Great Southern Copper plc (LON: GSCU) Step-Out Hole Confirms Expansion of Mostaza

Step-Out Hole 400m South Confirms Expansion of Mostaza Cu-Ag Discovery  

with Grades up to 6.55% Cu and 320g/t Ag

Mineralisation remains open to the South and at depth

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce the partial assay results of Hole CNG25-DD042 (DD042) recently received for its Phase III drilling programme at the Cerro Negro prospect, part of the Especularita Project.

Highlights:

·    Hole CNG25-DD042 tested the interpreted southern extension of Lens 2 some 300m along trend from the Mostaza mine

·   The hole intersected multiple zones or lenses of base and precious metals mineralisation including significant intercepts

o  21.8m @ 1.04% Cu and 52.26 g/t Ag From 73.0m, including

§ 6.0m @ 2.56% Cu and 128.16 g/t Ag From 79.0m, and

§ 1.6m @ 6.55% Cu and 319.75 g/t Ag From 82.0m

·    An upper Pb-Zn-Ag rich silica breccia zone includes grades up to 22.4g/t Ag, 1.9% Pb and 0.34% Zn similar to the mineralisation identified in the hanging-wall to Lens 2

·    DD-042 results extends the Mostaza system to over 400m of strike  length with mineralisation open to the south

·    Pb and Zn rich intervals add further evidence of a complex multiphase stacked system

·    Results pending for final diamond holes and all RC drill holes

·    Planning for fully funded Phase IV resource and exploration drilling is in progress

·    GSC holds option to 100% of the Cerro Negro project including the Mostaza mine

·    Project located at low elevation with excellent access to mining-related infrastructure

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: “These latest results for step-out hole DD042 continue to demonstrate the strong growth potential of the Mostaza copper-silver discovery. Intersecting mineralisation at this distance from the historic workings provides further evidence that the Mostaza Fault Zone hosts a much broader mineralised footprint than originally recognised and emphasises the significant exploration upside still to be tested along this emerging trend.

“Importantly, the presence of significant Pb and Zn mineralisation in DD042 within an upper silica-sulphide breccia zone underlain by high-grade chalcocite-clay mineralisation is consistent with the deposit architecture recognised below the Mostaza mine, reinforcing the exciting proposition that this represents a potential extension of the Mostaza Lens 2 trend towards the south.

“Exploration work to date has already traced mineralisation and alteration signatures for over two kilometres of the Mostaza Fault Zone, and this latest step-out hole is another important confirmation that the system remains open and continues to grow.

“Planning for our fully funded Phase IV drilling campaign is underway and will focus on in-fill drilling of Lens 2 between the Mostaza mine and the mineralisation intersected in holes DD07 and DD042, as well as additional step-out drilling aimed to extend the Lens 2 mineralisation trend further south along the Mostaza Fault Zone.  In addition, we look forward to continuing to test the broader potential of the Cerro Negro system.”

Hole CNG25-DD042 and the Lens 2 Extension:

Diamond drillhole DD042 was collared about 300m south and along trend of the Mostaza Lens 2 mineralisation (see Figure 1) and drilled towards the Southwest to test for extensions of a new mineralised trend discovered by holes CNG25-DD007, DD026 and DD027, that is interpreted to be the potential southern extension of Lens 2 – hence the “Lens 2 extension”.

Some of the previous holes intercepted high-grade mineralisation over significant widths, such as hole CNG25-DD007 which intersected 33m @ 1.96% Cu and 60.6 g/t Ag from 87m² (see Figure 1).

In contrast, hole DD026 intersected multiple narrow (<1m) intervals or lenses with grades up to 1.98% Cu and 94.1g/t Ag¹, suggesting the mineralisation might be pinching out or is overprinted by complex shearing and faulting.

Hole CNG25-DD042 was designed to test to the south of this structurally complex zone to demonstrate continuity (see Figure 1). The results are shown below. Near the top of the hole, it intercepted silica-sulphide assemblage mineralisation with pyrite-chalcopyrite-galena-sphalerite (see Figure 2) associated with silica-rich hydrothermal breccia on the margins of a coarse feldspar porphyry. At depth the hole intersected two intervals of crackle-veinlet and disseminated chalcocite-clay assemblage mineralisation (see Figure 2) and an aphanitic dyke as seen in Lens 2.

The results of hole DD042 suggest that the mineralisation may be coalescing and/or thickening to the south. This “pinch and swell” geometry, with alternating thick “lodes” separated by narrow “pinch-outs” is not uncommon for structurally-controlled ore deposits. Importantly, the mineralisation remains open and untested to the south under shallow cover. This area will also be targeted with step-out drilling in the Phase IV campaign.

The reported trend of significant Cu-Ag mineralisation intersected in drilling now measures over 400m in strike length, from the northern edge of the Mostaza mine to the intercept in hole DD042 (see Figure 1). This lies within the broader 2.5km trend of surface alteration and mineralisation (Mostaza Fault Zone) extending from Lens 1 to the south and defined by mapping, rock chip geochemistry and IP geophysics surveys³.

Figure 1: Location of step-out hole DD042 highlighting interpretation of the potential strike extent of the Lens 2 mineralisation trend. In-fill and extensional drilling plan for Phase IV also indicated.

Figure 2: Photographs of DD042 HQ drillcore illustrating the upper near-surface silica-sulphide breccia zone (a – c) with anomalous Pb-Zn-Ag mineralisation, and the lower chalcocite-clay zone (d – f) illustrating crackle vein and disseminated high-grade copper-silver mineralisation.

Note regarding Laboratory turn-around times

Chile is experiencing a rapid increase in exploration activity, coinciding with the summer exploration season in the high Andes (November to April) and further compounded by rising metal prices. Consequently, sample preparation times are significantly longer than in the past, with turnaround times of over two months now typical for assay results.

The Company has implemented strategies designed to improve the turn-around times and reduce costs including prioritising the sampling of clear mineral zones identified in the core and despatching these to the lab in advance of the remainder of the holes’ samples.

The Company does not intend necessarily to release assay results for individual or partial drillholes in a piece-meal fashion in the future, however, the results reported here for partially-sampled DD042 are considered highly significant due to their potential impact on the interpretation of the Mostaza deposit and the on-going planning of the Phase IV drilling programme.

  References:

1.    RNS 1749L (11 Dec 2025): Drilling extends mineralisation at Mostaza

2.    RNS 8896D (7 April 2025): Further Exceptional Assay Results from Mostaza

3.    RNS 9585U (12 Aug 2025): Final IP Geophysical Results Define Multiple New Drill targets at Cerro Negro

Enquiries: