Qatadah (may Allah have mercy on him) said that Umar bin Abd al-Aziz (may Allah have mercy on him) gave mines the status of rikaz, so khums was taken from them; then he sent another letter after it and prescribed zakah in it.
It is narrated from Abdullah bin Abi Bakr (may Allah have mercy on him) that Umar bin Abd al-Aziz (may Allah have mercy on him) took five dirhams out of every two hundred dirhams from mines. Abu al-Zinad (may Allah have mercy on him) said that Umar bin Abd al-Aziz (may Allah have mercy on him) prescribed a quarter of a tenth in mines, if it is not buried treasure; and if it is buried treasure, then there is khums in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7638