Gold has different host minerals:
•Au element
-native Au
-electrum
•Au tellurides
–Calaverite:
–Sylvanite:
–Nagyagite:
•In sulphide ore types they can be subdivided broadly into pyritic (pyrite±pyrrhotite) and arsenical types (pyrite+arsenopyrite±pyrrhotite).
•Arsenopyrite
–As-deficient
–contains high average concentrations of submicroscopic gold
–but doesnot contain inclusions of particulate gold.
•Arsenopyrite is coarser grained,
–less As-deficient to slightly As-rich
–contains low or negligible concentrations of submicroscopic gold,
–but contains inclusionsand fracture fillings of particulate gold.
•Pyrite also contains moderately high levels of submicroscopic gold the concentration of which is directly related to As content .
HGD
If gold is inside quartz, it will be visible (V.G.).
Usually, gold is in microscopic grains inside pyrite. The higher percentage of pyrite the higher the grade of gold.
This does not mean that every time you see pyrite it contains gold; only in gold "areas".
Not only within but also in their interstices, voids, and along cracks intersecting pyrite, quartz, and adularia. This is the case of the gold epithermal, low-sulfidation deposits.
Under the influence of various factors, rocks in which magma once solidified may eventually collapse. With constant exposure to winds, temperature fluctuations, precipitation or groundwater movement, precious metals "move away" from the indigenous deposit, settling in lowlands, crevices and other areas.
This is how placer deposits are formed. They, in turn, are divided into several types – by the nature of education.
Residual deposits (or near-demolition placers) are formed near the site of destruction of the bedrock of the indigenous deposit. Alluvial placers are those pieces of gold that were carried by a river stream to another place (most often they accumulate in river valleys).
Gold placers in old riverbeds can be formed from both near-drift placers and ancient alluvial deposits. They can be located anywhere, even far from existing rivers, as they can form long before their appearance.
I think they are but a lot is also associated to the magamatic intrusive pegmatites were they are disseminated in the country rocks also. Sometimes the alluvial gold is so much enriched in the stream beds and rigoliths that they form an important economic deposits.
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