Because of this saying of the Prophet (peace and blessings be upon him): "The mine (the blood of one who falls into it and dies) is without liability, and in rikaz (buried treasure) there is one-fifth (khums)."
So he (peace and blessings be upon him) distinguished between the two in what he mentioned, and he attributed the khums (one-fifth) to rikaz.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / Q7636
Rabi'ah bin Abi Abd al-Rahman (may Allah have mercy on him) narrates from more than one of their scholars that the Prophet (peace and blessings be upon him) granted Bilal bin al-Harith al-Muzani (may Allah be pleased with him) the mines of al-Qabaliyyah, which are in the direction of al-Fur'; and nothing but zakah is taken from those mines to this day.
Imam al-Shafi'i (may Allah have mercy on him) said: This is not among what the people of hadith regard as established; and even if they established it, it would contain no narration from the Prophet (peace and blessings be upon him) except his granting of them. As for zakah in mines rather than khums, that is not narrated from the Prophet (peace and blessings be upon him) on this matter. The Shaykh (may Allah have mercy on him) said: It is as al-Shafi'i said regarding the narration of Malik; and it has also been narrated from Abd al-Aziz al-Darawardi from Rabi'ah with a connected (mawsul) chain.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7636
Al-Harith bin Bilal bin al-Harith (may Allah be pleased with him) narrates from his father that the Messenger of Allah (peace and blessings be upon him) collected zakah from the mines of al-Qabaliyyah, and he (peace and blessings be upon him) granted the area of al-Aqiq to Bilal bin al-Harith (may Allah be pleased with him). When (the time of) Umar bin al-Khattab (may Allah be pleased with him) came, he said to Bilal (may Allah be pleased with him): The Messenger of Allah (peace and blessings be upon him) granted you this plot only so that you would work it. Then Umar bin al-Khattab (may Allah be pleased with him) distributed the plot of al-Aqiq among the people.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7637
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