A Stope Mining Design with Consideration of Hanging Wall When Transitioning from Open Pit Mining to Underground Mining for Sepon Gold Mine Deposit, Laos
Abstract
:1. Introduction
2. Geology of Gold and Copper Mineralization at Sepon Mineral District
2.1. Sepon Mineral District
2.2. Geology and the Structural Geology
2.3. Alteration and Mineralization
- Center on RDP intrusion: Cromie [29] has classified four types of alteration in the RDP intrusion centers (i.e., Phadan and Thenkham); (a) K-feldspar dominantly replaced the primary (igneous) plagioclase in the phenocryst and groundmass; (b) white mica with minor chlorite overprinted K-feldspar alteration; (c) quartz vein with K-feldspar alteration halo and later by quartz–chlorite–pyrite–molybdenite–chalcopyrite–bornite–hematite vein; and (d) massive barren quartz vein, vein stockwork. Cu and Mo sulfides are mainly present in the vein quartz and disseminated together with K-feldspar and white mica-chlorite alteration assemblages. However, the grades of Mo and Cu are slightly low, ranging from 40 to 1020 ppm Mo and <0.1 to 0.5% Cu, identifying sub-economic Mo and Cu mineralization in the SMD [30].
- Contact between wall rock lithologies and RDP: the contact between wall rock lithologies, particularly carbonated rocks (e.g., calcareous shale, limestone, and dolomitic limestone) of the Nalou and Kengkuek Formations, as shown in Figure 2, and RDP intrusions resulted in Cu-(Au) skarn mineralization. Cannell, Smith, Cromie, and Seedorff [27,29,31] classified three types of skarn mineralization and alteration assemblage: (a) garnet with minor pyroxene, biotite, and K-feldspar assemblages of prograde skarn, which were overprinted by (b) chlorite, epidote, hematite, calcite, and sulfide (e.g., chalcopyrite–bornite–molybdenite ± Au); and (c) quartz, calcite, fluorite, and pyrite ± Au in the later stage. Primary copper mineralization presented in semi-massive sulfide zones is typically less than 1% Cu, though locally, up to 5% Cu is present in chalcopyrite-rich zones [27]. Lower-grade gold, up to 1 ppm, was identified as a solid solution with chalcopyrite, and higher-grade gold, up to 290 ppm, occurred as a solid solution in pyrite crystal structure during the later stage.
- Sediment-hosted gold deposit: distal distance from the RDP intrusion centers, where the sets of dike and sill intruded calcareous mudstone and calcareous shale of the Discovery Formation presented sediment-hosted gold deposits as shown in Figure 2. Steep faults and secondary shallow to moderately dipping fault structures enhanced the RDP intrusions and elevated hydrothermal fluid ascending, resulting in intensely silicified carbonated rocks or jasperoid and decalcified shale. Gold is predominantly found as invisible gold or gold in solid solution with the crystallization of pyrite, most often disseminated and fracture controlled together with jasperoid and decalcified shale.
2.4. Geotechnical Rock Mass Characterization
3. Numerical Analysis
3.1. The Input Parameters
3.2. Model Construction
4. Results and Discussion
4.1. Surface and Slope Deformations
4.2. Comparison of Different Vein Widths
4.2.1. Yielded Elements
4.2.2. Displacement
4.3. Determined of the Various Stress Ratio
4.4. Different GSI Values
5. Conclusions
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- Surface and Slope Deformations: The stability of the slopes was thoroughly examined, revealing a factor of safety (FoS) of approximately 2.46. Displacement monitoring revealed that significant displacement primarily occurred in the hanging wall, while the footwall remained relatively unaffected. This highlights the importance of assessing and managing slope stability, particularly in relation to the hanging wall.
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- In the comparison of six different vein widths, the study evaluated six designs for open stope dimensions. The findings revealed that wider open stopes were associated with larger failure zones and higher displacements. On the other hand, narrower designs showed lower potential failure zones, although displacement gradually increased with larger stope sizes. Based on the analysis, it was determined that a stope dimension of 5 × 25 m was considered appropriate. This design struck a balance between avoiding excessive failure risks associated with larger dimensions and ensuring sufficient stability. By selecting this stope dimension, this study aimed to mitigate the potential for significant failures while also taking operational considerations into account.
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- Determination of Various Stress Ratios: The impact of different stress ratios on instability around the open stope was analyzed. Lower stress ratios showed relatively low instability, while higher stress ratios above 1.5 led to significant failure zones. The hanging wall side was found to be more prone to failure due to a weaker rock mass. This highlights the need to consider stress ratios and their influence on stability when designing and managing open stopes.
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- Different Geological Strength Index (GSI) Values: The geological strength index (GSI) was found to have a notable influence on safety factors and horizontal displacements. Lower GSI values resulted in larger failure zones and higher displacements, while higher GSI values (above 35) improved stability and reduced failure zones [37], particularly in the hanging wall. Understanding the GSI of the rock mass is crucial for assessing stability and implementing appropriate design strategies.
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- To ensure stability, it is recommended that the crown pillar thickness not be less than 40–50 m and the sill pillar have a minimum thickness of 5 m. These recommendations are based on the geological conditions specific to the mine and are essential for maintaining the stability of the underground workings.
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Geological Strength Index (GSI) | Zone/Rock Type | Unit Weight (MN/m3) | σci (MPa) | Tensile Strength (MPa) | Erm (GPa) | v | Hoek Brown Parameter | ||
---|---|---|---|---|---|---|---|---|---|
mb | s | a | |||||||
72 | Footwall RDP | 0.0275 | 82.58 | 15.59 | 17.43 | 0.3 | 6.167 | 0.0315 | 0.501 |
62 | Ore body | 0.0266 | 224.92 | 13.81 | 29.36 | 0.3 | 2.656 | 0.010 | 0.502 |
51 | Hanging wall Nodular shale | 0.0279 | 66.59 | 11.66 | 14.23 | 0.35 | 0.898 | 0.001 | 0.505 |
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Phaisopha, S.; Shimada, H.; Sasaoka, T.; Hamanaka, A.; Pongpanya, P.; Shorin, S.; Senthavisouk, K. A Stope Mining Design with Consideration of Hanging Wall When Transitioning from Open Pit Mining to Underground Mining for Sepon Gold Mine Deposit, Laos. Mining 2023, 3, 463-482. https://doi.org/10.3390/mining3030027
Phaisopha S, Shimada H, Sasaoka T, Hamanaka A, Pongpanya P, Shorin S, Senthavisouk K. A Stope Mining Design with Consideration of Hanging Wall When Transitioning from Open Pit Mining to Underground Mining for Sepon Gold Mine Deposit, Laos. Mining. 2023; 3(3):463-482. https://doi.org/10.3390/mining3030027
Chicago/Turabian StylePhaisopha, Seelae, Hideki Shimada, Takashi Sasaoka, Akihiro Hamanaka, Phanthoudeth Pongpanya, Seva Shorin, and Khounma Senthavisouk. 2023. "A Stope Mining Design with Consideration of Hanging Wall When Transitioning from Open Pit Mining to Underground Mining for Sepon Gold Mine Deposit, Laos" Mining 3, no. 3: 463-482. https://doi.org/10.3390/mining3030027