REG - Great Southrn.Copper - Final Drill Results for Mostaza Discovery

Great Southern CopperAnnouncement

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RNS Number : 1870C  Great Southern Copper PLC  28 April 2026

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.

 

 

28 April 2026

 

Great Southern Copper plc

("GSC" or the "Company")

 

Final Drill Results for Mostaza Copper-Silver Discovery Grade up to 4.23% Cu
and 233g/t Ag

 

Results confirm extension of Mostaza Cu-Ag deposit along strike and to depth

 

Great Southern Copper plc (LSE: GSCU), the company focused on
copper-gold-silver exploration in Chile, is pleased to report on the final
drill results for Phase III of its drilling programme at the Mostaza Cu-Ag
discovery, Cerro Negro prospect, Especularita Project, Chile.

 

Highlights:

·    Final Phase III assay results confirm down-dip and along strike
continuity of high-grade Cu-Ag mineralisation at Mostaza with significant
intervals including:

 

o  CNG25-DD045:17.0m @ 0.69% Cu, 48.2 g/t Ag from 184m, including;

·    3.4m @ 1.44% Cu, 103.8 g/t Ag from 189.55m, and

·    2.0m @ 1.1% Cu, 52.5 g/t Ag from 198m

 

o  CNG25-DD046:  2.0m @ 2.0% Cu, 123.7 g/t Ag from 187m, and

·    0.5m @ 4.24% Cu, 233 g/t Ag from 200.3m

 

·    Continuity of upper Pb-Zn+Ag mineralised breccia zone also confirmed

·    Phase III results exceed expectations including extending the Mostaza
deposit limits, discovering hanging-wall base-metal zone, recognising stacked
lens morphology and confirming IP chargeability as an exploration tool with
follow-up scout RC drilling

·    Interpretation of deposit architecture and Phase IV drill planning
continuing

·    Results for metallurgical samples awaited

·    GSC holds rights to own 100% of the Cerro Negro prospect

·    Prospect located at low elevation with excellent access to
infrastructure and mining services and along trend of major copper deposits
including Los Pelambres, Altar and El Pachon.

 

Sam Garrett, Chief Executive Officer of Great Southern Copper, said: "These
final Phase III results provide further confidence in the continuity of the
Mostaza copper-silver deposit, with drilling now consistently intersecting
mineralisation at increasing depths. The predictability of the deposit is
improving, providing a clear focus for the next phase of drilling, with the
system remaining open along strike and at depth.

 

"Importantly, the latest results indicate that the system appears to be
broadening at depth, with higher-grade copper-silver lenses hosted within a
wider mineralised envelope. This is consistent with our evolving geological
model and supports the potential for a larger-scale system.

 

"The Phase III drilling programme has exceeded expectations delivering highly
significant results including extending the known limits of high-grade
copper-silver mineralisation at Mostaza, identifying an anomalous
lead-zinc-silver zone in the hangingwall to the Mostaza deposit, recognising
the potential for stacked lens morphology and confirming base-metal
mineralisation associated with IP chargeability anomalies along the 2km long
Mostaza Fault Zone.

 

"Interpretation of the final results is on-going and will support planning for
the Phase IV drilling programme which will focus on the expansion of the
Mostaza resource, alongside the ongoing exploration of the broader Cerro Negro
system, that now measures over 2km along strike and 1 km in width."

 

 

Cerro Negro Prospect:

The Especularita Project, including the Cerro Negro prospect and high-grade
Mostaza Cu-Ag discovery, is located at low altitude within the Cretaceous-aged
coastal metallogenic belt of northern Chile that includes large scale copper
mines and projects including Carmen de Andacollo and El Espino (Figure 1).
Significantly, the project is also located within a northwest-southeast
trending structural lineament that links the project with notable large-scale
porphyry Cu-Au mines and projects, including Los Pelambres, Altar-Piuquenes
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 50km radius of over
50Mt of contained copper metal defined in mines and advanced projects.

 

Phase III Drilling Programme, Cerro Negro:

Phase III drilling was completed in January 2026, comprising 17 diamond
drillholes for 2,923 metres

and six scout RC holes for 1,032 metres(1). The Phase III programme was
designed to continue advancing the high-grade copper-silver discovery at
Mostaza by:

 

·    Infill and step-out drilling at the Mostaza deposit (Lens 2 and
adjacent lenses) to refine the

geometry and continuity of the high-grade Cu-Ag mineralisation.

 

·    Testing extensions of known mineralisation at nearby targets,
including Lenses 3, 4 and 5,

where previous drilling intersected strongly silicified but copper-leached
lens rocks indicative of underlying sulphide mineralisation.

 

·    First-pass scout RC drill testing of geochemical and geophysical
anomalies along the Mostaza Fault Zone to the south of the historic Mostaza
mine (Figure 2).

 

Figure 2: Drill plan summary for Cerro Negro prospect highlighting holes
reported in this release.

 

 

Final Drill Assay Results:

 

CNG25-DD037: Diamond drillhole DD037 was drilled as a 60 metre down-dip
step-out from DD031 which intersected 7.2m @ 2.58% Cu and 188.7 g/t Ag from
90.40m(2), and was designed to test the continuity of mineralisation at depth
(Figure 3).

 

The hole first intersected the down-dip extension of the shallow
silica-sulphide lens, characterised by highly anomalous base metals and
silver, grading 19m @ 0.17% Zn, 0.10% Pb and 3.9 g/t Ag. This interval is
consistent with previously reported polymetallic sulphide breccia horizons
developed in the hanging-wall to the main Cu-Ag mineralisation (Figure 3).

 

At depth, DD037 intersected the down-dip extension of the principal
chalcocite-clay Cu-Ag lens, returning 3.77m @ 0.83% Cu and 57.23 g/t Ag from
156.23m including: 1.3m @ 1.15% Cu and 82.98 g/t Ag from 156.23m.

 

A second, deeper Cu-Ag zone of 0.68m @ 0.82% Cu and 49.2 g/t Ag from 190m was
intersected, supporting the interpretation of a stacked lens type morphology
to the deposit. The lower footwall zone of the mineralised lens in this hole
appears to be faulted out by the interpreted Raven Fault resulting in a
narrower intercept width in this location (Figure 3) and further emphasising
the structural complexity of the deposit.

 

 

 

Figure 3: Cross-section for DD037 through the Mostaza deposit (looking
Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles
and grade down hole in DD037.

 

 

CNG25-DD045: Diamond drillhole DD045 was drilled as a 50m along-strike
step-out to the north of DD036, which returned a broad interval of 39.6m @
0.57% Cu and 34.7 g/t Ag from 170.6m (Figure 4). The hole also represents a
75m down-plunge step-out from DD022, which intersected anomalous Cu-Ag within
poorly developed lens alteration but no high-grade intervals.

 

DD045 intersected a similar sequence to that observed in DD036, comprising an
altered and mineralised dacite dyke with alteration and mineralisation
developed into both the hanging-wall and footwall of the dyke, and returning
significant intervals of:

 

·    17m @ 0.69% Cu and 48.21 g/t Ag from 184m, including;

o  3.4m @ 1.44% Cu and 103.8 g/t Ag from 189.55m, and

o  2.0m @ 1.10% Cu and 52.5 g/t Ag from 198m

 

The development of higher-grade zones within a broader mineralised interval at
the footwall contact is consistent with the emerging model of dacite-hosted
mineralisation, where higher-grade Cu-Ag lenses are preferentially developed
along dyke margins.

 

DD045 extends the mineralised system along strike to the northwest as well as
down-dip and highlights strong potential for further extensions of the Cu-Ag
mineralisation in these directions.

 

Figure 4: Cross-section for DD045 through the Mostaza deposit (looking
Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles
and grade down hole in DD037.

 

CNG25-DD046: Diamond drillhole DD046 was drilled as a twin to DD023, which did
not reach sufficient depth to test the interpreted mineralised lens during
Phase II drilling (Figure 5). The hole also represents an approximately 100m
down-plunge step-out from DD021 and was designed to test the continuity of
mineralisation at depth towards the southern extent of the Mostaza system.

 

DD046 intersected the down-dip extension of the shallow silica-sulphide base
metal lens, thereby confirming the exploration model and returning 24m @ 0.50%
Zn, 0.25% Pb and 3.8 g/t Ag from 93.45m. This interval is consistent with the
polymetallic (Pb-Zn-Ag) sulphide breccia lenses observed in previous drilling,
interpreted to occur in the hanging-wall to the main Cu-Ag mineralised lenses.

 

At depth, the hole intersected the down-dip extension of the principal
chalcocite-clay Cu-Ag mineralised zone, comprising 2.0m @ 2.0% Cu and 123.7
g/t Ag from 187m and an additional deeper interval of high-grade
mineralisation grading 0.5m @ 4.24% Cu and 233 g/t Ag from 200.3m (Figure 5).

 

The presence of multiple mineralised intervals, including high-grade
copper-silver mineralisation at depth, further supports the interpretation of
a vertically extensive, stacked lens system. The narrow, high-grade intercept
at the base of the hole suggests the potential for additional high-grade
lenses at depth.

 

DD046 materially extends the known footprint of the system down-dip and along
trend to the southwest, highlighting potential for further extensions with
ongoing drilling.

 

 

Figure 5: Cross-section for DD046 through the Mostaza deposit (looking
Northeast), Cerro Negro prospect. Core photos illustrate mineralisation styles
and grade down hole in DD037.

 

 

 

Next Steps:

 

All drilling assay results for the Cerro Negro Phase III drilling programme
have now been received. Detailed review and assessment of the geology and
geochemistry data is continuing, in conjunction with further surface mapping
and sampling, to assist with updating the Company's understanding and
interpretation of the deposit architecture for Mostaza.

 

Planning for a Phase IV drilling programme is in progress, focused on both
infill and step-out drilling at Mostaza to further define the geometry,
continuity and scale of the deposit, as well as definition of exploration
targets within the greater Cerro Negro mineral system. Ongoing detailed
interpretation is integrating geochemical, geophysical, geological and
structural datasets to refine the geological model and optimise targeting
ahead of the next phase of drilling.

 

Metallurgical test work is progressing with results expected to support the
assessment of processing characteristics of the Mostaza Cu-Ag deposit ore.

 

 

References:

1.    RNS 4907O (13th Jan 2026): Phase III Drilling Completed at Cerro
Negro

2.    RNS 1749L (11(th) Dec 2025): Drilling Extends Mineralisation at Cerro
Negro

 

 

 

Engage with the GSC management team directly by asking questions, watching
video summaries and seeing what other shareholders have to say. Navigate to
our Interactive Investor hub here: https://gscplc.com/ (https://gscplc.com/)

 

 

Enquiries:

 

 Great Southern Copper plc             https://gscplc.com/link/epzDBP (https://gscplc.com/link/epzDBP)
 Sam Garrett, Chief Executive Officer  +44 (0) 20 7138 3204

 SI Capital Limited
 Nick Emerson                          +44 (0) 1483 413500

 BlytheRay
 Tim Blythe / Megan Ray                +44 (0) 20 7138 3204

                                       GSC@blytheray.com

 

Subscribe to our news alert service: https://gscplc.com/auth/signup
(https://gscplc.com/auth/signup)

 

 

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 (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
(http://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.

 

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