Meteoric-type upper oxidation zone

Synonym

Supergene enrichment zone, Supergene deposit

Required Geological Setting

Neogene and Quaternary sediments

Occurrence

Minerals deposited at vadoze 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. Thickness of upper oxidation zones is up to 200-300 meters. Upper oxidation zones of sulfide deposits are mainly composed of sulfate minerals, and phosphate and/or carbonate minerals are oberved in cases phosphate ions are supplied from earth soils on the ground surface and carbonate ions from carbonate rocks such as limestone. Mixing seawater in the meteoric water results in formation of halogen mineral bearing marine-type upper oxidation zone. Upper oxidation zones of Cu deposits produce chalcanthite deposited at the condition of pH=3.5-4.0, antlerite at pH=4.0-5.0, brochantite at pH=5.0-6.0, malachite at pH=6.0-8.0, and tenorite above pH=8.0. For examples,

Cu²⁺ + SO₄^2- + 5H₂O = CuSO₄,5H₂O (Chalcanthite)

3Cu²⁺ + SO₄^2- + 4H₂O = Cu₃SO₄(OH)₄ (Antlerite) + 4H⁺

4Cu²⁺ + SO₄^2- + 6H₂O = Cu₄SO₄(OH)₆ (Brochantite) + 6H⁺

2Cu²⁺ + CO₃^2- + 2H₂O = Cu₂CO₃(OH)₂ (Malachite) + 2H⁺

Cu²⁺ + H₂O = CuO (Tenorite) + 2H⁺

Mineral Assemblages

• Chalcanthite - Antlerite
• Antlerite - Brochantite - Malachite
• Malachite - Tenorite