Nice observations
@Mr Kodal ! I guess the most important thing is to put into context the history of ***ong in a timeline. For instance, with Au priced at $35 in the 1930s and 40s, mining would have been very profitable (probably to the Japanese overlords at that time). By the 1960s, the $35 Au (pegged) price would have stressed the profitability of most mines and " high grading " would have been done to keep the mine operating.
Therefore, it's safe to say that there'll be plenty of < 6 g/t ore left. There'll probably be plenty of higher grade ore left too. There's no way that these very inefficient miners, in a third world country, would have extracted all of the Au. They probably extracted several million ounces though. There'll be millions of mineable ounces left and that's what has got Colin and Charles so excited. A similar deposit in Jiadong Peninsula China just reported 18MOz reserves, for instance.
Often the deeper you mine, the more chance the ore has to be baked (more chance of sulphides and less chance of oxides). However, I think there'll be a few years of cherry picking easy and high grade ore bodies before we come across technical problems. BMV and SAU are small companies too, so there's only really upside and very little on the expectations side of things.
