Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.048.

That zakah of rikaz and ma’din is obligatory is shown by a statement attributed to Abu Hurairah: “The Prophet, upon whom be peace, said: ‘There is no compensation for one killed or wounded by an animal, falling in a well, or because of working in mines; but, one-fifth (khums) is compulsory on rikaz.'” Ibn al-Munzhir confesses that he does not know anyone who contradicted this hadith except al-Hasan, who differentiates between what exists in the land of war and the Islamic land. The latter holds that if rikaz is found in the land of war, one-fifth (khums) is due, but if it is found in the Islamic land, it will be subject to the regular zakah.

Explaining it, Ibn al-Qayyim says that there are two interpretations of this statement:

The first interpretation is that whenever someone hires someone else to dig a mine for him and then he falls into it and is killed, there is no compensation for him. This view is supported by the Prophet’s saying: “There is no compensation for one who falls into a well or who is killed by an animal – (al-bi’r jubar, wa al-‘ajma’ jubar).”

The second interpretation is that there is no zakah on minerals. This view is supported by the Prophet’s saying: “…but one-fifth is compuslory on treasure – (wa fi az-zakah al-khums).” Thus, he differentiated between mineral (ma’din) and treasure (rikaz). He made zakah on rikaz compulsory because it is a wealth obtained without any cost or effort. He exempted minerals (ma’din) from zakah because they require both cost and effort for their mining.