Mineral explorer Widespread Energy says it is working with Crown Minerals on an agreement to prospect for an underwater rock phosphate resource which may be worth more than US$50 billion ($79.7 billion).

Widespread Energy and its parent company Widespread Portfolios applied in August 2007 for a prospecting licence over a 3048sq km area of the Chatham Rise, 600km east of Christchurch.

In a progress report to the NZX yesterday, Widespread said Crown Minerals had recently revised draft terms and conditions for the licence and the two parties were now working to seek agreement on the permit details.

Approval for the permit had been delayed while the Government developed guidelines for regulating marine mining and mineral exploration.

"We are most gratified by the progress that has recently been made in the licensing process as our project has, since it was first announced, attracted considerable interest from fertiliser industry participants, both local and overseas based," Widespread's Golden Bay-based director Chris Castle said.

Fletcher Challenge and other companies explored the Chatham Rise area first in the 1980s. Rapidly increasing superphosphate prices and advances in underwater mining technology meant mining submarine mineral deposits was now much more feasible, Castle said.

Widespread planned a two-year work programme that, if successful, would be followed by more detailed exploration and evaluation.

Nearly all rock phosphate being used by the New Zealand fertiliser industry was now imported from Morocco.