Sirius Minerals Regulatory News (SXX)

12 March 2012

Sirius Minerals Plc

("Sirius" or the "Company")

Assay Results for SM2

§ Two separate high grade seams of polyhalite intersected

§ Shelf Seam from 1,420m below surface consists of 46.9 metres thickness containing 32.6 metres of 83.1% polyhalite (24.0% K2SO4) including a section of 6.6 metres of 95.8% polyhalite (27.7% K2SO4)

§ Basin Seam from 1,535 metres below surface consists of 43.3 metres thickness containing 34.3 metres of 78.3% polyhalite (22.5% K2SO4) including a section of 6.8 metres at 99.2% (28.7% K2SO4)

§ Total polyhalite zones aggregated across the Shelf and Basin Seams consist of 90.2 metres of 74.6% polyhalite (21.6% K2SO4)

§ SM3 drilling on schedule and preliminary results expected to be available in the next few weeks

The Directors of Sirius Minerals Plc (AIM: SXX, OTCQX: SRUXY), the globally diversified potash development group, are pleased to announce the mineralogical assay results of the second borehole drilled at the York Potash Project.

Coring of SM2, the second new borehole in the York Potash Project, was completed on 21 January 2012. The polyhalite-bearing Fordon Evaporite Formation was present from 1,365.8 to 1,582.5 m below ground level ("bgl") - a total of 216.7 m - and was cored (85 mm diameter) with 100% recovery throughout almost its entire thickness. On 23 January 2012 preliminary results of the coring, based on visual inspection, were released. Results from the mineralogical analysis of the two seams have now been received and are summarised in this announcement.

Chris Fraser, Managing Director and CEO of Sirius said:

"The assay results of the second new hole in the York Potash Project have continued the strong results obtained from the drilling programme. These important results have confirmed the visual interpretation of the cores, the geological model and the world-class nature of the York Potash Project.

"While work continues on the drilling programme and interpretation of these results, the early indications of this work are that the new boreholes are confirming the geological continuity throughout the project area. The polyhalite grade and thickness found in SM1 and SM2 have both exceeded the assumptions used to determine the JORC Exploration Target of 3.3 to 6.0 billion tonnes of 67% to 94% polyhalite.

"The outstanding thicknesses, grade and continuity obtained in the drilling programme are being incorporated into the Detailed Scoping Study as soon as we receive them. However, through the combination of drilling delays, the better than expected scale and quality of the deposit together with the revised approach to accessing the ore body introduced by the quality of the Shelf Seam, we expect it will take until the end of April to optimise and complete the Detailed Scoping Study."

Polyhalite Analysis Process

Independent mineralogical analysis of the drill core has been carried out by the British Geological Survey ("BGS") at its Mineralogy, Petrology and Biostratigraphy Facility.

Each sample was dried, crushed and milled and homogenised; then a 4.5 g portion of the milled sample was micronised, and analysed by Quantitative X-Ray Diffraction ("QXRD"). This is a non-destructive method developed by BGS to assess the mineralogical composition of rock and ore samples (with a typical accuracy of ±2.5% for mineral concentrations >60%; ±5% for concentrations between 60 and 30%; ±10% for concentrations between 30 and 10% and ±20% for concentrations between 10 and 3%).

Polyhalite Analysis Results

Borehole SM1 intersected approximately 49.3 m (true thickness) at 66% polyhalite of which the three best sections aggregated 23.3 m (true thickness) at 95% polyhalite. The gangue was chiefly halite. Wireline logs show this intersection to be almost identical in mineral composition and vertical zonation to other historical drill holes in the area and are considered to be typical of polyhalite deposited in the basinal facies of the Fordon Evaporite Formation.

SM2 entered the Fordon Evaporites at 1,365.8 m bgl and, as expected, the Upper Subcycle was chiefly halite and anhydrite. The Middle Subcycle was intersected at 1,419.8 m bgl, at which point the first bed of massive polyhalite (the "Shelf Seam") was encountered. As identified in the field from visual examination of the core, the Shelf Seam extended from 1,419.8 to 1,453.6 m bgl with a cored thickness of 33.8 m. The unit dips at 0-10°from the horizontal. 'True' thickness, corrected for an average dip of 10° (to be conservative), is estimated to be 33.3 m.

The Shelf Seam grades down into lower quality polyhalite, some of which has been sampled and analysed. The entire analysed section has been subdivided, on the basis of grade and mineralogy, into several shorter zones as set out in the tables below. Although mostly massive polyhalite, it contains variable amounts of anhydrite (with minor halite) and a 14 m section with low levels (typically 1 to 5%) of trace minerals such as kieserite (magnesium sulphate), magnesite (magnesium carbonate), and howlite (calcium borosilicate).

A low grade section mainly comprising polyhalite mixed with variable amounts of halite - with occasional thin beds of massive polyhalite - extends from 1,453.6 m bgl to 1,535.3 m bgl (81.7 m cored thickness; equivalent to approximately 79 m 'true' thickness, corrected for a conservative estimate of average dip of 15°).

The Basin Seam was logged in the field as extending from 1,535.3 m bgl to 1,570.8 m bgl (35.5 m cored interval), and contains sections of massive high grade polyhalite interbedded with beds of more anhydrite-rich (with some halite) polyhalite. The unit displays dips of 0 to 20°. 'True' thickness (corrected for a conservative estimate of average dip of 15°) is estimated to be 34.3 m. The grade of the Basin Seam reduces both above and below its central section into lower quality mineral, that was also sampled and analysed in order to establish the boundaries more clearly. The whole analysed section has been subdivided, on the basis of grade and mineralogy, into several shorter zones as shown in the tables below.

The two principal polyhalite seams (and some overlying and underlying materials) were sampled at approximately 0.5 to 1 m intervals - taking care to isolate and analyse separately any partings of anhydrite or halite.

The Shelf Seam has, as described above, a sharp top contact but a gradational base and can be defined or subdivided in several ways for the purposes of presenting the results. From examination of the results, the greatest thickness of potential value is 46.9 m ('true' thickness) at 75% polyhalite. Within this can be defined 32.6 m ('true' thickness) at 83% polyhalite corresponding more or less with the best section of massive polyhalite described in the core log. This contained a high-grade section of 6.6 m ('true' thickness) at 95.8% polyhalite. The gangue mineral is chiefly anhydrite.

The Basin Seam has a gradational top and sharp basal contact, and again can be subdivided several ways. From examination of the results the greatest thickness of potential value is 43.3 m ('true' thickness) at 74% polyhalite. Within which can be defined 34.3 m ('true' thickness) at 78% polyhalite (again corresponding more or less with the best section described in the core log). This contained a high-grade section of 6.8 m ('true' thickness) at 99.2% polyhalite. The gangue mineral is chiefly anhydrite.

Thus it has been established by analysis that the field core log results announced on 23 January 2012, whilst accurately identifying the extent of higher grade mineral - underestimated the full thickness of the deposit (at ≥75% polyhalite).

The results are summarised as follows (quoted on a moisture free basis and on the basis that potential laboratory errors for quoted concentrations are typically +/- 2.5% for concentrations above 60%):

Shelf Seam 1,419.8 m below ground level and consists of 46.9 m (true-thickness) at 75% polyhalite, within which is a 32.6 m section at 83.1% polyhalite (including 6.6 m at 95.8%). Gangue mineral is chiefly anhydrite.

Basin Seam 1,535.3 m below ground level and consists of 43.3 m (true-thickness) at 74% polyhalite, within which is a 34.3 m section at 78.3% polyhalite (including 6.8 m at 99.2%). Gangue mineral is chiefly anhydrite.

SM2 POLYHALITE ZONE DETAILS (Borehole collar: 488424.89 E, 506841.87 N; 143.2m above sea level.)

Notes:

1) Cored section of Halite / Polyhalite from 1,475.92 to 1,530.32 mbRT (54.4m in total) was not analysed.

2) The various zones identified as 'A' to 'T' are used for convenience of description. It is not yet clear whether they can be correlated laterally to other drill hole results.

3) Depths are shown as metres below Rotary Table (mbRT), which was 4.4 m above ground level.

These results can be summarised in a number of ways to show some of the possible permutations of thickness and grade calculations. These have been adjusted for an approximation of true-thickness based on observed dips.

Polyhalite Exploration Model

A significant amount of historical data provided the foundation for the geological model for the York Potash Project. The western edge of the Zechstein Basin in the Fordon Evaporites is believed to have been a shallow-water depositional "Shelf", in which a condensed sequence of evaporites was deposited. Numerous holes have been drilled through the shelf for gas exploration and these repeatedly demonstrated the existence of a thick, relatively high grade (based on gamma logs), polyhalite seam.

The main part of the Fordon Basin, as explored by SM1 and by historical boreholes, is believed to have been a deeper water "Basinal" environment. It also contains a thick high grade deposit of massive polyhalite, towards the bottom of the sequence. This was confirmed by SM1 where Sirius intersected the seam and cored an aggregated thickness of 23.3 m (true thickness) of 95% polyhalite (27.5% K2SO4).

Prior to completion of SM2, there were two theories as to the nature of the transition zone between these two high grade deposits. One was that they were effectively the same deposit that was 'draped' over the ramp zone interface between the Shelf and Basinal areas. The second theory was that they were two separate deposits.

The Company believes that SM2 has proved that the second theory of the structure is correct. Moreover SM2 has proved that in this part of the transition zone, there are two seams of massive, high grade, polyhalite, approximately 82 m vertically apart and that these appear to represent overlapping Shelf and Basinal seams. The lateral extent of this zone of overlapping seams is, as yet, unknown and will be explored in future. Petrographic studies of the polyhalite have been commissioned to help understand better the environments of deposition.

The Shelf Seam appears to correlate with polyhalite beds known further west in the condensed 'Shelf' sequence and the Basin Seam appears to correlate, in terms of position in the sequence, with 'Basinal' facies polyhalite such as was intersected at SM1 (located approximately 5 kilometres to the east of SM2) and historical boreholes further east of SM2.

Geological Structure

The regional dip of the Fordon Formation is 5° or less, and the evaporites as a whole appear to be relatively flat-lying in North Yorkshire; although subject to localised disruption around primary (depositional) or secondary (tectonic) features. An ongoing programme of reinterpretation of reprocessed historical seismic data is improving the Company's understanding of this. For example, the work is defining the pattern of faults that affect the Fordon Evaporites while at the same time indicating significant areas of apparently flat-lying strata. This information, when finalised, will be used to assist mine planning.

The seismic reinterpretation has suggested the presence of a previously poorly-identified, minor fault in the Fordon Evaporites close to Borehole SM1. It is believed that this structure is the most plausible explanation of the steeper than expected core dips seen in the SM1 polyhalite seams - that otherwise display wireline log signatures, and hence internal seam structure, closely comparable with other historical boreholes in the area.

Observed dips in the polyhalite at SM2 are, as stated above, in the range 0 to 20°, and average dips of 10° and 15° were used to convert core intersections to "estimated "true thicknesses". These "upper end" figures were deliberately chosen to ensure conservative estimates of thickness. There is no indication in the SM2 cores of significant tectonic disturbance.

Observations on the Sneaton and Boulby Potash Seams

SM2 drilled through the upper sylvite-bearing Sneaton and Boulby seams without coring. This was for operational reasons. As reported, SM2 experienced problems with fluid loss zones above the evaporite sequence and it was deemed prudent to focus on reaching the polyhalite seam to ensure good recovery of the key target.

Wireline logging through the Sneaton and Boulby seams at SM2 enabled identification of the seam thickness, but since the hole was in poor condition (due to wash out) at the two sylvite horizons no reliable estimate of potash grade can be made. The seam depths and thicknesses are:

Note: Borehole is approximately vertical at this depth.

Within these thicknesses, "Higher grade" sections of 2.9 m and 2.3 m thickness were observed within the Sneaton and Boulby seams respectively.

Drilling Progress Update

Drilling progress at SM3, the Company's third hole at the York Potash Project, has been good with coring of most of the Boulby and Sneaton seams complete. After experiencing one fluid loss zone and applying a standard cementing tactic, coring of the Fordon Evaporites has now commenced. The Company expects to be able to announce the preliminary results in the next few weeks. Once the primary hole at SM3 is complete, a cored deflection from this hole is planned to be undertaken to obtain further information on the lateral consistency and continuity of the deposit.

Parts of the new coring rig (Rig 16) have begun to arrive and it is scheduled to begin work at SM6 (Newton House) in the next two weeks, once site preparation is complete. The current coring rig (Rig 5), is planned to move to SM4 (Gough) upon completion of the cored deflection at SM3.

Detailed Scoping Study ("DSS") Update

The confirmation of the existence, extent, and quality of the two distinct seams at SM2 has required some re-evaluation of how the project may be mined and this combined with the delays experienced at SM2 and the knock-on effect on SM3 has led to the completion of the DSS also being delayed. Although being progressed in parallel, the positive new results need to be incorporated into the DSS to optimise and complete it. Originally scheduled for completion at the end of March 2012, it is now expected to take approximately an additional month before a summary of the results of the DSS can be released.

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About Sirius Minerals Plc

Sirius Minerals is a globally diversified potash development company. Its primary focus is to bring on stream major potash mining facilities through the acquisition and development of projects overlying recognised potash deposits. Today it holds properties in the United Kingdom (North Yorkshire), the United States (North Dakota), and Australia (Queensland and Western Australia).

Incorporated in 2003, Sirius Minerals' shares are traded on the London Stock Exchange's AIM market. Its shares are also traded in the United States on the OTCQX through the use of a sponsored ADR facility. Further information on the Company can be found at www.siriusminerals.com.

Competent Person's Statement and Risk Statement

The information in this Stock Exchange Announcement that relates to Mineral Exploration results and Exploration Targets, together with any related assessments and interpretations, have been verified by and approved for release by Dr Frederick W. Smith FIMMM., CEng, CSci., a qualified geologist and full-time employee of FWS Consultants Limited ("FWS"), Environmental and Geological Consultants. Dr Smith, who is a Fellow of the Institute of Materials, Minerals and Mining (IMMM), has sufficient experience which is relevant to the style of mineralisation and type of deposit under consideration and to the activity which he is undertaking to qualify as a Competent Person for the purposes of the AIM Rules. Dr Smith and FWS consent to the inclusion of the information contained in this announcement and the respective references to them in the form and context in which they appear. The JORC Exploration Target estimates of quantity and grade are conceptual in nature and there has been insufficient exploration to define a Mineral Resource on the property and it is uncertain if further exploration will result in discovery of a Mineral Resource on the Property. The estimates are not a Reserve or Resource statement in accordance with an AIM recognised Standard and should not therefore be relied upon as such. It should be noted that the Pan European Code of Reporting or PERC (that IMMM co-wrote) released in December 2008 and JORC are essentially identical on all key aspects relating to compilation and quoting Exploration Targets.

Qualified Person

Graham Clarke, Operations Director for Sirius Minerals Plc, who has reviewed this update, has over 26 years experience in the potash mining industry. He has a Degree in Mining Engineering from the University of Nottingham and extensive operational experience gained at Cleveland Potash Limited in the mining of the Boulby Potash seam and the exploration and development of the upper Fordon polyhalite seam.