Again, I believe my assumptions are quite conservative.  For...

Again, I believe my assumptions are quite conservative. 

For starters, I use 1.5% Pb+Zn cut-off because historic holes weren't assayed for precious metals and I don't have a sense for what the barite concentrate may be worth.  If instead I were to use 1.5% ZnEq cut-off (inclusive of precious metals, copper and barite) you will see an expanded mineralized envelop and increased tonnage.

Also, I use very conservative lateral width assumptions.  If instead I use 75m lateral width (hard to do because not enough infill holes to get a sense of lateral continuity vs very strong continuity along strike), I get 6.75Mt.

400m strike * 75m width * 60m depth * 3.75t/m^3 = 6.75Mt.

Finally, my understanding is that the block model may incorporate separate cut-offs: one for the near-surface, open-pittable portion and the other for the under-ground portion.