RNS Number : 3799I Great Southern Copper PLC 16 June 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. 16 June 2026 Great Southern Copper plc ("GSC" or the "Company") Scout RC Drilling Commences at Artemisa South Copper-Gold Target AMT survey of lithocap completed Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce that scout RC drilling has commenced at the Artemisa South copper-gold target at the Especularita Project, while drilling recently completed at the nearby Victoria target has confirmed potential for a large-scale porphyry copper type alteration system. Highlights: Artemisa South · Scout reverse circulation ("RC") drilling commences at Artemisa South prospect, targeting old mine workings where structurally-controlled vein and disseminated type Cu-Au mineralisation occurs in granodiorite south of the La Colorada lithocap · No previous drilling of the target · GSC holds rights to own 100% of the Artemisa South target · Prospect located at low elevation with excellent access to infrastructure and mining services and along trend of more than 85Mt of contained copper hosted in major deposits including Los Pelambres, Altar and El Pachon. Victoria · Scout RC drilling completed at Victoria prospect with five holes drilled for 662 metres ("m") · First drilling beneath the La Colorada lithocap confirms porphyry-style alteration is strengthening toward the centre of the system, supporting the potential for a large-scale buried copper-gold porphyry target · Samples for all five holes at Victoria have been dispatched to ALS Laboratories in Santiago · GSC holds rights to own 100% of the Artemisa North target Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "Artemisa South is the final of four targets to be tested with scout RC drilling in the current porphyry exploration programme. This programme is testing beneath high-grade copper- gold-silver mineralisation identified in old workings, providing an opportunity to assess both the broader porphyry potential and the higher-grade mineralised structures observed at Artemisa South. "The identification of Porphyry-related phyllic alteration in drill chips at Victoria is very encouraging, particularly as this is the first hole to test beneath the La Colorada lithocap. These observations provide further support for our interpretation that the lithocap has the potential to host a large-scale porphyry copper alteration system. "Meanwhile, the AMT geophysics survey across the western half of the lithocap has been completed and results will be used in conjunction with the scout drilling and mapping to vector the next phase of drill targeting of the lithocap for buried porphyry copper deposits. "Exploration mapping and sampling activities are continuing at both the Cerro Negro and Viuda prospects as we prepare for the next phase of drilling activity in those areas." Artemisa South Porphyry and Breccia-Vein Target: The Artemisa South prospect is located south and west of the Victoria drill target along the southern margin of the La Colorada lithocap (Figure 1). The Especularita project is situated at low altitude within the Cretaceous-age coastal metallogenic belt, 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 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 50 million tonnes of contained copper metal defined in mines and advanced projects. Three scout RC holes are planned to test the Artemisa South target (Figure 2), consisting of structurally controlled zones of chlorite-sericite alteration with Cu oxides, trace sulphides and local quartz veins that are thought to be evidence of hydrothermal fluids flowing southwards from the Colorada Lithocap into the Quilitapia batholith. Drilling will test beneath artisanal mines to target continuation of the structural-controlled mineralisation. Victoria Cu-Au Target: Five scout RC holes were drilled at Victoria to test the prospect for evidence of porphyry copper-gold and high-grade breccia- vein mineralisation and to provide vectors towards targets beneath the undrilled Colorada lithocap (Figure 2). All holes were orientated perpendicular to target structures and inclined at 60-degree dip. Hole VIC26-RC010 was designed to test a zone of high-grade disseminated bornite mineralisation detected in historic artisinal workings that returned assay grades in rock chips up to 6.9% Cu and 1.85g/t Au1 . The hole intersected microdiorite with pervasive strong magnetite-biotite-clay alteration with traces of disseminated pyrite cut by occasional veinlets of carbonate and or quartz. VIC26-RC011 was oriented to cross a NW trending quartz vein with anomalous Au values and intersected microdiorite with pervasive strong magnetite-biotite-clay alteration with traces of disseminated pyrite cut by occasional veinlets of carbonate and or quartz. Locally minor Cu oxides observed over short intervals. Figure 2: Geology and drill hole plan map of the Artemisa South and Victoria, Especularita Project. Quartz-sulphide vein-breccia exposed in artisanal workings and considered a possible source of the abundant mineralised float was tested with hole VIC26-RC012. The hole intersected microdiorite with pervasive strong magnetite-biotite-chlorite- actinolite alteration with increasing biotite content with depth, with locally 1-5% pyrite and traces of chalcopyrite as disseminations and hairline veinlets, together with veinlets of quartz and/or carbonate. An additional artisanal mine working of vein-breccia with traces of Cu oxide (Espino vein) was tested with hole RC013 which intersected similar microdiorite with variable magnetite-biotite-chlorite alteration, minor to trace pyrite, and a couple of intervals of specular hematite that may represent the downward continuation of the specularite mineralisation seen in the workings. In contrast to other Victoria drill holes, VIC26-014 is located to the north of the Victoria Fault and was designed to test the edge of the postulated lithocap domain beneath an artisanal mine working with Cu oxides. The top of the hole intersected strong clay and chlorite altered microdiorite with traces of Cu oxide, including a 10m zone of quartz-sericite-pyrite alteration from 60m to 70m depth, before passing into strong magnetite-chlorite alteration with locally abundant disseminated and veinlet pyrite. This chlorite dominated alteration is interpreted as distinct from that seen to the south and is more like the alteration observed in Piedras Blancas2 and Artemisa North3, indicative of increasing hydrothermal alteration towards the La Colorada lithocap. Scout RC Drill Programme and Sampling: Scout RC drilling recently commenced at Artemisa South with a view to test the prospect for evidence of structurally- controlled copper-gold and high-grade breccia-vein mineralisation and to provide vectors towards targets beneath the undrilled Colorada lithocap. A preliminary test comprising up to 1000m of drilling in at least four scout RC holes is now underway to assess the alteration and mineralisation. The RC drilling method employed at Especularita collects samples on two-metre intervals. Representative samples for each sample of the cuttings are collected in large, individually numbered plastic bags, which are then weighed and transferred to a riffle splitter where they are separated (or split), and homogenised, three times into two duplicate samples of approximately 5kg each. GSC personnel then ticket and label one of the ±5kg samples using preprinted ticket books. Finally, standard reference materials ("SRM's" or "Standards"), blanks, and duplicate samples are inserted into the sample sequence for the purpose of quality assurance / quality control ("QAQC"). The second ±5kg sample is used for geological description and stored for reference. An additional reference selection of cuttings for each two-metre sample is also retained in labelled boxes as a record of the geology of the drill hole. At the completion of each hole, the representative 5kg samples are transported as a batch from the field and delivered by Company employees to ALS Laboratories in Santago or La Serena where they are prepared for analysis (weighed, dried, crushed and pulverised). A subsequent 200g split of each sample pulp is then forwarded to the ALS geochemical laboratory in Lima, Peru, where samples are assayed for gold (30g fire assay with ICP-AES finish, ALS code AU-ICP21) and 48 major, base and trace elements by ICP-MS following a four-acid digest (ALS code ME-MS61). RC drilling was selected for this initial proof-of-concept stage as it is faster and cheaper than diamond drilling and allows for the rapid testing of more targets for the same cost. Next Steps at Especularita: Geology and geochemical assay results for the scout RC drilling programme will be used to vector the next phase of exploration drilling at La Colorada. First assay results are expected within 2-4 weeks. Final results for leach tests are awaited for the Mostaza metallurgical test work conducted on Phase I and II high-grade core samples. Planning and permitting for Phase IV drilling at Cerro Negro, including Mostaza, are in progress. The AMT geophysics survey across the La Colorada lithocap is completed with data interpretation in progress. Planning for extensions to the Cerro Negro IP survey is also underway and anticipated to commence during June 2026. Detailed prospect-scale mapping and sampling are continuing at both the Viuda and Cerro Negro prospects. New results from this ongoing work, together with results of the planned geophysics surveys, will direct targeting for the next phase of drilling at both Cerro Negro and Viuda. References: 1. RNS Number 0445V (03 April 2023): Rock Chip Assay Grades up to 6.9% Cu and 1.85g/t Au in Outcrop from Victoria Prospect 2. RNS Number 0949B (20 April 2026): Drilling Commences with First Test of Porphyry Copper Targets at Especularita 3. RNS Number 1366E (13 May 2026): Scout RC Drilling Commences at Artemisa North Porphyry Copper Target 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/r6gBGP 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. 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