Further "exceptional" assay results from Mostaza

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

Highlights:

• Assay results received for diamond drill hole CNG25 DD007 with significant intervals including;

Ø 33m of 1.96% Cu and 60.6 g/t Ag from 87m, including

§ 3.85m of 5.21% Cu and 318.2 g/t Ag from 88m, and

§ 5.0m of 5.90% Cu and 99.2 g/t Ag, from 115m, including

• 2.5m of 9.67% Cu and 175.4 g/t Ag from 116.4m
• New results include highest reported assay grades of 16.15% Cu and 646 g/t Ag with four samples reporting copper assay grades over 8.0% Cu
• DD007 located 270m to the south of CNG25 DD001 which intersected;

Ø 20m of 3.31% Cu and 269.9g/t Ag from 27m1

• Mineralisation remains open in all directions including at depth
• Phase II resource and exploration diamond drilling in progress with a further 10 holes completed for 1,059m
• GSC holds an option to own 100% of the Cerro Negro project which includes the Mostaza mine
• Prospect located at low elevation with excellent access to infrastructure and mining services
• Recent funding round and the ongoing exercising of warrants provide a strong balance sheet for continuing exploration at Mostaza and the greater licence areas

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "The results for hole DD007 confirm that high-grade copper-silver mineralisation is continuing along trend south of the Mostaza mine which greatly enhances the potential value of this significant copper-silver deposit. The 16.15% copper result is the highest assay result received to date for the project.

"Phase II diamond drilling is well under-way with two rigs now in the Mostaza pit testing the depth extensions of the high-grade mineralisation identified in Phase I. To date all Phase II holes have intersected the mineralised lens to a vertical depth of 100m below the original hole 1 intercept.

"Excitement is building at Cerro Negro, but we have a lot of work ahead of us as we seek to demonstrate the size and economic value of this exciting high-grade deposit."

Results of diamond drillholes CNG25 DD006 - DD009

Diamond drillholes CNG25 DD006 - DD009 were designed to test extensions of the Mostaza Lens 2 mineralisation along trend to the south where discontinuous outcrops of silicified structurally-controlled mineralisation are mapped as Lenses 3 - 5 (Figure 1). Small-scale artisanal mining of these southern lenses is evident, however, no historical details of tonnages or grades have been previously reported.

All drill holes intersected zones of variably mineralised, structurally-controlled silicification over varying widths and significant assays are summarised in Table 1. Holes DD006, 008 and 009 all intersected shallow oxidised zones of lens with anomalous silver (>0.1 g/t Ag) associated with the oxidised silicious breccias indicating surface leaching of sulphides. Geological interpretation of the results are on-going.

Drillhole CNG25 DD006 was drilled to test below and along strike to the north of Lens 4. It intercepted several oxidised and reduced grade silicified mineralised zones of "proto-lens" with anomalous Cu and Ag up to 0.16% and 24.9 g/t respectively. (Figure 2)

Drillhole CNG25 DD007 was drilled below Lens 3 and intersected 33m of mineralised "lens" rock grading 1.96% Cu and 60.56 g/t Ag over the total intercept from 87m down-hole. The mineralised lens includes two distinct zones of high-grade Cu-Ag mineralisation including 3.9m at 5.21% Cu and 318.21 g/t Ag from 88m and 5.0m of 5.90% Cu and 99.2g/t Ag from 115m. The lower zone of high-grade Cu-Ag includes 2.5m at 9.67% Cu and 175.44 g/t Ag from 116m depth (see Figure 2). This interval includes the highest reported assay grades to date of 16.15% Cu and 646 g/t Ag with 4 samples reporting grades over 8.0% Cu. The mineralised Lens comprises silica-altered breccia clasts within a sulphide-rich matrix containing abundant chalcopyrite, bornite and pyrite (See Figure 3). The highest grades correlate with zones of semi-massive bornite-chalcopyrite mineralisation.

Drillhole CNG25 DD008 was drilled above and 20m to the north of DD007 to intersect the Lens 3 at a shallower depth. The hole intercepted a 21.4m zone of oxidised silicified breccia from surface with strong oxidised sulphides and anomalous Cu and Ag up to 0.51% Cu and 31.3 g/t Ag respectively. These results suggest that near-surface leaching has resulted in lower grades (Figure 3).

Drillhole CNG25 DD009 was drilled 100m to the south of the Mostaza Mine to test the southern extension of Lens 2 and intersected several zones of weakly altered and mineralised "proto-lens", suggesting the Lens has been structurally displaced in this location.

A summary of all Phase I significant assay results to date are shown in Table 1 below.

Table 1: Summary of significant intercepts for Phase I GSC drill holes DD001 - DD009.

The Mostaza Cu-Ag deposit, Cerro Negro

The high-grade Cu-Ag mineralisation at Mostaza is confined to a north-south trending, structurally-controlled lode, or "lens" (specifically Lens 2), of variable width (<4-20m) that dips moderately steeply to the west. Copper-silver mineralisation comprises abundant disseminations, veinlets, crackle networks, and breccia matrix fillings of hypogene black to dark metallic grey chalcocite with lesser bluish bornite and traces of chalcopyrite and pyrite

Host rocks to mineralisation comprise multiple generations of intensely altered breccia, tuffisite, and felsic intrusive. The mineralised lens is bounded 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, black volcanic lava flows and brecciated dacite that might be part of a large dome-diatreme complex outcropping to the east.

The Lens 2 mineralisation is interpreted to form one of a number of Cu-Ag rich bodies mapped at surface which have historically been interpreted as discrete "lenses" but which collectively have the potential to make up an anastomosing network of structurally controlled zones of mineralisation continuous over a strike length of greater than 1-2km.

Figure 1: Cerro Negro Project, Mostaza Mine: Simplified schematic plan map showing the location of GSC drillholes with respect to known mineralised lenses and historical drillholes. Holes reported in this press release are highlighted in yellow.

Figure 2: Cerro Negro Project: Schematic cross section through drillholes CNG25 DD006 and DD007 showing Cu-Ag intercepts and the preliminary geological interpretation and relationship to

historical drillholes. Note the extrapolations to the historical drillholes are based on assay results only because historical drill core and logs are unavailable.

Figure 3: Cerro Negro Project, Mostaza Mine. Photograph of core from hole CNG25 DD007 on the left and CNG25 DD008 on the right, with grades of Cu % and Ag g/t. The black, brown and bronze minerals filling the cracks and veins in hole DD007 are Cu-Ag bearing sulphides and sulfosalt minerals cutting intensely silicified rock. The core from hole DD008 is texturally similar but has been subject to near-surface oxidation and leaching leaving only Fe-oxides and clays with low grades of Cu and Ag.

Core preparation, sampling and assaying

The Mostaza 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 Molybdenum (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 are assessed to monitor the accuracy and precision of the assay data from ALS for the core samples.

Next steps

Phase II diamond drilling at Mostaza commenced on 11 March 2025 employing two rigs and is designed to include resource and exploration drilling. To date a further 10 holes have been drilled for the Phase II programme for a total of 1,059 metres. Within the Mostaza pit the Phase II drilling continues to intersect the high-grade Lens 2 mineralisation down to 100m below the DD001 intercept with mineralisation remaining open in all directions.

Metallurgical studies of drill core, stockpiles and tailings are also in planning stage. Extensions of mineralisation at depth and along trend under cover may also be targeted with geophysical surveys.

Table 2: Cerro Negro Project, Mostaza Mine; Mostaza drill core assay results for Cu, Ag and Sb, CNG25 DD006 - DD009.

Forward Looking and Cautionary Statements