A multidisciplinary approach considering geochemical reorganization and internal structure of tailings impoundments for metal exploration

Highlights

• Tailings impoundments might host considerable amounts of valuable elements.

• Tailings are complex structures affected by physical and geochemical reorganization.

• Reorganization generates zones of Cu depletion and enrichment.

• ERT and particle size distribution reveal structure of tailings efficiently in 3D.

• 3D structure provides understanding of economic potential and environmental impact.

Abstract

Tailings deposits may host a significant amount of valuable elements and are of great interest as exploration targets for secondary mining. However, elements are distributed heterogeneously within an impoundment due to physical and chemical processes during and after deposition. Therefore, the complex internal structure has to be considered for representative sampling, estimation of ore grade and to prevent dilution of metals by bulk mining. This study shows that a multidisciplinary approach and a combination of methods are necessary to take account of that. The selected methods comprise surface sampling with few drillings, geochemical and granulometric analysis and Electrical Resistivity Tomography (ERT). They provide important information for a quick decision about the economic viability of further detailed exploration or even secondary mining. In order to understand the internal structure of tailings impoundments collection of 1D and 2D information from sampling and drilling should be accompanied by 3D resistivity mapping. In this work, indications for particle size distribution, metal distribution and water movement were extracted from the different data sets and a model for the internal structure and re-organization is proposed. General zones of metal enrichment and depletion can be projected from the model. Furthermore, water saturation and movement as an important factor for acid rock drainage and tailings dam stability can be deduced for an estimation of the environmental risk of the tailings impoundment. Therefore, the proposed combination of methods covers both the estimation of economic potential as well as the analysis of environmental impacts of tailings deposits.

Introduction

In countries with long and intense mining history, deposition of residues of processed material after flotation or heap leaching has raised concerns in socioeconomic extent for several reasons (Edraki et al., 2014). After decades and centuries of mining, the residues cover large areas of the landscape with increasing extent of land consumption for the future. Furthermore, these residues might pose a risk to the environment, especially for the population, water and soil. Environmental impact might be caused by fluid interaction and leakage (acid mine drainage, AMD), geotechnical instability causing possible landslides, dam breaches, destruction of infrastructure or eolian transport of contaminated material into areas of e.g. agricultural use (Azam and Li, 2010; Dudka and Adriano, 1997; Evangelou and Zhang, 1995; Kalsnes et al., 2017; Kelm et al., 2009; Price, 2003; Smuda et al., 2008). Such problems are well known in the literature, the scientific community and the affected population, but remediation is a long and cost intensive process. It often requires political interference, especially when considering the large amount and dimensions of historic deposits.

In the past years, the idea of secondary mining has evolved (Lottermoser, 2010), considering a possible re-mining of old mining residues, such as residues from the flotation process (tailings) with a two-fold aim: First, ore grades decrease in present mining and processing and while mineral commodity prices increase. Therefore, the extraction of metals and economically strategic elements from old tailings with modern processing technique might not just be of financial interest, but also a way to decrease the potential supply risk for these elements. Secondly, decreasing the risk of hazards by re-depositing the material in modern and safe storage facilities. Even if the fluctuating market situation might not justify the reprocessing for financial reasons alone, the combination of both aims might increase the probability of a sustainable disposal of mining residues in a safer way for people and environment with no need of new environmental regulation measures.

To study these new kinds of anthropogenic deposits for a possible re-mining an interdisciplinary approach with various methods from different scientific fields should be considered, as there are several mechanisms affecting internal structure and reorganization of mining residues during and after deposition (Dold and Fontboté, 2001). Processes in the fields of sedimentology, hydrology and geochemistry take place, leading to complex and heterogeneous bodies, which should affect prospection or exploration strategies, namely sampling or drilling, significantly.

In the case of tailings storage facilities, the deposition process can be described as fluvial sediment transport, which is affected by several chemical and physical factors. Fluvial transport is mainly controlled by the ratio of vertical and horizontal force. It depends on the basin topography and geometry, particle size, particle shape and density, as well as water content, particle interaction or type of transport (turbulent or laminar) (Blight and Bentel, 1983; Kleinhans, 2002). With decreasing energy during transport, the coarse fraction is deposited first (gravel, sand) with amounts of fine-grained material (silt, clay). With further energy decrease, only fine particles remain in suspension. Therefore, the transport mechanism produces a lateral sorting of particle size according to the transport energy. With increasing transport length, particle size distribution tends to smaller grain sizes. Based on that, Blight and Bentel (1983) proposed a method to predict particle sizes depending on the distance from the spigot point. Furthermore, due to gravitation, there is vertical sorting within the flow according to Stoke's Law. These mechanisms produce more or less horizontal layers that are internally graded with fine-grained layers stopping vertical water migration and favoring lateral flow within coarse-grained layers. This affects the water migration within tailings material after deposition, which is an important factor for geochemical reorganization comprising oxidation, cation dissolution, migration and precipitation of elements.

To explore and understand such complex systems regarding element concentrations or ore grade, the aforementioned sedimentation structures, geochemical reorganization, and alteration processes have to be taken into account for a reliable economic potential estimation. Considering the amount of tailing deposits in historically active mining areas, detailed exploration of single deposits will be limited in cost and time investment. Therefore, the proposed efficient and comprehensive overview strategies might be more suitable in order to put prioritization of detailed exploration and rational decision-making regarding profitability and necessity of re-mining and re-deposition on a better basis.

For a good understanding, structures of tailings bodies need to be analyzed in three dimensions. For physical and chemical characterization, hand sampling or core drilling has been proven as a reliable method. However, bore hole data might be the most expensive and time consuming to acquire, with limits in horizontal resolution. Fast 2D and 3D geophysical methods such as electrical resistivity tomography (ERT) or ground penetrating radar (GPR) have been applied in various scientific fields. In this study, GPR has shown severe limits in depth with water saturated material, while ERT has been applied in the past 20 years with good results for mapping of the geometry of sedimentary units with different grain sizes for environmental studies, prospecting, in sedimentology or hydrology (Baines et al., 2002; Kemna et al., 2000; Maillol et al., 1999). Therefore, ERT seems to be well applicable on tailings deposits in order to map sedimentation structures and grain size distribution, as well as water content and migration.

In this study, an approach for analysis of tailings impoundments including sedimentological, geochemical and geophysical data is presented for rapid analysis of large impoundments, taking heterogeneity of the impoundments due to deposition and reorganization into account.