Third Drill Rig Mobilising to Cerro Negro

THIS ANNOUNCEMENT CONTAINS INSIDE INFORMATION AS STIPULATED UNDER THE UK VERSION OF THE MARKET ABUSE REGULATION NO 596/2014 WHICH IS PART OF ENGLISH LAW BY VIRTUE OF THE EUROPEAN (WITHDRAWAL) ACT 2018, AS AMENDED.  ON PUBLICATION OF THIS ANNOUNCEMENT VIA A REGULATORY INFORMATION SERVICE, THIS INFORMATION IS CONSIDERED TO BE IN THE PUBLIC DOMAIN.

11 November 2025

Great Southern Copper plc

("GSC" or the "Company")

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

Enquiries:

Notes for Editors:

About Great Southern Copper

Great Southern Copper PLC is a UK-listed mineral exploration company focused on the discovery of copper-gold-silver deposits in Chile. The Company has the option to acquire mining rights to 100% of Especularita project in the under-explored coastal belt of Chile that is prospective for large scale copper-gold-silver deposits. Chile is a globally significant mining jurisdiction being the world's largest producer and exporter of copper.

The Especularita Project is located in the coastal metallogenic belt of Chile which hosts significant copper mines and deposits, including Teck's Carmen de Andacollo copper mine, and boasts excellent access to infrastructure such as roads, power and ports. Significant historical small-scale and artisanal workings for both copper and gold are readily evident in the exploration project area. The coastal belt offers deposit type optionality for copper including porphyry and IOCG style deposits as well as newly recognised intrusive-related copper and gold deposits.

Great Southern Copper is strategically positioned to support the global market for copper - a critical battery metal in the clean energy transition around the world. The Company is actively engaged in exploration and evaluation work programmes targeting both large tonnage, low to medium grade Cu-Au as well as high-grade Cu-Ag-Au deposits.

Further information on the Company is available on the Company's website: https://gscplc.com

Competent Person Statement  

The information in this announcement that relates to exploration results is based on and fairly represents information reviewed or compiled by Mr Sam Garrett, a Competent Person who is a Member of the Australian Institute of Geoscientists and a Fellow of the Society of Economic Geologists. Mr Garrett is the CEO and a shareholder of Great Southern Copper PLC. Mr Garrett has sufficient experience that is relevant to the styles of mineralisation and types of deposit under consideration and to the activity being undertaken to qualify as a Competent Person as defined in the 2012 Edition of the "Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves". Mr Garrett has provided his prior written consent to the inclusion in this announcement of the matters based on information in the form and context in which it appears.

This announcement includes information that relates to Exploration Results prepared and first disclosed under the JORC Code (2012) and extracted from the Company's previous LSE announcements as noted, and the Company's Prospectus dated 20 December 2021. Copies of these announcements are available from the LSE Announcements page of the Company's website: www.gscplc.com.

The Company confirms that it is not aware of any new information or data that materially affects the information included within the Prospectus dated 20 December 2021.

Forward Looking and Cautionary Statements

Some statements in this announcement regarding estimates or future events are forward-looking statements. They include indications of, and guidance on, future earnings, cash flow, costs and financial performance. Forward-looking statements include, but are not limited to, statements preceded by words such as "planned", "expected", "projected", "estimated", "may", "scheduled", "intends", "anticipates", "believes", "potential", "predict", "foresee", "proposed", "aim", "target", "opportunity", "could", "nominal", "conceptual" and similar expressions. Forward-looking statements, opinions and estimates included in this report are based on assumptions and contingencies which are subject to change without notice, as are statements about market and industry trends, which are based on interpretations of current market conditions. Forward-looking statements are provided as a general guide only and should not be relied on as a guarantee of future performance. Forward-looking statements may be affected by a range of variables that could cause actual results to differ from estimated or anticipated results and may cause the Company's actual performance and financial results in future periods to materially differ from any projections of future performance or results expressed or implied by such forward-looking statements. So, there can be no assurance that actual outcomes will not materially differ from these forward-looking statements.