Highlights:

  • Middle Island Resources (ASX:MDI) expanded polymetallic anomalism across approximately six kilometres at the Bobija Project.
  • Peak rock chip and grab sample results returned up to 12.0g/t silver and 2.85% antimony.
  • The company identified visible stibnite mineralisation, a key ore mineral of antimony.
  • Exploration is targeting a carbonate replacement deposit (CRD) style mineral system.
  • The Kozila anomaly has been extended northward and remains open in multiple directions.

Perth-based explorer Middle Island Resources Limited (ASX: MDI) has materially expanded what it describes as a "major polymetallic target" at the Tisovik area within its 100% controlled Bobija Project in central-western Serbia. In a 13 May 2026 announcement, the company released assay results from a follow-up reconnaissance soil sampling program and from 20 rock chip and grab samples collected across the Red Rock and Kozila target areas.

The headline numbers are striking. Peak soil results returned 7.1 grams per tonne silver, 4,685 parts per million (ppm) lead, 969ppm zinc and 1,049ppm antimony. Rock chip and grab samples from outcrop, float and historic dump material returned up to 12.0g/t silver, 5,244ppm lead, 580ppm zinc and 28,500ppm antimony, the latter equivalent to 2.85% antimony. Several Red Rock samples contained coarse, visible radiating crystals of stibnite (Sb2S3), the principal sulphide ore mineral of antimony.

Crucially, the new data does not just lift the grade profile of an already-known anomaly. It extends the footprint of mineralisation across multiple new target zones. Middle Island said multi-element soil anomalism has now been identified along an east-west strike length of approximately six kilometres, taking in three distinct targets: Tisovik in the west, Red Rock in the centre and Kozila to the east. The previously identified Kozila soil anomaly has been pushed a further 300 metres to the north and now covers an area of roughly two square kilometres, 1.6 kilometres long by up to 1.5 kilometres wide, defined above a 200ppm lead-in-soil threshold.

Importantly, the anomaly remains open in multiple directions, particularly to the north where prospective limestone host rocks are interpreted to continue. The company said an approximate eight square kilometre untested area within the Bobija exploration licence will be the focus of the next phase of soil sampling.

What is a carbonate replacement deposit?

The silver-lead-zinc-antimony combination identified at Tisovik is, according to Middle Island, highly indicative of a carbonate replacement deposit, often abbreviated as CRD. The company explained that CRD systems form where hydrothermal fluids react with and replace carbonate rocks such as limestone, frequently producing large, high-grade sulphide bodies hosting significant silver, lead and zinc. The style is globally important and underpins a number of well-known producing mines worldwide.

Reinforcing the local prospectivity argument, Middle Island highlighted that its Bobija Project sits within a recognised Mining district. The operating Veliki Majdan silver-lead-zinc mine lies approximately 20 kilometres to the north-west, and the broader Western Tethyan Mineral Province in which the licences are located is home to multiple world-class copper, gold and silver deposits referenced in the company's announcement.

Details of the soil program

The latest reconnaissance soil program comprised 82 samples collected on north-south oriented lines spaced 400 metres apart, with samples taken every 100 metres along each line. Middle Island said the program was deliberately designed as a rapid first pass over a large, under-explored area surrounding the historic Tisovik Mine, with the goal of identifying coherent geochemical trends for follow-up rather than fully delineating individual anomalies.

Twenty rock chip and grab samples taken during the same field campaign added another layer of evidence. At Red Rock, dump and float samples returned 1.71g/t silver with 5,614ppm antimony and a separate dump sample returned 0.48g/t silver with 28,500ppm antimony, both accompanied by visible stibnite. At Kozila, an outcrop sample, K9572, returned 12.0g/t silver, 5,244ppm lead and 129ppm zinc from silicified, leached limestone located approximately 80 metres north-east of a previously undocumented exploration adit identified during field mapping.

The combination of widespread soil anomalism, outcropping mineralisation and historical workings is, in the company's view, strong evidence of a substantial mineralised system that has seen limited modern exploration.

Management commentary

MDI Chief Executive Officer Peter Spiers said: "These latest exploration results continue to demonstrate the emerging scale of the Bobija Project and reinforce our belief that Tisovik represents a significant polymetallic CRD-style mineral system. Defining multiple large zones of strong silver-lead-zinc-antimony anomalism over more than 6km of strike is highly encouraging at this early stage of exploration, particularly given the system remains open in multiple directions."

Mr Spiers also drew attention to antimony specifically. "The presence of stibnite mineralisation and high-grade antimony is especially exciting given the increasing strategic importance of antimony globally," he said. Antimony has been classified as a critical mineral in a number of jurisdictions in recent years, reflecting its use in flame retardants, certain alloys and military applications, and concerns about concentration of global Supply.

Next steps and other catalysts

Middle Island said the next phase of exploration will extend soil sampling northward into the roughly eight square kilometre untested area within the Bobija exploration licence, with the goal of testing the continuity and potential scale of the anomalous trends identified to date. Follow-up programs are expected to include infill soil sampling, gravity surveys aimed at detecting dense sulphide bodies, trenching and drill testing.

Soil work is being progressed in parallel with the company's drilling pipeline. Assay results are pending from a recently completed Phase 2 Reverse Circulation Drilling program at the Bobija mine area, as well as Phase 1 barium assay results, plus soil sampling results from the Priboj Project (Zabrnjica gold target and Jelaca copper target) and the Timok Project (Brodica gold targets).

About the Bobija Project and Middle Island

The Bobija Project is located in central-western Serbia, approximately 100 kilometres south-west of Belgrade. It comprises six mineral licences over a combined area of around 208 square kilometres, including three granted exploration licences (Bobija, Bobija East and Kamenita Kosa), an application for a fourth licence (Orovica), and a 10-year option to acquire the granted Bobija and Tisovik mining licences from a local company, Bobija doo Ljubovija.

Middle Island's wider Serbian portfolio comprises 14 licences either 100% owned or held under agreements with a path to 100% ownership and covering approximately 620 square kilometres across the Bobija, Timok and Priboj project areas. The company completed its maiden drilling program at Bobija in late 2025, generating results that included an intercept of 52 metres at 1.17g/t gold, 26.0g/t silver, 0.12% copper, 0.39% lead and 1.01% zinc from nine metres in hole BMLRC001, helping motivate the broader 2026 work program.

Why this matters to investors

For Middle Island shareholders, the Tisovik update is significant because it broadens the company's discovery thesis at Bobija from a single mineralised mine area to a district-scale opportunity stretching across six kilometres of strike. The Kozila and Red Rock targets sit within untested ground around historic workings, which combined with the CRD interpretation gives the company a clear, structured exploration target type to test with drilling.

Caveats apply. The results released on 13 May 2026 are surface geochemical and rock chip data, not drill assays, and exploration targets described in the announcement are conceptual in nature. The company itself notes there is insufficient information to establish whether further exploration will result in the determination of mineral resources. Investors should regard the announcement as an indication of target scale and quality, not as a resource statement.