Exploration Potential, page-121

Decided to “drill down” into the Fluid history…

Gold Deposition at Hill End – Paragenesis and Fluid Evolution

The Hill End goldfield formed during the Devonian period, with gold introduced during multiple deformation and metamorphic events. Detailed paragenesis studies, radiometric dating, and fluid inclusion analyses have established that the main gold deposition occurred post-peak metamorphism, during later deformation phases. Below is an expanded explanation of these key processes and stages.

1. Peak Metamorphism and Early Fluid Evolution (Stage I–II, ~370–365 Ma)

Metamorphic Context:

• Hill End’s host rocks (Silurian-Devonian turbidites, mainly sandstone, siltstone, and shale) underwent regional greenschist-facies metamorphism during the Tabberabberan Orogeny (~370–365 Ma).

• During peak metamorphism, these sediments experienced temperatures of ~350–400°C and pressures of ~3–4 kbar, causing ductile deformation and folding to form the Hill End Anticline.

• Fluids Generated: As metamorphic fluids were expelled from the sediment pile during peak burial, they were primarily high-temperature, low-salinity fluids rich in CO₂ and CH₄, with minimal metal content. These fluids created early barren quartz veins (Stage I) with chlorite ± pyrite but deposited little or no gold.

Structural Evolution:

• The folding generated open anticlines and synclines, creating conduits for later fluid flow.

• Vein development was initially minor – mostly quartz with occasional low-grade sulfides. These early veins formed along bedding planes and fold hinges but were pre-gold in nature.

2. Main Gold Deposition (Stage III & IV, ~356–343 Ma)

Fluid Influx and Structural Reactivation:

• After peak metamorphism, the fold structures and bedding-parallel vein systems were reactivated during late-stage deformation. This reactivation opened up dilation zones in the anticline limbs and along reverse faults, creating hydraulic conduits for fluid flow.

• During this period, gold-bearing hydrothermal fluids were introduced into these newly formed or reopened structural sites.

Radiometric Dating and Fluid Chemistry:

• Radiometric Dating:

o Dating of arsenopyrite and pyrrhotite associated with gold suggests that two main gold pulses occurred at Hill End:

o The first at ~356 Ma (corresponding to Stage III of paragenesis).

o The second at ~343 Ma (corresponding to Stage IV).

o These dates place the main gold events well after peak metamorphism, consistent with the notion that mineralization occurred during the waning stages of the Tabberabberan Orogeny.

• Fluid Inclusion Studies:

o Fluid inclusions trapped in quartz veins at Hill End reveal that gold was precipitated from low-salinity, CO₂-rich metamorphic fluids.

o Temperature and salinity data suggest that these fluids were:

o ~293–340°C with salinities of 1–4 wt% NaCl equivalent.

o Dominated by H₂O–CO₂–CH₄ fluids, with minor dissolved salts and sulphides.

o The combination of low-salinity fluids and CH₄-rich gas phases is characteristic of mesothermal orogenic gold systems, suggesting a deep-seated metamorphic source of fluids.

3. Gold Precipitation Mechanism: Stage III (~356 Ma) & Stage IV (~343 Ma)

Stage III (~356 Ma): Main Gold Pulse

• Structural Setting:

o During this stage, reactivation of the Hill End Anticline’s limb faults and bedding-parallel shears allowed the first major pulse of gold-bearing fluids to infiltrate the system.

o Quartz–sulfide veins formed preferentially along bedding-plane partings, reverse faults, and spur veins, particularly on the eastern limb of the Hill End Anticline.

o These zones of dilation created ideal sites for gold deposition.

• Fluid Composition and Precipitation:

o The fluids during this phase were low-salinity H₂O–CO₂–CH₄ fluids with minimal hypersaline components.

o Gold was precipitated primarily due to sulfidation reactions:

o Fluid-wallrock interactions caused Fe-rich sediments (slates and siltstones) to react with the sulfur-bearing fluids, resulting in the formation of arsenopyrite and pyrrhotite.

o Gold precipitated along with these sulfides, often as coarse free gold in the quartz veins.

o Visible gold (up to 1,000 g/t Au in places) occurred where fluid mixing or pressure drops led to rapid gold saturation.

• Associated Minerals:

o Quartz ± carbonates (ankerite, siderite)

o Sulfides (arsenopyrite, pyrrhotite, pyrite)

o Minor base metals (galena, sphalerite)

Stage IV (~343 Ma): Second Gold Pulse

• Structural Reactivation:

o A second episode of deformation occurred, reactivating existing vein structures and generating new brittle fractures in the fold limbs and reverse faults.

o During this phase, the main gold zones were enriched and additional quartz–sulfide veining occurred, often overprinting earlier veins.

• Fluid Characteristics:

o Similar to Stage III, the fluids were H₂O–CO₂–CH₄ dominated, but with slightly higher temperatures (~300–340°C).

o Increased CO₂ content in the fluids likely increased the solubility of gold complexes, allowing for further gold mobilization and reprecipitation along earlier vein structures.

• Gold Redistribution:

o Stage IV saw local remobilization of gold and formation of high-grade ore shoots where late-stage faults cut across the main bedding-parallel veins.

o These later ore shoots tend to plunge steeply and are characterized by high-grade pods with coarse gold and abundant sulfides.

4. Late-Stage Fluids and Minimal Gold Deposition (Stage V, Post-343 Ma)

Final Fluid Events:

• After the main gold deposition events, a later phase of low-temperature quartz–carbonate vein formation occurred during continued deformation and cooling of the system.

• These fluids were higher-salinity, hypersaline brines (10–12 wt% NaCl equivalent) with traces of lead, antimony, and arsenic, indicating a different fluid source or mixing event.

Hawkins Hill and Late Vein Formation:

• At Hawkins Hill (part of the Hill End system), these later veins introduced:

• Stibnite (Sb) – indicating minor antimony mineralization.

• Galena (PbS) – suggesting some base metal association.

Minimal Gold Addition:

• While these late-stage fluids introduced sulfides and minor carbonates, they contributed very little additional gold to the system.

• The bulk of Hill End’s gold had already been deposited during the earlier pulses (Stage III and IV), and these late-stage fluids had a more dilute gold tenor.

5. Summary: Key Timing and Fluid Processes at Hill End

• Stage I–II (~370–365 Ma): Early barren quartz veins formed during peak metamorphism with no significant gold.

• Stage III (~356 Ma): Main gold pulse introduced during fold reactivation, with gold precipitated by sulfidation from low-salinity, CO₂-rich fluids.

• Stage IV (~343 Ma): Second gold pulse during structural reactivation, enhancing earlier veins and forming high-grade shoots.

• Stage V (Post-343 Ma): Late-stage quartz–carbonate veins with minor Sb–Pb minerals, adding little gold.

These findings underscore that Hill End’s main gold endowment was emplaced during the waning stages of the Tabberabberan Orogeny, with structural reactivation providing the necessary fluid pathways and traps. The vein system is complex, with multiple generations of overprinting veins, but the highest grades correlate with the main gold pulses of Stage III and IV. This paragenetic understanding strengthens the case for continued exploration at depth, where additional gold-bearing fluid pulses may have extended the known high-grade shoots.