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Toms Gully Underground Project Acid and Metalliferous Drainage Management Plan

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Toms Gully Underground Project

Acid and Metalliferous Drainage

Management Plan

July 2019


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Document Control Record

Prepared by: Charles Hastie Approved by: Mark Qiu

Position: Manager Approvals and

Tenure Position: Director

Signed:

Signed:

Date: 15/07/2019 Date: 15/07/2019

Revision Status

Revision No. Description of

Revision

Date Comment Approved

1.0 First Issue 18/09/15 First issue released by GHD in 2015 for the purpose of the EIS draft.

2.0 Second Issue 8/08/18 Updated to reflect Draft EIS comments. Submitted for EIS Supplement

MQ


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Contents List of Figures ................................................................................................................................................ 5

List of Tables ................................................................................................................................................. 5

1. Introduction .......................................................................................................................................... 6

1.1. Purpose and Scope ........................................................................................................................ 6

2. Legal and Other Requirements ............................................................................................................. 9

2.1. Guidelines ..................................................................................................................................... 9

2.2. Environmental Corporate Governance ......................................................................................... 9

2.3. Inputs to the AMDMP ................................................................................................................. 10

2.4. Document Revision ..................................................................................................................... 10

3. Waste Rock and Tailings Characterization and Classification ............................................................. 12

3.1. Waste Rock Characterization ...................................................................................................... 12

3.1.1. Mineralogy .......................................................................................................................... 12

3.1.2. Oxide Waste Rock Dump..................................................................................................... 13

3.1.3. Sulphide Waste Rock Dump ................................................................................................ 13

3.1.4. Boxcut Waste Rock.............................................................................................................. 13

3.2.1. Mineralogy .......................................................................................................................... 14

3.2.2. Geochemistry ...................................................................................................................... 14

3.3. Waste Rock and Tailings Classification ....................................................................................... 15

3.4. PAF/NAF Estimated Volumes ...................................................................................................... 15

4. Conceptual Site Model ........................................................................................................................ 16

4.1. Background ................................................................................................................................. 16

4.2. Source ......................................................................................................................................... 16

4.2.1. SWRD................................................................................................................................... 16

4.2.2. OWRD .................................................................................................................................. 17

4.2.3. TSF1 ..................................................................................................................................... 17

4.2.4. TSF2 ..................................................................................................................................... 17

4.3. Pathway....................................................................................................................................... 18

4.3.1. Surface Water ..................................................................................................................... 18

4.3.2. Groundwater ....................................................................................................................... 19

4.4. Receptors .................................................................................................................................... 19


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4.4.1. Mt Bundey Creek ................................................................................................................ 19

4.4.2. Lake Bazzamundi ................................................................................................................. 19

5. Acid Mine Drainage Model and Balance ............................................................................................. 22

6. AMD Management .............................................................................................................................. 24

6 . 1 . AMD Risk Assessment ............................................................................................................. 24

6.2. AMD Management Strategy ....................................................................................................... 30

6.3. Controls ....................................................................................................................................... 30

6.3.1. Ore and Waste Rock ............................................................................................................ 30

6.3.2. Tailings ................................................................................................................................ 31

6.4. Site Drainage and Controls ......................................................................................................... 32

6.4.1. Maintenance of existing structures .................................................................................... 38

7. AMD Monitoring ................................................................................................................................. 38

7.1. Geochemical Monitoring ............................................................................................................ 40

7.1.1. Visual Methods ................................................................................................................... 40

7.1.2. Laboratory Analysis ............................................................................................................. 41

7.2. Surface Water Monitoring .......................................................................................................... 42

7.3. Groundwater Monitoring ............................................................................................................ 42

8. Contingency Planning.......................................................................................................................... 43

8.1. Overview ..................................................................................................................................... 43

8.2. Specific Measures ....................................................................................................................... 43

8.2.1. Tailings Management .......................................................................................................... 43

8.2.2. Waste Rock.......................................................................................................................... 43

8.2.3. ROM Pad / Ore .................................................................................................................... 44

8.2.4. Water Management ............................................................................................................ 44

9. Roles, Responsibilities and Training .................................................................................................... 45

9.1. Awareness, Training and Competence ....................................................................................... 45

9.2. Records, Reporting and Document Control ................................................................................ 46

10. References ...................................................................................................................................... 47

Appendix A: Site Geochemical Sampling Procedure ................................................................................... 49


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List of Figures Figure 1: Project Location ............................................................................................................................. 8

Figure 2: Site Layout .................................................................................................................................... 11

Figure 3: Conceptual Site Model at Toms Gully (GHD 2019) ...................................................................... 21

Figure 4: Location of waste material to be placed in the flooded pit ......................................................... 31

Figure 5: Systematic Floating Head Traverses to Deposit Tails. ................................................................. 32

Figure 6: Site Drainage ................................................................................................................................ 35

Figure 7: Surface Water Monitoring Locations ........................................................................................... 36

Figure 8: Surface Water Monitoring Locations Continued ......................................................................... 37

Figure 9: The process of adaptive management ......................................................................................... 40

List of Tables Table 1: 2019 TGU Materials Balance ......................................................................................................... 12

Table 2: Tailings Acid Base Accounting Summary - Median Results (GHD 2018) ....................................... 14

Table 3: Net Acid Generation (GHD 2018d) ................................................................................................ 22

Table 4: Average Annual Acid Mine Drainage Balance (GHD 2018d) ......................................................... 23

Table 5: AMD Risk, potential impact and management / mitigation control ............................................. 25

Table 6: Water management infrastructure and their catchment areas (GHD 2019) ................................ 34

Table 7: Water Storages at TGU Project (2019) .......................................................................................... 34

Table 8: Sampling Frequency ...................................................................................................................... 42

Table 9: Roles and Responsibilities ............................................................................................................. 45


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1. Introduction The Toms Gully Underground Project (TGU or the Project) is located within the Old Mt Bundey Station,

approximately 90 km south-east of Darwin. The mine was operational on occasions from 1988 and has

been in Care and Maintenance since November 2010 (Figure 1).

Primary Gold Ltd (PGO) acquired the Project in 2013 and has proposed to implement a resumption of

mining at TGU (Figure 2). The Project includes the following works to recommence underground mining

and ore processing on site:

• Construction of a new 1 gigalitre (GL) water storage dam (WSD),

• Construction of a new Boxcut and Decline,

• In-situ treatment of pit water,

• Treatment of the water displaced from the pit,

• Underground mining for approximately three to four years with all waste rock stored

underground or in-pit,

• Upgrade of the existing tailings storage facility (TSF2) to ANCOLD 2012 standard for potential

use a as water storage dam,

• Placement of existing tails from TSF1 and 2 (whether processed or not) in the pit under a water

cover,

• Upgrade of the processing circuit, and

• Establishment of a water and tailings treatment plant.

Acid and metalliferous drainage (AMD) (including neutral and saline drainage) from existing mine features

(TSFs, processing area, waste rock dumps, pits and evaporation ponds) are potentially impacting water

quality and downstream aquatic ecosystems.

The Environmental Impact Statement (EIS) Terms of Reference (ToR) issued by the Northern Territory

Environmental Protection Authority (NT EPA) for the Project requires that the EIS should contain a detailed

AMD Management Plan (AMDMP) that outlines how AMD risk will be managed on site. This AMDMP has

been developed to address the EPA’s recommendations in the ToRs as well as the NT EPAs comments on

the Draft EIS.

1.1. Purpose and Scope The purpose of this AMDMP (or, the Plan) is to describe the systems, processes and procedures used at

the Project to manage the overall risk of AMD being generated on site throughout operations (and

therefore into closure). It does so by classifying waste rock and tailings based on geochemical testing,


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providing a management strategy, and articulating management and monitoring procedures for the

handling and long term storage of waste rock and tailings on site.

This document outlines the objectives and methods for PGO to follow in pursuit of leading practice AMD

management. The principle objective is to manage AMD risk resulting from the oxidation of sulfidic

mineral waste material throughout operations, such that off-site environmental values are maintained

during operations and into closure.

The plan is applicable to Mining Leases (ML) MLN 1058, ML29812 and ML29814 and has been informed

by the Mineral Waste Geochemical Assessment (GHD 2019) shown in Appendices D, J and baseline

geochemistry and conceptual site model report undertaken by GHD (2019) Appendix N. As Toms Gully is

a brownfield site, it is noted that AMD is currently being generated at the sulfide and oxide WRDs, TSF1

and 2, the pit, and evaporation ponds 1 and 2. This Plan acknowledges the legacy AMD and incorporates

its ongoing management and monitoring into the AMD strategy for the Project.


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Figure 1: Project Location


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2. Legal and Other Requirements PGO submitted a Mining Management Plan (MMP) under the NT Mining Management Act to the Northern

Territory Department of Primary Industries and Resources (DPIR) in late 2013. Subsequently, on 28

February 2014, DPIR referred the Toms Gully Underground Project MMP 2013-2014 and 18 associated

documents to the NT EPA for consideration under the Environmental Assessment Act (EA Act).

The NT EPA subsequently determined that an Environmental Impact Statement (EIS) was required under

the EA Act and the terms of reference (ToRs) for the EIS were issued in October 2014.

PGO submitted the Draft EIS to the NT EPA. The NT EPA provided comments on the Draft EIS and

subsequent EIS Supplement submitted in response. This AMDMP is a revised version of the AMDMP

submitted with the EIS Supplement and incorporates the NT EPA/stakeholders comments as well as

results of updated studies and project changes.

2.1. Guidelines Content from the following industry guidelines were considered when preparing this Plan:

• AMIRA (2002). ARD Test Handbook. Project P387A Prediction and kinetic control of acid mine

drainage. Available at:

http://www.amira.com.au/documents/downloads/P387AProtocolBooklet.pdf

• Department of Foreign Affairs and Trade (DFAT) and Department of Industry Innovation and

Science (2016). Leading Practice Sustainable Development Program for the Mining Industry:

Preventing Acid and Metalliferous Drainage. Canberra. Available at:

https://industry.gov.au/resource/Documents/LPSDP/LPSDP-AcidHandbook.pdf

• The International Network for Acid Prevention (INAP) (2009). Global Acid Rock Drainage Guide.

Available at www.gardguide.com

• NT Environment Protection Authority (2013). Environmental Assessment Guidelines: Acid and

Metalliferous Drainage. Version 2.0. Available at:

https://ntepa.nt.gov.au/__data/assets/pdf_file/0011/287426/guideline_assessment_acid_meta

lliferous_drainage.pdf

2.2. Environmental Corporate Governance It is PGO’s mission to operate in an environmentally and socially responsible way, in order to minimise

their footprint and maximise benefits for their staff, shareholders and stakeholders well beyond the life

of their mines.

PGO formally endorsed their Environmental Policy in August 2015 and updated it in April 2017. The

Environmental Policy includes:

• A strong emphasis on rehabilitation

• Engaging widely with Project stakeholders

• Continuous improvement

http://www.amira.com.au/documents/downloads/P387AProtocolBooklet.pdf

https://industry.gov.au/resource/Documents/LPSDP/LPSDP-AcidHandbook.pdf

http://www.gardguide.com/

https://ntepa.nt.gov.au/__data/assets/pdf_file/0011/287426/guideline_assessment_acid_metalliferous_drainage.pdf

https://ntepa.nt.gov.au/__data/assets/pdf_file/0011/287426/guideline_assessment_acid_metalliferous_drainage.pdf


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• Cultural awareness

• Maximising shareholder value while balancing the quadruple bottom line of:

o Environmental sustainability

o Social equity

o Cultural Vitality

o Economic prosperity.

2.3. Inputs to the AMDMP This Plan has been informed by Appendix D, J and N (2019 Mineral Waste Assessment and 2019 Baseline

Geochemical Assessment respectively) and compiled using the following input documents and

information sources:

• A draft Toms Gully Underground Project description (Primary Gold 2015)

• The draft Terms of Reference for the Toms Gully Gold Project (NT EPA 2014)

• PGO exploration data from the 2014 drilling program

• Geotechnical results provided by PGO from the 2015 fieldwork

• Waste characterization analysis of 2018 PGO geotechnical Boxcut samples

• Geochemical data provided by PGO from the 2015 sampling and analysis program as

recommended in GHD (2015) report.

• Geochemical baseline assessment and conceptual site model (GHD 2019)

• NT EPA comments on draft EIS and EIS Supplement

• AMD Assessment: Toms Gully Boxcut Material

• Publicly available information.

2.4. Document Revision The Plan is a dynamic document and therefore should be revised annually (as required) and appended to

the updated Mining Management Plan (MMP) upon its re-submission to DPIR.


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Figure 2: Site Layout


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3. Waste Rock and Tailings Characterization and Classification Based on the current mine schedule, a summary of estimated ore and mineral waste (waste rock and

tailings) that require management at Toms Gully over the life-of-mine is shown in Table 1.

Current mine schedules shows underground development over the mine life totaling 21,769 metres,

ramping up and down; for an average of 454 metres per month.

As all waste rock from mining development will be treated as PAF managed by in pit and/or underground

placement, and all tailings will also be treated as PAF with all material placed in the pit. Tails will be placed

in the pit whether or not reprocessed to remove gold, mixed metal oxides, sulfur and silica. No

geochemical characterisation in real time is required for mineral waste handling and placement. Rather,

the geochemical sampling will provide an inventory and historic record for closure management and mine

legacy purposes.

Table 1: 2019 TGU Materials Balance

Year Ore mined Ore stockpiled Waste rock Tailings Existing Tailings Removal

1 - - 1,004,400 - -

2 385 385 156,997 - 187,500

3 220,666 27,062 162,062 193,998 187,500

4 241,116 18,183 138,418 249,996 -

5 284,357 52,544 47,317 249,996 -

6 135,579 - - 190,113 -

Total 884,103 NA 1,509,194 884,103 375,000

3.1. Waste Rock Characterization The data from both the SWRD and OWRD indicate that while there remains unoxidised sulfides and

weatherable acid forming sulfates in both dumps, the measured pyrite oxidation rates are proceeding at

a relatively benign pace that makes management of the oxidation products achievable under an

appropriate AMD strategy and management plan. Acidity loads from both structures can be effectively

managed using traditional water management solutions, including storage treatment and seasonal

dilution to reduce elements consistent with saline and metalliferous drainage to environmentally benign

concentrations for licenced offsite release (GHD 2018).

3.1.1. Mineralogy

The majority of minerals identified in the waste rock dumps were inert, being; quartz, mica and clays -

around 80% (in the RoM Pad), 75% (SWRD), and 85% (OWRD). The remaining balance of minor minerals

were classified as non-diffracting or unidentified; which are usually amorphous secondary minerals. The

presence of up to 11.1% dolomite and 1.9% calcite in the OWRD and up to 4.8% dolomite in the SWRD

indicates some neutralizing capacity is inherent in both structures (GHD 2018). See Table 5.5 of the 2018

Baseline Geochemical Report (Appendix C) for more details on Waste Rock acid base accounting results.


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3.1.2. Oxide Waste Rock Dump

The median pH1:5 value for the 114 samples analysed was 4.4 (classified as very low) which increases to

5.6 as pHOX when a rapid oxidant is introduced. A median EC1:5 of 96 μS/cm (classified as very low using)

suggests a very low potential for saline drainage, supported by a very low median chloride value of <10

mg/kg. The median total sulfur value for the 114 samples was 0.04%. The median chromium reducible

sulfur concentration was 0.01%. This would suggest the presence of a majority of acid-forming and non-

acid forming sulfate species in the total sulfur content reported from the OWRD. The NAPP value most

likely representative of the content of the OWRD is 0.4 kgH2SO4/t. when the median total sulfur NAPP

value of 0.9 kgH2SO4/t (or the jarosite adjusted value of 0.4 kgH2SO4/t) is considered along with the

median pHOX value of 5.6, waste rock in the OWRD may be seen as being potentially acid forming (low

capacity). Using the jarosite-adjusted NAPP value of 0.4 kgH2SO4/t, a density of 2.6 t/m3 (PGO 2013) and

GHD’s estimated volume of 3,967,800 m3, there remains a total potential acid load of around 3,761 tonnes

of H2SO4 in the OWRD (GHD 2018).

It is also important to note that sulfur concentrations are relatively low in the OWRD, and there are

pockets of neutralising carbonate present at up to around 11% (GHD 2018).

3.1.3. Sulphide Waste Rock Dump

The median pH1:5 value for the 79 samples analysed was 3.8 (classified as very low) decreasing to 3.2 as

pHOX when a rapid oxidant is introduced. A median EC1:5 of 1,340 μS/cm (classified as high), suggests a high

potential for saline drainage, despite the very low median chloride value of <10 mg/kg. The median total

sulfur value for the 79 samples was 0.62%, with the median chromium reducible sulfur value of 0.21%. As

for the SWRD, this would suggest the presence of a majority of acid-forming and non-acid forming sulfate

species in the total sulfur content reported from the SWRD. The NAPP value most likely representative of

the content of the SWRD is 13.5 kgH2SO4/t (GHD 2018).

Using the jarosite-adjusted NAPP value of 13.5 kgH2SO4/t, a density of 2.6 t/m3 (PGO 2013) and GHD’s

estimated volume of 3,267,800 m3, there remains a total potential acid load of around 115,031 tonnes of

H2SO4 in the SWRD. In summary, the SWRD contains a large potential acidity store and must be managed

to account for this (GHD 2018).

3.1.4. Boxcut Waste Rock

The 24 samples that were tested are predominantly NAF with less than 5% of samples reporting as

uncertain and none of the samples classifying as PAF. As over 95% of the tested samples reported as non-

acid forming, the mean and median values may also be classified as NAF. The kinetic NAG testing further

confirmed this low risk with no pH results below 4.5 (i.e. classified as acidic) and increasing pH values over

time in most cases.

The samples also pose a low risk for metalliferous (neutral) and saline drainage with metals exceedances

for aluminium and zinc generally within acceptable dilution factors. As a group, the results showed

negligible risk acid drainage risk with all but one sample reporting as non-acid forming. It should be noted

that the shallow depth samples taken directly from the boxcut area at depths of 3.1 m and 5.2 m showed


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more acidic pH values (5.7 and 5.4) than the mean for the group (7.2). This may indicate some surficial

cross-contamination with residual sulfidic material.

Low levels of metal leaching concentrations were found compared to the SSTVs with the highest

exceedance being 28 times the trigger. Considering dilution and natural attenuation factors, the

aggressive nature of the ASLP test, and that the majority of the samples returned results less than 10

times the trigger values, the material is deemed a relatively low risk of generating metalliferous drainage.

There remains a small risk of aluminum and zinc leaching at low concentrations based on the sample

analysis.

3.2. Tailings Characterisation

3.2.1. Mineralogy

Key reactive minerals reported by XRD analysis from the two tailings storage facilities (TSF1 and TSF2)

shows that unreacted pyrite and arsenopyrite remain in both tailings storage facilities, particularly at

depth in the unoxidised zone, while one near surface sample in TSF1 shows the presence of over 5%

jarosite; a secondary mineral that is sparingly soluble and contains acid-forming sulfate. Minor unreacted

dolomite is present in TSF2, consistent with the ore XRD data (GHD 2018).

3.2.2. Geochemistry

Geochemical results for the 18 samples analysed from TSF1 and the 11 samples analysed from TSF2 show

the pH1:5 is very low for TSF1 (3.0) and medium for TSF2 (6.2), they both become very low once a rapid

oxidant is added as pHOX. Chloride was below laboratory detection limits in both TSF1 and 2. EC1:5 was

very high for TSF1 (2,100 μS/cm) and TSF2 (2,110 μS/cm). There are higher total sulfur and chromium

reducible sulfur concentrations in TSF1 relative to TSF2; perhaps indicating an evolving process circuit over

time and/or ore imported from another site for processing. Neither TSF1 nor 2 contain sufficient

neutralising capacity relative to their maximum potential acidity (based on total sulfur) to offset acid

generation as demonstrated by a NPR of 0.3 and below. The NAG 7.0 approximates the SCR NAPP,

indicating the likely extent of net acid producing potential in the samples from unoxidised sulfides (GHD

2018).

The volume of emplaced tailings TSF1 and TSF2 as estimated by GHD using LiDAR, 12D software and

sampling logs are around 131,000 m3 and 90,000 m3. Using a density of 1.5 t/m3 (GHD 2018 pers. comm.),

estimated emplaced tonnages for TSF1 and TSF2 are around 196,500 and 135,000 tonnes respectively.

Jarosite-adjusted median NAPP total acidity loads for TSF1 and 2 based on these tonnages are

approximately 27,320 and 9,205 tonnes of H2SO4 respectively (GHD 2018).

In summary, TSF1 and TSF2 remain significant point sources of acidity based on the data presented in

Table 2:

Table 2: Tailings Acid Base Accounting Summary - Median Results (GHD 2018)

Units TSF1 TSF2

Count 18 11


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pH1:5 pH units 3.0 6.2

EC1:5 µS/cm 2,100 2,110

Chloride mg/kg 5 5

STOT % 5.1 3.0

STOT MPA kgH2SO4/t 155 92.4

ANC kgH2SO4/t 0.3 10.8

STOT NAPP kgH2SO4/t 155 45.4

STOT NPR ratio 0 0.3

SCR % 3.2 0.9

SCR MPA kgH2SO4/t 98.1 28.7

SCR NAPP kgH2SO4/t 97.3 9.2

% S as SCR % 57.4 44.3

pHOX pH units 2.2 3.0

NAG4.5 kgH2SO4/t 109 13.4

NAG7.0 kgH2SO4/t 118.5 23.9

Fizz value 0-5 0 1

3.3. Waste Rock and Tailings Classification An updated conceptual site model of the environmental geochemistry of the Tom’s Gully site was

produced based on the 2014/2015 and 2017 mineral waste results (GHD 2018). The mineralised geology

at Tom’s Gully includes sulfides that comprise between 10 and 40 per cent of the ore at Tom’s Gully. The

key sulfides are pyrite (FeS2) and arsenopyrite (FeAsS),with minor pyrrhotite (Fe(1-x)S where x = 0 to 0.2),

chalcopyrite (CuFeS2), loellingite (FeAs2), sphalerite (ZnS) and galena (PbS) are also present (Sener 2004).

This indicates that iron, arsenic, zinc and lead, at a minimum, may pose an environmental risk given the

level of historic disturbance on site (GHD 2018).

3.4. PAF/NAF Estimated Volumes All material (waste rock, tailings, ore, and existing stockpiles) onsite is considered to be PAF material.

Using considered assumptions and site geochemical knowledge, it has been calculated that around 95

tonnes of acidity (as CaCO3 equivalents) per year has accumulated within the pit. When converted to

tonnes of H2SO4 per year, this represents around 14 % of the mineral waste mass stored within the SWRD

oxidising each year at the rate reported herein using OxCon testing. Based on this back-calculation from

the pit acidity, total acid loads being generated from the SWRD that require active management are in

the order of 485 tonnes of H2SO4 (GHD 2018). As context, this would take around 20 truck and dog loads

of an 80% calcite blend of 80% purity to treat to a pH of 7.0 for release per year. Whilst these calculations

remain order of magnitude for input into this revised AMD Management Plan, they are provided as

indicative for context to show that the acidity being generated on site from key historic mineral waste

storage structures remains manageable (GHD 2018).


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4. Conceptual Site Model The mineralised geology at Tom’s Gully includes sulfides and has been discussed in terms of waste rock

and tailings chemistry in Section 3.3.

A simplified summary of the updated conceptual site model (CSM) from a background, source, pathway,

receptor model perspective is provided below Appendix N. See Figure 3 for the conceptual site model

schematic.

Table 5-26 in the 2018 Baseline Geochemical Report (EIS Supplement Appendix C) provides a summary of

the 2014/2015 and updated 2017 data that was used to develop the CSM.

4.1. Background Background surface water quality data at sampling upstream locations in Mt Bundey and Coulter Creeks

showed median values for all analytes below SSTVs (median values for PGO data from December 2016 to

March 2018).

The distal hanging wall (DHW) unit, along with the rock and soil samples collected from where the fresh

water dam is to be located to the west of SWRD returned analytical results that showed inert background

or baseline, non-mineralised conditions. This includes circumneutral median NAPP and NAG values, and

no element with a GAI exceeding a value of 3.

Background groundwater values (the Ridge Bore and Bore 11) include circumneutral to slightly alkaline

pH values, fresh, non-saline conditions with no individual element exceeding the ANZECC/ARMCANZ

(2000) stock value (GHD 2018b).

4.2. Source

4.2.1. SWRD

The SWRD contains an estimated 3.27 million m3 of mineral waste material, of which the bulk is

potentially acid forming. Mineralogical analysis shows that the SWRD contains up to 1.4% pyrite, 0.4%

arsenopyrite and up to 8.9% jarosite. Acid base accounting showed that the SWRD had a median NAPP

value of 16.2 kgH2SO4/tonne (13.5 kgH2SO4/tonne when adjusted for jarosite – assuming that all

nonsulfidic sulfur is present in that form). The median NAG value was 8.62 kgH2SO4/t. The sulfide oxidation

rate was shown to be relatively slow, with a median intrinsic acidity generation rate of 0.4

kgH2SO4/tonne/year (values were 0.9, 0.62, 0.4, 0.028 and 0.1 for a median of 0.4) (GHD 2018).

Therefore, whilst the SWRD is generating acidity, it would appear that the annual load is manageable. The

SWRD is also a saline and metalliferous drainage risk. Median historic surface water data from location

SWTG13, being surface water runoff from the SWRD prior to it entering the evaporation ponds showed

water quality consistent with sulfidic waste rock contact. However, median surface water quality collected

at SWTG9, located in a drainage line on the western side of SWRD prior to it entering Mt Bundey Creek


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downstream of SWTG1A was entirely compliant with respective SSTVs for all analytes. This suggests that

rainfall interacting with the rehabilitated western slopes of the SWRD was not entraining environmentally

deleterious elements (GHD 2018).

4.2.2. OWRD

The OWRD contains an estimated 3.97 million m3 of mineral waste material. Mineralogical analysis shows

that the OWRD contains up to 1.7% pyrite and up to 0.6% jarosite. It also contains carbonate as dolomite

(up to 11.1%) and calcite (up to 1.9%) (GHD 2018).

Acid base accounting showed that the OWRD had a median NAPP value of 0.9 kgH2SO4/tonne (0.4

kgH2SO4/tonne when adjusted for jarosite – assuming that all non-sulfidic sulfur is present in that form)

with a median NAG value of 1.1 kgH2SO4/t. The sulfide oxidation rate was shown to be very slow, with an

intrinsic acidity generation rate of <0.1 kgH2SO4/tonne/year. Therefore, whilst the OWRD is generating

acidity, it would appear that the annual load is small and manageable. The OWRD is also a saline and

metalliferous drainage risk (GHD 2018).

4.2.3. TSF1

TSF1 contains an estimated 131,000 m3 of tailings; or approximately 196,500 tonnes. Mineralogical

analysis shows that TSF1 contains up to 6.2% pyrite, up to 5.0% arsenopyrite, up to 0.4% marcasite (a

pyrite allotrope) and up to 5.5% jarosite. It also contains minor gypsum (up to 0.6%).

Acid base accounting showed that TSF1 had a median NAPP value of 155 kgH2SO4/tonne (139

kgH2SO4/tonne when adjusted for jarosite – assuming that all non-sulfidic sulfur is present in that form)

with a median NAG value of 119 kgH2SO4/t. Being finer grained material than that stored in the OWRD

and SWRD, the sulfide oxidation rate is somewhat faster, with an intrinsic oxidising zone, around the top

metre or so given the lack of oxygen replenishment in the lower, anoxic zone in the TSF; which comprises

around two-thirds of the facility. In addition, the upper horizons of the oxide zone would contain relatively

less sulfidic sulfur than the anaerobic zone given historic oxidation, though would contain jarosite as a

result. Therefore, whilst TSF1 is generating acidity, it would appear that the annual load is manageable.

TSF1 is also a saline and metalliferous drainage risk (GHD 2018).

4.2.4. TSF2

TSF2 contains an estimated 90,000 m3 of tailings; or approximately 135,000 tonnes. Mineralogical analysis

shows that TSF2 contains up to 3.9% pyrite, up to 3.9% arsenopyrite, and up to 0.1% jarosite. It also

contains minor gypsum (up to 0.8%) and up to 2.0% dolomite. Acid base accounting showed that TSF2 had

a median NAPP value of 45.4 kgH2SO4/tonne, with a median NAG value of 23.9 kgH2SO4/t. TSF2 is

generating acidity; however, it would appear that the annual load is relatively small and manageable. TSF2

is also a saline and metalliferous drainage risk (GHD 2018).

This risk was further shown for TSF1 and TSF2 via historic surface water quality data that significantly

exceeded the SSTVs for all analysed water quality parameters with the exception of lead (TSF1 and TSF2)

and manganese (TSF1). Median acidity levels of 1,100 and 1,600 mg/L CaCO3 equivalents respectively for

TSF1 and TSF2 would indicate significant sulfide and metal acidity in both structures (GHD 2018).


18

In summary, the key historic structures have a significant acid generation potential based on retained

(acid-forming secondary sulfo-salts) and potential acidity (sulfidic sulfur). However, the relatively slow

oxidation rates across the four key historic mineral waste storages suggest that appropriate management

and mitigation strategies are achievable to lower AMD risk to as low as reasonably practicable GHD 2018).

4.3. Pathway The three key pathways for potential transfer of contaminants from source to receptors are:

a) Sediment

b) Surface water (Old Decant Pond, Evaporation Ponds 1 and 2, Stormwater Pond, Pit Lake and drainage

lines) and

c) Groundwater

See the Updated CSM (pp. 117 – 128) of Appendix C for full details of contaminants in the AMD pathways.

4.3.1. Surface Water

The geochemical data suggests that contaminated runoff from the SWRD in Evaporation Ponds 1 and 2

has historically been discharged (via a discharge licence) into Mt Bundey Creek along the drainage line to

the north of Evaporation Pond 2. Visual evidence of perished discharge piping over the dam wall of

Evaporation Pond 2 would support this theory. Sufficient freeboard should therefore be retained in

Evaporation Ponds 1 and 2 to actively manage the risk of overtopping such that managed and licensed

release only may be undertaken. The flooding risk reported by GHD (2018a) should be considered when

determining appropriate sizing for the OWRD bund and any other structures that require re-sizing. In

particular, modelling by GHD (2018a) indicates that the Oxbow Wetland is expected to be inundated

during the 10-year average recurrence interval (ARI) flood event, with depths during the 100- year ARI

flood event exceeding three metres at some locations, albeit with relatively low velocities.

It is likely that saline and metalliferous acidic through flow and leachate from the OWRD is overtopping

the OWRD bund and discharging into Lake Bazzamundi during periods of extended rainfall and/or wet

season storm events. This is evidenced by acidity, sulfo-salts and environmentally deleterious elements

migrating down the drainage line between the OWRD bund, and into, Lake Bazzamundi as identified by

sediment and surface water data. The OWRD bund will therefore be increased in capacity to allow storage

volume for an appropriately risk-managed recurring storm event, consistent with appropriate water

management guidelines such as:

• Manual for assessing consequence categories and hydraulic performance of structures

(Queensland Department of Environment and Heritage 2016);and/or

• Structures which are dams or levees constructed as part of environmentally relevant activities

(Queensland Department of Environment and Heritage 2017).

Surface water and sediment shed from OWRD that is not overtopping the OWRD bund appears to be

managed reasonably effectively via the existing drainage line past TSF2 and into the Oxbow Wetland,

which is effectively acting as a retention basin. Sediment and surface water geochemical data has tracked


19

the reducing contaminant concentrations down the engineered drainage line until dilution effects within

the Oxbow Wetland further reduce contaminant concentrations (GHD 2018).

Following previously licenced wet season discharge at LDP1 (SWTG12), surface water monitoring (PGO

2018) suggests that the mixing zone within Mt Bundey Creek immediately below LDP1 (at SWTG 12) is all

but complete by the time the water reaches the Arnhem Highway Bridge (SWTG2) – where all key analytes

are below SSTVs except copper (GHD 2018).

Aquatic ecology monitoring (GHD 2018c) indicates that the aquatic fauna take slightly further downstream

to recover, with species impacted beyond SWTG2, though fully recovered to, or better than, background

levels by monitoring point SWTG3 – located around 300m upstream of the confluence with Coulter Creek.

4.3.2. Groundwater

GHD (2018b) reported that contamination, of at least shallow groundwater, has occurred around the

SWRD and OWRD, Evaporation Ponds 1 and 2 and TSF1 and TSF2, based on monitoring exhibiting localised

areas of elevated sulfate or low pH and elevated metals. It is possible that contamination to the west (well

G8 that showed pH, aluminium, cadmium, copper and nickel outside ANZECC/ARMCANZ (2000) stock

trigger values) and north (wells OB10 and 11 that showed EC and sulfate outside ANZECC/ARMCANZ

(2000) stock trigger values) of the SWRD and Evaporation Pond 2 extends through shallow aquifers to Mt

Bundy Creek, approximately 130 to 300 m to the northwest. Based on current relative groundwater

elevations, the hydraulic gradient is inwards to the pit, which is likely to capture groundwater flow from

beneath the various mine–related contamination sources noted above.

4.4. Receptors

4.4.1. Mt Bundey Creek

Once surface water leaves site, it passes via SWTG2 on Mt Bundey Creek at the Arnhem Highway Bridge.

The median water quality data for this sample location indicates that it is entirely compliant with all SSTVs,

except copper. Further downstream of SWTG2 on Mt Bundey Creek is SWTG3. The median water quality

data for this sample location indicates that it is entirely compliant with all SSTVs, with the exception of

zinc (0.0039 mg/L against the SSTV of 0.0031 mg/L) and copper (0.002 as against the SSTV of 0.0018 mg/L).

The furthest surface water sampling location is SWTG16, located some 15 kms downstream in Hardy’s

Lagoon. Interestingly, water quality in Hardy’s Lagoon shows that copper (0.003 mg/L) and EC (42 μS/cm)

exceed SSTVs. This may indicate influences other than Tom’s Gully, given the lack of sulfate in the water

(1 mg/L), a value that is below the upstream baseline of 2 mg/L.

4.4.2. Lake Bazzamundi

The five sediment samples collected within Lake Bazzamundi (TGSED18 to 22 inclusive) returned pH1:5

values that ranged between 3.7 and 5.1, EC values between 142 and 2,130 μS/cm, and STOT NAPP values

between 1.5 and 14.1 kgH2SO4/t. These data would infer overtopping of the bund during wet season with

AMD water containing sulfo-salts finding its way down the drainage path into Lake Bazzamundi. Water

quality in Lake Bazzamundi as per median historic data at SWTG5 shows a median pH value of 5.7, with

aluminium, cadmium, cobalt, copper, nickel, uranium and zinc exceeding their respective SSTVs. Median


20

sulfate (64 mg/L, acidity (7 mg/L CaCO3 equivalents) and EC (170 μS/cm) are approaching background

concentrations, likely as a function of dilution in the Lake Bazzamundi wetland.


21

Figure 3: Conceptual Site Model at Toms Gully (GHD 2019)


22

5. Acid Mine Drainage Model and Balance Based on the TGU Projects conceptual site model developed by GHD (2018), a mass balance of acid,

expressed in mass of CaCO3 that may be used to treat the water to neutralise the acidity was developed.

GHD (2018) identified the significant sources of potential acid generation as the sulfide waste rock dump,

oxide waste rock dump, TSF1 and TSF2 and estimated net acid generation. The ROM stockpile,

metallurgical tailings and waste rock were also identified as an acid and metalliferous drainage risk, but

as the risk was deemed lower it was not quantified in terms of net acid generation. The undisturbed

catchments and in situ rock were deemed as a neutral and not a significant source of acid generation. For

the purpose of the acid mine drainage model, the neutralising potential of sediment in surface water

storage was ignored.

The net acid generation of the different site land uses are summarised in Table 3. A nominal net acid

generation rate of 0.1 kg CaCO3/ tonne/year was adopted for the disturbed hardstand (predominately

the ROM stockpile) and pit areas. It was assumed that any new tailings produced would be lower risk than

the existing tailings.

The acid mine drainage model does not consider the any potential of seepage of water from WRDs and

surface water storages that may report offsite. The actual flow rates of seepage are difficult to quantify

at this stage, but are considered minor and of lower risk than any potential discharge of water directly

from surface water storages

Table 3: Net Acid Generation (GHD 2018d)

Land Use Net acid generation (kg CaCO3/tonne/year)

TSF1 34

TSF2 3.0

Sulfide WRD 0.4

Oxide WRD 0.1

Hardstand 0.1

Pit 0.1

Undisturbed 0.0

The forecast average annual acid balance for Toms Gully mine is summarised in Table 4 The potential acid

mine drainage is expressed in terms of mass of CaCO3 that may be used to treat the water to neutralise

the acidity.


23

Table 4: Average Annual Acid Mine Drainage Balance (GHD 2018d)

Acid flux Year ending June 2019

(tonne CaCO3)

Year ending June 2020

(tonne CaCO3)


(tonne CaCO3)


(tonne CaCO3)


(tonne CaCO3)

Inputs

Direct rainfall onto storages

0 0 0 0 0

Catchment runoff

419 499 507 512 487

Groundwater inflows

0 0 0 0 0

ROM ore moisture

0 0 0 0 0

Total inputs 419 499 507 512 487

Outputs

Evaporation 0 0 0 0 0

Uncontrolled off site discharge

1 1 1 1 2

Discharge from New WSD (or supply to third party)

5 9 11 12 13

Seepage losses 22 19 20 20 23

Dust suppression losses

0 0 0 0 0

Tailings moisture losses

152 297 291 277 61

Treated by WTP 693 208 200 198 218

Total outputs 873 534 524 509 317

Change in Storage

Surface water storages

-454 -36 -17 3 169

Total change in storage

-454 -36 -17 3 169


24

6. AMD Management PGO recognises that planning for closure is a fundamental component of mine planning (INAP2009, DTIR

2007). Therefore, identifying any PAF material either historical or within the context of future mine plans

and schedules is essential for effective management. Therefore PGO has developed AMD design and

operational controls to minimise the potential risks.

PGOs overall AMD strategy has been aligned to leading standard practice as per the following:

• subaqueous tailings / waste rock deposition – INAP 2009 Section 6.6.7; DITR 2007 Section 7.1.6;

DERM 1995 Section 7; DITR 2006, Appendix A; various MEND reports at: http://mend-

nedem.org/category/prevention-and-control/water-covers/

• Store-release TSF cover design - INAP 2009 Section 6.6.6, DITR 2007 Section 7.1.4, NT EPA 2013

Section 8; DERM 1995 Section 7; DITR 2006, Appendix A; various MEND reports at: http://mend-

nedem.org/category/prevention-and-control/dry-covers/

• Subterranean PAF waste rock storage – DITR 2006, Appendix A

• Drainage controls – DITR 2007 Section 7.1.1, DRET 2008

• Ongoing monitoring – INAP 2009, Chapter 8; DITR 2007, Chapter 8; NT EPA 2013 Section 9; DERM

1995 Section 7.2; various MEND reports at: http://mend-nedem.org/category/monitoring-

category

• Acid Mine Drainage – Environmental Notes on Mining, updated September 2009 (DMP 2009)

AMD Management for the TGU Project can be divided into two key strategies:

1. Managing existing AMD sources (WRDs, TSF, RoM Pad and Stockpiles, Evaporation Ponds and Pit)

2. Removal of existing AMD sources (TSF1 and 2)

3. Managing Proposed AMD sources (waste from underground mining, tailings and ore stockpiles)

6 . 1 . AMD Risk Assessment As part of the TGU Project’s NT EPA EIS Supplement, a risk assessment framework was developed for the

entire project. This included undertaking a risk matrix for potential AMD risks on site. See the EIS

Supplement for the comprehensive risk framework document and assessment. Risks associated with AMD

were taken from the Project Risk Framework and are summarized and addressed in Table 5

http://mend-nedem.org/category/prevention-and-control/water-covers/

http://mend-nedem.org/category/prevention-and-control/water-covers/

http://mend-nedem.org/category/prevention-and-control/dry-covers/

http://mend-nedem.org/category/prevention-and-control/dry-covers/

http://mend-nedem.org/category/monitoring-category

http://mend-nedem.org/category/monitoring-category


25

Table 5: AMD Risk, potential impact and management / mitigation control


26

Risk Potential Impact Mitigation / Management Control Timing Performance Indicator

Failure / overtopping

of TSF1 and/or TSF2

leading to

uncontrolled release

of tailings material

(Prior to tailings

removal)

Contamination of Mt

Bundey Creek and

downstream ecosystems

• Ensure appropriate freeboard in TSFs

at all times (weekly inspections during

operations)

• Tailings placement in the pit under a

water layer.

• Groundwater monitoring (to monitor

seepage)

• Implementation of AMDMP

• Implementation of Water

Management Plan (WMP)

• If re-processing of tailings material

occurs this will reduce the acid

producing profile of the tailings and

volumes of material

• Water treatment plant on site to treat

water for AMD

Design

Operations

Closure

• Removal of tailings

• Surface water

monitoring data

within Site Specific

Trigger Values (SSTVs)

• No incidents of

overtopping

Seepage from TSF1

or TSF2

Contamination of

surface waters and

groundwater quality and

downstream ecosystems

• Repurposing TSF1 (sediment basin)

and TSF2 (water dam) will address

existing seepage

• Engineered design

• Weekly Inspections

• Tailings removal from TSFs

• Treatment of water for AMD

• Groundwater monitoring bores

Design

Operations

Closure

Groundwater monitoring

data within SSTVs or

water quality

requirements

Runoff or seepage

from existing WRDs

Contamination of

surface water and

• Geotechnical inspection

• Continued use and management of

evaporation ponds

Design

Operations

Closure

Water monitoring data

within SSTVs


27

groundwater quality and

ecosystems

• Improvement of site drainage

(increase capacity of bunds and

ensure integrity of all drains leading to

evaporation ponds from WRDs)

• Use of suitable boxcut waste rock for

capping.

• Investigation and consideration of

long term treatment and associated

closure options for WRDs (e.g. capping

of WRDs)

• Water treatment plant onsite

• Groundwater monitoring bores

WRD investigation

program

Seepage from

evaporation ponds

Contamination of

groundwater

• Treat water ex-pit and use ponds for

short term storage

• Manage water inventory (water

balance across site)

• Containment and capture of

contaminated water and treatment

(via proposed water treatment plant)

• Ongoing identification of all sources of

contaminated water

• Surface and groundwater monitoring

with associated bores

Design

Operations

Closure


within SSTVs

Inappropriate

storage and disposal

of proposed waste

rock

• Contamination of

surface water and

groundwater

systems

• Baseline waste characterisation work

completed including the Boxcut

material.

Design

Operations

Monitoring of waste rock

movement and

positioning in pit.


28

• Storage outside of

footprint

• Waste rock deposited in base of pit

under a water covered during

operation and at closure

• Boxcut material suitable for capping

waste rock dumps


• Implementation of WMP

• On-going and weekly inspections of

project areas

• Assume all underground waste rock is

PAF

Geochemical sampling of

waste rock to identify

changes in chemistry.

Waste rock positioned

below water level.


within SSTVs

Indiscriminate use of

existing waste rock

for construction

• AMD leading to

contamination of

surface water and

groundwater

systems

• Storage outside of

footprint or

structure failure

• No disturbance to WRDs


• Implementation of WMP

• On-going and regular inspections of

project areas

• Assume all waste rock is PAF

Design

Operations

Monitoring of waste rock

movement and

positioning


within SSTVs

No disturbance to WRDs

Inappropriate

storage of ore on

ROM Pad or

elsewhere

AMD leading to

contamination of

surface water and

groundwater systems

• Implementation of AMD Management

Plan

• Maintenance and upgrade (if

necessary) of drainage controls and

surface drainage contours

• Operating procedures and mine

schedules

• Reprocessing of existing ore stockpiles

Operations Water monitoring data

within SSTVs

All ore stockpiles re-

processed

Monitoring of surface

water flow during rain

events.

Monitoring of ore pile

positioning on RoM pad


29

Delay in attaining full

submergence of PAF

waste rock in pit

AMD leading to

contamination of

groundwater systems

• Where practical waste rock positioned

in the lowest parts of the pit with the

stacker.

Closure Submergence of PAF

underground waste

material within 48 hours

Overtopping of pit

containing

submerged PAF

material and water

level fluctuations

Mixing of pit water with

surface water leading to

potential AMD products

released

• Investigate the establishment of an

insitu sulfate reducing bacteria system

thus reducing potential AMD

formation.

• Complete modelling to understand

the final water level height of the pit.

Closure Ensure 10 m of permanent

water cover


30

6.2. AMD Management Strategy As noted in the sections above, all waste rock and tailings generated on site will be treated as PAF and the

management is twofold; managing the existing AMD sources and minimizing the risk of proposed AMD

sources (waste rock, tailings and ore).

6.3. Controls As part of the AMD management strategy and minimizing the risk and volumes of AMD on site a number

of operation controls have been designed.

6.3.1. Ore and Waste Rock

• All waste rock is to be disposed in the pit.

• No future waste rock is to be deposited beyond the pit perimeter (Figure 4).

• No future waste rock is to be used for construction purposes, other than the DHW unit.

• No existing waste rock within the WRD will be used for construction purposes.

• Boxcut material will be visually monitored during excavation for the presence of sulfides. If

sulfides are identified material to be placed in the pit (under a water cover) to prevent acid

forming material being positioned on the waste rock dumps.

• The existing sulfide and oxide waste rock dumps are to be maintained to ensure their integrity.

• Waste rock will only be stored underground and/or on the ROM Pad prior to placement in the pit.

• Ore will only be stored underground and/or on the ROM Pad.

• During operations, waste rock will be placed flooded in the pit to minimize oxygen availability to

PAF waste rock. The water will work as an oxygen barrier. The dissolved oxygen concentration in

water is 8.6mg/L at 25ºC, which is approximately 25000 times lower than in the air. Organic

matter and other reduced compounds can rapidly consume the dissolved oxygen in the water,

which is then not available for sulfide oxidation (DMP 2009).

A design review is to be completed with each annual Mine Management Plan (MMP) that verifies the

AMD standards are being implemented for the MMP term.

From the baseline geochemical work it is estimated that the lag time for the breakdown of sulfides to

generate acid mine drainage is between 6 to 12 months (GHD 2018). Taking into consideration the lag

time of the PAF materials and to limit the egress of oxygen and water the following measures have been

adopted during operations:

• Placement of waste rock in the pit within 48 hours of excavating the material.

• Prior to pit emplacement position all waste rock on only the RoM or process area.

• Monitoring the pit water and if required adjust water quality by the use of lime, caustic or virtual

curtain.


31

Figure 4: Location of waste material to be placed in the flooded pit

At an operation level suitable locations for the underground placement of waste rock are to be

determined by the Mining Engineer in consultation with the Mine Manager. On a daily basis the

positioning of waste rock in the pit will be communicated to the stacker operator for implementation.

During operations surveying and sampling will occur to assess the depth of the material in the pit and acid

producing potential of the material this information will be used to inform the closure strategy for the pit.

6.3.2. Tailings

Design and operational controls for the tailings include:

• It is proposed tailings from TSF1 and TSF2 will be placed in the flooded pit (whether reprocessed or

not) within 18 months from the commencement of the Boxcut.

• All future tails will be placed in the already flooded pit.

• During operations, tailings will be placed in the flooded pit to minimize oxygen availability to PAF tails.

The water will work as an oxygen barrier. The dissolved oxygen concentration in water is 8.6mg/L at

25ºC, which is approximately 25000 times lower than in the air. Organic matter and other reduced

compounds can rapidly consume the dissolved oxygen in the water, which is then not available for

sulfide oxidation (DMP 2009).

• Deposition of tailings in the pit using a floating head to discharge tails 10m below the water surface.


32

• Tailings systematically discharged across the pit to create an even tails surface beneath the water

Figure 5.

• Tailings to be deposited with a minimum water cover of 25m.

• All metallurgical tailings to be treated as PAF and placed in the pit.

• Implementation of this AMD Management Plan and AMD site sampling procedure (refer to Appendix

S).

• Implementation of the TGU Water Management Plan that includes the AMD water monitoring

analytes.

• Implementation of an operating manual / procedure for tailings deposition.

• Maintaining a minimum freeboard across the pit via the treating of water displaced out of the pit.

• Weekly inspections of pit deposition during operations for freeboard.

• Surveying of tailings in the pit to assess tailings levels and evenness of distribution.

Figure 5: Systematic Floating Head Traverses to Deposit Tails.

6.4. Site Drainage and Controls As the OWRD and SWRD can generate contaminated water from runoff and seepage, therefore,

appropriately designed and operated water management structures are required. As outlined in the TGU

Water Management Plan, the design and operational level controls in place to manage water on site are

as follows:


33

• A series of run-on diversion bunds and run-off drains manage water on site to minimise the risk of

clean water being cross-contaminated by contaminated water (Figure 6). In addition bunds including

the OWRD will be upgraded to prevent overtopping.

• Mine affected water is diverted to stormwater ponds and non-mine affected water is transferred

through the site via the bunds and run off drains.

• Water captured within stormwater ponds is assessed through the established network of surface

water monitoring sites at TGU that would remain under the TGU Water Management Plan (refer to

Figures 7 - 8) and treated for discharge from site at locations approved under the required legislation.

The following operational controls will apply:

• The TSF water circuit is designed to be closed during operations

• Drainage from the ROM pad shall be directed to the Stormwater Sump which shall be managed to

prevent overflow

• Water from the Stormwater Sump shall not be released directly to the environment. It shall be utilised

in the process as first priority or treated.

• Details of routine operational and emergency water transfers are shown in the TGU Water

Management Plan.

• Monitor rainfall conditions and water levels across the site with the water treatment plant operated

to manage peak water periods to maintain constant water volumes across site

• Mine affected water will be treated using the Bioaqua Process (or contingency option) in the water

treatment plant and placed in the proposed water storage dam before being discharged or transferred

to a third party for agricultural use.

Controls requiring immediate action:

• The OWRD bund should therefore be increased in capacity to allow storage volume for an

appropriately risk-managed recurring storm event, consistent with appropriate water management

guidelines (GHD 2018)

• Runoff from the SWRD in Evaporation Ponds 1 and 2 has been discharged as per the discharge licence

into Mt Bundey Creek along the drainage line to the north of Evaporation Pond 2. Visual evidence of

perished discharge piping over the dam wall of Evaporation Pond 2 would support this theory.

Sufficient freeboard should therefore be retained in Evaporation Ponds 1 and 2 to actively manage

the risk of overtopping such that managed and licensed seasonal release only may be undertaken

(GHD 2018).

The catchment areas of each water management feature were delineated based on topographic

information. The land use of site, for the purpose of the site water balance, was delineated based on aerial

imagery and site observations. The catchment area and land use distributions for each water management

feature are summarised in Table 6.


34

Table 6: Water management infrastructure and their catchment areas (GHD 2019)

Catchment Hardstand (ha)

Oxide WRD (ha)

Pit (ha)

Sulphide WRD (ha)

TSF (ha)

Undisturbed (ha)

EP1 - - - 15.0 - -

EP2 - - - 10.6 - -

New WSD - - - - 10.5 -

Drainage Bund - - - 2.4 - 15.7

PWP - 22.5 - - - -

Stormwater Pond 0.5 - - - - -

Toms Gully Open Pit

10.4 - - - - 6.7

TSF1 - - 32.3 1.6 - -

TSF1 decant pond - - - 1.1 6.7 1.4

TSF2 - - - 1.1 2.1 0.1

New TSF - - - - 8.7 -

The capacity of surface water storages and the maximum surface areas were provided by Primary Gold.

Compared to the previous water balance report (Coffey 2015), the design of the New WSD has been

reduced to suit the revised water management system and its offline configuration. The geometric

properties of the surface water storages are summarised in Table 7.

Table 7: Water Storages at TGU Project (2019)

Water management feature

Capacity (ML) Spill level (m RL)

Maximum inundated area (Ha)

Shape factor

EP1 346 1029.35 4.67 5

EP2 354 1025.76 4.79 5

New WSD 1000 (once constructed)

Unknown 16.0 2

Drainage bund 5.0 Unknown 7.4 2

PWP 1.4 Unknown 0.03 3

Stormwater pond 12.5 Unknown 0.6 2

Toms Gully Open Pit 4660 1019.0 9.0 3

TSF1 (including decant pond)

135.2 Unknown 7.4 5

TSF2 408.9 (once upgraded)

1026.5 8.7 (once upgraded) 5

New TSF Unknown Unknown 9.0 5


35

Figure 6: Site Drainage


36

Figure 7: Surface Water Monitoring Locations


37

Figure 8: Surface Water Monitoring Locations Continued


38

6.4.1. Maintenance of existing structures

PGO recognises that the integrity of the existing structures on site needs to be maintained throughout

operations and into closure to minimise the risk of AMD. The following maintenance and monitoring is

proposed:

• Weekly visual inspections of drainage and sediment control

• Sufficient freeboard should be retained in Evaporation Ponds 1 and 2 to actively manage the risk of

overtopping such that managed and licensed seasonal

• Sufficient freeboard should be retained for TSF 1 and 2 to actively manage the risk of overtopping

while tailings are in place and if TSF 1 and 2 are repurposed.

• The OWRD bund should be increased in capacity to allow storage volume for an appropriately risk-

managed recurring storm event

• Ameliorate any eroded areas identified during visual inspection on run on and run off bunds, TSFs,

evaporation ponds and the sulfide and oxide WRDs

• Clean drainage lines surrounding key site infrastructure of observable secondary salts at the end of

dry season to minimise the risk of a contaminated ‘first flush’ event. Secondary salts would be treated

as PAF for management purposes and placed into the pit.

• Identify any areas of water ingress on the oxide and sulfide WRDs and ameliorate as necessary

• Monitor surface and groundwater as described in Section 6 and as per the TGU Water Management

Plan.

7. AMD Monitoring AMD monitoring provides feedback to confirm that the design and operational controls are effective for

their stated aim. In that regard, the following will be monitored:

• Tailings and waste rock (including ore) to validate the existing geochemical classifications and to

provide an historic inventory for site archives and legacy management

• Sources and use of construction materials

• Water (surface water and groundwater) (Figure 7 and 8)

PGO will utilise an adaptive management approach to meet the SSTVs developed for the site (refer to the

TGU Water Management Plan). The concept of adaptive management is a structured, iterative approach

to decision making with the ability to gradually reduce uncertainty over time through monitoring and

adapting to environmental, economic and social changes. In circumstances where potential impacts

cannot be entirely avoided, the adaptive management approach allows for an evaluation of the preferred

mitigation controls which can then be progressively improved and refined. This approach is particularly

relevant to the longer term management strategies for AMD sources such as the WRDs.

The process of adaptive management is shown in Figure 9. Specifically, the AMD monitoring program

would aim to:


39

• detect environmental change and, specifically, identify those changes resulting from the Project

• determine actual versus predicted change

• contribute to the assessment of the effectiveness of environmental management procedures

• provide data for the assessment of adherence to the environmental management plan, approval and

licence conditions

The AMD monitoring program would be reviewed annually and modified to assure continued

appropriateness. Reviews would consider the frequency and duration of monitoring and evaluate the

ongoing need for individual programs. Records of all monitoring activities would be retained to facilitate

auditing.

The following section provides an overview of the strategy and rationale, with Appendix A (providing the

detail for the in-situ material validation sampling and analysis. Note that the surface and groundwater

monitoring is wholly captured by the TGU Water Management Plan; therefore, it has not been reproduced

here.

Geochemical sampling and analysis on waste rock and tailings materials are to be undertaken using visual

and analytical methods. Each method is explained below.


40

Figure 9: The process of adaptive management

7.1. Geochemical Monitoring As the strategy adopted by PGO assumes that all ore and waste rock is PAF, the geochemical monitoring

program is for the purpose of maintaining an inventory of waste types to allow records to be maintained

for development of the TGU components of the site. The data will help facilitate closure and legacy

management strategies, plans and monitoring.

7.1.1. Visual Methods

The Site Manager or delegate will undertake weekly inspections of PAF management, PAF deposition and

water management structures to ensure their integrity. The Site Manager or delegate will also inspect to

ensure that no PAF material has been won from the engineered landforms for use in construction.

Records will be kept and photographic evidence of any management inconsistencies and structural

integrity failing captured, with the Mine Manager notified for action. Examples include evidence of

erosion and sediment transport downslope after a storm event, poorly maintained sediment traps, or a

ruptured run on bund. This is applicable to historic and future mineral waste and water management

structures.


41

On-site visual monitoring will be undertaken down-catchment of the toe of the WRDs, TSFs and ROM Pad

/ Process area, in addition to downstream of the WRDs, ROM Pad and TSF runoff dams. PGO Staff will be

alert to the formation of secondary salts on waste rock surfaces and in drainage lines, particularly in dry

season. The sources of the salts will be investigated to assist in determining future amelioration strategies

for the WRDs and site remediation for the ROM pad and TSFs.

Such salts are readily dissolved into solution during ‘first flush’ rain events and will compromise water

quality. In wet season, staff should note any discolouration of drainage water, particularly red-brown

(sometimes Fe, sometimes tannins), clear (often dissolved Al due to acidic pH conditions), or blue-green

(often dissolved Cu) rich discharge. The results of the visual inspections will be documented and

communicated to the Site Manager.

Excess sedimentation in flow channels or sumps may also be indicative of active erosion and material

transport and implies a lack of integrity of up-slope engineered management structures – which would be

investigated with remedial actions undertaken as required for stabilisation.

Material removed from drainage lines or sumps that are cleaned at the end of dry season will be managed

as PAF material and placed in the pit.

The supervising geologist will undertake visual inspection of development material (ore/waste) and

construction material collected for geochemical sampling according to Appendix A and note the presence

of any visual sulfidic material (particularly pyrite and/or arsenopyrite). This inspection will be documented

with the laboratory results cross referenced to the visual sample for data quality control and inventory,

and appropriate emplacement of the mineral waste material.

7.1.2. Laboratory Analysis

Following visual inspection, a subset of the development waste and tailings samples will be forwarded to

a NATA accredited laboratory to be analysed for:

• acid base accounting; and

• metals.

Details of this laboratory based analytical program are provided in Appendix A. The data will have two

purposes:

• supplementing the visual data generated; and

• establishing quantitative data for inventory and legacy management purposes.

The sampling and analysis frequency is provided in Table 8. The sample numbers in Table 8 are based on

an industry accepted formula provided in Equation 1 below (Price, 1997).

Equation 1: n = 25 * √x.

(Where x = Million tonnes (MT) of material per major lithological unit).


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Therefore, based on Equation 1, a rule of thumb for representative sampling is around 25 samples per

million tonnes of material - per major lithology. As all waste rock will be stored together underground or

in pit and each individual waste rock unit has been geochemically assessed, the ‘lithological’ units for

waste rock and tailings management purposes become simply ‘ waste rock’ and ‘tailings’.

Waste rock and tailings will be sampled and analysed approximately every month. The sampling frequency

will be reviewed upon the mine schedule and amended as required.

Table 8: Sampling Frequency

Mineral waste unit

Approx. life of mine tonnes

Approx. sample number required

Assumed mine life

Approximate sampling frequency

Waste Rock 1,509,194 48 48 ~1 per month

Tailings 884,103 48 48 ~1 every 40 days

7.2. Surface Water Monitoring The locations, sampling procedures, schedule and analytes for AMD surface water monitoring are entirely

consistent with the TGU Water Management Plan and are therefore not reproduced here. Analytes with

specific reference to AMD monitoring include pH, EC, acidity and alkalinity, sulfate and metals. Baseline

water quality data is also included in the TGU Water Management Plan.

Decreasing alkalinity is generally a good early indicator of deteriorating conditions in leachate from a WRD

containing PAF material, and can therefore be tracked as an ‘early warning’ mechanism.

Metals concentrations and declining pH values generally lag behind declining alkalinity; therefore,

corrective actions can be implemented early should alkalinity decline in the pit.

Other trends that highlight the onset of AMD include increasing sulfate, increasing sulfate / alkalinity ratio,

decreasing pH values and an increase in soluble metals as a result. Given that Toms Gully is a brownfields

site with known on site AMD water chemistry, the focus will be on improving trends as best as is

practicable through operations and ensuring offsite water chemistry compliance is maintained.

Given the development strategy adopted by PGO, the risks of the TGU Project creating new AMD impacts

on surface water quality are greatly reduced. Any deterioration in surface water quality will be

investigated to determine the source. Adherence to SSTVs and appropriate actions will be undertaken.

7.3. Groundwater Monitoring The locations, sampling procedures and schedule and analytes for groundwater monitoring are entirely

consistent with the TGU Water Management Plan. Analytes with specific reference to AMD monitoring

include pH, EC, acidity and alkalinity, sulfate and metals. Baseline groundwater quality data is also

included in the TGU Water Management Plan


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GHD (2018) concluded that contamination, of at least shallow groundwater, has occurred around the

SWRD and OWRD, Evaporation Ponds 1 and 2 and TSF1 and TSF2, based on monitoring exhibiting localised

areas of elevated sulfate or low pH and elevated metals. It is possible that contamination to the west and

north of the SWRD and Evaporation Pond 2 extends through shallow aquifers to Mt Bundy Creek,

approximately 130 to 300 m to the northwest. Based on current relative groundwater elevations, the

hydraulic gradient is inwards to the pit, which is likely to capture groundwater flow from beneath the

various mine–related contamination sources noted above. Therefore, the bulk of groundwater

contaminants leaching from point sources on site is reporting to the pit lake. The existing groundwater

monitoring network will be maintained with additional bores added as per recommendations from the

groundwater study (GHD 2018b).

8. Contingency Planning

8.1. Overview PGO will develop contingency plans for those failure modes where residual risk remains after the

application of AMD prevention and control approaches. A contingency plan will include targeted

monitoring, trigger levels for actions, and specific responses in case a certain event occurs. For example,

if a failure mode is the potential for increased AMD seepage from the pit, then monitoring can be

established for changes in seepage sulfate concentrations and/or acidity as an early indicator of potential

ARD formation. If significant increases in sulfate concentrations are measured, then contingency

measures such as additional drainage collection will be implemented.

PGO will develop contingency plans specific to AMD management at TGU which will include an

exceedance in the surface water monitoring results against site-specific trigger values at DP1 and DP2,

and at SWTG2. The approach will be to undertake a ‘root cause’ analysis whereby the causal link for the

water chemistry exceedance would be determined. Adaptive management would then seek to implement

an appropriate alternate management strategy to eliminate any future risk of a repeat, given the nature

of the incident.

Future revisions of this document will inform forward AMD risk management by providing feedback based

on additional water chemistry and geochemical monitoring to inform AMD risk, and therefore, any

adjusted management strategy.

8.2. Specific Measures

8.2.1. Tailings Management

Contingency measures for tailings management include the use of flocculants to enhance settlement.

8.2.2. Waste Rock

Controls for waste rock management include waste rock to be preferentially stored underground and

disposed in the pit. If parts of the Boxcut waste rock are unsuitable both geochemically or geotechnically

for waste rock dump capping this material will be placed within the pit beneath the water blanket.


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8.2.3. ROM Pad / Ore

Contingency measures for managing / stockpiling ore is for it to be stored temporarily underground for

AMD control if there is insufficient capacity on the ROM pad. This scenario is not anticipated.

8.2.4. Water Management

In the event of an emergency situation where freeboard is not maintained, water would be pumped

between facilities taking into consideration water chemistry. At the same time water treatment would be

ramped up to treat the excess water.

Should there be an emergency situation with the ROM pad stormwater sump with all contained water not

being able to be reused in the process; the water would be pumped to the evaporation ponds.

In the event of an emergency situation where the evaporation ponds exceeding their design capacity,

water would be treated and pumped to the Water Supply Dam.


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9. Roles, Responsibilities and Training The roles and responsibilities for the implementation of the AMD Management Plan are outlined in Table

9 below. This will be communicated to all relevant personnel during operations.

Table 9: Roles and Responsibilities

Task Responsible Accountable Consulted Informed

Implementation of, and compliance to AMDMP

General Manager Operations

Mine Manager Environment Manager

Geology Manager Managing Director Superintendent Mine Geology Senior Mine Geologists Environment Personnel Senior Mining Engineers Mining Contractors

Ongoing mineral waste characterisation and management

Mine Manager Geology Manager Environment Manager

Senior Mine Geologists


Ongoing water monitoring

Environment Manager

Environment Personnel

Mine Manager


Review and refine Rehabilitation Completion Criteria

Environment Manager

Environment Personnel

Geology Manager Mine Manager

General Manager Operations Environment Personnel Senior Mining Engineers Mining Contractors

9.1. Awareness, Training and Competence All senior geology, mining, processing and environmental personnel will have an understanding of AMD

through a site induction. All operational staff entering site, including contractors, are to be made aware

of the AMD Management Plan and the Water Monitoring Plan through a site induction, which would

include PAF material management.


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9.2. Records, Reporting and Document Control Records that are required to be held for this AMD Management Plan include:

• laboratory geochemical analytical results and geochemical monitoring reports

• tailings operating manual

• research and development reports (e.g. for long term management of WRDs)

• An inventory of all mineral waste placement which includes the following:

o quantities and nature of mineral waste located in specific areas within the pit and/or

underground

o the nature of emplacement

o quality control data as applicable

o materials that may be re-used at a later date, such as topsoil, NAF, AC, etc.

Records shall be maintained in accordance with PGO corporate policies and procedures, with all records

maintained into perpetuity to inform future site risk management.

Reporting would be undertaken consistent with approval requirements, and would include:

• geochemical data on mineral waste: to be included in annual updates of the AMD Management Plan

as an appendix to the Mining Management Plan under the NT Mining Management Act

• water monitoring data: to be included in a specific annual water report as required under the NT

Mining Management Act.


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10. References Australian National Committee on Large Dams Incorporated (ANCOLD) 2012. Guidelines on Tailings Dam:

Planning, Design, Construction, Operation and Closure

AMIRA (2002). ARD Test Handbook. Project P387A Prediction and kinetic control of acid mine drainage.

ANZECC/ARMCANZ (2000). Australian and New Zealand Guidelines for Fresh and Marine Water Quality.

Australia and New Zealand Environment and Conservation Council and Agriculture and Resource

Management Council of Australia and New Zealand, Canberra.

ANZECC/ARMCANZ (2000a). Australian Guidelines for Water Quality Monitoring and Reporting. Australia

and New Zealand Environment and Conservation Council and Agriculture and Resource Management

Council of Australia and New Zealand, Canberra.

Bowen H. J. M. (1979). Environmental Chemistry of the Elements. Academic Press, New York.

Commonwealth Government - Department of Industry, Tourism and Resources (2006). Leading Practice

Sustainable Development Program for the Mining Industry: Mine Closure and Completion. Canberra

Commonwealth Government - Department of Industry, Tourism and Resources (2007). Leading Practice

Sustainable Development Program for the Mining Industry: Managing Acid and Metalliferous Drainage.

Canberra

Commonwealth Government - Department of Resources, Energy and Tourism (2008). Leading Practice

Sustainable Development Program for the Mining Industry: Water Management. Canberra

Department of Mines and Petroleum (2009). Acid Mine Drainage – Environmental Notes on Mining,

updated September 2009. Government of Western Australia Available from:

http://www.dmp.wa.gov.au/Documents/Environment/ENV-MEB-220.pdf

GHD (2015). Toms Gully Mine Preliminary AMD Assessment and Waste Rock Classification. May 2015.

GHD (2015a). Toms Gully Project: Site Specific Trigger Values. April 2015.

GHD (2015b). TGU Underground Water Management Plan.

GHD (2018). Primary Gold Tom’s Gully Gold Project: Geochemical baseline and conceptual site model.

Report for Primary Gold Ltd.

GHD (2018a). Flooding Memorandum. Prepared for Primary Gold. Report for Primary Gold Ltd.

GHD (2018b). Tom’s Gully EIS – Baseline Studies. Groundwater Assessment and Modelling. Report for

Primary Gold Ltd.

http://www.dmp.wa.gov.au/Documents/Environment/ENV-MEB-220.pdf


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GHD (2018c). Tom’s Gully EIS – Baseline Studies. Aquatic Ecology Monitoring 2017. Report for Primary

Gold Ltd.

GHD (2018d). Tom’s Gully Mine Site Water Balance. Report for Primary Gold Ltd.

INAP (The International Network for Acid Prevention) (2009). Global Acid Rock Drainage Guide. Available

at www.gardguide.com.

Minesite Environmental Neutral Drainage (MEND). Various reports available at: http://mendnedem.

org/default/

Miller S. D. (1996). Advances in acid drainage: Prediction and implications for risk management. In:

Proceedings of the Third International and the 21st annual Minerals Council of Australia Environmental

Workshop, Newcastle, NSW. pp. 149-157.

NT EPA (2014). Draft Toms Gully Underground EIS Terms of Reference.

Price, W.A., (1997). Draft Guidelines and Recommended Methods for the Prediction of Metal Leaching and

Acid Rock Drainage at Mine sites in British Columbia. BC Ministry of Employment and Investment.

Primary Gold (PGOO) (2013). Primary Gold increases Tom’s Gully resource by 96% to 275,000 oz. ASX

release dated 23 April 2013.

Primary Gold (2015). Draft Toms Gully Underground Project Description.

Primary Gold (2015a). Unpublished water quality data.

PGO (2018). Routine surface water quality monitoring data December 2016 to March 2018. Provided by

PGO in March 2018.

Sener A.K. (2004). Characteristics, distribution and timing of gold mineralisation in the Pine Creek Orogen, Northern Territory, Australia. Unpublished PhD thesis, UWA.

Stewart W.A., Miller S.D. and Smart R. (2006). Advances in acid rock drainage (ARD) characterisation of

mine wastes. Proceedings of the 7th International Conference on Acid Rock Drainage (ICARD). Barnhisel

R.I (ed.), St Louis, America.

Queensland Department of Environment and Heritage (2016). Manual for assessing consequence

categories and hydraulic performance of structures.

Queensland Department of Environment and Heritage (2017). Structures which are dams or levees

constructed as part of environmentally relevant activities.

http://mendnedem/


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Appendix A: Site Geochemical Sampling Procedure

This has been placed in Appendix S of this EIS Addendum to the Supplement