The geochemical and geological processes resulting in ore body formation can significantly contribute to subsequent acid mine drainage potential. This paper will demonstrate the importance of incorporating an understanding of the ore forming process, in this case, for porphyry copper deposits, to acid mine drainage potential. The geochemical behavior of ore and gangue phase mineralogy on acid production will be discussed as well as how the ore content and mining design will lead to disposition of subgrade, but sulfide-bearing rock, into waste dumps with the resultant potential for maximum exposure to oxygenated water enhancing acid production. Further, a discussion will be provided as to how existing exploration and mining-related data can be beneficially employed to aid in predicting current or future acid mine drainage potential. Additionally, the use of historic acid rock drainage and mineralogical signatures, predating mining activities, such as associated with supergene enrichment processes, can be exploited to provide insight into future acid generation capacity.

1. Don Carpenter
Geochemist
ARCADIS-US
Brighton, MI

2. As a Statement of the Obvious Acid Mine
Drainage Can Be Problematic
Low pH (< 3.5 - potentially much lower)
Enhanced metals mobilization
 Readily detectable (pH)
 Visually apparent
 Significant environmental
impact
 Derived from mine waste tailings, abandoned
open pits, and underground mines

3. + +
• Emphasis on porphyry copper deposits
• Methodology applicable to other ore types – volcanogenic massive sulfides

4. • [Bullet 1]
• [Bullet 2]
Porphyry Deposits
Are the Most
Important Copper
Deposit Type
Most important in
Americas
Most important for
our clients

5. Feldspar phenocrysts set
in fine grained groundmass
Rapid crystallization – fluid
loss
Ore forming process
Massive, low grade metal
deposit
Mined by “bulk” methods
“Porphyry” has both a
Geochemical and Mining-Related
Connotation

6. Porphyry Deposits Are Enormous with Potential for
Proportionately Large Impacts

7. Mined, But Not Milled (Sub-economic), Rock Is
Placed in Proportionally Huge Dumps

8. Acid Mine Generation Results
Primarily from the Oxidation of Pyrite
Initiation Reaction(s)
4FeS2 + 14O2 + 4H2O → 4Fe+2 + 8SO4
-2 + 8H+
4Fe+2 + O2 + 4H+ → 4Fe+3 + 2H2O
4FeS2 + 15O2 + 2H2O → 4Fe+3 + 8SO4
-2 + 4H+
Propagation Reaction (pH < ~3.5)
FeS2 + 14Fe+3 + 8H2O → 15Fe+2 + 2SO4
-2 + 16H+

9. Copper Sulfide Oxidation Reaction
4CuFeS2 + 17O2 + 4H+ → 4Cu+2 + 4Fe+3 + 8SO4
-2 + 2H2O
 Contributes acidity through generation of ferric ion
 Contributes to the ferric ion induced “Propagation Reaction”
Hydrolysis of ferric ion
4Fe+3 + 12H2O = 4Fe(OH)3 + 12H+
Maybe less relevant because high copper content ore
is milled and not placed in mine dumps

10. Acid Generation Is Dependent on Availability of
Pyrite and Oxygen Supply
• Intrinsic buffering capacity of the rock
• Geological and geochemical understanding of ore deposit formation can aid in
identifying the waste types and their acid generation capacities
• Mining process
FeS2
H2SO4
O2

11. Porphyry Copper Formation Begins with Emplacement
of a Granitic Pluton
Si(OH)4, K+, Cu+, Fe+2, S-2
→ Ca+2Ca+2 ←
Granitic Country Rock
Porphyry
Quartz
K-Spar
Muscovite
Biotite
Chalcopyrite
Pyrite
Calcite
Epidote
Calcite
Epidote

12. Hydrothermal Alteration Results in Radial
Mineralogical and Compositional Zonations

13. The “Pyrite Shell” has the Attributes for
Significant Acid Mine Drainage Potential
Elevated pyrite
Potentially low copper
Within mine limit
But potentially outside
of ore zone
Disposed in mine
dumps

14. • Need to speak in a “common” technical
language
• Comfort factor and perhaps key differentiator
• Aids in understanding disposition
• Identifies location of disposal
• Often quantifies sulfide composition and
content
• May have performed acid generation tests of
dump leaching of low grade copper ore
These Terms Will
Be Recognized
by Mining Staff
and Often Are
Depicted on
Mine Maps

15. Historic Acid Generation Potential Can Aid in
Predicting Future Acid Release
 Presence of “leached gossan”
and supergene enriched ore
documents historic acid
production
 May be visually apparent
 Included on mine maps and
within geological logs and
reports
 Jarosite [KFe3(SO4)2(OH)6]
documents pH < 2.2
 Chalcocite [Cu2S] similarly low pH for the near total initial removal of copper
 Acid generation capacity may be exhausted
 Significant in non-oxidized portions of deposit

16. Examining Mine Data
and Use of Geochemical Modeling
Can Aid in Predicting Acid Potential
1. Assess mine maps and database for
rock/alteration types
2. Develop database of pyrite content
3. Identify location(s) of disposal
4. Estimate waste dump composition
5. Apply geochemical modeling to assess
constraints on acid generation and rate of
production

17. • As a statement of the obvious acid mine drainage can
be problematic
• Acid mine generation results primarily from the
oxidation of pyrite
• Hydrothermal alteration results in radial
mineralogical and compositional zonation
• Historic acid generation potential can aid in
predicting future acid release
• These terms will be recognized by mining staff and
often are depicted on mine maps
• Examining mine data and use of geochemical
modeling can aid in predicting acid potential

18. Imagine the result