Distinct zones of supergene processes can be identified at various depths. From the surface down they are the gossan cap, leached zone, oxidized zone, water table, enriched zone (supergene enriched zone) and primary zone (hypogene zone).
Gossan cap Pyrite (FeS2) has oxidised to form
goethite (FeO(OH)) and
limonite (FeO(OH)·
nH2O), Prospectors use gossan as an indication of ore reserves.
Leached zone Groundwater contains dissolved
oxygen and
carbon dioxide. As it travels downwards it oxidizes primary
sulfide minerals, concomitant with forming
sulfuric acid and
solutions of oxidized metals. For example, groundwater commonly interacts with pyrite (FeS2) to form an oxidized iron (FeO(OH)) and sulfuric acid (H2SO4), portrayed in the idealized chemical reaction below (intermediate steps omitted): : An intermediate in this process is
ferric sulfate (Fe2(SO4)3), which oxidizes pyrite and other sulfide minerals.
Oxidized zone Above the water table the environment is
oxidizing, and below it is
reducing. Solutions traveling downward from the leached zone react with other
primary minerals in the oxidised zone to form secondary minerals such as
sulfates and
carbonates, and
limonite, which is a characteristic product in all oxidised zones. In the formation of secondary carbonates, primary
sulfide minerals generally are first converted to sulfates, which in turn react with primary carbonates such as
calcite (CaCO3),
dolomite (CaMg(CO3)2) or
aragonite (also CaCO3,
polymorphic with calcite) to produce secondary carbonates. Soluble salts continue on down, but insoluble salts are left behind in the oxidised zone where they form. Examples of insoluble salts that are commonly found in the oxidized zone include
lead precipitates like
anglesite (PbSO4) and
pyromorphite (Pb5(PO4)3Cl);
copper precipitates like
malachite (Cu2(CO3(OH)2),
azurite (Cu3(CO3)2(OH)2), and
cuprite (Cu2O); and
smithsonite (ZnCO3).
Water table At the
water table the environment changes from an
oxidizing environment to a
reducing one.
Enriched zone Copper ions that move down into this reducing environment form a zone of supergene
sulfide enrichment.
Covellite (CuS),
chalcocite (Cu2S) and
native copper (Cu) are stable in these conditions and they are characteristic of the enriched zone. The net effect of these supergene processes is to move metal ions from the leached zone to the enriched zone, increasing the concentration there to levels higher than in the unmodified primary zone below, possibly producing a deposit worth mining.
Primary zone The primary zone contains unaltered
primary minerals. == Mineral alterations ==