Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 696

Multiple episodes of mineralization occur, with different stages involving l uids of dif-
fering composition and temperature, each producing its own suite of minerals. The l u-
ids which include extremely hot, acidic gases also cause 'alteration' - changes to the
composition of minerals comprising the host rocks. For example, feldspar minerals may
be changed to sericite as part of the alteration process. Alteration typically affects broad
zones, producing a halo surrounding the mineralized zones. As a result of geological his-
tory, including episodes of mineralization and alteration, the ore body and its surround-
ings can be classii ed into different rock types; different zones of weathering; different
stages of mineralization each with its own assemblage of minerals, grades or concentra-
tions of economically valuable metals; and zones of alteration. Each combination of rock
type, mineralization, and alteration will have its own characteristic suite of minerals. Some
may contain high concentrations of suli de minerals such as pyrite; some may contain little
or no suli de minerals. Some may contain alkaline minerals such as calcite with the poten-
tial to neutralize acid; others may contain no such minerals.
Economic geological evaluation of the ore body, using the results of drilling, core log-
ging, mineralogical identii cation, structural interpretation, and assaying, contributes to
the development of a geological model which shows the distribution in three dimensions
of each rock type, together with recognized zones and grades of mineralization and alter-
ation. Such a model provides the basis for quantitative evaluation of the ARD potential
and, in particular, for the selection of samples for testing. Multiple drill core samples are
selected from each designated unit (combinations of rock type and alteration zone) within
the proposed mine but outside the limits of the ore body. (In the event that stockpiling of
ore is proposed and there is a need to evaluate the risk of ARD from stockpiles, samples
would also be obtained from within the ore body). The number of samples collected for
each unit should be roughly proportional to the volume of that unit in relation to others.
In practice, the number of samples is usually limited by the availability of drill core sam-
ples. In any case, efforts are made to ensure that the samples collected are representative of
the units from which they are derived.
The total number of samples will depend on the size of the proposed mine, the number
of identii ed units, and the availability of drill cores. As a guide, the number of samples
for a small gold mine would typically be 40 to 60, while for a large porphyry copper mine,
400 samples or more may be required to adequately characterize the situation. It should be
recognized that the distribution of drill holes is directed mainly at delineation and evalua-
tion of the ore body. Accordingly, units outside the ore body, particularly the more distant
ones, may not be well represented in drill cores. Geotechnical drilling for slope design pur-
poses may provide additional cores from some of these units. However, it is likely that the
sampling density will be low in the outer units compared to those close to the ore body.
This problem is usually addressed not by additional drilling for geochemical purposes, but
by geological mapping, supplementary sampling, and re-evaluation of the ARD potential,
once exposures become available in the mine.

Each combination of rock type,
mineralization, and alteration will
have its own characteristic suite
of minerals.

The total number of samples
will depend on the size of the
proposed mine, the number
of identifi ed units, and the
availability of drill cores.

Static Geochemical Testing
While essentially similar, geochemical testing procedures and terminology differ from
country to country, particularly between Australia and North America. The following dis-
cussion is based mainly on Australian practice, which also predominates in Asian countries.
The following static tests are used to screen samples of waste rock and tailings so that
they can be classii ed in terms of their acid-generating potential:
●

Saturated paste pH and electrical conductivity (EC) tests, as described previously;

●

Acid-base accounting - tests to determine net acid production potential (NAPP), and

●

Net acid generation (NAG) test.