Exploration at Kutcho Creek, located 110 kilometres east of Dease Lake, B.C., has
delineated a polymetallic, massive sulphide deposit with possible mining reserves of 13.9
million tonnes. In 1979, then owner, Esso Minerals Luc., submitted a proposai to
govemment in order to develop the Kutcho site. Upon review of the proposal by the
B.C. Mine Development Steering Committee, several issues were raised including the
management of certain rock assemblages that were capable of generating acid.
Preliminary acid generation studies indicated that footwall waste rock would generate
acid and that the hanging wall material would consume acid.
The potential for acid generation at the Kutcho Creek project has become a key
environmental issue with respect to obtaining the Stage II Approval-in-Principle. TO
alleviate thesec oncernst,h e present owners,S umacM ines Ltd. and HomestakeM ineral
Development CO., embarked upon a jointly funded research program to investigate the
potential for acid rock drainage (ARD) at Kutcho Creek and the viability of blending
waste rock to mitigate ARD.
PreliminaryARD studiess uggestedth at blendingp otentially acid generatingw aster ock
with potentially acid consuming waste rock in a carefully designed waste rock dump
would mitigate acid generation. This was viewed as a unique opportunity to study the
effectivenesso f waste rock blending to c.ontrolA RD. Becauseo f its unique character
the waste rock blending research has attracted the interest of the Minera1 Opportun@
Program of the Canada-British Columbia Minera1 Development Agreement. Rescan
Environmental Services Ltd. was selected as the consultant to assist with the
development of this research program and to carry out the testwork.
Research conducted to date includes static tests (acid base accounting) and kinetic
experiments (laboratory humidity cells and on-site pilot scale weathering tests). The
rationale and objectives for each test were as follows:
- Acid base accounting tests examined the chemical and physical composition of
representativew aster ock typest o determinet he balanceb etweena cid producing
minerais (sulphides) and acid consuming minerals (carbonates) and the
consequenpt otential to generate acid. - Laboratory humidity ce11 tests examined the rates and factors controlling acid
generation and acid consumption in a controlled weathering environment for the
various types of rock and for blended samples of waste rock. Results were used
to confirm the potential for acid generation identified by previous acid base
accounting tests, and to provide a comparison to the large scale tests being
conducted on-site. - Lastly, large scale weathering tests consisted of three biended waste rock pads
(20 tonnes each) which were intended to simulate acid generation behaviour of
blended waste rock piles under natural weathering cotiditions.
Acid base accounting test results confirmed that footwall rocks are potentially acid
generating and that hanging wall rocks have the potential to neutralize acid.
Laboratory humidity ce11 tests composed of blended ratios of waste rock material
demonstrated that the rate of acid generation yas signi&ntly reduced. For the
blended test cells the rate of neutralization, at least over the short term, was sufficient to
maintain a slightly alkaline pH in the leachate thereby greatly reducing metal leaching.
Results from the blended field test pads are not as conclusive. It was discovered that
leachate quality was dependent upon flushig rates as opposed to the actual acid
generation rate. During the second year of observation, leachate from two of the three
pads was consistently neutral in pH. Conversely, the third pad produced an acidic pH.
This has been attributed to a reduction in flushing of the waste rock due to the inclusion
of a soi1 cap. As a consequence, acid products were allowed to accumulate, resulting in
a lower pH. In other words, the low pH did not necessarily reflect increased acid
generation rates.
Long term predictions for acid generation have been made and are based on the
sulphate production rates from the humidity ce11te stwork which have been extrapolated
employing mathematically derived “best-fit” curves. Predictions, based on these
extrapolations, show that the neutralization potential could be consumed in as early as
100 weeks with acidic conditions continuing until ail sulphides were oxidized. Ideally, in
order to completely obviate acid generation in any samplé, there should be sufflcient
neutralizing potential (NP) in the sample (assuming only half of it is.available) that the
NP and sulphide component are consumed concurrently. Therefore, based on our long
term extrapolations, blending of Kutcho waste rock, even at l:l, would not be a viable
ARD mitigation strategy.
The overall objective for this research program was to develop a management plan
utilizing information obtained during the study, to ensure acid generation would not
cause environmental degradation. A separate analysis has determined that the costs
associated with blending large amounts of waste material in a full scale waste dump
would be excessive. An alternative ARD mitigation strategy, the disposal of reactive
waste material in a flooded impoundment, is generahy accepted as thè. most feasible
method for long term disposal of reactive mine wastes for Kutcho Creek
