I think this is based on what Victor Goncalves wrote:

The historical Sigma mine, for example, produced over 4.4 million ounces of gold, with 80% of the ounces coming from what they called the Main Plug. The Sigma mine reached the 6,100 foot level and the Lamaque mine only to the 3,400 foot level, leaving what I believe is a 3,000 foot untested zone below Lamaque mine, where an additional 4.6 million ounces was mined, totaling nearly 9 million ounces of gold.