First published by Money Metals here: https://www.moneymetals.com/news/2018/01/30/worlds-largest-silver-mines-001399

The world’s two largest silver mines have seen their productivity decline substantially due to falling ore grades and rising costs. Gone are the days when silver mines could produce silver at 15-20 ounces per ton. Today, the Primary Silver Mining Industry is likely producing silver at an average yield of 4-5 ounces per ton.