Geochemical Predictions of Metal Leaching and Acid Generation: Geologic Controls and Baseline Assessment

ABSTRACT
An essential component in the evaluation of a mineral deposit
or a mine development project is the consideration of environmental
impact, particularly the leaching of deleterious elements such as
As and many of the transition metals or of Acid Rock Drainage
(ARD) generation. A major part of this assessment is the potential
for the materials within the mineral deposit to degrade land or water.
In order to make this assessment testwork or modeling programs
are conducted to predict the magnitude of impact and to assess future
changes. Very often this work is carried out late in a development
timetable and can lead to frustrating and costly delays for the developer.
However, the prediction of geochemical impacts from a mineral
deposit can be qualitatively determined at an early stage in development
and used to execute a more efficient impact assessment.
The processes that can lead to geochemical impact from a mineral
deposit are generally related to the geologic characteristics of
the ore and host rocks. For example ARD is related to the presence
of acid generating phases, especially Fe-bearing sulfides or sulfate
minerals coupled with the lack of buffering agents. Consequently,
the potential for metal leaching can be qualitatively based on the
altered wallrock mineral assemblage that constitutes the highest proportion
of waste rock and exposed wallrock. Geochemical reactions,
like metal leaching, are natural processes and as such they can be
interpreted in the context of geologic understanding.
This paper presents field and mineralogical observations from
several contrasting ore deposit types, common to the Great Basin,
along with associated laboratory assessment of geochemical leaching.
A practical guideline is discussed for the field assessment of
geochemical impacts that can be made during the exploration and
pre-mine development phases of a project. The inclusion of geologic
knowledge to that of engineering design and analytical testing
and modelling is recommended in order to reduce the costs and
delay associated with environmental assessment during pre-mine
development.