Gold Mineralization Depth in Hill End and NSW Goldfields: Modern Insights (OpenAI Deep Research)
Introduction: Hill End (NSW, Australia) is a historic goldfield famous for its rich quartz-vein gold, including the Holtermann Nugget. An older view held that the richest gold “shoots” were confined to shallow depths (within ~50–100 m of surface), possibly due to supergene enrichment (secondary concentration by weathering).
Here we examine whether recent geological studies and exploration (post-1977) support or refute that view.
Key questions include: (1) Are most high-grade gold lenses still thought to occur within the top 50–100 m? (2) Have modern programs found significant primary gold deeper than 100 m? (3) What do recent geochemical/mineralogical studies say about the supergene enrichment model? (4) What drilling or exploration data exist for gold beyond 100 m at Hill End or similar NSW goldfields? Below is a summary of current understanding, with references to government surveys, academic research, and exploration reports.
Near-Surface Gold Concentration (50–100 m Depth)
Historical observations: Early mining at Hill End (especially the Hawkins Hill–Reward area) extracted exceptional gold grades from near-surface levels. Many of the bonanza gold pockets were found only 40–50 m below surface, coinciding with the weathered zone . Contemporary accounts (e.g. Harper 1918) noted that rich “plates” of gold were mined at shallow depths, and that gold content appeared to drop off in deeper, unoxidized portions . This gave rise to the idea that gold was enriched near surface.
Modern confirmation: Recent geological reviews affirm that significant near-surface remobilization of gold occurred at Hill End due to weathering. This process produced supergene-enriched zones in the upper 50–100 m of the veins (especially at Hawkins Hill) . For example, composite mine sections show very high-grade ore shoots concentrated in the top levels . The famous Beyers and Holtermann specimen (a 286 kg mass of gold in quartz) was recovered around 40 m depth – a level now interpreted as a supergene-enriched zone .
Geological Survey studies by Downes & Seccombe (2003) explicitly note that weathering caused enrichment of some near-surface zones . This indicates current understanding still supports the idea that the richest gold lenses were largely shallow, formed by secondary enrichment over primary (“hypogene”) mineralization.
Other NSW examples: The shallow enrichment phenomenon is not unique to Hill End. At the Adelong Goldfield (southern NSW), modern drilling has shown a similar pattern: a leached near-surface zone underlain by a supergene-enriched zone. Shallow rotary air blast holes in one area found little gold, yet a slightly deeper hole intersected 6 m @ 6.68 g/t Au about 60 m down, near old workings that averaged >30 g/t . This suggests older miners capitalized on a narrow enriched horizon close to the water table. In summary, both historic evidence and recent studies confirm that most bonanza gold shoots occurred within roughly the top 50–100 m at Hill End and comparable NSW deposits, consistent with supergene enhancement in the upper oxidized zone.
Primary Gold Mineralization at Greater Depths
Despite the shallow bonanzas, modern exploration has revealed substantial primary gold at depth in these districts. At Hill End, historic workings on Hawkins Hill extended only to ~200 m depth in places , and many stopes did not chase ore far below the oxidized zone. However, drilling since the 1980s–2000s has intersected gold-bearing veins well below 100 m. For instance, Hill End Gold Ltd (ASX:HEG) drilled beneath old stopes and delineated new resources. By 2010, they outlined ~0.9 Mt @ 8.6 g/t Au (about 10.4 tonnes of gold) in the Hawkins Hill–Reward area, extending known mineralization below the shallow historic mines . Much of this resource lies in primary (unweathered) rock, demonstrating that gold-bearing veins do continue at depth, albeit generally narrower or lower grade than the supergene-rich patches.
In the nearby Hargraves field (also part of the Hill End Trough), modern work has similarly proven deeper mineralization. Hill End Gold’s drilling at the Big Nugget Hill deposit outlined 221,000 oz Au to an average depth of ~200 m . The deposit remains open at depth and along strike, indicating significant primary gold extends well beyond the historical shallow workings . Company reports note that multiple stacked quartz reef zones occur in an en-echelon pattern down the anticline, and additional veins are expected at depth as drilling progresses . These findings refute any notion that gold “runs out” completely below 100 m – instead, economic grades have been discovered in deeper structural positions.
Other NSW lode gold districts show a similar trend. At the Lucknow Goldfield near Orange (historic production >400,000 oz @ ~100 g/t), it was long suspected that rich ore shoots might plunge to greater depths. In 2020, Talisman Mining began the first deep diamond holes ever drilled at Lucknow. Hole LUDD001 reached 621 m depth, targeting down-plunge extensions below the old 19th-century stopes . It intersected quartz-carbonate veins with sulphides well below the mined levels, including modest grades like 0.8 m @ 2.34 g/t Au at ~467 m . While no new bonanza was hit in that hole, the presence of gold and sulphide stringers confirms the system continues at depth . A second hole is underway to test another deep target zone . This modern drilling underscores that significant primary gold mineralization can exist beyond 100 m, and explorers are actively probing these depths in NSW.
Supergene Enrichment: Recent Geochemical Insights
The supergene enrichment model for Hill End and similar gold deposits has been largely validated by recent research. Geological and geochemical studies conclude that the exceptionally high grades near surface were boosted by secondary processes. Specifically, meteoric water circulation and oxidation of sulphides caused gold remobilization and re-precipitation just below the water table . Downes & Seccombe (2003) report clear evidence of “significant supergene enrichment” in parts of the vein system due to weathering . They cite old mine data showing enriched “clayey” or gossanous zones yielding spectacular assays between roughly 30 m and 60 m depth . These zones often contained very pure gold (high fineness), consistent with gold that was re-deposited after leaching of silver in the oxidized environment – a hallmark of supergene processes.
No major modern study has fundamentally challenged the supergene model for Hill End’s shallow gold; rather, it has been reinforced by new analyses. Researchers like P. Seccombe and co-workers have examined fluid inclusions, stable isotopes, and mineralogy of the Hill End veins. They found multiple stages of gold deposition: an initial deep orogenic (hypogene) phase, followed by later episodes including possible surface-driven events . While most of the gold formed during Carboniferous metamorphic fluid pulses, the final stage involved remobilization during uplift and weathering, enriching the upper levels . For example, gold fineness and inclusion patterns change from depth to surface, suggesting secondary addition of gold in the topmost portions (this matches observations at other slate-belt gold deposits as well). In summary, recent geochemical and mineralogical research strongly supports the supergene enrichment model: the near-surface bonanzas at Hill End and similar NSW goldfields were enhanced by later weathering, rather than representing the full grade of the primary ore at depth . No evidence has emerged to refute this; instead, the model helps explain why grades decrease below ~100 m as the influence of supergene processes wanes.
Deep Drilling and Ongoing Exploration (>100 m)
Exploration activity in the past few decades provides direct data on gold distribution beyond 100 m depth. Numerous drilling programs have targeted below the old workings in the Hill End district:
•Hill End (Hawkins Hill–Reward): Hill End Gold Ltd conducted extensive underground drilling in the 2000s, extending the known veins. This led to a JORC resource down to ~200 m depth as mentioned above . High-grade intersections were encountered beneath the historical stopes, confirming that some ore shoots continue down-dip (though generally narrower). As of 2024, the project (now held by Vertex Minerals) is still being actively explored. Vertex recently acquired an underground drill rig to probe “mid-depth” targets 80–200 m below the existing Reward resource, aiming to expand it . Notably, the current resource drilling had only gone ~50 m below the main adit level, so deeper potential remains untested . The new drilling will assess whether additional high-grade shoots persist further down the anticline fold limbs.
•Hargraves (Big Nugget Hill): Drilling in 2010–2012 tested this field along strike and at depth. Holes confirmed that gold-bearing reefs extend at least ~200 m down. One diamond hole (HGFD01) intersected a spectacular vein with 47.6 g/t Au over 0.1 m at ~98 m downhole, demonstrating that coarse gold pockets were not limited to the very shallow levels (this vein was likely a steeply plunging shoot similar to those mined nearer surface) . The Big Nugget Hill resource remains open, and additional sub-parallel structures (e.g. the Gundowda Shear) have been identified for drilling, indicating a larger mineralized system at depth . In short, ongoing exploration around Hill End is finding new gold zones beyond the historical 100 m depth horizon.
•Other NSW Goldfields: As discussed, projects like Lucknow and Adelong have seen renewed drilling. At Lucknow, Talisman’s deep holes are investigating several hundred meters below old mine levels . Early results show the primary mineral system (quartz-sulphide veins) persists down-plunge, though so far grades are much lower than the near-surface ores . At Adelong, 3D Resources Ltd recently noted that some past shallow holes likely missed mineralization that lay just a bit deeper. Their 2022 drilling confirmed a supergene gold zone around 40–70 m depth, beneath a depleted surface layer . These findings have opened up new targets along old reef lines that were not fully tested at depth. For example, historic Adelong mines like Challenger and Donkey Hill ended where oxide ore was exhausted, but unmined primary extensions are now being explored down-dip and along strike .
Summary of exploration results: The collective drilling evidence from Hill End, Hargraves, and analogous NSW fields shows that gold mineralization is not strictly confined to the top 100 m. Rich supergene shoots did cluster near surface, but primary gold-bearing structures continue deeper into fresh rock. In many cases, grades do diminish with depth (as the ore transitions to refractory sulphidic material and loses the supergene boost ). Nonetheless, significant gold resources have been proven below 100 m, and companies are actively pursuing deeper targets. This modern exploration refutes the old assumption that “there’s no gold at depth” in these areas – on the contrary, economic mineralization has been found and can likely be expanded with further drilling .
Conclusion
Modern geological studies and exploration largely affirm that Hill End’s spectacular gold was concentrated at shallow depths, but they also reveal that gold does extend deeper in these systems. The consensus is that the highest grades (bonanza lenses within ~50–100 m of surface) resulted from supergene enrichment during weathering . This model has been substantiated by recent research, which finds clear geochemical fingerprints of secondary gold concentration near the old water-table . However, the idea that gold mineralization “cuts off” at 100 m is refuted by drilling – substantial primary gold has been discovered below that depth in Hill End and comparable NSW goldfields. Since the late 20th century, multiple projects have intersected gold-bearing veins well into unoxidized rock, delineating resources down to ~200 m or more . These primary zones are generally lower in grade than the supergene-rich cap, but they remain economically significant and are targets for ongoing exploration.
In summary, most of the historic riches were indeed near-surface, but modern exploration confirms that gold exists at depth – it simply transitions from supergene-enhanced bonanzas up top to more typical orogenic gold grades below. The supergene enrichment model is widely accepted for Hill End, and no recent evidence has overturned it. Meanwhile, new drilling results continue to extend the known gold mineralization deeper, ensuring that the full vertical extent of theseVictorian-era goldfields is still being unraveled by today’s geoscientists .
Sources:
•Downes, P.M. & Seccombe, P.K. (2003). A Review of the Hill End Mineral System, NSW. Geological Survey of NSW (Evolution of the Hill End Trough Project). – Discusses vein geometry, deformation history, and notes supergene enrichment of near-surface ore zones .
•Wilkins, C. & Quayle, M. (2021). Structural Control of High-Grade Gold Shoots at the Reward Mine, Hill End, NSW.Economic Geology, 116(4): 909–935. – Detailed structural study of Hill End’s Reward deposit; documents shallow-plunging ore shoots and provides context for high-grade zone distributions (implying limited vertical extent).
•Hill End Gold Ltd – various ASX releases (2010–2012) on Hill End and Hargraves projects. For example, June 2012 release reporting 221k oz @ 3.1 g/t resource to ~200 m depth at Big Nugget Hill, Hargraves ; July 2011 quarterly report noting deep drilling and multiple vein sets open at depth .
•3D Resources Ltd (2022). Adelong Goldfield Exploration Update. – Notes near-surface grade depletion and a supergene-enriched zone at depth, with drilling intercepting 6 m @ 6.7 g/t beneath previously low-grade shallow holes .
•Talisman Mining Ltd (2020). Lucknow Project Drilling Update. – First deep drill results at Lucknow goldfield; confirms the structure at >400 m depth with modest gold in sulphidic veins, indicating primary mineralization below historic shallow stopes .
•Porter GeoConsultancy (2021). Hill End Goldfield – Ore Deposit Description. – Summarizes the geology and mining history: historic mining to ~200 m depth, 56 t Au produced, and modern resource delineation below old workings .
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