It seems to be report on mining from a specific area in the Andes of South America. It's one of the region's riches which is constituted, doubtlessly, with sulphur fields and deposits, of which, according to comprehension, reserves have more than 50 million tonnes, an estimated and not exaggerated quantity.

A mine is a resource. It's an asset on the books of the owning entity. My take is that there needs to always be some periodic accounting of the value of the remaining useful life of the mine. Over time, the reserves in a mine are depleted. The LOA report is a report indicating the (remaining) life of (any) asset. The LOA report can be used to support the valuation of the asset on the owning entity's books. The LOA report and the name mine life (remaining life of the mine) would, specifically, represent the current valuation of the undepleted mine reserves, the remaining mine life.

(I have a background in managerial accounting in the industrial manufacturing industry, not specifically in minerals or metal mining. I kind of backed into this using my general experience working in accounting.)

Reference page 40 of the below-linked document.