Drilling Extends Copper Mineralisation at Mostaza

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-lithium exploration in Chile, is pleased to provide an update on its Phase 1 diamond drilling programme at the Cerro Negro project in northern Chile, where drilling at the historical Mostaza mine commenced on 5 January 20251.

Highlights:

• Eight diamond holes completed (867m) to date in Phase 1 drilling at Mostaza, Cerro Negro
• All holes intersected visual evidence of high-grade copper mineralisation comprising abundant disseminations, veins, crackle-vein networks and breccia-hosted chalcocite-bornite-chalcopyrite in zones up to 32.3m downhole width* (Figures 1 and 2)
• The mineralised intercept for CNG25 DD001 closely correlates with the width of the mineralised intercept in historical drill hole EDH-25 (33m of 0.53% Cu, including 6m of 1.05% Cu and 81g/t Ag, and 9m of 0.92% Cu and 73g/t Ag2), confirming that Lens 2 occurs as a west-dipping tabular body that is open at depth
• Step-out drill holes CNG25 DD002-005 confirm that Lens 2 mineralisation extends further to depth and along strike of historical drill intercepts at the Mostaza mine
• Lens 2 mineralisation potentially thickening with depth
• Continuity of mineralisation along trend up to 270m south of the mine is confirmed with significant copper intersections identified in exploration holes CNG25 DD006, 007, and 008 targeting outcropping lenses 3 and 4
• Hole CNG DD007 intersected 32m* of high-grade copper mineralisation from 87m
• Samples for holes DD001 - 005 have been dispatched to ALS laboratories for assaying
• Drilling is now underway on hole CNG25 DD009.

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "Our Phase I diamond drilling programme at Mostaza has been very successful to date with three of the four objectives achieved, namely, the width of mineralisation reported in historical drill holes is confirmed, mineralisation extending at depth and along strike of historical holes below the Mostaza mine is confirmed, and copper mineralisation extending along trend to the south of the mine is also confirmed.

All that remains is to receive the assay results from the laboratory which we are hopeful will confirm historical high-grade copper-silver grades and establish the gold grade of the system. Mineralisation in core includes evidence of intense chalcocite-bornite-chalcopyrite mineralisation of high-sulphidation type suggestive of what could be the upper levels of a porphyry system.

The progress to date is highly encouraging and is delivering exciting results which will positively impact our on-going exploration focus and plans at Mostaza and Cerro Negro."

Diamond drill campaign at Cerro Negro

As previously reported, CNG25 DD001, was drilled to 103.5m depth and intersected 20m* of intensely altered and mineralised rock between 27m and 47m depth down-the-hole (Figure 1) confirming both the continuity and thickness of Cu-Ag mineralisation in Lens 2 below the historical open pit and reported in historical hole EDH-253.

Subsequent drill-holes CNG25 DD002 to 005 were designed as step-out holes from hole DD001 to test the continuity of Lens 2 mineralisation along strike and to depth of the historical drilling (the "Mine Zone"). All holes successfully intersected Lens 2 (Figure 3) with down-hole widths* of visible mineralisation reported as follows;

CNG25 DD001: 20m* of visible copper mineralisation from 27.00 to 47.00m

CNG25 DD002: 4.3m* of visible copper mineralisation from 38.90 to 43.20m (off-set by dyke)

CNG25 DD003: 18.8m* of visible copper mineralisation from 36.60 to 55.40m (thickening with depth)

CNG25 DD004: 3m* of visible copper mineralisation from 28.70 to 31.70m (off-set by dykes/faults)

CNG25 DD005: 23.5m* of visible copper mineralisation from 32.90 to 56.40m (thickening with depth)

Drill holes CNG25 DD006 to 008 targeted mineralisation below Lenses 3 and 4 (the "South Zone", Figure 4) located 270m along trend to the south of the Mostaza mine. These lenses were not effectively drill tested previously, with historical holes terminated short of the target. All three exploration holes intersected visible copper mineralisation (Figure 2) confirming the continuity of these lenses at depth. Importantly, DD007 intersected a 32m* down-hole interval of variably intense chalcocite-bornite-chalcopyrite mineralisation (Figure 5). A summary of the visual mineralised intervals includes;

CNG25 DD006: 8.6m* of visible copper mineralisation from 124.40 to 133.06m

CNG25 DD007: 32.3m* of visible copper mineralisation from 87.00 to 119.30m

CNG25 DD008: 21m* of visible copper mineralisation from 4.5 to 25.5m

In all cases mineralisation is of high-sulphidation style and mineralogy and occurs as abundant disseminations, veinlets, crackle networks and breccia matrix fillings of hypogene chalcocite, bornite, chalcopyrite and lesser pyrite, associated with minor quartz-baryte gangue and intense quartz-alunite-dickite/pyrophyllite alteration that overprints earlier pervasive illite-montmorillonite alteration (see Figures 1 and 2).

Zonation in the copper mineralogy is evident with mineralisation in the northern holes (DD001-005, Figure 1) being dominated by chalcocite-bornite and the southern holes (DD006-008, Figure 2) being higher in chalcopyrite and bornite. This mineralisation is the same as seen in the bottom of the open-pit and historical stockpiles where rock sampling by GSC returned assay grades up to 4.64% Cu and 177ppm Ag4.

The mineralisation and its associated alteration occur in structurally controlled "lenses" or "lodes" of felsic porphyry, "tuffisite" and polymictic rock-flour matrix breccia. In places the lenses may be bounded by faults and or cut by narrow inter-mineral to late-mineral andesitic dykes with intensely clay-altered margins.

These are preliminary observations, and detailed core logging and geological interpretation is ongoing.

Figure 1: Cerro Negro Project, Mostaza Mine: Photographs of drill core from CNG25 DD001. Left: Crackle and veinlet hosted chalcocite-bornite (cc-bo; black mineral) cutting illite and locally quartz-alunite-dickite/pyrophyllite altered polymictic breccia (30.4m depth). Right: 5cm thick vein of steely-black chalcocite (cc) mineralisation (33.2m depth). Bottom: Intense crackle and disseminations of black chalcocite (cc) in polymictic breccia (36.7 to 37.2m depth). All drill core is HQ diameter 6.35cm thick. Note: core photos illustrate examples of the style and textures of copper mineralisation observed within the mineralised intervals and are not necessarily representative the entire mineralised zone.

Figure 2: Cerro Negro Project, Mostaza Mine: Photographs of drill core from CNG25 DD007, over 200m south of mineralisation shown in Figure 4. Upper Left: chalcopyrite (cpy) dominated breccia matrix within intensely silicified rock (113m depth). Upper Right: intense disseminated, crackle and breccia matrix mineralisation of purple bornite (bo) and yellow chalcopyrite (116.4 to 118.4m depth). Lower Left and right: Intense dissemination and breccia infill by bornite-chalcocite, bo-cc, (90.4m depth) and chalcopyrite, cpy, (103m depth). All drill core is HQ diameter 6.35cm. Note: core photos illustrate examples of the style and textures of copper mineralisation observed within the mineralised intervals and are not necessarily representative the entire mineralised zone.

The wall rocks on either side of the lens are dominated by red polymictic rock-flour matrix breccias with textural characteristics suggestive of a phreatic or phreatomagmatic origin, including fluidisation and injection textures, lack of sorting, and possible juvenile fragments. On the west side the red polymictic breccia is in faulted contact with regionally extensive black monomictic volcanic breccias. To the east the red breccia forms part of a large dome-diatreme complex typical of the upper parts of porphyry-epithermal systems.

Drilling at Cerro Negro continues with drillhole CNG25 DD009 planned to test the "Middle Zone" between holes DD001-005 (Mine Zone) and DD006-008 (South Zone). See Figure 4.

The core is being processed and evaluated for sampling. The Company looks forward to providing further updates as results arise.

Figure 3: Cerro Negro Project, Mostaza Mine: Long section of Lens 2 with circles showing the location of where phase 1 drillholes are projected to intersect the hanging wall of the mineralised lens. The exact intersection points are subject to confirmation. Completed drill holes are shown with their final end-of-hole (EOH) depths. The apparent thickness of visible copper mineralisation is shown in bold below each intersection point. Note that mineralisation remains open at depth and along strike. Historical resource blocks, underground mine workings, and channel sample and drillhole assay results (coloured) are shown for reference. (GSCU has not been able to independently verify the historical channel sample or drill hole assay results).

Figure 4: Cerro Negro Project, Mostaza Mine: Plan map showing the approximate locations of mineralised lenses at surface, historical drill holes, and current GSC drilling. Note the scale.

Figure 5. Cerro Negro Project, Mostaza Mine: Preliminary cross section across Lenses 3, 4 and 5. Note that mineralisation remains open at depth and along strike. Historical drillhole assay results (coloured) are shown for reference.

• RNS 5778T (16 January 2025): First drill hole at Cerro Negro
• RNS 3120K (31 October 2024): Cerro Negro Results Up To 4.64% Cu and 177ppm Ag

Forward Looking and Cautionary Statements