RNS Number : 1870C Great Southern Copper PLC 28 April 2026 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. 28 April 2026 Great Southern Copper plc ("GSC" or the "Company") Final Drill Results for Mostaza Copper-Silver Discovery Grade up to 4.23% Cu and 233g/t Ag Results confirm extension of Mostaza Cu-Ag deposit along strike and to depth Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to report on the final drill results for Phase III of its drilling programme at the Mostaza Cu-Ag discovery, Cerro Negro prospect, Especularita Project, Chile. Highlights: · Final Phase III assay results confirm down-dip and along strike continuity of high-grade Cu-Ag mineralisation at Mostaza with significant intervals including: o CNG25-DD045:17.0m @ 0.69% Cu, 48.2 g/t Ag from 184m, including; · 3.4m @ 1.44% Cu, 103.8 g/t Ag from 189.55m, and · 2.0m @ 1.1% Cu, 52.5 g/t Ag from 198m o CNG25-DD046: 2.0m @ 2.0% Cu, 123.7 g/t Ag from 187m, and · 0.5m @ 4.24% Cu, 233 g/t Ag from 200.3m · Continuity of upper Pb-Zn+Ag mineralised breccia zone also confirmed · Phase III results exceed expectations including extending the Mostaza deposit limits, discovering hanging-wall base- metal zone, recognising stacked lens morphology and confirming IP chargeability as an exploration tool with follow- up scout RC drilling · Interpretation of deposit architecture and Phase IV drill planning continuing · Results for metallurgical samples awaited · GSC holds rights to own 100% of the Cerro Negro prospect · Prospect located at low elevation with excellent access to infrastructure and mining services and along trend of major copper deposits including Los Pelambres, Altar and El Pachon. Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "These final Phase III results provide further confidence in the continuity of the Mostaza copper-silver deposit, with drilling now consistently intersecting mineralisation at increasing depths. The predictability of the deposit is improving, providing a clear focus for the next phase of drilling, with the system remaining open along strike and at depth. "Importantly, the latest results indicate that the system appears to be broadening at depth, with higher-grade copper-silver lenses hosted within a wider mineralised envelope. This is consistent with our evolving geological model and supports the potential for a larger-scale system. "The Phase III drilling programme has exceeded expectations delivering highly significant results including extending the known limits of high-grade copper-silver mineralisation at Mostaza, identifying an anomalous lead-zinc-silver zone in the hangingwall to the Mostaza deposit, recognising the potential for stacked lens morphology and confirming base-metal mineralisation associated with IP chargeability anomalies along the 2km long Mostaza Fault Zone. "Interpretation of the final results is on-going and will support planning for the Phase IV drilling programme which will focus on the expansion of the Mostaza resource, alongside the ongoing exploration of the broader Cerro Negro system, that now measures over 2km along strike and 1 km in width." Cerro Negro Prospect: The Especularita Project, including the Cerro Negro prospect and high-grade Mostaza Cu-Ag discovery, is located at low altitude within the Cretaceous-aged coastal metallogenic belt of northern Chile that includes large scale copper mines and projects including Carmen de Andacollo and El Espino (Figure 1). Significantly, the project is also located within a northwest- southeast trending structural lineament that links the project with notable large-scale porphyry Cu-Au mines and projects, including Los Pelambres, Altar-Piuquenes and El Pachon (Figure 1). Figure 1: GSC's Especularita project is located at low altitude within the coastal metallogenic belt of northern Chile and within 50km radius of over 50Mt of contained copper metal defined in mines and advanced projects. Phase III Drilling Programme, Cerro Negro: Phase III drilling was completed in January 2026, comprising 17 diamond drillholes for 2,923 metres and six scout RC holes for 1,032 metres1 . The Phase III programme was designed to continue advancing the high-grade copper-silver discovery at Mostaza by: · Infill and step-out drilling at the Mostaza deposit (Lens 2 and adjacent lenses) to refine the geometry and continuity of the high-grade Cu-Ag mineralisation. · Testing extensions of known mineralisation at nearby targets, including Lenses 3, 4 and 5, where previous drilling intersected strongly silicified but copper-leached lens rocks indicative of underlying sulphide mineralisation. · First-pass scout RC drill testing of geochemical and geophysical anomalies along the Mostaza Fault Zone to the south of the historic Mostaza mine (Figure 2). Figure 2: Drill plan summary for Cerro Negro prospect highlighting holes reported in this release. Final Drill Assay Results: CNG25-DD037: Diamond drillhole DD037 was drilled as a 60 metre down-dip step-out from DD031 which intersected 7.2m @ 2.58% Cu and 188.7 g/t Ag from 90.40m2 , and was designed to test the continuity of mineralisation at depth (Figure 3). The hole first intersected the down-dip extension of the shallow silica-sulphide lens, characterised by highly anomalous base metals and silver, grading 19m @ 0.17% Zn, 0.10% Pb and 3.9 g/t Ag. This interval is consistent with previously reported polymetallic sulphide breccia horizons developed in the hanging-wall to the main Cu-Ag mineralisation (Figure 3). At depth, DD037 intersected the down-dip extension of the principal chalcocite-clay Cu-Ag lens, returning 3.77m @ 0.83% Cu and 57.23 g/t Ag from 156.23m including: 1.3m @ 1.15% Cu and 82.98 g/t Ag from 156.23m. A second, deeper Cu-Ag zone of 0.68m @ 0.82% Cu and 49.2 g/t Ag from 190m was intersected, supporting the interpretation of a stacked lens type morphology to the deposit. The lower footwall zone of the mineralised lens in this hole appears to be faulted out by the interpreted Raven Fault resulting in a narrower intercept width in this location (Figure 3) and further emphasising the structural complexity of the deposit. Figure 3: Cross-section for DD037 through the Mostaza deposit (looking Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles and grade down hole in DD037. CNG25-DD045: Diamond drillhole DD045 was drilled as a 50m along-strike step-out to the north of DD036, which returned a broad interval of 39.6m @ 0.57% Cu and 34.7 g/t Ag from 170.6m (Figure 4). The hole also represents a 75m down-plunge step-out from DD022, which intersected anomalous Cu-Ag within poorly developed lens alteration but no high-grade intervals. DD045 intersected a similar sequence to that observed in DD036, comprising an altered and mineralised dacite dyke with alteration and mineralisation developed into both the hanging-wall and footwall of the dyke, and returning significant intervals of: · 17m @ 0.69% Cu and 48.21 g/t Ag from 184m, including; o 3.4m @ 1.44% Cu and 103.8 g/t Ag from 189.55m, and o 2.0m @ 1.10% Cu and 52.5 g/t Ag from 198m The development of higher-grade zones within a broader mineralised interval at the footwall contact is consistent with the emerging model of dacite-hosted mineralisation, where higher-grade Cu-Ag lenses are preferentially developed along dyke margins. DD045 extends the mineralised system along strike to the northwest as well as down-dip and highlights strong potential for further extensions of the Cu-Ag mineralisation in these directions. Figure 4: Cross-section for DD045 through the Mostaza deposit (looking Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles and grade down hole in DD037. CNG25-DD046: Diamond drillhole DD046 was drilled as a twin to DD023, which did not reach sufficient depth to test the interpreted mineralised lens during Phase II drilling (Figure 5). The hole also represents an approximately 100m down-plunge step-out from DD021 and was designed to test the continuity of mineralisation at depth towards the southern extent of the Mostaza system. DD046 intersected the down-dip extension of the shallow silica-sulphide base metal lens, thereby confirming the exploration model and returning 24m @ 0.50% Zn, 0.25% Pb and 3.8 g/t Ag from 93.45m. This interval is consistent with the polymetallic (Pb-Zn-Ag) sulphide breccia lenses observed in previous drilling, interpreted to occur in the hanging-wall to the main Cu-Ag mineralised lenses. At depth, the hole intersected the down-dip extension of the principal chalcocite-clay Cu-Ag mineralised zone, comprising 2.0m @ 2.0% Cu and 123.7 g/t Ag from 187m and an additional deeper interval of high-grade mineralisation grading 0.5m @ 4.24% Cu and 233 g/t Ag from 200.3m (Figure 5). The presence of multiple mineralised intervals, including high-grade copper-silver mineralisation at depth, further supports the interpretation of a vertically extensive, stacked lens system. The narrow, high-grade intercept at the base of the hole suggests the potential for additional high-grade lenses at depth. DD046 materially extends the known footprint of the system down-dip and along trend to the southwest, highlighting potential for further extensions with ongoing drilling. Figure 5: Cross-section for DD046 through the Mostaza deposit (looking Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles and grade down hole in DD037. Next Steps: All drilling assay results for the Cerro Negro Phase III drilling programme have now been received. Detailed review and assessment of the geology and geochemistry data is continuing, in conjunction with further surface mapping and sampling, to assist with updating the Company's understanding and interpretation of the deposit architecture for Mostaza. Planning for a Phase IV drilling programme is in progress, focused on both infill and step-out drilling at Mostaza to further define the geometry, continuity and scale of the deposit, as well as definition of exploration targets within the greater Cerro Negro mineral system. Ongoing detailed interpretation is integrating geochemical, geophysical, geological and structural datasets to refine the geological model and optimise targeting ahead of the next phase of drilling. Metallurgical test work is progressing with results expected to support the assessment of processing characteristics of the Mostaza Cu-Ag deposit ore. References: 1. RNS 4907O (13th Jan 2026): Phase III Drilling Completed at Cerro Negro 2. RNS 1749L (11th Dec 2025): Drilling Extends Mineralisation at Cerro Negro Engage with the GSC management team directly by asking questions, watching video summaries and seeing what other shareholders have to say. Navigate to our Interactive Investor hub here: https://gscplc.com/ Enquiries: Great Southern Copper plc https://gscplc.com/link/epzDBP Sam Garrett, Chief Executive Officer +44 (0) 20 7138 3204 SI Capital Limited Nick Emerson +44 (0) 1483 413500 BlytheRay Tim Blythe / Megan Ray +44 (0) 20 7138 3204 GSC@blytheray.com Subscribe to our news alert service: https://gscplc.com/auth/signup 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. 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