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).
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.