Report on Geotechnical Assessment & Proposed New Storage ...

Report on Geotechnical Assessment

Tailings Structures, Waste and Oxide Dumps & Proposed New Storage Dam

Toms Gully Mine Site Arnhem Highway, Northern Territory

Prepared for Primary Gold Limited

Project 78205.00 August 2015

Document History

Document details

Project No. 78205.00 Document No. 1

Document title Report on Geotechnical Assessment

Tailings Structures, Waste and Oxide Dumps & Proposed New Storage Dam

Site address Toms Gully Mine Site, Arnhem Highway, Northern Territory

Report prepared for Primary Gold Limited

File name 78205.00 Report_Rev0

Document status and review

Revision Prepared by Reviewed by Date issued

0 A Gane Michael J Thom 30 June 2015

1 A Gane Michael J Thom 28 August 2015

Distribution of copies

Revision Electronic Paper Issued to

0 1 0 Primary Gold Limited (Mr Clay Gordon)

0 1 0 Primary Gold Limited (Mr Clay Gordon)

The undersigned, on behalf of Douglas Partners Pty Ltd, confirm that this document and all attached

drawings, logs and test results have been checked and reviewed for errors, omissions and

inaccuracies.

Signature Date

Author A Gane 28 August 2015

Reviewer M J Thom 28 August 2015

Douglas Partners Pty Ltd

ABN 75 053 980 117

www.douglaspartners.com.au

Unit 2, 14 Caryota Court

Coconut Grove NT 0810

PO Box 36858

Winnellie NT 0821

Phone (08) 8948 6800

Fax (08) 8948 6899

Report on Geotechnical Assessment Project 78205.00 Tailings Structures, Waste & Oxide Dumps, & New Dam June 2015

Table of Contents

Page

1. Introduction ................................................................................................................... 1

2. Background Information ................................................................................................ 1

3. Site Description ............................................................................................................ 2

3.1 Tailings Dam No. 2 .............................................................................................. 2

3.2 Oxide and Sulphide Waste Dumps ...................................................................... 3

3.3 New Dam Site ...................................................................................................... 3

3.4 Tailings Dam No. 1 and Decant Dam ................................................................... 4

4. Topography and Geology ............................................................................................. 4

5. Site Observations ......................................................................................................... 5

6. Proposed Development ................................................................................................ 8

7. Comments .................................................................................................................... 8

7.1 Dam No. 2 ........................................................................................................... 8

7.2 Oxide and Sulphide Waste Dumps ...................................................................... 9

7.3 New Dam Site .................................................................................................... 10

8. Limitations .................................................................................................................. 11

Appendix A: Notes About this Report

Drawing A1 – Site Location Plan

Drawing A2 – Site Layout and Photograph Locations

Site Photographs

Appendix B: Client Supplied Drawing

Geological Sketch form Previous DP Report

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Report on Geotechnical Assessment

Tailings Structures, Waste and Oxide Dumps & Proposed New Dam

Toms Gully Mine Site, Arnhem Highway, NT

1. Introduction

This report by Douglas Partners Pty Ltd (DP) presents the results of a geotechnical assessment of

existing and proposed infrastructure at Tom’s Gully Mine Site (Tom’s Gully), located west of the

Arnhem Highway, near Mount Bundy Creek, Northern Territory. The work was requested by Mr Clay

Gordon on behalf of Primary Gold Limited (PGL) and was carried out in general accordance with DPs

proposal dwn15006, dated 24 April 2015.

Historically, Tom’s Gully Mine operated as an open pit until circa 2007, when it progressed to

underground operations. In circa 2011 work at the site ceased. It is understood that PGL now wish to

re-commence underground mining operations at the site. With this regard, the aims of this study

are to:

• determine if existing Tailings Dams No. 2 is ‘fit for purpose’ and provide advice on management

requirements for this structure, primarily from a stability and seepage point of view;

• determine if the existing tailings solids in Dams No. 2 are suitable for re-use as filling to raise the

tailings dam walls, which are simple earth embankments;

• assess existing oxide and waste dump erosion and overall stability, and provide advice on

management requirements to minimise the risks of instability and degradation of slopes by

erosion;

• assess current stormwater management methods at the base of the oxide and waste dump and

provide advice for improving these methods, if required; and

• undertake a preliminary assessment of the site of a new water storage dam.

This assessment comprised a review of previous DP reports relating to the site and other historical

information, followed by a site visit to visually assess the various existing infrastructure detailed above

and the proposed site of the new storage dam. The results of the review and site visit are presented

herein and are followed by geotechnical engineering comments relating to the aims listed above.

2. Background Information

The following reports, prepared by DP in 2004 and 2006 were reviewed as part of this study:

• Report on Geotechnical Assessment, Existing Tailings Storage Structures, Tom’s Gully Gold

Mine, NT, Prepared for Renison Consolidated Mines NL, Project 26785, dated December 2004.

• Report on Geotechnical Assessment, Tailings Storage Structures, Tom’s Gully Gold Mine, NT,

Prepared for Toms Gully Mining Pty Ltd, Project 26785A, dated December 2006.

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Whilst not specifically reviewed for this study, the following historical information was reviewed for the

previous DP assessments and pertinent information was included in the DP reports referenced above:

• Geotechnical and Hydrogeological Investigation for Proposed Mine at Tom’s Gully, NT – Report

S8237/1-AF, March 1988, prepared by Coffey Partners Pty Ltd for Kinhill Engineers Pty Ltd.

• Tom’s Gully Tailings Retreatment Project, Notice of Intent for a Proposed Tailings Storage,

Geotechnical Report – Ref 0237, October 1993, prepared by D E Cooper & Associates Pty Ltd for

Kakadu Resources Limited.

• Tom’s Gully Tailings Retreatment Project, Site Investigation and Design Report – Ref 0237,

October 1993, prepared by D E Cooper & Associates Pty Ltd for Kakadu Resources Limited.

• Tom’s Gully Mine, NT, Notice of Intent for Tailings Retreatment Project, November 1993, prepared

by Signet Engineering Pty Ltd for Kakadu Resources Limited.

• Tom’s Gully Tailings Retreatment Environmental Management Plan, May 1994, prepared by D E

Cooper & Associates Pty Ltd for Kakadu Resources Limited.

3. Site Description

The location of Tom’s Gully is shown on Drawing A1 in Appendix A and the layout of the mine is

shown on Drawing A2, also in Appendix A. Descriptions of existing infrastructure and the sites of

proposed infrastructure that are relevant to this study, are given below. The descriptions are based on

current knowledge as well as information from the previous DP reports.

Further site specific details gathered during the site visit are presented in Section 5.

3.1 Tailings Dam No. 2

Tailings Dam No. 2 is a storage area located in the north east region of the mine site and consists of:

• a 280 m by 300 m rectangular earth filled “turkey’s nest” dam;

• a decant tower dyke constructed from the centre of the western wall into the centre of the storage

area; and

• a spillway linking the storage to the open pit midway along the western wall of the dam.

Dam No.2 was constructed in 1994 for placement of retreated tailings from Dam No. 1, located in the

southwest region of the mine site, and was used for a six month period in 1995 and for a further six

month period in 1999 / 2000. Discharge of tailings into Dam No. 2 was primarily from the crest of the

northern and western walls. The north western corner of the storage area is filled to practical capacity.

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3.2 Oxide and Sulphide Waste Dumps

The Oxide Dump is located to the south of the open pit and review of the contour drawing supplied by

PGL indicates the dump is about 24 m higher than the natural surface at its highest point.

The dump is primarily composed of rocky filling, presumably from the open pit. The material observed

during the inspection appeared to be predominantly excavated weak meta-siltstone and shale. It was

also apparent by white staining that minor amounts of salt were leaching from the rocky filling and

crystallising on the surface of the fill material. The entire dump was covered in vegetation of tall grass

clumps, bushes and shrubs.

Overall the oxide dump did not have any indicators of potential slope stability issues but there were

some areas where minor erosion had occurred and where a toe drain around the southeast corner of

the dump had failed by overtopping.

The sulphide dump is located to the west of the open pit and evaporation ponds. Review of the

contour drawing indicates the dump is consistently about 20 m higher than the natural surface.

The dump appears to be rocky filling, that has probably also been excavated from the open pit. The

material observed on the slopes of the dump during the inspection appeared to be excavated meta-

siltstone and meta-sandstone. Overall, the material on the sulphide dump was noticeably stronger

than the oxide dump material indicating that it was possibly removed from an area of the pit where

higher strength rocks were more prevalent (see Section 4).

The sulphide dump had been re-vegetated and was therefore covered in established low to medium

size eucalypts, with patchy grass clumps between trees. Atop the dump, turkey bush and other

shrubs and bushes typical of dry conditions were dominant.

As with the oxide dump, the sulphide dump did not have any indicators of potential slope stability

issues or significant erosion issues. There were some areas where erosion had occurred but they

were very minor. Overall the sulphide dump appeared very stable.

3.3 New Dam Site

The new dam site is located to the west of the sulphide dump and Dam No. 1 and the Decant Dam. It

is in an area characterised by undulating hilly terrain (refer to the client supplied contour drawing).

Minor creeks and seepage lines intersect the site of the new dam and they all eventually flow to the

north or north west towards Mount Bundy Creek.

The dam site is generally covered by typical vegetation of eucalypts, spear grass, shrubs and bushes

with pandanus and melaleucas close to any seasonally water logged areas.

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3.4 Tailings Dam No. 1 and Decant Dam

Although not strictly the subject of this assessment, a description of Dam No. 1 and the adjacent

Decant Dam is given below for completeness.

Tailings Dam No. 1 is located in the south west region of the mine site and is bounded by the following

structures:

• the southern toe of the sulphide waste dump;

• a saddle dam approximately 6 m high at the south western extremity of the storage area;

• a saddle dam approximately 3 m high in the south eastern region of the storage area with a small

water retention area in the gully upstream of this saddle dam; and

• a splitter dyke between the tailings and the decant pond to the east of the storage area.

Dam No. 1 was constructed in 1988 and was used to store tailings until open cut mining operations

ceased in 1991. Some of the tailings were pumped from Dam No. 1 in 1995 and further tailings were

removed in 1999 / 2000.

The Decant Dam is located to the west of Dam No. 1 and is bounded by the following structures and

natural terrain:

• the southern toe of the sulphide waste dump;

• a splitter dyke between the decant pond and the tailings storage to the west; and

• natural hill slopes to the south and east.

The Decant Dam was constructed in 1988 and was used to store decant water from the adjacent

tailings in Dam No. 1.

4. Topography and Geology

The current site topography is shown on contour plan supplied by the client, presented in Appendix B,

and it is dominated by low rounded hills, predominantly orientated approximately north to south and

surrounded by low lying areas of minor floodplain.

The main surface drainage feature is Mount Bundy Creek which flows from west to east approximately

200 m north of the mine site area. Runoff from the mine site area is generally towards Mount Bundy

Creek. Surface water from the north and west sides of the mine site drain directly into the creek, while

surface water from the east side of the mine site drains to the east into the lower reaches of Mount

Bundy Creek. Surface water from the south side of the mine site is intersected by open swales and

directed to the north and eventually Mount Bundy Creek or at some locations it is allowed to flow into a

low lying storage dam further to the south (referred to as ‘Lake Bazamundi’) that is used for the

pastoral lease that adjoins the mine site.

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Few well defined stream channels are visible on the contour plan, and it is likely that while sheet flow

from the ridges occurs during periods of intense rainfall, infiltration is probably rapid, and much of the

post-rainfall runoff occurs by subsurface flow in the deep weathered soil profile. Most of the low-lying

land remains waterlogged throughout the wet season indicating that the surface soil layers comprise

low permeability materials.

A geological sketch of the mine lease area from DPs previous report is presented in Appendix B. The

sketch shows that Dam No. 1 and the Decant Dam, and Dam No. 2 are located within the Wildman

Siltstone sequence. The eastern contact between the Wildman Siltstone and the Mt Goyder Syenite

runs north-south along the eastern side of the mine site approximately 400 m east of the Decant Dam

and the northern contact runs east-west approximately 100 m to the south of Dam No. 2.

Reference to geological information indicates that Wildman Siltstone predominantly comprises

laminated shale, siltstone, and sandy siltstone with minor dolomite, quartz sandstone and quartzite.

Mount Goyder Seyenite is predominantly coarse grained pink seyenite, a rock similar to granite.

5. Site Observations

A walkover of key and easily accessible areas of the site was carried out by an experienced

geotechnical engineer from DP, escorted by a representative from PGL. Photographs taken during

the site walkover are presented in Appendix A, with their locations marked on Drawing A2, which

shows the location where each photograph was taken. Observations made during the walkover are

summarised in Table 1.

Table 1: Observations Made During Site Walkover

Area Location

No. Observations

No. 2 D

am

1

Dried surface of grey, silty fine to medium grained sand tailings.

Gradient of outer slope on west wall is approximately 1(vertical) to

1(horizontal) and it is covered in moderately dense vegetation consisting of

tall grass clumps, various shrubs, saplings and small trees.

The gradient of the inner slope appears to also be 1(v) to 1(h).

Based on the ground slope it appears that the tailings are about 2 m to 3 m

deep and there is about 0.3 m of freeboard to the crest of the west wall.

The dam wall is about 3 m to 4 m wide and mostly covered by vegetation

except for a small width near the centre which allows partially obstructed

access to vehicles.

2

The northern embankment of Dam No. 2 has similar conditions to those

described above, in terms of vegetation, height and embankment slope.

The area beyond the northern embankment is low lying and it is understood

that it generally becomes inundated during the wet season. The low lying

area flows into Mount Bundy Creek, which is further to the north.

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Table 1 Continued

Area Location

No. Observations

No. 2 D

am

3

Dried, desiccated surface of grey, silt, clay and fine sand sized tailings.

The eastern embankment of Dam No. 2 generally has similar conditions to

those described at Location 1 in terms of vegetation, height and embankment

slope. The inner slope of the embankment is also heavily vegetated.

It is apparent that the discharge into the tailings dam was from the west

embankment as the tailings are significantly finer on the east side of the dam.

Sulp

hid

e W

aste

Du

mp

4

Re-vegetated sulphide waste dump that appears to consist of rocky fill

material with slopes formed at about 1(v):3(h) to 1(v):4(h) and covered in

sparse small to medium eucalyptus trees.

No obvious global instability issues were observed at any of the locations

visited. Some minor slumping of loose surface rocky filling had occurred,

primarily where the slope had been exposed to excessive surface water

flows.

5

Eastern end of sulphide waste dump where some minor erosion has occurred

on north slope of the dump where surface water flows down to the southern

evaporation pond.

Minor earthworks have been carried out in this are to re-direct surface flows

away from the slope.

New

Da

m S

ite

6

The proposed new dam site is located east of the sulphide waste dump

location and it appears to cover the area below about RL 1030 m.

In general, the ground surface in this area is relatively flat or gently sloping

towards the north and northwest, towards Mount Bundy Creek.

A small low area exists at the southern end of the proposed new dam

footprint, and this low area was inundated at the time of the site visit. It was

apparent that this area was probably inundated year round.

A mostly dry creek bed was present at the western end of this low area and

conditions exposed in the near vertical banks of the creek comprised thin

sandy silt / clay overlying thinly bedded to laminated low to medium strength

siltstone. The strike of the siltstone foliation was approximately north to south

and it was generally dipping down to the west at about 45º.

Other seepage lines / minor dry creek beds were observed in the vicinity of

the proposed new dam site and conditions were similar to those detailed

above, with relatively shallow bedrock that dipped at about 30º to 45º to the

west.

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Table 1 Continued

Area Location

No. Observations

Dam

No. 1 a

nd

Decant D

am

7

The area south of existing Dam No. 1 appears to have been used as a borrow

area to source material possibly for use in construction of the Dam No. 1

earth embankment.

This area is within the area of the proposed new dam that will be flooded if it

is built. Some minor erosion and deposition of sediment has occurred in this

area and appears to have been caused by rainfall and surface water runoff.

The southern embankment of Dam No. 1 appears to be stable and there are

no signs of significant erosion, slumping or other stability related issues.

Oxid

e D

um

p

8

No obvious global instability issues were observed at any of the locations

visited on the Oxide Dump. Some minor slumping of loose surface rocky

material had occurred but it was minimal and it was apparent that it had

primarily been caused by excessive surface water flowing off the dump.

A swale drain skirting the southwest and southeast toe of the oxide dump

showed signs of excessive erosion and in some areas, particularly the

southern corner, it was apparent that the drain had been overtopped by

surface run-off during the wet season and water has escaped the drainage

system into ‘Lake Bazamundi’, a seasonally inundated area.

It appeared that some mineral leaching from the oxide dump had occurred in

this area, evident by staining of the surface, salt crystals at the surface, and

some dead vegetation downslope of the dump.

Exis

tin

g

Infr

astr

uctu

re

9

The existing ore crushing and processing facilities are located south of the

open pit and Dam No. 2. The equipment appears generally in good order.

Some minor erosion of earthworks has occurred at the site over time,

presumably due to the excess surface water flows experienced on site during

the wet season. The erosion has not led to any potential stability issues at

the site. It was apparent that minor remedial works to improve surface

drainage and minimise effects of erosion had been carried out at the site.

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6. Proposed Development

It is understood that PGL wish to re-commence underground mining operations in the existing decline

at the site, which is at the east end of the open pit.

In order to access the decline it will be necessary to remove some 2.7 GL of mine water from the pit

and it is proposed to construct a new earth embankment dam to the east of the mine for storage of this

water. Exact details on the size, proposed location and height of the earth embankment were not

known at the time of reporting but it is understood that it will probably be located in the saddle of two

ridges to the north where the land surface slopes down towards Mount Bundy Creek. It is also

understood that the dam will be about 10 m to 14 m in height and the crest of the embankment will be

tentatively within the 1,032 m to 1,036 m natural surface contour (refer to client supplied drawing in

Appendix B). The initial purpose of the new dam will be to store mine water but at later stages it will

also be used for stormwater management.

Processing of mined ore will be carried out at the existing infrastructure south of the open pit and Dam

No. 2. It is proposed to store tailings from the processing operations in Dam No. 2 after the dam walls

are raised and it is currently proposed to raise the dam walls with the existing Dam No. 2 tailings.

The existing sulphide and waste dumps will not be added to for new mining operations but they will be

managed so that risks associated with slope instability and erosion are minimised.

The comments given herein may require reconsideration when exact development details are known if

any of the assumptions made by DP, as detailed above and in the comments section below, are not in

accordance with the final development plan.

7. Comments

7.1 Dam No. 2

The profile of the crest and outer slope of Dam No.2 is regular with the dam wall being 3 m to 4 m high

and the crest approximately 4 m to 5 m wide. The slopes and crest comprise rocky filling and

therefore they are quite resistant to erosion. There were no visible signs of seepage at the toe of the

outer face, nor were there any visible signs of excessive erosion or slope instability. Any potential

seepage from the current storage area should be arrested by a combination of the tailings deposited

against the dam walls and the clay core in the dam walls. The dam walls appeared to be stable.

Small trees, bushes and shrubs were growing on the outer batters of the dam wall, as well as across

the entire width of the crest and on the inner slope of the dam walls in places. They will provide

erosion protection for the earth and rock filled slopes. However, it is possible that the roots of these

trees may penetrate the embankment and provide possible seepage paths, which could lead to piping

through the dam walls. The trees and bushes on the crest are also restricting vehicle access to the

dam. Therefore, it is suggested that all trees and bushes are removed from the crest and all trees and

bushes greater than the height of the embankment (ie, greater than about 4 m height) are removed

from the inner and outer batters. The trees and bushes should be cut at ground level and removed

with the roots left in place, so as not to disturbed the embankment. The trees should also be treated

so they don’t regrow or they should be continually trimmed during periodic mine maintenance work.

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If the tailings dam is raised then it may be necessary to remove all vegetation from the dam wall, but

this will depend upon the strategy adopted, which is discussed below.

Based on this inspection and review of previous reports, it is considered that Dam No. 2 will be fit for

the purpose for disposal of tailings, provided that dry tailings are removed to allow for storage of new

tailings or provided that the capacity of the dam is increased by raising the height of the existing walls.

Use of the storage area should also be in accordance with current industry practice to manage the

deposition of the tailings.

The existing walls of Dam No. 2 could be raised by either extending the earth filling outwards or

inwards, whilst maintaining the same slope and a new crest width that is at least vehicle width (ie 3 m

to 4 m) so that vehicle access is possible for maintenance. Extending inwards will require removal of

tailings down to the toe of the inner slope for some 2 m to 3 m depth so that new filling can be placed

on a competent foundation. Removal of the material from the inner slope may be problematic

because it is likely to be wet, weak and may liquefy at depth, leading to continual collapse back up

against the embankment halting excavation.

Whilst specific slope analysis has not been carried out for this assessment, it is envisaged that if

similar rocky filling is used to form the new embankment, than an additional height of say 2 m to 3 m

above the current level and at the current slope should be feasible without compromising the

embankment stability. Before construction commences, a detailed design of the new embankment

should be completed and the design should allow for a low permeability clay core with external rocky

filling for erosion protection and stability.

The new inner or outer slope should be ‘keyed in’ to the existing slope during construction and it will

be important to key in the new clay core to the existing core to maintain continuity of the low

permeability zone.

Inspection of the existing tailings was limited to the upper 0.5 m of material during this assessment,

but based on this it appears that the tailings near the original outlet mostly comprise fine to medium

sand sized particles. Further to the east the material becomes finer and silt and clay sized particles

are dominant. These sand, silt and clay dominant materials are unlikely to be suitable for re-use in

construction of the new embankment as they will readily erode and their overall stability will be

inadequate for the relatively steep slope of the existing embankment.

Better quality fill material for raising the dam wall could be sourced from either the existing borrow

area, south of Dam No. 1, or from numerous sources that are likely to exist within the new dam site.

Alternatively, material could be carted from off site, but this is likely to be less cost effective.

7.2 Oxide and Sulphide Waste Dumps

The oxide and sulphide waste dumps appear to consist primarily of rocky filling that has been formed

at relatively steep slopes. The sulphide dump has been re-vegetated and is mostly covered in well-

established eucalypts. The oxide dump is mostly covered in grass, shrubs and some bushes.

Because of the rocky filling and the cover of vegetation the slopes are quite resistant to erosion and

they also appear stable. Aside from some areas of minor erosion there were no visible signs of slope

instability, nor were there any potential seepage issues noted at the time of the inspection. Although it

10 of 12


should be noted that the inspection was carried out during the dry season and conditions could vary

during the wet season when significant rainfall events will be expected.

Even with the apparent stability and erosion resistance, there are some minor drainage improvements

to the waste dumps that could be carried out and then monitored and repaired as required during

routine mine maintenance operations. These include:

• Minor earthworks to improve drainage at locations where erosion has occurred, such as at

Location 5. The drainage should be improved by forming a cut-off drain at the slope crest or a

small bund, so that surface water is directed away from slope face.

• Re-building the swale drain at the toe of the oxide dump in areas where it has been over-topped

and eroded and drainage water has been released into the natural drainage system, such as at

Location 8. The swale drain at Location 8 should be re-shaped and deepened, if required by

further excavation or by forming a bund at the crest of the drain on the downstream side.

7.3 New Dam Site

It should be noted that this is only a preliminary assessment of the new dam site based on a limited

site walkover, and it is recommended that a detailed assessment be carried out before design of the

dam is completed. The detailed assessment should include investigation by site walkover, drilling of

boreholes primarily in the dam footprint, excavation of test pits in specific areas of importance (ie in the

dam footprint or in potential borrow areas identified during the site walkover), then laboratory testing to

define material properties such as compaction characteristics and permeability. The investigation

should be followed by analysis of the geological conditions, development of geological models, an

assessment of the founding conditions and slope stability analysis of the proposed dam wall.

Assessment of the flow characteristics of the dam wall and underlying geological conditions should

also be considered.

This assessment is also limited by available surface levels contours of the site, which only extend just

beyond the mine lease boundary (refer to client supplied drawing in Appendix B).

Geological references indicate that conditions in the vicinity of the dam site predominantly comprise

quaternary aged alluvial soils to the north and close to Mount Bundy Creek, and tertiary to quaternary

aged unconsolidated sand and ferruginous clayey sand to the south, upslope of Mount Bundy Creek.

This is broadly similar to the conditions observed on site during the inspection, although it must be

noted that the inspection covered a limited area only close to Locations 6 and 7. In the areas

inspected, shallow bedrock was also observed and it predominantly comprised thinly bedded to

laminated metamorphosed siltstone that dipped at about 45 degrees and had a strike approximately

north to south. It was apparent that this siltstone belonged to the Proterozoic age Wildman Siltstone

unit of the Mount Partridge Group, which is shown on the geological maps of the area. No faults or

other major discontinuities are shown in the area on the geological references, and evidence of such

features was also not observed on site.

As discussed in Section 6, it is anticipated that the dam will be located in the saddle of two ridges to

the north of the mine lease where the land surface slopes down towards Mount Bundy Creek. With a

dam wall of about 10 m to 14 m the resulting water level in the lake will be about RL 1,032 m to

RL 1,036 m. Based on this preliminary work by DP it is considered that a dam at this location will be

11 of 12


feasible, provided that adequate geotechnical investigation is carried out to eliminate geotechnical

risks. Based on current limited information there do not appear to be any significant geotechnical risks

associated with the construction of a dam that would preclude it from being built, however further

investigation is required for confirmation. The current known geotechnical issues at the location are

typical of those that would be encountered on any dam construction site (ie topsoil to be removed,

some weak subgrade soils, excavation difficulties and sources of filling material).

Based on the conditions observed on site, particularly close to the existing borrow area at Location 7,

it is considered that fill material that is suitable for the dam wall construction should be able to be

sourced from within the resultant lake footprint. Sourcing material from within the footprint will also

marginally increase the lake capacity. As detailed above, the actual material sourced should be

assessed by test excavations and drilling and subsequent laboratory testing.

Preparation of the dam foundation soils will be required prior to placement of fill material for the dam

wall. Based on the recent inspection it is anticipated that foundation preparation requirements will not

exceed normal requirements for such a project. It is expected that the preparation will essentially

comprise stripping of vegetation, topsoil then grubbing of roots, followed by re-compaction of the

disturbed natural soils. Depending upon the eventual design it may also be necessary to key in a clay

embankment core and / or scarify the natural soils to ensure a smooth shear surface or a path for flow

is not created between the filling and the natural subgrade soil.

8. Limitations

DP has prepared this report for this project at Toms Gully, Northern Territory. The work was carried

out under DP’s Conditions of Engagement. The report is provided for the exclusive use of PGL for this

project only and for the purpose(s) described in the report. It should not be used for other projects or

by a third party. In preparing this report DP has necessarily relied upon information provided by the

client and / or their agents.

The results provided in the report are indicative of the conditions at the specific investigation locations,

and only to the depths investigated and at the time the work was carried out. Sub-surface conditions

can change abruptly due to variable geological processes and also as a result of anthropogenic

influences. Such changes may occur after DP's investigation has been completed.

DP's advice is based upon the conditions encountered during this investigation. The accuracy of the

advice provided by DP in this report may be limited by undetected variations in ground conditions.

The advice may also be limited by budget constraints imposed by others or by site accessibility.

This report must be read in conjunction with all of the attached notes about this report and should be

kept in its entirety without separation of individual pages or sections. DP cannot be held responsible

for interpretations or conclusions made by others unless they are supported by an expressed

statement, interpretation, outcome or conclusion given in this report.

This report, or sections from this report, should not be used as part of a specification for a project,

without review and agreement by DP. This is because this report has been written as advice and

opinion rather than instructions for construction.

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The contents of this report do not constitute formal design components such as are required, by the

Health and Safety Legislation and Regulations, to be included in a Safety Report specifying the

hazards likely to be encountered during construction and the controls required to mitigate risk. This

design process requires risk assessment to be undertaken, with such assessment being dependent

upon factors relating to likelihood of occurrence and consequences of damage to property and to life.

This, in turn, requires project data and analysis presently beyond the knowledge and project role

respectively of DP. DP may be able, however, to assist the client in carrying out a risk assessment of

potential hazards contained in the Comments section of this report, as an extension to the current

scope of works, if so requested, and provided that suitable additional information is made available to

DP. Any such risk assessment would, however, be necessarily restricted to the geotechnical

components set out in this report and to their application by the project designers to project design,

construction, maintenance and demolition.

Douglas Partners Pty Ltd

Appendix A

Notes About this Report

Drawing A1 – Site Location Plan

Drawing A2 – Site Layout and Photograph Locations

Site Photographs

July 2010

Introduction These notes have been provided to amplify DP's report in regard to classification methods, field procedures and the comments section. Not all are necessarily relevant to all reports. DP's reports are based on information gained from limited subsurface excavations and sampling, supplemented by knowledge of local geology and experience. For this reason, they must be regarded as interpretive rather than factual documents, limited to some extent by the scope of information on which they rely. Copyright This report is the property of Douglas Partners Pty Ltd. The report may only be used for the purpose for which it was commissioned and in accordance with the Conditions of Engagement for the commission supplied at the time of proposal. Unauthorised use of this report in any form whatsoever is prohibited. Borehole and Test Pit Logs The borehole and test pit logs presented in this report are an engineering and/or geological interpretation of the subsurface conditions, and their reliability will depend to some extent on frequency of sampling and the method of drilling or excavation. Ideally, continuous undisturbed sampling or core drilling will provide the most reliable assessment, but this is not always practicable or possible to justify on economic grounds. In any case the boreholes and test pits represent only a very small sample of the total subsurface profile. Interpretation of the information and its application to design and construction should therefore take into account the spacing of boreholes or pits, the frequency of sampling, and the possibility of other than 'straight line' variations between the test locations. Groundwater Where groundwater levels are measured in boreholes there are several potential problems, namely: • In low permeability soils groundwater may

enter the hole very slowly or perhaps not at all during the time the hole is left open;

• A localised, perched water table may lead to an erroneous indication of the true water table;

• Water table levels will vary from time to time with seasons or recent weather changes. They may not be the same at the time of construction as are indicated in the report; and

• The use of water or mud as a drilling fluid will mask any groundwater inflow. Water has to be blown out of the hole and drilling mud must first be washed out of the hole if water measurements are to be made.

More reliable measurements can be made by installing standpipes which are read at intervals over several days, or perhaps weeks for low permeability soils. Piezometers, sealed in a particular stratum, may be advisable in low permeability soils or where there may be interference from a perched water table. Reports The report has been prepared by qualified personnel, is based on the information obtained from field and laboratory testing, and has been undertaken to current engineering standards of interpretation and analysis. Where the report has been prepared for a specific design proposal, the information and interpretation may not be relevant if the design proposal is changed. If this happens, DP will be pleased to review the report and the sufficiency of the investigation work. Every care is taken with the report as it relates to interpretation of subsurface conditions, discussion of geotechnical and environmental aspects, and recommendations or suggestions for design and construction. However, DP cannot always anticipate or assume responsibility for: • Unexpected variations in ground conditions.

The potential for this will depend partly on borehole or pit spacing and sampling frequency;

• Changes in policy or interpretations of policy by statutory authorities; or

• The actions of contractors responding to commercial pressures.

If these occur, DP will be pleased to assist with investigations or advice to resolve the matter.

July 2010

Site Anomalies In the event that conditions encountered on site during construction appear to vary from those which were expected from the information contained in the report, DP requests that it be immediately notified. Most problems are much more readily resolved when conditions are exposed rather than at some later stage, well after the event. Information for Contractual Purposes Where information obtained from this report is provided for tendering purposes, it is recommended that all information, including the written report and discussion, be made available. In circumstances where the discussion or comments section is not relevant to the contractual situation, it may be appropriate to prepare a specially edited document. DP would be pleased to assist in this regard and/or to make additional report copies available for contract purposes at a nominal charge. Site Inspection The company will always be pleased to provide engineering inspection services for geotechnical and environmental aspects of work to which this report is related. This could range from a site visit to confirm that conditions exposed are as expected, to full time engineering presence on site.

Client: Site Location Project No. 78205.00

Office: Darwin Drawn by: A Gane Geotechnical Assessment Drawing No. A1

Scale: NTS Date: 25 Jun 2015 Toms Gully, Arnhen Highway, NT Revision: 0

Primary Gold Limited

N

Site Location

Darwin

Client: Site Layout and Photograph Locations Project No. 78205.00

Office: Darwin Drawn by: A Gane Geotechnical Assessment Drawing No. A2

Scale: NTS Date: 25 Jun 2015 Toms Gully, Arnhen Highway, NT Revision: 0

Primary Gold Limited

Location No.

N

Site Feature

Project No.

78205.00

Date:

20 May 2015 Client: Primary Gold Limited

Photo 1, Location 1: Looking southeast across Dam No. 2

Photo 2, Location 1: Looking at typical outer embankment slope of Dam No. 2

Site Photographs

Toms Gully Mine Site

Project No.

78205.00

Date:

20 May 2015

Photo 3, Location 2: Crest of Dam No. 2 north embankment

Site Photographs


Client: Primary Gold Limited

Photo 4, Location 2: Looking north at outer slope of north embankment and towards area of

wet season inundation

Project No.

78205.00

Date:

20 May 2015

Photo 5, Location 3: Looking west from east end of Dam No. 2 showing fine sand and clay sized tailings

Photo 6, Location 3: Looking east at heavily vegetated inner slope of east embankment

and dry desiccated clay tailings on surface

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 7, Location 4: Looking southwest downslope from north end of sulphide waste dump

Photo 8, Location 4: Looking northwest downslope from north end of sulphide waste dump

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 9, Location 4: Looking north downslope from north end of sulphide waste dump

Photo 10, Location 4: Looking south along east slope of sulphide waste dump from north end of the dump

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 11, Location 4: Looking south across sulphide waste dump from north end

Photo 12, Location 5: Looking west along eroded section of sulphide waste dump

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 13, Location 5: Looking east along eroded section of sulphide waste dump

Photo 14, Location 5: Looking north down slope at eroded section of sulphide waste dump

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 16, Location 6: Looking west or northwest downstream of dry section of creek

Site Photographs



Photo 15, Location 6: Inundated low point at head of dry creek that is in the general area of the proposed New Dam,

and appears towards flow in a northwest direction to Mount Bundy Creek

Project No.

78205.00

Date:

20 May 2015

Photo 17, Location 6: Looking downstream in dry section of creek showing exposed bedrock in creek bed dipping at

about 45 degrees

Photo 18, Location 7: Looking southwest of southern end of Dam No. 1

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 19, Location 7: Eroded gully downslope or south of Dam No. 1

Photo 20, Location 7: Looking north towards Dam No. 1

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 21, Location 8: Looking south from crest of south corner of oxide dump. Swale drain in

foreground and 'Lake Bazamundi' in background, a seasonally waterlogged area.

Photo 22, Location 8: Swale drain.

Site Photographs



Project No.

78205.00

Date:

20 May 2015

Photo 23, Location 9: Existing ore crushing infrastructure.

Photo 24, Location 9: Existing shed west of crushing infrastructure.

Site Photographs



Appendix B

Client Supplied Drawing

Geological Sketch form Previous DP Report

PRiMJ\RY GOLD

Mt Bundy Project

Toms Gully Mine

Site contours over ALOS image 0

1 : 10000 Map Projection: MGA Zone 52 (GOA 94)

Pnni sae: A3 1:10,oo:> This ""I> is c:opynglt.~ {2015), oi f'Nnaty Gold l.od

Contour intervals 2m

1020 --- 1030

--- 1040

--- 10m

--- 1050

500m

, -l

' f 5" ' /'--""" D

x ___, / r ./~ I . --- I

. . . . . . . . " . . . . . . . . . . . . .. ... .... x'- -

r. ~·/ ·1· .eo: · ·-:-:-: -:. -: -: -: ·1

1

: > x I r . . ....... j... I

! . . .: >oe~~N~: o~\ : :!" '1. x /

l . . . . . ..._, " x ' \ . . . .. ·y .... ',· . ·. ·// · .·.· .· ~ - ~--·:'.S · ~ 'x ~ . . . . . . . . - ... -:-:--~ x .· /

;:: : : : t: : : : : : : : . : : : : : : : : : : >' :.~ " -. • J ••••• . • •• ••••••••• ·i · x \

. ~L~ ~o.1 _ DAM :-~~:-:-:-:·} x

. · . · ./" I:~ :- :- :- :- :- :- :- :- :- :- x 7 <( ) .. . · 1·. . . . . . . . . . . . . 5 x . . . . . .. .... ·{ .... ... .... t: x / _,.---:--: . : : . : . : . : 1~ . : . : . : . : . : . : . 1 ,' ~ ' . . .. I ... ..... . . . ......... -; rt

. -:)it . : . : . : . : . : . : . : . : . : . :/ ~ . . ~- . . : .: . :- :- :- :- :- :- > §

I I

LEGEND

I: . :: WILDMAN SILTS1DNE

fXXl MT GOYDER SYENITE QU S GRANITE

f?J~%1 APUTE

D ALWVIUM/COWJ\ltUM

.. 11 FERRUGINOUS OLJTIBOPS

0 IOOm

SCALE

·t ..:::::> ( I: x u--..._·· i: ,J xi

\ X !:

Sydney, Newcastle

Brisbane, Melbourne,

Perth, Wyong

Singleton, Campbelltown,

Townsville, Cairns, Darwin.

TITLE GEOLOGICAL SKETCH MAP

TOM'S GULLY GOLD MINE 1 CLIENT RENISON CONSOLIDATED MINES NL

DRAWN BY: DSF \ SCALE: As Shown

APPROVED BY:

I OFFICE DARWIN

PROJECT No: 26785 DRAWING No: 2

DATE: December 2004

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x

i

I

I i

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