Step-Out Hole Confirms Expansion of 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.

10 March 2026

Great Southern Copper plc

("GSC" or the "Company")

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 deposit geometry observed in DD042 comprising an upper Pb-Zn rich silica-sulphide breccia and lower Cu-Ag rich chalcocite-clay lens is consistent with that observed and recently reported for holes in Lens 21 and suggests the mineralisation intersected here could potentially represent the southern extension of the Lens 2 mineralisation discovered beneath the Mostaza mine. Consequently, in-fill drilling designed to potentially connect the Lens 2 mineralisation between these two centres will be a focus of the Phase IV programme.

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:

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-silver deposits in Chile. The Company has the option to acquire mining rights to 100% of Especularita project in the under-explored coastal belt of Chile that is prospective for large scale copper-gold-silver deposits. Chile is a globally significant mining jurisdiction being the world's largest producer and exporter of copper.

The Especularita Project is 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 the exploration project area. 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.