RE:stonnage shrinkage
The shrinking tonnage is related to the economics of mining.
When you start to develop the mine, you make a decision on how far down you are going to go. That fixes the size of the excavation at the top.
For example, if you have an ore body that is 100 metres across, and you decide to mine to the 200 metre level, you will start with a 500 metre wide excavation (2 x 200 metres for sloping the sides back at 45 degrees plus 100 metre ore body).
In ZEN's case that excavation would be 500 metres wide and 40 metres deep before they hit ore, because the top of the ore body is 40 metres below ground. The cost of that excavation is mine development cost, and is part of the CAPEX.
If they plan the mine to be 300 metres deep, then the width of the hole at the top becomes 700 metres, and the amount of material to pull out for the mine development is almost doubled, adding a big cost to the CAPEX.
The pit optimisation software evaluates various scenarios and comes up with the best economics, usually by maximising the NPV. The 660,000 tonnes is the best case scenario, go deeper than that and the extra cost of mine development and mining outweighs the extra benefit.