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Martina Frei:

Composition, Formation and Leaching Behaviour of Supergene, Polymetallic Ores from the Sanyati Deposit (Zimbabwe)

A Case Study

[Zusammensetzung, Bildung und Laugungsverhalten der supergenen polymetallischen Erze der Lagerstätte Sanyati (Simbabwe): Eine Fallstudie]

2011. 227 pages, 102 figures, 36 tables, 21x30cm, 840 g
Language: English

(Sonderhefte Reihe D - Geol. Jahrb., Heft 9)

ISBN 978-3-510-95992-1, paperback, price: 39.80 €

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Keywords

base metalCopper ore polymetallic ore supergene process stratigraphy sampling host rock major element minor element secondary mineral crystal chemistry metal bonding heap leaching Zimbabwe

Contents

Synopsis top ↑

Copper is won from the supergene ore of the polymetallic Sanyati deposit in north-western Zimbabwe in a heap leaching–solvent extraction – electrowinning (HL – SX–EW) process. However, copper recovery is below expectations. The composition, formation and leaching behaviour of the supergene ore was therefore studied using mineralogical (optical microscopy, SEM, XRD), geochemical (XRF, EMPA, AAS, ICP –OES, LA– ICP –MS), and experimental methods in order to unravel the processes responsible for unsatisfactory copper recovery.

The supergene ore bodies of Sanyati developed in a warm humid climate since the Miocene (LISTER 1987) and began to form inselbergs during the Pliocene erosion cycle. The ore bodies represent an immature and therefore very heterogeneous oxidation zone with rudimentary secondary sulphide ore lenses developed at its base. Beside physical (unfavourable grain size distributions) and technical (high compaction of the heap leach pad) aspects, the textural and mineralogical characteristics of the ores lead to metal retention during the heap leaching process. Significant amounts of copper (and other base metals) are retained by the formation of iron oxides and iron oxy-hydroxides (primarily goethite and haematite), which are ubiquitous in the supergene ore.

These "invisible" base metal levels are significantly higher compared to those reported from other deposits (SCOTT 1986, 1992). The observed distributions of base metals (and miscellaneous other elements) in goethite- and haematite-rich decay textures of sulphide minerals demonstrate that they do not contain a geochemical fingerprint of their precursor sulphide phase. Goethite-rich areas of the decay texture are generally enriched in copper (Cu), zinc (Zn) and arsenic (As), as well as selected trace elements (gallium (Ga), germanium (Ge), selenium (Se), silver (Ag), cadmium (Cd) und antimony (Sb)) and are depleted in lead (Pb) compared to haematite-rich areas. The base metal content of goethite and haematite is in the same range in run-of-mine ore and ore from the heap leach pad that has been leached for several years. Extraction experiments on run-of-mine ore show that 19 % of Cu and 6 % of Zn are adsorbed to the surfaces of limonite phases. The remaining 81 % of Cu and 94 % of Zn are fixed in the lattices of limonite phases (predominantly goethite and haematite). Leaching and adsorption experiments show that these phases remain partly undissolved under the conditions used on the heap leach pad (H2SO4, pH 1.5 – 2). Limonite phases reprecipitate from the dissolved component and coprecipitate base metals that are therefore lost as an output of the leaching process.

Inhaltsbeschreibung top ↑

Aus den supergenen Erzen der polymetallischen Lagerstätte Sanyati (NW-Simbabwe) wird im „Heap Laching – Solvent Extraction – Electrowinning –Verfahren (HL – SX – EW)“ hochwertiges Kupfer gewonnen. Das Kupferau s bringen blieb bisher jedoch hinter den Erwartungen zurück. Um genauere Erkenntnis für die Gründe des geringen Ausbringens zu erlangen, wurde die Zusammensetzung, die Bildung und das Laugungsverhalten der supergenen Erze mit mineralogischen (Polarisationsmikroskopie, SEM und XRD), geochemischen (XRF, EMPA, AAS, ICP–OES und LA–ICP–MS) sowie experimentellen Methoden untersucht.

Die Bildung der supergenen Erzkörper erfolgte seit dem Miozän (LISTER 1987). Rezent stehen diese Erzkörper als Inselberge in einer seit dem Pliozän gebildeten Erosionsmorphologie. Die Verwitterungsprofile der Erzkörper sind nur rudimentär zoniert. Die Mineralisation der Oxidationszone ist sehr heterogen und eine Zementationszone ist nur reliktisch als vereinzelte Erzlinsen ausgebildet. Neben physikalischen (ungünstige Korngrößenverteilung) und technischen Gründen (zu hohe Erzkompaktion auf den Laugungsbetten), führen mineralogische und texturelle Eigenschaften zu einer Fixierung von Metallen im Erz. Wertmetalle wie Kupfer (Cu) und Zink (Zn), aber auch Blei (Pb) und Arsen (As) sind an Limonitphasen (hauptsächlich Goethit und Hämatit) gebunden, die Hauptmineralbestandteile der supergenen Erze sind. Diese „unsichtbaren“ Metallgehalte in Hämatit und Goethit sind in Sanyati deutlich höher als die in vergleichbaren Lagerstätten beschriebenen (SCOTT 1986, 1992). Die Elementverteilungen in goethit- und hämatitreichen Abbautexturen von Sulfid mineralen enthalten keinen geochemischen „Fingerabdruck“ der Ausgangssulfide.

Cu, Zn und As (sowie die Spurenelemente Gallium (Ga), Germanium (Ge), Selen (Se), Silber (Ag), Cadmium (Cd) und Antimon (Sb)) sind im allgemeinen in goethitreichen Bereichen der Abbautexturen angereichert, während Bleigehalte (Pb) in hämatitreichen Bereichen erhöht sind. Die Buntmetallgehalte in Goethit und Hämatit sind in Roherz sowie gelaugtem Erz etwa gleich. Extraktionsexperimente mit Roherz ergeben, daß 19 % des Cu und 6 % des Zn adsorptiv an die Limonitphasen gebunden sind. Der restliche Anteil ist im Gitter der Limonitphasen fixiert. Laugungs- und Adsorptionsexperimente zeigen, daß ein Teil der Limonitphasen unter den Bedingungen der technogenen Laugung gelöst wird. Davon wird ein Teil des mobilisierten Cu und Zn durch Copräzipitation mit Goethit und Hämatit jedoch wieder fixiert. Aus diesen Gründen geht ein signifikanter Teil der Wertmetalle dem Ausbringen verloren.

Table of contents top ↑

1 Introduction 9
1.1 Object and aims of this study 9
1.2 Hydrometallurgical base metal production from leachable deposits 11
1.2.1 Supergene deposits, recovery techniques, economic importance: an overview 11
1.2.2 HL–SX–EW, a hydrometallurgical route for Cu recovery 17
1.2.2.1 Pyrometallurgical vs. hydrometallurgical route – advantages and restrictions 17
1.2.2.2 HL–SX–EW process for Cu recovery 18
2 The Sanyati Cu ore deposit 21
2.1 Geographical situation 21
2.2 Geological and morphological setting 22
2.2.1 Regional structural situation 22
2.2.2 Lithostratigraphy 24
2.2.3 Morphological situation 26
2.3 Distribution and composition of primary mineralisation 28
2.4 Distribution and composition of the supergene mineralisation 30
2.4.1 Terminology of supergene mineralisations 30
2.4.2 The Sanyati supergene mineralisation 31
2.5 Mining and beneficiation activities at the Sanyati mine 31
2.5.1 Historical development 31
2.5.2 The mining process 32
2.6 HL–SX–EW at the Sanyati mine 33
3 Field- and laboratory work 35
3.1 Fieldwork 35
3.1.1 Sampling ore bodies and run-of-mine ore dump 35
3.1.2 Sampling of host rock 36
3.1.3 Sampling of the heap leach pad 37
3.2 Analytical and experimental methods 38
3.2.1 Sample preparation 38
3.2.2 Leaching experiments 38
3.2.2.1 Leaching experiments of the water-soluble fraction (V0) 39
3.2.2.2 Leaching experiments of the H2SO4-soluble fraction (VR, V15, V7, V1, V6) 40
3.2.2.3 Partial extraction experiments (V12 – V14) 41
3.2.2.4 Experimental adsorption of base metals to goethite (experiments V8 – V11) 42
3.2.3 Phase analysis (Pol. Mic., SEM, XRD) 42
3.2.4 Chemical analyses (XRF, EMPA, LA–ICP–MS, ICP–OES, F–AAS and G–AAS) 43
4 Weathering products and processes at Sanyati 47
4.1 Weathering products of host rocks with (proximal)
and without (distal) the influence of sulphide decay 47
4.1.1 Mineralogical and geochemical composition of fresh host rock 47
4.1.2 Mineralogical characteristics of weathered host rock distal and
proximal to the zone of sulphide decay 47
4.1.3 Geochemical characteristics of weathered host rocks distal and
proximal to the zone of sulphide decay 49
4.1.3.1 Geochemical changes during weathering processes 49
4.1.3.2 Metal signature 53
4.2 Formation of supergene ore in the oxidation zone 55
4.2.1 Breakdown reactions of primary sulphide ore 55
4.2.2 Composition of the secondary sulphide ore 59
4.2.3 Composition of the supergene ore 60
4.2.3.1 Mineralogical characteristics 61
4.2.3.2 Geochemical characteristics 65
4.2.4 Composition of groundwater in the open pits 71
4.2.5 Neoformation of sulphates in the open pits 71
4.3 Summary of Chapter 4 72
5 Leaching products and processes on the heap leach pads 74
5.1 Characteristics of the run-of-mine ore (ROM) 74
5.2 Composition of the leach pad ore (LPO) 76
5.3 Composition of the acid solution used for leaching 80
5.4 Neoformation of phases during leaching 81
5.5 Summary of Chapter 5 86
6 Composition of goethite and haematite in run-of-mine and leach pad ore 87
6.1 Chrystal chemistry of goethite and haematite 87
6.2 Boxwork texture and chemistry (microprobe analysis) 89
6.2.1 Development, preservation, classification and geochemistry of relict decay textures of sulphides 89
6.2.2 Element distribution in goethite-rich and haematite-rich zones of boxwork textures 101
6.2.3 Concentrations in run-of-mine ore (ROM) and leach pad ore (LPO) 106
6.3 Trace element chemistry of goethite- and haematite-rich zones (laser ablation analysis) 107
6.4 Comparison of element distributions of goethite and haematite in three base metal and lead deposits 109
6.5 Summary of Chapter 6 111
7 Laboratory leaching behaviour of the supergene ore 112
7.1 Leaching experiments with ROM and LPO (H2SO4-soluble fraction) 112
7.1.1 Percolation experiments 112
7.1.2 Leaching experiments under idealised conditions 115
7.1.3 Metal production rates and rate equations 117
7.2 Extraction experiments on goethite- and haematite-rich supergene ores 119
7.3 Adsorption experiments of copper onto goethite 121
7.4 Summary of Chapter 7 123
8 Formation of “invisible” base metal concentrations in supergene goethite and haematite and their consequences for the leaching process 124
8.1 Development of the oxidation zone in Sanyati and the composition of the supergene ores 124
8.2 Distribution of base metals in sulphide decay textures – colloform textures as a proxy for the "invisible" base metal contents in goethite and haematite 126
8.3 Fixation of base metals by goethite and haematite 128
8.3.1 Adsorption to goethite and haematite 128
8.3.2 Lattice incorporation of base metals in goethite and haematite 133
8.3.3 Jarosites/plumbojarosite 134
8.4 Consequences of the base metal retention on the extraction success of heap leaching 134
9 Conclusions and general perspective 137
10 References 141
Annexes 153
Annex A: List of abbreviations and quality control of geochemical methods 155
Annex B: Sample lists and results of XRF analysis 161
Annex C: Results of the laboratory experiments 193
Annex D: Results of EMPA and LA–ICP–MS 207
Annex E : Results of the chemical composition
of the leaching acid 225