Deduction for prospecting expenses allows phased tax relief limited to income from commercial exploitation. A tax deduction is allowed for qualifying expenditure on prospecting, extraction or development of specified minerals, disbursed during the year of commercial production and up to four preceding years; the deduction is granted as annual instalments equal to one tenth of such expenditure across ten relevant previous years, limited to income from commercial exploitation, with carry forward of unallowed instalments (subject to a ten year cap).