Lower oxidation zone

Synonym

Meteoric supergene, Meteoric secondary enrichment

Required Geological Setting

Neogene and Quaternary sediments

Occurrence

Minerals deposited at capillary fringe zones above water tables by reduction reactions from components in the high-pH meteoric ground water produced at the Gossan zone near the ground surface. Mineral species depends on reductive conditions of enclosing rock types and pH of ground waters at the deposition sites. The deeper depositon sites are, the more reductive. Thus mineral species associations depend on the depth of deposition sides. Supergene enrichment zones are composed of following zones in descending order: 1) Gossan zone, 2) Upper oxidation zone, 3) Lower oxidation zone, and 4) Secondary sulfide zone. The unaltered 5) Primary ore zone usually exists below the Secondary sulfide zone. The upper oxidation zone may distribute at laterally distant places from the Gossan zone as dissolved components in the meteoric water are transferred by ground water. Lower oxidation zones of sulfide deposits are mainly composed of oxide minerals and native elemets. Lower oxidation zones of Cu deposits in capillary fringe zones produce cuprite deposited above pH=3.0, and those on water tables produce copper above pH2.0. For examples,

Cu²⁺ + 2H₂O = Cu₂O (Cuprite) + 4H⁺

Mineral Assemblages

• Cuprite - Copper