Pyriteââfool's goldâ or misunderstood mineral

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© Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 26, No. 1, January–February 2010

FeaturePyrite—’fool’s gold’ or misunderstood mineral?

Craig D. BarrieSchool of Geography and

Geosciences, University of

St Andrews, St Andrews,

Fife, KY16 9AL UK

[email protected]

Pyrite (FeS2) is the most abundant sulphide mineral in the Earth’s crust, and with its characteristic shape and appearance, is easily one of the most recognizable. Yet, pyrite has a somewhat tarnished reputation with many considering it to be waste material and of little economic importance, never-mind interest. This assessment however, is misleading and a considerable volume of current research focuses upon the unique characteristics and formation of pyrite. Indeed, with its potential for revealing the very foundations of life, as well as maximizing the discovery and exploitation of society’s future metallic resources, pyrite may be of much greater interest and benefit than many geologists realize.

Pyrite (FeS2) or ‘fool’s’ gold as it is commonly known,

due to its yellowish, metallic appearance being similar to gold (Au), can be found in virtually every rock type and is hosted in a myriad of environments, from metamorphic terranes and estuarine basins to me-teorites and microscopic bacteria. The crystal habit most people instinctively associate with pyrite is cubic (Fig. 1A) although ‘pyritohedrons’ (irregular dodeca-hedrons) (Fig. 1B) are also a common sight in muse-ums and shops. Other less well recognized, but equal-ly common forms include spheroid (Fig. 1C), framboid (Fig. 1D) and colloform (Fig. 1E) textures. These lat-ter textures, while less well known, are representative of primary pyrite precipitation and can, therefore, be useful indicators of initial nucleation and pre-defor-mation growth conditions. In terms of size, pyrite crystals vary from over 30 cm in the metamorphosed sulphide deposits of Ducktown, Tennessee, USA to a few nanometres in magnetotactic bacteria. One of the key characteristics of pyrite, unique amongst the common sulphide minerals, is its refractory nature which allows crystals to survive relatively unchanged up to temperatures of ~743 °C.

Pyrite, therefore, has a widespread distribution, is stable across a wide range of conditions and preserves an array of crystal habits dependent upon both ini-tial nucleation conditions and post-depositional his-tory (Fig. 1). However, to many geologists, pyrite is considered as, at best, an annoyance, for while it

may be widespread, it is currently of little economic worth. For instance, in the mining industry, pyrite is ‘gangue’ material and often ends up in waste stock-piles; not only costing companies in extraction costs but also in remediation to negate any potential acid mine drainage (AMD). In fact, many mining opera-tions consider pyrite (particularly where crystals are micrometre framboidal textures, Fig. 1D) to be haz-ardous, due to its potential for spontaneous oxidation and combustion of stockpiles. With these issues in mind it is little wonder then that ‘fool’s gold’ is con-sidered an apt name by many of those extracting the Earth’s raw resources. Yet this view is misrepresenta-tive, for in terms of furthering our understanding of the evolution of both life and metallic resources, py-rite is of the utmost interest and importance.

The literature on both the beneficial and detri-mental nature of pyrite is vast, encompassing an ar-ray of disciplines and research fields, and this article could never hope to discuss this range. Instead, three areas of current research, in which pyrite plays a fundamental role in furthering our understanding, are discussed below.

The building blocks of life?

When people consider the probable building blocks responsible for the initial origins of life, the majority would probably place pyrite fairly low down the list

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of essentials, if even considering it at all. Yet, what may be surprising is that theories have been circulat-ing since the 1980s which suggest that iron sulphide, specifically FeS

2, may have served as the principal

catalyst for the initial formation of life. Currently, there are two main competing theories in the ‘origin of life’ debate, ‘RNA-first’ and ‘metabolism-first’. Both schools of thought have their supporters and detrac-tors. It is not possible to challenge either school of thought here, but it is possible to discuss how pyrite formation, coupled with other factors in the early, pri-mordial oceans may have given rise to life assuming the operation of ‘metabolism-first’ processes.

The basic premise of this theory assumes that the formation of initial organic matter occurred as a re-sult of the reactions generated when hydrogen sul-phide (H

2S), released from beneath the ocean crust,

bonded with ferrous sulphide (FeS) present within primordial oceans. This process can be considered similar, if not analogous, to modern day hydrother-mal vents or ‘black smokers’ (Fig. 2A) in oceanic crust, where superheated (< 400 °C) fluids seeping from beneath the crust interact with cold seawater precipitating mounds of sulphide minerals. The ini-tial iron sulphide reactions in the primordial ocean

would have released both energy and hydrogen at-oms and potentially amino acids and hydrocarbons. This organic matter could potentially adhere to the surfaces of newly formed pyrite crystals, using them as catalysts to drive further reactions. The continued growth of organic material, though, would require nutrients, sourced from the surrounding waters and subsequently metabolized for energy, hence the name ‘metabolism-first’.

This theory, that pyrite represents the precursor to life, does have a few complications. The first is-sue is that hydrothermal vents (Fig. 2A), which generate pyrite and other sulphide minerals on the present day seafloor generally have fluid tempera-tures of ~100–400 ºC. This is problematic because most workers consider temperatures of ~110–140 ºC the very limits at which present day life can survive, above which it would literally rip itself apart before it even got started. However, a solution to this problem is the ‘chemical garden’ theory, proposed from the discovery of tubular, pyrite textures (Fig. 2B–D) at the Silvermines Zn-Pb-Ba deposit, County Tipperary, Ireland. These textures are suites of pyrite tubes, most less than ~1 mm in diameter (Fig. 2C–D), thought to represent another type of hydrothermal vent. These vents have similar suggested origins to ‘black smok-ers’ (Fig. 2A) but are on a much smaller scale and with anticipated lower temperatures of ~100 ºC, far more amenable to fostering life. Therefore, in an ex-pansion to the earlier, much simpler ‘catalyst’ model, the ‘chemical garden’ model envisages that these miniature pyrite vents could have initially served as

Fig. 1. Images highlighting the various, different crystal habits of pyrite. A. Sketch and photomicrograph of crystals of pyrite with the characteristic cubic habit. B. Sketch and photomicrograph of pyritohedron (irregular dodecahedron) crystals of pyrite. C. Photomicrograph of a flattened pyrite spheroid or disc (commonly referred to as pyrite dollars or suns). D. High resolution scanning electron microscope (SEM) image of a large pyrite framboid texture (i) and an adjacent cluster of smaller framboid textures (ii). All of the framboids preserve the characteristic ‘raspberry’ appearance. E. Reflected light (RL) image of pyrite (py) forming a layered, colloform texture surrounded by chalcopyrite (cp) and silicate phases (si).

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‘vials’, or even the first ‘cell membranes’, rather than purely catalysts. These ‘cell membranes’ would con-centrate the chemicals generated from the sulphide reactions in an enclosed, but also accessible environ-ment.

However, in order to induce organic matter with-in these vents, conditions would need to have been warm (vents will be ~100 ºC), hydrogen rich (reac-tions generating FeS

2 produce H

2 as a product) and

alkaline in contrast to the cold, acidic waters of the primordial oceans. These conditions, although mostly covered, bring us to our next problem; black smoker type vents (Fig. 2A) are not alkaline; they generate acidic fluids, with pH values as low as ~3. This prob-lem, however, was potentially solved in 2000 when the Lost City hydrothermal field was discovered near the Mid Ocean Ridge (MOR) in the Atlantic Ocean. The vents at the Lost City, rather than being acidic like most black smokers, are alkaline and pump out fluids at a modest ~200 °C, forming 60-m tall car-bonate towers. Although overly large for the purposes required to fit the ‘chemical garden’ theory, these vents are surprisingly analogous to the conditions en-visaged for life in the miniature vents at Silvermines (Fig. 2B–D).

Even were this process, involving pyrite as either a ‘catalyst’ and/or ‘cell membrane’, to be proven be-yond all doubt in the laboratory, detractors will say, and rightly so, that this is still conjecture about the conditions ~3.5 billion years ago. However, that is not to say that this research is not worth doing. On the contrary this apparent conjecture is indispensable and were it proven that pyrite could be used to gener-ate life, in any shape, form or size, in the laboratory it would be ground breaking, elevating pyrite from ‘fool’s gold’ to the ‘gold standard’ at least in ‘origin of life’ debates.

Evolving ore deposits?

Although pyrite is widespread throughout the Earth’s crust, it is often concentrated in greatest abundance within sulphide ore deposits. As a general rule, and as discussed previously, pyrite is considered, by most mining operations, to be ‘gangue’ material only ex-tracted due to its intermingled nature with the eco-nomic sulphide minerals the mines want to extract (e.g. chalcopyrite (CuFeS

2), sphalerite (ZnS) or galena

(PbS)). The history and evolution of sulphide ore de-posits, however, is rarely simple, primarily because most form at plate margin settings. This depositional environment often means that at some stage in an ore deposit’s history it is likely to become involved in oro-genic, mountain building events resulting in deforma-tion and/or metamorphism of the ores. This is evident world-wide, with hundreds of ore bodies located in the orogenic belts of the Scandinavian Caledonides

(Fig. 3) and the Iberian Massif amongst others. De-formation and metamorphism are rarely consistent across orogenic belts (Fig. 3), or even within indi-vidual ore deposits for that matter, resulting in the preservation of a wide array of pyrite textures from primary framboids (Fig. 1D) and colloforms (Fig. 1E) in relatively undeformed ores, to annealment, recrys-tallization and growth textures (Fig. 3D) and most regularly brittle (Fig. 5A, G) or ‘pulverization’ tex-tures in metamorphosed ores.

The response of sulphide minerals, within polyme-tallic ore deposits, to increases in P–T conditions var-ies widely. The ‘soft’ economic sulphide minerals (i.e. sphalerite, galena) commonly remobilize, losing any inherited crystal shape, and deform in a ductile man-ner at relatively low temperatures (<200 ºC) (Fig. 3). In contrast, pyrite often remains unchanged, preserv-ing equant crystals even at temperatures >400 ºC (Fig. 3A–C, G). This unique refractory nature of py-rite, amongst the common sulphide minerals, means that it has the potential to preserve information about the post-depositional and possibly primary precipita-tion and growth history of ore deposits.

Historically, and for that matter currently, most geologists consider pyrite a hard, rigid mineral which deforms by brittle mechanisms (Fig. 3A), if at all, within the Earth’s crust. This generalization has been, and still is, made upon the basis of observation during

Fig. 2. A. Sulphide spires forming part of the ‘Bio 9’ black smoker hydrothermal vent, 9ºN on the East Pacific Rise. (Credit: Stace Beaulieu, Woods Hole Oceanographic Institution). B. Pyrite chimney with a globular true exterior; two later rims coat this; an authigenic pyrite coat, and partly surrounding this, tabular crystals of barite (Silvermines deposit, Ireland). C–D. Small pyrite vent structures, with a framboidal centre, and a fine euhedral authigenic rim surrounded by baryte, with minor pyrite (Silvermines deposit, Ireland). (Figures B–D are reproduced from Boyce et al., Nature, 1983, v.306, pp.545–550; with the permission of A.J. Boyce).

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reflected light microscopy and back scatter electron (BSE) imaging where pyrite either appears apparently undeformed (Fig. 3B–C) or more often dominated by cataclasis (Fig. 3A). This idea is ingrained in the lit-erature and while partly true, is not reflective of the full story. Pyrite can and does deform in a ductile manner, even at temperatures as low as ~200ºC, however, this is rarely reported because it is far more difficult to detect this behaviour than in the other sulphide minerals. This is partly due to the ability of pyrite crystals to retain their characteristic shape even at relatively high P–T conditions and partly be-cause standard reflected light analysis does not ac-tually reveal evidence for internal distortion within opaque minerals (Fig. 3A–D). Indeed, all of the sam-ples shown in Fig. 3, although appearing fractured or undeformed, have been deformed by ductile mecha-nisms. This evidence is often only apparent when Transmission Electron Microscopy (TEM) or Scanning Electron Microscope (SEM) based techniques are used for analysis. The latter technique involves orientation contrast (OC) imaging (Fig. 3E, G) which produces qualitative grey-scale images of plastic distortion in crystals coupled with quantification using electron backscatter diffraction (EBSD) (Fig. 3F, H). These techniques readily reveal evidence for ductile defor-

mation in notoriously problematic opaque minerals.Although this knowledge, about the behaviour

and nature of pyrite and indeed the other sulphide minerals, is interesting from an academic viewpoint, how does it make pyrite a beneficial mineral within ore deposits? Well, on purely economic grounds this knowledge doesn’t. But, if geological implications are considered then pyrite is essential in understanding deformed ores. In order to fully exploit ore deposits, and therefore maximize profits, mining companies need to understand not only the general processes by which ore deposits form, but in orogenic settings, the processes by which they deform. This knowledge is required because deformation and metamorphism can alter primary sulphide textures, differentially mo-bilize the minerals present and change the trace ele-ment composition of both ore and gangue minerals. As pyrite is the most stable and persistent mineral in ore deposits and is also commonly intermixed with the economic sulphide minerals (Fig. 3) it can act as a guide to understanding how, why and where these metallic minerals have been distributed post-deformation.

Information about post-depositional behaviour is often preserved prior to peak metamorphic condi-tions when pyrite is still relatively rigid and often

Fig. 3. Geological map of the Scandinavian Caledonides indicating the different nappes (allochthons) forming the orogenic belt and the location of a few of the ore deposits (white dots) associated with orogenesis. Seven ore deposits are highlighted (red dots) indicating the range of metamorphic conditions prevalent across the Caledonides. A. Reflected light (RL) image indicating brittle textures prevalent within pyrite grains at the Bleikvassli deposit (6). B–C. RL images revealing the lack of deformation evidence within pyrite grains at both the Løkken (1) and Gjersvik (3) deposits, despite metamorphic temperatures of ~400 ºC. D. RL image revealing overprinting of chalcopyrite (cp) and sphalerite (sp) by pyrite (py) in the Klingenberg Mine (Røros orefield). E–G. Orientation contrast (OC) images indicating ductile deformation (apparent from grey-scale variation) within pyrite grains at Sulitjelma (5) and Nye Storwatz (Roros orefield) respectively. F/H. Electron backscatter diffraction (EBSD) maps quantifying the ductile distortion within the pyrite grains (in degrees) shown in E and G.

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experiencing porphyroblastesis, engulfing the weaker phases and textures present in the ores (Fig. 3D). This overprinting, as with garnet porphyroblasts in other settings, provides information about textures present prior to growth and also, potentially, the timing at which porphyroblastesis occurred relative to any oth-er events. Indeed, pyrite can, in some cases, overgrow cores of the very earliest, pre-deformation sulphide textures indicative of conditions of initial mineraliza-tion. Primary textures such as framboids have been used in some studies to predict redox conditions dur-ing initial precipitation while sulphur isotope analysis of colloform textures have been used to determine changing sulphur source conditions in subsequent mineralized layers and hence through time.

All of these textures tell a story and were deposits to lack pyrite, as many on the extraction side may prefer, deciphering information about initial precipi-tation and growth as well as subsequent deformation would be far more difficult (Fig. 3). Therefore, while pyrite may be considered by many to be waste, based upon its economic worth, this argument is skewed; ignoring the geological worth of pyrite in understand-ing ore deposits.

‘Fool’s gold’ or invisible gold?

In nature it is very much a rarity, and equally so in the sulphides, to recover mineral samples which are pure and composed of only their major constituent elements. In reality the majority of minerals tend to sequester at least some trace elements during nu-cleation and growth. Pyrite is known to sequester a

multitude of different trace elements, with probably the most well known and problematic being arsenic (As) (Fig. 4C). Not all substituted trace elements are unwanted, indeed, some are highly sought after and pyrite can sequester nickel (Ni), antimony (Sb), cop-per (Cu) silver (Ag) and gold (Au) amongst others (Fig. 4). These trace elements can be major metal sources, accounting for significant percentages of ore reserves within deposits. As an example the Pascua Au-Ag-Cu deposit in Northeastern Chile contains gold reserves estimated at 274 Mt, 50 per cent of which is trapped as so called ‘invisible’ Au within the lattice of pyrite crystals. Similarly, the Suurikuusiko gold deposit, Lapland contains pyrite with an average gold abundance of ~46 ppm, an amount which accounts for ~22.7 per cent of total Au reserves at Suuriku-usiko.

Gold can be present within ore deposits in two forms. The first and most well known type of gold is ‘free’ Au and it is this type which often sparks ‘gold rushes’. Free Au often forms as layers or grains (Fig. 4A, B) which can accumulate into nuggets or lodes and can either be pure gold or most often alloys with other metals such as Ag (electrum) (Fig. 4A, B). Free Au is visible either to the naked eye or stand-ard microscope techniques and can occur intermixed with pyrite. The second type of gold is called ‘invisible’ Au, a term used to describe the accumulation of Au within growing crystals by two different processes. The first process is where atoms of Au are seques-trated into the lattice of growing crystals as a trace element (Fig. 4E, F) while the second is where grains of ‘free’ Au are trapped as micro-inclusions within

Fig. 4. A. BSE image of a mass of Au alloy (electrum (E)) in a barite vein (B) cutting across pyrite (P) (Curraghinalt deposit, N. Ireland). B. BSE image of electrum (bright, arrowed) present along domain boundaries in pyrite (P) (Curraghinalt deposit, N. Ireland). C. RL, BSE and trace element maps of colloform pyrite. Individual trace element maps for As (red) and Sb (blue) are shown as well as an As-Sb overlay indicating coupled incorporation (purple). Quantified results are shown in the BSE image in weight per cent (Greens Creek deposit, Alaska). D–E. BSE images of pyrite-rich ore showing laser-ablation craters with measured concentrations of Au and Te shown in ppm (Huangtuliang deposit, China). (Figures A–B and D–E are reproduced from Cook et al., Chemical Geology, 2009, v.202, pp.101–121 with the permission of N.J. Cook).

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growing crystals. The only common sulphide miner-als which accumulate significant and therefore, eco-nomically interesting quantities of Au are pyrite and arsenopyrite (FeAsS).

In order for trace element invisible Au to be ini-tially sequestrated into the crystal lattice of pyrite, most studies suggest the substitution must be twofold, involving uptake of As atoms as well as Au. This mechanism is generally assumed to occur whereby As substitutes for S at the pyrite growth surface, which in turn promotes the substitution of Au (as Au+) for Fe. This is not the universally accepted mechanism though, and other studies suggest mechanisms in-volving the uptake of Au and an array of other trace elements. Distribution of ‘invisible’ Au in pyrite-rich ore deposits is variable and the highest Au-bearing pyrite concentrations are generally within carlin- and epithermal-type deposits. In contrast, but with a few exceptions, ore deposits with an orogenic history, such as those forming the Scandinavian Caledonides (Fig. 3) are often Au-bearing pyrite poor. This lack of Au in pyrite within orogenic belts is suggested to result from remobilisation of the Au out of pyrite crystals as a result of elevated P–T conditions during orogenesis.

Although orogenic ore deposits are often Au-bear-ing pyrite poor this does not mean that gold has necessarily been lost from ore deposits. Indeed, if Au was sequestered into pyrite grains during an earlier, pre-deformation, stage in the ore deposit’s history then orogenesis may have actually remobilized and concentrated Au into lodes within low strain defor-mation zones. Thus, pyrite grains can, as discussed earlier, be used as a guide to determining the history of mineralization including how, when and where Au remobilization and accumulation may have occurred within ore deposits.

This area of research is, understandably, of wide interest to those seeking to discover future reserves of metallic minerals, especially those as rare and valuable as Au. This relationship also quashes any misconception that pyrite, along with arsenopyrite (another non-economic sulphide mineral), is always unwanted material, indeed as a carrier of potentially large reserves of gold, it may be the mineral everyone in the future is actually looking to discover. These

studies suggest that pyrite is not only a contender for the modern day equivalent of the ‘philosophers stone’ but also is the key mineral in understanding the deformation history of ore deposits and therefore, maximizing their exploitation.

Summary

Pyrite is commonly known by the nickname ‘fool’s gold’ due to its yellowish colour and metallic lustre. To many geologists this name is an apt one as while the appearance of pyrite may be appealing to collec-tors it is generally considered of little or no economic worth. However, while these assertions may be true in some contexts they do pyrite an injustice, as to some this mineral is an important, if not key, tool. Indeed, pyrite may not only be the key to understand-ing the ‘origin of life’ and the formation of society’s future metallic resources but in an ironic twist of fate may be a principal carrier of actual gold—Au. Hardly the characteristics expected of a mineral long derided as ‘fool’s’ gold.

Suggestions for further reading

Cook, N.J. & Chryssoulis, S.L. 1990. Concentration of ‘invisible gold’ in the common sulfides. Canadian Mineralogist, v.28, pp.1–16.

Chouinard, A., Paquette, J. & Williams-Jones, A.E. 2005. Crystallographic controls on trace-element incorporation in Auriferous pyrite from the Pascua epithermal high-sulfidation deposit, Chile–Argen-tina. Canadian Mineralogist, v.43, pp.951–963.

Craig, J.R. & Vokes, F.M. 1993. The metamorphism of pyrite and pyritic ores: an overview. Mineralogical Magazine, v.57, pp.3–18.

Craig, J.R., Vokes, F.M. & Solberg, T.N. 1998. Py-rite: physical and chemical textures. Mineralium Deposita, v.34, pp.82–101.

Russell, M.J. & Hall, A.J. 1997. The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front. Journal of the Geological Society, v.154, pp.377–402.

Whitfield, J. 2009. Origin of life: Nascence man. Na-ture, v.459, pp.316–319.

Pyriteââfool's goldâ or misunderstood mineral

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