Mount Isa Mines Rehabilitation Material Sampling and Analysis Program for Closure Planning Matt Landers1, Greg Maddocks1, Candice Nucifora2, Kristian Mandaran2, Jason Jones2, Amanda Forbes3, Ward Wilson4, Steven Newman5
1RGS Environmental Pty Ltd, PO Box 3091, Sunnybank South, Queensland, Australia, 4109, [email protected] 2Mount Isa Mines, Glencore Pty Ltd, Australia 3Deswik Mining Consultants Pty Ltd, Level 22, Riparian Plaza, 71 Eagle St, Brisbane, QLD 4000, Australia 4Unsaturated Soils Engineering Ltd., 960 Massey Court NW, Edmonton, AB, Canada, T6R 3S8 5Newman Engineering Pty Ltd, Melbourne, Australia
Abstract Glencore’s Mount Isa Mines Limited (MIM) has completed the second phase of the rehabilitation materials sampling and analysis plan (RMSaAP) to identify and quan- tify potential materials that can be used to rehabilitate the large tailings storage facility (TSF) with an earthen cover system sourced from adjoining hills. Samples, collected from a test pit/ drilling program, were analysed to evaluate chemical/ physical proper- ties of regolith within major rock/ soil types. Results were used with Deswik so ware to develop a stratigraphic 3-D overburden model to delineate rehabilitation borrow sourc- es and schedule the TSF cover placement, and to undertake unsaturated zone modelling to evaluate the cover performance. Keywords: Soil Cover | Deswik | Mount Isa Mines | Rehabilitation | TSF | AMD | Mine planning
Introduction period of weathering, as ANC is depleted, the Glencore’s Mount Isa Mines Limited (MIM) neutral pH drainage may contain elevated operates the Mount Isa open cut and under- concentrations of some elements including ground copper (Cu) and zinc-lead (Zn/Pb) sulfate, manganese and zinc. Only when the mines near Mount Isa in Queensland, Aus- ANC is depleted would the tailings then be- tralia. Tailings from processed Cu and Zn-Pb gin to produce acid drainage. ore is contained in a 1,315 ha shallow (< 35 e main environmental risks posed by m deep) multi-cell valley ll tailings storage the tailings are the potential for (i) the physi- facility (TSF). cal movement of tailings or sur cial e ores- Static geochemical testing conducted by cent salts by aeolian processes, and (ii) the MIM over the life of mine (LoM) veri es that release of acid, major ions, metals and metal- the Cu, and Zn-Pb tailings have variable sul- loids from the tailings solids and the perco- de content and diverse mineralogy with sig- lation of these elements in solution into and ni cant acid neutralising capacity (ANC) in through the surrounding regolith, and to a some samples. e tailings are predominantly lesser degree through the tailings that may re- classi ed as potentially acid forming (PAF) sult in surface seeps, percolation to basement using static analytical methods. Kinetic leach regolith below the TSF and/or deeper perco- testing using free draining columns and hu- lation to bedrock. It is assumed tailings solids midity cell tests simulating tailings placed on and solutes would not be released from the the surface of the TSF suggest that the tailings TSF in surface water as the TSF is designed samples are likely to remain non-acid form- with su cient storage capacity. ing (NAF) for ≈7 years under aerial exposure, e overall management goal for the TSF and much longer than this under anoxic satu- is to maintain ongoing tailings deposition on rated conditions within the TSF. During this the tailings beach and then implement reha-
Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors) 21
01_Mining for closure BOOK.indb 21 9/3/18 10:22 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”
bilitation strategies a er the nal placement A soil horizons for each test pit, and select in- of tailings before sur cial ANC is depleted tervals from the core. in the tailings. is management approach e geochemical analytical methods in- should ensure that potential adverse envi- cluded continual downhole hyperspectral ronmental impacts are minimised as the PAF mineralogy by HyLoggerTM, pH/ EC, NAPP, tailings will continue to produce neutral pH CEC/ ESP, metal(loid)s analysis and nutrient drainage. In practice the decommissioning and organic carbon content. Physical analyti- and rehabilitation of the TSF will occur pro- cal methods included texture, porosity, den- gressively as TSF cells reach maximum design sity, particle-size-distribution (PSD), point storage capacity. load and abrasion testing, Emerson aggregate erefore, the current rehabilitation ob- analysis, Atterberg limits, hydraulic conduc- jectives for the TSF are to cover the tailings tivity and soil water characteristic curves. with an earthen cover system which is (i) safe e results from physical testing (parti- to humans and wildlife (ii) non-polluting (iii) cle-size-distribution, soil water characteris- stable, and (iv) able to sustain native vegeta- tic curves [SWCC] and saturated hydraulic tion. e earthen cover system design cur- conductivity) for the material types within rently proposed for the TSF comprise the the borrow areas were used to undertake - placement of a 1.5 m thick geochemically in- niteelement unsaturated ow modelling us- ert borrow material layer directly on the tail- ing SEEP/W (formerly VADOSE/W) (GEO- ings with a 0.5 m growth media layer overly- SLOPE International, 2016; GEO-SLOPE ing the borrow material. International, 2008a, b). e SWCC used in the modelling were corrected for particle Methods size according to the Bouwer-Rice correc- To achieve the rehabilitation objectives, MIM tion procedure (Bouwer and Rice, 1984) and is implementing a progressive RMSaAP. e tted with the equation by Van Genuchten work described in this study (Phase 2 of the (1980). e hydraulic conductivity function RMSaAP) aims to identify and quantify the (suction [kPa] versus hydraulic conductivity potential materials that can be used for re- [m/s]) was estimated using the measured ksat habilitation and landform design at MIM: and extrapolated using the Fredlund method at a broad scale within the project area. e (Fredlund et al., 1994). A two-dimensional earthen cover system requires soil and rego- (2-D) slice through the TSF with overlying lith to be sourced from adjoining hills that cover system, including growth medium (Fig. include volcanic, sedimentary and metamor- 1), was simulated under transient conditions. phic units overlain by ferrosol, rudosol and e models comprise a climatic ux-bound- chromosol soil pro les. A test pit and drilling ary on the surface which allows true climatic program was undertaken to collect samples conditions to be applied (rainfall, evapora- to (i) evaluate the variability of the chemical tion and evapotranspiration), with several and physical properties of the regolith within ux boundaries within each cover layer (and the major rock and soil types, and (ii) estab- within the tailings) to determine in ltration lish the depth and degree of weathering in (and capillary rise) rates. Since the tailings the hills (iii) used as input to numerical un- are de-watered there is no water table within saturated zone modelling to evaluate prob- the TSF; therefore, the lower boundary con- able cover performance (iv) map-out poten- dition of the models was simulated with unit tial borrow areas (shallow areas to strip soil) hydraulic gradient. Similarly, lateral inter ow and borrow pits to mine deeper regolith, and of water above the tailings was allowed to exit (v) to schedule the movement of the soil and the model via seepage face. As part of the as- regolith onto the TSF. sessment, the cover material thickness and e drilling and test pit program included texture were varied, and runo and in ltra- 30 test pits and ve PQ diamond core drill tion values provided as performance indica- holes placed within each of the major mate- tors. rial types or potential borrow areas. Samples Deswik.CAD so ware (Deswik.CAD were collected (nominally) from the O, E and v2017.2) was used to develop a concept level
22 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
01_Mining for closure BOOK.indb 22 9/3/18 10:22 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”
rial for rehabilitation, and no substantial dif- ferences in the pH, EC, NAPP or metal(loid) content in topsoil (0 to 0.5 m bgl), subsoil (0.5 to 2.0 m bgl) or deeper soil and regolith below 2.0 m. ere may be issues in the soil cover sys- tem relating to chemical imbalances (e.g. CEC) and biological aspects such as nutrient de ciencies, particularly if the upper topsoil layer is diluted with subsoil. Samples that were composited prior to analysis had the fol- lowing results: • Soil from 0.to 0.5 m bgl had the follow- ing averages pH 6, EC 45 µS/cm, CEC 5.4 meq/100g, ESP 0.5%, TKN 197 mg/kg, Bray P 2.2 mg/kg and TOC 1,283 mg/kg. • Soil from 0.5 m to 3 m bgl had the follow- ing averages pH 7.5, EC 62 µS/cm, CEC 4.9 meq/100g, ESP 0.3%, TKN 78 mg/kg, Bray P 0.8 mg/kg and TOC 766 mg/kg. • Regolith > 3 m bgl had the following averages pH 8.3, EC 53 µS/cm, CEC 1.3 meq/100g, ESP 0% (n=7) and 25% (n=1), TKN was BDL, Bray P was BDL and TOC Figure 1 VADOSE/W model geometry and mesh. was BDL (n=7) and 400 mg/kg (n=1). From the perspective of the physical material stratigraphic 3-D overburden model incor- properties, the surface material in all borrow porating a 2 m deep soil pro le and underly- areas is mostly non-dispersive and is domi- ing regolith. Mining of soil and regolith from nated by material with low clay content. e borrow areas and borrow pits and schedul- nes content decreases substantially with ing options for placement onto the TSF were depth in drill core. e physical sampling evaluated with varying plant and equipment and analytical program veri es a wide range using Deswik.LHS. in measured PSD in the test pit samples. e PSD on the crushed drill core samples pro- Results and Discussion vides some qualitative comparison between Geochemical and physical properties of sample types and was not intended to de ne soil and regolith the PSD of as-mined rehabilitation units. Whereas it is probable the sur cial test pit e results of the analytical program have samples would yield material with a suitable indicated that there are no fatal aws related nes content this may not be attained from to the chemical and physical properties of the deeper regolith units. However, because of upper pro le of the regolith (comprising soil the low permeability of the underlying tail- and sub-soil to ≈ 2 m bgl) or the ability to ings water that does percolate through the win extremely weathered to weathered mate- cover will be stored there in the pore voids. rial from borrow pits that could extend from e deeper regolith sampled from drill core the surface of the hills down to ≈ 420 m ASL may be amenable to use as “general ll” that (nominal elevation selected for the base of the comprises the 2.0 m layer of geochemically borrow areas as it would be above the nal inert material in the concept cover system: tailings beach and TSF embankments). the general ll may yield clay, slit and sand ere are no geochemical constraints to cobble size material in a soil cover system (pH, EC, TS, ANC and soluble and total when blasted. is is assumed because of the metal(oids) to using test pit or borrow mate- highly fractured nature of the regolith, low
Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors) 23
01_Mining for closure BOOK.indb 23 9/3/18 10:22 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”
rock strength and measured permeability. Rehabilitation scheduling One aspect of the evaluation of the pro- e material balances for the borrow sources, posed soil cover system which is not able to derived using Deswik.CAD, are presented in be modelled is that the soil cover will under- Tab. 1. e key results are that approximately go continual physical and chemical weather- 500 ha of borrow areas were identi ed in the ing, and a soil pro le with increasing nes study (Fig. 2), but approximately 1,500 ha content will develop within the soil cover of area is required to be rehabilitated. is over the tailings as nes are leached through means that the topsoil (speci cally) and un- the soil cover. derlying subsoil recovered from the 500 ha e volume of soil from the designated will need to be diluted i.e. if 60 cm of topsoil borrow areas (≈ 485 ha) is in the order of 3 is stripped from the borrow areas it would be Mlcm (short of the required 9 Mlcm) but this spread at a depth of 20 cm and it is uncer- could be increased to the desired 9 Mlcm by tain if this would enable sustainable growth increasing the stripping depth from 0.5 m to of vegetation without amendment of the as 1.5 m bgl. Bgl. is would result in “dilution” constructed soil cover. e rehabilitation of the chemical, physical and biological pa- schedules (in respect to the time over which rameters, and functions, of the as-placed soil the cover could be placed) are based on as- (as outlined in the above bullet points). sumptions needing testing and veri cation, for example the: Earthen Soil cover modelling • geotechnical properties of the tailings are e placement of a soil cover comprising not known and there are major (untested) soil and regolith sourced from borrow pits assumptions related to the time it will take around the perimeter of the MIM will shed for the tailings to consolidate so a soil cov- runo during the large rainfall events (e.g. > er can be placed on the TSF; and, 25 mm/day rainfall events) but will generally • the time it will take to construct the soil allow rain to percolate into the soil cover due cover (and therefore nancial cost) is a to the sandy / loam texture soil and the coarse function of gaining access to tailings and particle size distribution of the weathered to the eet that is used, therefore it is neces- heavily weathered regolith present to a depth sary to understand the physical proper- of 30 m bgl within the hills around the MIM ties of the tailings and TSF and determine TSF. e variable surface runo from the soil what type of equipment can (cannot) be cover ranges from: 0% for rainfall events < 10 used to construct a soil cover on the tail- mm per day; 16.5% for rainfall events > 10 ings. mm and < 30 mm per day; and, 42% for rain- fall events > 30 mm per day. Equipment for sourcing and placing soil and e modelling results verify that the regolith is assumed to include 1 x Komatsu amount of runo is reduced substantially if 1250 excavator and CAT 725G (20 t capacity) the slope of the soil cover is reduced because trucks and requires 14 to 15 years to complete ponding of water on a at surface allows more placement of the cover system. e Deswik water to percolate into the soil cover. Since landform haulage scheduling veri es there average annual evaporation (≈3 m/ yr) ex- are di erences in the order of 20,000 to 30,000 ceeds average annual rainfall (≈430 mm/ yr) hours truck time for the two borrow pit loca- by a factor of > 7 and since the tailings have tion options (Fig 2) but substantial di erenc- very low permeability almost all rain entering es in the order of 200,000 hours for the two the soil cover will leave the cover system as sequencing options (Fig 3). e Deswik mod- evaporation and or transpiration. el can be used to evaluate additional schedul- e coarse and blocky nature of the ing options including changing borrow area deeper regolith material may act in part as a locations, con gurations, haul routes, equip- capillary break reducing upward migration ment speci cations and placement strategies. of soluble weathering products but may also Similarly, the nancial aspects of the current allow some lateral movement of water within Deswik scheduling model can also be built-in the cover during large rainfall events. to the model.
24 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
01_Mining for closure BOOK.indb 24 9/3/18 10:22 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”
Figure 2 proposed borrow pit locations and scheduling options: Option 1 all borrow areas (le ) Option 2 N1 and N2 excluded (right).
Figure 3 Scheduling variations: strip sequence north to south (le ) versus no sequence (right).
Conclusions the tailings and should support a vegetative e ndings of the RMSaAP work program cover. Additional detail to support the gen- informed the work that is scheduled for 2018 eral conclusion of the work and limitations and 2019 that includes additional test pits and around this statement are summarised below. drill holes in each borrow pit to enable the Acknowledgements development of a 3D regolith model that will quantify the volume of each major rehabili- e authors thank Tshwane University of tation unit from topsoil through to the par- Technology (TUT) for hosting the ICARD | tially weathered regolith that could be mined IMWA | MWD 2018 Conference. from each borrow pit. When all borrow pits References have been drilled, sampled and analysed the detailed regolith models for each borrow area Bouwer H, Rice R (1984). Hydraulic Propertiesof will be used to develop a detailed earthen Stony Vadose Zones. Ground Water, 22(6). soil cover system construction schedule to Deswik Mining Consultants Australia Pty Ltd be completed with optimisation around the (2017) Deswik.CAD v2017.2 so ware https:// placement of materials and equipment eet www.deswik.com/ used to construct the cover system. Fredlund DG, Xing A, Huang S (1994) Predicting e approach being implemented by the permeability function for unsaturated soils MIM will provide a robust, reliable and trans- using the soil-water characteristic curve. Cana- parent method of calculating the timing and dian Geotechnical Journal, 31, p 533 - 546 cost of construction and the probable perfor- Geo-Slope International Ltd. (2008a) Seepage mance of the earthen soil cover system. e Modeling with SEEP/W 2007 – An Engineer- overall conclusion of this work is that the soil, ing Methodology. ird Edition, GEO-SLOPE sub-soil and extremely weathered to partially International Ltd., March. weathered regolith in the hills adjacent to the Geo-Slope International Ltd. (2008b) Vadose MIM TSF would be suitable to encapsulate Zone Modeling with VADOSE/W 2007 – An
Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors) 25
01_Mining for closure BOOK.indb 25 9/3/18 10:23 AM 11th ICARD | IMWA | MWD Conference – “Risk to Opportunity”
Engineering Methodology. ird Edition, GEO- van Genuchten M (1980) A Close-form Equation SLOPE International Ltd., March. for Predicting the Hydraulic Conductivity of Geo-Slope International Ltd. (2016) GeoStudio Unsaturated Soils. 2016. Version 8. Online. www.geo-slope.com
26 Wolkersdorfer, Ch.; Sartz, L.; Weber, A.; Burgess, J.; Tremblay, G. (Editors)
01_Mining for closure BOOK.indb 26 9/3/18 10:23 AM