Great Southern Copper plc (LON: GSCU) Third Drill Rig Mobilising to Cerro Negro to Commence Scout RC Drilling

Drilling to test Mostaza Fault Zone and hanging wall mineralisation where new rock chip samples grade up to 2.56 % Cu and 293 g/t Ag

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce that a third drill rig is being mobilised to its Cerro Negro prospect, part of the Company’s Especularita Project in Chile, and will commence scout reverse circulation (“RC”) drilling of targets along the Mostaza Fault Zone (“MFZ”) as well as newly identified outcropping mineralisation in the hanging wall volcanics, where recent mapping has identified oxide copper mineralisation.

Highlights:

·      Phase III drilling at Cerro Negro has expanded with the mobilisation of a third drill rig to site

·     Scout RC drilling to commence initial testing of the MFZ, 1.5 kilometres (“km”) south of the Mostaza mine

·      Drilling to also test newly identified oxide copper mineralisation in the hanging wall of the MFZ

·      Results for 22 outcrop samples collected in the hanging wall volcanics return grades up to 2.56% Cu and 293 g/t Ag

·    Programme comprises up to 2,000 metres (“m”) of RC drilling, which is designed to accelerate testing of multiple high-priority prospects and expand the scale of the Cerro Negro mineralised system

·    Two diamond drill rigs are currently in operation, testing extensions to GSC’s high-grade Cu-Ag discovery at Mostaza mine

·    GSC holds option to own 100% of the Cerro Negro prospect, including the Mostaza mine

·    Prospect located at low elevation with excellent access to infrastructure and mining services

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: “Exploration at Cerro Negro has taken its next exciting step-up with the addition of a third drill rig, joining the two diamond drill rigs currently in operation on site. This expanded drilling capacity will enable us to rapidly test the multiple high-priority exploration targets that we have identified along the Mostaza trend through mapping, geophysics and geochemistry programmes.

“The new rig will commence scout RC drilling of the Mostaza Fault Zone, south of the Company’s Mostaza mine. It will also test recently identified zones of oxide copper located in the hanging wall volcanics to the fault, which have not previously been drill-tested and have the potential to significantly extend the scale of near-surface copper mineralisation in the Cerro Negro system.

“With copper and silver prices continuing to push historical highs, this new development will significantly accelerate our ability to evaluate the broader-scale potential of the system and continue building on the excellent results achieved to date.”

Scout RC Drilling:

The initial scout RC drillholes, up to a total of 1,000m, will test a series of outcropping lenses located along the southern extension of the Mostaza Fault trend, 1-1.5km south of the historic Mostaza Mine. These lenses show strong silica alteration with crackle breccia and boxwork textures characteristic of surface-leached mineralised lenses elsewhere at Cerro Negro.

Surface sampling of the leached silicification has returned assays of up to 86.4 g/t Ag², with anomalous molybdenum (Mo), copper (Cu), arsenic (As) and antimony (Sb) – a geochemical signature consistent with the high-grade mineralisation at the Mostaza Mine. Due to intense surface leaching, copper grades are subdued in outcrop but are interpreted to represent the uppermost, weathered portions of mineralised lenses (Figure 1a).

The targets coincide with gradient array IP chargeability and conductivity anomalies extending along the trend³.

Rock Chip Sampling Results:

Scout RC drilling will also test newly identified oxide copper mineralisation outcropping in the hanging wall (west-side) of the MFZ  with a minimum 1,000m of additional RC drilling. Although outcrop in this area is predominantly obscured by gravel and talus deposits draining from the dacite dome to the east, windows of outcropping brecciated andesitic volcanic rock with oxide copper mineralisation are mapped and sampled along a 1.5km trend south of the mine which is open to the south along the fault trend (Figure 1b).

Assay results for 22 samples collected over a 1.5km trend in the west hanging wall area (Figure 2) yield grades up to 2.56% Cu and 293 g/t Ag, with 10 samples (45%) being anomalous in copper (>0.1% Cu) and 15 samples (68%) being anomalous in silver (>5 g/t Ag). These results also point to potential leaching of copper in the oxide, near-surface environment.

Figure 1: Oxidised silicified lens rock within the Mostaza Fault Zone (MFZ) south of the Mostaza pit (l); oxide copper in fractured and brecciated volcanics within the MFZ hanging wall (r).

Figure 2: Map of Ag-Cu-Mo-Sb rock chip geochemistry for the Cerro Negro Prospect, Especularita. The Mostaza Fault Zone (MFZ) is well defined by anomalous Mo geochemistry. To the west, the hanging wall oxide copper zone is a new target for near-surface oxide copper deposits to be tested with RC drilling.

Phase III Diamond Drilling:

The Phase III diamond drilling programme is on-going with two drill rigs targeting the Mostaza high-grade Cu-Ag mineralisation below the existing open pit as well as testing extensions to the mineralisation in the Lens 3-5 area⁴. As at the end of October 2025, 1,340m of drilling has been completed in 10 drill holes and 572 samples have been delivered to ALS Laboratories in Santiago for geochemical analysis. Assay results are pending. Logging, processing and sampling of core is on-going in conjunction with geological interpretation and modelling of the Mostaza Cu-Ag deposit.

Drill hole CNG25-DD036 is currently in progress in the north end of the Mostaza pit as a re-drill (twin-hole) to CNG25-DD033 which intersected two mineralised lenses before being prematurely terminated (due to rig malfunction) at 197.9m. The breakdown occurred whilst the hole was in chalcocite-mineralised lens rock[1] (Figure 3) that appears to represent a third, newly discovered lens. Re-drill hole DD036 will aim to test the full width of this deeper mineralised lens.

To the south, CNG25-DD035 is drilling beneath Lens 5 which hosts oxide copper in outcropping silicified dacite overprinted by quartz crackle-breccia.

Figure 3: Newly identified mineralised lens rock in HQ core samples from hole CNG25-DD033. Examples of coarse black disseminated and crackle-hosted chalcocite(?) occurs in silicified dacitic breccia at 192.8m (l) and 193.1m (r). The hole terminated prematurely in this mineralised lens rock at 197.9m. These samples have not been assayed and are for reference only, and no guidance for copper grade is provided. Refer to Cautionary Note below.

Cautionary Note:

Photographs of un-assayed drill core used in this RNS are for reference purposes only to describe the style of mineralisation encountered in the drill hole and do not provide information as to the metal grade of the rock where that rock has not yet been sampled and assayed. Readers are cautioned not to interpret metal grades or mineralisation widths from photographs of core or visual descriptions where no corresponding assays are reported or yet available.

  References:

1.    Cautionary Note

2.    RNS 5192G (29 April 2025): Exploration Results Extend Cerro Negro

3.    RNS 9584U (12 August 2025): Geophysics Define New Cerro Negro Drill Targets

4.    RNS 7559 (10 September 2025): Phase III diamond drilling underway at Cerro Negro

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