Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining

Sulfate minerals are absent from tailings in the non-operational TSF1 at Mount Keith.

“Bea et al. (2012) demonstrate that sulfate minerals are absent from tailings in the non-operational TSF1 at Mount Keith.”

• cites Bea et al., 2012 sourceDisputed

  The Mineralogy of Mount Keith Tailings section says sulfate minerals occur in surface efflorescences and are less common in TSF1 than in TSF2—not absent from TSF1. It specifies that they are restricted to the upper few centimeters of the tailings deposit. The Model Parameters section says TSF1 operated from 1994 to 1996 and was sampled in 2006, consistent with its being non-operational at sampling. (LLM, conf=0.94)

  Checked by a reasoning model — may be wrong.

  Experimental — accuracy not yet measured

In situ carbonation of mineral waste from ultramafic mines has been documented previously.

“In situ carbonation of the mineral waste from ultramafic mines has been documented previously (Wilson et al., 2006(Wilson et al., , 2009a(Wilson et al., ,b, 2011;;Pronost et al., 2012;Bea et al., 2012;Beinlich and Austrheim, 2012), but systematic measurement and modeling of this phenomenon have not previously been attempted on the scale of a large operating tailings facility.”

• cites Wilson et al., 2006 sourceNot checked — no full text of the source
• cites Pronost et al., 2012 sourceNot checked — no full text of the source
• cites Bea et al., 2012 sourceSupported

Systematic measurement and modeling of in situ carbonation of mineral waste from ultramafic mines had not previously been attempted on the scale of a large operating tailings facility.

“In situ carbonation of the mineral waste from ultramafic mines has been documented previously (Wilson et al., 2006(Wilson et al., , 2009a(Wilson et al., ,b, 2011;;Pronost et al., 2012;Bea et al., 2012;Beinlich and Austrheim, 2012), but systematic measurement and modeling of this phenomenon have not previously been attempted on the scale of a large operating tailings facility.”

• cites Wilson et al., 2006 sourceNot checked — no full text of the source
• cites Pronost et al., 2012 sourceNot checked — no full text of the source
• cites Bea et al., 2012 sourceSupported

MIN3P was modified by Bea et al. (2012) to better represent dynamic atmospheric conditions by incorporating energy balance equations and vapour transport.

“MIN3P was recently modified by Bea et al. (2012) to better represent the influence of dynamic atmospheric conditions on reaction progress by incorporating energy balance equations and vapour transport.”

• cites Bea et al., 2012 sourcePartly supported

  The Model Development section says this work further enhanced MIN3P with water-vapor diffusion and atmospheric boundary conditions, supporting the claim about representing dynamic atmospheric conditions. It says the energy balance equation had already been implemented in work cited as Bea et al. (2011), rather than being incorporated as part of this 2012 work. (LLM, conf=0.88)

  Experimental — accuracy not yet measured

Secondary efflorescent minerals are more likely to form in an evaporative horizon at the contact between tailings and the atmosphere.

“secondary efflorescent minerals are more likely to form within an evaporative horizon at the contact between tailings and the atmosphere”

• cites Acero et al., 2007 sourceNot checked — no full text of the source
• cites Acero et al., 2009 sourceNot checked — no full text of the source
• cites Bea et al., 2012 sourceSupported

Trends of decreasing primary gangue mineral abundance can be caused by dilution as secondary minerals precipitate from tailings water, solutes, and atmospheric gases.

“Bea et al. (2012) have shown that such trends can be caused by dilution of primary gangue minerals as secondary minerals precipitate from tailings water, solutes and atmospheric gases.”

• cites Bea et al., 2012 sourceSupported

Conversion of hydromagnesite to magnesite requires tens to hundreds of years.

“Zhang et al. (2000) estimate that the conversion of hydromagnesite to magnesite requires tens to hundreds of years.”

• cites Zhang et al., 2000Supported

Hydromagnesite precipitates within mine tailings at the Mount Keith Nickel Mine as a direct result of mining operations.

“The hydrated Mg-carbonate mineral, hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O], precipitates within mine tailings at the Mount Keith Nickel Mine, Western Australia as a direct result of mining operations.”

• Also stated in: § 1. Introduction · plain

  “At the Mount Keith Nickel Mine, Western Australia, the hydrated Mg-carbonate mineral, hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O], develops as a weathering product within ultramafic mine tailings.”

At least 80% of the carbon in hydromagnesite at Mount Keith is sourced from the modern atmosphere.

“Our radiocarbon results indicate that at least 80% of carbon in hydromagnesite is sourced from the modern atmosphere.”

Dissolution of atmospheric CO2 into mine tailings water at Mount Keith is kinetically limited.

“Stable isotopic results indicate that dissolution of atmospheric CO 2 into mine tailings water is kinetically limited, which suggests that the passive rate of carbon mineralization could be accelerated.”

Approximately 39,800 tonnes per year of atmospheric CO2 are being trapped and stored in tailings at Mount Keith.

“Based on our assessment, approximately 39 800 t/yr of atmospheric CO 2 are being trapped and stored in tailings at Mount Keith.”

• AssumptionRests on an assumption: The tailings samples collected along the accessible paths and perimeter are representative of the entire TSF2 and can be used to estimate total CO2 fixation at Mount Keith.

The CO2 trapped and stored in tailings at Mount Keith represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions.

“This represents an offsetting of approximately 11% of the mine's annual greenhouse gas emissions.”

Hydromagnesite and halite abundances in tailings of all ages are greatest within 0 to 25 cm of the surface.

“The abundances of both hydromagnesite and halite in tailings of all ages are greatest within 0 to 25 cm of the surface”

The most common and abundant efflorescent phases at Mount Keith are secondary hydromagnesite, sulfate minerals, and halite.

“The most common and abundant efflorescent phases at Mount Keith are secondary hydromagnesite, sulfate minerals and halite.”

SEM images commonly show fine hydromagnesite crystals precipitating at the surface of serpentine grains in Mount Keith tailings.

“SEM images of Mount Keith mine tailings commonly show fine crystals of hydromagnesite precipitating at the surface of grains of serpentine (Fig. 2).”

The highest halite concentrations occur at shallow depths, typically above 25 cm.

“As with hydromagnesite, the highest concentrations of halite occur at shallow depths, typically above 25 cm (Fig. 5).”

F14C values for carbonate minerals at Mount Keith range from 0.004 to 1.052.

“The F 14 C values for carbonate minerals at Mount Keith range from 0.004 to 1.052”

Sulfide minerals have not been detected by XRD in Mount Keith tailings to a limit of approximately 0.5 weight percent.

“Sulfide minerals have not been detected with XRD to a limit of approximately 0.5 wt.%.”

Within weeks to months of deposition, desiccation cracks begin to appear in the upper few centimeters of new tailings flows.

“Within weeks to months of tailings deposition, desiccation cracks begin to appear within the upper few centimeters of new tailings flows.”

The approximately 35-meter tall W1 riser was buried in tailings to a depth of 19 meters at the time of sampling.

“The ~35-meter tall W1 riser (near sampling sites 1 and 2, Fig. 1a) was buried in tailings to a depth of 19 m at the time of sampling.”

After approximately 6 to 12 months, many more desiccation cracks have propagated within hardened efflorescences on tailings surfaces.

“After approximately 6-12 months, many more cracks will have propagated within the hardened efflorescences on tailings surfaces (Fig. 3d).”

At Mount Keith, hydromagnesite, halite, hexahydrite, and blödite dominate the efflorescences, commonly with lesser amounts of epsomite, konyaite, löweite, and gypsum.

“At Mount Keith, hydromagnesite, halite, hexahydrite [MgSO 4 •6H 2 O], and blödite [Na 2 Mg(SO 4 ) 2 •4H 2 O] dominate these efflorescences, and are commonly associated with lesser amounts of epsomite [MgSO 4 •7H 2 O], konyaite [Na 2 Mg(SO 4 ) 2 •5H 2 O], löweite [Na 12 Mg 7 (SO 4 ) 13 •15H 2 O], and gypsum [CaSO 4 •2H 2 O].”

The mean abundance of hydromagnesite within the upper 25 cm increases with time since tailings deposition.

“Quantitative mineralogical results also show that the mean abundance of hydromagnesite within the upper 25 cm increases with time since deposition of tailings.”

Hydromagnesite abundance becomes relatively constant below 25 cm depth in tailings of each age.

“The abundance of hydromagnesite becomes relatively constant at depths below 25 cm in tailings of each age.”

Desiccation cracks are a common feature at the surfaces of mine tailings in TSF2.

“Desiccation cracks are a common feature within mine tailings at the surfaces of TSF2.”

Former surfaces and adjacent horizons are enriched in hydromagnesite compared with the hydromagnesite-poor horizons that separate them.

“It is particularly notable that these former surfaces and the horizons adjacent to them are enriched in hydromagnesite compared to the hydromagnesite-poor horizons of tailings that separate them.”

Mine tailings deposits are commonly less than 1 meter deep within the exterior overflow cells and at the edges of the large central cell of TSF2.

“However, mine tailings deposits are commonly less than 1 m deep within the exterior overflow cells and at the edges of the large central cell of TSF2.”

Dissolved inorganic carbon in tailings water has δ13C VPDB from -8.16‰ to -6.20‰, average -7.36‰.

“Dissolved inorganic carbon in tailings water gives values of -8.16‰ ≤ 13 C VPDB ≤ -6.20‰ ( 13 C av = -7.36‰).”

Sanderite, starkeyite, pentahydrite, and carnallite are less common in Mount Keith efflorescences and have only been observed at low abundance.

“Sanderite [MgSO 4 •2H 2 O], starkeyite [MgSO 4 •4H 2 O], pentahydrite [MgSO 4 •5H 2 O], and carnallite [KMgCl 3 •6H 2 O] are less common and have only been observed at low abundance.”

If tailings remain exposed for a significant time, desiccation cracks can deepen as efflorescent sulfate minerals are leached, leaving hardened blocks and layers of hydromagnesite-cemented tailings.

“If the tailings are left exposed for a significant amount of time, the desiccation cracks can become deeper features as efflorescent sulfate minerals are leached from the surficial tailings, leaving behind hardened blocks and layers of hydromagnesite-cemented tailings (Fig. 3e).”

Processing plant water has δ13C VPDB values of -6.91‰ and -4.60‰, average -5.76‰.

“Water from the processing plant gives values of -6.91‰ and -4.60‰ ( 13 C av = -5.76‰).”

In some instances, hydromagnesite infills fine cracks and broad fissures in serpentine grains.

“In some instances, this hydromagnesite infills both fine cracks and broad fissures in serpentine grains (Fig. 2a).”

Bedrock magnesite isolated from tailings by selective acid dissolution has δ13C VPDB from -6.79‰ to -1.23‰ and δ18O VSMOW from 10.33‰ to 18.98‰.

“Bedrock magnesite, isolated from tailings by selective acid dissolution, gives values of -6.79‰ ≤ 13 C VPDB ≤ -1.23‰ and 10.33‰ ≤ 18 O VSMOW ≤ 18.98‰”

Hydromagnesite and sulfate minerals are not detectable in the fine sand and coarse sand layers beneath the surficial samples in 06MKP1.

“Hydromagnesite and sulfate minerals are not detectable in the layers of fine sand (4) and coarse sand (5) beneath the surficial samples.”

CO2 selectively extracted from hydromagnesite has F14C from 0.572 to 1.052, average 0.921.

“Analysis of CO 2 selectively extracted from hydromagnesite is characterized by 0.572 ≤ F 14 C ≤ 1.052 (average F 14 C = 0.921).”

Hydromagnesite was present in the anoxic samples from sites 1 and 30 at abundances comparable to those in the unsaturated zone.

“Hydromagnesite was present within the anoxic samples from sites 1 and 30 at abundances comparable to those in the unsaturated zone, suggesting that hydromagnesite is relatively insoluble within the saturated zone of TSF2 and would persist at depth following burial.”

For brucite, median abundance and MAD decline to 0 wt.% in all surficial tailings aged 0 to 7-8 years.

“This effect is particularly marked for brucite, for which the median abundance and MAD decline to 0 wt.% for all surficial tailings aged 0 to 7-8 years.”

The serpentine decrease is proportionally minor compared with the complete loss of brucite.

“This is, however, a proportionally minor decrease in the abundant serpentine minerals when compared to the complete loss of brucite.”

Median serpentine abundance declines by about 10 wt.% from 25 cm to 0 cm depth.

“Median values for the abundance of serpentine decline by approximately 10 wt.% over the depth interval from 25 cm to 0 cm.”

Brucite is typically present at 1.0-2.5 wt.% in deeper, less weathered TSF2 samples.

“Contrastingly, brucite is typically present at 1.0-2.5 wt.% abundance throughout the deeper, less weathered samples from TSF2.”

Brucite and serpentine abundances are relatively constant below about 25 cm in tailings aged 0 to 7-8 years but decline rapidly within the upper 25 cm.

“Abundances of brucite and serpentine minerals are relatively constant below a depth of approximately 25 cm in tailings aged 0 to 21 7-8 years, but decline rapidly within the upper 25 cm of the tailings.”

Median abundances of mined bedrock calcite, dolomite, magnesite, and carbonate-bearing hydrotalcite minerals show no consistent trends with depth or time.

“median values show no consistent trends in their distribution with depth or time.”

A carbonate-bearing iowaite specimen has a stable isotopic signature similar to magnesite specimens.

“One specimen of a hydrotalcite-group mineral (carbonate-bearing iowaite) has a similar stable isotopic signature to the specimens of magnesite.”

None of the Mount Keith hydromagnesite data fall within the fields for precipitation in equilibrium with atmospheric CO2 or mine camp sewage.

“None of the data for hydromagnesite at Mount Keith fall within the fields defined for mineral precipitation in equilibrium with atmospheric CO 2 or mine camp sewage.”

Compositional data show a consistent trend of decreasing serpentine-group mineral and brucite abundance toward tailings surfaces.

“The compositional data for these minerals display a consistent trend toward decreasing abundance with proximity to tailings surfaces (Fig. 7).”

Secondary Ca-bearing minerals such as gypsum and anhydrite are relatively uncommon in Mount Keith tailings.

“Secondary Ca-bearing minerals (i.e., gypsum and anhydrite), which could form as a consequence of calcite (and dolomite) dissolution, are relatively uncommon in the tailings at Mount Keith”

Bedrock carbonate mineral abundances stay relatively constant compared with secondary minerals such as hydromagnesite.

“In almost all cases, the abundances of bedrock carbonate minerals stay relatively constant in comparison to results for secondary minerals such as hydromagnesite”

The decrease in brucite and serpentine abundance coincides with increasing hydromagnesite abundance.

“The decrease is coincident with increasing abundance of hydromagnesite.”

Magnesite in four hydromagnesite-bearing tailings samples gives F14C values between 0.024 and 0.365.

“Magnesite in these samples gives F 14 C values between 0.024 and 0.365.”

The model predicts no serpentine dissolution because supersaturated conditions existed throughout the tailings pile in the simulations.

“The model predicts no serpentine dissolution, as supersaturated conditions existed throughout the tailings pile in the simulations.”

Assuming CO2 supply is limiting and aqueous flow follows Darcy's law driven by the natural elevation gradient, the maximum aqueous CO2 flux would be insufficient to carbonate all brucite in TSF2.

“Assuming that the rate of CO 2 supply is the limiting factor, and that flow is governed by Darcy's law and driven by the natural elevation gradient at Mount Keith, it is estimated that the maximum CO 2 flux in the aqueous phase would be insufficient to carbonate all the brucite in TSF2”

• AssumptionThe authors state this as an assumption: The rate at which CO2 can be supplied is the limiting factor for the effectiveness of injecting DIC-charged water, air or flue gas.
• AssumptionThe authors state this as an assumption: Aqueous-phase flow is governed by Darcy's law and driven by the natural elevation gradient at Mount Keith.

At the average tailings deposition rate of about 50 cm per year, approximately 45% of the initial brucite remains un-carbonated after burial.

“Model calculations suggest that at the average rate of tailings deposition at Mount Keith (i.e., ~50 cm/yr), approximately 45% of the initial abundance of brucite remains un-carbonated after burial (Fig. 10), which is consistent with quantitative mineralogical results.”

The model predicts brucite is completely removed within about 170 days in the upper 2 cm of the tailings.

“The model predicts relatively rapid brucite dissolution in the near surface environment, with brucite completely removed within ~170 days in the upper 2 cm of the tailings.”

In all simulations, most hydromagnesite precipitation is predicted to occur within the top 25 cm of the tailings.

“In all simulations, the majority of hydromagnesite precipitation is predicted to occur within the top 25 cm of the tailings”

About 4 wt.% hydromagnesite is predicted to form in the upper 2 cm during that time.

“Correspondingly, ~4 wt.% hydromagnesite is predicted to form within this horizon during this time.”

Hydromagnesite acts as a cement binding fine and larger grains of serpentine and magnetite together.

“Hydromagnesite cementing both fine and larger grains of serpentine and magnetite together.”

Six magnesite specimens give F14C values from 0.005 to 0.365.

“Six specimens of magnesite give values of 0.005 ≤ F 14 C ≤ 0.365.”

The magnesite specimen with the highest F14C value, 0.365, comes from the 10-year-old surface of TSF1.

“In fact, the specimen of magnesite with the highest F 14 C value (i.e., 06MKG10-5-1-mags with F 14 C=0.365) comes from the 10-year old surface of TSF1.”

The average hydromagnesite precipitation rate for the upper 10 cm of TSF2 is estimated at about 4 wt.% per year.

“An average hydromagnesite precipitation rate for the upper 10 cm of tailings of ~4 wt.%/yr is estimated for TSF2 based on quantitative mineralogical data and modeling results.”

There is no evidence that the initial influx of process water has elevated DIC relative to tailings waters.

“There is no evidence to suggest that the initial influx of process water has an elevated DIC content relative to tailings waters.”

Stable carbon isotopic data indicate that CO2 ingress into Mount Keith tailings water is limited by slow diffusion and dissolution of atmospheric CO2 into saline, moderately high pH waters below pH 11.

“Our stable carbon isotopic data indicate that the rate of CO 2 ingress into Mount Keith tailings water is limited by the slow kinetics of diffusion and dissolution of atmospheric CO 2 into saline, moderately high pH waters (i.e., pH < 11).”

Textures in field samples indicate that a small percentage of the serpentine is carbonated.

“Textures observed in field samples, however, indicate that a small percentage of the serpentine is carbonated.”

The 7-8-year-old surface of TSF2 at sampling site 9 is heavily cemented with hydromagnesite.

“The 7-8-year old surface of TSF2 at sampling site 9 (showing sample 06MK9-6), which is heavily cemented with hydromagnesite.”

The modeled influx of replenished process water with atmosphere-equilibrated DIC can drive hydromagnesite formation at depth without direct atmospheric CO2 transport.

“The modeled influx of replenished process water, containing atmosphere-equilibrated DIC, is able to drive formation of hydromagnesite at depth without invoking the transport of CO 2 directly from the atmosphere.”

Two highly pure magnesite samples from wasterock at sites 53 and 54 give F14C values between 0.005 and 0.009.

“Two of these analyses are of highly pure samples of magnesite found in wasterock at sampling sites 53 and 54 (i.e., 06MK53 and 06MK54), which give F 14 C values between 0.005 and 0.009.”

Mass balance calculations indicate that about 87% of the magnesium in hydromagnesite comes from brucite dissolution, with the remainder from pore water.

“Mass balance calculations indicate that ~87% of magnesium contained in hydromagnesite is sourced from brucite dissolution, with the balance from magnesium initially present in the pore water.”

Modeling predicts tailings waters may become temporarily undersaturated with respect to hydromagnesite after rainfall, which could dissolve and redistribute this phase at depth.

“Modeling also predicts that tailings waters may become temporarily undersaturated with respect to hydromagnesite following rainfall events, which could lead to dissolution and redistribution of this phase at depth.”

A tailings deposition rate of 25 cm per year would allow more than 80% of the brucite to be carbonated before burial.

“Adoption of a tailings deposition rate of 25 cm/yr would allow more than 80% of the brucite to be carbonated prior to burial (Fig. 10).”

Most work on carbon mineralization has focused on rapid, large-scale ex situ methods for trapping and storing CO2 at industrial point sources.

“Since carbon mineralization was first proposed as a method for storing CO 2 (Seifritz, 1990), most of the work on this subject has focused on the development of rapid, large-scale ex situ methods for trapping and storing CO 2 at industrial point sources (reviewed in Huijgen andComans, 2003, 2005;IPCC, 2005;Sipilä et al., 2008;Power et al., in review).”

• cites Huijgen and Comans, 2003, 2005Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites IPCC, 2005Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Sipilä et al., 2008Not checked — no full text of the source

Passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO2.

“Thus, passive sequestration via enhanced weathering of mineral waste can capture and store a significant amount of CO 2 .”

To estimate the total amount of CO2 captured within secondary carbonate mineral phases at a mine, the mineralogy of its tailings storage facilities must be well constrained.

“In order to estimate the total amount of CO 2 captured within secondary carbonate mineral phases at a mine, it is necessary that the mineralogy of its tailings storage facilities be well constrained.”

Geoengineering strategies that accelerate CO2 uptake into tailings water could further reduce or completely offset net greenhouse gas emissions at Mount Keith and many other mines.

“Implementation of geoengineering strategies that accelerate the uptake of CO 2 into tailings water could further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines.”

Tailings in the overflow cells of TSF2 had weathered without new tailings or process water for periods of one, three, and between seven and eight years before sampling in 2006.

“Tailings in the various overflow cells of TSF2 have been permitted to weather, without addition of new tailings or process water, for periods of one, three, and between seven and eight years prior to sampling in 2006.”

The passive rate of carbon mineralization at Mount Keith could be accelerated.

“Stable isotopic results indicate that dissolution of atmospheric CO 2 into mine tailings water is kinetically limited, which suggests that the passive rate of carbon mineralization could be accelerated.”

Accounting for the amount of atmospheric CO2 trapped and stored within hydromagnesite at Mount Keith could be used to offset the mine's greenhouse gas emissions.

“Consequently, accounting of the amount of atmospheric CO 2 that is being trapped and stored within this mineral could be used to offset the mine's greenhouse gas emissions.”

The mining operation at MKD5 produces approximately 370,000 tonnes of greenhouse gases (CO2 equivalent) each year.

“The mining operation at MKD5 produces approximately 370 000 t of greenhouse gases (cited as CO 2 equivalent) and approximately 11 Mt of ultramafic tailings each year (BHP Billiton, 2005).”

• cites BHP Billiton, 2005Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Geostatistical methods that rely on regular and predictable geological structures cannot be used to predict the mineralogy of a mine tailings pile.

“As a result, geostatistical methods that rely on the presence of regular and predictable geological structures cannot be used to predict the mineralogy of a mine tailings pile.”

• AssumptionRests on an assumption: Mine tailings mineral content is generally heterogeneous because ore mineralogy, processing procedures, and tailings management practices change frequently.
• AssumptionRests on an assumption: The mineralogy of tailings piles changes over time as tailings react with the atmosphere and with meteoric and process waters to form secondary mineral phases.

The mining operation at Mount Keith produces approximately 11 million tonnes of ultramafic tailings each year.

“The mining operation at MKD5 produces approximately 370 000 t of greenhouse gases (cited as CO 2 equivalent) and approximately 11 Mt of ultramafic tailings each year (BHP Billiton, 2005).”

• Also stated in: § 4.3. Consequences of timing and depth of tailings deposition for hydromagnesite formation · plain

  “Mining at Mount Keith produces approximately 11 Mt of tailings each year.”

• cites BHP Billiton, 2005Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Accelerating carbonation of mine tailings at Mount Keith has the potential to completely offset the net greenhouse gas emissions of this mine.

“Accelerating the carbonation of mine tailings at Mount Keith has the potential to completely offset the net greenhouse gas emissions of this mine.”

Only the ages of tailings sampled from near the surfaces in the central regions of TSF2 are well constrained, as zero or one half years.

“Contrastingly, only the ages of tailings sampled from near the surfaces in the central regions of TSF2 are well constrained (as zero or one half years).”

The textural relationship between hydromagnesite and gangue minerals such as serpentine confirms that hydromagnesite is a secondary weathering product at Mount Keith.

“The textural relationship between hydromagnesite and gangue minerals such as serpentine confirms the secondary origin of hydromagnesite as a weathering product at Mount Keith.”

The burial of the W1 riser represents an average deposition of approximately 2 meters of tailings per year from TSF2 commissioning to the time of sampling.

“This represents an average deposition of approximately 2 m of tailings each year from the time TSF2 was commissioned up until the time of sampling.”

The textures in which hydromagnesite infills cracks and fissures in serpentine suggest that hydromagnesite may form by replacement of serpentine.

“The latter textures suggest that hydromagnesite may be forming by replacement of serpentine.”

The observed hydromagnesite textures are similar to dense hydromagnesite and dypingite cements described by Wilson et al. (2009a) from depth in tailings at the Clinton Creek chrysotile mine, Yukon, Canada.

“These textures are similar to those described by Wilson et al. (2009a) for dense cements of hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O] and dypingite [Mg 5 (CO 3 ) 4 (OH) 2 •5H 2 O] that form at depth within tailings at the Clinton Creek chrysotile mine, Yukon, Canada.”

• cites Wilson et al. (2009a)Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

At sampling site 30, water-saturated samples collected with a backhoe excavator indicate that they came from beneath the vadose zone.

“At sampling site 30, samples collected with a backhoe excavator were saturated with water, indicating that they had been collected from beneath the vadose (unsaturated) zone.”

These anoxic samples may reflect a prevalent compositional change within the deeper, anoxic tailings located in the saturated zone of TSF2.

“These samples may reflect a prevalent compositional change within the deeper, anoxic tailings that are located within the saturated zone of TSF2.”

The F14C results for highly pure bedrock carbonate minerals are consistent with 14C-free carbon plus the expected level of atmospheric contamination from vacuum-line gas separation and graphite preparation.

“These results for highly pure bedrock carbonate minerals are consistent with 14 C-free carbon and the level of atmospheric contamination expected from gas separation in our vacuum line and preparation of graphite samples.”

Sulfate minerals precipitate in mine tailings because of oxidative weathering of primary sulfide minerals.

“Sulfate minerals precipitate in mine tailings as a consequence of oxidative weathering of primary sulfide minerals”

• cites Jambor and Blowes, 1998Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Jambor et al., 2000 sourceNot checked — no full text of the source

Estimates of hydromagnesite abundance made predominantly from vadose-zone samples should still apply to deeper, saturated tailings.

“Thus, estimates of hydromagnesite abundance, made predominantly from samples collected from within the vadose zone, should still apply to deeper, saturated tailings.”

In the analyses, the same ages known for surface deposits were attributed to the deeper tailings.

“Following this line of reasoning, we have attributed the same ages known for the surface deposits to the deeper tailings in our analyses.”

Well-sorted layers of clay- and silt-sized tailings over sand-sized tailings likely indicate mass settling within tailings flows during distinct depositional events.

“These well-sorted layers are likely indicative of mass settling within tailings flows during distinct depositional events [see upper 2 m of the profile at sampling site 1 (i.e., 06MKP1 within 0.5-year old tailings)], which is illustrated in Fig. 3a and 3b.”

In parts of TSF2, only the upper few meters of tailings are unsaturated.

“This suggests that in parts of TSF2, only the upper few meters of tailings are unsaturated.”

After 10 years of uniformly thick deposition, tailings deposits in TSF2 would be 5.0 meters deep.

“After 10 years (i.e., from January 1997 to October 2006) of uniformly thick deposition, tailings deposits would be 5.0 m deep.”

If the annual mass of tailings were deposited uniformly throughout TSF2, it would be approximately 50 centimeters deep.

“If this mass of tailings were deposited uniformly throughout TSF2 it would be approximately 50 cm in depth.”

Hydromagnesite precipitation is an ongoing process in TSF2.

“Thus, hydromagnesite precipitation is an ongoing process in TSF2.”

Deep grid samples in TSF1 and near the risers in TSF2 most likely access tailings deposited within one year of the last surface deposition.

“As such, deep grid samples in TSF1 and, similarly, near the risers in TSF2 would most likely be accessing tailings deposited within one year of last deposition at the surface.”

The relative abundances of hydrated sulfate minerals measured in the laboratory may not reflect their abundance in the field.

“Thus, the relative abundances of hydrated sulfate minerals measured in the laboratory may not reflect abundance in the field.”

Most hydromagnesite data are consistent with precipitation in equilibrium with DIC in process or tailings water at about 25°C.

“However, most of the data for hydromagnesite are consistent with precipitation in equilibrium with DIC in process/tailings water at approximately 25˚C.”

At the circumneutral pH of Mount Keith tailings waters, bedrock carbonate dissolution proceeds most readily for calcite, then dolomite, and least readily for magnesite.

“Under the circumneutral pH conditions (i.e., pH of 5 to 8) that dominate within the tailings waters at Mount Keith, dissolution of bedrock carbonate minerals will proceed most readily for calcite, then dolomite, and least readily for magnesite (Palandri and Kharaka, 2004).”

• cites Palandri and Kharaka, 2004 sourceNot checked — no full text of the source

The assumed minimum δ18O value reflects the lower range of monthly rainwater data for Perth, the only Western Australian GNIP site.

“This value reflects the lower range of monthly rainwater data for Perth, which is the only Western Australian sampling site in the Global Network of Isotopes in Precipitation (IAEA/WMO, 2012).”

• cites IAEA/WMO, 2012Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• AssumptionThe authors state this as an assumption: Tailings process water, derived from local groundwater, has a minimum δ18O VSMOW value of about -7‰.

Tailings from operating mines like Mount Keith have four potential carbon sources: atmospheric CO2, bedrock carbon from gangue carbonate minerals, industrial additives, and organic carbon.

“There are four potential sources of carbon in tailings from operating mines like Mount Keith: (1) atmospheric CO 2 , ( 2) bedrock carbon from gangue carbonate minerals, (3) carbon from industrial additives, and (4) organic carbon”

δ13C VPDB between -10‰ and +1‰ with δ18O VSMOW around 10‰ to 20‰ commonly reflects metamorphic magnesite isotopic composition.

“Values for 13 C VPDB between -10‰ and +1‰ with 18 O VSMOW in the range of approximately 10‰ to 20‰ commonly reflect the isotopic compositions of metamorphic magnesite”

• cites Kralik et al., 1989Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Hansen, 2005Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

CO2 trapping beneath crusts occurs only where CO2 is generated at depth by biodegradation of organics or by carbonate dissolution during biologically mediated sulfide oxidation.

“This process only occurs where CO 2 is being generated at depth within tailings as a consequence of biodegradation of underlying organics or dissolution of carbonate minerals during biologically mediated oxidation of sulfide minerals.”

Sulfate minerals in surface crusts suggest that older tailings flows around the TSF2 perimeter may occasionally be exposed to fresh tailings water from the interior.

“The presence of sulfate minerals in surface crusts suggests that the older tailings flows around the perimeter of TSF2 may occasionally be exposed to fresh tailings water from the interior region of the storage facility.”

Adding significant new crystalline mass to previously unaltered tailings dilutes mineral abundances.

“Significant input of new crystalline mass to previously unaltered tailings results in a dilution effect in mineral abundances.”

Without new tailings process water input, rainfall events likely leach sulfate minerals from the surface of TSF1 within 10 years of deposition.

“Without new input of tailings process water, rainfall events likely leach sulfate minerals from the surface of TSF1 within 10 years of deposition”

• cites Jambor et al., 2000 sourceNot checked — no full text of the source

Serpentine-group mineral carbonation in alkaline mine tailings produces similar textures in brucite-free tailings.

“Carbonation of serpentine-group minerals in alkaline mine tailings is known to produce similar textures in brucite-free mine tailings (Wilson et al., 2009a).”

• cites Wilson et al., 2009aNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Brucite weathering produces hydromagnesite and other hydrated Mg-carbonate minerals under Earth-surface temperature and pressure conditions.

“Weathering of brucite is known to produce hydromagnesite and other hydrated Mg-carbonate minerals under conditions of temperature and pressure that prevail at the Earth's surface”

• cites Hostetler et al. 1966Not checked — no full text of the source
• cites Xiong and Lord 2008 sourceNot checked — no full text of the source

Hydromagnesite forms as a secondary alteration mineral.

“This is consistent with our interpretation that hydromagnesite forms as a secondary alteration mineral.”

An F14C value of 1.06 reflects the atmospheric 14C concentration in 2006, the year of sampling at Mount Keith.

“A value of F 14 C = 1.06 reflects the 14 C concentration of the atmosphere in 2006, the year of sampling at Mount Keith (Levin et al., 2008).”

• cites Levin et al., 2008 sourceNot checked — no full text of the source

The observed decreases in serpentine and brucite abundance are very likely due to dissolution producing secondary Mg-bearing minerals and dilution by incorporation of water and atmospheric gases into newly formed minerals.

“These results imply that the observed decreases in the abundances of both serpentine and brucite are very likely the result of two processes: (1) dissolution to produce secondary Mg-bearing minerals such as hydromagnesite, and (2) a dilution effect caused by incorporation of water and atmospheric gases into these newly formed minerals.”

Hydromagnesite is less resistant to dissolution in acidic solutions than magnesite.

“Hydromagnesite is less resistant to dissolution in acidic solutions than magnesite (Königsberger et al., 1999).”

• cites Königsberger et al., 1999 sourceNot checked — no full text of the source

Precipitation of carbonate and evaporite minerals within tailings pores likely helps control the rate and extent of mineralization.

“Precipitation of carbonate and evaporite minerals, such as hydromagnesite and halite, within tailings pores likely participates in controlling the rate and extent of mineralization.”

Hydromagnesite overgrowths on magnetite may help limit sulfide oxidation and carbonate mineral dissolution in Mount Keith tailings.

“This textural feature may play a role in limiting sulfide oxidation and carbonate mineral dissolution within Mount Keith tailings.”

Brucite carbonation can be rapid at near-ambient pressure and temperature, even in saline and alkaline brines similar to Mount Keith tailings water.

“Recent carbonation experiments (e.g., Wilson et al., 2010;Zhao et al., 2010;Hövelmann et al., 2012;Harrison et al., 2013) indicate that brucite carbonation can be rapid at near-ambient pressures and temperatures, even in saline and alkaline brines similar to the tailings water at Mount Keith”

• cites Wilson et al., 2010 sourceNot checked — no full text of the source
• cites Harrison et al., 2013 sourceNot checked — no full text of the source

The Ni-dolomite isotopic signature is consistent with an origin as fracture coatings and vug linings in oxidized ore.

“This is consistent with their origin as fracture coatings and vug linings in oxidized ore (Grguric, 2003).”

• cites Grguric, 2003Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Net CO2 sequestration in mine tailings can occur only when atmospheric carbon, directly or indirectly from modern organic matter, is fixed in carbonate mineral crystal structures.

“Thus, net sequestration of CO 2 can only occur in mine tailings when atmospheric carbon (either directly from the atmosphere or indirectly from modern organic matter) is fixed within the crystal structures of carbonate minerals.”

Efflorescence formation can be viewed as a self-limiting process that controls the depth to which alteration minerals extend.

“Thus, formation of efflorescences may be viewed as a selflimiting process that controls the depth to which alteration minerals may extend.”

Net CO2 sequestration in hydromagnesite requires a non-carbonate mineral source for both magnesium and carbon.

“However, this would not constitute net sequestration of CO 2 within hydromagnesite, which requires a non-carbonate mineral source for both magnesium and carbon (Wilson et al. 2009a).”

• cites Wilson et al. 2009aNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Including temperature dependence for dypingite-DIC fractionation would likely broaden the field to include all remaining hydromagnesite data.

“Based on comparison with carbon isotopic fractionation factors between other carbonate minerals and DIC, it is likely that inclusion of temperature dependence for dypingite-DIC fractionation (Wilson et al., 2010) would increase the breadth of this field to include all the remaining data for hydromagnesite.”

• cites Wilson et al., 2010 sourceNot checked — no full text of the source

The significant 18O enrichment of hydromagnesite relative to magnesite suggests precipitation in an evaporative environment.

“The significant enrichment of 18 O in hydromagnesite relative to magnesite suggests precipitation in an evaporative environment.”

Cracks and fissures in efflorescences more likely allow limited gas exchange between deeper tailings and the atmosphere after crusts form.

“Instead, it is more likely that the ubiquitous cracks and fissures that texture efflorescences allow for limited gas exchange between the deeper tailings and the atmosphere once efflorescences have formed.”

Weathering of serpentine-group minerals and brucite is an alternative magnesium source in TSF2.

“Weathering of serpentine-group minerals and brucite represents an alternative source of magnesium in TSF2.”

Most stable isotopic data for magnesite fall within the metamorphic range and are consistent with a metamorphic origin at Mount Keith.

“The majority of the stable isotopic data for magnesite fall within this range of values (Fig. 8) and are consistent with a metamorphic origin for this mineral at Mount Keith.”

In Mount Keith ore, brucite commonly forms coronas with magnetite that encompass sulfide blebs.

“Grguric (2003) describes a common texture within Mount Keith ore whereby brucite forms coronas with magnetite that encompass sulfide blebs.”

• cites Grguric, 2003Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The absence of brucite is explained by hydromagnesite overgrowths on magnetite grains.

“Thus, the absence of brucite is explained by the (otherwise perplexing) overgrowths of hydromagnesite on magnetite grains (observed in Fig. 3b).”

The anticipated δ13C range is broader than shown because of seasonal temperature variations at Mount Keith.

“Thus, the anticipated range of δ 13 C values is expected to be broader than shown owing to seasonal temperature variations at Mount Keith.”

Quantitative mineralogical results, microscopy and all simulations suggest that most carbon fixation at Mount Keith occurs through brucite carbonation.

“Quantitative mineralogical results, microscopy and all simulations suggest that the majority of carbon fixation at Mount Keith occurs via carbonation of brucite.”

Improving brucite carbonation efficiency so that all brucite deposited annually in TSF2 is carbonated would offset the mine's emissions by about 20-60%.

“Improving the efficiency of brucite carbonation, to ensure that all the brucite deposited annually within TSF2 were carbonated, would offset the mine's emissions by ~20-60% (Harrison et al. 2013).”

• cites Harrison et al. 2013 sourceNot checked — no full text of the source

The very small 14C amount in Ni-dolomite sample 06MKNi-dol is consistent with typical vacuum-line atmospheric contamination for 14C-free bedrock carbonate minerals.

“The very small amount of 14 C measured in Ni-dolomite sample 06MKNi-dol (F 14 C = 0.004) is consistent with the amount of atmospheric contamination typically observed in our vacuum line for samples of 14 C-free bedrock carbonate minerals.”

The majority of hydromagnesite at depth in TSF2 was initially precipitated near the tailings surface and has been reworked upon burial.

“Therefore, it is more likely that the majority of the hydromagnesite documented at depth within TSF2 was initially precipitated near the tailings surface, but has become reworked upon burial.”

The tailings produced at Mount Keith in one year have the capacity to bind approximately 4 Mt of CO2.

“The amount of tailings produced at Mount Keith in one year has the capacity to bind approximately 4 Mt of CO 2 (after Lackner et al., 1995 andestimates from Harrison et al., 2013).”

• cites Lackner et al., 1995 sourceNot checked — no full text of the source
• cites Harrison et al., 2013 sourceNot checked — no full text of the source

Modeling results indicate that hydromagnesite at depth is more likely due to burial and reworking of material originally precipitated near the tailings surface than to fresh DIC in replenished process water.

“Based on agreement with mineralogical data, the modeling results indicate that the presence of hydromagnesite at depth is more likely attributable to burial and reworking of material originally precipitated near the tailings surface as opposed to introduction of fresh DIC in replenished process water”

Globally, mineral waste from mines has the potential to sequester on the order of 400 Mt of CO2 per year.

“Globally, there is potential to sequester on the order of 400 Mt of CO 2 /yr in the mineral waste from mines (Power et al., 2013).”

• cites Power et al., 2013Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

CO2-rich gas streams may be more effective than circulating DIC-charged waters for delivering CO2 to tailings.

“These results imply that the use of CO 2 -rich gas streams may therefore prove more effective than circulation of DIC-charged waters.”

Enhancing carbon mineralization to completely carbonate brucite in the tailings would offset 20-60% of the mine's emissions, which are 370 kt per year CO2 equivalent.

“Enhancing the rate of carbon mineralization at Mount Keith to completely carbonate the brucite within tailings would offset between 20 and 60% of the mine's emissions (i.e., 20-60% of 370 kt/yr CO 2 equivalent emissions; Harrison et al., 2013).”

• cites Harrison et al., 2013 sourceNot checked — no full text of the source

Even if the process water flux doubled, DIC initially present in the process water would become progressively depleted along the flow path due to hydromagnesite precipitation.

“Moreover, even if the process water flux were doubled, the concentration of DIC initially present in the process water would become progressively depleted along the flow path as a consequence of hydromagnesite precipitation.”

Harrison et al. (2013) reported a roughly 240-fold increase in brucite carbonation rates in slurry reactors when CO2 was raised from atmospheric levels to 10%.

“For example, Harrison et al. (2013) reported a ~240-fold increase in brucite carbonation rates in slurry reactors when the concentration of CO 2 was increased from atmospheric levels (~0.04% CO 2 ) to 10%.”

• cites Harrison et al. (2013) sourceNot checked — no full text of the source

Accelerating mineral carbonation in Mount Keith tailings to use 11% of their total capacity would completely offset the mine's greenhouse gas emissions.

“Accelerating the rate of mineral carbonation within Mount Keith tailings to make use of 11% of its total capacity to capture and store atmospheric CO 2 would completely offset the greenhouse gas emissions produced by this mine.”

These values are consistent with vacuum-line contamination of samples with F14C = 0, as expected for bedrock magnesite formed by carbonate alteration of serpentinized komatiite.

“These values are consistent with vacuum line contamination of samples with F 14 C = 0, as expected of bedrock magnesite that formed by carbonate alteration of serpentinized komatiite (Barrett et al., 1977;Grguric et al., 2006).”

• cites Barrett et al., 1977 sourceNot checked — no full text of the source
• cites Grguric et al., 2006Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Radiocarbon results indicate that stable isotope data for Mount Keith carbonate minerals cannot uniquely identify the carbon source in hydromagnesite.

“Importantly, our radiocarbon results indicate that stable isotope data for Mount Keith carbonate minerals cannot be used to uniquely identify the source of carbon in hydromagnesite.”

Almost all CO2 mineralized as hydromagnesite originated from the atmosphere.

“based on our radiocarbon results, it can be concluded that almost all CO 2 mineralized as hydromagnesite originated from this source.”

• AssumptionRests on an assumption: The atmosphere is the only large reservoir of modern carbon at Mount Keith.

Mg-carbonate minerals forming from such carbon-limited solutions precipitate in isotopic equilibrium with 13C-depleted DIC.

“Mg-carbonate minerals that form from such carbon-limited solutions precipitate in isotopic equilibrium with 13 C-depleted DIC (Figs. 8 and 9).”

The highest hydromagnesite F14C values are consistent with a modern source for approximately 100% of DIC-derived carbon.

“Thus, the highest values in this range are consistent with a modern source for approximately 100% of the DIC-derived carbon in hydromagnesite.”

Experiments show kinetic 13C depletion during atmospheric CO2 dissolution and aqueous CO2 hydroxylation to bicarbonate in saline, moderately alkaline solutions emulating Mount Keith tailings water.

“Experiments by Wilson et al. (2010) and Harrison et al. (2013) demonstrate that kinetic depletion of 13 C occurs during dissolution of atmospheric CO 2 and hydroxylation of aqueous CO 2 to form bicarbonate in saline and moderately alkaline (pH < 11) solutions designed to emulate tailings water at Mount Keith.”

• cites Wilson et al., 2010 sourceNot checked — no full text of the source
• cites Harrison et al., 2013 sourceNot checked — no full text of the source

On the scale of the entire tailings impoundment, only negligible hydromagnesite precipitation may occur from lateral recharge of DIC-rich process water.

“On the scale of the entire tailings impoundment, only negligible precipitation of hydromagnesite may occur due to lateral recharge of DIC-rich process water.”

The finest fractions of less acid resistant bedrock carbonate minerals may react during CO2 extraction from hydromagnesite.

“Furthermore, the finest fractions of less acid resistant bedrock carbonate minerals such as calcite, dolomite, and hydrotalcite-group minerals may be reacting to some extent during extraction of CO 2 from hydromagnesite.”