Abstract
Mining activities inevitably result in changes to the environment and have the potential to
cause negative impacts. This work investigates and emphasises the role of geology as the
primary control on the environmental issues related to mining activities, whether current or
historic. The knowledge and understanding of geological and geochemical factors associated
with a particular deposit is crucial in ensuring the prevention and/or minimisation of the
environmental impacts of mining operations. Such knowledge is essential for the responsible
environmental management of mines. The research presented in this thesis is applied in nature
and focuses on mesothermal and associated placer gold deposits located in the South Island of
New Zealand.
Arsenic and antimony are two metalloids commonly associated with mesothermal deposits,
where they are mainly present as minerals arsenopyrite and stibnite, respectively. The
mobilisation of these metalloids from deposits is facilitated by near-neutral pH and the
greenschist facies rocks hosting mesothermal deposits are characterised by generally high acid
neutralising capacity thanks to the presence of carbonate minerals. Arsenic and antimony are
known for their toxicity at low levels (e.g. < 0.01 mg/L in water) and therefore their elevated
concentrations in waters and solid mine residues and soils are the main environmental
concerns with regards to the mining of mesothermal deposits.
The presence of metalloids has been studied as part of this work in both active as well as
historic mining settings in four different locations. At the active Globe Progress mine in the
West Coast, metalloid signatures of mine waters were found to have evolved from Sb/As < 1
to above 1 suggesting that the mobilisation of antimony has proportionally increased with
regards to arsenic over the course of over two years since the mine’s opening. The impact of
metalloids on aquatic fauna was investigated in a long-term field study and not enough
evidence was gathered to suggest their involvement in the decrease in the ecosystem’s health
which was found more likely to be attributable to the repeated increased turbidity events in
the receiving stream.
The choice of mining methods as well as climate as a factor in determining the environmental
aspects is also recognised here with the change in mining method from underground to open
cast being responsible for the change in the type of ore mined and therefore also the related
metalloid signatures. An historic mining processing method of roasting of sulphide-rich ore
(especially in an Edwards roaster including an As saving system) was found to influence the
mobility of metalloid-bearing mine residues governed by the presence of soluble arsenolite
and immobilisation of metalloids through the formation of relatively stable secondary
minerals at sites where no roasting in the Edwards roaster has been performed in the past. For
example, immobilised As and Sb-bearing residues at the Big River mine were found to
contain up to ~20 wt% As and 3.5 wt% Sb. The presence of localised acidic pH conditions
was recognised as an important control on the immobilisation of metalloids, ensuring the
stability of some of the secondary mineral phases (e.g. scorodite).
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Turbidity or suspended solids load in waters has been a major environmental issue in New
Zealand since the beginning of mining operations in the country. Even though the problem is
widely-recognised, not much is known with regards to what controls the levels of turbidity
produced and their rates of settling. The geological factors recognised as important in the
study of five paleoplacer deposits from Central Otago include the abundance of clay minerals,
which is partially dependant on the presence of altered basement rocks in the Central Otago
setting as well as the mode of transport and deposition of the sediments. Additional physical
factors such as the level of cementation of a deposit were also found to be an important
control on turbidity production and dissipation.
The appropriate management of active mine sites is crucial in ensuring that the activities are
performed in as safe a manner as possible from the environmental point of view. Today,
modern mines operate extensive environmental management and monitoring systems and
actively work towards improving the existing schemes. The evaluated waste rock
management system at the Globe Progress mine, designed to help keep the metalloids on site
and prevent their release via waters percolating through waste piles, was found to be working
well. Overall, the system correctly categorises waste rocks into two types depending on their
predicted arsenic content, and therefore their level of environmental sensitivity, followed by
correct handling and storage in appropriate waste piles.
The management of historic sites involves the evaluation of their environmental impact on the
local environment which should also include an assessment of any potential health and safety
risks with regards to the visiting public, which is not always considered at historic sites in
New Zealand. In addition, a potential conflict between historical preservation and
environmental management has been recognised. On the other hand, the widely-perceived
conflict between mining and conservation values has been demonstrated to not always be the
case with examples of unique saline habitats forming at two historic placer mining sites in
Central Otago. The natural rehabilitation of these sites was found to contribute to the
enhancement of the sites’ long-term biodiversity suggesting that natural succession may be
important for the establishment of stable and robust ecosystems.
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