Hypogene (geology)
In ore deposit geology, hypogene processes occur deep below the earth's surface, and tend to form deposits of primary minerals, as opposed to supergene processes that occur at or near the surface, and tend to form secondary minerals.
At great depth the pressure is high, and water can remain liquid at temperatures well above 100 °C. Hot aqueous solutions originating in the magma contain metal and other ions derived from the magma itself, and also from leaching of surrounding rocks. Hypogene deposition processes include crystallization from the hot aqueous solutions rising through the earth's crust, driven by heat provided by the magma.
Major dissolved components are chlorine, sodium, calcium, magnesium and potassium, and other important components include iron, manganese, copper, zinc, lead, sulfur (as SO42− or S2− or both) carbon (as HCO3− and CO2) and nitrogen (as NH4). Most ore fluids contain chloride as the dominant anion.
As the solutions rise the temperature and pressure fall. Eventually a point is reached where the minerals start to crystallise out. Minerals formed in this way are called primary, or hypogene, minerals. Sulfur is a common component of the fluids, and most of the common ore metals, lead, zinc, copper, silver, molybdenum and mercury, occur chiefly as sulfide and sulfosalt minerals. Examples of primary minerals formed in this way include the sulfide minerals pyrite (FeS2), galena (PbS), sphalerite (ZnS), and chalcopyrite (CuFeS2).
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