Why are there no giant nickel porphyry deposits like those of copper, or SEDEX deposits, like those of zinc? Why is all the nickel found in direct magmatic intrusions from the mantle to the surface? Why isn't more nickel found from epithermal deposits in which water runs down to a deep seated magmatic intrusion, picks up the metals, and deposits them on the surface? Also, as I understand it, the copper found in the Andean deposits are from water going down and picking up metal from the melting continental crust sitting above the subducting oceanic crust. It's not coming from any mantle rocks. Is that correct, and if so, why don't we get nickel deposits in the Andes too?