Greenhouse Gas Emissions and Production Cost Footprints in Australian Gold Mines

Journal of Cleaner Production 267 (2020) 122118

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Journal of Cleaner Production

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Greenhouse gas emissions and production cost footprints in Australian gold mines

* Sam Ulrich a, b, , Allan Trench c, b, d, Steffen Hagemann b a CSA Global, 3 Ord Street, West Perth, Western Australia, 6005, Australia b Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia c UWA Business School, 35 Stirling Highway, Crawley, Western Australia, 6009, Australia d CRU Group, Chancery House, 53-64 Chancery Lane, London, WC2A 1QS, United Kingdom article info abstract

Article history: Australia has a globally significant gold mining sector for which both greenhouse gas (GHG) emissions Received 22 June 2019 data and production cost data are often available on an individual mine basis. Establishing relationships Received in revised form between GHG emissions and production costs has the potential to shape the future of the gold industry 5 March 2020 in Australia through greater focus upon cleaner, efficient production. GHG emissions data from Australian Accepted 6 May 2020 gold mines reveal consistent, significant relationships between gold grade and GHG emissions intensity Available online 14 May 2020 per ounce. Higher gold grades are associated with lower GHG emission intensity per ounce. Differences Handling editor: Kathleen Aviso in both emissions intensity and gold grades exist between open pit mines, underground mines and those operations which source ore from both open pit and underground. Open pit gold mines have the highest Keywords: GHG emissions intensity but lowest costs. Underground mines have the lowest GHG emissions intensity Greenhouse gas emissions but costs that are above open pit mines. Australian gold mines exhibit declining gold grades that are Production costs predicted to continue over the coming decade. These projected lower gold grades will lead to higher GHG Gold emissions intensities in the absence of other GHG abatement interventions. Significant opportunities Energy consumption exist, to materially reduce GHG emissions at Australian gold mines with interventions underway. Broader Footprints adoption of solar and wind energy, changing how underground mines are cooled and the introduction of electric vehicles and mining fleet, especially in underground mines, are key impact areas. © 2020 Elsevier Ltd. All rights reserved.

1. Introduction scrutiny is on the industry’s environmental impact by regulators, investors and the general public. Australia is the second-largest producer of gold in the world, This study focuses on GHG emissions of gold mines in Australia. and this sector accounts for 1.1% of Australia’s annual GHG emis- However, the authors make reference where appropriate to the sions. As such, reducing emissions in Australia’s gold sector and the required energy consumption for gold production. It is the ﬁrst cost of doing so is of critical importance for the sustainability of the study to investigate whether relationships exist between GHG sector. Gold is important to Australia’s economy being its sixth- emissions and energy consumption with reported costs of pro- largest export of goods and services valued at A$19.1 billion in duction, All-in Sustaining Costs (AISC), the source of the gold 2018 or 4.4% of total exports (Department of Foreign Affairs and mined, whether via open pit (OP), underground (UG) or both (OP & Trade, 2019). UG) and the individual mine power source. It builds upon previous A better understanding of GHG emissions and the measures that sector-based studies. can be taken to reduce them, allows Australia’s gold mine owners to The world gold industry produces over 3000 tonnes of mined improve their environmental credentials in times when far greater gold per annum. China is the largest gold producer, producing 401.1 tonnes (12.90 million ounces) in 2018 (China Gold Association, 2019). Australia is the second-largest, producing 315.1 tonnes (10.13 million ounces) of gold in 2018 (Department of Industry * Corresponding author. Centre for Exploration Targeting, The University of Innovation and Science, 2019). Other signiﬁcant gold producing Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia. nations are Russia, USA and Canada (U.S. Geological Survey, 2018). E-mail address: [email protected] (S. Ulrich). https://doi.org/10.1016/j.jclepro.2020.122118 0959-6526/© 2020 Elsevier Ltd. All rights reserved. 2 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118

Australian gold production is sourced from around 63 different Australia is heavily reliant on non-renewable fuels for electricity mining operations (December 31, 2018), comprising 46 gold-only generation, accounting for over 80% of electricity generated and 17 gold-plus other metals deposits, with approximately 54% (Table 3). The coal contribution has dropped from over 80% to 60%, of mines operating underground, 22% as open pits and 24% mining and natural gas has increased to 21%. The renewable energy from both open pit and underground sources in 2018. contribution has increased from 10% to 17% in the last decade, with Australia’s GHG emissions and energy consumption for the year an increase in electricity from wind and solar. ending June 30, 2018 were 533.7 million tonnes of carbon dioxide The existing knowledge base on gold production costs (AISC) equivalent (Mt CO2-e) (Department of the Environment and Energy, and the significant relationship to grade focused upon UG mines 2018) and 6172 PJ (Department of Environment and Energy, 2019a) (Ulrich et al., 2019). The current study extends grade-cost analyses respectively. The Australian gold mines in this study represent over to all Australian gold mines. 90% of Australia’s annual gold production, produced 5.9 Mt CO2-e or There remains considerable opportunity to deepen the under- 1.1% of Australia’s total GHG emissions and consumed 67.1 PJ or 1.1% standing of the variations in GHG emissions and production effi- of Australia’s total energy consumption. ciency in Australian gold mining, especially given the unique role of In this study gold mining refers to the processes of mining, gold in global financial markets (Baur and Oll, 2019) and the eco- processing, concentrating and smelting to produce gold dore bars nomic significance of the industry to Australia. at the mine before sent for refining at a third party (Fig. 1). 2. Data and methods 1.1. Study context This section summarises the GHG emissions, energy consumption, gold production statistics and costs data, its treatment and There is limited academic literature explicitly relating to GHG analysis. This study uses data from January 1, 2014 to June 30, 2018. emissions and energy consumption in gold mining. Previous The start date is governed by the introduction of the AISC measure, studies are based on company reported data or life cycle assess- by the World Gold Council in June 2013 (World Gold Council, 2013) ment (LCA) studies. The company reporting studies explain the and subsequently reported by most gold producers in Australia main driving factors of GHG emissions in gold mining, with quan- from 2014. All data were aggregated into a tabular database for titative assessments of the relationship to ore grade. The effect of analysis. declining gold grades over time highlights the sustainability issue The evolution of cost-reporting in the global gold industry, of increasing GHG emission footprints per unit of gold produced. specifically the recent widespread adoption of AISC as a cost pro- The LCA studies provide an understanding of the relative contri- tocol, makes systematic investigation into the relationships be- bution of individual production inputs to energy and carbon foot- tween production cost-efficiency, GHG emissions-intensity and prints. The LCA studies typically include some Scope 3 emissions, energy consumption possible. whereas the company reported studies only Scope 1 and Scope 2 emissions (Fig. 1). However, no studies provide a quantitative breakdown on a mine by mine basis or show the impact of initia- 2.1. GHG emissions and energy consumption data tives implemented to reduce GHG emissions. Summaries of these studies GHG emissions and energy consumption as intensities are GHG emission and energy consumption data were sourced from in Table 1 and Table 2, respectively. the Australian National Greenhouse and Energy Reporting (NGER) Some studies are specific to parts of the mining process, such as scheme data (Clean Energy Regulator, 2016, 2017, 2018, 2019), re- energy consumption used in ore comminution (Ballantyne and ported on the Australian financial year basis; 1 July to 30 June; and Powell, 2014; Ballantyne et al., 2012), greenhouse gas emissions from company sustainability data reported on either the Australian of the mining fleet (Peralta et al., 2016) and reducing greenhouse financial year or calendar year basis. Based on the assumption that gas emissions by different blasting approaches (Goswami and the gold mines are in a steady-state of production, the specific, Brent, 2016). intra-year timing of the GHG emission and energy consumption

In Study Not in Study Scope 1 – Direct GHG emissions Scope 3 – Other indirect emissions Mining Processing Concentrating Refining and Downstream uses and smelting recycling Jewellery

99.99% Gold

Onsite electricity generation emissions

Investment Gold doré bars Scope 2 – Indirect electricity emissions Recycled gold

Electronics

Scope 3 upstream – suppliers, good and services Excluded from study

Fig. 1. Deﬁnition of Scope 1, 2 and 3 GHG emissions in the gold industry supply chain within this study. Note the Scope 3 (upstream) emissions e suppliers, goods and services are excluded from this study. Modiﬁed after the World Gold Council (2019). S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 3

Table 1 Previous studies related to gold mining and GHG emissions intensity.

Study kg CO2-e/kg AuEq kg CO2-e/t ore Reference 1991e2006 World average GHG emissions in gold mining* 11,500 21.7 1 1991e2006 Australian average GHG emissions in gold mining* 14,100 36.0 2 1991e2008 World weighted average GHG emissions in gold mining* 13,900 NA 3 LCA study value extracted from Ecoinvent 2.0, EMPA/ETH Zurich# 16,991 NA 4 LCA open pit gold mining study at a head grade of 3.5 g/t gold# a e non-refractory ore, b e refractory ore 17,560a 61.7a 5 26,840b 77.2b LCA of conceptual gold ISL mining and metal production at 2.0 g/t gold head grade# 28,989 NA 6 LCA cradle to the gate global warming potential of gold mining# 12,500 NA 7 LCA of gold mining in the Philippines (Process B cyanidation with CIL)* 53,500 706.2 8 LCA impact assessment of gold mining in China of refractory ore at a head grade of 4.3 g/t gold (midpoint values of study)# 55,500 212 9 2016 World average GHG emissions in gold mining* 23,300 74.6 10

Notes: GHG Emissions intensity presented in kilograms of carbon dioxide equivalent per kilogram of gold equivalent produced (kg CO2-e/kg AuEq) and kilograms of carbon dioxide equivalent per tonne of ore processed (kg CO2-e/t ore). * studies that only include Scope 1 and Scope 2 emissions. # studies that include Scope 1, Scope 2 and Scope 3 emissions. References: 1. Mudd (2007a);2.Mudd (2007b);3.Mudd (2010);4.Hagelüken and Meskers (2010);5.Norgate and Haque (2012);6.Haque and Norgate (2014);7. Nuss and Eckelman (2014);8.Tamayao et al. (2017);9.Chen et al. (2018); 10. Tost et al. (2018).

Table 2 Previous studies related to gold mining and energy consumption.

Study GJ/kg AuEq GJ/t ore Reference

1991e2006 World average energy consumption in gold mining* 143 0.31 1 1991e2006 Australian average energy consumption in gold mining* 123 0.31 2 1991e2008 World weighted average energy consumption in gold mining* 149 NA 3 LCA open pit gold mining study at a head grade of 3.5 g/t gold# a e non-refractor ore, b e refractory ore 199a 0.70a 4 304b 0.87b LCA of conceptual gold ISL mining and metal production at 2.0 g/t gold head grade# 347 NA 5 LCA cradle to gate global energy consumption of gold mining# 208 NA 6 LCA of gold mining in the Philippines (Process B cyanidation with CIL)* 666 8.79 7 LCA impact assessment of gold mining in China of refractory ore at a head grade of 4.3 g/t gold (midpoint values of study)# 146 0.56 8

Notes: Energy consumption presented as an intensity in gigajoules per kilogram of gold equivalent produced (GJ/kg AuEq) and gigajoules per tonne of ore processed (GJ/t ore). * studies that only include Scope 1 and Scope 2 emissions. # studies that include Scope 1, Scope 2 and Scope 3 emissions. References: 1. Mudd (2007a);2.Mudd (2007b);3. Mudd (2010);4.Norgate and Haque (2012);5.Haque and Norgate (2014);6.Nuss and Eckelman (2014);7.Tamayao et al. (2017);8.Chen et al. (2018).

Table 3 Australia’s electricity generation mix, by main fuel types.

Percentage of main non-renewable and renewable fuels in electricity generation

Fuel 1989e90 1994e95 1999e00 2004e05 2009e10 2014e15 2017e18

Coal 78 80 83 79 71 63 60 Natural Gas 99810182121 Total Non-renewables 90 90 91 91 91 87 83 Wind eeee256 Hydro 10 987556 Solar eeeee24 Total Renewables 10 10 9 9 9 13 17

Source: Department of Environment and Energy (2019a). reporting data will not materially impact changes to the annual less than 10% of the gold produced annually in Australia. data compilation. Data on individual mine energy mixes, whether from the elec- Data available in the NGER are GHG Scope 1 and Scope 2 tricity grid, generated on-site or some combination thereof, are emissions measured in tonnes of carbon dioxide equivalent (t CO2- primarily sourced from company reports or direct queries to the e) and Net Energy Consumed in gigajoules (GJ). Scope 3 emissions companies. are not available and routinely not reported in the mining industry. The definition of Scopes 1, 2 and 3 GHG emissions in relation to the gold industry are illustrated in Fig. 1. 2.2. Gold output and production cost data A limitation of NGER data is that it is reported by a corporate group, which may, therefore, contain one or more gold mines Mine production and costs data are sourced from company within composite reported data. Where companies provided data quarterly and annual reports. Production statistics include; millions by individual mine within their sustainability reports, these data of ore tonnes processed annually (Mtpa), head grade of the ore sources are used in preference to the NGER corporate group data. processed, the source of the ore whether OP, UG or OP & UG and the Not all gold mines or companies meet the thresholds for costs of sustaining gold production (AISC). reporting under the NGER scheme. Companies with smaller gold Quarterly data are used when the annual data do not correspond mines, or with a single gold mine, may not meet the reporting with the annual NGER data reporting period. In these instances, the thresholds. The authors estimate that these occurrences represent weighted average head grade (weighted by ore tonnes processed) and the weighted average AISC (weighted by gold produced) are 4 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 calculated over the appropriate period, enabling annual data Weisberg, 1982) and leverage data points (Huber, 1981) on the comparison. Weighting of head grade and AISC is necessary when data confirmed the three identified combined Fosterville and Sta- combining two or more mines to match an NGER corporate group. well data points as outliers, and so is the fourth Fosterville point. For the gold mines with base metal by-products (Boddington, The outliers are excluded from the final regression results. Cadia, Mt Carlton, Peak, Deflector and Telfer), the number of equivalent gold (AuEq) ounces produced is calculated and a co- 3. Results product AISC estimated based on the revenue contribution between gold and base metals. This section divides the results of the analyses between GHG The average gold head grade of ore processed is the most emissions and energy consumption. routinely reported gold grade statistic. Two companies (Gold Fields and AngloGold Ashanti) report the average recovered gold grade, which is after processing. The average recovered grade is used in 3.1. GHG emissions the absence of the average head grade. For gold mines with by- products, an equivalent gold head grade is calculated using the The study comprises a total of 110 data points with GHG emis- average head grades of the gold and other metals and average gold sions, including outliers. and other metals prices received by the company. Insufficient data Summary statistics of the GHG emission intensities, for all the are available to calculate an average equivalent gold head grade for data (All Data), differentiated by the source of the ore, i.e. OP, UG or the Mt Carlton and Deflector mines so the average gold head grade OP & UG, are presented on a per tonne of ore processed and on a per values are used. troy ounce (oz) and per kilogram (kg) of gold equivalent (Table 5). Not all mines in Australia report AISC in US Dollars, the majority The average GHG emissions intensity per unit of gold produced in report in Australian Dollars. The quarterly or annual AISC reported Australia (Table 5) has increased by 40% since the study by Mudd in nominal Australian Dollars is converted to US Dollars using the (2007b) (Table 1). average Australian Dollar to US Dollars foreign exchange rate re- A subset of all the data representing a cross-section of individual ported by the Reserve Bank of Australia (2019). Australian gold mines (Subset of Mines) most recently reported All AISC values are corrected for inflation and presented on a GHG emissions data (31 data points including an outlier), on a per real basis in US Dollars per ounce (US$/oz) with a reference date of tonne of ore processed, on a per oz and per kg of gold equivalent is June 30, 2018. Australian quarterly Consumer Price Index (CPI) data presented in Table 6. Depending on the data source and reporting sourced from the Australian Bureau of Statistics (2019) are used to period of this subset ranges from July 1, 2016 to June 30, 2018. The correct the nominal AISC values. subset represents more than 90% of the gold produced in Australia In linking the gold grade and real AISC to the GHG emissions and annually. energy consumption, three statistical outliers are identified The different types of mines have different GHG emission in- (Table 3), shown on subsequent figures for reference (see Fig. 4). tensity ranges (Fig. 2), with OP mines having the highest average These three outliers are due to the Fosterville and Stawell mines, and median GHG emissions. UG mines have the lowest average and GHG emissions and energy consumption, being reported on a median GHG emissions (Fig. 2), and OP & UG mines have average combined basis and occur due to the average gold head grades and and median GHG emissions between the OP and UG mines (Fig. 2). real ASIC data for the two mines being very different (Table 4). A A comparison is made between all available data (Fig. 2A) and a fourth suspect point associated with Fosterville in the year ending subset of mines (Fig. 2B) which represents the most recently re- June 30, 2018 is also treated as an outlier. Additionally, these mines ported GHG emissions of the individual mines in the dataset. It is are in the State of Victoria which has a far higher emissions factor undertaken to determine if there are material differences over time (kg CO2-e/kWh) for consumed grid electricity compared to the in the data, which does not appear to be the case. other States and Territories (see Section 3.2), which would also The zones of overlap in GHG emissions intensity between OP explain the higher GHG emissions intensities observed. and UG mines is primarily due to the average grade of the ore processed. The Mt Carlton OP mine has the lowest GHG emissions intensity and an average grade (>5 g/t Au) comparable to many UG 2.3. Statistical analysis mines. Similarly, the lowest grade UG mines have grades comparable to OP mines. An analysis of strip ratios (ratio of waste rock to Regression analysis is undertaken using IBM SPSS Statistics ore) in OP mines where available showed no relationship to GHG Version 25 (SPSS). Highest correlations are observed using lines of emissions intensity, indicating the GHG emissions from OP mining best fit based on the power function (y ¼ axb) in Microsoft Excel. In activities have less of an impact than changes in the gold grade SPSS a natural logarithm transform is calculated for millions of ore processed. tonnes processed, GHG emissions intensity, energy intensity and Mining companies report their GHG emissions either as Scope 1 average gold equivalent head grade. Linear ANOVA regression is and Scope 2 emissions or as an emissions intensity, either on a per undertaken on the transformed data. tonne of ore processed or troy ounce/kilogram of gold produced. Statistical testing for outliers and influential data points during Results based on a per tonne of ore processed (Fig. 3) does not regression using studentized residuals, Cook’s distance (Cook and take into account the gold grade. On a per tonne of ore processed

Table 4 Average and weighted average gold head grade (g/t) and real AISC (US$/oz) for Fosterville and Stawell gold mines.

Mine Average Gold Head Grade (g/t) Real AISC (US$/oz)

30-Jun-2015 30-Jun-2016 30-Jun-2017 30-Jun-2015 30-Jun-2016 30-Jun-2017

Fosterville 5.47 6.88 10.58 1009 770 552 Stawell* 1.65 1.46 1.46 1083 1229 1757 Weighted Average 3.36 3.79 7.19 1036 863 633

Note: Stawell was transitioning to care and maintenance during this time. S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 5

Table 5 GHG emissions for entire Australian gold mines emissions dataset differentiated by the source of the gold ore.

GHG All Data OP OP & UG UG Emissions kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg t ore AuEq AuEq t ore AuEq AuEq t ore AuEq AuEq t ore AuEq AuEq

Number 104 106 106 31 31 31 31 33 33 42 42 42 Minimum 19 226 7282 19 226 7282 22 460 14,802 28 227 7306 Maximum 117 1365 43,886 55 1365 43,886 72 1041 33,463 117 839 26,989 Median 41 577 18,560 33 812 26,121 39 619 19,896 71 474 15,234 Mean 49 616 19,802 33 777 24,992 41 648 20,845 68 471 15,150 Standard 23 240 7716 10 299 9617 12 141 4530 22 157 5038 Deviation

Note: Excludes problematic data associated with the Fosterville and Stawell mines (4 data points).

Table 6 GHG emissions for the subset of the Australian gold mines emissions dataset differentiated by the source of the gold ore.

GHG Subset of Mines OP OP & UG UG Emissions kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg kg CO2-e/ kg CO2-e/oz kg CO2-e/kg t ore AuEq AuEq t ore AuEq AuEq t ore AuEq AuEq t ore AuEq AuEq

Number 29 30 30 8 8 8 10 11 11 11 11 11 Minimum 21 227 7306 21 281 9026 23 520 16,726 30 227 7306 Maximum 97 1093 35,144 46 1093 35,144 65 1041 33,463 97 835 26,831 Median 38 612 19,688 35 681 21,898 36 669 21,507 71 480 15,417 Mean 48 635 20,426 35 736 23,674 39 702 22,585 66 495 15,905 Standard 22 228 7330 10 284 9121 12 156 5017 24 188 6030 Deviation

Note: Excludes problematic data associated with the Fosterville and Stawell mines (1 datum point).

Fig. 2. Box and whiskers plots of GHG emissions intensity (kg CO2-e/oz) by the source of the mine’s ore. Notes: Fig. 2A e All data. Fig. 2B e Subset of Mines representing a cross- section of Australian mines most recently reported GHG emissions data. basis, large tonnage operations have the lowest GHG emission in- grade gold mines have lower GHG emission intensities on a per troy tensities, whereas small tonnage operations have higher GHG ounce basis compared to lower grade mines. Generally, the higher emission intensities. Underground mines have higher GHG emis- grade mines are UG mines, and the lower grade mines are OP sion intensities per tonne of ore processed compared to OP or OP & mines. The regression and correlation statistics in Table 8 between UG operations (Fig. 3). The regression and correlation statistics in gold head grade and GHG emissions per ounce show the relation- Table 7 between millions of ore tonnes processed and GHG emis- ship between the two variables to be statistically signiﬁcant sions per ore tonne processed, show the relationship between the (p < 0.05). The relationship between GHG emissions intensity and two variables to be statistically signiﬁcant (p < 0.05). The resultant the average gold head grade is a power function (y ¼ axb). The regression equations in Table 7 for all the data and the subset of resultant regression equations for all the data and the subset of mines are very similar. mines are very similar. The results on a per a troy ounce of gold equivalent produced Due to the different GHG emission intensity ranges observed in accounts for the gold grade of the ore processed (Fig. 4). Higher Fig. 2, regression is undertaken on the different ore sources (Table 9 6 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118

A 160 B 160 OP OP 140 OP & UG 140 OP & UG e/t ore) - UG -e/t ore) UG 2 2 120 120

100 100

80 80

60 60 y = 59.587x-0.280 y = 58.597x-0.282 R² = 0.568 R² = 0.584 40 40

20 20 GHG Emissions Intensity GHG Emissions Intensity (kg CO GHG Emissions Intensity GHG Emissions Intensity (kg CO

0 0 0 1020304050 01020304050 Ore Tonnes Processed (Mt/yr) Ore Tonnes Processed (Mt/yr)

Fig. 3. GHG emissions intensity per ore tonne milled (kg CO2-e/t ore) against the ore tonnes milled (Mt/yr). Notes: Figure A e All data. Figure B e Subset of Mines representing a cross-section of Australian mines most recently reported GHG emissions data.

A 1,400 B 1,400 OP OP 1,200 OP & UG 1,200 OP & UG UG UG -e/oz -e/oz Au Eq) -e/oz -e/oz AuEq) 2 1,000 2 1,000 Outlier Outlier

800 800

600 600

400 400 y = exp(6.846)x-0.504 y = exp(6.853)x-0.520 R² = 0.748 R² = 0.814 200 200 GHG Emissions Intensity GHG Emissions Intensity (kg CO GHG Emissions Intensity GHG Emissions Intensity (kg CO 0 0 048121620 048121620 Average Gold Head Grade (g/t AuEq) Average Gold Head Grade (g/t AuEq)

Fig. 4. GHG emissions intensity (kg CO2-e/oz) versus average gold head grade (g/t). Notes: Fig. 4A e All data. Fig. 4B e Subset of Mines representing a cross-section of Australian mines most recently reported GHG emissions data. The outliers relate to the Fosterville and Stawell mines (see Fig. 4A and Section 2.3).

Table 7 Australian Gold Mining GHG emissions per ore tonne regression statistics.

Statistic All Data Subset of mines

Number of data points 104 29 a (scaling coefficient) exp(4.088) ¼ 59.6 exp(4.072) ¼ 58.7 b (decay coefficient) 0.280 0.282 Regression equation GHG/t ¼ exp(4.088)x 0.280 GHG/t ¼ exp(4.072)x 0.282 Standard error 0.298 0.287 F statistic 133.987 37.895 Statistical significance (p) <0.001 <0.001 Pearson correlation coefficient (R) 0.754 0.764 Coefficient of determination (R2) 0.568 0.584

Notes: The F statistic is the variation between sample means/variation within the samples. and Fig. 5). Plotting GHG emissions intensity per ounce versus real AISC, Ore sourced from OP mines has a greater decay coefﬁcient with a horizontal and vertical line representing the average real (0.691) compared to the mines sourcing ore from UG (0.500) or AISC and average GHG emissions intensity respectively (Fig. 6), OP & UG (0.474). Grade changes in mines sourcing their ore from allows for a matrix view of the data. The resulting GHG Emissions OP lead to greater percentage increases or decreases in GHG Intensity-Production Cost (GEIPC) matrix indicates whether mines Emissions intensity per ounce compared to mines sourcing their have low or high GHG emissions per ounce and are either low or ore from OP or OP & UG. high cost. However, the data needs to be adjusted for the different S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 7

Table 8 of OP data points), with high-grade OP mines in the low GHG Australian Gold Mining GHG emission intensity per ounce regression statistics. emissions intensity, low-cost quadrant (e.g., Mt Carlton Fig. 9). Low- Statistic All Data Subset of mines grade OP mines are in the high GHG emissions, low-cost quadrant

Number of data points 100 28 (e.g., Boddington Fig. 9) with some of the low-grade mines in the a (scaling coefficient) exp(6.845) ¼ 940 exp(6.853) ¼ 947 high cost, high GHG emissions intensity quadrant (e.g., Edna May b (decay coefficient) 0.504 0.520 Fig. 9). 0.504 0.520 Regression equation (kg CO2-e/oz ¼ ) exp(6.846)x exp(6.853)x Standard error 0.205 0.173 F statistic 291.314 113.741 3.1.3. Open pit and underground mines Statistical significance (p) <0.001 <0.001 The OP & UG mines are focussed around the mean GHG emis- Pearson correlation coefficient (R) 0.865 0.902 sions intensity and real AISC values, extending into the high cost, Coefficient of determination (R2) 0.748 0.814 high GHG emissions intensity quadrant as the gold grade decreases Notes: The F statistic is the variation between sample means/variation within the (e.g., Telfer Fig. 9). There are few OP & UG mines in the low GHG samples. emissions intensity, low-cost quadrant.

3.2. Energy consumption sources of ore (OP, UG or OP & UG), as they have different GHG emissions intensity to costs footprints (Fig. 7). Open pit mines have The study comprises a total of 99 data points with energy con- the highest GHG emissions intensity, followed by combined OP & sumption, including outliers. The electricity source breakdown for UG mines, with UG mines having the lowest GHG emissions in- the subset of mines, 31 data points comprising of 38 individual tensity (Table 10). Open pit mines have the lowest average real mines are presented in Table 11. AISC, OP & UG mines the highest average real AISC and UG mines An analysis of the reported energy consumption data for average real AISC is between the OP and OP & UG mines (Table 10). Australian gold mines is undertaken in the same manner as the An alternative depiction of the subset of mines GHG emissions analysis of the GHG emissions. However, there is a problem, dis- intensity data is in the form of a cost curve or aggregate supply cussed in the following paragraphs with the reported energy con- curve (Ballantyne and Powell, 2014). The subset of Australian mines sumption data, which does not allow a meaningful analysis and that report GHG emissions can be presented as a GHG emissions comparison between mines. intensity curve (Fig. 8). Underground gold mines feature promi- Ballantyne et al. (2012), in a study of energy consumption nently in the lowest GHG emissions intensity quartile corre- attributable to comminution, observed that there are in- sponding to 94% of the AuEq ounces mined (Fig. 8). OP gold mines consistencies in the reporting of energy consumption data when produce 58% of the AuEq ounces in the highest GHG emissions comparing mines connected to the national electricity grid and intensity quartile (Fig. 8). those that generate electricity on-site by the combustion of diesel The relationship between the gold grade of the ore processed, and/or gas. Companies are generally reporting diesel and gas en- and GHG emissions intensity is seen in Fig. 4 on an overall basis and ergy consumption by multiplying the volume consumed by the in Fig. 5 by the different mined ore sources. Additionally, there is a calorific heating value of the fuel (Ballantyne et al., 2012). The relationship between gold grade and costs (Ulrich et al., 2019), calculation of energy consumption for diesel and gas by this which is illustrated using the GEIPC matrix (Fig. 9). method will give an anomalously higher energy intensity than grid- supplied electricity (Fig. 10). The utilised mechanical energy from 3.1.1. Underground mines the diesel and gas would be a more comparative variable to elec- The UG mines are predominantly in the low GHG emissions trical energy consumption. The diesel and gas heating value (pri- intensity quadrants (76% of UG data points) (Fig. 9). Higher grade mary energy) would need to be adjusted by the generator efficiency UG gold mines are in the low GHG emissions intensity, low-cost to make a fair comparison. The detail to undertake this adjustment quadrant (e.g., Gwalia Fig. 9), with lower-grade UG mines being is not available in the datasets. in the high cost, low GHG emissions intensity quadrants (e.g., Grid supplied electricity has a higher carbon intensity than Sunrise Dam Fig. 9). The Cadia UG mine plots on its own far from diesel and gas (Fig. 11), which is as expected, with coal-fired power the cluster of other UG mines. Cadia is a low-cost, low-grade mine being the most significant contributor to the electricity grid. The (Fig. 9) due to using a bulk tonnage UG mining method called panel outliers in Fig. 11 have the highest carbon intensities, reflecting that caving (Newcrest Mining Limited, 2018), which is unique compared the Fosterville and Stawell mines are in the State of Victoria, which to all the other UG mines within the dataset. has a higher emission factor (kg CO2-e/kWh) for the consumption of electricity from the grid compared to other States and Territories 3.1.2. Open pit mines (Department of Environment and Energy, 2019b) due to brown coal The OP mines are predominantly in the low-cost quadrants (77% providing 76% of Victoria’s grid electricity in 2018 (Department of

Table 9 Australian Gold Mining GHG emission intensity per ounce regression statistics by mined ore source.

Statistic OP OP & UG UG

Number of data points 31 28 41 a (scaling coefficient) exp(6.889) ¼ 981 exp(6.814) ¼ 910 exp(6.882) ¼ 975 b (decay coefficient) 0.691 0.474 0.500 0.691 0.474 0.500 Regression equation (kg CO2-e/oz ¼ ) exp(6.889)x exp(6.814)x exp(6.882)x Standard error 0.231 0.098 0.216 F statistic 92.232 92.016 51.128 Statistical significance (p) <0.001 <0.001 <0.001 Pearson correlation coefficient (R) 0.872 0.883 0.753 Coefficient of determination (R2) 0.761 0.780 0.567

Notes: The F statistic is the variation between sample means/variation within the samples. 8 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118

A 1,400 B 1,400 OP OP & UG 1,200 1,200 e/oz e/oz AuEq) e/oz e/oz AuEq) - - 2 2 1,000 1,000 y = exp(6.814)x-0.474 R² = 0.780 800 800 y = exp(6.889)x-0.691 R² = 0.761 600 600

400 400

200 200 GHG Emissions Intensity GHG Emissions Intensity (kg CO GHG Emissions Intensity GHG Emissions Intensity (kg CO 0 0 0123456 012345 Average Gold Head Grade (g/t AuEq) Average Gold Head Grade (g/t AuEq)

C 1,400 D 1,400 UG OP 1,200 00Outlier 1,200 OP & UG

e/oz e/oz AuEq) UG - -e/oz -e/oz AuEq) 2 2 1,000 1,000

800 800 y = exp(6.882)x-0.500 R² = 0.567 600 600

400 400

200 200 GHG Emissions Intensity GHG Emissions Intensity (kg CO GHG Emissions Intensity GHG Emissions Intensity (kg CO 0 0 0 5 10 15 20 024681012 Average Gold Head Grade (g/t AuEq) Average Gold Head Grade (g/t AuEq)

Fig. 5. GHG emissions intensity (kg CO2-e/oz) against gold grade (g/t), differentiated by the source of the ore. Fig. 5A ¼ Open pit; Fig. 5B ¼ Open pit and underground; Fig. 5C ¼ Underground; Fig. 5D ¼ Resultant regression equation lines. The outliers relate to the Fosterville and Stawell mines (see Fig. 4A and Section 2.3).

Environment and Energy, 2019a). completed its Tanami Power project, comprising 450 km of natural gas pipeline and two gas power plants (62 MW). Switching the 4. Discussion mine from diesel fuel power generation to gas is estimated to reduce GHG emissions by approximately 20% (Newmont Mining This section discusses measures to reduce GHG emissions at Corporation, 2018) or 35,400 t CO2-e. ’ mines in Australia, the effect of declining gold grades and the Gold Fields Limited s (Gold Fields) Granny Smith gold mine in- maturity of gold mines on GHG emission footprints on a unit of gold tends to install an 8 MW solar power generation system with a produced basis. 2 MW backup battery, to offset approximately 18 GWh of gas generated power (Gold Fields Limited, 2019b). It is forecast to reduce GHG emissions at Granny Smith by approximately 10% or 4.1. Interventions to reduce GHG emissions 9500 t CO2-e (Gold Fields Limited, 2019a). At Gold Fields’ Agnew gold mine a new hybrid energy microgrid Below are case studies of measures being undertaken at is to be established combining wind, solar, gas and battery storage. Australian mines to reduce GHG emissions. The microgrid is forecast to provide the Agnew mine with greater than 50% of renewable energy over the long term and reduce GHG 4.1.1. Energy sourcing emissions by approximately 40,700 t CO2-e annually (Gold Fields Changing the energy source for power generation at the mine is Limited, 2019a, c), which is over 50% of 2014e2017 annual re- the primary method being deployed by mining companies in ported GHG emissions. Australia, including gold miners, to reduce GHG emissions. That is, Sandﬁre Resources Limited built a solar power plant and 6 MW emissions reduction involves either switching to an energy fuel lithium-ion battery at its Degrussa copper-gold mine in 2016. In the source with lower GHG emissions such as natural gas from diesel year ending June 30, 2018, the solar power plant provided 17% of and/or substituting some of the energy generated by fossil fuels power requirements and saved 12,959 t CO2-e of GHG emissions with renewables such as solar and/or wind. (Sandﬁre Resources NL, 2018). Newmont Corporation at its Tanami gold mine in 2019 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 9

A 1,600 chilling plant is to be installed as part of the Gwalia Extension High Cost High Cost Project. Low GHG High GHG

1,400 4.1.3. Mining fleet electrification An area for future GHG emission reductions is through the use of 1,200 electric vehicles (EVs). In 2018 BHP Group Limited started a 12- month trial of electric light vehicles at its UG Olympic Dam copper-gold-uranium operation (BHP Group Limited, 2018). In 1,000 Mean AISC 2019, Safescape launched its prototype EV light vehicle, the Bortana OP EV, to undergo a three-month trial at Fosterville gold mine 800 (Safescape, 2019). OP & UG An all-electric underground mining fleet has benefits beyond UG the immediate reduction of GHG emissions and improvement in air

Real AISC Real AISC (US$/oz) 600 Outlier quality for workers by reducing airborne diesel particulate matter. That is, an all-electric ﬂeet leads to a reduction in mine ventilation 400 requirements and hence reduces the overall power required,

emissions lowering capital and operating costs (Hiyate, 2019). 200 GHG Low Cost Low Cost 4.1.4. Impact of current, planned and possible reductions of GHG

Low GHG Mean High GHG emissions 0 0 200 400 600 800 1,000 1,200 1,400 1,600 Reducing the present annual GHG emissions at the Agnew,

GHG Emissions Intensity (kg CO2-e/oz AuEq) Granny Smith, Gwalia and Tanami mines by the measures in Sec- tions 4.1.1 and 4.1.2 and recalculating their emissions intensity B 1,600 e High Cost High Cost footprint leads to reductions in GHG emissions intensity of 7% 52% Low GHG High GHG (Table 12 and Fig. 12). The order of the mines with the ﬁve lowest 1,400 GHG emissions intensities in Fig. 8 would alter to Agnew, Gwalia, Mt Carlton, Granny Smith and Tanami. An estimate of the opportunity for Australia’s gold mines to 1,200 reduce their GHG emissions in the future is possible using the previously described subset of mines as a base case. The possible 1,000 reduction in GHG emissions is between 9200e41,000 t CO2-e. The Mean AISC lower end of the range reﬂects the average reduction from

800 OP installing a solar plant and the upper end based on the Agnew mine OP & UG from installing a solar and wind energy plant. The total reduction in UG GHG emissions achievable is between 0.26 and 1.15 Mt CO2-e,

Real Real AISC (US$/oz) 600 Outlier equivalent to between 4.4 and 19.5% based on the 5.9 Mt of CO2-e produced in the year to June 30, 2018. Additional reductions are 400 possible by installing a chiller plant over an ammonia refrigeration plant at underground mines. The effect of EV’s in UG mines could provide a considerable reduction in GHG emissions directly and 200 indirectly through lower energy consumption for mine ventilation. Low Cost Low Cost Additional reductions could be possible at the mines that generate

Low GHG Mean GHG emissions High GHG 0 electricity from diesel by switching to gas. 0 200 400 600 800 1,000 1,200 1,400 1,600 GHG Emissions Intensity (kg CO2-e/oz AuEq) 4.2. Declining ore grades

Fig. 6. GEIPC matrices - Australian Gold Mining GHG emissions intensity (kg CO2-e/oz) versus real production costs (AISC US$/oz). Notes: Fig. 6A e All data. Fig. 6B e Subset of Mudd (2007a, 2007b, 2009) showed that gold grades had mines representing a cross-section of Australian mines most recently reported GHG declined, in Australia and the world, over time. The 2014e2018 emissions data. The box around the intersection of the means is one standard deviation average GHG emissions intensity (19,800 kg CO2-e/kg Au) of this from the mean real AISC and mean GHG emissions intensity. The outliers relate to the study (Table 4), shows a 40% increase compared to the 1991e2006 Fosterville and Stawell mines (see Fig. 4A and Section 2.3). average (14,100 kg CO2-e/kg Au) of Mudd (2007b) (Table 1). This increase in emissions intensity is primarily due to declining ore IGO Limited at its Nova nickel-copper-cobalt operation intends grades. to build a solar photovoltaic facility into its diesel fuel power plant. Irrespective of the mining method, over time, technology can It is estimated that the solar plant will reduce diesel consumption provide permanent reductions in the costs of unit processes in by 3,000,000 L and reduce GHG emissions by approximately 5200 t mines, translating to lower economic cut-off grades. The best CO2-e per annum (Independence Group NL, 2018). example of this was the introduction of carbon-in-pulp (CIP) and carbon-in-leach (CIL) processing in the 1980s allowing the mining and processing of lower grade gold ores (Huleatt and Jaques, 2005). 4.1.2. Mine ventilation These technology improvements are cumulative over time, putting St Barbara Limited installed a chilling adsorption plant at their long-term downward pressure on grades. Gwalia gold mine in 2015. The chiller plant uses waste heat from Lower gold head grades can also result from high gold prices as the power plant to cool the UG mine, replacing a traditional previously uneconomic ore becomes mineable due to lower eco- ammonia refrigeration plant, saving an estimated 4000 to 5000 t nomic cut-off gold grades within a life of mine plan. However, the CO2-e per annum (St Barbara Limited, 2018). A second adsorption long-term decline in grade observed overprints this effect, which 10 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118

A 1,600 BC1,600 1,600 High Cost High Cost High Cost High Cost High Cost High Cost Low GHG High GHG Low GHG High GHG Low GHG High GHG 1,400 1,400 1,400

1,200 1,200 1,200 Mean 1,000 1,000 1,000 AISC Mean AISC 800 Mean 800 800 AISC 600 600 600 Real AISC (US$/oz) AISC Real Real AISC (US$/oz) AISC Real Real AISC (US$/oz) AISC Real 400 400 400 OP OP & UG UG 200 200 200 Low Cost Low Cost Low Cost Low Cost Low Cost Low Cost Mean Mean GHG emissions emissions emissions Mean GHG Low GHG High GHG Low GHG Mean GHG High GHG Low GHG High GHG 0 0 0 0 400 800 1,200 1,600 0 400 800 1,200 1,600 0 400 800 1,200 1,600

GHG Emissions Intensity (kg CO2-e/oz AuEq) GHG Emissions Intensity (kg CO2-e/oz AuEq) GHG Emissions Intensity (kg CO2-e/oz AuEq)

Fig. 7. GEIPC matrices - GHG emissions intensity (kg CO2-e/oz) to costs (US$/oz) footprints by mined ore source. Notes: Fig. 7A - OP mines footprint. Fig. 7B-OP& UG mines footprint. Fig. 7C e UG mines footprint. The box around the intersection of the means is one standard deviation from the mean real AISC and mean GHG emissions intensity.

Table 10 et al. (2019) in a study on grade-cost relationships in Australian Average GHG Emission Intensities and real AISC by the mine ore source. underground mines showed that 13 out of 19 UG mines studied Mine Ore Source GHG Emissions Intensity AISC US$/oz AuEq were processing high-grade ore in the four preceding years than the currently reported Ore Reserves, being on average 18.5% lower, kg CO -e/oz AuEq kg CO -e/kg AuEq 2 2 signalling lower future mined and processed grades. OP 777 24,992 787 Using the regression equations of the different grade-GHG OP & UG 648 20,845 1007 emissions intensity relationships for the different mine ore sour- UG 471 15,150 920 All Sources 616 19,802 910 ces (Fig. 5) the percentage change in GHG emissions intensity per ounce for a certain percentage change in head grade can be calculated (Table 13). The percentage change in GHG emissions may only be short term. intensity per ounce is greater under a reducing grade scenario In Australian gold mines, Schodde (2017) observed that the compared to an equivalent percentage increase in gold grade. weighted average gold head grade declined by 25% from 2.44 g/t Au The observed 25% decline in gold head grade from 2006 to 2017 in 2006 to 1.83 g/t Au in 2017. Schodde (2017) forecasts for existing by Schodde (2017) would represent a 22.0%, 14.6% and 15.5% in- & gold mines the average head grade to decline a further 44% to crease in the GHG emissions intensity for OP, OP UG and UG 1.02 g/t by 2029. After 2029 the average gold head grade of existing mines, respectively. A 44% decrease in gold head grade across the mines is expected to rise to over 5 g/t Au as most large low-grade Australian gold industry by 2029 as forecast by Schodde (2017) OP mines either close or convert to high-grade UG mines. Ulrich would lead to a 49.2%, 31.6% and 33.6% increase in the GHG

Fig. 8. GHG emissions intensity curve (kg CO2-e/oz) versus cumulative Australian gold production (in troy ounces). Lowest quartile (25%) 513 kg CO2-e/oz AuEq (16,493 kg CO2-e/kg AuEq). Median (50%) 619 kg CO2-e/oz AuEq (19,901 kg CO2-e/kg AuEq). Third quartile (75%) 811 kg CO2-e/oz AuEq (26,074 kg CO2-e/kg AuEq). Notes:1. Gwalia; 2. Mt Carlton; 3. Agnew; 4. Granny Smith; 5. Tanami; 6. Peak; 7. Fosterville; 8. Carosue Dam; 9. Cracow; 10. Sunrise Dam; 11. Mount Monger; 12. St Ives; 13. Combined Jundee, Kalgoorlie and Paulsens; 14. Tropicana; 15. Combined Duketon North and South; 16. Plutonic; 17. Combined Central Murchison, Fortnum, Higginsville and South Kalgoorlie; 18. Tomingley; 19. Kalgoorlie (KCGM); 20. Mungari; 21. Combined Mt Magnet Operations and Edna May; 22. Thunderbox; 23. Matilda-Wiluna; 24. Paddington; 25. Cadia Valley; 26. Darlot; 27. Ravenswood; 28. Boddington; 29. Telfer; 30. Mt Rawdon; 31. Cowal. S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 11

1,600 25 High Cost High Cost Grid Low GHG High GHG Telfer High Grade Low Grade Gas 1,400 Diesel Gas Sunrise Dam 2 20 Hybrid R = 0.955 1,200 Edna May Outlier

1,000 Mean AISC 15 Diesel Boddington 800 R2 = 0.960 Gwalia UG 10 Grid Real AISC Real AISC (US$/oz) 600 2

Real AISC(US$/oz) OP & UG R = 0.830

OP Energy Intensity (GJ/oz AuEq)

400 Outlier 5 Mt Carlton Cadia 200 Low Cost Low Cost Low GHG High GHG

High Grade Mean GHG emissions Low Grade 0 0 0 200 400 600 800 1,000 1,200 1,400 1,600 0 200 400 600 800 1,000 1,200 1,400 1,600

GHGGHG Emissions Emissions Intensity Intensity (kg CO CO2-e/oz2-e/oz AuEq) AuEq) GHG Emissions Intensity (kg CO2-e/oz AuEq)

Fig. 9. GEIPC matrix e All data - GHG emissions intensity (kg CO2-e/oz) against real Fig. 11. GHG emissions intensity (kg CO2-e/oz) against energy intensity (GJ/oz) by the AISC (US$/oz) by ore source and gold grade. Point size is reﬂective of average gold head mine power source. The outliers relate to the Fosterville and Stawell mines (see Fig. 4A grade (g/t). The outliers relate to the Fosterville and Stawell mines (see Fig. 4A and and Section 2.3). Section 2.3).

Table 11 Subset of mines -Percentage of mines and gold equivalent produced by the mines electricity source.

Electricity Source Number of Mines Percentage of Mines Percentage of AuEq oz produced

Grid 15 39% 45% Gas 11 29% 30% Diesel 8 21% 12% Grid & Gas 1 3% 8% Diesel & Gas 3 8% 5%

25 Table 12 Grid Current and planned annual GHG emissions at Australian Gold Mines. Gas Mine Annual GHG Emissions (t CO2-e) GHG Emissions Intensity Diesel (kg CO2-e/oz Au) 20 Hybrid Current Reduction Future Current Future Change

Outlier Agnew 78,000 40,700 37,300 323 154 52% Granny Smith 96,700 9500 87,200 333 300 10% Gwalia 61,000 4500 56,500 227 210 7% 15 Tanami 176,800 34,500 142,300 422 340 19%

Notes: Annual GHG emissions rounded to nearest 100 t CO2-e.

10 emissions intensity for OP, OP & UG and UG mines, respectively. The differences between the recently processed gold head Energy Energy Intensity (GJ/oz AuEq) grades and the current reported Ore Reserves grades in UG mines observed by Ulrich et al. (2019), would lead to a near term increase 5 Gas in the GHG emissions intensity of 10.8% on average for those un- R2 = 0.929 derground mines. Grid R2 = 0.723 Diesel R2 = 0.903 0 4.3. Maturation of the Australian gold industry 0 2 4 6 8 10 12 14 Average Gold Head Grade (g/t) The modern Australian gold mining era began in the 1980s Fig. 10. Energy intensity (GJ/oz) against the average gold head grade (g/t) of ore facilitated by the introduction of carbon-CIP and CIL processing processed by the mine power source. The outliers relate to the Fosterville and Stawell allowing the mining and processing of lower grade gold ores mines (see Fig. 4A and Section 2.3). (Huleatt and Jaques, 2005). Most Australian gold mines start as an open pit mine (Phase 1), then if feasible at the end of the economic life of the initial OP, UG 12 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118

1,600 1,600 High Cost High Cost Low Emissions Moderate Emissions High Emissions Low GHG High GHG 1,400 1,400 High Cost 1,200 1,200

Agnew 1,000 1,000 Granny Smith Mean AISC 800 800

Tanami OP Moderate Cost OP & UG 600 Gwalia UG Real AISC (US$/oz)

Real Real AISC (US$/oz) 600 Outlier 400 400 Low Low Cost

200 OP OP & UG 200 UG Outlier Low Cost Low Cost 0 0 200 400 600 800 1,000 1,200 1,400 1,600

Low GHG Mean GHG emissions High GHG 0 0 200 400 600 800 1,000 1,200 1,400 1,600 GHG Emissions Intensity (kg CO2-e/oz AuEq)

GHG Emissions Intensity (kg CO2-e/oz AuEq) Fig. 14. GEIPC matrix - GHG emissions intensity (kg CO2-e/oz) by real AISC (US$/oz) sectors for the subset of mines. The central sector with the grey background is the Fig. 12. GEIPC matrix e Reduction in GHG emissions intensity (kg CO2-e/oz) for the Agnew, Granny Smith, Gwalia and Tanami mines compared to Fig. 6B the subset of industry standard sector. The outlier is the Fosterville mine (see Fig. 4A and Section mines GHG emissions intensity versus real production costs (AISC US$/oz). Notes: 2.3). Subset of mines representing a cross-section of Australian mines most recently reported GHG emissions data. The box around the intersection of the means is one standard deviation from the mean real AISC and mean GHG emissions intensity. The outlier relates to the Fosterville mine (see Fig. 4A and Section 2.3). mining commences, and in many instances, additional ore is sourced from smaller satellite open pits and/or pit cutbacks (Phase 2). As the mining of satellite open pits come to an end, the mining Table 13 operation progresses to a wholly UG mining operation (Phase 3). Percentage change in GHG Emissions intensity as gold head grade changes. Many of Australia’s longer-life gold mines have transitioned Percentage change in Gold Head Grade Percentage change in GHG from OP to UG mining, such as Agnew, Carosue Dam, Fosterville, Emissions Intensity Granny Smith, Gwalia, Jundee, Kanowna Belle and Sunrise Dam. OP OP & UG UG Not all gold mines go through all phases in their economic life; for

40 þ42.3 þ27.4 þ29.1 example, smaller shorter life gold mines may only be OP (e.g., 30 þ27.9 þ18.4 þ19.5 Matilda mine). Some mines are mostly UG with only a small starter 20 þ16.6 þ11.2 þ11.8 pit operating for a few months such as Andy Well and Deﬂector. 10 þ7.5 þ5.1 þ5.4 The effect of gold mine maturation on GHG emissions intensity ee e 0 and costs is illustrated as a conceptual gold mine through time þ10 6.4 4.4 4.7 þ20 11.8 8.3 8.7 (Fig. 13) using the GHG emissions intensity to costs footprints for þ30 16.6 11.7 12.3 the different ore sources identiﬁed in Fig. 7. þ40 20.7 14.7 15.5 Based on the average GHG emissions intensity and real AISC values displayed in Table 10, as a mine transitions from phase 1 to

900 1,200 Mean GHG Emissions Intensity 800 1,000 700 Mean Real AISC 600 800 -e/oz -e/oz AuEq) 2 500 600 400

300 400 Real AISC Real AISC (US$/oz) 200 200 GHG Intensity GHG Intensity (kg CO 100 Phase 1 Phase 2 Phase 3 OP Mining OP & UG Mining UG Mining 0 0 0 2 4 6 8 101214161820 Years

Fig. 13. Relationship between GHG emissions intensity (kg CO2-e/oz AuEq) and real AISC (US$/oz AuEq) with the mined ore sources of a conceptual gold mine through time. The 20- year mine life represents an indicative mine life for a major gold mine in Australia. Between years nine and ten the mine transitions from phase 1 to phase 2 and between years 13 and 14 the mine transitions to phase 3. S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 13

Table 14 The subset of mines e GHG/AISC Sector descriptions.

GHG AISC Mines Comments Emissions

Low Low Gwalia, Mt Carlton High-grade gold mines: The Gwalia mine is one of the highest-grade UG gold mines with an average gold head grade for 2014e18 of 8.90 g/t Au. The Mt Carlton mine is the highest-grade OP gold mine with an average gold equivalent head grade for 2014e18 of 5.23 g/t AuEq. The average gold head grade for all underground and open pit mines over the study period was 5.66 g/t and 1.94 g/t Au, respectively. Moderate Agnew, Granny Smith Moderately high-grade (5.5e7.5 g/t Au), large tonnage (>1 Mtpa) UG gold mines: The Agnew and Granny Smith mines average gold head grades are 6.43 g/t Au and 5.54 g/t Au, respectively. In previous reporting periods, the Gwalia and Tanami underground mines reported in this sector when Gwalia’s average gold head grade dropped to 7.30 g/t Au in 2015, and Tanami’s average gold head grade was >5.5 g/t Au in 2014 and 2015. In 2015, the Mt Carlton OP mine also reported in this sector when its average gold equivalent head grade dropped to 4.25 g/t AuEq, compared to its average gold equivalent head grade for 2014e18 of 5.23 g/t AuEq. High None Small tonnage (<0.3 Mtpa), high-grade UG operations are most likely to display low emissions but high costs. No mines reported from the current or previous reporting periods. The absence of data is likely indicative of small scale mining not exceeding the minimum NGER reporting thresholds in terms of GHG emissions and energy consumption. Moderate Low Cadia, Fosterville Underground caving gold mines: Cadia is a large, bulk tonnage, panel cave UG mine, processing >20 Mtpa, with an average gold equivalent head grade for 2017e18 of 1.64 g/t AuEq. The Fosterville mine is a high-grade underground gold mine with an average gold head grade of 18.05 g/t Au in the 2018 reporting period. Fosterville is identiﬁed as an outlier, at the data evaluation stage; as such, it is not certain whether it is moderate emissions or whether it may sit within the low emissions sector. Moderate Carosue Dam, Cracow, Kalgoorlie (KCGM), Mount Monger, Mungari, Approximately half of the mines are in this ‘Industry standard’ sector. Northern Star Resources’ combined Jundee, Kalgoorlie Operations and They comprise of an even split between OP, UG and OP & UG mines. Paulsens mines, Peak, Ramelius Resources’ combined Edna May mine and Although collectively, Northern Star Resources’, Ramelius Resources’ and Mt Magnet operations, Regis Resources’ combined Duketon North and Regis Resources’ operations plot here, the individual mines in the South operations, St Ives, Tanami, Thunderbox, Tomingley, Tropicana combined reporting groups may not all plot within this sector. Reporting of the individual mines would resolve this uncertainty. High Darlot, Matilda-Wiluna, Plutonic, Westgold Resources’ combined Central Low-grade (<2.75 g/t Au) OP & UG gold mines and low-grade (<3 g/t Au), Murchison, Fortnum, Higginsville and South Kalgoorlie mines, Sunrise non-caving UG gold mines: Sunrise Dam is a low-grade underground Dam mine with an average gold head grade of 2.37 g/t Au from 2014 to 2018, compared to the 2014e18 average gold head grade of all underground mines being 5.66 g/t Au. Although collectively, Westgold Resources’ operations report here, the individual mines in the combined reporting group may not all plot within this sector. Of note, all of Westgold Resources’ mines are classiﬁed as combined OP & UG mines. High Low None Large tonnage (>3 Mtpa) low-grade gold mines: In the 2016 and 2017 reporting periods, the Cowal OP mine plotted in this sector. Most likely to be OP mines. Moderate Boddington, Cowal, Mt Rawdon, Ravenswood Low-grade (approximately <1.5 g/t Au) and generally large tonnage (>3 Mtpa) OP and OP & UG gold mines: Most likely to be OP mines. Boddington is Australia’s largest OP gold mine, processing >40 Mtpa at an average gold equivalent head grade for 2014e18 of 0.95 g/t AuEq. Cowal and Mt Rawdon are OP mines processing >6 Mtpa and >3 Mtpa, respectively. Ravenswood is a low grade (1.19 g/t Au) OP & UG mine, which processed >2.4 Mtpa. High Telfer Low-grade (approximately <0.90 g/t Au), large tonnage (>3 Mtpa) gold mines: Most likely to be OP or OP & UG. Telfer is a large, low-grade bulk tonnage OP & UG mine processing >20 Mtpa, with an average gold equivalent head grade for 2017e18 of 0.88 g/t AuEq. In 2016, before the Edna May mine was collectively reported as part of Ramelius Resources’, it had reported within this sector when its average gold head grade dropped to 0.82 g/t Au.

phase 2, GHG emissions intensity is reduced by 18%, and real AISC mining methods. Open pit mines generally have lower cut-off increases 30%. As the mine transitions further to phase 3, its GHG grades, whereas UG mines have higher cut-off grades due to be- emissions intensity reduces by a further 27% and real AISC de- ing a selective mining method, with smaller volumes of ore mined creases by 9%. The overall transition from phase 1 to phase 3 on and processed for the same amount of product. average leads to a decrease in the GHG emissions intensity of 40% and an 18% increase in real AISC. The primary influence on the reduction in GHG emissions in- 4.4. Characterisation of gold mines by their GHG emissions tensity from OP to UG is the increase in the gold grade processed, intensity and AISC due to the change in economic cut-off grades for the different The type of mine (OP or UG), the gold grade and scale (number 14 S. Ulrich et al. / Journal of Cleaner Production 267 (2020) 122118 of tonnes of ore processed), allows the characterisation of a gold companies will need to change the energy source mix of their mine in terms of their GHG emissions intensity and AISC. Utilising mines and look to use electric vehicles and mining fleet to lower the subset of mines dataset, a three by three, nine sector GEIPC both total emissions and their GHG emission footprints per ounce. matrix can be constructed using the values one standard deviation from the mean GHG emissions intensity and AISC as the boundary Declaration of competing interest for either low or high emissions and low or high AISC (Fig. 14). The threshold boundaries between the sectors will vary depending on The authors declare that they have no known competing the time period selected. financial interests or personal relationships that could have Generally, the low GHG emissions intensity sectors are mines appeared to influence the work reported in this paper. with high gold head grades, the high GHG emissions intensity & sectors are large, low-grade OP or OP UG mines (Fig. 14). The CRediT authorship contribution statement central sector coloured light grey represents what could be termed as the industry-standard sector (Fig. 14). The low-cost sectors Sam Ulrich: Conceptualization, Methodology, Validation, generally represent high-grade gold mines and the high-cost sec- Formal analysis, Investigation, Visualization, Resources, Data cura- tors low-grade gold mines (Fig. 14). Details of each GHG/AISC sector tion, Writing - original draft. Allan Trench: Supervision, Writing - are described in Table 14. review & editing. Steffen Hagemann: Supervision, Writing - review & editing. 5. Conclusions Acknowledgements Establishing relationships between GHG emissions and production costs has the potential to shape the future of the gold in- This manuscript benefited from constructive comments and dustry in Australia through greater focus upon cleaner, efficient open discussion with Centre for Exploration Targeting and CSA production. Characteristics of a gold mine such as the type of mine, Global colleagues. The authors would like to thank Dirk Baur, the whether it is OP, UG or OP & UG, its average gold head grade and the reviewers and the editor for their constructive feedback to improve scale of the mine can indicate if the mine is of high or low GHG the quality of this manuscript. emissions intensity and AISC versus the whole industry dataset. Sam Ulrich thanks those gold mining companies who have Generally, high-grade mines have low GHG emission intensities shown, and continue to show, an active interest in this research. and are lower cost. Large low-grade mines have high GHG emissions intensities and higher costs. Open pit mines have the highest GHG emissions intensity on average but have the lowest costs. References Open pit and underground mines have lower GHG emissions in- Australian Bureau of Statistics, 2019. December quarter 2018 consumer price Index tensity than OP mines but are the highest cost. Underground mines Australia. Australian Bureau of Statistics, p. 40 pp. http://www.ausstats.abs.gov. have the lowest GHG emissions intensity, with their costs higher au/ausstats/meisubs.nsf/0/F771E5E03E8E70DFCA258391001B1452/$File/ than OP mines but lower than OP & UG mines. For the long life gold 64010_dec%202018.pdf. accessed 14 February 2019. Ballantyne, G.R., Powell, M.S., 2014. 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