Mineralisation Extended at Monolith Target

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration at its Especularita Project in Chile, is pleased to report results from recent channel and rock chip sampling programmes at the Monolith Target ("Monolith"), Cerro Negro Prospect.

Highlights:

• All 15 exploration channel samples host Cu-Ag mineralisation
• Channel sampling results include;

o CH025: 6 m @ 0.29% Cu and 34.8 g/t Ag

o CH030: 4m @ 0.53% Cu and 47.5 g/t Ag

o CH033: 4m @ 0.38% Cu and 53.2 g/t Ag

• Rock chip assay grades range up to 1.5% Cu and 95.8 g/t Ag
• Copper and silver grades depleted due to surface weathering and leaching
• Vein, crackle-breccia and stockwork style Cu-Ag mineralisation very similar to Mostaza
• Mineralisation defined over a 700m by 400m area and open to the south and west
• GSC holds rights to own 100% of the Cerro Negro prospect, including the Mostaza mine

Sam Garrett, Chief Executive Officer of Great Southern Copper, said:

"These exciting results from the Monolith target provide further compelling evidence that the high-grade copper-silver mineralisation identified in drilling at Mostaza now extends across a much broader system at Cerro Negro.

"The combination of strong channel sample grades, high-grade rock chips, and widespread alteration demonstrates the scale potential of the system. Importantly, the surface mineralisation is strongly weathered and leached, reinforcing our belief that higher-grade sulphide mineralisation may be preserved at depth.

"The copper-silver mineralised system identified at Monolith remains open to the south and west, highlighting the significant growth potential of the target. While work in these areas is continuing, we are now ready to commence scout drilling at Monolith as part of the Phase IV drilling programme. This work will be untaken alongside continued efforts to demonstrate the broader district-scale potential at Cerro Negro."

Monolith Cu-Ag Target:

The Monolith high-grade Cu-Ag target is located. approximately 400m to the southeast of the Mostaza mine at Cerro Negro, which itself comprises part of GSC's Especularita project in northern Chile (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 mine, 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 a 50km radius of over 50Mt of contained copper metal defined in mines and advanced projects.

Results of Surface Mapping and Sampling:

Geology mapping and channel sampling are being undertaken at the Monolith target to follow up previously reported high-grade rock chip results, which returned assay grades of up to 271 g/t Ag and 2.05% Cu¹.

Assay results for 13 rock chip grab samples range in grade up to 1.5% Cu and 95.8g/t Ag (Figure 2). The samples are notably also anomalous in As-Ba-Sb-Mo-Pb-Zn, consistent with the high-level epithermal to intermediate sulphidation environment interpreted for this type of mineralisation.

A total of 15 rock channel samples (comprising a total of 57 rock chip samples) were collected across key structures and alteration zones, all of which returned anomalous copper and silver values, confirming the continuity of mineralisation across the target area. Significant results include (Figure 2):

• CH025: 6m @ 0.29% Cu and 34.8 g/t Ag
• CH030: 4m @ 0.53% Cu and 47.5 g/t Ag
• CH033: 4m @ 0.38% Cu and 53.2 g/t Ag
• CH032: 6m @ 26.5 g/t Ag
• CH023: 6m @ 21.8 g/t Ag

These results demonstrate consistent Cu-Ag mineralisation over metre-scale widths at surface. The predominance of oxide copper mineralisation and elevated silver values is interpreted to reflect near-surface leaching, suggesting potential for higher-grade sulphide mineralisation at depth, analogous to the mineralisation intersected in drilling at Mostaza.

This work has significantly expanded the known extent of mineralisation, defining a coherent alteration and mineralised footprint measuring approximately 700m by 400m, in area and open to the south and west, where further exploration work is continuing. Copper-silver mineralisation is hosted in dacitic to rhyo-dacitic flow-banded lavas (interpreted as a resurgent dome setting) and is characterised by copper oxide development associated with crackle networks of quartz veinlets, as well as larger quartz-baryte veins and breccia systems (Figure 4). Significantly, mineralisation observed at the Monolith is consistent with the style of mineralisation associated with similar dacitic rocks at Mostaza, located approximately 400m to the northwest.

The quartz veining is closely associated with pervasive silica-clay alteration, forming a broad alteration envelope that hosts the mineralised structures (Figure 3). These zones form an anastomosing network of veins, breccias and stockworks, developed along the main ridge and extending down the western flank of the hill. The geometry and distribution of these structures indicate strong structural control, likely related to the broader Mostaza Fault Zone.

Figure 2: Summary map of the Cerro Negro prospect with rock chip sample locations (Ag g/t) (l) with alteration map of the Monolith target highlighting significant channel rock chip results (r).

Figure 4: High-grade Cu-Ag mineralisation at the Monolith occurs in silica-flooded breccia zones (upper left and lower left), shear-fault zones with baryte veins (upper right) and crackle-vein breccia zones (lower right) within silica-clay alteration haloes to the larger baryte-quartz-copper-silver veins.

Next Steps:

Mapping and sampling continues to the south and west of the current work, where the veins trend toward a significant zone of mapped breccia (Valle Escondida Breccia), which is also yet to be sampled.

An extension to the earlier IP geophysics survey is being planned to cover the Monolith zone, and planning and permitting are in progress to include scout RC drilling within the Company's Phase IV drilling programme for Cerro Negro.

References:

• RNS 1932D (14th October 2025): High Grade Ag-Cu Samples Expands Monolith Target

Forward Looking and Cautionary Statements