Supergene Enrichment of Porphyry Copper Deposits
Definition from Evans, 1993: “leaching of valuable elements from the upper parts of
mineral deposits and their precipitation at depth to produce higher concentrations.”
Definition from Guilbert and Park, 1985: Supergene enrichment occurs when oxidizing
acids dissolve metal ions from the “protore” and redeposits it in more reducing, basic
areas, i.e. below the water table. This results in an oxidized zone on top (gossan), a
supergene zone beneath and the hypogene (protore) beneath that.
For this to happen, the rock (starting material) needs to be porous, permeable and have a
large source of pyrite.
Why do we need pyrite?
2 FeS2 + 7O2+ 2H2O 2FeSO4 (aq) + 2H2SO4
2FeSO4 (aq) + H2SO4 + 0.5 O2 Fe2(SO4)3 aq H2O
2 FeS2 + 7.5 O2+ 4H2O Fe2O3 +4H2SO4
2 Fe+2 (aq) + ½ O2 + 2H2O Fe2O3 +4H+
2 CuFeS2 + 8.5 O2 +2H2O Fe2O3 +2Cu+2 + 4 SO4-2 + 4H+
2 CuFeS2 + 8 Fe2(SO4)3 +8H2O CuSO4 + 17 FeSO4 + 8 H2SO4
During the precipitation phase, the pyrite is again important, because Cu replaces Fe. In
5 FeS2 + 14 Cu +2 + 14 SO4-2 +12 H2O 7 Cu2S + 5 Fe+2 + 24 H+ 17 SO4-2
Reference: Fig 17.13 from Guilbert and Park
Example of supergene enrichment at La Caridad, Sonara, Mexico
[Chalcocite (Cu2S) 79.8% Cu; Covellite (CuS) is 66.4% Cu]
Chalcocite at top where Cu+2/HS ratios higher; Covellite below where Cu+2/HS- are
Kaolinite and chlorite generally replaces feldspar and micas in both oxidized and
Ref. Fig 17-14: Modern Profile, New Guinea