Muwatta Malik: Book 17, Hadith 9

Malik said, “In my opinion, and Allah knows best, nothing is taken from what comes out of mines until what comes out of them reaches a value of twenty gold dinars or two hundred silver dirhams. When it reaches that amount there is zakat to pay on it where it is on the spot. Zakat is levied on anything over that, according to how much of it there is as long as there continues to be a supply from the mine. If the vein runs out, and then after a while more becomes obtainable, the new supply is dealt with in the same way as the first, and payment of zakat on it is begun on it as it was begun on the first. Malik said, “Mines are dealt with like crops, and the same procedure is applied to both. Zakat is deducted from what comes out of a mine on the day it comes out, without waiting for a year, just as a tenth is taken from a crop at the time it is harvested, without waiting for a year to elapse over it.”