komatite for dummies..., page-8

One more for the road...

Basal Contact

If you've read a lot of announcements about nickel exploration you've probably noticed that this term 'basal contact' comes up frequently. I thought it might worth going into a bit more detail about what it is. It's time for a confession. I said something in my original post that wasn't strictly true. I wanted to keep it simple at first but now that we've covered the basics I'll set the record straight.

The part I'm referring to is the analogy of a glass of water mixed up with dirt. It's true that the elements contained in the original magma are relatively evenly distributed. But not so much for the additional material that comes from melting the walls of the host rock as the magma flows through it. Rather than being free flowing like water the magmatic intrusion is relatively thick and viscous, a bit more like cold honey. For that and a couple of other reasons, as the flow proceeds through the host rock channel and picks up material (and hopefully sulfur) it's not mixed in evenly. You will see the host rock material in greater concentrations near where the magma contacts the host rock. This is what they are referring to when they talk about 'basal contact'. The part about nickel sulfides settling to the bottom is still quite correct. It's a slow process and time gives it a chance. But gravity only goes in one direction so there's no force at play to send other elements up apart from buoyancy.

A basal contact is at the base when the komatite is formed but given that a lot can happen in a few hundred million years that may not be where it is now. Geological events like shearing and folding can cause the basal zone to shift in relation to the main komatite body so you need a way to find it. Magnetic surveys are good for finding the komatite itself (which is what BHP used to target their holes) but are no help in finding the basal contact zone (Which is why it would have been blind luck if BHP had hit a sulfide zone).

That's where EM conductance comes in. Ni sulfide deposits happen to be good electrical conductors compared to most rocks. So if we know we've got high MgO komatite, and we have a sulfidated host rock and we find an EM anomoly nearby that's where we stick our hole.

Review

If you learned something from these last few posts can I make a suggestion. Armed with some new knowledge go back and read SGQ's announcement from 16th Aug "St George Identifies New Nickel Prospect". Then read it again and keep on reading it until you have one of those 'ahah' moments.

http://www.asx.com.au/asxpdf/20120816/pdf/4281x8sfzjq38k.pdf

On page 3 there's an excellent couple of paragraphs under the heading "NICKEL MINERALISATION AND EXPLORATION" that will give further background. But read the main body of the announcement describing the characteristics of the prospect.

This is an example of an advanced prospect where you'd expect to intercept primary mineralization.

What BHP were doing was nothing of the sort. They were confirming that the prospectivity exists over a wide area. They were using magnetic surveys which as I pointed out above will not point them to the basal contact zone and I'm pretty sure if I know that they knew it and a whole lot more.