I think Nyrstar wont be impressed by the in-situ value of IBG,...

I think Nyrstar wont be impressed by the in-situ value of IBG, which is clearly competitive compared to peers. In order to achieve their medium term goal to get up to a ratio of 50% mining, 50% smelting, they have specified potential takeover candidates:

"We want mines that will lower our cash costs over all in zinc. We currently have a target in mining to be at or below $1,000 per tonne and any mine that will help us lower that cost significantly will be welcomed."

http://www.mineweb.com/mineweb/view/mineweb/en/page36?oid=121467&sn=Detail&pid=102055

The one million dollar question is, can IBG meet this expectation? Very likely that IBG is aware of Nyrstars requirements and this is why the BFS is delayed.