RNS Number : 0949B Great Southern Copper PLC 20 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. 20 April 2026 Great Southern Copper plc ("GSC" or the "Company") Drilling Commences with First Test of Porphyry Copper Targets at Especularita Scout RC drill programme targets margins of La Colorada lithocap 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 Piedras Blancas (White Stones) prospect located on the western margin of the La Colorada lithocap, in the Especularita project ("Especularita"). Highlights: · Drilling to test porphyry Cu target at Piedras Blancas with initial three scout reverse circulation ("RC") holes · Holes targeting silica-sericite-pyrite altered rocks with overprinting silica-pyrite stockwork alteration indicative of porphyry copper-related phyllic alteration zone · Target outcrops over 1km in diameter but extends under gravel cover to east and south · Anomalous molybdenum up to 49 ppm Mo in rock chips from leached alteration zone · Drilling aims to vector follow-up drilling toward potential potassic copper-rich porphyry zones · No evidence of historical drilling at Piedras Blancas · GSC holds the rights to own 100% of the Piedras Blancas 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: "We are very excited to start our new drilling campaign at Piedras Blancas as it represents the Company's first drill test of a porphyry copper target at Especularita. "An initial three scout RC holes are planned, which will test a window of outcropping porphyry-style alteration enclosed by gravels and scree. The alteration is characteristic of the phyllic zone of porphyry copper systems with pervasive silica-sericite- pyrite overprinted by stockworks of pyrite and quartz-pyrite fracture veins. The surface geology is highly oxidised and leached, so we want to get below the oxide zone and confirm what's there and use that information to potentially vector toward the potassic copper-rich core of the porphyry system." Piedras Blancas Porphyry Cu Prospect: Piedras Blancas is located at the northwestern margin of the La Colorada lithocap that comprises the Company's principal porphyry copper target within its flagship Especularita project (Figure 1). The Especularita project is located 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 50Mt of contained copper metal defined in mines and advanced projects. Scout RC Drilling Programme: Scout RC drilling has commenced at Piedras Blancas (White Stones), where the overlying La Colorada lithocap is interpreted to have been eroded away, exposing possible porphyry-style phyllic alteration comprising intense quartz-sericite-pyrite overprinted by stockworks and crackle veinlets of quartz-pyrite mineralisation (Figure 2). Intense weathering and oxidation of the surface rocks have resulted in depleted (leached) copper signatures for rock chip samples in the area. However, molybdenum, which is a useful pathfinder element for porphyry copper mineralisation, is less mobile in the oxide environment and exhibits strong anomalism with assay results up to 49ppm Mo from 13 rock samples of the alteration zone. The drilling target is further defined by a prominent broad zone of de-magnetisation in the drone magnetics survey1 which is a feature consistent with pervasive magnetite-destructive phyllic alteration in porphyry copper settings. On a local prospect scale, however, subtle magnetic features are evident which will be tested with this drilling programme (Figure 3). An initial test of three scout RC drill holes (Figure 3) has been designed with the aim of determining the depth of oxidation and leaching and confirming that the alteration observed in outcrop corresponds at depth to the phyllic zone of a porphyry copper system. The inner phyllic zone, or "shell", may host lower grade copper mineralisation, however, the results for this drilling are aimed to provide strong vector indicators for possible follow-up drilling targeting the potassic core of the porphyry system which typically comprises higher copper grades and may be located at depth or laterally beneath the gravel cover (or both). Figure 2: Oxidised quartz-sericite-pyrite (QSP) alteration overprinted by quartz-pyrite stockwork veining exposed in outcrop at Piedras Blancas ("White Stones") prospect, Especularita. Alteration is characteristic of the phyllic alteration zone in porphyry copper systems. Next Steps: Following the completion of the Piedras Blancas scout RC drilling programme, the rig will proceed to the Artemisa and Victoria prospects to complete similar drill tests targeting porphyry-related copper mineralisation. Final results for Phase III diamond drilling at Mostaza are still awaited, as are results for metallurgical testwork conducted on Phase I and II high-grade core samples. Planning and permitting for Phase IV drilling at Mostaza are in progress. Geophysics surveys are being planned for both the Cerro Negro prospect (extension of 2025 IP survey) and the La Colorada lithocap (AMT), anticipated to commence in May 2026. Detailed prospect-scale mapping and sampling are continuing at the Viuda prospect. New results from this ongoing work, together with results of the recent ground magnetic survey, will direct targeting for the next phase of drilling at Viuda. Figure 3: Preliminary plan for scout RC drilling at Piedras Blancas illustrating geology (left) and Vertical Signal Magnetic Intensity (right). References: 1. RNS 7231B (06 June 2023): Magnetic survey commences at Especularita. 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/epzQ9P 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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