Secondary enrichment of mineral deposits
Secondary enrichment of mineral deposits is a natural geological process that enhances the concentration of valuable minerals near the Earth's surface. This process, also referred to as downward enrichment or supergene enrichment, occurs when metals in their soluble forms are dissolved and then redeposited in a more concentrated state, thus increasing the ore-grade level of existing deposits. For secondary enrichment to take place, there must be an exposed metallic ore deposit, typically found in the oxidation zone above the groundwater table.
During this process, minerals such as pyrite undergo weathering, leading to the formation of ferric sulfate and sulfuric acid, which act as solvents to dissolve other primary sulfide minerals. As mineral-rich solutions migrate downward, they can precipitate metal oxides or carbonates in the oxidation zone. Below this zone, in the reduction zone, the dissolved metals interact with the unchanged sulfide minerals of the primary deposit, resulting in a supergene enrichment zone. A notable feature of this process is that it primarily affects deposits of metals like copper and silver due to their lower ionization potentials compared to iron.
Secondary enrichment is prevalent across many non-glaciated regions worldwide and is particularly exemplified by the supergene enrichment zones found in the porphyry copper deposits of the western United States. This natural enrichment process plays a significant role in the economics of mining and the availability of metal resources.
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Secondary enrichment of mineral deposits
Secondary enrichment is an ore-deposition process that involves the dissolution and redeposition of an ore mineral. This process concentrates the mineral, creating an enriched deposit or zone that is often more valuable than the original deposit from which the mineral was derived.
Definition
Secondary enrichment is a naturally occurring, near-surface process whereby a valuable substance is dissolved, carried downward in solution, and redeposited in a concentrated state. This ore-deposition process is also known as downward enrichment or supergene (literally, “origin from above”) enrichment. does not create a new and separate mineral deposit; rather, it concentrates and makes more valuable an existing deposit. Some low-grade mineral deposits have been made ore-grade as a result of secondary enrichment.
Overview
For secondary enrichment to occur, there must be an exposed or near-surface deposit of a metallic ore mineral, and the mineral must be present in a soluble form. Many metallic sulfides (minerals composed of sulfur and a metal or metals) combine with oxygen as they to become sulfates. The sulfates tend to dissolve more readily in water than the original sulfides. Pyrite (FeS2), an iron sulfide mineral present in most metallic mineral deposits, contributes to the dissolution process. Pyrite weathers to form ferric sulfate (2Fe2(SO4)3) and sulfuric acid (H2SO4), powerful solvents that help to break down the other primary sulfide ore minerals. and solution take place in the zone, the portion of the mineral deposit that is above the table. At the surface, a cap of ferric hydroxide often forms a residual surface deposit. Called a gossan or “iron hat,” this rusty-looking deposit may contain enough iron to be mined as an ore.
The mineral-rich solutions percolate downward through the oxidation zone. If these solutions are near saturation, they may precipitate metal oxides or carbonates above the groundwater table. Just below the groundwater table, the solutions enter an environment where oxidation cannot take place and the sulfide ore minerals of the primary deposit have remained essentially unchanged. There, in the reduction zone, these sulfide minerals interact with the dissolved metals to create what is known as a supergene enrichment zone. When a metal in solution encounters a mineral that contains a metal with a greater ionization potential (that is, one with a stronger tendency to enter into solution), the metals exchange places. Copper, for instance, has a lower ionization potential than iron; when a copper-bearing solution encounters a primary deposit of an iron sulfide mineral such as pyrite, iron begins to enter into solution while copper is deposited. The pyrite changes to chalcopyrite (CuFeS2), which in turn becomes bornite (Cu5FeS4), then covellite (CuS), and finally chalcocite (Cu2S), the most common supergene sulfide. Iron in primary sulfide deposits appears to be the controlling metal in enrichment reactions. Because copper and silver are the only common metals with lower ionization potentials than iron, supergene enrichment rarely occurs in deposits of other metals.
Secondary enrichment has occurred in most of the nonglaciated land areas of the world. The supergene enrichment zones found within the coppers of the western and southwestern United States are classic examples.
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