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.

18 August 2025

Great Southern Copper plc

("GSC" or the "Company")

Final Phase II Drilling Results Extends High-grade Mostaza Cu-Ag Deposit

New Mineralised Zone Identified at Lens 1

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce final drilling results from the Phase II programme at Cerro Negro including first drill results from Lens 1, located approximately 150 metres ("m") north of Lens 2 at Mostaza.

Highlights:

·    Final Phase II drill assay results confirm continuity of high-grade Cu-Ag mineralisation at Mostaza with significant intervals including;

o  CNG25 DD019: 7.2m @ 1.47% Cu and 122.3 g/t Ag from 99.9m, including;

·    1.4m @ 3.75% Cu and 319.1 g/t Ag from 103.5m

o  CNG25 DD020: 8.4m @ 1.49% Cu and 121.4 g/t Ag from 80.1m, including;

·    3.2m @ 2.63% Cu and 239.7 g/t Ag from 80.1m

·    First confirmed mineralised intercepts from Lens 1, 150m north of Lens 2 at Mostaza include;

o  CNG25 DD025: 3.0m @ 0.91% Cu and 61.7 g/t Ag from 28m including;

·    1m @ 1.38% Cu and 88.6 g/t Ag from 29m

·    25 holes completed in Phases I and II with all holes intersecting the target mineralisation zone

·    7 holes demonstrate evidence of "leached" copper from the silicified lenses

·    Metallurgical and structural studies in progress

·    Phase III drilling planned to commence in late August, subject to rig availability, targeting Lenses 3 - 5 and extensions of Lens 2

·    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: "These high-grade copper-silver results continue to build our understanding of the geometry and continuity of the mineralisation at Mostaza. Although holes DD021 to DD023 did not encounter high-grade mineralisation, they provide valuable geological information, helping to refine our model.

"We are also excited to report the first mineralised intercept from Lens 1 in DD025 - our first ever drill intersection into Lens 1, which provides encouragement for follow-up drilling. These results represent an important step in our broader campaign to define a potentially significant cluster of mineralised lenses at Cerro Negro.

"Drill planning is well advanced for our Phase III programme, which we hope to commence in late August, subject to drill rig availability. This is an exciting time for GSC as we begin the next step to attempt to explore the growth potential of this high-grade asset."

Phase II Drilling:

Drillholes CNG25 DD019-020 and 022-025 were designed to further test the continuity of high-grade copper-silver mineralisation within the Lens 2 deposit and to provide a first-pass assessment of Lens 1, located approx.150m north of the Mostaza pit (Figure 1). All results have now been received for the Phase II drilling programme.

Figure 1: Cerro Negro Project. Plan view showing the open pit and extensions of mineralisation at depth and along trend to the north and south. Drillholes in this release are labelled.

Mostaza Lens 2 (DD019-20 & 022-023):

Drillholes CNG25 DD019 and DD020 were drilled as 80m and 40m down-dip step-outs from CNG25 DD004 (Figure 1) and were designed to test the hypothesis that the Mostaza mineralisation is truncated by a fault at shallow levels on the northern side of the pit but remains continuous at depth.

Both holes intersected strongly altered and silicified "lens" rock hosting disseminated and crackle-hosted sulphide mineralisation dominated by chalcocite and lesser bornite, returning significant intervals of;

o  DD019:  7.2m @ 1.47% Cu and 122.3 g/t Ag from 99.9m, including

§ 1.4m @ 3.75% Cu and 319.1 g/t Ag from 103.5m

o  DD020:  8.4m @ 1.49% Cu and 121.4 g/t Ag from 80.1m, including

§ 3.2m @ 2.63% Cu and 239.7 g/t Ag from 80.1m

These results provide strong evidence for the down-dip continuity of the deposit, which remains open at depth (Figure 2a). Table 1 provides a summary of significant intercepts for all holes drilled to date.

Drillhole CNG25 DD022 was drilled as a 40m step out along strike to the north-west of DD020. It intersected multiple zones of altered "proto-lens" with anomalous Cu-Ag but no high-grade intercepts.

Drillhole CNG25 DD023 was drilled as an exploratory hole testing the southern extension of the deposit. The hole intersected a broad zone of "lens" rock which displays evidence of leaching consistent with its low Cu-Ag grade. This may be a result of post-mineral fault displacement which is also evident. Structural studies of the core are in progress to evaluate the potential effects of fault and dyke displacement of the mineralisation.

Ongoing interpretation of the drill core suggests that the mineralisation forms as a tabular, northwest-dipping deposit (Figure 2a-d), distinct from and with higher Cu-Ag grades than the historically mined steeply dipping lens structures evidenced in the pit wall and tested by historical drilling. Integration of recent and historical drill data, surface geological mapping, and geophysical surveys (IP and AMT) has significantly improved the understanding of the deposit's architecture.

This evolving geological model is guiding drill planning and target generation for the next phase of exploration at both Mostaza and along trend.

Lens 1 (DD024-025):

Holes CNG25 DD024 and DD025 represent the company's first drill tests beneath the interpreted Lens 1, approximately 150m north of the Mostaza open pit (Figure 1).

DD025 returned a shallow intercept of 3.0m @ 0.91% Cu and 61.7 g/t Ag from 28m, including 1.0m @ 1.38% Cu and 88.6 g/t Ag from 29m. This marks the first confirmed mineralisation beneath Lens 1 and provides encouragement for follow-up drilling (Figure 2b).

DD024 intersected anomalous Cu and Ag up to 0.23% Cu and 10.0 g/t Ag within structurally controlled silicified breccias similar to those seen at Mostaza, suggesting favourable host rocks are present.

Next steps:

Detailed logging of the core, including structural and petrographic studies, is in progress to better define the deposit model and assist with targeting of Phase III exploration drilling. Samples from mineralised intercepts in the Phase II programme have also been included within the preliminary metallurgical study which is ongoing.

In addition, the discovery of multiple zones of less altered and mineralised lenses in the hanging-wall of the deposit is highly encouraging and these will be investigated for their potential to zone into high-grade lenses at depth and as such potentially develop a "stacked lens" type system.

The success of the 2D pole-dipole IP and resistivity modelling at identifying the high-grade mineralisation plus dykes and fault structures suggests that a follow-up programme of in-fill survey IP lines may be considered.

Planning of Phase III drilling is well advanced and anticipated to commence in late August, subject to rig availability. Drilling will target extensions to the high-grade Lens 2 mineralisation, specifically in the Lenses 3, 4, and 5 areas, where previous holes were drilled too shallowly and intersected oxidised and copper-leached lenses.

Figure 2: Preliminary drill-section interpretations for the Mostaza deposit, Cerro Negro. Mineralisation intervals are reported as down-hole widths.

Table 1: Summary of significant drill intercepts for Phase I and II drill programmes, Mostaza mine, Cerro Negro Project. Mineralised interval meterage is reported as down-hole widths.

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.