Thanks Gosouth, much appreciated. Yes, I found this on block...

Thanks Gosouth, much appreciated.

Yes, I found this on block caving underground at North Parkes:

http://www.ausimm.com.au/content/docs/northparkes_example_paper.pdf

The table of production grades for 2010 and 2011 in the appendices shows a head grade for those two (apparently profitable) years of about 0.8% Cu and 0.5 g/t Au, ie. about 1.1% copper equivalent or 1.8g/t gold equivalent.

That is an incredibly low grade to be making money on underground, but I am aware that with a suitably configured large orebody block caving is a significanlty cheaper bulk-tonnage underground mining method. As the paper notes, the orebody would not have been economic using conventional stoping methods.

From the limited (but very useful) geological descriptions we have had so far of the porphyry section of ICG's first drillhole it is apparent that there are brecciated sections within the porphyry that will carry significantly higher grades than the overall interval. I am sure that will be enough of a taster to encourage ongoing work.

But with regard to the predominant 1-2% total sulphide content, we will just have to wait and see what that carries. I have worked on orebodies where it is all in the breccia veins, and the finer grained 2-5% sulphide overprint in the highly altered host rocks to these breccia veins carries no grade at all.

Equally within the porphyry body itself, it's overall shape and dimensions are uncertain, and it may be a complex, of several phases of intrusion, not all of which need be mineralised, and those that are will not necessarily be homogenously mineralised. Though thinking about that, the widespread presence of gold in the breccia veins right across this property suggests that the porphry is widely mineralised at depth.