St Barbara Boosts 15-Mile Processing Hub Gold Resources by 20% to 2.5 Million Ounces

Extensive Validation of Historical Drilling Unlocks Resource Growth

St Barbara has completed an extensive validation process on a large volume of historical drilling data from the mid-to-late 1980s across the 15-Mile Processing Hub Project, which includes the 15-Mile Mine, Old Mitchell Mine (formerly Cochrane Hill), and Old Austen Mine (formerly Beaver Dam). This dataset, consisting of 436 drill holes totaling 71,000 metres, had not been previously included in Mineral Resources estimates by Atlantic Gold Corporation or St Barbara. The drilling primarily served as infill, enhancing drill spacing and spatial coverage within existing mineralised zones rather than defining new targets.

The validation involved a thorough review of historical reports, database audits comparing original and digital records, assessment of sampling and analytical methods, umpire assay comparisons, and alignment with recent infill drilling by St Barbara. The team confirmed the historical data's suitability for inclusion in the updated Mineral Resources Estimate. This effort has significantly increased data density across the deposits, enabling the use of more advanced estimation techniques previously unsupported by the limited data.

Transition from Multiple Indicator Kriging to Ordinary Kriging Reflects Improved Data Quality

The updated Mineral Resources Estimate reflects a methodological shift in St Barbara's resource modeling at the 15-Mile Processing Hub. Previously relying on Multiple Indicator Kriging (MIK), the substantial increase in validated drilling data now supports Ordinary Kriging (OK) estimation methodology.

This transition indicates greater confidence in mineralisation geometry and continuity. OK allows for modeling constrained geological wireframes that more accurately represent gold distribution across the three deposits. Managing Director and CEO Andrew Strelein attributed the 0.5 million ounce resource increase to updated geological interpretations, revised estimation methods, and the inclusion of historical drilling data, which together enabled more precise lode modeling and a stronger Mineral Resources Estimate.

15-Mile Mine Leads Resource Growth with Enhanced Lode Definition and Larger Pit Shells

The flagship 15-Mile Mine saw the most significant upgrade due to improved data and geological interpretation. The historical drilling dataset for this mine included 119 drill holes covering about 17,000 metres. Combined with previously validated drilling, the total data increased from 725 holes (95,000 metres) to 844 holes (112,000 metres), a 16.4% increase in holes and 17.9% increase in metres.

Revised geological models improved the definition and continuity of multiple mineralised lodes. While the resource within the historical pit shell remains similar, the updated model supports larger optimised pit shells, notably for the Egerton-Maclean pit, expanding contained gold within economically mineable zones. The new resource classification for 15-Mile Mine stands at 35.7 million tonnes at 1.2 g/t gold containing 1,390,000 ounces, up from 26.5 million tonnes at 1.0 g/t containing 860,000 ounces as of 31 December 2025.

Old Mitchell Mine Adjusted with Selective Geology-Constrained Interpretation

Old Mitchell Mine (formerly Cochrane Hill) underwent significant data augmentation and remodelling. Historical drilling included 104 holes (13,000 metres), plus validation of 23 holes from 2019 (5,200 metres) and 3 holes from 2024 (1,400 metres). Total data increased from 329 holes (53,400 metres) to 459 holes (73,000 metres), a 39.5% increase in holes and 36.6% in metres.

Using geology-constrained Ordinary Kriging, the interpretation became more selective, reducing tonnes but increasing average grade. The contained gold within the optimised pit shell remains broadly unchanged. The updated resource classification totals 16.5 million tonnes at 1.2 g/t gold containing 660,000 ounces, compared to 20.9 million tonnes at 1.1 g/t containing 690,000 ounces as of 31 December 2025, reflecting enhanced geological confidence.

Old Austen Mine Gains from Depth Continuity Despite Pit Limit Constraints

Old Austen Mine (formerly Beaver Dam) benefited from historical drilling and revised interpretation. Historical data included 213 holes (41,000 metres), expanding total drilling from 184 holes (29,000 metres) to 397 holes (70,000 metres), a 115.8% increase in holes and 141.4% in metres.

Improved geological models increased confidence in mineralisation continuity at depth. Although constrained pit inventory remains consistent due to a nearby river limiting depth, the model shows greater down-dip continuity and potential beyond current pit limits, which may influence future mine planning. The updated resource classification is 10.2 million tonnes at 1.4 g/t gold containing 460,000 ounces, compared to 12.0 million tonnes at 1.2 g/t containing 470,000 ounces as of 31 December 2025.

15-Mile Processing Hub Consolidated Resource Totals 62.4 Million Tonnes at 1.2 g/t Gold

The combined Mineral Resources Estimate for the 15-Mile Processing Hub, covering all three deposits, now totals 62.4 million tonnes at 1.2 grams per tonne gold, containing 2.5 million ounces. This represents a 20% increase or 0.5 million ounces compared to the prior estimate. The 15-Mile Mine contributes 1,390,000 ounces, Old Mitchell Mine 660,000 ounces, and Old Austen Mine 460,000 ounces.

Resource classification includes 17.6 million tonnes measured at 1.2 g/t (700,000 ounces), 38.8 million tonnes indicated at 1.2 g/t (1,450,000 ounces), and 6.1 million tonnes inferred at 1.1 g/t (230,000 ounces). These estimates are pit-constrained using a US$2,500 per ounce gold price assumption and underpin near-term mine planning and feasibility studies.

Group Mineral Resources Reach 5.4 Million Ounces Across Atlantic and PNG Operations

This upgrade raises St Barbara's total group-wide Mineral Resources Estimate to 5.4 million ounces of gold, including 2.5 million ounces from the 15-Mile Processing Hub and 2.9 million ounces attributable to the New Simberi operation. The company’s asset base spans Atlantic Canada and Papua New Guinea, supporting its development and operational strategies.

St Barbara continues to enhance value through exploration, development studies, and operational improvements. The 15-Mile Processing Hub Project highlights the benefits of thorough data validation and remodelling in uncovering additional resource potential. The diverse resource base offers flexibility to prioritize projects with the highest return prospects.

Study Updates and Feasibility Timeline Set Through Mid-2027

St Barbara plans to advance the 15-Mile Processing Hub Project through detailed study phases. An updated Pre-Feasibility Study incorporating the upgraded Mineral Resources Estimate and enabling an updated Ore Reserves Estimate is expected by the end of Q1 September FY27. The full Feasibility Study is scheduled for completion and announcement in Q4 June FY27.

This phased approach integrates updated technical data and economic parameters. The company has achieved key milestones including environmental permit approvals and a final investment decision on the Touquoy Restart in April 2026. The upcoming Pre-Feasibility Study Update will provide critical reserve and economic insights, setting the stage for potential final investment decisions pending board approval and market conditions.

Expanded Drilling Program and New Targets Planned for FY27

Improved geological modeling has identified new drill targets at the 15-Mile Mine, planned for evaluation and drilling during FY27. These targets reflect enhanced understanding of lode geometry and mineralisation controls from the recent data validation and remodelling work.

The drilling program underscores St Barbara’s commitment to fully testing the 15-Mile Processing Hub’s mineral potential. Positive drill results could further expand the resource inventory and extend mine life. Investors will closely monitor FY27 drilling outcomes, which may influence the final resource estimate incorporated into the Feasibility Study due in Q4 June FY27.