The Diavik waste rock project: Initial geochemical response from a low sulfide waste rock pile

Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053 wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1 year. Sulfate concentrations reached 2000 mg L⁻¹ when ambient temperatures were >10 °C, and decreased as ambient temperatures declined to <0 °C. The pH decreased to <5, concomitant with an alkalinity minimum of <1 mg L⁻¹ (as total CaCO₃), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were <0.36 and <0.11 mg L⁻¹, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13 mg L⁻¹), Ni (<7.0 mg L⁻¹), Co (<1.5 mg L⁻¹), Zn (<0.5 mg L⁻¹), Cd (<0.008 mg L⁻¹) and Cu (<0.05 mg L⁻¹) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite [Fe_1−xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite [CuFeS₂] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO₄, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock.     

The Diavik Waste Rock Project: Particle size distribution and sulfur characteristics of low-sulfide waste rock

Three large-scale instrumented waste rock piles were constructed at the Diavik Diamond Mine in the Northwest Territories, Canada. These experimental waste rock piles (test piles) are 15 m high and are part of an integrated field and laboratory research program to characterize and compare low-sulfide waste rock and drainage at various scales. During test pile construction, samples of the <50 mm fraction of waste rock were collected from two types of waste rock that are segregated during mining operations based on S content. The samples were analyzed for S content and particle size distribution. One test pile contained waste rock with an average of 0.035 wt.% S in the <50 mm fraction, within the operational S target of <0.04 wt.% S for the lower S waste rock type. The second test pile contained waste rock with an average of 0.053 wt.% S in the <50 mm fraction, lower than the operational S target of >0.08 wt.% S for the higher S waste rock type. The third test pile has a low permeability till layer and a low sulfide waste rock thermal layer covering a core of waste rock with average 0.082 wt.% S in the <50 mm fraction, which is within the operational S target of >0.08 wt.% S for the higher S waste rock. Particle size distributions for the lower and higher S waste rock are similar, but the higher S waste rock has a higher proportion of fine-grained particles. Sulfur determinations for discrete particle sizes of the <50 mm fraction illustrate higher S concentrations in smaller particles for both the lower S waste rock and the higher S waste rock. Similarly, S concentrations calculated for the >10 m scale, from composite blast hole cuttings, are lower than those calculated for the <50 mm scale. Acid–base accounting using standard methods and site-specific mineralogical information was used to calculate the ratio of neutralization potential to acid generating potential. A comparison of calculation approaches to pH and alkalinity data from humidity cell and test pile effluent suggest that ratios are very sensitive to the calculation method. The preferred calculation method was selected by comparing calculation results to pH and alkalinity data from humidity cell effluent collected over 95 weeks and test pile effluent collected over five field seasons. The preferred acid–base accounting values were obtained by calculating the average neutralization potential divided by the average acid potential of a sample set. This approach indicates that waste rock with >0.05 wt.% S is of uncertain acid-generating potential and effluent data indicate this waste rock generates acidic effluent; whereas lower S waste rock does not produce acidic effluent, consistent with the acid–base accounting predictions.     

The Diavik Waste Rock Project: Geochemical and microbiological characterization of low sulfide content large-scale waste rock test piles

Two experimental waste-rock piles (test piles), each 15 m in height × 60 m × 50 m, were constructed at the Diavik diamond mine in Northern Canada to study the behavior of low-sulfide content waste rock, with a similarly low acid-neutralization potential, in a continuous permafrost region. One test pile with an average of 0.035 wt.% S (<50 mm fraction; referred to as Type I) and a second test pile with an average of 0.053 wt.% S (<50 mm fraction; referred to as Type III) were constructed in 2006. The average carbon content in the <50 mm fraction of waste rock in the Type I test pile was 0.031 wt.% as C and in the Type III test pile was 0.030 wt.% as C. The NP:AP ratio, based on the arithmetic mean of particle-size weighted NP and AP values, for the Type I test pile was 12.2, suggesting this test pile was non-acid generating and for the Type III test pile was 2.2, suggesting an uncertain acid-generating potential. The Type I test pile maintained near-neutral pH for the 4-year duration of the study. Sulfate and dissolved metal concentrations were low, with the exception of Ni, Zn, Cd, and Co in the fourth year following construction. The pore water in the Type III test pile contained higher concentrations of SO₄²⁻ and dissolved metals, with a decrease in pH to <4.7 and an annual depletion of alkalinity. Maximum concentrations of dissolved metals (20 mg L⁻¹ Ni, 2.3 mg L⁻¹ Cu, 3.7 mg L⁻¹ Zn, 35 μg L⁻¹ Cd, and 3.8 mg L⁻¹ Co) corresponded to decreases in flow rate, which were observed at the end of each field season when the contribution of the total outflow from the central portion of the test pile was greatest. Bacteria were present each year in spite of annual freeze/thaw cycles. The microbial community within the Type I test pile included a population of neutrophilic S-oxidizing bacteria. Each year, changes in the water quality of the Type III test-pile effluent were accompanied by changes in the microbial populations. Populations of acidophilic S-oxidizing bacteria and Fe-oxidizing bacteria became more abundant as the pH decreased and internal test pile temperatures increased. Irrespective of the cold-climate conditions and low S content of the waste rock, the geochemical and microbiological results of this study are consistent with other acid mine drainage studies; indicating that a series of mineral dissolution–precipitation reactions controls pH and metal mobility, and transport is controlled by matrix-dominated flow and internal temperatures.     

The Diavik Waste Rock Project: Measurement of the thermal regime of a waste-rock test pile in a permafrost environment

The interior thermal regime of a field-scale experimental waste rock pile in the Northwest Territories, Canada, was studied. Test pile construction was completed in the summer 2006, and temperature data was collected continuously since that time to February 2009. The temperature data indicates the test pile cooled over the study period, with an average heat energy release of −2.5 × 10⁴ and −2.6 × 10⁴ MJ in 2007 and 2008, respectively. The mean annual air temperature (MAAT) at the site was −8.9 °C during the period between 2006 and 2009, with a permafrost table at a depth of 4 m in bedrock away from the pile. Because of this cold environment, the upward movement rate of the 0 °C isotherm into the test pile at its base was approximately 1.5 m a⁻¹ during 2007 and 2008. Thermistor strings installed immediately below the base of the test pile showed the test-pile basal temperatures remained near and below 0 °C during the study period. Furthermore, due to low rates of sulfide mineral oxidation, elevated temperatures in the interior of the test pile were not observed. The average air velocity in the pore space in July 2007 and 2008 was about one third of that during January of each year based on temperature distributions. Therefore, due to higher air velocity during the winter, it is expected that heat transfer is greater during winter.     

Water quality impacts from waste rock at a Carlin-type gold mine,Elko County,Nevada

Changes in water quality in the North Fork of the Humboldt River, Nevada are caused by weathering of waste rock from an inactive Carlin-type gold mine. Review of historical water-quality data, monthly water sampling, and continuous monitoring of water-quality parameters were used to quantify these impacts. River water pH, which ranged between 7 and 8, did not show statistically significant variation from upstream of the mine to downstream. Several constituents, most notably sulfate, calcium, and magnesium, showed statistically significant increases in dissolved-ion concentrations. These data, along with geochemical modeling, suggest that oxidation of sulfide minerals and in situ acid neutralization by carbonate host rocks are occurring. Large increases in dissolved-ion concentrations were observed twice a year—during spring snow melt and the onset of the winter precipitation season. These spikes are likely caused by flushing of pore waters that have reacted with waste rock during months-long periods when shallow groundwater recharge is not occurring.     

Laboratory and numerical modeling of water balance in a layered sloped soil cover with channel flow pathway over mine waste rock

Macropores developed in barrier layers in soil covers overlying acid-generating waste rock may produce preferential flow through the barrier layers and compromise cover performance. However, little has been published on the effects of preferential flow on water balance in soil covers. In the current study, an inclined, layered soil cover with a 10-cm-wide sand-filled channel pathway in a silty clay barrier layer was built over reactive waste rock in the laboratory. The channel or preferential flow pathway represented the aggregate of cracks or fissures that may occur in the barrier during compaction and/or climate-induced deterioration. Precipitation, runoff, interflow, percolation, and water content were recorded during the test. A commercial software VADOSE/W was used to simulate the measured water balance and to conduct further sensitivity analysis on the effects of the location of the channel and the saturated hydraulic conductivity of the channel material on water balance. The maximum percolation, 80.1% of the total precipitation, was obtained when the distance between the mid-point of the channel pathway and the highest point on the slope accounted for 71% of the total horizontal length of the soil cover. The modeled percolation increased steadily with an increase in the hydraulic conductivity of the channel material. Percolation was found to be sensitive to the location of the channel and the saturated hydraulic conductivity of the channel material, confirming that proper cover design and construction should aim at minimizing the development of vertical preferential flow in barrier layers. The sum of percolation and interflow was relatively constant when the location of the channel changed along the slope, which may be helpful in locating preferential flow pathways and repairing the barrier.     

Application of seepage flow models to a drainage project in fractured rock

Summary Various theoretical approaches are used to model groundwater flow in fractured rock. This paper presents the application of several approaches to the restoration of the drainage of Rofla tunnel, Grisons, Switzerland. In this tunnel it became necessary to take measures against the washing out of calcium carbonates from the tunnel lining cement, because the calcium carbonate clogged up the existing drainage tubes leading to increased rock water pressures on the inside arch of the tunnel. Drainage boreholes were drilled on a section of the tunnel and their influence on the water pressures was monitored. On the basis of the geological survey different seepage flow models were established to reproduce the measured water pressures. The models were then used to predict the future water pressures acting on the tunnel lining after restoration. Thus, the efficacy of the different drainage proposals could be predicted and therefore optimised. Finally, the accuracy of the predictions is discussed and illustrated using the measurements in the test section.     

Mineralogical and geochemical investigations of silicate-rich mine waste from a kyanite mine in central Virginia: implications for mine waste recycling

A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na₂O content (1.12–2.66 wt%), TiO₂ content (0.63–1.68 wt% TiO₂) and Fe content, expressed as Fe₂O₃ (up to 1.37 wt%). Major element compositions of samples were dominated by SiO₂ (87.894–90.997 wt%), Al₂O₃ (6.759–7.741 wt%), Fe₂O₃ (1.136–1.283 wt%), and K₂O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.     

Use of sample color to estimate oxidized Fe content in mine waste rock

 Color, a readily perceived feature of natural earth materials, including mine waste, often represents compositional variation as a result of oxidative processes involving Fe. Near surface samples from excavated trenches in a mine-waste rock pile were collected to investigate the relationship between color and contents of Fe, Cu, and S. The silt+clay fraction (<0.05 mm) was isolated from recognizable colored material of the bulk sample for determination of pH, total and extractable Fe and Cu, and bulk mineralogy. Rock fragments within the pile exhibited coatings of crystalline gypsum and amorphous Fe. These coatings result from weathering (secondary products) and play important roles in surface reactions of waste rock piles, such as adsorption of anions (SO₄ ^2–) or coprecipitation of Fe with Cu. The correlation between color (Hurst method) and extractable Fe was high. Although color is influenced by site conditions such as original mineral composition, materials handling, weathering conditions etc., the results suggest that color measurements may provide an inexpensive and rapid estimation for secondary iron compounds and associated sorbed elements. Received: 5 April 1998 · Accepted: 30 June 1998     

Environmental characterisation of coal mine waste rock in the field: an example from New Zealand

Characterisation of mine waste rock with respect to acid generation potential is a necessary part of routine mine operations, so that environmentally benign waste rock stacks can be constructed for permanent storage. Standard static characterisation techniques, such as acid neutralisation capacity (ANC), maximum potential acidity, and associated acid–base accounting, require laboratory tests that can be difficult to obtain rapidly at remote mine sites. We show that a combination of paste pH and a simple portable carbonate dissolution test, both techniques that can be done in the field in a 15 min time-frame, is useful for distinguishing rocks that are potentially acid-forming from those that are acid-neutralising. Use of these techniques could allow characterisation of mine wastes at the metre scale during mine excavation operations. Our application of these techniques to pyrite-bearing (total S = 1–4 wt%) but variably calcareous coal mine overburden shows that there is a strong correlation between the portable carbonate dissolution technique and laboratory-determined ANC measurements (range of 0–10 wt% calcite equivalent). Paste pH measurements on the same rocks are bimodal, with high-sulphur, low-calcite rocks yielding pH near 3 after 10 min, whereas high-ANC rocks yield paste pH of 7–8. In our coal mine example, the field tests were most effective when used in conjunction with stratigraphy. However, the same field tests have potential for routine use in any mine in which distinction of acid-generating rocks from acid-neutralising rocks is required. Calibration of field-based acid–base accounting characteristics of the rocks with laboratory-based static and/or kinetic tests is still necessary.     

Spatial statistics for predicting fluid flow through a single rock fracture

Steady-state laminar flow through single rock fractures is predicted in terms of spatial statistics computed from the arrangement of voids and contact areas within the fracture. Within the voids, aperture is assumed to be constant. One statistic measures how often pixels alternate from void to contact area in the rows parallel to the flow direction. Two others measure the dispersion of voids in the rows and columns of the pattern. Fractures with complexity typical of observed data are simulated. Flow through patterns with 80% voids is predicted in terms of a linear combination of the three statistics. Using an extended model involving one of the three statistics, flow through patterns with other void fractions is predicted.The author did this work at the Earth Sciences Division at Lawrence Berkeley Laboratory, Berkeley, California. It is part of a Ph.D. thesis which was submitted at Stanford University.     

Assessing the acidic potential of waste rock in the Akara gold mine,Thailand

Acid mine drainage (AMD) is the environmental issue that generates the greatest public concern regarding the mining industry. Thus, characterization of mine waste rock according to acid generation potential is necessary for mining operations to ensure proper waste rock storage and to avoid future adverse environmental effects. Therefore, this study was conducted to estimate the potential of AMD generation in the largest operating gold mine in Thailand by using acid base accounting and net acid generation tests. Representative samples of six types of waste rock classified by mining geologists for mineral processing and waste dumping were collected for this study: volcanic clastic, porphyritic andesite, andesite, silicified tuff, silicified lapilli tuff, and sheared tuff. Under various conditions, experimental results indicate that only silicified lapilli tuff and shear tuff are potentially acid-forming materials. The results indicate that AMD generation may possibly occur a long time after mine closure due to the lag time of the dissolution of acid-neutralizing sources. Acidic generation from some waste rocks may occur in the future based on environmental conditions, particularly the oxidation of sulphide minerals by the combination of oxygen and water. Therefore, a proper design for waste rock dumping and storage is necessary to reduce the risk of AMD generation in future. It is advisable to install a surface management system to control the overland flow direction away from the waste dump area and tailing storage facility and to install a second water storage pond next to the main storage pond to store the spilled water during storms and the rainy season. A water quality monitoring plan that focuses on disturbed areas such as water storage ponds and mine pits should be put in place.     

Oxygen influx and geochemistry of percolate water from reactive mine waste rock underlying a sloping channelled soil cover

An ideal engineered soil cover can mitigate acid rock drainage (ARD) by limiting water and gaseous O₂ ingress into an underlying waste rock pile. However, the barrier layer in the soil cover almost invariably tends to develop cracks or fractures after placement. These cracks may change water flow and O₂ transport in the soil cover and decrease performance in the long run. The present study employed a 10-cm-wide sand-filled channel installed in a soil barrier layer (silty clay) to model the aggregate of cracks or fractures that may be present in the cover. The soil cover had a slope of 20%. Oxygen transport through the soil cover and oxidation of the underlying waste rock were investigated and compared to a controlled column test with bare waste rock (without soil cover). Moreover, gaseous O₂ transport in the soil cover with channel and its sensitivity to channel location as well as the influence of the saturated hydraulic conductivity of the channel material were modeled using the commercial software VADOSE/W. The results indicted that the waste rock underlying the soil cover with channel had a lower oxidation rate than the waste rock without cover because of reduced O₂ ingress and water flushing in the soil cover with channel, which meant a partial soil cover might still be effective to some extent in reducing ARD generation. Gaseous O₂ ingress into the covered waste rock was more sensitive to the channel location than to the saturated hydraulic conductivity of the material filling the channel. Aqueous equilibrium speciation modeling and scanning electron microscopy with energy dispersive X-ray analysis indicated that secondary minerals formed as a result of the oxidation of the waste rock included gypsum and goethite in the covered waste rock and schwertmannite and other Fe oxides in the uncovered waste rock. The findings of the study provided insight into the effect of channel flow on O₂ transport and oxidation of the covered waste rock, which may help to improve soil cover design and construction to minimise the generation of preferential flow in the barrier layer.     

Failure of a mine waste dump in Zimbabwe: Causes and consequences

A combination of factors are considered important in causing the failure and subsequent development of a flowslide of a gold mine waste dump (or tailings dam) at Arcturus, near Harare, Zimbabwe. These factors comprise poor basal drainage, steep perimeter walls, saturation of the walls and basal sediments through continued spigoting of slurry during a period of heavy rainfall, and the effect of this saturation on the tailings. Properties of the tailings, eyewitness accounts, documentary evidence, and site characteristics are discussed. The failure and subsequent development of a fatal 300-m flowslide are reconstructed in a five-phase developmental model. The general applicability of the results is discussed.     

含煤地层岩石吸水性分析——以陕北杭来湾煤矿为例

为了查明影响含煤地层岩石吸水性的主要影响因子及其对于岩石吸水性的影响关系,在陕北杭来湾煤矿含煤地层取岩样,根据岩样的水理性质测试及X射线衍射试验数据,通过灰色关联分析及逐步回归分析对岩石吸水性的影响因子进行对比分析,结果表明：孔隙度及伊利石、高岭石、钾长石、绿泥石、斜长石的矿物含量为岩石吸水性的主要影响因子;利用逐步回归分析方法建立了岩石吸水性的回归方程,精度较高,可以较好地反映该矿区含煤地层岩石的吸水性与其主要影响因子之间的量化关系。     

全尾砂-废石混合回填膏体流动特性变化规律

质量浓度、废石掺量是影响尾砂-废石混合回填体流动特性的主要因素,研究这些因素对膏体流动性的影响规律,对于控制膏体流动性具有重要意义。采用改进的小型直剪装置进行混合回填体直剪试验,以黏聚力、内摩擦角、抗剪强度为依据,综合得出了混合回填膏体流动性变化规律。试验结果表明,膏体在受力变形前期呈较为明显的似弹性性质,随废石掺量增加,位移-剪应力曲线中阶梯状变化趋势增加;回填体黏聚力取决于质量浓度,内摩擦角受质量浓度和废石掺量影响程度基本相同,两者协同作用;质量浓度在80%～88%范围时,黏聚力随质量浓度增大呈非线性增大,内摩擦角与质量浓度呈S型变化趋势;抗剪强度随质量浓度的增大而增加,在82%～84%区间抗剪强度变化显著,流动性出现突变。     

Micro- and macro-behaviour of fluid flow through rock fractures: an experimental study

Microscopic and macroscopic behaviour of fluid flow through rough-walled rock fractures was experimentally investigated. Advanced microfluidic technology was introduced to examine the microscopic viscous and inertial effects of water flow through rock fractures in the vicinity of voids under different flow velocities, while the macroscopic behaviour of fracture flow was investigated by carrying out triaxial flow tests through fractured sandstone under confining stresses ranging from 0.5 to 3.0 MPa. The flow tests show that the microscopic inertial forces increase with the flow velocity with significant effects on the local flow pattern near the voids. With the increase in flow velocity, the deviation of the flow trajectories is reduced but small eddies appear inside the cavities. The results of the macroscopic flow tests show that the linear Darcy flow occurs for mated rock fractures due to small aperture, while a nonlinear deviation of the flow occurs at relatively high Reynolds numbers in non-mated rock fracture (Re?>?32). The microscopic experiments suggest that the pressure loss consumed by the eddies inside cavities could contribute to the nonlinear fluid flow behaviour through rock joints. It is found that such nonlinear flow behaviour is best matched with the quadratic-termed Forchheimer equation.     

Noble gas and isotope geochemistry in western Canadian Arctic watersheds: tracing groundwater recharge in permafrost terrain

In Canada’s western Arctic, perennial discharge from permafrost watersheds is the surface manifestation of active groundwater flow systems with features including the occurrence of year-round open water and the formation of icings, yet understanding the mechanisms of groundwater recharge and flow in periglacial environments remains enigmatic. Stable isotopes (δ¹⁸O, δD, δ¹³C_DIC), and noble gases have proved useful to study groundwater recharge and flow of groundwater which discharges along rivers in Canada’s western Arctic. In these studies of six catchments, groundwater recharge was determined to be a mix of snowmelt and precipitation. All systems investigated show that groundwater has recharged through organic soils with elevated P_CO2, which suggests that recharge occurs largely during summer when biological activity is high. Noble gas concentrations show that the recharge temperature was between 0 and 5 °C, which when considered in the context of discharge temperatures, suggests that there is no significant imbalance of energy flux into the subsurface. Groundwater circulation times were found to be up to 31 years for non-thermal waters using the ³?H-³He method.