Phase III Diamond Drilling Underway at 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.

10 September 2025

Great Southern Copper plc

("GSC" or the "Company")

Phase III Diamond Drilling Commences at Cerro Negro

Drilling planned to follow up on Phases I and II high-grade results and test new targets

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce that Phase III diamond drilling has commenced at Cerro Negro, part of the Especularita project.

Highlights:

· Phase III diamond drilling commenced at Cerro Negro

· Up to 2,000m drilling planned to test extensions to high-grade Cu-Ag results in Phase I and II

· First holes to target extensions to discovery hole DD007 which intersected;

o  3.85m of 5.21% Cu and 318g/t Ag from 88m, and

o  5.00m of 5.59% Cu and 99.2g/t Ag from 115m, within a broader interval of

o  33.0m of 1.96% Cu and 60.6g/t Ag from 87m1

· Drilling is also planned beneath shallow, leached intercepts at Lenses 3, 4 and 5, including a 22.5m leached intercept in hole DD012 which terminated in;

o  3m @ 3.43% Cu and 164.7 g/t Ag from 24m2

· Subsequent drillholes to target geochemical and geophysical targets along trend to the south

· The Company 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: "We are very excited to commence the full funded Phase III drilling at Cerro Negro, which is the next step towards extending the deposit beyond the existing Mostaza mine pit.

"Our knowledge of the deposit is significantly improved as a result of the earlier drilling, particularly in regards to structure, geology and the oxidation and leaching processes acting on the mineralisation near to surface. Drilling beneath the oxidised 'leached' zone in the Lenses 2 to 5 area is a priority for this programme and, if successful, will confirm the wider potential of mineralisation in the area and will significantly expand the priority areas for follow up drilling.

"We are also evaluating the addition of a second drilling rig, again subject to rig availability, to ensure momentum is maintained and to accelerate progress on the programme. We look forward to updating the market on Phase III results as they become available."

Phase III Drilling:

Approximately 2,000m of diamond drilling is planned for the Phase III programme which will focus on priority targets adjacent to and along strike from the high-grade copper-silver mineralisation discovered at the Mostaza Mine (Lens 2). In particular, drilling will initially focus on extending known mineralisation beneath and along strike of earlier holes that appear to have intersected shallow, leached portions of the lens rock which are depleted in copper but elevated in silver suggesting that copper has been remobilised out of the rock ("leached") during the near-surface oxidation process.

Later stages of Phase III drilling will target geochemical and geophysical anomalies up to 1.5 km south along trend. This staged approach allows GSC to build on the established high-grade system at Mostaza while systematically testing the broader trend for additional Cu-Ag mineralised bodies.

Lens 2, DD007 results:

The mineralisation intercepted in hole CNG25-DD007, which returned 33m @ 1.96% Cu and 60.6 g/t Ag from 87m, including the highest drill assays to date of 16.15% Cu and 646 g/t Ag1, is interpreted to represent a possible extension of the Lens 2 mineralisation identified in the Mostaza open pit. The mineralised Lens comprises silica-altered breccia clasts within a sulphide-rich matrix containing abundant chalcopyrite, bornite and pyrite. The highest grades correlate with zones of semi-massive bornite-chalcopyrite mineralisation. Phase III drilling will test the along-strike and down-dip continuity of this lens, which remains open in all directions.

Lens 4:

Up to 400m south of Mostaza, mapping and drilling confirm a zone of silicified, structurally controlled mineralisation with evidence of artisanal workings. CNG25-DD012 intersected 22.5m of leached lens rock from surface, transitioning sharply to sulphide mineralisation with 3m @ 3.43% Cu and 164.7 g/t Ag2. Phase III drilling will test this sulphide mineralisation at depth and along strike.

Lens 5:

Surface sampling at Lens 5 has returned assay grades up to 0.66% Cu and 86.3 g/t Ag from leached, oxidised outcrops of strongly silicified lens rock3. Geophysics highlights coincident chargeability and resistivity anomalies at depth4. Phase III drilling will provide the first drill test of this target.

Lens 2 - Mostaza mine (In-fill & Step-out):

In-fill drilling around the historical Mostaza open pit will refine the geometry and grade continuity of the newly-discovered west-dipping high-grade Cu-Ag deposit. Phase III step-out drilling will target extensions to the mineralisation at depth and along strike.

Figure 1: Phase III drill plan, Cerro Negro prospect, Especularita Project, Chile.

References:

1. RNS 2553F (7 April 2025): Further "exceptional" assay results from Mostaza

2. RNS 5588M (12 June 2025): Cerro Negro Drilling Confirms Outstanding Grades

3. RNS 5192G (29 April 2025): Exploration results extends Cerro Negro

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

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.