawkins Hill vein system but did not contribute significantly to the gold endowment.

o The primary gold had already been emplaced during Stages III and IV.

6. Exploration and Economic Implications

Deep High-Grade Potential

• The presence of two main gold pulses suggests that deeper untested zones may host additional high-grade shoots formed during Stage IV or remobilized gold.

• Modern exploration at Hill End should target down-plunge extensions of known high-grade structures, as similar systems (Bendigo, Fosterville) exhibit continued ore shoot continuity to >1 km depth.

Structural and Fluid Model Validated

• Understanding the timing and chemistry of the fluids refines the exploration model by highlighting the importance of targeting late-stage structures, fold hinges, and fault intersections where fluid trapping and sulfidation occurred.

• Analogy with Fosterville and Bendigo:

o Similar late-stage fluid pulses and high-grade shoot formation have been observed in Victoria’s Bendigo and Fosterville deposits, where gold grades increased dramatically at depth as new structural zones were intersected by gold-rich fluids.

Conclusion: Why These Findings Matter

1. Post-Peak Timing Supports Deeper Potential:

• Since the gold deposition occurred after peak metamorphism, there is a high likelihood that structurally controlled zones at greater depths may host further mineralization.

2. Fluid Chemistry Confirms Mesothermal Model:

• The low-salinity, CO₂-rich fluids are classic for mesothermal orogenic gold systems, indicating that Hill End’s mineralization fits a well-defined exploration model.

3. Two Pulses = Double Enrichment:

• The presence of two gold pulses suggests multiple opportunities for structural zones to be enriched, increasing the probability of discovering additional high-grade zones.

4. Exploration Strategy Aligned with Fluid Evolution:

• Future drilling should target structures intersecting known vein sets at depth, where second-phase enrichment is likely to have occurred during the 343 Ma pulse.

These insights provide a strong scientific rationale for targeting deeper high-grade extensions of known ore shoots at Hill End, enhancing its potential as a major high-grade gold system with unexplored upside at depth