Impurity heterogeneity in natural pyrite and its relation to internal electric fields mapped using remote laser beam induced current Jamie S. Laird a,b,c,⇑ , Ross Large b , Chris G. Ryan a,b,c a CSIRO, Earth Science and Resource Engineering, Clayton, Victoria, Australia b Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Hobart, Tasmania, Australia c School of Physics, University of Melbourne, Parkville 3010, Victoria, Australia article info Article history: Received 24 July 2012 Received in revised form 13 December 2012 Accepted 15 December 2012 Available online 8 February 2013 Keywords: PIXE Laser beam induced current Geology Gold Ore genesis abstract Regions of band-bending in naturally occurring semiconducting sulfides are thought to drive electro- chemical reactions with passing fluids. Metal bearing fluids within the right pH range interact with the electric fields at the surface resulting in precious metal ore genesis, even in under-saturated solutions. Metal reduction at the surface occurs via field assisted electron transfer from the semiconductor bulk to the ion in solution via surface states. Better understanding the role these regions and their texturing play on nucleating ore growth requires imaging of electric field distributions near the sulfide surface and cor- relation with underlying elemental heterogeneity. In this paper we discuss PIXE measurements made on the CSIRO Nuclear Microprobe and correlate elemental maps with laser beam induced current maps of the electric field distribution. Ó 2013 Published by Elsevier B.V. 1. Introduction Naturally occurring semiconducting metal sulfides such as pyr- ite possess electrical properties which depend on their crystal structure as well as formation sequence, subsequent diagenesis, metamorphic activity and natural weathering [1,2]. These settings and their influence on paragenesis results in a wide variety of hab- its, impurity levels and their heterogeneity throughout the mineral. Large impurity gradients in natural samples are not uncommon particularly close to hydrothermally altered rims. Impurity hetero- geneity, structural imperfections and small changes in stoichiome- try can lead to large variation in electrical properties [2–4]. Illustrating the scale of spread in natural assemblages, an early compilation by Pridmore et al. on the resistivity of natural pyrite found n-type samples to vary over four orders of magnitude, with p-type samples tending to have higher resistivities but a smaller spread [1]. Likewise, low co-ordination surface sites due to conchoi- dal fracture for example can result in bandgap narrowing and heterogeneous electrical properties at the surface [5,6]. In theory then, the band structure constants including minimum gap, the electron and hole Fermi levels, positions of the conduction and va- lence band edges as well as their density of states can all vary with- in a single mineral phase [7]. Of interest to this work however is the occurrence of neighbour- ing p and n-type regions with a shared boundary most commonly seen in zoned pyrites and their relationship to gold ore-genesis. Such regions have been observed by Marion et al. [8] and exist in many zoned pyrites such as those from Carlin and Bendigo-type deposits [9,10]. Semiconductor theory tells us that these metallur- gically clean interfaces result in the establishment of an in-built micro-junction. The resultant potential difference across the junc- tion can be characterised by its open circuit voltage V oc or a short circuit photocurrent if suitably irradiated by light. Similarly, band offsets between mixed sulfide phases such as pyrite and galena can also lead to heterojunctions. A complex mineral assemblage can be thought of as a three dimensional circuit which controls the flow of free carriers [2]. Any electric fields established close to the surface are able to interact with passing fluids resulting in enhanced abiotic and biotic oxidative attack and electroless auto- catalytic adsorption [11,12] of precious metals like Au and Ag (an electrochemical process). In this process, micro-galvanic action [13,14] between the p and n-regions of the junction results in the dissolution of the n-type material with electrons liberated passing to the p-side via a diffusion current where they are transported to the surface and quantum mechanically tunnel via surface states to reduce noble metal ions in solution. Under short circuit conditions prevalent due to the fluid shunting the device, the junction be- comes slightly forward biassed i.e. the diffusion current outweighs the drift current. In this work we attempt to observe these micro-junctios in pyrite and correlate their existence to PIXE measurements of local 0168-583X/$ - see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.nimb.2012.12.046 ⇑ Corresponding author at: CSIRO, Earth Science and Resource Engineering, Clayton, Victoria, Australia. Tel.: +61 383448375. E-mail address: [email protected](J.S. Laird). Nuclear Instruments and Methods in Physics Research B 306 (2013) 249–252 Contents lists available at SciVerse ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
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Nuclear Instruments and Methods in Physics Research B 306 (2013) 249–252
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier .com/locate /n imb
Impurity heterogeneity in natural pyrite and its relation to internal electricfields mapped using remote laser beam induced current
Jamie S. Laird a,b,c,⇑, Ross Large b, Chris G. Ryan a,b,c
a CSIRO, Earth Science and Resource Engineering, Clayton, Victoria, Australiab Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Hobart, Tasmania, Australiac School of Physics, University of Melbourne, Parkville 3010, Victoria, Australia
a r t i c l e i n f o a b s t r a c t
Article history:Received 24 July 2012Received in revised form 13 December 2012Accepted 15 December 2012Available online 8 February 2013
Regions of band-bending in naturally occurring semiconducting sulfides are thought to drive electro-chemical reactions with passing fluids. Metal bearing fluids within the right pH range interact withthe electric fields at the surface resulting in precious metal ore genesis, even in under-saturated solutions.Metal reduction at the surface occurs via field assisted electron transfer from the semiconductor bulk tothe ion in solution via surface states. Better understanding the role these regions and their texturing playon nucleating ore growth requires imaging of electric field distributions near the sulfide surface and cor-relation with underlying elemental heterogeneity. In this paper we discuss PIXE measurements made onthe CSIRO Nuclear Microprobe and correlate elemental maps with laser beam induced current maps ofthe electric field distribution.
� 2013 Published by Elsevier B.V.
1. Introduction Of interest to this work however is the occurrence of neighbour-
Naturally occurring semiconducting metal sulfides such as pyr-ite possess electrical properties which depend on their crystalstructure as well as formation sequence, subsequent diagenesis,metamorphic activity and natural weathering [1,2]. These settingsand their influence on paragenesis results in a wide variety of hab-its, impurity levels and their heterogeneity throughout the mineral.Large impurity gradients in natural samples are not uncommonparticularly close to hydrothermally altered rims. Impurity hetero-geneity, structural imperfections and small changes in stoichiome-try can lead to large variation in electrical properties [2–4].Illustrating the scale of spread in natural assemblages, an earlycompilation by Pridmore et al. on the resistivity of natural pyritefound n-type samples to vary over four orders of magnitude, withp-type samples tending to have higher resistivities but a smallerspread [1]. Likewise, low co-ordination surface sites due to conchoi-dal fracture for example can result in bandgap narrowing andheterogeneous electrical properties at the surface [5,6]. In theorythen, the band structure constants including minimum gap, theelectron and hole Fermi levels, positions of the conduction and va-lence band edges as well as their density of states can all vary with-in a single mineral phase [7].
Elsevier B.V.
and Resource Engineering,
ing p and n-type regions with a shared boundary most commonlyseen in zoned pyrites and their relationship to gold ore-genesis.Such regions have been observed by Marion et al. [8] and exist inmany zoned pyrites such as those from Carlin and Bendigo-typedeposits [9,10]. Semiconductor theory tells us that these metallur-gically clean interfaces result in the establishment of an in-builtmicro-junction. The resultant potential difference across the junc-tion can be characterised by its open circuit voltage Voc or a shortcircuit photocurrent if suitably irradiated by light. Similarly, bandoffsets between mixed sulfide phases such as pyrite and galenacan also lead to heterojunctions. A complex mineral assemblagecan be thought of as a three dimensional circuit which controlsthe flow of free carriers [2]. Any electric fields established closeto the surface are able to interact with passing fluids resulting inenhanced abiotic and biotic oxidative attack and electroless auto-catalytic adsorption [11,12] of precious metals like Au and Ag (anelectrochemical process). In this process, micro-galvanic action[13,14] between the p and n-regions of the junction results in thedissolution of the n-type material with electrons liberated passingto the p-side via a diffusion current where they are transported tothe surface and quantum mechanically tunnel via surface states toreduce noble metal ions in solution. Under short circuit conditionsprevalent due to the fluid shunting the device, the junction be-comes slightly forward biassed i.e. the diffusion current outweighsthe drift current.
In this work we attempt to observe these micro-junctios inpyrite and correlate their existence to PIXE measurements of local
250 J.S. Laird et al. / Nuclear Instruments and Methods in Physics Research B 306 (2013) 249–252
mineralogy and impurity heterogeneity. Arsenian pyrites fromOtago New Zealand were chosen as native gold is often associatedwith As rich pyrites and in particular fracture lines and borders be-tween mixed sulfide phases. The assemblage consisted of a blackshale host with large 0.5–2 mm euhedral grains scattered about.Heterogeneity in the electric-field distribution near the surface ofthese grains was mapped using Remote laser beam induced cur-rent (LBIC) [15] microscopy. By correlating PIXE and LBIC measure-ments we hope to observe p-n junctions responsible for goldprecipitation.
Fig. 2. PIXE elemental images of the dominant electrically active impurities inpyrite mapped across an approximate size of 3 mm wide by 1 mm high. Asdominates the electrical properties anywhere across the mineral bar the smallinclusions of chalcopyrite (see Cu image).
2. Particle induced X-ray emission (PIXE)
Technique minimum detection levels in the ppm range offeredby PIXE make it ideally suited to investigate elemental heterogene-ity in natural minerals that lead to variation in electrical properties[16]. To this end a 3 MeV proton beam from the University of Mel-bourne Pelletron was used on the CSIRO NMP [17] to map minorand trace element concentrations across several Otago pyritegrains (only one is shown here). Two large area hyperpure GeX-ray detectors are mounted at ±45� to either side of the mineralface. Only the minor and trace level distributions in relation tothe Fe distribution will be discussed here. The MicrodaQ data col-lection system in x-stage step mode [18] was used to scan a�3.5 mm � 1.5 mm region in 2 lm steps encompassing both thelong euhedral and smaller subhedral grain shown in the photo-graph in Fig. 1. Accumulated charge at each pixel was set at 50pC to ensure ppm sensitivity across common impurities found inmetal sulfides. A 300 lm Al filter damps contributions from Feand S to reduce pileup for the applied beam currents of 3–5 nA.The second X-ray detector included a 250 lm Be filter to collectall elements including S in the sulphide. As such it is of little inter-est in this paper. Results generated with GeoPIXE [19] on the tracechannel are summarised in Figs. 2 and 3. The elements shown arethose known to alter the electrical behaviour of pyrite [1]. Ni hasnot been included as it has recently been shown by Lehner et al.to form a deep level [20] and is not expected to play a large rolein determining local dopant type.
3. Remote laser beam induced current
The remote LBIC method uses dual micromanipulator probesplaced on either side of the mineral under study to capture a por-tion of the modulated lateral photocurrent induced by a focused1–2 lm 633 nm laser spot as it is scanned across its surface [21].Due to the large leakage current (or small dynamic resistance) inthese natural junctions in addition to shorting material around
Fig. 1. Photograph of the Otago arsenian pyrite sample studied in this work. Thesample dimensions are approximately 3 mm long by 1 mm high. Note the locationof the electrical probes contacting both extreme edges made for LBIC measurementsto ensue.
the edges etc., this current is typically buried in the noise requiringthe use of sophisticated AC methods and lock-in-amplifiers [21]. Atypical LBIC response for this ‘‘remote’’ configuration is bipolar asnoted in Fig. 3. Also shown is the equivalent circuit for the junctionand ohmic contacts. A full description of the system used in thisstudy is given elsewhere [21]. Note that LBIC measures an AC shortcircuit current or photovoltage phasor with both real and imagi-nary components. All images presented here are the magnitudeof the short circuit photocurrent vector Fig. 4.
An LBIC short-circuit photocurrent image of the entire regionshown in Fig. 1 as a function of laser modulation frequency is givenin Fig. 5 (scale is on the bottom). For reasons discussed elsewhere,the large component close to the probe contacts shown in the75 Hz image is due to thermoelectric currents due to high laserpowers. At higher modulation frequencies this component de-creases due to the minerals thermal mass and the structurerevealed should be that due to electric fields near the surface dueto impurity and mixed sulphide heterogeneity.
4. Discussion
For brevity the full analysis cannot be discussed here and amore full analysis will be published later. PIXE tells us that themajority of both grains contains an As rich overprint related tometamorphic activity late in paragenesis. These As impurities formshallow acceptor levels in pyrite resulting in a p-type dopingthroughout most of the crystal bulk [1]. Likewise, the elemental
Fig. 3. False colour RGB images generated with GeoPIXE to display associations between Fe–As–Cu (colour wheel on the right side). Dashed circles are areas of smallchalcopyrite inclusions expected to generate heterojunctions.
Fig. 4. (Top) Equivalent electrical circuit for an np junction in pyrite or heterojunction between two mixed sulfide phases such as pyrite and chalcopyrite. (Bottom) Thebipolar remote LBIC signal measured across the junction (–xj to xj assuming a symmetric doping profile) as the laser is scanned from left to right. Also shown is the chargestored on the depletion edge which constitutes its capacitance Cj. In the LBIC signal a zero exists at the weighted average centre of the junction.
Fig. 5. The modulation dependence of the magnitude of the LBIC photocurrent phasor over the entire pyrite grouping shown in figure 1. The higher frequency image moreaccurtaely portrays the electric field in the mineral as thermoelectric effects induced by laser heating are dampened.
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maps of Co displayed in Fig. 2 indicates n-type doping but its lowconcentration relative to As results in local p-type behaviour. Cuon the other hand is indicative of CuFeS2 (chalcopyrite) which isan n-type semiconductor.
Besides the large thermoelectric component in the LBIC images,of Fig. 5, numerous smaller hot-spots can be seen scatteredthroughout the mineral grains at higher modulation frequencies.The smaller grains to the far left contains a high number. Theseare likely the result of either spatial gradients in As–Ni or due tothe presence of mixed sulfide boundaries such as chalcopyrite-pyr-ite. The higher LBIC response around the top edges appears to beassociated with a strong gradation in As typical of zoned pyrite[22]. Chalcopyrite is a narrow band n-type semiconductor andreadily forms a heterojunction with pyrite. Note that a laser probeof 633 nm means only the top 100–200 nm’s is probed with LBIC[21]. Grains below this seen with PIXE will not show up in LBICwhich might explain the absence of any junction response closeto the grouping of dashed circles on the far right of the sampleas seen in Fig. 3. Overall correlation between the fields and elemen-tal maps in this sample is poor, partly in response to (a) the sheertextual complexity in impurities known to alter electrical proper-ties, (b) the complexity of the remote LBIC method and (c) thedifferent depth scales probed by each method. It is worth notinghowever that the second sample analysed resulted in a morestraight forward correlation and does indicate that gold preferen-tially deposits on the p-cathode side of junctions formed betweenpyrite and chalcopyrite (to be published).
More work is required to improve the modelling of LBIC to bet-ter aid in understanding its response in complex mineral assem-blages of the type examined here. To this end, TCAD simulationson a range of natural assemblage ‘‘scenarios’’ have begun. In paral-lel with this work we are also pursuing analysis on textually dis-similar samples and developing a cryogenic LBIC system to vastlyimprove sensitivity thereby avoiding thermoelectric currents. Infuture we hope to make real progress on the role natural junctionsplay in metal ore genesis.
5. Conclusion
We have attempted to correlate elemental maps made withPIXE to electric-field maps measured using remote LBIC on an arse-nian pyrite. However, the mineral complexity in this case togetherwith subtelties in the LBIC method have resulted in limited successalthough strong fields in the mineral have been imaged and arelikely a result of either pyrite-chalopyrite heterojunctions andstrong spatial gradients in the As concentration around the rims.
Acknowledgments
The authors gratefully acknowledge a large Australian Educa-tion Infrastructure Fund (EIF) grant for some equipment purchasesrelated to this work. We would also like to thank Roland Szymanskifor his tireless dedication to the Pelletron accelerator.
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