Secondary sulfide zone

Synonym

Meteoric supergene, Meteoric secondary enrichment

Required Geological Setting

Neogene and Quaternary sediments

Occurrence

Minerals deposited at saturated zones below 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 secondary sulfide 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 secondary sulfide zones is 10-100 meters. The upper part of secondary sulfide zones at all ranges of pH is composed of chalcocite, the middle part at the condition of pH=0-8.0 is composed of covellite. The lower part of secondary sulfide zones, the lowest part of secondary enrichment zones, is composed of secondary bornite, chalcopyrite, and chalcocite. For example,

2Cu²⁺ + HS^- = Cu₂S (Chalcocite) + H⁺

Mineral Assemblages

• Chalcocite, Covellite, Bornite, Chalcopyrite