The Influence of Textural Variation and Gangue Mineralogy on Recovery of Copper by Flotation from Porphyry Ore – A Review A F Cropp1, W R Goodall2 and D J Bradshaw3 ABSTRACT Mineralogy and critically the texture of both the ore and gangue phases are of paramount importance in the flotation recovery of copper from porphyry ores. Mineralogy and texture will define the theoretical grade recovery curve for a feed ore. Actual grade-recovery falling below this may be due to texture and gangue influences, whilst desired grade-recovery above this line will not be possible unless the characteristics of the feed are altered. This review discusses examples of porphyry copper deposits where process mineralogy has been used to characterise these textural and gangue influences on recovery by flotation. More specifically, the influence exerted by grain size, liberation, association and elemental distribution is explored. The implications of these on overall plant philosophy and operation are highlighted, along with the subsequent potential impact on recovery.
INTRODUCTION Porphyry copper deposits are of critical importance to the global supply of copper (and other metals), and it is estimated that up to 60 per cent of the world’s copper comes from these deposits (Sinclair, 2007). Deposits are typically very large (Keeney, Walters and Kojovic, 2011) and can have over a billion tonnes of ore (McMillan and Panteleyev, 1980), with copper grades running typically around 0.2 to two per cent. Sillitoe (2010) provides an excellent global overview of porphyry deposits (Figure 1). Porphyry copper ores can be considered to have four key zones: potassic, sericitic (phyllic), argillic and propylitic. These may or may not be overprinted by subsequent weathering, leaching and supergene enrichment. Each of these zones and alteration envelopes display different mineralogy and texture, and therefore different processing characteristics. An understanding of these can be used to predict likely flotation response, and then to guide relevant test work to validate the theories, feed the geometallurgical block model and refine the operation. Linking the macro (ie metres to kilometres orebody and block model scale) knowledge of geology and features with those of the micro (ie micrometre to millimetre mineral textures and mineralogy) textures and other influences on flotation will help: •• understand and predict copper flotation concentrate grade and recovery
•• understand and predict causes of dilution to concentrate or losses to tailings •• inform identification of test work. This paper gives a brief overview of the general structure and geometry of porphyry copper deposits to provide context to more focused discussion on the control exerted by ore and gangue texture and mineralogy. Seven mineralogical factors influencing copper recovery to the flotation concentrate are discussed, namely: fine-grained copper minerals; locked copper minerals in composite particles; surface coatings on valuable minerals; gangue recovered in composite particles; gangue recovered through entrainment; floating gangue minerals with activated surfaces; and deleterious elements distributed in copper minerals. Other aspects of processing porphyry ores are covered more specifically elsewhere for other processes such as comminution (Yildirim et al, in press) or hydrometallurgy (Fander, 1994; Baum, 1996; Baum, 1999; Allen et al, 2007), and are therefore not considered in the scope of this review. The textural and gangue influences presented here are done so within the context of an idealised porphyry copper model. In reality, however, different formation and alteration histories mean porphyry ores around the world will vary from this model. They will each have their own unique combination of challenges; however, there are many common threads, such as those discussed here.
1. MAusIMM, Senior Consultant, MinAssist Pty Ltd, Suite 246, 135 Cardigan Street, Carlton Vic 3053. Email:
[email protected] 2. MAusIMM, Principal Consultant/Director, MinAssist Pty Ltd, Suite 246, 135 Cardigan Street, Carlton Vic 3053. Email:
[email protected] 3. MAusIMM, Professor, Julius Kruttschnitt Mineral Research Centre, University of Queensland, 40 Isles Road, Indooroopilly Qld 4068. Email:
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FIG 1 - Global map of the location, age, deposit type and principal metals of major porphyry copper deposits (Sillitoe, 2010).
STRUCTURE AND GEOMETRY Porphyry orebodies are dominantly structurally controlled, with condensation of supercritical fluids derived from a crystallising magma reservoir resulting in porphyritic intrusions (Berger et al, 2008). These are typically intermediate to acidic intrusive rocks, often granitic or dioritic, and the accompanying fluids result in a stockwork of fine fractures and veins (Titley, 1995). Mineralisation commonly centres around this quartz- and monazite-rich stockwork, and around intrusive dykes and breccia zones (Petruk, 2000; McMillan,
1991). Successive concentric zones of hydrothermal alteration, each with distinct mineralogy, typify the basic structure of a porphyry deposit. These zones have a central potassic core that may be associated with the bulk of the primary ore, surrounded progressively by sericitic (phillic), argillic and propylitic zones (Lowell and Guilbert, 1968). Often overprinting this is later supergene enrichment resulting from weathering and oxidation (Bulatovic, Wyslouzil and Kant, 1998; Petruk, 2000). Figure 2 aims to summarise these in an idealised model, although depth of emplacement, composition of intrusion(s) and host rocks,
FIG 2 - Idealised structure of a porphyry copper deposit. Overprinting the basic zonation is alteration from weathering and leaching, and a potentially enriched supergene blanket. An indication of the typical depth is also provided (modified from Guilbert and Lowell, 1974). 280
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A
B
C
FIG 3 - Example of a north-south section of zonation, mineralisation and subsequent supergene alteration at La Escondida (Robb, 2005). fluids, pressure, time, water table, weathering and erosion give each deposit a unique set of characteristics. Figure 3 presents an example of a cross-section of the La Escondida deposit from (Robb, 2005), highlighting the structure, zonation, alteration and mineralisation. Skarn-type deposits can also often be found associated with porphyry deposits, and whilst these can be large (they can exceed 1 Bt) they are typically smaller in volume than the porphyry ore. Butte Montana, Bingham Canyon Utah and Ok Tedi Papua New Guinea (PNG) are all examples of porphyry deposits with closely associated skarns, which in some cases are mined and processed together (British Geological Survey, 2007).
Zonation The four zones common to most porphyry copper deposits are those of the potassic, sericitic (phillic), argillic and propylitic, although they may not always be present and/ or there may be other zones (Lodder et al, 2010; Jorge, 1998; John, 1978; Cooke et al, 2006). These distinct zones with their characteristic mineralogy have different processing requirements, and (Yildirim et al, in press) provide a table summarising the principal alteration-mineralisation types and some of their processing implications (with specific reference to grinding and flotation), reproduced here in (Table 1):
TABLE 1 Principal alteration types with typical mineralogy, impact on a grind-float circuit and inferred economic potential (Yildirim et al, in press). Alteration type (alternative name)
Position in system (abundance)
Key minerals
Principal sulfide assemblages (minor)
Economic potential
Expected effect on mill throughput
Expected effect on flotation
Potassic (K-silicate)
Core zones of prophyry Cu deposits
Biotite, K-feldspar
Pyrite, chalcopyrite, bornite, digenite and chalcocite
Main ore contributor
High
Low
Chloritie-sericite (sericite-claychlorite)
Upper parts of porphyry Cu core zones
Chlorite, sericite/ illite, hematite
Pyrite, chalcopyrite
Common ore contributor
Moderate
Moderate
Sericite (phyllic)
Upper parts of porphyry Cu core zones
Quartz, sericite
Pyrite, chalcopyrite, enargite, tennantite, bornite, sphalerite
Weak to moderate ore contributor
Moderate
Moderate
Advanced argillic
Above prophyry Cu deposits, constitutes lithocaps
Quartz, alunite, pyrophyllite, dickite, kaolinite
Pyrite, enargite, chalcocite, covellite
Locally constitutes ore
Low
High
Marginal parts of systems below lithocaps
Actinolite, heamatite, magnetite
Pyrite, sphalerite, galena
Barren, unlikely to be ore grade
N/A
N/A
Deep, including below prophyry Cu deposits (un-common)
Albite/oligioclase, actinolite, magnetite
Typically absent
Barren, unlikely to be ore grade
N/A
N/A
Propylitic
Sodic-calcic
Core photographs
Note: the photographs are taken from Highland Valley Copper mine (British Columbia, Canada), except the sodic-calcic alteration, which is from a Cu porphyry prospect in Turkey. THE SECOND AUSIMM INTERNATIONAL GEOMETALLURGY CONFERENCE / BRISBANE, QLD, 30 SEPTEMBER - 2 OCTOBER 2013
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•• Potassic zone – this central potassium silicate core is typified by an assemblage of secondary biotite-muscovitesericite-orthoclase feldspar-magnetite, found associated with most of the ore in the stockwork, and along veins and fractures. The biotite is often accompanied by chlorite, with the chlorite diminishing and the biotite getting coarser closer to the core. Chlorite, quartz, phlogopite, anhydrite and pyrite may also be found. Chalcopyrite and bornite are the primary copper minerals, and pyrite may be present. Being close to the core and typically deeper, this zone is often subjected to less weathering and supergene alteration than shallower zones (depending on depth of erosion); so much of the primary ore mineralogy and rock structure of the host and porphyry is often retained. •• Sericitic (phyllic) zone – the typical assemblage in this zone is quartz-sericite-pyrite-chlorite. Sericitisation is often associated with main orebody, and the veins are typically filled with quartz and other silicates. Flaky sericite is commonly found around these veins in strong association with kaolinite and montmorillonite, and more rarely with calcite and epidote. High quantities of pyrite can be found. Copper is typically found in bornite (and to a lesser degree chalcopyrite), and a suite of secondary copper minerals including enargite and tennantite. •• Argillic zone – this clay-rich zone contains a quartzalunite-kaolinite assemblage, with feldspars (orthoclase), biotite and hornblende (commonly altering to sericite). Montmorillonite, illite, epidote and minor chlorite may also be found. The ore and mineralogy of the argillic zone may (or may not) have undergone either weathering and leaching (lowering grade); or supergene enrichment (improving grade), depending on depth and the location of the water table. •• Propylitic zone – a larger range of alteration minerals are presented in this zone due to the typically shallower depth and resultant higher degree of weathering, with chloriteepidote-calcite assemblage being typical, along with feldspar altering to sericite, and other clays. Epidote is characteristic of this zone and is often found in veins along with calcite. Magnetite commonly alters to hematite. The propylitic zone is unlikely to contain copper at ore grade, and is therefore not discussed in detail in this paper.
Supergene overprinting Supegene alteration of the porphyry structure through weathering, oxidation and groundwater leaching results in changes of the mineral assemblages, and can lead to potential supergene enrichment. It can also be responsible for an increase in the amount and type of clay minerals encountered. The upper portions of porphyry copper deposits have commonly intensely oxidised (Hartley and Rice, 2005) and leached, with metals being dissolved into solution and carried deeper into the deposit where they may be precipitated in concentration; area is typically referred to as the ‘leached cap’. Primary mineral replacement is common, clay minerals are abundant and the rock structure and strength may be depleted; making milling easier but potentially increasing fines and reducing the rate of flotation. Below this, an envelope of supergene enrichment may form when metals from the upper oxidised cap have been dissolved, transported down and precipitated below the water table. If this occurs, this supergene ‘blanket’ can comprise a major portion of the ore; for example, at La Escondida (Chile) this represents around 65 per cent of the copper with grades up to 3.5 per cent (Augustithis, 1994; 282
Berger et al, 2008). It should, however, be noted that not all instances of oxidation will result in the formation of a supergene cap, such as at Afton (Canada), where oxidation did not improve Cu grade (Kwong, 1987). Figure 2 is used to indicate how the oxidised and supergene zones can overprint the original porphyry structure, influencing mineralogy and processing characteristics. The depth of the zones, level of the water table, extent of weathering and leaching, and any subsequent alteration will influence which zones are actually overprinted and to what degree, so this is an idealised model. The zones and alteration, and the overall grade distribution, leads commonly to an inability to selectively mine zones (Butcher, 2009), so different ores may be processed together.
Skarn Broadly there are two categories of skarn: endoskarns, which developed within an igneous intrusion, and exoskarns, which formed within sedimentary rocks. Skarns are defined by their mineralogy, which include a variety of calcite and calciumbearing silicate minerals, typically garnet and pyroxene dominated. Phylosicilicates such as talc are common, and can present challenges to flotation rates. The polymetallic replacement found in skarns will often surround the main intrusive porphyry complexes with radiating offshoots (Berger et al, 2008).
CHARACTERISTIC MINERALOGY The mineralogy of porphyry copper deposits is varied and Schwartz (1966), McMillan and Panteleyev (1980), Petruk (2000), Sinclair (2007) and Sillitoe (2010) provide excellent mineralogical references, whilst Berger et al (2008) provide a mineralogical summary of specific deposits around the world. Titley (1995) presents a description of the primary rock type hosts (silicate and carbonate) and the influence they have on resultant mineralogy. Table 2 aims to summarise the key minerals commonly found in the three main ore zones: potassic, sericitic and argillic.
ORE TEXTURES Geological setting and alteration history will influence the texture of the ore. Amstutz (1961) introduces the concept of classifying ore grain textures and linking them to processing characteristics (Table 3), whilst Butcher (2010) describes the size distribution of ore minerals as either equigranular or inequigranular and provides an example of textural change through oxidation (Figure 4). By way of clarification, in this paper a ‘grain’ is classed as a single mineral, whilst a ‘particle’ is made up of one or more mineral grains. In Table 3, for example, microstructure Type 1b shows a single particle containing five mineral grains (four black grains and one white grain). These examples highlight how ore texture will directly influence the: •• ideal particle size for the flotation feed •• degree of liberation of the target mineral(s) at that ideal particle size •• phase-specific surface area of the target mineral(s) available for bubble interaction •• amount of fines •• number and type of composite particles. Evans (2010) expands on these textural variations, pointing out that in considering ore texture there may well be more than one ore mineral of interest due to the deportment. Copper in porphyry deposits, for example, may deport to
THE SECOND AUSIMM INTERNATIONAL GEOMETALLURGY CONFERENCE / BRISBANE, QLD, 30 SEPTEMBER - 2 OCTOBER 2013
THE INFLUENCE OF TEXTURAL VARIATION AND GANGUE MINERALOGY ON RECOVERY OF COPPER BY FLOTATION FROM PORPHYRY ORE
TABLE 2 Overview of some minerals common to porphyry copper deposits, with an indication of the zone(s) in which they typically occur (compositions from http://mindat.org). Assemblage Primary Cu-bearing minerals
Secondary Cu-bearing minerals
Other sulfides
Gangue
Typical minerals
Chemical composition
Potassic
Sericitic
Chalcopyrite
CuFeS2
Bornite
Cu5FeS4
Diginite
Cu9S5
Enargite
Cu3AsS4
Tennanite
(Cu,Fe)12As4S13
Chalcocite
Cu2S
Covellite
CuS
Malachite
Cu2(CO3)(OH2)
Native Cu
Cu
Cuprite
Cu2O
Enargite
Cu3AsS4
Chrysocolla
(Cu,Al)2H2Si2O5(OH)4•n(H2O)
Pyrite
FeS2
Sphalerite
ZnS
Galena
PbS
Biotite
K(Mg,Fe)3(Al,Fe)Si3O10(OH,F)2
K- Feldspar
KAlSi3O8
Muscovite
KAl2(AlSi3O10)(OH)2
Sericite
KAl2Si3O10(OH)2
Quartz
SiO2
Chlorite
(Mg,Fe)6Al2Si2O10(OH)8
Illite
K0.65Al2.0[][Al0.65Si3.35O10](OH)2
Phlogopite
KMg3(AlSi3O10)(OH,F)2
Anhydrite
CaSO4
Actinolite
Ca2(Mg,Fe)5Si8O22(OH)2
Epidote
Ca2Al3(SiO4)3(OH)
Albite
NaAlSi3O8
Kaolinite
Al2Si2O5(OH)4
Montmorillonite
(Na,Ca)0.33(Al,Mg)2(Si4O10(OH)2.nH2O
Calcite
CaCO3
Dolomite
CaMg(CO3)2
Alunite
KAl3(SO4)2(OH)6
Pyrophillite
Al2(Si4O10)(OH)2
Hornblende
Ca2(Mg,Fe)4Al(Si7Al)O22(OH,F)2
Diaspore
AlO(OH)
Dickite
Al2(Si2O5)(OH)4
Tourmaline
(Ca,K,Na)(Al,Fe,Mg,Mn,Li)3(Al,Cr,Fe,V)6(BO3)Si6O18(OH,F)4
Corundum
Al2O3
Topaz
Al2(SiO4)(F,OH)2
Magnetite
Fe Fe2 O4
Hematite
Fe2O3
2+
3+
Argillic
Indicate common mineralogy; indicate less common mineralogy. THE SECOND AUSIMM INTERNATIONAL GEOMETALLURGY CONFERENCE / BRISBANE, QLD, 30 SEPTEMBER - 2 OCTOBER 2013
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TABLE 3 Examples of various common ore textures that can impact processing, from Evans (2010) based on the work by Amstutz (1961). Type
Typical microstructure
Description
Type 1a
Simple intergrowth or locking type; rectilinear or gently curved boundaries. Most common type, many examples.
Type 1b
Mottles, spotty or amoeba-type locking or intergrowth. Simple common pattern; many examples.
Type 1c
Graphic, myrmekitic or eutectic type. Common; examples: chalcopyrite and stannite; quartz and feldspars, etc.
Type 1d
Disseminated, emulsion-like, drop-like, buckshot or peppered type. Common; examples: chalcopyrite in sphalerite; and sericite etc in feldspars.
Type 2a
Coated, mantled, enveloped, corona-, rim-, ring-, shell- or atoll-like. Examples: chalcocite or covellite around pyrite, sphalerite, galena.
Type 2b
Concentril-spherulitic or multiple shell type. Fairly common; examples: uraninite with galena, chalcopyrite, bornite.
Type 3a
Vein-like, stringer-like or sandwichtype. Common; examples: molybdenitepyrite; silicates; carbonates; phosphates, etc.
Type 3b
Type 3c
Lamellae, layered or poly-synthetictype. Less common; examples: pyrrhotite-pentlandite; chlorites-clays, etc. Network, boxwork or Widmanstattertype. Less common; examples: heamatite-ilmenite-magnetite; bornite or cubanite in chalcopyrite etc.
multiple minerals, as is demonstrated by Evans’ modified sketch from Butcher (2010) (Figure 5), each of which may have equigranular or inequigranular distribution, have multiple textures and/or have undergone later alteration; leading to further textural complexities.
SEVEN MINERALOGICAL FACTORS INFLUENCING COPPER-RECOVERY TO THE FLOTATION CONCENTRATE Texture of particles in a float feed plays a pivotal role in both Cu- recovery and grade in the concentrate. Theoretical curves can be generated to indicate the maximum grade-recovery possible for a given feed ore, and if the actual concentrate plots below this curve (as inevitably happens), this is either a result of feed texture and mineralogy or the operating conditions of the flotation circuit (Figure 6). A fundamental limit is set by this theoretical grade-recovery curve; no amount of changes to plant conditions (reagents, pH, etc) will improve the flotation concentrate Cu grade-recovery beyond this curve, due to the physical limitation set by the texture 284
FIG 4 - Ore textures can be equigranular (right) with grains of similar size, or inequigranular where ore minerals have two or more size distributions (left and centre). The bottom row of examples show of how pristine (top row) textures may be altered by groundwater and oxidation (Butcher, 2010).
FIG 5 - Example of how copper deportment across multiple minerals and textures will directly influence target grind size and liberation potential (Evans, 2010). of the feed ore (and in particular liberation, surface area and deportment). To improve the grade or recovery beyond this theoretical curve, changes need to be made to the feed; for example to increase liberation. The operating conditions of the flotation circuit are not considered in this discussion, but the typical influences of mineralogy and texture have been summarised in Table 4; each of which is then discussed in further detail in following sections: •• Coarse particles can be composites (ie contain multiple grains) of copper-bearing minerals and gangue minerals. Recovering these will increase recovery but lower the grade, as the locked gangue will, by necessity, also be recovered. Rejecting these will mean that grade is not compromised but the target grains will be lost to tailings, thereby reducing overall recovery. •• Fine particles largely comprise liberated grains. Fine ore particles can have a lower chance of particle-bubble collision and therefore have a reduced chance of being recovered. Very fine gangue material may be collected by entrainment and recovered to the final concentrate. •• Middling particles are typically considered to be fully or partially liberated, and therefore be amenable to separation in a flotation circuit, with the ore minerals being recovered without significant degradation of grade; however, this is not always the case. Surface coatings can reduce the susceptibility of an ore mineral’s surface to bubble adherence, reducing recovery, whilst concentrate grade may be lowered when the surface of gangue minerals become activated (for example by Cu ions) causing them to float.
THE SECOND AUSIMM INTERNATIONAL GEOMETALLURGY CONFERENCE / BRISBANE, QLD, 30 SEPTEMBER - 2 OCTOBER 2013
THE INFLUENCE OF TEXTURAL VARIATION AND GANGUE MINERALOGY ON RECOVERY OF COPPER BY FLOTATION FROM PORPHYRY ORE
FIG 6 - Left: ore texture defines the theoretical grade recovery curve. Particle images are used to show how high target mineral recovery will typically also mean recovery of gangue minerals, reducing the grade. Right: if actual grade/recovery is less than the theoretical, then operational conditions may be changed to improve this (1). If grade/recovery above the theoretical curve is required, then the texture of the feed will need to change (2). TABLE 4 Summary of seven common causes for lower than anticipated Cu grade or recovery in the flotation concentrate. The graph is used to highlight each of these and how particle size relates to ore or gangue material. Losses to tailings
Recovery by size: losses/dilution
1) Fine Cu-minerals 2) Locked Cu-minerals 3) Surface coatings on valuable minerals
Dilution in concentrate 4) Gangue composites 5) Entrained gangue 6) Activated gangue 7) Deleterious element distribution
1. Fine copper minerals MacDonald et al (2011) describe how common copper minerals larger than 50 µm are at Kennecott. Primary copper sulfide grains of chalcopyrite and bornite can be as large as 1 mm in some disseminated porphyry ores; however, most are smaller (Anderson, Scholz and Strobell, 1955). Indeed a considerable percentage (up to 70 per cent) of chalcopyrite and bornite can occur in grains of
