Hi HappyIn response to your question, as you say the Cobar...

Hi Happy
In response to your question, as you say the Cobar deposits typically have a great vertical extent i.e. they go deep underground, sometimes to 1000+ metres. Whilst this is great (and often results in the orebodies being very valuable/profitable), it means that the process of defining the ore body is progressive - the mining company typically will initially define a commercial resource, start mining and then progressively drill deeper from platforms as the mine goes deeper - they do it this way as attempting to define the down hole limits of the resource at the start would be prohibitively expensive - drilling deep holes is not cheap! What this means of course is that the process of finding the ore is ongoing - on day one of mining no one knows exactly how deep it goes and how long the mine will operate, and fingers crossed they just keep finding ore as they mine deeper and deeper and they just keep finding ore/mining. This is quite different to shallow orebodies where they can define the limits of the orebody quite accurately and cost effectively before mining starts. Disclaimer - I'm not a driller or a miner however this is my understanding of the process.