Nuclear Instruments and Methods in Physics Research B45 (1990) 327-332 North-Holland

327

PIXE ANALYSES TO DETERMINE THE TRACE-ELEMENT CONCENTRATIONS IN A SERIES OF GALENA (PbS) SPECIMENS FROM DIFFERENT LOCALITIES

Karen J. REESON , I) Chris J. STANLEY 2), Chris JEYNES , ‘) Geoff GRIME 3) and Frank WATT 3,

Ii Department of Electronic and Eleczrical Engineering, Univetsity of Surrey, Guildford, Surrey, GCJ2 .WH, England

-” Department of MineraFoa, Natural History Musatm, &‘romwelI Road, So& Kensington, London, S W7 MD, Eqkzpld

‘) Department of Nuclear Physics, University of Oxford Keble Road, Oxford, England

Micro-PIXE (proton-induced X-ray emission) at 1 irn lateral resolution and EPMA (electron-probe microanalysis) have been used to examine a series of naturally occurring galena (PbS) specimens from a number of localities &here silver has been produced. The results show little evidence for the solid solution of silver in gaIena and indicate that the silver is present as discrete mineral grains of silver-bearing sulphosalts. Even around the martins of a silver-bearing tetrahedrite, where it has clearly been replaced by galena, the silver levels in the galena appear close to background.

1. Introduction

The presence of silver in lead deposits, consisting predominantly of galena (PbS) and known as “ argentif- erous gale&‘, has been documented for centuries. Re- cently, debate has centered on the nature of the silver in these deposits and on whether it occurs in solid solution within the matrix through a coupled substitution such as;

Ag+ + (Sb3+ + Bi”) * 2Pb2*,

or whether it occurs in the form of submicroscopic, or larger, inclusions of discrete silver-bearing minerals dis- tributed throughout the galena matrix [1,2] and in the gangue minerals. Tetrahedrite is extremely common in lead-zinc deposits and has the general formula of Cu,,(Fc, Zn),Sb,S,, [3]. A number of chemical sub- sti~~tions are possible, however, including Ag for Cu and As, or Bi for Sb, such that a general tetrahedrite formula is best given as [4]

where Sb > As + Bi, and TRG = trigonal planar, TET = tetrahedral and SM = semimetal.

Sphalerite (ZnS) is also commonly found associated with galena, but has rarely been found to contain silver other than as silver-bearing inclusions.

As the detection limits for proton-induced X-ray emission (PIXE) [5-71 are lower than those for electron-probe microanalysis (EPMA) due to the lower background bremsstrablung radiation, a study of galena minerals for traces of Ag and Sb seemed a good candi- date for PIXE examination. The value of PIXE analysis in ore mineralogy has been demonstrated by Cabri [6,7] and Sie et al. [8], who have shown that it can be used to

~l~a-~83X~~/~O3_5~ Q Else-&r Science Publishers B.V. (North-Holland)

encompass a wide range of objectives, ranging from mineral beneficiation, through ore genesis, to trace-ele- ment analysis.

In this study, a number of galena specimens from British localities, some of which are known from old mining records to have produced large amounts of silver, in addition to lead, were examined by PIXE and EPMA. The specimens in which silver was detected by PIXE were then reanalysed using a scanning 1 pm proton-beam spot to map, in two dimensions, the distri- butions of the different trace elements within the region of interest.

2.1Experiment2l

Galena samples were chosen from nine localities (table I), sectioned with a diamond saw, mounted in epoxy resin and thin-sectioned. They were then polished using successively finer grades of diamond paste and buffed with y-alumina. The sections were examined optically to define regions of interest which were then marked. In every section at least one area was free from detectable (i.e. > 0.5 pm) inclusions. In one of the sec- tions (specimen 6) a second area was also defined where a large grain of tetrahedrite was partially replaced by galena. Prior to PIXE and EPMA analysis all the speci- mens were coated with a thin carbon film to prevent electrical charging.

Initially EPMA was used in an attempt to locate silver. This was done with a Cambridge Instruments Microscan IX microprobe at the British. Museum (Natu- ral History) using 20 kV accelerating voltage and 25 nA on the Faraday cage. Pure PbS and Ag standards were employed. Silver was not detected (above the detection

IV. PIXE, PIGE

328 K.J. Reeson et al. / Trace element c~~~en~ra~~on~ in a series of PbS specimens

Table 1 Locality and mineralogy of galena specimens analysed by PIXE and EPMA

Specimen Locality Inclusions (from optical examination)

2 Pettergill none 3 Clan&l Burn minor 4 Trial shaft, none

Harwood Valley 5 Stronchullin none

6(i) Gilson’s Cove (trial), none Port Quin, Cornwall

6(n) Gilson’s Cove (trial), tetrahedrite, Port Quin, Cornwall sphalerite

7 Greystone Quarry none

8 Wheal Kitts, Lydford none 9 Wheal Mary Ann, minor chalcopyrite,

Menheniot, Cornwall tetrahedrite 10 Combe Martin none

limit of about 100-200 ppm (0.01-0.02 wt.%)) in any of the areas analysed. However, silver-bearing inclusions such as tetrahedrite were identified.

The initial PIXE analysis was carried out at the University of Surrey using a 2 MeV, 200 pA stationary proton beam focussed down to a spot size of = 50 pm. To ensure that the correct area of the specimen was being analysed (since galena- does not fluoresce under the proton beam), a small hole was made in a glass

cover slip and placed over the area marked for analysis. Thus, when beam fluorescence could no longer be ob- served, the beam was on the designated area.

From these preliminary analyses, two specimens (6 and 9) in which silver was detected were selected for further analysis with the scanning proton microprobe at the University of Oxford [9,10], using 3 MeV protons focussed down to a spot size of 1 urn at a beam current of 100-200 pA. The X-rays were detected using a Link Systems 80 mm2 Si(Li) detector (resolution 140 eV at 5.9 keV). The beam was scanned over an area of 200 x 200 pm and the marked area was selected by examina- tion of the secondary-electron image. To reduce the count rate from the lead lines, a thick filter of 2.5 mm of perspex was used. This absorbed X-rays with energies < 5 keV, and cons~uen~y it was not possible to map the S dist~bution. By windowing the appropriate lines in the X-ray spectrum above 5 keV it was possible to build up elemental maps of the armlysed region. These elements were Ag, Cu, Zn, Fe, Ni, Sb, Te and Ca (9, lines), and Pb (the main L, line). Once the elemental maps had been collected the ,beam scanning was switched off and selected areas were analysed with a stationary proton beam. The PIXE spectra were analysed using the code PIXAN [ll] and normalized to the nominal Pb concentration. Subsequent to PIXE analy- sis, these areas were also analysed by EPMA.

3. Results and discussion

Fig. 1 shows the elemental maps of Ag, Cu, Pb, Sb, Fe and Zn for area 2 of specimen 6 with a reflected-light

Fig. 1. Optical micrograph (a) and elemental maps (b) of area 2 of specimen 6. Each map represents a scan size of 200 X

vs inclusions of tetrahedrite (T) and sphalerite (S) in a matrix of galena (G). The Pb and Fe maps show the regions concentrated around the margins of the tetrahedrite, where it is replaced by galena.

200 urn and where Fe is

Fig. 1 (continued).

IV.

330 K.J. Reeso~l et al. / Trace element concentrations in a series of PbS specimens

photomicrograph of this area. The photomicrograph shows that the area contains inclusions of tetrahedrite (T) and sphalerite (S) in the galena (G) matrix. It is clear from the elemental mapping that the silver is concentrated in the tetrahedrite phase with only back- ground levels being detected in the galena and sphalerite. This is true even around the margins of the tetrahedrite grain where it is replaced by galena and where diffusion of silver and antimony into the galena might be ex- pected to have occurred. Apart from silver, the tetra- hedrite also contains iron and zinc impurities and it is ~terest~g to note that, while zinc appears to be uni- formly distributed, iron appears to be concentrated around the margins where the tetrahedrite is replaced by the galena (shown by the Pb and Fe maps in fig. 1). Examination of the sphalerite (S) grains shows the presence of copper, iron and minor antimony impuri- ties, all of which appear to be evenly distributed.

Fig. 2 illustrates the distribution of the elements Ag, Pb, Cu, Fe, Zn and Sb for area 1 of specimen 9, together with a reflected-light photomicrograph. Apart from galena (G), this area contains an inclusion of the copper-iron sulphide chalcopyrite (C) in the upper right-hand comer (associated with a minor amount of tetrahedrite) and a fracture (F) running at 45 o from the left-hand comer towards the chalcopyrite grain. Ex- amination of the silver distribution over this 200 X 200 pm area reveals that the silver is concentrated in small (10-15 pm) localized areas which lie near and parallel

to the fracture (fig. 2). The copper and antimony distri- butions show a similar pattern of high concentrations, associated with the fracture. PIXE analysis using a stationary 1 pm beam gives 0.41 wt.% silver in region I close to the fracture, while in region II, in the upper left-hand corner of the maps, the silver concentration is measured at background levels. It appears, therefore, that, as with specimen 6, the silver detected resides as discrete inclusions (minute in this case) of Ag, Cu and Sb bearing sulpho-salts which are associated with a fracture in the galena, and thus may have been formed at a later stage in the paragenesis.

4. Conclusions

The marked areas of specimens 6 from Gilson’s Cove (trial) and 9 from Wheal Mary Ann were the only areas in which silver was detected above background. In both cases the silver was found as inclusions of argen- tian tetrahedrite (specimen 6) and as other Cu-, Ag-, Sb-bearing sulpho-salts (specimen 9) too small for posi- tive identification. We cannot rule out minor solid solution since the background of the silver signal is l&h, due to the presence of the lead lines. However, it is interesting that production statistics from the years 1852-1874 for Wheal Mary Ann give a total of 1103 097 ounces of silver from 24099 tons of lead ore which yielded 16429 tons of lead [12]. This is equivalent to

Fig. 2. Optical micrograph (a) and elemental maps (b) of area 1 of specimen 9. Each map represents a scan size of 200 X 200 pm and shows an inclusion of chalcopyrite (C) and a fracture (F) in the galena matrix. The arrow indicates the presence of small Ag-, Cu- and Sb-rich inclusions which run parallel to the fracture (region I). Regions I and II mark the areas where PIXE analysis was carried out

using a stationary proton beam.

K.J. Reeson et al. / Trace element concentrations in a series of PbS specimens 331

Fig. 2 (continued).

IV. PIXE, PIGE

332 K.J. Reeson et al. / Trace element concentrations in a series of PbS specimens

1819 ppm Ag. It seems probable, therefore, that in these examples most of the silver produced came from silver- bearing inclusions within the galena.

The authors would like to thank the UK Science and Engineering Research Council for their partial support of this work. They would also like to thank Karen Bray for her patience and help in typing this manuscript.

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