Actually, the grades they are including in the resource estimates are probably less than or similar to the grades of the material they have been using as input to the mill over the last quarters. So by taking these grades they accurately reflect what resources they have to let the mill run economically at today's metal prices.
For a 1300 t/d oxide sulphide plant they would need less than .5 mln tonnes per year, so this plant can run the first 5 years on the M+I. However the potential resources are up to 10 times as high, which could provide high grade material for another 50 years.
All this doesn't include the byproducts zinc and lead and assumes gold and silver prices remain as low as they are. When gold and silver prices increase in proportion to the cost of extraction, they could lower the grades and this would increase the available reources.
From the perspective of financial feasibility this is a good approach in my opinion.
