The resource is part of the Late Permian evaporite succession on the western edge of the Zechstein Basin. The full Zechstein sequence was deposited over 5–7 million years and represents multiple influxes and subsequent evaporation of seawater in a topographic low with restricted connection to the Zechstein sea.

The polyhalite deposit itself sits within the EZ2 Fordon Evaporite sequence, a significant basin-infilling cycle. This sequence varies between 30 and 200 m thick across the project area, thickening to the east, towards the centre of the basin. The polyhalite itself appears to be formed by syn-sedimentary metasomatism or back-reaction of pre-existing sulphates; gypsum/anhydrite, with potassium and magnesium-enriched marine brines.

The deposit is a stratiform sedimentary unit displaying significant lateral continuity. It is composed of two high grade seams; the Shelf Seam and the Basin Seam. The Shelf Seam exists on the basin margin and thickens in the central part of the project area, this forms the Indicated Resource. It is bounded by intergrown haliteanhydrite-polyhalite below and anhydrite above. The Basin Seam is deeper and bound by halite. Both seams pinch out to the west, the Basin seam against the Basin Ramp, and the Shelf seam further west towards the edge of the palaeo-coastline.

On a smaller scale, the deposit is modelled as a series of laterally continuous intervals within the Shelf Seam, potentially representing discrete variations within the seawater chemistry of the Zechstein Sea at the time. Three zones of high grade polyhalite are distinguished, separated by anhydrite-dominated bands. The mineralogy and texture within evaporites can be complex, with primary depositional and secondary overprinting effects common throughout. Polyhalite is found alongside numerous other evaporite sulphate, chloride and borate minerals ranging from gangue-level to trace-level.

Mining at the Woodsmith Mine will adopt simple and conventional bulk mining methods to enable maximum extraction and low-cost operations. From the base of the shafts we can mine in all directions, maximising our ability to optimise ramp up from initial production. Using a combination of room and pillar and drill and blast mining methods we can excavate the thicker areas of the seam up to a height of 40 metres.

The company is constructing a mine to realise the potential of our world-class deposit. The exceptional nature of our resource means that all mine development will be excavated entirely within the polyhalite seam. This will result in a 1:1 mining ratio ensuring that every tonne of ore mined becomes a tonne of POLY4.

At the Woodsmith Mine site, two deep shafts are being constructed to access the polyhalite in the centre of the mineral reserve: the production shaft reaching a depth of 1,594 metres and the service shaft reaching a depth of 1,565 metres. Once constructed the production shaft will be fitted out with two Blair multi-rope winders, each capable of hoisting 6.7 Mtpa of polyhalite ore for a combined capacity of 13 Mtpa. A third shaft, the Mineral Transport System (MTS) access shaft will also be constructed. The MTS access shaft is being constructed to facilitate construction of the MTS.

The polyhalite ore will undergo a simple process of crushing, grinding, granulation, screening and drying to produce the finished POLY4 (Sirius Minerals’ trademarked polyhalite product) granular product.

- Crushing: Run of mine ore is nominally crushed to below 50mm.

- Grinding: Crushed ore ground to pass 0.2mm.

- Granulation: Binder and water added to polyhalite powder and granulated to form POLY4 granules.

- Screening: POLY4 granules screened. Oversize and undersize removed for reprocessing.

- Drying: POLY4 granules dried to final moisture content, ready for storage and delivery.