"Developing the growing 1.4Moz Pilbara Gold Project"
Positive gold recoveries enhance
Pilbara Gold Project potential
ASX Announcement
12 February 2019
ASX Code DEG
➢ Breakthrough +90% recovery confirmed for "pyrite dominated sulphide" mineralisation at Withnell - the key sulphide deposit at the project - likely to improve project economics.
FRA Code WKN 633879
➢
Average indicated recoveries for mineralisation types:
ABN 65 094 206 292
Executive Chairman
Simon Lill
-
➢ Ongoing optimisation seeking to further improve recoveries
Technical Director & Operations Manager
-
➢ High gold recovery maintained at coarser grind size significantly reducing grinding requirements and likely the associated power costs.
Andy Beckwith
Non-executive Directors Peter Hood
Brett Lambert
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➢ Simplified process flowsheet developed, GR Engineering Services now refining plant design, layout and costings. All components to be conventional industry accepted, targeting operating costs equal to or less than industry average for the relevant operation scale.
Steve Morris
➢
Diamond drilling has been underway at Withnell since mid-January.
Company Secretary Patrick Holywell
Craig Nelmes
➢ An RC rig is due to arrive shortly to commence at Toweranna. Priority resource extension drilling will target Toweranna, Mt Berghaus and Mallina during the first half of the year..
Exploration Manager Phil Tornatora
Technical Director Andy Beckwith commented:
CONTACT DETAILS Principal & Registered Office Level 3, 22 Railway Rd, Subiaco WA 6008
"The 90% recovery on the Withnell sulphide mineralisation is an outstanding improvement making Withnell a fundamental long-term baseload to the project. We are increasingly more confident that Withnell will also provide a long term high-grade underground mining opportunity.
PO Box 2023
Subiaco WA 6905
GR Engineering are working on the plant design and costings and from this we will be re-evaluating pit optimisations and the potential to add an underground mining component.
admin@degreymining.com.au
T +61 8 6117 9328
F +61 8 6117 9330
The simplified processing flowsheet, rapid extraction times and reduced grinding requirements, together with on-going exploration success, point to a significantly enhanced operation. We are currently assessing the optimal project."
www.degreymining.com.au
Pilbara Gold Project - Unlocking Shareholder Value
Summary Metallurgy Results
De Grey is pleased to report on positive metallurgical test work undertaken across the Pilbara Gold Project ("PGP"), located immediately south of Port Hedland in the Pilbara Craton of Western Australia (Figure 1).
Detailed metallurgical test work has been on-going since the release of the August 2017 Scoping Study and has recently culminated in the breakthrough results achieved at Withnell, the largest resource at the PGP with rapidly expanding underground potential. This work has involved detailed metallurgical test work on existing drill core and the drilling of dedicated PQ diamond core holes at various deposits for additional sampling material. ALS Metallurgy Services have carried out the testing under the supervision of GR Engineering Services Limited (GRES), with input from De Grey technical staff. All test work was completed to standard industry accepted practices.
Overall average recoveries
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• +94% - Oxide material
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• +92% - Free milling fresh material
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• +90% - Pyrite dominated sulphide material
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• Fast CIL extraction rates of between 6-24 hours
Gravity
Comminution
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• Rock properties indicate low to high hardness and moderate abrasion
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• Coarser grind size improvement from 75-90µm to 106-150µm
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• Coarser grind size leads to expectation of reduced power consumption
Sulphide flotation
Oxidation of Sulphide Concentrate
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• Sulphide flotation concentrate to be oxidised
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• Pressure oxidation (POX) preferred
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• POX requires only small vessel with capacity of 50kt-100kt per annum
Plant Design - underway by GRES
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• All plant componentry is expected to be conventional and industry standard - "tried and tested"
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• Crush, grind, flotation, gravity, POX, CIL tanks, elution and tail disposal
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• Simplified processing flowsheet, rapid extraction times and reduced grinding requirements point to a larger scale throughput which is expected to provide reduced operating costs
Figure 1 Pilbara Gold Project, location plan
Oxide Samples
Composite samples from the oxide domain were assessed, from the following deposits:
Calvert, Mt Berghaus, Camel, Roe and Dromedary.
The test work delivered positive results for each of the oxide domains with an average gold recovery of 94%. Two grind sizes, ranging from 106 and 150µm were used with room to further optimise. All samples demonstrated rapid extraction rates with leach residence times of 6-24 hours. The coarse grinding size of between 106-150µm is expected to provide cost savings due to reduced power requirements and lower grinding media consumption.
The Mt Berghaus sample showed a high gravity recovery component at 49% with Camel at 20%, indicating a gravity circuit could provide benefits to the processing circuit. Wingina and Withnell results are based on previously undertaken test work. Results are summarised in Table 1.
In general, the oxide domains in all deposits assessed to date will require a simple typical oxide CIL processing flowsheet used widely throughout the gold mining industry in Australia.
Table 1
Summary Oxide Metallurgy Test Work
OXIDE
|
AVERAGE GOLD RECOVERY
|
ROCK PARAMETERS
|
Deposit
|
No. Samples
|
GravityDirect CILFloat CILFloat Oxidise
%
%
%
CIL %
|
Grind SizeBwiRwi
Abrasion
microns
kWh/t
kWh/t
|
Calvert
|
2
|
10.3
|
95.4
|
106 - 150
|
11.6
|
13.5
|
0.0937
|
Camel
|
2
|
20.3
|
96.1
|
106 - 150
|
15.8
|
10.5
|
0.2922
|
Roe
|
1
|
1.6
|
94.7
|
106 - 150
|
10.3
|
8.43
|
0.0346
|
Dromedary
|
1
|
1.8
|
86.9
|
106 - 150
|
10.6
|
11.1
|
0.0214
|
Withnell
|
8
|
11.5
|
90.4
|
75 - 106
|
14.4
|
9.3
|
0.1023
|
Wingina
|
2
|
26.8
|
97.3
|
75
|
Mt Berghaus
|
1
|
49.8
|
88.0
|
106
|
17.2
|
14.8
|
0.0784
|
Fresh Samples
Fresh core samples were assessed for the following deposits:
Withnell, Calvert and Mt Berghaus
The Withnell samples were chosen as the priority to concentrate on the pyrite dominant sulphide mineralisation assessment as it is currently the largest sulphide deposit at the project. The other pyrite dominant sulphide rich deposits include the Calvert, Roe, Dromedary and Calvert, all situated along the Withnell Trend. Mallina is expected to be sulphide rich mineralisation and this deposit is currently being evaluated with new drill core recently submitted to ALS Metallurgy. Results are summarised in Table 2.
Most importantly, the Withnell results have shown that high gold recovery can also be maintained from the fresh pyrite sulphide dominant mineralisation at coarser grind size. Previously proposed fine grinding has been eliminated in favour of a coarser sulphide flotation with a resultant small mass pull (5-9%) into sulphide concentrate followed by oxidation and standard CIL leaching to extract the contained gold. The preferred oxidation process is pressure oxidation (POX). Importantly, the POX component is expected to be significantly smaller with annual capacity of 50kt-100kt depending on final throughput volumes due to the strong volume reduction into the final sulphide concentrate.
Overall, the flotation, pressure oxidation (POX) and cyanide leach test work provided a peak recovery of 95.9% from the concentrate, so when combined with gravity and sulphide flotation recovery provides an overall average gold recovery of +90% of the contained gold. The fresh tail is also proposed to be processed through the standard CIL tanks to ensure the highest recovery of gold is extracted.
The simplification of the processing circuit (Figure 2) together with coarser grind size is considered a substantial improvement, on the previously proposed flotation and ultrafine grind methodology. Reduced operating costs are expected with this simplified flow sheet and importantly enhanced recovery. Results are summarised in Table 1.
Table 2
Summary Fresh Metallurgy Test Work
FRESH
|
GOLD RECOVERY
|
ROCK PARAMETERS
|
Deposit
|
Nos Samples
Tested
|
Gravity
%
|
Direct CIL
%
|
Float CIL
%
|
Float Oxidise
CIL
%
|
Grind Size
microns
|
Bw i
kWh/t
|
Rw i
kWh/t
|
Abrasion
|
Calvert
|
2
|
Not Tested
|
15.8
|
70.6%
|
75 - 212
|
19.2
|
20
|
0.1774
|
Withnell
|
12
|
3
|
69.5
|
75.0
|
91.5%
|
75 - 150
|
17.4
|
20.3
|
0.203
|
Mt Berghaus
|
1
|
Not Tested
|
95.0
|
75
|
17.5
|
19.8
|
0.1318
|
Figure 2
Simplified processing circuit
Additional Test Work
Ongoing test work is planned to improve the overall knowledge of all the deposits. This work will include similar detailed comminution, recovery and where appropriate sulphide flotation test work as required.
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• Metallurgical test work on the Toweranna and Mallina deposits with purpose drilled PQ core recently submitted to ALS Metallurgy
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• Plant design, capital and operating costs underway with GRES (engineering consultants)
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• Open pit and underground evaluation using new operating costs and recoveries and assessment to increase plant scale planned with CUBE (mining consultants)
Additional evaluation of the Withnell Deeps underground mining potential is currently being advanced with remodelling of the resources below the open pit and this will be followed by mine design and costing. Diamond drilling recommenced in mid-January and this +4000m drilling program is planned to continue through H1 CY2019 to increase resources at Withnell.
For further information:
Simon Lill (Executive Chairman) or
Andy Beckwith (Technical Director and Operations Manager)
De Grey Mining Ltd
Phone +61 8 6117 9328
admin@degreymining.com.au
Competent Person
The information in this report that relates to exploration results is based on, and fairly represents information and supporting documentation prepared by Mr. Andrew Beckwith, a Competent Person who is a member of The Australasian Institute of Mining and Metallurgy. Mr. Beckwith is a consultant to De Grey Mining Limited. Mr. Beckwith has sufficient experience that is relevant to the style of mineralisation and type of deposit under consideration and to the activity being undertaken to qualify as a Competent Person as defined in the 2012
Edition of the "Australasian Code for Reporting of Exploration Results, Mineral Resource and Ore Reserves".
Mr. Beckwith consents to the inclusion in this report of the matters based on his information in the form and context in which it appears.