A guide to leading practice sustainable development in mining

Geochemical assessment aims to identify the distribution and variability of key
geochemical parameters (such as sulfur content, acid neutralising capacity and
elemental composition) and acid generating and element leaching characteristics. A
basic screening level investigation is essential and should commence at the earliest
possible stage. The need and scope for detailed investigations will depend on
the findings of initial screening. Since some studies such as leach tests or sulfide
oxidation rate measurements require a long time frame to provide the necessary
data, it is important to initiate this work well ahead of key project milestones.
Reference to other mining operations in the region, particularly those situated in the
same stratigraphic or geological units may provide empirical information on the likely
geochemical nature of similar ore types and host and country rocks. The Pine Creek
Geosyncline in the far north of Australia is renowned for its propensity for acid mine
drainage potential in virtually all gold mines that operated in the 1970s and 80s.
Early indications can also be provided by exploration drill core where it is leading
practice to log key indicators such as sulfide and carbonate type, abundance
and mode of occurrence. All samples should be analysed for total sulfur content
as a minimum, and include key environmental elements in all drill core assays.
Mineralogical investigations should examine the type and mode of occurrence of
sulfide and carbonate minerals.
A number of procedures have been developed to assess the acid forming
characteristics and metal leaching behaviour of mine materials. The most
widely used screening method is based on the Acid Base Account (ABA) which
is a theoretical balance between the potential for a sample to generate acid and
neutralise acid. The simplest form of the ABA is known as the Net Acid Producing
Potential (NAPP).
Some sulfur minerals do not generate acid (but may contribute to metalliferous
drainage), and there are different forms and reactivities of AMD generating minerals
and AMD neutralising minerals. As a result, there is a level of inherent uncertainty in
prediction based solely on the theoretical ABA. Mineralogical investigations, elemental
analysis, sulfur and carbonate speciation, acid neutralising capacity, reactivity, and the
Net Acid Generation (NAG) test (a rapid direct oxidation procedure) are used to address
this uncertainty. AMD prediction is greatly enhanced by using a combination of tests,
in particular independent tests such as NAPP and NAG.
Sampling
Sample selection is a critical task and must be given careful consideration at all
stages of a project. Samples should represent each geological material that will
be mined or exposed and each waste type, for current and projected mine plans.
Sampling design normally utilises drill hole cross-sections through the deposit.
The number and type of samples will be site-specific and will depend on the phase of
project development, but must be sufficient to adequately represent the variability/
heterogeneity within each geological unit and waste type. Factors such as grainsize,
structural defects, alteration, brecciation, veining, etc., must therefore be considered
in sample selection. As a minimum requirement, through the exploration phase to
final feasibility, all drill hole samples should be assayed for total sulfur.
A GUIDE TO LEADING PRACTICE SUSTAINABLE DEVELOPMENT IN MINING 47