> The collision theory is one of four contested theories for the formation
> of the moon. It is a better theory, actually, than the other three,
> but its appeal has less to do with explaining crustal abundances, but
> rather, it explains the siderphillic, H2O, and voltile element depletion
> of the moon itself. This is a very involved subject (ie, the formation
> of the moon), so I will stop here. (S. R. Taylor's article in Geochimica
> Cosmochimica Acta (1987) is a excellent spot for the compulsively
> interested to continue reading on this subject.)
>
> Gold, silver, platinum, copper, and iron are considered to be depleted
> in the crust of the Earth with respect to cosmic abundances. The
> reason we see these siderphillic elements at all is because the initial
> differentiation of the Earth during its formation was an imperfect
> process. The accretion of the Earth's core didn't manage to suck up
> all of the siderophillic metals, which for us is most fortunate! :-)
Maybe I'm off-track with respect to your arguement here, but you *seem*
to be saying that "What's in the crust was already in the crust" If I'm
wrong then tell me I'm an idiot and I can't read your arguement properly.
Crustal abundances of minerals are mainly due to hydrothermal processes.
The evolution of hot water from diagenesis[sediment compaction] or
volcanic bodies (from contact with groundwater or the evolution of
their own supercritical fluids) dissolves large amounts of low
concentrations of metals. The passage of these fluids through preferred
pathways (usually fractures or weak points in the rocks) leads to
deposition from fluid reaction with the surrounding rocks (altering pH
and solubility) or temperature reduction as the fluid migrates away from
the hot body. Also, another method of forming minerals in the crust
is relatively "dry". Involving the separation of immiscible sulphides as
a mafic magma cools. So, pretty much, as long as there is volcanic activity,
there will be minerals to mine. However, with the exception of massive
layered intrusions such as the Bushvelt complex in South Africa, most of
the mineralization likely to ON THE EARTH is likely to be due to the
properties of water.
Brad.