Exceptional Drilling Results Continue at Mostaza.

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.

24 June 2025

Great Southern Copper plc

("GSC" or the "Company")

Exceptional Cu-Ag Drill Results Continue from Phase II Drilling at Mostaza, Cerro Negro

Including CNG25 DD015: 6.0m at 6.61% Cu and 542.4g/t Ag from 43m

Highest assay grades up to 10.4% Cu and 672g/t Ag

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

Highlights:

·    Assay results received for diamond holes CNG25 DD015 - DD018 and DD021 with highest significant results including;

Ø CNG25 DD015:   9.9m of 5.47% Cu and 442.5g/t Ag from 41.15m, including

o  6.0m of 6.61% Cu and 542.4g/t Ag from 43m, including

o  2.0m of 8.18% Cu and 669.0g/t Ag from 44m

Ø CNG25 DD016:   7.5m of 4.48% Cu and 342.6g/t Ag from 89m, including

o  0.5m of 10.4% Cu and 503.0g/t Ag from 96m

·    Highest assay result received includes 10.4% Cu and 672g/t Ag

·    Five lines of induced polarisation (IP) and audio frequency magneto-telluric (AMT) geophysics surveys completed testing the Mostaza trend over 2.5km

·    Phase III exploration drilling at Mostaza is in advanced planning stages and will be guided by recent pole-dipole IP and AMT geophysics surveys, along with surface mapping and geochemistry data

·    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, CEO for Great Southern Copper in Chile, said: "These spectacular jewellery box copper and silver grades are our best results to date and continue to highlight the exciting potential of the Mostaza deposit at Cerro Negro. Besides the very high grades, the deposit also benefits from being near surface and is located at low altitude with excellent access to key infrastructure and mining support services.

"Planning for the Phase III exploration drilling at Mostaza is well advanced and will rely on results from recently completed pole-dipole IP and AMT geophysics surveys as well as our surface mapping and geochemistry results.

"We are very excited to progress the opportunity to continue to grow and develop this significant copper-silver deposit."

Phase II drilling results for CNG25 DD015 to 018, and DD021

Diamond drillholes CNG25 DD015 to DD018, and DD021 are part of GSC's Phase II Mostaza drilling programme and were oriented to test the continuity of geology and grade of the northwest ("NW") dipping "Manto" of copper-silver mineralisation recently discovered below and west of the historical Mostaza Mine (see Figure 1 and 2).

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 discovered "Manto" beneath the open pit. Arrows indicate that mineralisation remains open. Holes in this release are labelled.

The Phase II holes were drilled using a "fan" pattern below the old mine workings to effect an approximate grid-pattern of intercepts intersecting the deposit with a nominal spacing of approximately 20-25 metres (m) along strike (NE-SW) and 20-50m down dip of the plane of the manto (NW-SE).

All drillholes in this release intersected silicified intermediate-argillic altered and variably mineralised rock at depths where the deposit was expected, indicating that the manto comprises a tabular body striking NE-SW and dipping at 50 degrees to the NW (see Figures 1, 2), although the exact architecture of the deposit is still to be determined. This preliminary manto interpretation, however, is in contrast to the original mine target which was assumed to be a steeply-dipping mineralised structure. Drilling to date defines the copper-silver manto measuring approximately 145m down-dip and 145m along strike with mineralisation remaining open both along strike and at depth.

Hole DD015 was drilled to intersect the manto at approximately 25m down-dip from hole DD013 which intersected 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 37m¹. Hole DD015 intercepted the same style of mineralisation with abundant disseminated and crackle-hosted chalcocite-dominant mineralisation in strongly altered and locally brecciated "lens" rock (see Figure 3, 4). Assay results returned 9.9m @ 5.47 % Cu and 442.5 g/t Ag from 41.15m, including 3.0 m @ 7.59 % Cu and 612.7 g/t Ag from 43m. The highest grade intersected was 1m @ 8.29% Cu and 672 g/t Ag from 45m down the hole. Despite the shallow depth below the current surface (approx. 50m), recent petrographic studies confirm the high-grade mineralisation is hypogene (ie, not supergene enrichment).

Hole DD016 targeted the manto approximately 30m down-dip from DD015 (Figure 3, 5) and intersected a broad interval of mineralisation including 10.7m @ 3.28 % Cu and 248.9 g/t Ag from 85.8m. This includes two intervals separated by a barren dyke near the top (hanging wall) of the manto. The upper interval includes 1.2m @ 1.2 % Cu and 76.3 g/t Ag from 85.8m. The lower interval is higher-grade and returns 6.7m @ 4.9 % Cu and 381.2 g/t Ag from 89.8m. The maximum assays were of 0.5m @ 10.4 % Cu and 503 g/t Ag from 96m.

Preliminary sections (Figures 4-6) suggest that this hole and hole DD015 cut the mineralisation orthogonally indicating a true thickness of the deposit of approximately 10m in these locations.

Hole DD017 intersected a broad interval of 13.1m @ 1.18 % Cu and 48.7 g/t Ag from 71m cut by metric intervals of barren dyke near the hanging wall of the manto. The upper interval of manto included 0.6m @ 1.11 % Cu and 100g/t Ag from 71m. The lower interval is higher grade and intersected 8.1m @ 1.76 % Cu and 68.1 g/t Ag from 76m (Figure 3, 6). This interval also includes anomalous zinc up to 2.67% Zn and lead up to 1.34% Pb.

Hole DD018 (see Figures 3, 6) is the deepest hole drilled to date. Like holes DD016 and 017, the hole intersects an upper and lower interval of manto separated by a barren dyke. The entire thickness of manto returns 16.6m @ 0.46 % Cu and 42.3 g/t Ag from 118.9m, including an upper interval of 3.6m @ 0.75 % Cu and 65.4 g/t Ag from 118.9, and a higher-grade lower interval of 5.8m @ 0.81 % Cu and 78.1 g/t Ag from 129.7m, including 2.5m @ 1.07 % Cu and 101.6 g/t Ag from 133m.

Hole DD021 intersected almost 16m of altered proto-lens cut by dykes. However, only a few metres of mineralised lens were observed from 99.9m with maximum values of 0.84 % Cu and 41.4 g/t Ag (Figure 4). It is unclear if this is a local zone of weaker mineralisation.

Broad zones of low grade Ag (>0.1 g/t Ag) are strongly anomalous in the hanging-wall and coincides with weaker zones of alteration - referred to as "proto-lens". Significantly, low-grade silver (>0.1 g/t Ag) also forms a halo enveloping the copper-silver manto. Therefore, the silver-enriched proto-lenses may be significant as "markers" for further stacked copper-silver lenses potentially discoverable along trend or down dip.

Figure 3: Cerro Negro Project, Mostaza Mine. Photograph of core from hole CNG25 DD015 on the left DD016 in the middle, and DD017 and DD018 on the right, with grades of Cu % and Ag g/t. The black mineral is mostly chalcocite with stromeyerite inclusions and lesser tennantite, digenite and bornite.

                        Figure 4: Cerro Negro Project, Mostaza Mine. Schematic cross section DD013-DD021, showing the location of reported Phase I and Phase II drill holes. The blue box shows the intercepts in this release.

Figure 5: Cerro Negro Project, Mostaza Mine. Schematic cross section DD001-DD016 showing the location of reported Phase I and Phase II drill holes. The blue box shows the intercepts in this release.

Figure 6: Cerro Negro Project, Mostaza Mine. Schematic cross section DD013-DD018 showing the location of reported Phase I and Phase II drill holes. The blue box shows the intercepts in this release.

Table 1: Summary of significant intercepts for Phase I and Phase II GSC 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 and lesser tennantite, digenite, bornite, chalcopyrite, pyrite and local galena and sphalerite. The chalcocite contains inclusions of stromeyerite, a 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.

The Company has not been able to independently verify the results of historical drilling or mine channel samples. 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 5588M (12 June 2025): Cerro Negro drilling confirms outstanding grades

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.