Gold is concentrated in certain geothermal solution formed when magma is injected into water bearing rocks. The magmatic heat sets up hydrothermal circulation. These hot fluids dissolve minerals and rocks and the solutions become metal rich. If certain conditions prevail, the solutions will preferentially transport gold in the aqueous state (as a solution), as a chloride complex for example. The decompression and cooling of this gold rich fluid results in the precipitation of native gold.
Therefor gold is usually found in metamorphic rocks that have been affected by hydrothermal solutions.
Finally I may add that in addition to its presence in metamorphic rocks, sometimes gold gets weathered out. Then, because of its large density gold gets transported by streams and concentrated into what are called placer deposits. These are fluvial sediments that occur along the banks of streams and rivers.
These are very good questions and if you want to learn more you should look up the periodic chart and learn more about metallic bonding, covalent bonding and ionic bonding between atoms. also in an economic Geology book you may want to look up gold deposits and learn more about where these deposits occur.
These are very valuable questions, especially for jewelers!Gold usually forms during volcanic activity. The magma chamber (where hot lava comes near the surface) below a volcano serves as a source of hydrothermal solutions (water heated by the hot earth that carries elements and minerals). These solutions travel through mineralized tunnels through rock up to the surface of the earth. When the water cools near the surface, minerals, like gold, crystallize.
(This is similar to when you try and stir sugar into ice cold lemonade.
You have to stir harder than if the lemonade is warm, right? The sugar remains in crystal form because the liquid is too cold to dissolve it.)
Gold can be found in volcanic regions like plate boundaries. They are also moved around the earth by plate tectonics, folding, subduction, uplift, etc. and found in sedimentary mineral deposits called Placer deposits. A Placer is a deposit of heavy minerals concentrated by mechanical means. A good example of this is in rivers and streams. Water, the mechanical mean, carries lots of sediment down the river channel. Gold drops out of the sediment load when the water slows down (around a bend or when the slope of the riverbed decreases)
because it is so heavy. The Placer deposit forms where all of the gold drops out. Gold is usually found in rivers and streams because water is the most efficient/fastest way of moving rocks and minerals.
Because gold is typically deposited by moving water, which carries many other different types of rock, it's not always found near a particular type of rock.
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