The Oxidized Ores of the Main Sulfide Zone, Great Dyke, Zimbabwe: Turning Resources Into Minable Reserves

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The oxidized ores of the Main Sulfide Zone, Great Dyke, Zimbabwe: Turning resources into minable reserves – mineralogy is the key !

Thomas Oberthür Frank Melcher, Malte Junge, Marek Locmelis, Peter Buchholz, Herwig Marbler, Dennis Kraemer, Gila Merschel, and Michael Bau Outline of Talk

The Great Dyke of Zimbabwe

Main Sulfide Zone, Great Dyke

Pristine ores - Geochemistry - Mineralogy

Oxidized ores - Geochemistry - Mineralogy - Metallurgical options

4 Hartley Mine

Ngezi Mine

Unki

100 km Mimosa Mine

Mafic Sequence

Ultramafic Sequence

1 Stratigraphy of the Great Dyke

upper Mafic Sequence, + 1150 m middle lower Gabbros, Norites, Gabbronorites MSZ Bronzitite Ultramafic Sequence, + 2300 m succession Dunites/Serpentinites, Harzburgites, Olivine-bronzitites, Pyroxenites, Dunite Websterite succession Chromitites

1 Great Dyke, Zimbabwe

MSZ

1 Pt Sulfide,

Pd metal subzone Base Ni, Cu PGE subzone PGE

Vertical zonation pyroxenite or “offsets“ Main Sulfide Zone, Mimosa Mine, Great Dyke (Prendergast; 3 Wilson) cpy po

moncheite

sperrylite cpy

Mimosa MIM-603 Hartley, SD-04

Hartley Mine GD-11-D

MoS2 moncheite

cooperite pn Platinum-group (contains up to 3000 ppm Pd !) minerals po (PGM) cpy 50 µm Pt-Fe alloy PtS Grains from concentrates

(electric pulse, Univ. Leoben)

PtAs2 (Pt,Pd)(Te,Bi)2 PGM - Main Sulfide Zone

• Cooperite - Braggite • Moncheite • Maslovite • Sperrylite • Merenskyite • Michenerite • Rh-Ir-Ru-Pt-sulfarsenides • Kotulskite • Laurite • Pt-Fe-alloy • "PtSnS"

4 ~ 70 % are Pt-minerals ! PGM by dominant PGE (%)

Pt 67.7

Ir Ru Rh Pd 2 3.7 19.8 6.6

n = 781

2 Regional variation – PGM proportions – Great Dyke

Bi,Te > As

Hartley (Pt,Pd)-bismuthotellurides

Mhondoro Pt-Fe alloys

Ngezi

Unki others

Mimosa PtAs2 (PGE)-sulfarsenides

As > Bi,Te Pd in pentlandite (ppm) example : Unki

10 Pt peak = BMS-1 13

19 Pd peak 310 310 cm

28 Base of MSZ 31 0 500 1000 1500 2000

R-13 ~ 80 % of the Pd is hosted in pentlandite !

1 Distribution of PGE

PtFe Laurite [RuS2] PGE are bimodally distributed

– Pt, Os, Ru: discrete PGM

– Pd, Rh, Ir: in sulfides (mainly pentlandite)

13 PGM / PGE in the exogenic cycle

MSZ MSZ PGM pristine oxidized

(Pt,Pd)(Bi,Te)* 30-70% PGE oxides +

PtAs2 ~ 25 % (Pt,Pd,Ni)S ?

Pd in pentlandite ?? ~ 80% of Pd total Oxide ores • Great Dyke ~ 250 Mt of resources ! • Bushveld some 100 Mt (Platreef, MR, UG-2)

• Characterization of the mineralogy of the PGE • Development of new processing technologies (“conventional“ PGE recovery only ≤ 30% ! )

3 Oxide ores, Hartley Platinum Mine

Resources: ~ 250 Mt of ore !

1 Hartley Mine, Zimbabwe 1 Ngezi test pit Sampling “Old Wedza“ (Mimosa) MIMOSA (MIM 08-13) Martin Prendergast‘s outcrop ppm / ppb 0 1000 2000 3000 4000 5000

Cu Ni

0.75

1 Oxide Ores

• Geology • Geochemistry • Mineralogy NGEZI (NRC146)

ppm / ppb ppm / ppb 0 1000 2000 3000 4000 5000 1000 2000 3000 4000 5000 6000

Ni 2 Cu

3 Pt Pd Cu 4 Au Ni

5 1,45 m 1,45 Pt 8.00 m 6

Au Pd 7

Oxidized MSZ Oxidized MSZ Hartley Mine (HOP-20x) Ngezi (NRC 146)

1 Pt

Pt/Pd = 2.43 (n=9)

oxide

MSZ Pt/Pd = 1.28 (n=12) sulfide

Pd Au Geochemical trends: Sulfide ores  oxide ores

1 2 MSZ: gains/losses during weatheringduring MSZ: gains/losses HOP - 20x 20x CD vs.

relative change [%] - -100 02+SD 100 -50 50 0 - 04 Si Ti Al Fe Mn gain Mg Ca Na K S LOI 234 V Cr After After (1967).Gresens Al = constant Co Ni loss Cu Zn (Os) Ir Ru Rh Pt Pd Au Oxide Ores

• Geology • Geochemistry • Mineralogy

Sample Depth Degree of PtAs2 (Pt,Pd) Pt-Fe (Pt,Pd)  PGM Oxide minerals Sulfides metres oxidation S alloys (Bi,Te)

MHR 4-4.5 1 1 - - 2 Goethite, chromite None 194 MHR 6.5-7 - - 5 - 5 Goethite, hematite, None 195 magnetite, chromite MHR 10-10.5 9 - 1 1 11 Goethite, hematite, Sparse relicts 196 magnetite, rutile in silicates MHR 15-15.5 4 12 - 28 44 Magnetite, Po, pn, cpy, py. 197 chromite Incipient oxidation at rims

2 Sperrylite SEM image Relict PGM

Sperrylite BSE image polished section

50 µm

smec HOP-102 HOP-103

goe

50 µm Cooperite SEM image opx

cpx Relict sulfides

talc

Relict pentlandite: 127 ppm Pd up to 6500 ppm Pd up to 450 ppm Pt cpy

1 HOP-206 Replacements coop

mich

mer

Replacement of (Pt,Pd)(Bi,Te)*

HOP-206 (5910a) Ngezi Adit A (5711b) PGE - “Oxides“ Ngezi Mine

Complete replacements (or neoformations ?)

Ngezi Adit A (both 5711b) talc

cpx

opx mag Fe- serp mag Pd-S FeOOH

Pt-S +

100 um 50 um

PGE in FeOOH, Mn-Co-OH, smectites

(Pt, Pd) Sn opx amph 3 ca. 200 ppm Pt Mn-Co-Ni-OH

Pd-Pt smectite Pt Ni-Cu-cl

20 um 20 um Mimosa Mimosa

Mn-Ni-Co-OHx = 200-400 ppm Pt ! FeOOH Mn-Ni-Co-OH

Ni-chl 50-80 ppm Pd

Opx (Pd,Pt) <1 µm Pt(Fe?) <1 µm

FeOOH FeOOH = (<15 ppm Pd, Pt) 50-80 ppm Pd !

Pt(Fe?) <1 µm

PGE in FeOOH

and in Mn-Co-OH Ptx-neoformation ? 2 Pt and Pd in Mn-Co and Fe-hydroxides

Hydroxides

Pt-bearing 100000 Fe hydroxides Mimosa

10000 Mn-Co hydroxides MIM-11 Hartley MIM-12 HOP-206 HOP-207 1000 HOP-209

Pt Pt ppm MIM-11 Mn Mn-Co hydroxides Mimosa HOP-207 Mn HOP-209 Mn 100 MIM-10 Mimosa Hartley

10 10 100 1000 10000 Pd ppm

Mimosa and Hartley Mineralogy of PGE/PGM in oxide ores

1. Relict PGM 2. Relict sulfides 3. PGE-oxides/hydroxides replacing PGM

4. Ptx (neoformation) 5. PGE in FeOOH 6. PGE in Mn-Co-OH 7. PGE in silicates (smectites)

Mineralogical balance ? Pt distribution MSZ sulfide ores oxidized ores bimodal polymodal

1) discrete PGM ~ 25% PtAs2 & (Pt,Pd,Ni)S (Pt,Pd)(Bi,Te)* PGE - “oxides“ > 95% Ptx (neoformation) Pt > Pd in FeOOH Pt > Pd in Mn-Co-OH 2) Pt in pn and py Pd > Pt in smectites < 5% ~ 75%

3 Pd distribution

MSZ sulfide ores oxidized ores bimodal polymodal

1) discrete PGM (Pd,Pt,Ni)S ~ 10% (Pd,Pt)(Bi,Te)* ~20% PGE - “oxides“ Pt > Pd in FeOOH Pt > Pd in Mn-Co-OH 2) Pd in pentlandite Pd > Pt in smectites

~80% ~ 40% ca. – 50%

4 PGM / PGE in the exogenic cycle

MSZ MSZ PGM pristine oxidized

(Pt,Pd)(Bi,Te)* 30-70% PGE oxides +

PtAs2 ~ 25 % (Pt,Pd,Ni)S ?

Pd in pentlandite ?? ~ 80% of Pd total

1 Metallurgical options for oxide ores

Method Recovery • Grainger Bros. (1926) Bulk - Gravitation << 50% (?) • Hartley Mine (1997-99) Bulk - Flotation 30 % (?) • TML Process BHP – lab scale Bulk – Roasting (300-425°C)

(1992/1994) Br° in NaBr in H2SO4 Pt = 85 % leach (70°C, 2h) Rh = 65 % CIP Au = 95 % problems: costs of chemicals; aggressive acids; environmental issues

• Evans Bulk – various/hydrometallurgy ? (2002) – pyrometallurgy (ConRoast) ?

3 Metallurgical options for oxide ores

BGR – (1) Establish mineralogical balance of PGE in the ores – (2) Metall. tests → mechanical pre-concentration (?)

Step-wise crushing & milling Electric pulse disintegration Sieving Gravity separation Magnetic separation Preconcentration Selective leach

1 Metallurgical options for oxide ores

BGR – previous work

Preconcentration Selective leach NO

Current work: Total leach experiments (JU Bremen) - hydrometallurgy - (1) one-step leach using siderophores: < 1% Pt recovery (2) two-step leach, inorganic acids + siderophores, unbuffered: 20 – 40 % Pt recovery (3) Bio-leaching (ongoing work, BGR labs) (4) Inorganic acids – ongoing work – BGR/DERA + JU Bremen Conventional processing of oxidized PGE ores problematic due to:

missing BM sulfide association (Oberthür et al., 2013)

Occurrence of naturally-floating Concentrate dilution; gangue (Becker et al. 2014) 15-30% Recovery Polymodal distribution of PGE (Oberthür et al., 2013) Therefore

Hydrometallurgical leaching experiments with organic and inorganic lixiviants (Jacobs University Bremen, Germany) Leaching experiments with siderophores (biologically produced organic ligands)

Siderophores / Metallophores:

• low molecular weight organic molecules

Structure of desferrioxamine B (DFO-B) • exuded by plants (phytosiderophores) and Yoshida et al. (2004) bacteria (microbial siderophores) to cope with nutrient deficiency or toxicity in natural systems

• In modern environments mainly produced to bind and mobilize Fe(III), which is needed as a micronutrient

But: High affinity for platinum and palladium Holmes et al. (2005) Leaching experiments with siderophores (biologically

produced organic ligands) Oxidized MSZ Oxidized Platreef Great Dyke Bushveld 100 Oxidized MSZ (Great Dyke): 80 • Pt recovery up to ~80%

• Selective leaching possible 60 • Low Pd and Ni mobilization

Oxidized Platreef (Bushveld): leached[%] • Very low recoveries 20 irrespective of leaching time, reagent concentrations, preleach, etc. 0 Pd Pt Ni Pd Pt Ni [%] leached by hydrochloric acid pretreatment (24h) [%] leached by siderophores (48h) [%] remaining in solid residue Siderophore experiments demonstrate principal viability of bioleaching ! (leaching with biogenic compounds) ! for the treatment of oxidized PGE ores Leaching experiments with inorganic lixiviants

Oxidized MSZ Oxidized Platreef Great Dyke Bushveld 2B-Platinum Leach: 100 • Leach based on inexpensive inorganic reagents 80 • Initial lab-scale tests indicated very 60 promising leaching efficiencies

[%] leached [%]

0 Pd Pt Ni Pd Pt Ni [%] remaining in solid residue [%] leached by 2B-platinum leach (60°C; 1h) Test of applicability in optimization and upscaling experiments Samples totalling 150 kg of oxidized PGE ore Heap ore from the Mogalakwena Mine, South Africa Heap of oxidized PGE http://southafrica.angloamerican.com ore: 6 mio tonnes

Nickel, Cr, Au, Pt and Pd grades of the oxidized PGE ore samples used for Samples were only optimization and upscaling experiments crushed, Pt Pd Ni Cu Cr [µg/kg] [µg/kg] [mg/kg] [mg/kg] [mg/kg] one sample was milled

Average (n=6) 1184 924 2050 2088 798 First results – upscaling experiments

2B-Platinum leach:

Upscaling experiments conducted on oxidized heap ore

• Experiments at 20°C, 40°C and 80°C

• Time series: samples taken after 1h, 2h, 4h, 8h

• Upscaling experiments:  Up to 78% Pt and 55% Pd into solution Extraction of PGE from oxide ores by hydrometallugical methods

Advantages:

1. Cheap and simple surface mining Challenges: 2. Pt + Pd are amenable to leaching by various acids 1. Heterogeneous mobilization of PGEs from oxidized PGE ores 3. Heap-leach/ tank-leach is possible → need for better understanding of 4. Inexpensive reagents mineralogy and PGE deportment 5. Good/acceptable recoveries 2. Optimization of process 6. Expensive processing steps like (pre-treatment, one or multiple step fine-milling, flotation, smelting, leach (modified Kell process?), matte-leaching are not needed chemicals, temperature, etc.)

Hydrometallurgical leaching technologies may be applicable to other oxidized ore deposits Ex Africa semper aliquid novi

….PGM grains…. weathered PGE ores - MSZ

weathered pristine ore ore

Evans 2000

Processing of oxidized PGE ores

 Surficial ore - cheap mining  Large portions of oxidized ore already excavated and on heaps  Heap ore broken and classified

New approach: Hydrometallurgical treatment of oxidized ores Leaching experiments with organic and inorganic lixiviants (Jacobs University Bremen, Germany)