De Grey Mining achieves up to 96.3 % gold recovery at Hemi

by Leena Thomas
Image: De Grey Mining Limited

De Grey Mining Limited (ASX: DEG) has announced excellent metallurgical results from initial metallurgical testwork of Brolga mineralisation at the Hemi gold discovery in Western Australia.

The company reported that Initial metallurgical testwork on Brolga mineralisation has achieved excellent gold recoveries. DEG reported Oxide 93.0 % recovery based on CIL leach and Fresh 96.3 % recovery based on sulphide flotation, oxidation, and CIL leach.

De Grey said that the testwork flowsheet comprised a conventional carbon in leach (CIL) circuit to treat oxide mineralisation. The company said that an industry accepted circuit comprising sulphide flotation to generate a concentrate prior to oxidation was used to treat fresh mineralisation.

DEG said that additional samples from Brolga and Aquila covering oxide, transitional and fresh mineralisation will be tested to assess variability and to optimise the flowsheet. The previous testwork conducted on other regional resources (2.2Moz) outside Hemi indicates that the Hemi testwork flowsheet would be suitable to treat those deposits.

The company said that additional metallurgical testwork results are expected later in the current September quarter.

Mallina gold project

De Grey’s Mallina Gold Project is located approx. 60km south of Port Hedland in the Mallina Province of Western Australia.

The project hosts numerous large deposits, of which significant gold deposits include Withnell, Mallina, and Toweranna. Other prospects include Hemi, Antwerp, and Scooby prospect.

Hemi prospect

Hemi is a discovery under 30m of transported cover, with two zones of strong, broad sulphide rich alteration with high grade gold mineralisation partially defined in the Aquila and Brolga zones. Hemi prospect comprises of the initial discovery Section A, as well as Section B, Section C, and Section D.

The Aquila Zone represents a 30-50m wide high-grade gold zone (>5g/t) defined on Section 30,000E (local grid) to 180m below surface. The Brolga Zone is a substantially wider alteration zone, potentially up to 200m wide and is currently defined by RC drilling over 160m of strike.

Metallurgical work at Hemi

The company reported that initial metallurgical testwork was conducted by the leading global testing and analysis company, ALS Metallurgy, under the supervision of GR Engineering Services Limited (GRES) and with input from De Grey’s technical team.

Composites from DD holes

DEG said that comprehensive testwork is currently in progress on composites obtained from two diamond drill holes (HEDD001 and HERC011D) at Brolga.

The company said that three of the composite samples contain fresh mineralisation and another composite comprises transitional mineralisation.

DEG reported that composite 4, comprising fresh mineralization, has completed a full suite of testwork including crushing, grinding (150μm to 53μm), multi-element analysis, gravity separation, whole of ore cyanide leaching and CIL treatment, sulphide flotation, oxidation and cyanide leaching of oxidised flotation concentrate and cyanide leaching of flotation tailings.

6 additional DD holes completed

The company reported that an additional six dedicated larger diameter (PQ and HQ) diamond core holes (HEDD004 – HEDD009) have been completed to provide larger volume samples within the oxide, transition and fresh domains throughout the deposit.

DEG said that these holes have been drilled and are currently being sampled and transported to ALS Metallurgy in Perth and will provide a detailed assessment of metallurgical variability across the deposit. The company said that the detailed results of this comprehensive program are expected later this current September quarter.

Oxide mineralisation from RC holes

The company reported that twelve samples of oxide mineralisation were taken from two RC holes (HERC034 and HERC035) drilled at Brolga.

The company said that each sample was separately tested using conventional CIL testwork by a 24 hour bottle roll cyanide extraction method.

DEG said that the samples selected covered a range of mineralisation styles including sediments, mafic intrusive and intermediate intrusive. The company reported that head grades varied between 0.1 g/t Au and 4.1 g/t Au. The average grade of all samples was 1.5 g/t Au.

De Grey reported a metallurgical recovery of gold varied between 85% and 98% at an average metallurgy recovery of 93%.

Fresh mineralisation from DD holes

DEG said that an industry accepted circuit comprising sulphide flotation to generate a concentrate prior to oxidation was used to treat fresh mineralisation

The company reported that flotation circuit generated a gold-rich sulphide concentrate that was oxidised using pressure oxidation (POX). The concentrate represented approximately 10% of the ore feed to the flotation circuit. That is, approximately 10% mass pull. Oxidised flotation concentrate from the POX circuit was treated by CIL to recover gold. The company noted that other methods of oxidation will also be assessed during ongoing test work.

It may be noted that POX is an established treatment process used worldwide on large scale gold deposits such as Hemi. Examples of operations that are currently using a POX circuit include: Macraes, Porgera and Lihir in the Asia Pacific region, various Carlin deposits in the USA and Alacer Gold’s Copler Mine in Turkey.

Four composite samples selected

The company reported that four composite samples were selected for initial metallurgical testwork as they contained similar levels of sulphide mineralisation. Composite 1 is partially oxidised and forms part of the transition zone. Composite 4 was selected for the initial assessment due to the largest volume of sample material.

96.3% recovery from composite 4

De Grey said that composite 4 comprised fresh mineralisation taken from across two diamond drill holes HEDD001 and HERC011D at Brolga.

DEG said that the testwork on composite 4 comprised sulphide flotation of a gold-rich concentrate followed by pressure oxidation of the concentrate which is then subjected to conventional CIL to recover gold. In addition, to increase the overall gold recovery, the tailings from sulphide flotation were also subjected to CIL to recover gold not included in the flotation concentrate.

DEG reported that overall gold recovery of 96.3% was achieved on the composite 4 sample with the sample flowsheet including pressure oxidation.

The company said that composites 1 through 3 were tested using flotation and CIL using the same flowsheet with the results of pressure oxidation pending.

The table below shows metallurgical recoveries for these three composites prior to treatment through POX compared with Composite 4, also prior to treatment through POX.

DEG said that a recovery of 99% was achieved through POX treatment for Composite 4 and overall recoveries for Composites 1 to 3 are also expected to be high after POX treatment.

The company noted that the transition sample, composite 1, produced a lower flotation recovery relative to composites 2 to 4, which comprise fresh mineralisation. However, after the flotation tailing was treated using CIL, the overall recovery for composite 1 was greater than 95%. This is a very encouraging result for transition mineralisation and given the head grade of composite 1 was 1.4 g/t Au.

De Grey said that the testwork and optimisation of each stage of processing is ongoing. The company said that further testwork will include optimising the grind size, flotation grade, mass pull, flotation recovery and flotation tail recovery.

Latest updates and next steps

De Grey reported that independent consultant engineers (GRES) are evaluating the results to establish a preferred processing plant, components, design including layout, operating costs and capital requirements.

The company said that further ongoing metallurgical test work is continuing with an additional 6 dedicated diamond core holes with processing and metallurgical testwork underway. DEG said that additional sampling of the other deposits at Hemi is also currently underway.

Management comments

De Grey Mining Managing Director, Glenn Jardine said: “The gold recovery achieved in the initial metallurgical testwork of oxide, transition and fresh mineralisation at Brolga is very encouraging. The testwork significantly de-risks the potential development of Hemi as a Tier 1 gold project in a Tier 1 jurisdiction.

The testwork flowsheet used for Hemi can also be applied to De Grey’s other regional resources in the Mallina Basin.

Hemi is a growing resource and contains a combination of oxide, transitional and fresh sulphide mineralisation. We will continue to increase our understanding of the scale and metallurgical properties of each of these domains across the deposit with ongoing testwork.

Hemi’s excellent location, along a major transport, gas and power corridors, all within 60km of Port Hedland, is a major advantage for a Tier 1 gold resource and development project.”