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.

12 June 2025

Great Southern Copper plc

("GSC" or the "Company")

First Assay Results from Phase II Drilling at Cerro Negro Confirm Outstanding Cu-Ag Grades

Drillhole CNG25-DD013 Intersected up to 4% Cu and 578 g/t Ag

Exploration holes confirm significant extension of high-grade Cu-Ag system south of Mostaza

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is extremely pleased to report high-grade assay results from Phase II diamond drilling at the Mostaza Mine, Cerro Negro.

Highlights:

·    Assay results received for holes CNG25-DD010 - DD014 with significant intervals including;

Ø DD013: 5m @ 3.04% Cu and 322.4 g/t Ag from 37m, within a broader zone of

§ 13.9m @ 1.74% Cu and 153.4 g/t Ag from 26.1m

Ø DD012: 3m @ 3.43% Cu and 164.7 g/t Ag from 24m, within a broader zone of

§ 20m @  32.1g/t Ag from 7m (copper and silver leached in surface oxide zone)

Ø DD010: 19m @ 30.5g/t Ag from 6m, including

§ 8.8m @ 58g/t Ag from 10.2m (copper and silver leached in surface oxide zone)

·    CNG25-DD012 confirms mineralisation up to 500m south of the Mostaza mine

·   Exploration holes DD010, DD012 and DD014 intersected broad zones of anomalous silver in silicified leached rock near surface stongly indicative of the potential for high-grade sulphide Cu-Ag mineralisation  at depth

·    Phase III drilling is planned to target beneath the broad leached mineralisation zones

·  Pole-dipole induced polarisation (PDIP), gradient, and audio frequency magneto-telluric (AMT) geophysical surveys are underway in advance of Phase III drilling

·    Further assays pending on holes CNG25 DD015 - DD025

·    GSC holds option to own 100% of the Cerro Negro project 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:

"The exciting results at Mostaza continue with drilling below the historical Mostaza mine intersecting further outstanding high-grade Cu-Ag mineralisation that remains open along strike and at depth.

"In addition, our exploration drilling, designed to test for mineralisation beyond the historical pit, also intersected significant mineralisation over 500 metres south of the mine. Holes DD010, 012 and 014 have intersected broad zones of anomalous silver hosted in highly leached rocks near surface, which we believe represent the possible oxidised tops of sulphide-rich, high-grade Cu-Ag mineralised lens material at depth.

"Our very exciting drill results, together with the recently started geophysical surveys, preliminary petrography, and metallurgical test work, are setting the stage for an exciting Phase III drill programme.

"We look forward to sharing more exciting results as they become available."

Results of diamond drillholes CNG25 DD010 - DD014

Phase II diamond drilling was designed to test continuity of the high-grade mineralisation intercepted in Phase I drilling at the Mostaza mine and to test new areas along strike to the north and south (see Figures 1, 2, 3).

Drillhole CNG25-013 was drilled along strike to the south of and roughly parallel to Phase I drillhole CNG25-DD001 that intersected 20m @ 3.31 % Cu and 269.9 g/t Ag from 27m, including 7m @ 5.72 % Cu and 489.6 g/t Ag from 31m¹ (see Figure 3).

Hole DD013 intercepted abundant disseminated and crackle-hosted chalcocite-dominant mineralisation in strongly altered and locally brecciated "lens" rock (see Figure 4). Assay results returned 13.9m @ 1.74% Cu and 153.4 g/t Ag from 26.1m, including 5m @ 3.04% Cu and 322.4 g/t Ag from 37m, and a highest grade of 1m @ 4% Cu and 578 g/t Ag from 30m. Table 1 summarises the Phase I and II significant intercepts to date.

These results are continuing to add to GSC's understanding of the architecture of the mineralised system at Mostaza, with updated modelling and interpretation suggesting that the high-grade mineralisation intersected in holes DD001, DD003, DD005 and DD013 (see Figures 2 - 3) is not part of the subvertical Lens 2 that outcrops in the mine - which was the focus of historical mining and resource estimation and GSC's original drill targeting. Instead, the information provided by new drilling  suggests that GSC may have discovered the top of a previously unrecognised tabular body (or "manto") dipping at some 50° degrees to the west. Importantly the manto is untested, close to surface, and open at depth and along strike.

Drillhole CNG25-DD011 was drilled to the north of the Mostaza open pit to test for a possible strike extension of Lens 2. Although the hole cut numerous intervals of alteration (Figure 4) with anomalous values of Cu and Ag, no high-grade mineralisation was cut, suggesting Lens 2 may have "pinched out" along strike or it could have been offset by a fault.

Exploration drillholes CNG25-DD010, 012 and 014 tested strike extensions of the Mostaza style mineralisation up to 500 metres south of the mine, where large outcrops of silicified structurally controlled mineralisation are mapped as Lens 4 (Figure 1 and 2). Historical artisanal mining of this southern lens is evident, however, no historical details of the workings, nor tonnages or grades are available. All drillholes intersected zones of strongly oxidised, structurally-controlled silicification over varying widths. Significant assays are summarised in Table 1.

Hole DD012 is the southernmost hole drilled to date. It intersected approximately 22.5 metres of strongly oxidised "lens" and "proto-lens" from surface with consistently anomalous but low-grade Cu and Ag thought to have been depleted by intense near-surface leaching. The leached interval transitions abruptly to hypogene sulphide mineralisation with 3m @ 3.43 % Cu and 164.7 g/t Ag which may reflect pre-leaching grades in the oxide zone (see Figure 5).

Hole 010 intersected similar intensely oxidised and leached rock (see Figure 4) strongly depleted in Cu but consistently anomalous in Ag (19.0m @ 30.5 g/t Ag from 6.0m) suggesting potential for higher grades of copper and silver at depth. The hole also intersected shallow underground mine workings from 10.2 - 14.2m down-hole. This is highly encouraging for a deeper exploration hole into the interpreted sulphide zone below.

Phase III drilling

The next planned phase of drilling will test this promising lens 3 - 5 area for high-grade mineralisation at depth below the leached oxide zone and historical workings as well as testing other exploration targets further to the south where rock chip samples by GSC have reported highly anomalous copper and silver in outcrop².

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

Figure 2: Cerro Negro Project, Mostaza Mine. Long section looking east showing the projection of mineralised bodies at depth. Note the recently interpreted "Manto" beneath the open pit. Arrows indicate that mineralisation remains open. Holes in this release are labelled.

Figure 3: Cerro Negro Project, Mostaza Mine. Long section orthogonal to Manto 1 showing the location of reported Phase I and Phase II drill holes. The blue box shows the intercepts in this release.

Figure 4. Cerro Negro Project, Mostaza Mine. Photograph of core from hole CNG25 DD013 on the left and CNG25 DD010, 011 and 012 on the right, with grades of Cu % and Ag g/t.

Figure 5: Hole CNG25 DD012, core illustrating the sharp transition from oxidised leached core (with anomalous silver) to primary, sulphide-rich lens material with high-grade copper-silver mineralisation at 24m depth down-hole.

Table 1: Summary of significant intercepts for Phase I and Phase II drill holes.

About the Mostaza Cu-Ag deposit, Cerro Negro Project

High-grade Cu-Ag mineralisation at Cerro Negro is hosted by structurally controlled elongate lodes of variable width (<4-20m) that strike between north and north-east. Historical mining at the Mostaza mine exploited steeply dipping to subvertical lodes such as "Lens 2". In addition to the steeply dipping "lenses", geological interpretation of recent drill results has identified a previously unrecognised and untested tabular lode, or "manto", below the Mostaza pit and dipping about 50 ° to the west.

Copper-silver mineralisation in the lenses and manto comprises abundant disseminations, veinlets, crackle networks and breccia matrix fillings of hypogene chalcocite with lesser tennantite, digenite, bornite, chalcopyrite, pyrite and local galena and sphalerite. The chalcocite contains inclusions of stromeyerite, an Ag-Cu sulphide.

Host rocks to mineralisation comprise multiple generations of intensely altered breccia, tuffisite and felsic intrusive. In the mine, the mineralised lodes are bound to the east and west by faults that are typically invaded by narrow (1-2m) andesite to rhyolite dykes. Outside of the bounding dykes and faults, the enclosing wall rocks consist of an east-dipping bedded sequence of fresh volcanic lava flows and brecciated dacite that might be part of a large dome-diatreme complex outcropping to the east.

In addition to the mineralisation seen in the Mostaza mine, several Cu-Ag rich bodies have been mapped at surface.  Historically these were interpreted as discrete "lenses", but collectively they have the potential to form an anastomosing network of mineralised bodies continuous over a strike length of up to 2km or more.

Core preparation, sampling and assaying

The Cerro Negro diamond drill core is collected from site by GSC staff and transported to the Company's nearby sampling facilities where it is then processed for geological, geotechnical, petrophysical and geochemical data. Sampling intervals are selected principally on geologic boundaries and may vary in length up to 2m. The core is cut into two halves using an electric diamond brick saw with half-core samples each allocated a unique identifier code and bagged-tagged separately. Samples for each complete hole are transported by GSC personnel to ALS laboratories in Santiago, Chile for sample preparation (drying, weighing, crushing and grinding). A final 200g aliquot of each sample is forwarded by ALS to their laboratories in Lima, Peru where it is assayed for Au (by 30g fire assay with ICP-AES finish method) plus a suite of 48 base metals and trace elements including Cu and Mo (by four acid digest ICP-MS method).

Sampling and assaying QAQC protocols employed by the company for this drilling project include routine insertion of standard reference materials including standards and blanks. Results for each SRM is assessed to monitor the accuracy and precision of the assay data from ALS for the core samples.

Cautionary note:

The Company cautions that all references to "resources", "mineral resources", or "mineral resource estimates" in this RNS are historical and were prepared and reported prior to the implementation of the JORC code and therefore are non-JORC compliant. The Company advises that the resource categories used in the historical estimates, for example "measured, indicated, demonstrated and inferred", may not have the same meaning or degree of confidence as current JORC categories. Historical records indicate that the quoted non-JORC resources were calculated using the polygonal method based on underground sampling, 1024m of drilling in 25 holes, and geological cross-sections and level plans. GSC is presenting this information for historical context only and is not treating it as a current mineral resource estimate.

References to mineralised grade intervals in drill holes are considered apparent thicknesses as there is not sufficient geological information to calculate true thickness of mineralised intercepts at this time.

References:

1.            RNS 5182Y (26 February 2025): Cerro Negro assays return 20m of 3.3% Cu & 270g/t Ag

2.            RNS 5192G (29 April 2025): Exploration results extend Cerro Negro

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 deposits in Chile. The Company has the option to acquire rights to 100% of two projects in the under-explored coastal belt of Chile that are prospective for large scale copper-gold deposits. Chile is a globally significant mining jurisdiction being the world's largest producer and exporter of copper.

The two, early-stage Cu-Au projects comprise Especularita and San Lorenzo Projects, both 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 both exploration project areas. 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.