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Photo by Francois Jacamon, 2005

Surficial and supergene ore-forming processes. Photo by Francois Jacamon, 2005. Photo by Francois Jacamon, 2005. Gold-panning (orpaillage) is an ecological catastrophe for the Amazonian rainforest. Abandoned gold-panning site in the middle of the rain forest.

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Photo by Francois Jacamon, 2005

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  1. Surficial and supergene ore-forming processes Photo by Francois Jacamon, 2005

  2. Photo by Francois Jacamon, 2005

  3. Gold-panning (orpaillage) is an ecological catastrophe for the Amazonian rainforest Abandoned gold-panning site in the middle of the rain forest Million of hectares of virgin forest disappear every year due to intense deforestation and gold mining activities

  4. Laterite deposits • Forming in tropical climates • Rich in kaolitnite clay + Fe-Al oxides • Al-ore “Bauixite” enrichmentm Ni, Cu, Fe, Mn, Au PGE • Stable cratons >100 Ma to develop • up to 150 m of regolith • Eluviation profile removal of clay and solutes • Illuviation profile acc. of matter • Pedolite u. part no protholith tex. Preserved (Al) • Saprolite l. part some protholith tex. Preserved (Al) • Saprock l.m. part <20 % weat. (Au, Cu, Ni, PGE(?))

  5. Ore forming principles during lateritization • Dissolution / hydration of primary minerals • Oxidation due to meteoric water infiltration • Hydrolysis i.e. H+-metasomatism

  6. Hydrolysis and acid hydrolysis • Breakdown of aluminosilicates (feldspar, quartz, pyroxens etc) (acid hydrolysis) • Si4+ + 4H2O = H4SiO4 + 4H+ • Other minerals are reprecipitating as a result of hydrolysis e.g. • Al3+ + 3H2O = Al(OH)3 + 3H+ • Gibbsite

  7. 25 oC

  8. Bauxite formation Gibbsite Al(OH)3 Boehmite gAlO.OH Diaspore aAlO.OH

  9. Bauxite formation Feldspar KAlSi3O8 Si-loss Al2Si2O5(OH)4 Kaolinite Si-loss Gibbsite Al(OH)3

  10. Ni deposits in laterite profiles • Ultramafic protholith • Peridotite >40 mole% Ol • Decomposition of Olivine releases Ni-ions • Reconcentration in smectite clay, talc, serpentine by cation exchange rections forming “Garnierites”

  11. Cementation front weathering front Gold in laterites Percolating meteoritic fluids (dissolution of Au, Ag) (Au, Ag) AgCl, AgCl2-, AgCl32- (Water plan) Low pH High Eh AuCl4- Recrystallisation and concentration of isolated Au or associated with goethite in the pedolith (Pedolith) Au, FeOOH (Saprolith) Fe2+ neutral pH low Eh Mafic bedrock

  12. Au-PGE deposits in laterite profiles • feldspar-quartz rich /Acid protholith. (but also mafic rocks) • Relatively arid climates (but not desert either) • Acid and strongly oxidizing topsoil (little or no organic matter) • Chemical barrier maybe at ground water table. Neutral pH, more reducing

  13. Clay

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