As an Addendum to my earlier post of Saturday, June 19, 2010 8:34:13 PM
I have again “Borrowed” some “Officialistic” explanations to explain this sequences of events properly. I think that it’s a very important part in doing researches (especially on geological maps) to find the “Better” areas that bore the best chances of releasing some of the “Larger” Gold pieces into the surrounding environment for you to find, and so increase your “Luck”.
Metamorphic grades.
In the Barrovian sequence (described by George Barrow in zones of progressive metamorphism in Scotland), metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals in rocks of “Shaly”, aluminous origin:
Low grade ------------------- Intermediate --------------------- High grade
Greenschist ------------- Amphibolite ----------------------- Granulite
Slate --- Phyllite ---- Schist --------- Gneiss -----------------------Migmatite
(partial melting) >>>-----------melting>>>-------------------------------melted.
Chlorite zone.
----------------Biotite zone.
----------------------------Garnet zone.
----------------------------------------Staurolite zone.
--------------------------------------------------------Kyanite zone.
----------------------------------------------------------------------Sillimanite zone.
Hydrothermal Metamorphism.
(This process produces mainly “Bonanza” or High grade smaller gold ore vein systems.)
Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions.
The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of water in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.
Epithermal Metamorphism.
(This process produces mainly “Bullion” Gold/Silver type vein systems.)
Epithermal Au /Ag deposits form in the near surface environment, from hydrothermal systems typically within 1.5 km of the Earth’s surface.They are commonly found associated with centres of magmatism and volcanism, but also form in Shallow marine settings.
Hot spring deposits, and both liquid and vapour dominated geothermal systems are commonly associated with epithermal deposits.
Epithermal Au deposits are commonly considered to comprise one of three subtypes:
high sulphidation, intermediate sulphidation, and low sulphidation, each denoted by characteristic alteration of mineral assemblages, occurrences, textures, and, in some cases, characteristic suites of associated geochemical elements (e.g. Hg, Sb, As, and Tl). Base metal (Cu, Pb, and Zn) and sulphide minerals may also occur in addition to pyrite and native Au or electrum. In some epithermal deposits, notably those of the intermediate-sulphidation subtype, base metal sulphides may comprise a significant ore constituent.
Cheers---Colin.
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