According to the info, the mill is capable of processing 20,000 ounces per month. That does not mean that it will do all that work. And the gold in the mine probably will not be mined in one fell swoop.
Then there's that pesky $8,000,000 need to get the mill going.
"Speaking to the cooperativistas of La Joya in his thick Texas accent, Mr. Petty pledged to transform their mud-brick village at the base of a mountain containing 13 million ounces of gold and silver into a thriving modern community."