Maiden Resource Estimate

11 June 2012

Sirius Minerals Plc

York Potash Inferred Resource - 1.35 billion tonnes of 88.7% Polyhalite

§ World-class nature of York Potash confirmed from just the initial three drill holes, delivering the largest and highest grade Polyhalite resource globally

§ Inferred Resource of 1.35 billion tonnes of 88.7% Polyhalite (25.6% K2SO4) within an area representing only 2% of the revised project area

§ High grade mineralisation within Inferred Resource of 0.94 billion tonnes of 94.0% Polyhalite (27.2% K2SO4)

The Directors of Sirius Minerals Plc (AIM: SXX, OTCQX: SRUXY) ("Sirius" or the "Company"), the globally diversified Potash development group, are pleased to announce the receipt of an NI 43-101 compliant Mineral Resource Estimate for its flagship York Potash Project. The Inferred Resource is 1.35 billion metric tonnes of 88.7% Polyhalite (25.6% K2SO4) and contained within this, is high grade mineralisation of 0.94 billion metric tonnes of 94.0% Polyhalite (27.2% K2SO4).

Chris Fraser, Managing Director and CEO of Sirius said:

"This maiden Resource estimate is another key milestone in the development of the world-class York Potash Project. With just three drill holes, we have delivered the world's largest and highest grade resource of Polyhalite - the unique multi-product Potash ore.

"Our Detailed Scoping Study released in April demonstrated the potential value of a significant Polyhalite resource at York Potash, and this estimate confirms both the scale and quality of the resource underpinning that study. With only 690 million metric tonnes of mineable Polyhalite ore needed to deliver 50 years of the Detailed Scoping Study production profile, the maiden Resource from just 2% of the revised project area demonstrates the scale and significance of this world-class deposit. Our fully funded drilling programme is now focussed on resource definition to underpin financing of the Project as quickly as possible."

The NI 43-101 compliant Mineral Resource Estimate was prepared by Dr Henry Rauche and Dr Sebastiaan van der Klauw, Qualified Persons under NI 43-101 of ERCOSPLAN Ingenieurgesellschaft Geotechnik und Bergbau mbH ("ERCOSPLAN") of Erfurt, Germany. ERCOSPLAN has extensive experience in resource characterisation and preparation of NI 43-101 Technical Reports on Potash deposits around the world.

INFERRED MINERAL RESOURCE ESTIMATE

1) Deductions of 15% for the Shelf Seam and 7.5% for the Basin Seam have been made for barren zonesand tectonic impact.

The Inferred Resource was estimated using basic polygon shapes around and between the three new holes drilled by Sirius (SM1, SM2, and SM3). The area of minerals within these polygons represents only 17.2km2, which is just 2% of the revised York Potash project area (reduced to 795km2 due to the removal of the low level mineralisation areas on the western and southern edges of the deposit). Extensive historical drilling and seismic information has been used to model the Polyhalite deposit, however for the calculation of this resource estimate, ERCOSPLAN has excluded this historical drilling data.

As the York Potash drilling programme continues and consistent results are obtained from future cores recovered, the Company expects this resource estimate will significantly increase in size and confidence level as material is upgraded into the Indicated and Measured Resource categories. The goals of the drilling programme currently underway at York Potash continue to be refined. However, with confirmation of the world-class nature of the resource, the focus of this programme is now moving to achieve sufficient resource definition to underpin financing of the Project. 

Drilling Progress Update

Preliminary coring results from SM4 (Gough) indicate the Shelf Seam at a high grade and mineable thickness of approximately 5 metres. The location of this drill hole was designed to provide regional depositional structure information of the overall deposit. The recently received reinterpretation of the legacy seismic data suggested that SM4 was close to the eastern edge of the shelf and also there was likely to be an absence of the basin seam. The preliminary coring results of SM4 have validated this seismic interpretation and as a result provided valuable information on the extent and nature of the transition zone in the deposit and the eastern extent of the Shelf Seam - SM4 is over 6.5km South East from SM3. 

Hydrogeological testing has been conducted at SM6 (Newton House). This testing of the water structure was needed to confirm the pressure and flow of water for shaft freezing considerations and design of the sub-level hoisting station. The results of this test work continue to be analysed, however initial interpretation confirms viability of the underground development concept detailed in the Detailed Scoping Study. Testing has been completed and drilling at SM6 resumed with coring expected shortly.

For further information, please contact:

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. It is developing the flagship York Potash Project in North Yorkshire, United Kingdom (North Yorkshire) and holds properties in 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 a sponsored ADR facility. Further information on the Company can be found at www.siriusminerals.com.

Qualified Persons

The mineral resources were estimated by Dr. Henry Rauche, Ph.D., EurGeol, Managing Director, CEO ERCOSPLAN and Dr. Sebastiaan van de Klauw, Ph.D., EurGeol., Consulting Geologist, ERCOSPLAN. Dr. Rauche and Dr. van de Klauw are Qualified Persons for the purposes of National Instrument 43-101 Standards of Disclosure for Mineral Projects of the Canadian Securities Administrators, and have reviewed and verified the data disclosed in this Stock Exchange Announcement.

The information in this Stock Exchange Announcement that relates to Mineral Exploration results, 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.

Graham Clarke, Operations Director for Sirius Minerals Plc, who has reviewed this Stock Exchange Announcement, 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.

National Instrument 43-101

The NI 43-101 is a codified set of rules and guidelines for reporting and displaying information relating to minerals properties owned by, or explored by, companies and is supervised by the Canadian Securities Administrators. The NI 43-101 covers metalliferous, precious metals and solid energy commodities as well as bulk minerals, dimension stone, precious stones and mineral sands commodities.

The NI 43-101 is broadly comparable to the Joint Ore Reserves Committee Code (JORC Code) which regulates the publication of mineral exploration reports on the Australian Stock Exchange and the South African Code for the Reporting of Mineral Resources and Mineral Reserves (SAMREC). The NI 43-101 is more prescriptive in terms of the manner in which mineral exploration reporting is presented, although the content of the technical reports, and the scientific rigors to which the mineral resource classifications within them are put, are often very similar.

Forward Looking Statements

This announcement contains certain "forward-looking statements". These statements, which sometimes use words such as "anticipate", "believe", "intend", "estimate", "expect" and words of similar meaning, reflect the Company's intention, beliefs and current expectations and involve a number of risks, uncertainties and assumptions that could cause actual results and performance to differ materially from any expected future results or performance expressed or implied by the forward-looking statement. There can be no assurance that such statements will prove to be accurate and actual results and future events could differ materially from those anticipated in such statements. Accordingly, readers should not place undue reliance on forward-looking statements.

Forward-looking statements involve known and unknown risks, future events, conditions, uncertainties and other factors which may cause the actual results, performance or achievements to be materially different from any future results, prediction, projection, forecast, performance or achievements expressed or implied by the forward-looking statements. Such factors include, amongst others, the interpretation and actual results of the specific studies undertaken by expert consultants, changes in project parameters as plans continue to be refined, future costs estimates, possible variations in grade or recovery rates, failure of equipment or processes to operate as anticipated, the failure of contracted parties to perform, labour disputes and other risks of the mining industry, delays in obtaining governmental approvals or financing or in the completion of exploration, as well as those factors disclosed in the company's publicly filed documents.

Sirius does not undertake to update any forward-looking statements that are included herein, except in accordance with applicable securities regulations.