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.

The Woodsmith team is further developing the engineering to optimise the configuration of the project, recognising the multi-decade life of the mine. Particular attention is on those aspects identified at the outset of Anglo American’s ownership – namely, the sinking of the two main shafts, the development of the underground mining area, and the changes required to accommodate both increased production capacity and the more efficient and scalable mining method of using only continuous miners; such improvements will also require the installation of additional ventilation earlier in the development of the underground mining area.

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.

Woodsmith’s entire infrastructure has been planned and designed for minimal intrusion on this unique protected landscape. The mining area will be underground, and the mined ore will not be visible on surface, as it will be transported by conveyor through a 37-kilometre tunnel to the Teesside port materials-handling facility. All mineshaft workings too are being completely hidden underground, and the number and size of the buildings on the surface have been reduced to a minimum.

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.