Understanding How Ore Body Formation Aids in Predicting Acid Mine Drainage Potential

Don CarpenterGeochemist ARCADIS-USBrighton, MI

As a Statement of the Obvious Acid Mine Drainage Can Be Problematic

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

Systematic Analysis of Acid Generation Process Aids in Predicting Potential

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• Emphasis on porphyry copper deposits

• Methodology applicable to other ore types – volcanogenic massive sulfides

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Porphyry Deposits Are the Most Important Copper Deposit Type

Most important in Americas

Most important for our clients

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

Porphyry Deposits Are Enormous with Potential for Proportionately Large Impacts

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

This material is readily susceptible to exposure to infiltrating, oxidizing water

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+

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 ion4Fe+3 + 12H2O = 4Fe(OH)3 + 12H+

Maybe less relevant because high copper content ore is milled and not placed in mine dumps

Acid Mine Generation Can Also Result from Oxidation of Other Sulfides

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

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

Hydrothermal Alteration Results in Radial Mineralogical and Compositional Zonations

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

• 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

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

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

Understanding Ore Deposit Formation Aids in Predicting Acid Mine Drainage Generation Potential

• 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

Imagine the result