Upgraded Mineral Resource Estimate

7 May 2013

Sirius Minerals Plc

Increase and upgrade of York Potash Mineral Resource

·; Increase of Total Mineral Resource to 2.66 billion tonnes at an average grade of 85.7% polyhalite within an area representing 7% of the York Potash Area of Interest.

·; Upgrade of 820 million tonnes of polyhalite at an average grade of 87.3% to Indicated Category

·; Upgrade to Indicated Resource provides basis for a Reserve estimate expected later in the year

The Directors of Sirius Minerals Plc (AIM: SXX, OTCQX: SRUXY) ("Sirius" or the "Company"), are pleased to announce an updated Mineral Resource for the York Potash Project. The updated York Potash Mineral Resource comprises an Indicated Mineral Resource of 820 million tonnes of polyhalite at an average grade of 87.3%; an Inferred Mineral Resource of 1.84 billion tonnes at an average grade of 85.1% and a total Mineral Resource of 2.66 billion tonnes at an average grade of 85.7% polyhalite.

The updated JORC compliant Mineral Resource has been prepared by independent consultants SRK Consulting (UK) Ltd ("SRK") and reflects the completion of the SM11 borehole, deflections SM11A and SM11B and analysis of the chemical assays from SM11 that were provided by the British Geological Survey confirming reliability of the wireline data. In total the Company has now completed over 16,000m of drilling and this, along with information from additional historical holes, has been used by SRK to derive the estimates presented here.

Chris Fraser, Managing Director and CEO of Sirius said:

"The scale and quality of the York Potash deposit has been known about for some time, but the upgrade to an Indicated status is another key milestone for the Project and one that will give further comfort in the strength of the Project and this globally significant deposit."

Mineral Resource

The Mineral Resource is summarised in the table below and has been reported using the guidelines and definitions set out in the 2004 edition of the JORC Code. The upgrading of a substantial part of the Resource to the Indicated category is based on the updating of the geological model following the successful completion of SM11 and the deflections 11A and 11B which have helped demonstrate the extent, quality and importantly the continuity of the polyhalite mineralisation over mineable distances.

The consideration that this mineralisation is potentially economic is supported by the results of the Project Study Update released on 3 December 2012 which demonstrated the viability of the construction of the York Potash Project and the operational parameters.

The York Potash Project is contained within the Permian Zechstein evaporites, which have been mined extensively for potash and halite in both the UK and north-western Europe. SRK's Mineral Resource Estimate occurs as two main polyhalite horizons, an upper Shelf Seam and a lower Basin Seam. 

SRK created a geological model for the polyhalite horizons based on the hangingwall and footwall contacts of these horizons as delineated in drilling supported by drillhole logging information, wireline gamma data and the results of Quantitative X-Ray Defraction (XRD) analysis from which the polyhalite grades were derived. The analytical data was then composited into 1.5 metre composites and subjected to a statistical and geostatistical analysis following which Ordinary Kriging was selected as the main grade interpolation method. The resulting interpolated grades were validated through visual checks and a comparison of the mean input composite and output model grades. SRK is confident that the interpolated block grades are a reasonable reflection of the available sample data.

Mineral Tenure

Mineral Rights in the United Kingdom generally belong to either the present freehold surface owner or a previous owner who may have reserved them. In order to extract the minerals, permission is required from the mineral owner usually under the terms of a lease. Sirius is in the process of entering into agreements with respective landowners and has already reached agreement on some 88% of the Mineral Resource outlined above and has no reason to expect that this process will not result in all the required agreements being made in due course.

Drilling Update

Rig 16 was moved from the South Shaft Site (SM11) to York Potash's final planned drilling site on 21 April 2013 and drilling operations commenced at the North Shaft Pilot Hole (SM14) on 25 April 2013. The top casing has been successfully installed to protect near surface aquifers and coring of the sedimentary overburden from 250-1,240mbRT is now underway. The core extracted will be analysed and the data collected will assist with shaft design.

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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). 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 information in this press release that relates to Mineral Resources is based on information compiled under the direction of Dr Mike Armitage C Eng, who is a Member of the Institute of Materials, Minerals and Mining which is a 'Recognised Overseas Professional Organisation' (ROPO) included in a list promulgated by ASX from time to time.

Dr Armitage is a full time employee of SRK and has sufficient experience which is relevant to the style of mineralisation and type of deposit under consideration and to the activity which he has undertaken to qualify as a Competent Person as defined in the 2004 Edition of the 'Australasian Code for Reporting of Exploration Results, Mineral Resources and Ore Reserves' (the JORC Code) and for the purposes of the AIM Rules. Dr Armitage has reviewed this press release and consents to the inclusion in the press release of the matters based on his information in the form and context in which this appears.

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, is a Fellow of the Institute of Materials, Minerals and Mining (IMMM) 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.