Conference Paper

Saprolite Slope Design at the Rosebel Gold Mine

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The Rosebel Gold Mine, located in Suriname, is comprised of eight open pits which have been developed to varying depths. Due to their origin and tropical climate setting, the rocks throughout the site are deeply weathered with saprolite and transition (sap-rock) extending to depths greater than 70 m. Existing interim saprolite and transition slopes have been excavated in several of the operating pits. The performance of these slopes is extremely variable due to the impact of relict structures, groundwater, intense rainfall, and protolith. A detailed geotechnical investigation program was undertaken during 2013 through 2014 to provide slope design configurations that could be practically implemented in this high-rainfall tropical environment using the capabilities of the equipment on site. This paper documents the methodology used to generate the saprolite and transition slope designs and the implementation requirements. The slope design approach involved a detailed review of existing pit slope failures in various geotechnical settings along with findings from the geotechnical drilling program, to aid in the estimation of strength parameters of the materials and analysis of controlling factors on slope stability. A series of simple numerical models were then generated to support the slope designs. One of the controlling factors was found to be orientation of relict structures and foliation. Groundwater control was highlighted as another controlling factor. Back-analysis indicated that the most critical period from a groundwater perspective was when the mine floor was located at the base of the saprolites. This is due to elevated pore pressure in the toe of the saprolite slopes. As mining progresses into the more permeable transition material, passive drainage of the transition layer acts as a natural drain beneath the saprolites. Identification of this process enabled mine plans to be modified such that the natural drainage could be used to depressurize the slopes. Adjusting mine plans to take advantage of these natural processes reduced the need for a more complex and costly dewatering system.

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... However, the proportion of saprolite mining also reduced significantly around 2016 as several pits advanced into the transition and fresh rock. Interception of the more permeable transition and upper fresh rock has been shown to promote underdrainage and depressurisation of the overlying saprolite (Abrahams et al. 2015) and this is a likely cause of reduced seasonality in failure frequency. As mining continued into the fresh rock, the seasonality of failures is less evident than during mining of the saprolite and transition, as evidenced by the relatively linear cumulative failure curve from mid-2017 onwards. ...
... An alternative would be to undertake probabilistic design analyses. Figure 6 is a scatter plot of design cohesion versus friction angle from various sites in Australia, Africa, Madagascar and Suriname (Abrahams et al. 2015); the data is presented in this way to demonstrate the range of values for the datasets considered. Generally, the cohesion and friction angle form a cluster in the 10-35 kPa and 25-35° range respectively. ...
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Mining in residual soils is a characteristic of some open pit mines, particularly those mines in the tropical and sub-tropical regions. With residual soils’ prevalence on the earth’s surface almost as common as that of sedimentary rocks (Wesley 2013), mining in such soils requires special understanding of the behaviour and characteristics of the residual soil to determine slope designs that are both safe and economic. Due to the presence of relict structures, and the relatively low strength of the residual soils and weathered rock, design slope angles in these materials should be developed by blending the results of the kinematic assessments of geologic structures with rock mass stability analyses and traditional soil mechanics (Newcomen & Burton 2000). It is thus imperative that geotechnical designs should be site/location-specific and based on soil’s field performance, back-analyses and risk zoning. Understanding the variability of these materials is important for developing robust designs. This paper outlines the different aspects that are to be considered when conducting slope designs in residual soils, and in particular, saprolites, and summarises shear strength data from various mine sites that highlights the uncertainty associated with these parameters. Keywords: saprolite, residual soils, slope design, uncertainty, variability, reliability
... In addition, the conceptual scheme of a stratiform bedrock inherited from a single-phase weathering palaeoprofile since the Miocene (Dewandel et al. 2006;Koïta et al. 2013) allows questions of groundwater occurrence and flow anisotropy in shallow hard-rock aquifers to be tackled. The model stands for two superposed compartments, the saprolite or alterite layer (Abrahams et al. 2015) and the saprock or fissure layer (Fig. 1a), the latter enhanced by some vertical fault zones allowing for a deeper flow component (Dickson et al. 2015). Underneath the main valleys, other fault zones isolated from the saprock are assumed to be non-conductive. ...
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Randomness of fracture networks still makes channelized flow a challenge to track in hard-rock aquifers. While not underestimating geological and hydrological criteria that are also handled here through mapping exercises, this study raises an issue of water quality encountered in lifelong boreholes. Chemical classification checked against a recent conceptual model of bedrock aquifers gives birth to a new typology of groundwater in a complex granitic aquifer system located in the SW of Ivory Coast (West Africa). Major ion chemistry, borehole completion data, digital elevation model and satellite images are used to interpret the geochemical water facies as an expression of connexions between the saprolite and the saprock, or transient insulation. From major ions ratios, cumulate mineralization, carbonate equilibrium, stable isotopes, the maturation of ground waters and mixing between bedrock layers are described at seasonal and local scales. The results highlight some vertical feeding of the water table into the main saprock aquifer owing to shortcuts through the saprolite, along with the existence of dead-ends in the hydraulically active fracture network. Also, some influence of fault zones, either drain or barrier, is confirmed on the (Ca, Mg) bicarbonate water facies within the saprock.
... Remarkably, the VWPs shows a trend of strong downward gradient once mining advanced through the transition material into rock allowing the sap-trans overlying material to under drain easily between March and May 2016 in Figure 5. Also, elevated pore pressure response to rainy season can be seen clearly from January to August, 2017. It is however noted, that this strong downward gradient is not pronounced in all parts of the mine, which might be related to the significant role of rock fabrics and relict structures (Abrahams et al., 2015). Staggered benches were mined in the past to lower pore pressures locally in a narrower saprolite slopes while another part of the pit was operating in rock slopes at different elevation prior to opening up the adjacent saprolite slope segment. ...
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IAMGOLD Rosebel Gold Mines N.V. Rosebel Gold Mines is located in a tropical environment with high rainfall. The operation comprises several open pits with extensive slopes in saprolite and transition materials to a depth of more than 100m. The design of safe and economically viable slopes becomes challenging in the saprolite and transition materials due to the extremely variable thickness of the weathering profile, the high rainfall, and the complexity of relict structures. Interim slopes have performed variably and have been subjected to several types of failure mechanisms in saprolite and transition slopes. Documenting the slope height versus the slope angle relationship of the historical failures and stable areas of the interim slopes has provided valuable insight of slope performance. This paper presents an empirical approach to define slope design parameters and to provide an empirical guide to designing sustainable final walls.
... Various studies have shown that this simplification can result in reduced SRF estimates compared to full 3D analyses, as plane strain conditions increase overall kinematic freedom within 2D models (Cavounidis 1987;Chugh 2003;Albataineh 2006;Cala et al. 2006;Jiang et al. 2008). Despite these limitations, two-dimensional analysis is still widely used throughout geotechnical mine design as a simplification of three-dimensional behaviour (Hormazabal et al. 2013;Abrahams et al. 2015;Wen et al. 2015;Wolter et al. 2015;Argumedo et al. 2016;Tuckey et al. 2016). While geotechnical analysis in this study has applied 2D simplifications of the Ok Tedi pit, the same geostatistical methodology would also be applicable for full 3D analysis. ...
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With the increased drive towards deeper and more complex mine designs, geotechnical engineers are often forced to reconsider traditional deterministic design techniques in favour of probabilistic methods. These alternative techniques allow for the direct quantification of uncertainties within a risk and/or decision analysis framework. However, conventional probabilistic practices typically discretize geological materials into discrete, homogeneous domains, with attributes defined by spatially constant random variables, despite the fact that geological media display inherent heterogeneous spatial characteristics. This research directly simulates this phenomenon using a geostatistical approach, known as sequential Gaussian simulation. The method utilizes the variogram which imposes a degree of controlled spatial heterogeneity on the system. Simulations are constrained using data from the Ok Tedi mine site in Papua New Guinea and designed to randomly vary the geological strength index and uniaxial compressive strength using Monte Carlo techniques. Results suggest that conventional probabilistic techniques have a fundamental limitation compared to geostatistical approaches, as they fail to account for the spatial dependencies inherent to geotechnical datasets. This can result in erroneous model predictions, which are overly conservative when compared to the geostatistical results. Link:
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The Proton Magnetic Resonance (PMR) or Nuclear Magnetic Resonance (NMR) method, coupled with geometrical aquifer modelling, has been used to create a map of groundwater reserves over a 270 km(2) study area in a weathered basement setting. Most of the reserves are contained in a stratiform multi-layer aquifer whose geometry is influenced by the weathering front. The depths to the interfaces determined by PMR are considered and validated by comparison with the geometrical approach. Water contents and decay times of the PMR signal for each weathered layer are compared with the hydrogeological model. The results of the study show a decrease in water content from the top downwards for the three main aquifer layers (respectively : unconsolidated alterite, and an upper and a lower fissured zone). The groundwater reserves (80% in the fissured zone and 20% in unconsolidated alterite) represent approximately three years of average infiltration.
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The weathered layers of crystalline rocks form aquifers that are of prime interest for water supply in hard rock areas. These weathering profiles generally develop under both stable geodynamic conditions (weathering rate ≫ erosion rate) and a hydrolysing climate. They are composed of thick stratiform layers that follow the paleo-landscape (paleotopography) and thus present a gently dipping sequence at a regional scale.The structure and the hydrodynamic properties of the weathering profile of a granitic area (53 km2 Maheshwaram catchment, state of Andhra Pradesh, India) were characterized in detail and mapped from observations on outcrops, 80 vertical electric soundings and lithologs from 45 borewells in which flowmeter measurements and injection tests were also performed to characterize the hydraulic conductivities of the conductive fissure zones.The structure of the weathering profile results from a multiphase process: an ancient weathering profile was partly eroded, down to its fissured layer. It was later re-weathered more or less parallel to the current topographic surface. This peculiar structure is linked to the geodynamic history of the Indian Peninsula that underwent alternate weathering and erosion-dominated phases. The profile is thus composed, from top to bottom, up to a total depth of 35 m, of sandy regolith saprolite (1–3 m), 10–15 m of laminated saprolite containing unusual preserved fissures and a 15–20 m thick fissured layer.By comparing various case studies in similar terrain, a generalized 3-D geological and hydrogeological conceptual model of granite-type aquifers (granites, gneisses, etc.) is proposed. In granite-type rocks, single weathering or multiphase weathering and erosion processes induce similar geological structures. A few main geological differences arise from the comparison of single phase and multiphase weathering profiles: (i) in the later, the respective thicknesses of the various layers can be deeply modified, (ii) the ancient fissured layer can be re-weathered, and the resulting new laminated layer may contain quite well preserved and conductive ancient fissures, and (iii) the upper part of the fissured layer is more densely fissured. As a result of the same weathering processes, the fissures from the fissured layer of single phase or multi-phase profiles exhibit very similar hydraulic conductivities, and show both a higher density of conductive fissures at the top of the layer. The preserved ancient fissures within the laminated layer of a multiphase profile are partly obliterated by the recent weathering; their hydraulic conductivity is thus significantly reduced. Nevertheless, they do significantly contribute to borewell yield. These weathering-induced fissures provide most of the aquifer permeability.The use of such a conceptual model for the precise geological mapping of the weathering structure appears to be a prerequisite for groundwater development and management in hard-rock areas as it answers to several key issues.
Liberia is recovering from a 14 year civil war and only 51% of the rural population has access to safe drinking water. Little hydrogeological knowledge survives in Liberia, increasing the difficulty in successfully siting new boreholes. An understanding of the local hydrogeological environment is therefore needed to improve borehole site selection and increase success rates. This research provides a semi-quantitative characterization of the hydrogeological environment of the basement aquifer in Lofa county, Liberia. Based on literature review and analysis of borehole logs, the study has developed a conceptual hydrogeological model for the local conditions, which is further characterized using 2D geoelectrical sections. Groundwater is predominantly obtained from the saprolite and underlying fractured bedrock, but specific capacities (median 281lh(-1) m(-1); 25th and 75th percentile of 179 and 490 lh(-1) m(-1), respectively) are constrained by the limited thickness of the saturated saprolite. This study has shown that the groundwater resources in the crystalline basement in this part of Liberia conform to the general conceptual model, allowing standard techniques used elsewhere for siting and developing groundwater to be used.
The Rosebel gold district is hosted in a Paleoproterozoic greenstone belt of the Guiana Shield and has many characteristics that enable classification of the ores as an orogenic gold deposit. Host rocks have undergone several phases of deformation. However, gold deposition occurred late in the structural history of the belt, and is considered part of a late regional metallogenic event with respect to the geotectonic evolution of the Guiana Shield.Economic gold mineralization is hosted in felsic to mafic volcanic rocks and two sedimentary successions that are differentiated into turbiditic and arenitic depositional packages. The detailed lithostratigraphic characterization and the geochemistry enable the correlation of the local rock types with the Paramaka, the Armina, and the Rosebel formations respectively. The Rosebel district comprises eight discrete gold deposits distributed along two major structures. The northernmost structure is a sub-vertical WNW–ESE shear zone that preserves evidence of dextral strike–slip followed by normal faulting. The southern structure is an east–west reverse fault along which gold deposits are mainly hosted in the footwall. Gold mineralization is associated with quartz vein arrays developed along pre-existing structural heterogeneities, such as stratigraphic contacts and fold hinges. Four main sets of veins are recognized in the district: shear veins, north–south tension veins, stacks of north-dipping tension veins, and anticline-hosted tension veins. Mineralized quartz veins are typically associated with a wallrock alteration assemblage comprising sericite, chlorite, carbonate, tourmaline, pyrite, pyrrhotite, and plagioclase.The presence of a WNW–ESE dextral strike–slip structure, an east–west reverse fault, and north–south tension veins are consistent with the formation of a Riedel system during a simple shear event. All vein sets cut deformed sedimentary rocks that were deposited in a pull-apart basin, which together with their planar geometries, indicates a late-Transamazonian orogen timing for gold mineralization. Extensive dating of other orogenic gold deposits in the Guiana Shield has revealed ages between 2.023 and 1.955Ga for gold mineralization, which is broadly coeval with the onset of late-Transamazonian calc-alkaline volcanism and plutonism (2.05–1.88Ga) south of the Guiana Shield.
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