RE:Deep dive on rock sample for Waayrah -- VMS targetI loved the bit on mass wastage as that really helped me understand the whole "supergene enrichment" thing. Ties in with the whole gold cap that can really shift economics for these kind of deposits in this area of the world...
Tim Neall: No. A conglomerate is a rock composed of coarse rounded clasts, pebbles in effect, in a fine grained sandy matrix. This rock is a product of weathering of massive sulfide mineralization. The reason the hemimorphite forms bands running through it is that you get a lot of mass wastage during gossan formation. When the sulfide oxidizes, it generates sulfuric acid. That sulfuric acid will leach out some of the components of the rock and this results in a loss of mass or mass wastage, which is why this material is less dense from the original. Mass wastage results in voids that then fill up with other secondary minerals.
Peter Bell: Are we talking about a specific gravity of 2.0 here or something?
Tim Neall: The density of the original massive sulfide would have probably been about 4, maybe a little bit higher. After weathering, it is probably reduced to around 2-2.5.
Peter Bell: That's a significant change in density of the rock. Does that affect geochemistry or mineralogy and how does it affect the exploration geology?
Tim Neall: Initially, oxidation causes a loss of soluble metals. Particularly alkali metals, zinc and copper but also some iron. Eventually you get a completely leached cap with higher gold values because gold is not soluble and remains behind. Just imagine if you lost 50% of the material in a piece of rock but kept whatever gold was present - it doubles the gold grade. The white material in the photograph is hemimorphite, a zinc silicate mineral, and this represents the concentration of zinc. This may appear to contradict what I said earlier but what actually happens is that some of the metal leached out from close to the surface is later redeposited at depth, close to the base of the oxidized zone.
Peter Bell: Simple, I like it.