High Grade Ag-Cu Samples Expands Monolith Target

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.

14 October 2025

Great Southern Copper plc

("GSC" or the "Company")

High Grade Ag-Cu in Rock Samples Expands Monolith Target at Cerro Negro, Especularita

Second drill rig arrives at Mostaza to expand Phase III drilling programme

Great Southern Copper plc (LSE: GSCU), the company focused on copper-gold-silver exploration in Chile, is pleased to announce that the results of its channel rock chip sampling programme at the Monolith Cu-Ag prospect ("Monolith") significantly enhances the mineralised footprint at Cerro Negro, and that a second diamond drill rig has arrived on site at the Mostasa Mine ("Mostaza").

Highlights:

· 48 rock channel samples collected at Monolith with assay grades up to 271g/t Ag, 2.05% Cu

· Samples average 35.4g/t Ag and 0.31% Cu despite evidence of surface leaching

· Results define a zone of mineralisation greater than 50 metres ("m") wide

· Mineralisation open to the south and southeast where exploration is continuing

· Second diamond drill rig arrives on site to advance Mostaza Phase III drill programme

· Company holds the option to earn 100% of the Cerro Negro prospect

· The Especularita Project is located at low elevation with excellent access to mining infrastructure and services

Prospect-scale mapping and sampling have continued across the Cerro Negro prospect with the aim of determining the potential for the Mostaza style high-grade, structurally-controlled Cu-Ag mineralisation to extend along strike to the south and east and to investigate potential for, and vector towards, porphyry Cu style mineralisation.

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "With copper and silver prices at record highs, these high-grade results are both exciting and highly significant since they further demonstrate and expand the scale potential of the Cerro Negro Cu-Ag prospect. With Phase III drilling underway at Mostaza, the team is also working in parallel to advance exploration across additional targets within the Cerro Negro prospect area to expand the mineralised footprint.

"The Company has previously reported results of high-grade Cu-Ag in rock chip samples along the Mostaza Fault up to 2.5 kilometres to the south of the Mostaza mine. These results now extend that work to the east of Mostaza, reinforcing our confidence in the broader district-scale potential of the system.

"A second diamond drill rig has arrived on site to further advance the Phase III drill programme at Mostaza. Rig 2 will initially target extensions and in-fill to the Lens 2 mineralisation. The Company looks forward to updating the market as drilling progresses and results are received"

Figure 1: Location map of GSC's Especularita Project and Mostaza mine in Chile.

Monolith Cu-Ag prospect:

This prospect-scale mapping and sampling work follows on from reconnaissance sampling at Monolith reported in 20241 which first identified the Monolith prospect. The Monolith prospect, where artisanal miners recovered high-grade Cu-Ag from baryte-rich veins extending over 200m, is located approximately 400m to the southeast of the Mostaza mine (Figure 2, 3). The copper and silver-rich baryte veins form an anastomosing network within a broader alteration zone of silica-clay altered dacitic wall-rock up to 50m wide. High-grade Cu-Ag mineralisation is identified as green and black copper oxides on fracture surfaces, in quartz veinlets, stockworks and vein breccias and as disseminations within the altered dacite (Figure 4). In places, the silicified host-rocks are variably eroded, producing spires or "monoliths" of strongly Cu-Ag mineralised rock. Significant limonitic-goerthitic boxworks after sulphide suggest evidence of leaching of sulphides at surface.

The mineralised dacitic rocks are very similar in appearance to those observed at the bottom of the Mostaza pit and in the Lens 5 area.

Mapping and sampling is continuing at Monolith and across the Cerro Negro prospect to assist with defining further targets for exploration drilling.

Sampling and results:

Samples were collected as 1-2m channel chip samples with the exception of one sample (991641) which was taken from mine mullock. All rock samples collected are located using hand-held GPS units (WGS84 zone 19S datum, UTM coordinate system) and allocated a unique sample number using a ticket-book system.

Samples are transported from the field and delivered by Company employees to ALS Laboratories in Santiago, where they are prepared for analysis (weighed, dried, crushed and pulverised). A subsequent 200g split of each sample pulp is then forwarded to the ALS geochemical laboratory in Lima, Peru, where samples are assayed for gold (30g fire assay with ICP-AES finish, ALS code AU-ICP21) and analysed for 48 major, base and trace elements by ICP-MS following a four-acid digest (ALS code ME-MS61).

Copper assay grades for the channel chip samples range up to 2.09% Cu with 55% of samples being anomalous in copper (>0.1% Cu) - although the oxidised and leached nature of the dacitic host rock suggests that a lower threshold may still be anomalous. Silver assay grades range up to 271g/t Ag with 85% of samples anomalous in silver (>1.0g/t Ag) and 75% of samples assaying >10g/t Ag. Strongly elevated silver grades with low copper is further evidence of potential leaching of copper in the oxide profile at Monolith.

Phase III drilling update:

Four diamond drill holes (CNG25-DD026-29) have been completed to date testing the Lenses 3-5 area. A second diamond rig has arrived on site to advance the programme and will initially test in-fill and extensions to the Lens 2 mineralisation.

Figure 2: Summary map of rock chip assay results at Cerro Negro for silver (L) and copper (R).

Figure 3: New and previously reported assay results for silver (L) and copper (R) at the Monolith target, Cerro Negro prospect, Especularita.

Figure 4: Examples of visible copper mineralisation outcropping at the Monolith target, Cerro Negro.

References:

1. RNS 3120K (31 Oct 2024): Cerro Negro Results up to 4.64% Cu and 177g/t Ag.

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.