A definitive feasability study looks at whether a project can be completed given a range of legal, economic, technological and other factors.
"The completion of a definitive feasability study for such a large scale project is a very detailed process, and there is a large amount of complex information from various suppliers, consultants and engineering firms that needs to be compiled, reviewed and then integrated into the final DFS," it said in a statement today.
"While this process is time consuming, it is vitally important for a project with an expected life of over 100 years that it is robust."
Shares in the company fell as much as 25 per cent to 10.5p per share at the open, before recovering slightly to 12.7p in mid-morning trade.
Sirius has secured planning approval for the £1.7bn mine which will be used to produce potash, a form of fertiliser, between Whitby and Scarborough in North Yorkshire. It recently agreed an "offtake" agreement with Chinese importer and exporter Huaken International, which will help it secure funding for the project.
Chris Fraser, managing director, said: "We are continuing to build a very robust business that will be capable of strong returns throughout the many cycles it will experience."