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G lk superior where vast iron resources were used to

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Unformatted text preview: during crystallization of a magma deep underground  ­the minerals can be separated from the host rock and concentrated into faults which is why you’ll often see veins of these types of minerals  ­can also occur as massive deposits where typically the minerals are separated through gravitational settling and other processes 3) precipitation from lake or sea water to form sedimentary mineral deposits a) Evaporite deposits ­minerals are carried in solution in water and then evaporated over time  ­salts, borax, gypsum, potassium b) Iron deposits (actually iron ­oxides):  ­e.g. Lk. Superior where vast iron resources were used to make steel for generations  ­ these deposits are strange in that the modern amount of iron in seawater is actually quite small and the total amount of oxygen locked up in these deposits is ~20x the volume present in the modern atmosphere Q: how could you get such rich deposits of iron?  ­were formed 2 billion years ago when atmos. was not oxygen rich and so iron did not oxidize at the surface but instead entered the oceans where it combined with O2 created during photosynthesis from algae producing iron ­rich deposits  ­eventually iron content of ocean would grow and kill off algae and you develop a layer of iron poor silica deposits on the ocean floor  ­algae eventually re ­establishes and a new iron ­rich layer deposits 4) flowing streams drop out minerals along the shore and bed to form placer deposits  ­differences in density cause minerals to be sorted and deposited in flowing water  ­these minerals are not formed in situ (in the stream) but eroded out of the host rock and deposited in the stream Q: what is the most famous placer mineral?  ­gold! but can also find platinum, zircon...
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