Gossan zone

Synonym

Supergene enrichment zone, Leached zone

Required Geological Setting

Neogene and Quaternary sediments

Occurrence

The oxide-rich zone formed by oxidation and chemical dissolution of minerals by meteoric water under the oxidizing condition near the ground surface. Components dissolved at the gossan zone are transferred with descending high-pH meteoric ground waters forming supergene enrichment zones. 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. Gossan zones are formed at outcrops of metal ore deposits of Fe, Cu, and Mn, and the thickness of gossan zones is up to several hundreds meters. Sulfides and metals in the deposits are oxidized by meteoric water, and the meteoric water descends as the low-pH groundwater rich in sulfate ions and carbonate ions. For examples,

2FeS₂ (Pyrite) + 9O₂ (Oxigen in meteoric water) + 2H₂O (Meteoric water) = 2FeO(OH) (Goethite) + 4SO₄^2- + 2H⁺

4CuFeS₂ (Chalcopyrite) + 17O₂ (Oxigen in meteoric water) + 6H₂O (Meteoric water) = 4FeO(OH) (Goethite) + 4Cu²⁺ + 8SO₄^2- + 8H⁺

ZnS (Sphalerite) + 2O₂ (Oxigen in meteoric water) = Zn²⁺ + SO₄^2-

PbS (Galena) + 2O₂ (Oxigen in meteoric water) = Pb²⁺ + SO₄^2-

This oxidation reaction is enhanced by Fe and S-oxidizing bacteria. Goethite deposits by this reaction, and the meteoric water is mixed with Cu²⁺ ion, sulfate ion, and H⁺. The meteoric water may be rich in phosphate ion originated from earth soils on the ground suraface, carbonte ion from carbonate rocks such as limestone, and halogen ions from the mixed seawater.

Mineral Assemblages

• Limonite