Gold assessment in micas by XRF using synchrotron radiation

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ISOTOPE GEOSCIENCE

ELSEVIER

Chemical Geology 124 (1995) 83--90

Gold assessment in micas by XRF using synchrotron radiation M.J. Basto a,., M.O. Figueiredo b, F. Legrand c, p. Chevallier c, Z. Melo d, M.T. Ramos e a Laborat6rio de Mineralogia e Petrologia, lnstituto Superior T~cnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal b Centro de Cristalografia e Mineralogia, lnstituto de lnvestigafao Cient{fica Tropical, Al. Afonso Henriques, 41-4°, 1000 Lisboa, Portugal Laboratoire pour l'Utilisation du Rayonnement ElectromagnJtique (LURE), Centre Universitaire Paris-Sud, 91405 Orsay Cedex, France a Departamento Central de Estudos e Andlises lndustriais, INETI, Azinhaga dos Lameiros, 22, 1699 Lisboa Codex, Portugal Cemro de F£sica Atdmica, Unversidade de Lisboa, Av. Gama Pinto, 2, 1699 Lisboa Codex, Portugal Received 4 October 1993; accepted after revision 22 January 1994

Abstract

The detection of sub-trace concentrations (ng g- t) of heavy elements in geological samples by X-ray fluorescence may be achieved by the use of :synchrotronradiation (SXRF). Minimum detection limits range from a few/zg g- ~to tenths of ng g- 1, depending on the atomic number of the analyte vs. the matrix mean atomic number. Recently, the assessment of sub-trace Au contents in lepidolites from the Gonfalo lithium mine, northern Portugal, using SXRFA was described, and the optimal excitation energy established at 13 keV. New results from muscovite and lepidolite micas from various localities in Portugal are now reported. Experiments were performed at the LURE (Laboratoire pour l'Utilisation du Rayonnement Electro-magu6tique), DCI (Dispositif de Collision sous Igloo ) storage-ring with a Si (Li) detector, using local data handling and processing programs. A complex population of trace elements gives rise to interferences on the diagnostic Au-L~ line, namely from K-lines of lighter elements (Zn-K~,Ge-Ka, Ga-K~) and from the L-spectraof Ta and W, which were accounted for in data processing. Unfortunately, the lack of suitable standards currently hinders the presentation of reliable absolute figures for Au contents.

1. Introduction The improvement of analytical methods to determine gold in geological :samples at sub-trace levels has received increased attention in recent years (e.g., Cook and Chryssoulis, 1990), particularly for the identification of so-called "invisible gold", trapped in the crystal structure of c~a'fier minerals (Chryssoulis et al., 1987). Two major difficulties have to be faced in such analyses: firstly, the detection limits of the techniques usually employed are insufficient to enable the determination of bulk concentrations at the levels which are likely to be present; secondly, there is a need to obtain * Corresponding author. 0009-2541/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSD10009-2541 ( 95 ) 0 0 0 2 6 - 7

spatial resolution during analysis, especially when Au is hosted by sulphide minerals in complex ores. For many years, fire assay has been the classical analytical procedure for the determination of gold, although it is widely recognised that the method requires extensive sample manipulation and necessitates an experienced operator to give reliable results. Recently, modern instrumental techniques including micro-PIXE (Halden et al., 1990), synchrotron X-ray fluorescence (SXRF) microprobe ( Kucha et al., 1993 ) and slurry nebulisation ICP-MS (Totland et al., 1993) have been shown to offer the advantage of avoiding complex sample pretreatment, and in some cases, no preparation is required at all. Low detection limits, and in the case of the microbeam techniques, the ability to

84

M.J. Basto et al. / Chemical Geology 124 (1995) 83-90

provenances in Portugal, some of them related to identifiable gold mineralisation.

acquire spatially resolved data, have the potential to overcome the two main difficulties identified above. In the case of SXRF, significant analytical advantages can be attributed to the particular properties of the synchrotron excitation source. Synchrotrons offer a high-brilliance, linearly polarised beam of X-rays, with the possibility of tuning the excitation energy by using a suitable monochromator, thus improving significantly the analytical sensitivity. The very low angular divergence of the synchrotron X-ray beam also provides an efficient means of in-situ non-destructive microanalysis, making possible the study of minute samples and the mapping of the analyte. As a development of a previous SXRF study of subtrace gold hosted by lepidolites from a single lithium granite pegmatite (Figueiredo et al., 1993), results are now presented of an extension of this work to other phyllosilicate minerals from various localities and

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2. Sample characterisation The mica samples studied came from the granitic massif, northern Portugal and were selected from museum specimens (Geology and Mineralogy Museums of the Technical University in Lisbon). Lepidolites (2M2 polytype) originate from the Castanho lithium mine, southwest of Gon~alo near Guarda, the locality studied previously by Figueiredo et al. (1993). These micas derived from granitic aplite-pegmatites containing lithium, tin, tantalum, niobium and beryllium mineralisation (Teixeira et al., 1962; Ramos, 1981). Muscovites (2M~ polytype) came from three localities: (1) Cerdeirinha tungsten mine at Serra de Arga,

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a well-known gold site since the Roman times (SilvaCarvalho and Veiga-Ferreira, 1954; Dias, 1984); (2) nearby Louredo Church at Vieira, not clearly related to any known mineralisafion (Cotelo-Neiva and Chorot, 1945; Noronha and Ribeiro, 1983); and (3) Jales gold mine (specific location unknown), also discovered during the Roman occupation of the Iberian Peninsula and, since then, the main Au-Ag deposit in the region (Ramos, 1983). Muscovite from Serra de Arga is the only mica present in a medium-grained granite, while the other two orginate from pegmatites.

3. Experimental The conventional mTangement of the X-ray fluorescence station at the LURE, installed on the bendingmagnet beam line labelled D15 of the DCI storage ring, was used. Operating conditions of the ring were 1.85-

GeV positron energy and 300-mA current with 70-hr half-life. Monochromatic excitation was obtained using a pyrolitic graphite double-crystal composed of a flat, highly oriented mosaic crystal (FWMH = 0.4), followed by a curved crystal (Brissaud et al., 1989). The fluorescence spectra were recorded with a Si(Li) detector (26-mm 2 area and 150-eV FWHM at 5.9 keV) placed in the plane of the storage ring at 90 ° to the incident beam, in order to minimise Compton and Rayleigh scattering contributions to the detected background in the spectrum. To attenuate the counting rate and to reduce pile-up effects in the detector, an aluminium absorber foil was placed in front of the monochromator. A constant acquisition time of 103 s was used. The experiments were always performed in air, thus minimising the spectral contributions of the major light elements A1 and Si, the fluorescence lines of which are heavily absorbed in air before reaching the detector. These and other major or minor elements (Li, F, Mn, K, Na, Rb) were deter-

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mined by chemical methods. The sample area selected for irradiation ( ~ 50/xm 2 to 1 mm 2) was positioned with the aid of a laser beam. Detected spectra were stored in an IBM AT3 microcomputer using a Nucleus PCA 4K multichannel card. Previously written data-handling programs (Wang, 1988, Chevallier et al., 1991) were used for peak assignment, deconvolution of complex lines, reduction of continuous background caused by scattering and "Bremsstrahlung" in the sample, and computation of peak area under each analytical K- or L-line. These programs account for escape and sum peaks, as well as

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at the time of mineral formation and during subsequent modification caused by pressure/temperature variations (Cerny and Burt, 1984). These crystal structures display a diversity of interstitial sites ranging fi'om, (a) tetrahedral sites with a coordination number (CN) of four, dominantly occupied by Si and AI, plus Fe, Ga, Ti(?) and Ge, through, (b) octahedral CN 6 sites, filled by A1, Li, Fe, Mn, Mg, Ti and other cations with suitable ionic radii and valence such as So, V, Ni, Zn, Ta, Nb, W, Sn, Ge, rare-

earth elements (REE), to, (c) the large interlaminar site filled by K, with minor abundances of both alkaline cations Rb, Na, Cs and divalent cations of large radius, such as Ba, Ca and Pb. REE ions also enter the interlaminar space in mica structures where TI may also be hosted (Figueiredo and Basto, 1990). The structural locii for As and Au are not yet fully assigned. Indeed, the minute concentration of the latter element hinders the application of spectroscopic techniques like EXAFS (extended X-ray absorption fine

M.J. Basto et al. /Chemical Geology 124 (1995) 83-90

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structure), which would otherwise be capable of checking both valence and coordination of Au in mica-type silicates•

5. Results

Lepidolites from Gon~alo have a common chemical signature, always containing Mn and Fe as minor components, and V, Zn, Ga, Ge, Rb, Nb, Cs, La, Ce, Ta, W, Au and TI as trace or subtrace elements.

The chemical constitution of the muscovites differs in the relative proportion of minor and trace elements when compared to the lepidolites (Fig. 1). Mica from Vieira contains Sc and a higher proportion of tungsten, and Jales mica has a significant Pb content, while none of them contains detectable levels of REE. Two muscovite sheets from Vieira were carefully scanned, some of the collected spectra being plotted in Fig. 2. Despite their similarity, a difference in the Ni content is noticeable. Interferences on the diagnostic Au-L,~ line are illustrated in Fig. 3 and were adequately accounted for

M.J. Basto et al. / Chemical Geology 124 (1995) 83-90

during the analysis of SXRF spectra. Various muscovite fragments from Serra de Arga were also analysed, their chemical compo:~itions being remarkably similar (Fig. 4). As well as the elements found in other micas studied here, these muscovites contain As and Ba in quite significant proportions. The computation of selected peak areas and the assessment of provisional analytical values were undertaken using the SYME © (Wang, 1988) and EPAIS2 ~ computer programs, the latter modified from an original version written by P. I,agarde and I. Brissaud for PIXE analyses (Abbas, 199 it). However, the lack of suitable standards prevented the calculation of quantitative analytical data. In an attempt to find geochemically useful correlations, plots of peak areas were undertaken for several trace elemen~ts in muscovite samples from Vieira. The peak area of each elemental diagnostic line was normalised to the K~ line of Mn, one of the minor elements determined l~y chemical methods that presents a relatively constant concentration in all samples. The combined W + Ta content plotted against the strategic elements Ga and Au, showed a linear correlation with Ga, and a much poorer linear relationship to Au (Fig. 5a and b); conversely, the Au vs. TI plot showed no clear correlation (Fig. 5c).

6. Final comments

Synchrotron X-ray fluorescence spectrometry is a high-sensitivity technique for the chemical analysis of geological materials at a microprobe scale, whose advantages have already been emphasised for silicatehosted elements (Halden et al., 1990; Jaklevic et al., 1990; Figueiredo et al., 1993). In fact, minute samples can be studied without damage, and local concentration ranges efficiently mapped within the same sample, two features of utmost importance in the analysis of archaeological artefacts, ores and minerals (Lu et al., 1989). The results of this pioneering study strengthen those arguments, also emphasising the low detection levels which can be attained - unquestionably relevant when considering the very low concentration of precious elements in most geological samples. The difficulty in presenting a reliable quantitative assessment of element,; at such very low concentrations (tenths of ng g - 1 to a few pg g - 1) stems mainly from the unavailability of suitable reference materials, a

89

drawback that hopefully will be overcome in the near future. The possibility of determining Au contents of a few ng g-1 hosted by lepidolite and muscovite micas in rocks surrounding mineralised masses may open new perspectives to geochemical prospecting for gold deposits. The establishment of correlations between elemental contents could be used as an indirect sign of Au traces. Indeed, the good linear correlation of Au vs. W + Ta may be indicative of the uptake of heavy metals by aluminuous micas. It is our intention, therefore, to extend the present study by analysing more mica sampies collected at the same type-localities and at other ore deposits, and attempt to relate geochemical variation to mechanisms of ore genesis.

References Abbas, K., 1991. Utilisation du rayonnement synchrotron pour la caracttrisation d'~ltments traces par fluorescence X. Ph.D. Thesis, University of Paris VI, Paris. Brissaud, I., Wang, J.X. and Chevallier, P., 1989. Synchrotron radiation induced X-ray fluorescence at the LURE. J. Radioanal. Nucl. Chem., 131: 399-413. Cerny, P. and Burr, D.M., 1984. Paragenesis, crystallochemicai characteristics, and geochemical evolution of micas in granitic pegmatites. In: S.W. Bailey (Editor), Micas. Mineral Soc. Am., Rev. Mineral., 13: 257-297. Chevallier, P., Abbas, K. and Sainfort, P., 1991. Determination of trace elements in aluminium with synchrotron radiation induced X-ray fluorescence. X-ray Spectrom. 20: 293-295. Chryssoulis, S.L., Cabri,,L.J. and Salter, R.S., 1987. Direct determination of invisible gold in refractory sulphide ores. In: R.S. Salter, D.M. Wyslouzil and G.W. McDonald (Editors), Proceedings of a International Symposium on Gold Metallurgy, Winnipeg. Pergamon, Toronto, Ont., pp. 235-244. Cook, N.J. and Chryssoulis, S.L., 1990. Concentrations of "invisible gold" in the common sulphides. Can Mineral., 28: 1-16. Cotelo-Neiva, J. and Chorot, J.L., 1945. Some gold deposits from Alto Minho, Portugal. Estud. Notas Trab. Serv. Fomento Min., 1:190-265 (in Portugese). Dias, G., 1984. Syntectonic Hercynian granites from Ponte de Lima area, northern Portugal: geochemical evolution. Mem. Not., Univ. Coimbra, 98:9-33 (in Portugese). Figueiredo, M.O. and Basto, M.J., 1990. Thallium crystal chemistry in natural compounds. Acta Crystallogr., A46:C-262 (abstract). Figueiredo, M.O., Basto, M.J., Abbas, K., Melo, Z., Ramos, M.T. and ChevaUier, P., 1993. Synchrotron X-ray fluorescence analysis of trace elements in light silicate minerals. X-ray Spoctrom., 22:248-251. Haiden, N.M., Hawthorne, F.C., Durocher, J.J.G., Smith, G.S., Gallop, D.M. and McKee, J.S.C., 1990. High energy K-line PIXE spectra of Au-bearing minerals. Am. Mineral., 75: 956-962.

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