Porphyry copper deposits form near shallow porphyritic intrusions where hydrothermal fluids interact with the intrusion and surface waters. Characteristic alteration minerals like chlorite, epidote, clays and micas form from the reaction of hydrothermal solutions with igneous rock silicates. Primary sulfide minerals like chalcopyrite and bornite near the intrusion are oxidized in the supergene zone to form secondary copper minerals and oxides, leaving a gossan at the surface. In contrast, epithermal deposits form at even shallower depths dominated by surface waters and produce a variety of ore minerals including gold, silver, lead and zinc.

1. Porphyry Copper
Supervision by: Dr. Ahmed M. ZayedSupervision by: Dr. Ahmed M. Zayed
Edited by : Mostafa Abdel Azim MasoudEdited by : Mostafa Abdel Azim Masoud
Beni-Suef University - EgyptBeni-Suef University - Egypt

2. Copper Resources

6. Idealized Porphyry Alteration/Mineralization (Lowell and Gilbert, 1970(

9. Potassic Alteration
High Grade Ore
Bornite
Chalcopyrite
Porphyry Ore and
Alteration Textures
Phyllic Alteration

12. SupergeneHypogene
Chalcopyrite
Bornite
Malachite
From Hypogene to Supergene

13. Sulfide minerals are unstable in
the presence of oxygenated
groundwater. Primary (hypogene)
sulfides react to form secondary
(supergene) sulfides, and then
supergene oxygen-bearing
minerals such as oxides,
carbonates, sulfates, and
phosphates, depending on the
anions that are available in the
groundwater. At the surface,
red/orange colored iron oxides
(gossan) are left behind and
become a marker for sulfide
mineral prospecting.
See Table 19.3, page 382 for
names of some minerals in the
oxidized part of the supergene
zone.

14. Gossan at the Lavender Pit overlook, Bisbee Arizona.

15. Oxidised zone
Primary zone
Enriched zone
Leached zone
Mineralized gravel
Mineralized bedrock
Barren gravel
Supergene enrichment
FeS2 + H2O + 7/2O2= Fe2+
+ 2SO4
2-
+ 2H+
2Fe2+
+ 2H2O + 1/2O2 = Fe2O3 + 4H+
+ 2Cu+
+ H2O = Cu2O + 2H+
2Cu+
+ SO4
2-
= Cu2S + 4O2
Leached zone – acidity creation
CuFeS2 + 4O2= Fe2+
+ Cu2+
+ 2SO4
2-
Oxidised zone – Fe and Cu oxides, acidity creation
Enriched zone – reduction and sulphide deposition

16. The silicates in igneous rocks
(feldspar, hornblende and
micas) are altered by reaction
with hydrothermal solutions to
form characteristic alteration
minerals:
Propylite (chlorite and epidote
form)
Argillite (clay minerals form)
Sericite (mica forms from
clays)
Secondary or Supergene Hydrothermal
Minerals

23. Porphyry copper deposits Epithermal deposits
originate in or nearby porphyritic
intrusives which were saturated
by water as they intruded in
shallow areas of the crust (less
than 6 km depth(.
The developing hydrothermal
systems are characterized by the
interaction, to different degrees,
of fluids coming directly from
the parental intrusive and from
meteoric surface waters.
Hydrothermal deposits
formed at shallow depths at
depths of less than 1 km in a
hydrothermal system
dominated by surface waters
with only a small contribution
by magmatic fluids.
Epithermal deposits include a
wide variety of ores: Au, Au-
Ag, Ag, Pb, Zn, Cu, Sn, Sb, U
and Hg.
Comparison between the the porphry copper
deposits and the epithermal deposits

24. Schematic diagram oft he tectonic setting of porphyry, epithermal and skarn deposits | Image Credit: MBG