Study area: Garpenberg mine and Lappberget orebody

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Project partners and consortium structures

The STOICISM Consortium is led by a major industrial mineral producing com-pany (Imerys minerals Ltd) and consists of 17 partners from 8 different European countries. Key contributors on this multidisciplinary platform include several univer-sities, specialized SMEs & corporations, an industry association, as well as applied technology and research institutes. The project is structured in 9 work packages (WP) with associated tasks and with clearly identified milestones and outcomes. The 6 first work packages are complementary and correspond to the main steps of the whole in-dustrial materials supply chain (Figure 1.4).
This work is incorporated within the framework of WP2 « beneficiation » the leader of which is the Université de Lorraine (UL). The others partners involved in this work package are:
• Imerys Minerals Ltd, the wholly-owned UK entity of Imerys, world leader in mineral-based specialties for industry, active in 47 countries with more than 240 industrial locations. Imerys is the lead participant in the project and ensure that the industrial partnership, with its academic support, meets the aims and objectives of the project on time and within the budget. They are involved in all of the Work Packages, thus providing leadership and continuity throughout the project.
• University of Exeter’s Camborne School of Mines (CSM), which has an interna-tional reputation for research related to the understanding and management of the Earth’s natural processes, mineral resources, energy, and the environment. The core of Exeter’s involvement is participating in WP1 (Extraction) but it also participates across WP2, investigating opportunities for renewable energy.
The strategic objective of WP2 is to develop and apply new beneficiation techniques to allow the use of low quality DE, perlite and kaolin ore and to recover CRMs from waste streams. This is expected to reduce waste production and increase useful mine volumes. UL led this activity with support and input from Imerys and CSM. The specific objectives of WP2 are to:
1. Advance technological development of high intensity flotation devices for better flotation recovery with investigation of pulsation, vibration and ultrasonic fields. Thus a considerable diminution of energy and reagents consumption by unit volume of material processed will be obtained.
2. Develop a process for selective flocculation of DE to remove impurities.
3. Identify alternative and new uses for industrial mineral waste streams.
4. Identify processes to extract useful CRMs from raw materials and waste such as physical separation, advanced flotation technology, and embrittlement by high power electromagnetic pulses.
5. Assess opportunities for the use of renewable energy in the beneficiation process stages.
This PhD thesis is linked to the fourth objective through the task 2.6 untitled “CRM Recovery”.

Task 2.6 CRM Recovery

This task is dedicated to CRM recovery from waste streams of kaolin production of the St Austell kaolin workings of Imerys in Cornwall, UK. T2.6 itself is subdivided into specific subtasks:
• T2.6.1 Analysis of materials streams and definition of the samples to be characterised: analyse raw materials/waste streams and evaluate their avail-ability to extract the critical CRMs.
• T2.6.2 Identification of valuable minerals and metals: identify the CRM-bearing minerals and the degree of dissemination in the ore and waste streams to evaluate the way of processing. The classical and advanced analytical techniques for mineral characterisation (XRD, high resolution SEM and TEM, etc.) will be used by UL to identify the mineral composition and mineral associations. University of Exeter will apply the new QEMSCAN technique to establish the mineral and size by size associations in order to direct to the best separation method.
• T2.6.3 Feasibility studies of pre-concentration by physical methods and extraction of CRMs. UL will test the physical separation techniques such as gravity (i.e., centrifuge) and high intensity magnetic separation, advanced flotation technique and leaching (all available in STEVAL pilot plant) to extract the CRMs from various type of streams. For low grade rare metals-containing ore (W, Sn, Ta, Nb, REE) a high capacity pre-concentration method will be searched and tested.
• T2.6.4 Feasibility studies on advanced flotation technique for low grade and fine grained ores: For low grade and fine grained ores (or waste products) the high intensity flotation machine will be used. In order to decrease the reagent consumption, P4 will develop a new formulation of flotation reagents based on the synergistic effects in the mixed collector system. Thus, the combination of new formulations of surfactants with pulsed fields in the high intensity flotation machine will result in the enhanced separability of CRMs from waste streams. In particular it will allow rare metals (W, Ta and Nb) to be processed economically from low grade, high volume ores.
• T2.6.5 “Go/No Go” evaluation: UL will carry out a “Go/No Go” evaluation at M24.
• T2.6.6 Validation of CRMs extraction methods at laboratory and pilot scale: UL will validate the promising CRM extraction methods at laboratory and pilot scale.
• T2.6.7 Assessment of scale-up potential for the technologies developed: Imerys and UL will assess the potential for scale-up of the technologies developed based on the results of (M36-M42).
1.2 Geological background
The Cornubian granite province is well known for its historically important deposits of tin and base metals (Manning and Hill, 1990), and for its current international importance as a major producer of china clay (kaolin).

The Cornubian Sn-W province

Tin is distributed on the continents in relatively narrow continent-sized belts (Schuil-ing, 1967). For instance, most of tin deposits across Europe are related to Variscan granites (Figure 1.5). The major tin belt passes from the Cornubian massif (Redruth, Camborne, etc.) in UK through the French Massif Central (Echassières) and con-tinues towards Italy with tertiary tin deposits (Elbe, Tuscany and Scilly). Other tin belts are also related to varsican granites, like the one which pass from Northen Por-tugal (Panasqueira) through Spain or the one around the Bohemian massif (Cinovec-Altenberg). Indeed most of these tin deposits are associated with Sn-W cupolas located at the top of tardi- to post-orogenic plutons emplaced at shallow depths (Jébrak and Marcoux, 2008).
Figure 1.5: Spatial relationship at distinct geological scales between Variscan granites and Sn-W deposits. (a) Tin belts in Europe and their spatial connection to the Variscan belt modified from Schuiling (1967). Main Sn-W deposits are from the PROMINE project (BRGM, 2011).
(b) Outline geologic map of south west England peninsula showing the location of the major plutons of the Cornubian batholith, the kaolinised areas and the active kaolin pits, modified from Černý et al. (2005). (c) Distribution of the felsite dykes (elvans) and Sn-W mineral lodes around and within the St Austell rare-metal granite after Bray and Spooner (1983).
The batholith of SW England was formed during late Variscan orogeny in the Late Carboniferous to Early Permian (270-300 Ma) intruding deformed Devonian-Carboniferous terrestrial to marine sediments. The batholith has a 200-km-long ex-tension (Willis-Richards and Jackson, 1989), comprising six major and several minors granite bodies. The major bodies, from east to west, are Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land’s End and the Scilly Isles (Figure 1.5b).
The outcrops are dominated by biotite-bearing monzogranite, with minor intrusions of tourmaline-bearing aplites, pegmatites and local fine-grained facies (Manning et al., 1996). In addition there is some occurrences of other granite types, resulting from multiple intrusive episodes, such as the lithium-mica granites or the topaz-bearing granite in the Tregonning-Godolphin Granite and the St Austell Granite (Manning and Exley, 1984; Manning and Hill, 1990; Manning et al., 1996). The batholith was affected by several episodes of alteration including quartz-tourmaline veining associated with greisening, intrusion of rhyolite dykes, quartz-hematite veining. Kaolinisation is the last alteration event, which is believed to have a meteoric origin (Psyrillos et al., 1998), and was relatively extensive in the western part of the St Austell pluton where the majority of the active kaolin pits are located (Figure 1.5b).
According to Willis-Richards and Jackson (1989) there is a close spatial association between the batholith and the rich Sn, Cu mineralization of the Cornubian Peninsula. Most of the ores of Sn, Cu, W, Zn and As came from vein deposit type minerali-sation, along or parallel to the axis of the batholith (Moon, 2010). Within the St Austell granite zones of intense kaolinisation are spatially related to swarms of steeply dipping, quartz-tourmaline cassiterite wolframite sheeted veins with greisen (quartz-muscovite-tourmaline topaz wolframite) alteration selvages (layered alteration zones), observable at Goonbarrow pit (Bray and Spooner, 1983). These mineral lodes are lo-cated South (and North) of the St Austell granite, but some of them actually occurred within the St Austell granite kaolinised area (Figure 1.5b and Figure 1.6). Figure 1.5 illustrate the multi-scale spatial relationship between Variscan granites and Sn-W de-posits from the Variscan belt to the St Austell district. This shared relationship is confirmed by the numerous similarities between the St Austell rare-metal granite and several rare-metal granites worldwide, that will be discuss further in Section.

The St Austell rare-metal granite

Rare-metal granites are so-called because of their mineralisations being dissemi-nated through the granites rather than concentrated in layers, lodes, etc. The St Austell rare-metal granite share numerous characteristics with other granites of SW England, summarised in many reference papers and textbooks (Floyd et al., 1993; Manning and Hill, 1990; Manning et al., 1996). These granites were originally de-scribed by four different granite types, all observed in distinct SW England granites: biotite granite, lithium mica granite, tourmaline granite and topaz granite (Hill and Manning, 1987). Additional granite types were added later based on field and textural observations within the St Austell granite, indicating complex late-stage magmatic and hydrothermal processes (Manning et al., 1996). The St Austell rare-metal granite is composed of six major granite types, each unit being characterised by a specific mineral assemblage and textures (Manning et al., 1996), see Figure 1.6.
The biotite granite (BG) is the main lithology, which represent up to 70% of the outcrop of the St Austell granite, exposed in the Western Area, at the eastern part of the pluton and intermittently within the Central Area. It corresponds to the megacrys-tic biotite granite observed at Land’s End and Dartmoor (Manning and Exley, 1984; Manning et al., 1996). It is coarse grained, mainly composed of quartz, K-feldspar and micas with tourmaline and topaz as minor phases. Biotite is the main mica, but mus-covite is also present. The main accessory phases are rutile, topaz, apatite, monazite, cassiterite, zircon and uraninite (Manning et al., 1996). The main host for the LREE differs depending on the authors. Works of Jefferies (1985) on Carnmenellis biotite granite describe monazite as the main LREE-bearing mineral accounting for approx-imately 75% of the total LREE content. In contrast microprobe analysis on samples from Cornubian granites by Alderton et al. (1980) show significant concentrations (up to 0.5%) of LREE in other phases, particularly apatite, but also zircon and sphene.
The lithium mica granite (LMG) has a similar grain size and texture than the bi-otite granite (Manning et al., 1996). It is characterized by the presence of lithium mica (zinnwaldite) and plagioclase (albite component). The accessory minerals are present as inclusion in zinnwaldite (apatite, monazite, zircon and rutile) or in plagio-clase (apatite, fluorite and secondary micas). Additional alteration products include fine-grained topaz aggregates, and replacements of zinnwaldite by tourmaline (Manning et al., 1996).
The tourmaline granites were subdivided Manning et al. (1996) in a tourmaline granite (TG), it-self regrouping a globular quartz and medium coarse-grained equigran-ular facies, and in a fine-grained tourmaline granite (FGTG) with fine-grained (<0.5 mm) equigranular facies. The globular quartz facies is characterized by a consider-able textural variation. It contains large quartz grains, phenocrysts of K-feldspar, zinnwaldite with a fine grained groundmass of quartz, plagioclase, zinnwaldite, tour-maline and topaz. The accessory phases are limited to apatite and rutile. On the contrary, the fine-grained facies is an equigranular rock with euhedral albite, lithium mica (zinnwaldite). Accessory minerals include monazite, apatite, zircon and rutile with some occurrences of arsenopyrite and cassiterite (Manning et al., 1996).
The topaz granite (TZG) is medium grained, characterized by euhedral-subhedral fluorine-rich topaz, interstitial lithium mica, albite, plagioclase feldspar with perthitic orthoclase, and subhedral quartz (Manning and Hill, 1990). It contains a smaller proportion of accessory phase but a wider variety including sub-economic minerals (Manning et al., 1996). The accessory minerals are apatite, amblygonite, zircon, Nb-Ta oxides (columbite-tantalite and ilmenorutile) and uraninite. According to (Manning and Exley, 1984), the field relations indicates a separate origin of the topaz granite. The mineralogy of the six granites is summarized in Table 1.2.

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GEOLOGICAL BACKGROUND

St Austell granite (Bray and Spooner, 1983). Whereas two different types of miner-alisations, a Sn breccia-stockwork in the Treliver area, generally unenriched in base metals, and a E-W veins complex enriched in base metals have been identified north of the St Austell pluton (Camm and Dominy, 1999; Camm and Moon, 2001). In addition, Müller and Halls (2005) describe an intrusive tourmaline breccia in biotite granite at Wheal Remfry where zoned rutile is the principal mineral hosting Sn (up to 1.88%) and also includes high W (up to 1.95%) and Nb (up to 2.05%) domains.

Similarities with granites-based kaolin deposits worldwide

From all the magmatic units of the St Austell pluton, the topaz granite is the youngest, and the most geochemically different, relatively enriched in Li, Rb and with a much higher Nb/Zr ratio (Manning and Hill, 1990). Nonetheless, there are close similarities between the mineralogical assemblages described above and the accessory minerals assemblage of evolved granite elsewhere described as peraluminous high to intermediate phosphorus granites (Linnen and Cuney, 2005). All the St Austell granite types are felsic and weakly peraluminous (1.16>A/CNK>1) granites. However if Li2O is included, the molecular ratio A/CNKL is reduced, to less than 1 in the case of topaz granite (Manning et al., 1996). The comparison of the P2O5=SiO2 ratio of the St Austell granites with other rare-metal granites shows that the St Austell granites belong to the intermediate phosphorous granite (IHP) type (Figure 1.5a). The relatively low Zr and Th contents of St Austell granites, visible in Figure 1.7b are characteristic of non-peralkaline rare-metal granites.
Figure 1.7: Trace-element characteristics of peraluminous high phosphorous (PHP), pera-luminous intermediate phosphorous (IHP), peraluminous low phosphorous (PLP) and per-alkaline granites (PLK). (a) Silica-phosphorus diagram. (b) Zirconium-thorium diagram. B series: Beauvoir granite, Massif Central; YCH: Yichun granite, Jiangxi, China. Abbreviations for St Austell granites are given in the text. Based on data from Manning et al. (1996) and the review of Raimbault et al. (1995).
two-micas granites, corresponding to the S-type, rich in hydroxylated minerals (mus-covite) and enriched in lithophile elements such as Nb, Ta, Li, Be, P , F (Jébrak and Marcoux, 2008). Among these granites some display similarities with the St Austell granite and are associated with Sn, W, and Ta-Nb disseminated mineralisations (Černý et al., 2005). Some of this granites are associated to kaolin deposits which are also known to display similarities with Cornish kaolin deposits (Wilson and Jiranek, 1995; Wilson et al., 1997).
According to Scott et al. (1998), the Yichun granite in China is one of the closest rare-metal granite, in terms of lithology. It comprises also several magmatic units in-cluding a biotite granite and a Li-mica (lepidolite) granite such as the St Austell granite. The Li-micas granite is the most evolved magmatic unit (cf. St Austell topaz granite) and display a similar accessory assemblage of amblygonite/montrebasite, apatite, flu-orite, Mn-tantalite, microlite, Ta-cassiterite, ilmenite, monazite, zircon, sphalerite and topaz (Belkasmi et al., 2000; Yin et al., 1995). The Beauvoir topaz-lepidolite albite granite (Massif Central, France) is also very similar to the St Austell topaz granite and display a Sn-Li-Ta-Nb-Be disseminated mineralisation with an accessory minerals assemblage composed of topaz, apatite, amblygonite, columbite-tantalite, microlite, zircon, uraninite and sphalerite (Cuney et al., 1992; Raimbault et al., 1995). Further similarities exist with the Podlesí granite in Czech Republic which is composed of three magmatic units, including biotite granite, and two Li-mica granites. All gran-ite types contain disseminated Ta-Nb-Ti-W-Sn minerals including rutile, cassiterite, Fe-columbite, ixiolite and ferberite (Breiter et al., 2007).
All the above-cited deposits are considered as potential source for Nb, Ta, Sn or W and some of them are actually mined for these metals. For instance the Yichun granite is mined for Ta and Li (Schwartz, 1992) and the Beauvoir granite is known to produce cassiterite (800 g/t Sn) and columbite (190 g/t Ta plus 120 g/t Nb) as by-product of the kaolin production (Pohl, 2011).
By analogy with the Beauvoir granite it is possible to estimate the most prospective Nb-Ta-Sn and REE target. Rare-element chemistry of the Beauvoir granite shows that the concentration of most metallic elements including Nb, Ta, Sn and W is very high and increase with the Li contents from the bottom to the upper part of the granite body (Cuney et al., 1992). The outcrop surface dimensions of the Beauvoir Granite (around 14 k m2) are considerably less than even the smallest occurrence of topaz granite, but it correspond to the top of the cupola, which is the most enriched part of the orebody in terms of rare-metals, due to a combination of magmatic process and interaction with meteoric water (Raimbault et al., 1995). It is therefore the upper parts of the topaz granite that should have the greatest potential for disseminated magmatic mineralization. The exact morphology and the intern organisation of the topaz granite is poorly constrained but Manning and Hill (1990) suggested that the individual out-crops of topaz granite may be connected at depth. The field relations indicate that the Nanpean stock represents a position well below the roof of the granite body and that the Hensbarrow stock represents the roof. Thus, from a pure metallogenic perspective, the Hensbarrow stock, which represents the upper part of the St Austell topaz granite, is the most prospective target zone for Ta, Nb and Sn within the topaz granite.

Table of contents :

Chapter 1 – Introduction
1.1 Mining-induced seismicity mechanisms
1.2 Monitoring of mining-induced seismicity and hazard assessment
1.3 Motivation, strategy and structure of this thesis
Chapter 2 – Study area: Garpenberg mine and Lappberget orebody
2.1 An introduction to Garpenberg mine
2.2 Geological setting and initial stress state
2.2.1 Lappberget orebody and weakness zones
2.2.2 Initial stress state and elastic rock mass properties
2.3 Mining method and sequencing
2.4 Geophysical and geotechnical monitoring in Lappberget
2.4.1 Extensometer data
2.4.2 Strain measurements
2.4.3 Microseismic data
2.5 Seismic activity and observed damage
Chapter 3 – Seismic data processing
3.1 Routines of seismic data acquisition and processing
3.1.1 Type of recorded seismic signals
3.1.2 Challenges and common errors in daily data processing
3.2 Picking consistency evaluation – The Wadati analysis
3.3 Evaluation of microseismic network performances
3.3.1 EMAP algorithm methodology
3.3.2 EMAP application to Lappberget microseismic network
3.4 Considerations about the extension of the analyzed area
3.5 Seismic source parameters estimation
3.5.1 Considerations on source parameters uncertainties
Chapter 4 – Rock mass response to mining
4.1 Spatiotemporal behavior of microseismic activity and mine blasts
4.1.1 Seismic sequences and clusters
4.2 Analysis of seismic source parameters
4.2.1 Temporal variation in b-value
4.3 What drives seismicity?
4.4 Analysis of geotechnical observations
4.5 Summary and discussion
Chapter 5 – Numerical modelling
5.1 Numerical modelling techniques
5.2 Model choice and strategy
5.3 Description of the model
5.3.1 Model geometry and boundaries
5.3.2 Model meshing
5.3.3 Initial and boundary conditions
5.3.4 Modelled elements and mechanical effect of paste fill
5.3.5 Constitutive laws and mechanical properties
5.3.6 Simulated mining sequence
5.4 Comparison with in situ geotechnical measurements
5.5 Model results and interpretations
5.5.1 Analysis of stress distribution
5.5.2 Analysis of strain distribution
5.5.3 Analysis of plastic zones and influence of weak geological materials
5.5.4 Temporal evolution of model parameters
5.6 Discussion and conclusion
Chapter 6 – Combined analysis of seismicity and numerical modelling
6.1 Relating induced seismicity with geomechanical modelling
6.2 Strategy of comparison in our work
6.3 Qualitative comparison at large-scale
6.3.1 Plastic zone and seismic activity
6.3.2 Instability criteria and seismic activity
6.4 Quantitative comparison at small-scale
6.4.1 Model and seismic parameters at punctual locations
6.4.2 Model and seismic parameters at spheres location
6.5 Summary and conclusion
Chapter 7 – Summary, conclusions and perspectives
7.1 Microseismic and geotechnical data analysis and interpretation
7.2 Numerical modelling and mining-induced seismicity
7.3 General perspectives
Bibliography

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