Supergene Enrichment of Porphyry Copper Deposits

Document Sample
Supergene Enrichment of Porphyry Copper Deposits Powered By Docstoc
					                  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?

Leaching reactions
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+

Or

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
general:

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
lower.

Kaolinite and chlorite generally replaces feldspar and micas in both oxidized and
enriched zones.

Ref. Fig 17-14: Modern Profile, New Guinea

				
DOCUMENT INFO