PREDICTION
AND PREVENTION OF ACID ROCK DRAINAGE FROM A GEOLOGICAL AND MINERALOGICAL
PERSPECTIVE
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project 1.32.1
October 1993
EXECUTIVE
SUMMARY
The aims of
this project on prediction and prevention of acid rock drainage
from a geological and mineralogical perspective are:
- to clarify
the susceptibility of different types of ore deposits to acid
rock drainage problems
- to investigate
the relative weatherability of common sulphide minerals; and
- to examine
the role of a field geologist in acid rock drainage assessment.
especially during mineral exploration.
Based on a
literature survey and supplementary field investigations, seven
common types of metallic mineral deposits occurring in western Canada
are ranked according to acid rock drainage potential. The susceptibility
of a deposit to develop acid rock drainage problems is largely determined
by the net acid-generating potential of the prevalent mineral assemblages,
accessibility of the reacting minerals to the weathering agents
and potential mediation by iron-oxidizing bacteria. Generally, the
alteration mineral assemblages about an orebody reflect well the
acid rock drainage potential of the relevant rock types. A formula
for assessing acid rock drainage potential based on field estimates
of mineral abundance is suggested.
To determine
the relative weatherability of common sulphide minerals, a kinetic-type
weathering experiment using polished thin sections as starting materials
has been conducted. Detailed examination of the prevalent sulphides
in the sections with petrographic and scanning electron microscopes,
augmented by electron microprobe analysis and determination of leachate
chemistry by induction coupled plasma mass spectrometry (ICPMS)
indicate the importance of trace element composition in affecting
the stability of individual sulphides. Where different sulphides
are in contact with each other, electrochemical processes are likely
to be operative, further complicating the relative reactivity of
the sulphides. Thus, a general weathering sequence for the common
sulphides may not exist except at a local scale. To predict the
rate of acid generation and metal release associated with sulphide
oxidation, further work to clarify the interdependence of geochernical
reactions, microbial mediation and electrochemical processes involved
is required.
Well-trained
to collect and interpret mineralogical and geological data, a field
geologist can readily assess the acid rock drainage potential of
geologic materials based on field observations. A practical checklist
for field assessment of acid rock drainage potential during mineral
exploration is suggested and the tasks of a geologist in acid rock
drainage assessment at various phases of a pre-mine development
project are briefly described.
In conclusion,
in as much as acid rock drainage is a product of natural water-rock
interaction, its prediction and prevention relies on a thorough
understanding of the local geology and mineralogy. The involvement
of a geologist in acid rock drainage assessment 'in every stage
of a mining project may lead to a significant reduction in the cost
of tackling the serious environmental problem and is thus highly
recommended.
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