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Zechstein evaporite deposits - Woodsmith, Boulby, Billingham |
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UK |
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Polyhalite Potash Anhydrite Halite
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The areally extensive Woodsmith polyhalite deposit is hosted within the western marginal sections of the thick Permian Zechstein evaporite sequence of central and western Europe. It is located to the south and southeast of the town of Whitby in North Yorkshire, England, with the main shaft to the underground operations being ~6 km south of the town centre and just over a kilometre WSW of the hamlet of Sneatonthorpe. The deposit and mine encroach upon the North York Moors National Park (#Location: 54° 26' 13"N, 0° 37' 30"W).
Other significant deposits within the same evaporite succession include the historic Billingham Anhydrite Mine near Stockton on Tees, 45 km WNW of Whitby, that was operated between 1927 and 1971 (#Location: 54° 35' 52"N, 1° 15' 43"W), and the Boulby Mine which is located 16 km WNW of Whitby and 35 km ESE of Middlesbrough. The latter has exploited potash and rock salt from an upper seam, but now produces substantial tonnages of Polyhalite from a layer that lies 150 to 170 m lower (#Location: 54° 33' 5"N, 0° 49' 32"W).
This record will concentrate on the stratigraphic succession within the western Zechstein sequence in the UK, detailing the distribution of mineralised units. The geology and mineralisation of the three mines/projects Woodsmith, Billingham and Boulby are included in the description of the respective host units below.
Polyhalite [K2Ca2Mg(SO4)4•2H2O], the most significant commodity in this section of the Zechstein, is a hydrated calcium, magnesium and potassium sulphate evaporite mineral used as a fertiliser.
Polyhalite is interpreted to have formed by synsedimentary metasomatism, or back-reaction, of pre-existing sulphates. This is inferred to have occurred as a result of the dehydration of an earlier gypsum bed to form anhydrite, which was simultaneously transformed to polyhalite by reaction with more evolved potassium- and magnesium-enriched marine brines. These evolved brines are interpreted to have formed as a result of further evaporative concentration since the original gypsum was deposited, or alternatively to be fluids that formed, and flowed in from, nearby shallow salt pans in adjacent sulphate platform areas.
Sirius Minerals plc began assessing heritage data from oil and potash (sylvite) exploration in May 2010, and acquiring extensive mineral tenement in the Whitby-Scarborough area, with the aim of outlining a resources of high grade potash sulphate fertiliser. Funds were raised in early 2011 to continue this evaluation, which included studying 46 historic petroleum wells and drill holes that had cut the prospective sequence, both onshore and offshore. A program of 17 new onshore drill holes was commenced in July 2011, outlining the scale and purity of the polyhalite resource. By May 2013, 4200 m of core from 16 000 m of drilling, along with detailed wireline logging, had delineated a potential resource of 2.66 Gt @ 85.7% polyhalite. Planning approval from the various authorities had been obtained by October 2015 and a definitive feasibility study published in March 2016, followed by commencement of construction in May 2017. Difficulties in raising funds for the completion of stage I of the project led to Sirius Minerals, and hence the project, being acquired by Anglo American plc in March 2020. Development has since continued, with the first production expected in 2027 (Anglo American 2023). To avoid visual pollution in the National Park, infrastructure has included low profile buildings, and an underground winder/shaft head, surrounded by forest, as well as a 37 km conveyor tunnel at a depth of 340 m to transport ore to the 'material handling facility' at Wilton and the nearby port at Teesside. No mineral will come to surface in the National Park. The deposit is largely flat lying at a depth of ~1550 m below surface and will be mined by room and pillar utilising continuous miners. The mineralised unit covers an area of ~796 km2, of which ~271 km2 is onshore and the remaining 535 km2 is offshore.
Regional Setting
The Woodsmith, Boulby and Billingham deposits are located on the western margin of the Late Permian Zechstein Basin which extends across the North Sea into Europe, and as far east as Poland, Lithuania and western Belarus. The Zechstein Basin is characterised by an evaporite sequence of up to 1.5 km thickness overlying the Permian arid red bed Rotliegendes succession. For a summary of the development of the Permian basins in Europe and the regional setting of the Zechstein sequence see the Regional Setting section and accompanying map of the Kupferschiefer-Zechstein record.
The full Zechstein sequence was deposited over a period of 5 to 7 million years and represents multiple influxes and subsequent evaporation of seawater in a topographic low with restricted connection to the Zechstein sea. As such, deposition in the Zechstein sequence was cyclic in nature. During high sea level stands, when there were no barriers to the open ocean, carbonate shelves formed around the margins the basinal sea, with desert and sabkha conditions on the landward side behind. However, when access to the open ocean was cut off, wedges of minerals such as gypsum and anhydrite were deposited on the shelf edge or ramp. Sections of the anhydrite beds are thought to have subsequently been altered in the NE Yorkshire area to form the complex mineral polyhalite. During periods when the basin remained isolated, more complete evaporation would allow minerals to be deposited in the basin centre, with slow seepage through the shelf edges replenishing fluids, and allowing deposition of thick columns of halite and associated evaporites. Reconnection with the ocean initially allowed deposition of ‘shelf’ gypsum/anhydrite deposits and the start of the next cycle.
Stratigraphy and Evaporite Deposition
Five overall Zechstein cycles containing sylvite [KCl], halite, gypsum/anhydrite and related evaporites have been recognised within the Zechstein sequence, with the bulk of the sylvite being in the third and fourth cycles. The Fordon Evaporites of the underlying second cycle (EZ2) contain up to 400 m of salts in NE Yorkshire, including the two main layers or seams of polyhalite, interpreted to represent a lower basin floor and upper shelf deposit. However, these cycles are rarely complete and are usually dominated by carbonate deposition.
The Zechstein sequence transgression ended the early Permian continental Rotliegendes phase and initiated a prolonged period of marine conditions. In the eastern UK, the western shoreline of the Zechstein Sea is inferred to be only slightly to the west of the current limit of Permian outcrop. Applying a sequence-stratigraphic approach, the five cycles are divided into seven Zechstein evaporite-carbonate sequences (ZS), which can be summarised as follows, after Stone, P., Millward, D., Young, B., Merritt, J.W., Clarke, S.M., McCormac, M. and Lawrence, D.J.D. 2010, updated 2016. British Regional Geology: Northern England; British Geological Survey; the BGS Lexicon of Named Rock Units; Smith and Crosby (1979); Cooper, Whitbread and Irving (2007) and other sources:
ZS1, which unconformably overlies the aeolian 'Yellow Sands' Formation of the early Permian Rotliegendes desert that is composed of weakly cemented, mainly medium- and fine-grained, cross-stratified, yellow aeolian sand or sandstone. ZS1 comprises, from the base, the:
• Marl Slate Formation - which is 5 to 6 m thick, and began with an initial rapid transgression of the Zechstein Sea that flooded wide areas of desert. This event deposited a sequence largely composed of alternating bands of dark grey and black silty, dolomitic mudstone that are finely laminated and often bituminous, with pyrite, galena and sphalerite that coat bedding planes, and local thin beds of dolostone and dolomitic limestone. This formation is interpreted to be equivalent to the Kupferschiefer of Poland and Germany.
• Raisby Formation, traditionally known as the Lower Magnesian Limestone, which only outcrops over a narrow width. It is composed of cream, brown or grey dolostone and pure, grey limestone, although the latter is rare. It is 48 m thick in the type locality, but generally ranges from 20 to 40 m, and locally as much as to 73 m, and has been divided into three units:
i). the lowermost of which is composed of 15 to 30 cm thick beds of dolomitic to almost pure limestone composition, with common laminated argillaceous layers containing galena, pyrite and sphalerite, particularly towards the base;
ii). a middle unit, predominantly composed of grey to buff dolostone with local calcitic dolostone that is finely crystalline and thinly bedded in 5 to 10 cm thick layers. Bedding is grossly planar, but may be very uneven and nodular in fine detail, and has abundant argillaceous stylolitic bedding laminae films. Although predominantly stratified it is brecciated towards the top, and is often interbedded with gypsum; and
iii). the upper unit which is a buff to brown dolostone occurring as irregular, lenticular, up to 45 cm thick beds. Stylolites and brecciation are less common than in the middle unit.
ZS2, the base of which is marked by a thin sequence of fine-grained, clastic dolostone, sometimes known as the Transition Beds that represents the transition to initial lowstand deposition of the ZS2 shallow-water shelf environment. This transition led to the gradual extinction of much of the earlier fauna and the accumulation of their shells on the shelf edge in sufficient numbers to form an elongate bank on which a reef-forming fauna could gain a foothold. The lower parts of the reef are composed of massive dolomitic limestone and dolostone with prolific brachiopods, bivalves and polyzoa. As the reef reached a maximum height of ~60 m, the shallower water encouraged rapid growth of calcareous algae, whilst increasing salinity led to the continued gradual extinction of much of the earlier fauna. Consequently, the uppermost parts of the reef are largely of algal origin and contain a wide variety of stromatolites.
• The Ford Formation, which is 1 to 150 m, but generally 100 to 120 m thick, and both includes and overlies the transition beds, and is composed of variable facies. The bulk of the formation is located on the western, landward side of the reef, occurring as back-reef and lagoonal beds within a shallow and protected lagoonal setting. In this setting, a varied sequence of granular ooidal and pisolitic carbonates accumulated, keeping pace with the upward growth of the reef. These lagoonal facies are almost universally dolomitised and recrystallised to dolostone crystals as much as 5 mm across. In contrast, the basinward, or eastern side of the reef is characterised by fore-reef talus aprons and off-reef beds, where deposition was comparatively muted. The exception is in the immediate vicinity of the reef edge where wedge-shaped, fore-reef talus aprons developed from detrital material eroded from the reef front. Towards the end of the Ford Formation, a sea level decline of several metres terminated reef growth and deposition of the up to 150 m thick Hartlepool Anhydrite Formation, a dense aggregates of very finely crystalline laths that form nodular masses of almost pure anhydrite. The Zechstein Sea by then presented a very shallow-water, hypersaline and inhospitable shelf-edge regime.
ZS3, which commenced with deposition of the Roker Formation on the shelf and adjacent basin slope, marking the onset of the next major marine transgression:
• The Roker Formation, (also known as the Kirkham Abbey Formation) which is up to 80 m thick, is composed of the,
i). Concretionary Limestone, a thinly bedded granular dolostone that is locally recrystallised and contains calcite concretions that may locally be as large as 'cannon balls'. It has been subdivided into a lower, ~15 m thick unit containing abundant concretions, and an upper, up to 20 m or more thick unit, in which they are absent. To the east, it has a maximum thickness of 95 to 100 m, where it is interpreted to represent the basin slope equivalent of the Roker Formation, with a gradational, interdigitating contact with the underlying Hartlepool Anhydrite Formation.
ii). Hartlepool and Roker Dolostones, which are almost entirely composed of soft, granular and ooidal, cross- and ripple-bedded dolostone with little significant variation in lithology. The character of the dolostone is consistent with deposition in a shallow-water shelf environment, although the presence of rip-up clasts and minor erosion surfaces imply subaerial emergence partly within the intertidal zone. The base of the dolostone is diachronous, from where it rests conformably on and interdigitates with the Concretionary Limestone Formation, to a gradational contact with the underlying Hartlepool Anhydrite Formation offshore, with an equivalent dissolution residue contact to where it overlies the Ford Formation onshore. To the west, the sequence grades into the,
iii). Edlington Formation, which comprises 0 to 65 m of red-brown calcareous mudstone, with subordinate siltstone and sandstone, and beds of gypsum or anhydrite at its base.
ZS4, the character of which, following the the Roker Formation, infers a period of marked sea-level oscillations, as evidenced by evaporites that contains sedimentary features characteristic of both shallow and deepwater conditions. This sequence is composed of two main facies,
i). the Fordon Evaporite Formation, a thick and varied sequence of evaporites including anhydrite and halite, with some gypsum and dolostone. The formation is 396 m thick in the type section, and varies between 30 and 200 m across the Woodsmith deposit area. Within the formation, the Fordon Evaporites are only preserved in the subsurface offshore where they reach a thickness of up to 90 m and consist primarily of gypsum, halite and anhydrite with some dolostone. They are interpreted to have originally extended as far west as the present coastline, but have been largely removed by dissolution, leaving only the Seaham Residue and producing significant foundering and collapse brecciation of the overlying succession. The Fordon Evaporite Formation is inferred to represent the lowstand facies marking the onset of Zechstein Sequence 4.
ii). the Seaham Residue, which is up to 9 m thick at its type locality and consists primarily of insoluble clay and silt residue after the limestone, dolomitic clay and evaporites of the laterally equivalent Fordon Evaporite Formation.
Woodsmith polyhalite deposit area.
The Fordon Evaporite Formation is the host to the Woodsmith polyhalite deposit where the formation can be subdivided into Upper, Middle and Lower sub-cycles. Each contains identifiable mineral zones (Stewart, 1949; 1963) that can be correlated over significant distances within the western Zechstein of the UK (Colter and Reed, 1980). The evaporites can also be divided into lateral facies that comprise a western 'shelf' facies, predominantly anhydrite and and an eastern 'basinal' facies characterised by thick halite in the middle sub-cycle. The two are separated by a basin slope, or ramp that trends irregularly north-south. These vertical sub-cycles and lateral facies are manifested as two vertically separated, high grade seams,
a). a deeper Basin Seam which is low in the Middle Sub-cycle, bounded both above and below by halite, and lenses out against the Basin Ramp which forms its western margin; and
b). a shallower Shelf Seam, immediately below the Upper Anhydrite on the basin margin, that persists further west than the Basin Seam, becoming more condensed and pinching out towards the edge of the palaeo-coastline. It thickens in the central part of the Woodsmith area, and is bounded by a unit composed of intercalated halite-anhydrite-polyhalite below, and an anhydrite bed above.
Both seams are underlain by the Basal Halite of the Lower Sub-cycle, which is thick on the shelf and Basin Ramp, but thins markedly further to the east. The Basal Halite and Basin Seam are separated by the thin, black, Lower Halite band.
The Basin and Shelf seams are vertically separated by a 'Transition Zone', characterised by intercalated halite with K and Mg chlorides and sulphates (sylvite [KCl], kainite [KMg(SO4)Cl•3H2O] and kieserite [MgSO4•2H2O]), immediately east of the Basin Ramp, concentrated at the top of the Middle Sub-cycle. This 'Transition Zone' also corresponds to the lateral overlap of the two seams, and includes intervals parallel to the seams that vary from low, to marginal and high grade polyhalite and mixed polyhalite and halite.
The overlying Shelf Seam is the lowest member of the Upper Sub-cycle, immediately underlying the Upper Anhydrite, and is overlain, in turn, by the uppermost 'Upper Anhydrite and Halite Member'.
A mixed sylvite /sylvinite-bearing unit, the Gough Seam, has been identified at the top of the Upper Subcycle above the Shelf Seam [NOTE: sylvinite is a mixture of sylvite and halite].
The Shelf Seam appears to laterally fade eastward into the basin into a zone of diffuse K-salts, sylvite, kieserite and kainite.
Representative intersections of the two seams and transition zone include (after Smith, Dearlove, Kemp, Bell, Milne and Pottas, 2014):
• Shelf Seam - 11.1 m @ 97.1% polyhalite, within a broader zone of 23.3 m @ 95.0% polyhalite. This intersection is well onto the shelf, to the west of the Transition one and Basin Seam;
• Shelf Seam - 6.6 m @ 95.8% polyhalite, within a broader underlying section of the Transition Zone of 32.6 m @ 83.1% polyhalite;
- 16.2 m @ 95.9% polyhalite within a broader underlying section of the Transition Zone of 25.2 m @ 87.5% polyhalite; and
- 23.02 m @ 93.0% polyhalite within a broader underlying section of the Transition Zone of 46.9 m @ 83.0% polyhalite.
• Basin Seam - 6.8 m @ 99.2% polyhalite within a broader overlying section of the Transition Zone of 34.3 m @ 78.3% polyhalite. and
- 11.1 m @ 97.1% polyhalite within a broader interval one of 23.3 m @ 95.0% polyhalite to the east of the main Transition Zone and Shelf Seam.
In addition to halite, anhydrite and dolomite, a variety of minerals are associated with the polyhalite, principally kieserite and magnesite, in addition to a range of strontium (celestine) and boron bearing minerals (kalistrontite [K2Sr(SO4)2] in the Shelf facies polyhalite seam).
ZS5 - Deposition of the Fordon Evaporite largely completed the filling of the Zechstein Basin. Subsequent deposition occurred in a relatively shallow water environment, likely <20 m deep, below the tidal zone, with little or no basin floor relief. The sequence comprised:
• The Seaham Formation, which has a sharp, conformable contact with the underlying Fordon Evaporite Formation, varies from 1 to 31.5 m in thickness, and predominantly comprises, and is characterised by, thin-bedded limestone, which includes calcite mudstone/wackestone, some interbedded coquina, packstone, grainstone, mudstone and concretionary limestone, with some dolostone. The whole formation has been severely disrupted and locally brecciated by the foundering arising from solution collapse of the underlying Fordon Evaporite.
• The Billingham Anhydrite Formation, divided into the Upper, Main and Lower seams of anhydrite that comprise 4.5 to 6 m of massive anhydrite averaging 90% purity.
Billingham Anhydrite Mine.
The Billingham Anhydrite Formation was exploited by the Imperial Chemical Industries (ICI) at the Billingham Mine near Stockton, 45 km WNW of Whitby, between 1927 and 1971 as feedstock for the company's chemical works to manufacture of sulphuric acid and ammonium sulphate fertiliser.
Boulby Mine.
The Boulby Mine is developed within the:
• Boulby Halite and Boulby Potash formations, which progressively overlie the Billingham Anhydrite Formation. These units are currently (2024) exploited for potash and rock salt (the latter for de-icing UK roads in winter) at the Boulby Mine, which is located 16 km WNW of Whitby. Mine construction had commenced in 1969, with the first product delivered in 1973. The Boulby Potash Formation was initially mined for potash alone from 1973 by Cleveland Potash, a subsidiary of ICI, before being jointly owned with Anglo American, and then with De Beers. The operation was subsequently acquired in 2002 by Israeli Chemicals Ltd (ICL) who has operated it to the present (2024). Operations are both below onshore terrane, but also extend up to 8 kilometers offshore.
NOTE: Polyhalite, is also mined at Boulby, and is now the main product, but is extratced from a layer that lies 150 to 170 m beneath the Boulby Potash Formation at a depth of ~1200 m below surface, assumed to be the Fordon Evaporite. Polyhalite hoisted in 2023 amounted to 1.009 Mt, extracted by room and pillar using continuous miners. In 2023, a total of 87.6 thousand tonnes of salt were sold.
The polyhalite seam falls within two lateral zones: i). a Western Zone that averages ~15 m in thickness and is divided into a higher polyhalite grade western domain and a lower polyhalite grade eastern segment; and ii). an Eastern Zone which in 2023 was under technical review, with planned operations, over time, to augment and eventually supplant the Western Zone (ICL Annual Report, 2023).
At the Boulby Mine, the Billingham Anhydrite Formation is directly overlain by the Boulby Halite Formation which has a total thickness of ~40 m, but thickens offshore to the east. It is increasingly anhydritic towards the base, but grades upwards into orange-brown halite with interstitial grey clay, to a 1 to 2 m thick bed of pure orange-pink halite ~8 to 10 m below the top of the unit. This is overlain above a sharp contact by 3 to 4 m of a brownish-grey halite, locally speckled with small red sylvite crystals, and an upper 10 to 50 cm that is markedly more anhydritic. The contact with the overlying Boulby Potash Formation is sharp and generally planar. The thickness of this potash bed averages 7 m in the Boulby Mine, but is very variable, sometimes ranging from ~<1 to as much as 20 m over a lateral distance of 20 to 30 m. It is composed of greyish pink to orange sylvinite which has a texture closely resembling the foliation of a metamorphic gneiss, and encloses rounded fragments of wall rock. The foliation is strongly folded in places and does not parallel the stratigraphic contacts. As such it is inferred to represent flowage rather than bedding. It has a very undulose contact with the overlying halite-enriched anhydritic and argillaceous transition zone to the Carnallitic Marl at the base of the overlying Rotten Marl Formation. The Potash Formation is composed of variable proportions of sylvite and halite with minor clay and anhydrite, and trace magnesite, pyrite and quartz. Under high differential stress sylvite is known to flow more readily than halite and is differentiate into high grade bands. Consequently the proportions of the constituent minerals influences the physical character and grade of mineralisation. This interpreted plastic deformation also has a marked effect on the thickness and potash grade, which ranges from complete absence in some areas, to up to 20 m in thickness and grades of up to 60% KCl in others. The average grade mined at Boulby is 34% KCl (21% K2O), with 45 to 55% halite. The seam has been mined at depths of between 1200 and 1500 m, and underlies thick Triassic aquifers that persist to a depth of ~1150 m and contains brine under high pressure.
• The Rotten Marl Formation, which grades upward from the Boulby Potash Formation via the transition zone mentioned above. It represents the first real clastic input into the basin but is only preserved in the south-east of the region. It is a dull, dark red-brown, silty mudstone commonly with scattered halite crystals, and is cut by a network of veins containing fibrous halite and gypsum. Where well developed, it has been subdivided into the:
i). Carnallitic Marl Formation, which is 10 to 17 m thick and reddish-brown, mainly composed of dehydrated clay minerals after mudstones and siltstones with variable but low grade carnallite [KCL•MgCl2•6H2O], and locally up to 60% halite. The latter occurs as halite/sylvinite veins and distinct pods and crystals in the clay. The formation contains little or no carbonate and would be more aptly termed a salt-clay. Carnallite is rarely found in the Boulby area, but is seen in drill holes further to the south.
ii). Upgang Formation, a 3 to 4 m thick, distinctly laminated, dark grey, dolomitic shale, with local magnesite. It has a gradational upper boundary, but a sharp contact with the underlying Carnallitic Marl.
ZS6 - The Rotten Marl Formation, and where differentiated, the Upgang Formation, is overlain by what has been termed the Upper Anhydrite and then the thicker Upper Halite. In more recent literature, these are described as the,
• The Sherburn Anhydrite Formation which rests on magnesite of the Upgang Formation or mudstones and siltstones of the Carnallitic Marl Formation. It comprises 2 to 15 m thick, eastward thinning well bedded, very fine grained, pale grey anhydrite bed containing lenses of sylvite. Distinct bands of halite pseudomorphs and some of sylvite that are both after gypsum, are evident.
• The Sneaton Halite Formation, which is up to 55 m thick in the Whitby area, but thins rapidly westwards and is absent below Wilton and Teesside, whilst it thickens gently and relatively uniformly eastward to a maximum on land of about 60 m. It comprises five main recognisable units:
i). Halite, 5 to 7 m thick, that is colourless to pale pink, medium- to coarse-grained, faintly rhythmically colour-banded, with delicate laminae of fine-grained grey anhydrite;
ii). Halite, 8 to 12 m thick, colourless, pink or amber, mainly medium-grained, equigranular, commonly weakly colour banded, with a few laminae and thin 'beds' of fine grained grey anhydrite and grey or red-brown anhydritic mudstone;
iii). Upper Potash, 0 to 8 m thick, composed of colourless, grey, brown or mottled sylvinite, variably mixed with medium to coarse grained halite and sylvite in a generally concordant mesh of grey anhydrite and dark grey-green carbonaceous clay;
iv). Mudstone that is 14 to 23 m thick and halitic, occurring as a heterogeneous, faintly layered mixture of colourless, mainly coarsely crystalline, euhedral to subhedral halite, in a distended matrix of brick-red silty mudstone or argillaceouss siltstone;
v). Halite, 1.8 to 4.9 m of colourless to amber, medium to coarse grained halite, with scattered films and patches of fine-grained grey anhydrite and red, silty mudstone.
ZS7 - which comprises the,
• The Roxby Formation - 0 to 130 m of reddish brown mudstone and siltstone, and subordinate sandstone, with gypsum and anhydrite being common towards the base. The Roxby Formation includes, within its lower sections the,
i). Sleights Siltstone, composed of 2 to 4 m of basin margin red siltstone with minor interbedded halite;
ii). Littlebeck Anhydrite Formation, which is 1 to 2 m thick and sits on the Sleights Siltstone. It comprises pink-grey, finely crystalline and commonly unevenly laminated anhydrite with local discontinuous laminae of red-brown silty marl, and passes upward into red-brown mudstones and siltstones of the Roxby Formation.
This sequence is overlain by a Triassic clastic sequence which commences with the Sherwood Sandstone Group, formerly known as the Bunter Sandstone, composed of red, yellow and brown sandstones and pebbly sandstones with lesser amounts of conglomerate and minor mudstone and siltstone, which varies from as little as 90, to >600 m in thickness. This is unconformably overlain by the Mercia Mudstone Group, which varies up to 1350 m in thickness and is composed of dominantly red, less commonly green-grey, mudstones and subordinate siltstones with thick halite-bearing units in some basinal areas, as well as thin beds of gypsum/anhydrite and sandstone.
Whilst the sequence-stratigraphic approach employed by Stone et al., (2010/2016) divides the succession into seven Zechstein sequences (ZS1 to 7), as outlined above, sections of the literature, including descriptions of the Woodsmith deposit, split the same succession into five carbonate-evaporite cycles (EZ1 to 5), as follows:
• EZ1 - Marl Slate, Raisby, Ford and the Hartleypool Anhydrite formation;
• EZ2 - Roker/Concretionary Limestone, Fordon Evaporite formations, and Seaham Residue;
• EZ3 - Seaham, Billingham Anhydrite and Boulby Halite and Potash formations;
• EZ4 - Rotten Marl (Carnallitic and Upgang), Sherburn Anhydrite and Sneaton Halite formations;
• EZ5 - Sleights Siltstone, Littlebeck Anhydrite and Roxby formations.
Reserves and Resources
Billingham - Over the 44 years in which the mine operated, >30 Mt of anhydrite were extracted and used principally for the manufacture of fertilizers and cement. Room and Pillar mining was employed, with an extraction efficiency of only ~50%, which means that half the anhydrite was left unmined. The mine extends over a distance of ~1.6 km east to west and 3.2 km north to south.
Boulby - Although the ICL website (viewed June, 2024) states that "to date, ICL has estimated that one billion tonnes (1 Gt) of polyhalite resources are available at ICL Boulby", the ICL Annual Report (2023) records the following JORC compliant estimates as at 31 December, 2023 at a cut-off of 12% K2O;:
Indicated Mineral Resource - 38.9 Mt @ 13.3% K2O;
Inferred Mineral Resource - 9.3 Mt @ 13.3% K2O;
Probable Ore Reserve - 7.6 Mt @ 13.5% K2O;
NOTE: Mineral Resources are exclusive of Ore Reserves. The Mineral Resource, only includes the western zone, as described above.
TOTAL Reserve + Resource at Boulby - 55.8 Mt @ 13.33% K2O.
Woodsmith - Current published Ore Reserves and Mineral Resources as at 31 December 2023 (Anglo American plc
Ore Reserves and Mineral Resources Report, 2023) were:
Shelf Seam
Measured + Indicated Mineral Resource - 230 Mt @ 81.5% Polyhalite;
Inferred Mineral Resource - 810 Mt @ 82.3% Polyhalite;
Proved + Probable Ore Reserve - 290 Mt @ 88.8% Polyhalite.
Basin Seam
Inferred Mineral Resource - 960 Mt @ 86.3% Polyhalite;
NOTE: Mineral Resources are reported as additional to (exclusive of) Ore Reserves. Note also that Polyhalite contains 14% K2O.
TOTAL Reserve + Resource at Woodsmith - 2.290 Gt @ 84.72% Polyhalite = 11.86% K2O.
The most recent source geological information used to prepare this decription was dated: 2023.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Woodsmith Boulby Billingham Anhydrite
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Anglo American plc 2023 - Crop Nutrients: in Ore Reserves and Mineral Resources Report 2023, pp. 17-19, 86-89.
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Cooper, A.H., Whitbread, K. and Irving, A.M., 2007 - Environment Agency: Durham Permian Sections: in British Geological Survey, Internal Report, CR/07/117. 52p.
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Sirius Minerals plc 2016 - Geology and Resource: in Polyhalite Project Summary Document Technical Report prepared for Sirius Minerals plc (269p.) pp. 756-769.
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Smith, D.B. and Crosby, A., 1979 - The regional and stratigraphical Context of Zechstein 3 and 4 Potash Deposits in the British Sector of the Southern North Sea and Adjoining Land Areas: in Econ. Geol. v.74, pp. 397-408.
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Smith, F.W., Dearlove, J.P.L., Kemp, S.J., Bell, C.P., Milne, C.J. and Pottas, T.L. 2014 - Potash - Recent exploration developments in North Yorkshire: in Hunger, E., Brown, T. J. and Lucas, G. (Eds.), 2014 Proceedings of the 17th Extractive Industry Geology Conference, EIG Conferences Ltd, Edge Hill University, Ormskirk, Lancashire , September 2012, pp. 45-50.
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Tucker, M.E., 1991 - Sequence stratigraphy of carbonate-evaporite basins: models and application to the Upper Permian (Zechstein) of northeast England and adjoining North Sea: in Journal of the Geological Society, London v.148, pp. 1019-1036.
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Woods, P.J.E., 1979 - The Geology of Boulby Mine: in Econ. Geol. v.74, pp. 400-418.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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