Sept 14 (Reuters) - Sphere Minerals Ltd SPH.AX :
* Executed agreement to grant exclusive option to Al Rawda Resources Limited to purchase an 80% interest in Sphere Mauritania SA
* Purchase price payable if option is exercised is US$10 million in cash plus a life of mine royalty of 2.5% of fob revenue
* Says US$10 million cash consideration is payable in three equal tranches
* Option is in effect for a three month period during which time al rawda will undertake detailed due diligence which may, or may not, lead to transaction
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