Here is a detailed synthesis based on the images you’ve provided from the Discoveries in the Tasmanides 2024 Bulletin 76, focusing on grade distribution with depth at Hill End, particularly in the Reward Mine and surrounding structures.
Hill End – Grade Distribution with Depth
1. Shallow vs Deep Mineralization: Is High Grade Limited to Upper 100m?
There’s strong historical evidence that early mining at Hill End exploited extremely high-grade gold lenses, particularly within 50–100m of surface, much of it hosted in supergene-enriched zones. However, the new data and diagrams you’ve provided show that:
• High-grade gold (>10 g/t) continues well below 100m in primary mineralization zones.
• The Reward Shaft intersects mineralized veins at the 640 Level (~286m deep), and gold shoots remain well-developed even deeper, down to levels 671–695.
• JORC resource data (2023) reports an average gold grade of 16.72 g/t Au from 419,000 tonnes, with the inferred portion (typically deeper) averaging 17.28 g/t, higher than the shallower indicated portion at 15.54 g/t.
This demonstrates that grade does not drop off below the supergene zone—in fact, deep primary zones can be just as rich or richer.
2. Insights from Reward Shaft and Deep Veins (Paxton’s System)
• Paxton’s Vein System (within Reward Shaft) continues between 640 and 695 Levels, meaning mineralization persists from ~286m to ~340m vertical depth.
• Shoots in this range (like Phillipson’s and Amalgamated) had stoping widths of 1.5–2.5m with bonanza grades historically reported at over 1,000 g/t in narrow veins.
• Figure 25 from the Bulletin shows a dense network of 14 bedding-parallel veins, some separated by just 5–25m stratigraphically, stacked over 360m of metaturbidites. The image shows clear structural control on grade persistence.
This spatial continuity, along with ongoing grade at depth, supports the idea that high-grade gold in Hill End extends well beyond traditional shallow zones.
3. Structural Controls on Vertical Extent and Localization
• The Reward Mine is located in the eastern limb of the Hill End Anticline, where east-dipping bedding-parallel veins intersect steeply west-dipping reverse faults.
• These intersection zones localize high-grade gold shoots due to dilation and fluid flow concentration—particularly in narrow, weak, shale-rich beds.
• The folding (D2) and later faulting (W-dipping structures) create duplex zones and saddle reefs where gold grades spike. These structures continue down-dip and down-plunge.
So, structural intersections—not depth alone—control grade. When favorable vein/fault geometries persist at depth, so does high-grade mineralization.
4. Vein Continuity, Thickness, and Depth Distribution
• Veins like Paxton’s, Mica, Phillipson’s, and Star of Peace show strike and dip continuity over hundreds of meters, as seen in the stratigraphic section.
• However, vein thickness ranges from cm to ~75 cm, and multiple veins may be grouped into zones (e.g. Mica + Paxton’s), reaching broader mineable widths.
• Mineralization is bounded to certain stratigraphic units (black slate beds or shale-dominated metaturbidites) and is variably persistent—i.e., spotty in places, but very high grade where developed.
This suggests that grade variability is not just a function of depth but also of stratigraphic control and structural dilation zones.
5. Interpretation of Grade Distribution from Paragenesis and Cross-Sections
• The paragenesis table (from Lu et al. 1993) shows multiple stages of mineralization:
• Stage II–IV include quartz-pyrrhotite-pyrite-chlorite-carbonate veins at ~230–280°C, with CH₄ as the dominant fluid gas.
• These correlate with continued deformation and vein reactivation, which produced many of the crosscutting high-grade shoots.
• The cross-sections show steep reverse faults intersecting gently dipping quartz veins—this geometry is optimal for fluid flow, gold deposition, and grade buildup.
• The Reward Mine is analogous in structure and mineralization to Fosterville and Bendigo, though perhaps less intense in fault-reactivation-induced bonanzas.
Thus, Hill End’s high grades are not just relics of shallow enrichment but are tied to complex structural and paragenetic history that continues to generate gold shoots at depth.
Conclusion
• Hill End is not a shallow-only system. While supergene enrichment exists, primary high-grade gold (>10 g/t) continues well below 100–200m, especially in structurally favorable zones like Reward Shaft.
• Grade distribution is highly structural, with intense gold shoots at intersections of bedding-parallel veins and W-dipping reverse faults.
• JORC data and vein maps confirm continuity of mineralization at depth, and the inferred resource is both deeper and higher in grade than the shallower indicated category.
Hill End’s vertical potential remains significant, particularly in areas where the fold/fault framework is best preserved. Further deep drilling along the anticline limbs and hinge zones is highly warranted.
I read that paper a while ago. Decided to get AI to add it to my supergene enrichment query. Above is what came back.
(20min delay)