Long Term
Acid Generation Studies: Cinola Project British Columbia
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Project 1.19.1
March 1994
EXECUTIVE
SUMMARY
In 1987, City
Resources (Canada) Limited initiated a program of acid generation
testing as part of feasibility studies for the Cinola Gold Project
located on the Queen Charlotte Islands, off the north coast of British
Columbia. The program was designed to quantify acid generation potential
and evaluate waste rock management options for the proposed open
pit gold mine. In 1990, the Mine Environment Neutral Drainage (MEND)
Program and the BC Acid Mine Drainage Task Force began funding further
monitoring of tests to evaluate long term weathering conditions
in five humidity cells, five leach columns and four on-site waste
rock pads. The study was concluded in 1993.
The project
area experiences mild damp maritime conditions. Most precipitation
occurs in the winter, primarily as rain. Bedrock in the area is
mantled by thin till deposits. The Cinola deposit was formed by
localization of relatively low temperature hydrothermal fluids along
a fault system. During mineralization, the surrounding conglomerates,
sandstones, siltstones and mudstones were altered to clay minerals
(kaolinite and illite, a low temperature form of muscovite) and
pyrite was disseminated throughout the rock mass at concentrations
typically in the range of 1 to 2%. Calcite occurs at similar or
lower concentrations. Several rock types were classified as potentially
acid generating based on conventional acid-base accounting.
The study yielded
important data on long term weathering trends for tests conducted
at different scales. Five humidity cells (four of which contained
acid generating waste rock) were continued for 140 weeks and showed
three distinctive stages in pH and sulphate production rate. In
Stage 1, leachate pH was near 7.0 and sulphate production was slow
(less than 20 mg SO4/kg/week). As readily available neutralizing
minerals were removed, leachate pH decreased to less than 3 and
sulphate production increased rapidly (to greater than 1000 mg SO4/kg/week).
In Stage 3, sulphate production slowly decreased and leachate pH
slowly increased to between 3 and 4. After more than 100 weeks of
leaching, sulphate production decreased by one or two orders of
magnitude and leachate continued to be acidic (leachate pH<4).
Trends in waste rock pad leachate were less apparent due to annual
flushing events related to late summer and fall rainfall. However,
a qualitatively similar long term decrease in sulphate production
and increase in leachate pH was observed. After five years, the
waste pad leachate continued to be strongly acidic for three test
piles and concentrations of iron, copper, zinc and arsenic were
indicative of acidic weathering conditions.
The leach column
study evaluated the delay in release of heavy metals and acid drainage
achieved by addition of varying concentrations of crushed limestone
to waste rock. Long term trends were very similar to humidity cells.
Two simple relationships were defined relating limestone content
(ie. neutralization potential) to (1) time elapsed until the first
indications of net acid generation were observed; and (2) time elapsed
until peak acid generation was observed. Release of zinc followed
the first type of relationship whereas release of iron and copper
followed the second relationship. The study showed that although
limestone addition delayed full acid generation and release of copper
and iron according to an exponential relationship, significant zinc
loads may be released in pH-neutral drainage several years before
acid drainage is produced.
When comparing
different types of weathering tests, the study demonstrated that
the small- and large-scale tests produced similar results for qualitative
long term trends, sulphide oxidation reaction kinetics, and average
sulphate release rates expressed as estimates of exposed particle
surface areas. The test results were dissimilar for metal release
rates probably due to complex mineral precipitation controls operating
under widely varying solid-to-liquid ratios. These conclusions apply
to the Cinola epithermal deposit where sulphide mineralogy is simple
relative to complex deposits containing two or more different types
of iron sulphide in a variety of forms.
It was concluded
that further research is needed to address a number of issues. These
include: evaluation of solid weathering products; long term kinetics
of weathering in mineralogically complex materials; long term kinetics
of weathering in marginally-acid generating materials; long term
leachate quality for sulphide-depleted weathered waste rock; application
of small-scale test results to large scale waste rock piles; and
evaluation of the effect of heterogeneities in mixing of calcareous
and potentially acid generating waste rock.
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