Primary and Secondary, Hypogene and Supergene Minerals

Minerals are primary when they form directly by crystallisation from molten magma, precipitation from magmatic fluids or sublimation from gases. They are secondary when they form from alteration of primary rocks, by weathering, metamorphic or hydrothermal processes. Whether they are primary or secondary is determined by their formation, not by their composition, and the same mineral species may occur sometimes as a primary mineral and sometimes as a secondary mineral.

The terms hypogene and supergene are closely associated with, but not synonymous with, primary and secondary. Hypogene processes are processes that occur deep within the earth, and supergene processes occur near the earth’s surface. Hypogene processes tend to form deposits of primary minerals, from ascending fluids, and supergene processes tend to form secondary minerals from descending fluids.

Ascending hot aqueous solutions originating in the magma contain ions derived from the magma itself, and also from leaching of surrounding rocks. 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 primary minerals. Sulphur is a common component of the fluids, and most of the common ore metals, lead, zinc, copper, silver, molybdenum and mercury, occur chiefly as sulphide and sulphosalt minerals. Examples of primary minerals formed in this way include pyrite, galena, sphalerite and chalcopyrite (Wiki).

Processes due to circulation of meteoric water (water derived from snow and rain) are supergene processes. The descending meteoric waters oxidise the primary sulphide ore minerals and leach metals from the oxidised ore. As the fluids descend the dissolved substances may eventually precipitate to form two zones of secondary minerals, one above and the other below the water table. Conditions above the water table are usually oxidising. Secondary minerals that are stable under these conditions include malachite, azurite, cuprite, pyromorphite, and smithsonite. Beneath the water table conditions are usually reducing, and secondary minerals that form here are dominantly sulphides, such as covellite and chalcocite. Even native metals such as copper may precipitate in this environment as secondary minerals (R&M 78-419). This region is called the zone of supergene enrichment, because the processes here produce secondary sulphides making the metal content of the rock higher than that of the primary ore.

All these processes take place at essentially atmospheric conditions, 25°C and atmospheric pressure (Wiki).

Below the zone of supergene enrichment the original primary minerals remain unchanged.

In order for supergene enrichment to occur the ore deposit must contain iron sulphides and metals such as copper and silver that can undergo enrichment. The deposit must be permeable to permit percolation of the mineral solutions, and the oxidised zone cannot contain carbonate rocks and other precipitants that hinder the formation of soluble sulphates. The supergene deposits can form only where oxygen is excluded, as it is below the water table.

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