- Email: [email protected]
Proton-microprobe analyses of platinum-group element and gold grains
.__b!B
&
ELSEVIER
RIONil B
Beam Interactions with Materials 8 Atoms Nuclear Instruments
and Methods in Physics ResearchB 130 (1997) 666-670
Proton-microprobe analyses of platinum-group element and gold grains M. Maetz a3*, B. Orberger b*c,K. Traxel d, P. Wathanakul
e, S. Pitragool
f
a Max-Planck-lnstitutf~r Kernphysik. P.O. Box 103980, 69029 Heidelberg. Germany h Laboratoire de Gkochmnologie, lnstitut de Physique du Globe de Paris, Universite’ Denis Diderot. Paris, Frunce ’ LEPCAT. Unicersitl de La Rochelle. La Rochelle. France * Physikalisches Institut, Unioersitiit Heidelberg, Heidelberg. Germuny ’ Department qj’General Sciences, Kasetsart Unioersity, Bangkok, ThaiLund ’ Department of Geological Sciences, Chiang Mai Uniuersiv, Chiang Mai. Thailund
Abstract Noble metal grains were separated from sediments of a paleoterrace of the Mekong river, northeast Thailand. Preliminary scanning-electron microscopy investigations show that all levels of the sediment are characterized by the coexistence of Au and Pt grains. This coexistence in one geological site is rare. Proton-microprobe analyses were carried out in order to detect trace elements which are indicative of their source region(s) and the physico-chemical conditions during formation. Special attention was given to the trace-element contamination of the used absorbers and to the influence of the response function of our Si(Li)-detector on the data evaluation. Both effects play an important role in judging the reliability of our results. 0 1997 Elsevier Science B.V.
1. Introduction
Platinum-group minerals (PGM) have long been considered as resistant to chemical attack during weathering. Three interacting processes have been proposed for the formation of alluvial noble metals: (a) simple erosion departing from the hostrock (e.g. ultramafic for PGM and granite for Au) and translocation of the particles [I]; (b) erosion followed by overgrowth of PGE/Au which were dissolved in the meteoric water [2-41; (c> complete dissolution and neocrystallization of PGM/Au from aqueous solutions [5].
* Corresponding author: Tel: +49 6221 516210; 6221 516540; e-mail: [email protected]
fax:
+49
The paleoplacer of the ancient Mekong terrace consists mainly of Pt-Fe and Au-Ag alloys. The objective of this study is the determination of the trace-elements contents, particularly the other noble metals (Rh, Ir, OS, Ru, Pd) and those indicative of the geologically favourable source region (e.g. As, Hg, Cu, Ni, etc.). This helps to constrain the conditions of the formation. Trace elements down to levels of a few ppm can be analysed by the proton microprobe. Platinumgroup elements have been analysed by PIXE mainly in sulfides [6-91 or in fluid inclusions [lo]. Native gold contains traces of Ag (1.4-30%), Hg, Bi, Cu, Pd, As and Mn [ll]. This study presents the first PIXE analyses on platinum-group element (PGE) alloys and tries to enhance the knowledge of trace
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO168-583X(97)00263-2
661
M. Maetz et al. / Nucl. Instr. and Meth. in Phys. Res. B 130 (1997) 666-670
elements in native gold. Some detector artefacts like ghost lines and bumps and secondary fluorescence phenomena that distort the evaluation of some traceelement data will be discussed.
over 1 m from the surface (sample distance: Each level contains Au and Pt alloys.
3. Mineralogy, methods
sample preparation
20 cm>.
and analytical
2. Geological setting The Mekong river forms the border between Laos and the northern and northeastern part of Thailand. Its ancient river bed was probably located south of the actual stream, in the region of Loei. The geology around the studied area consists of sedimentary rocks of marine or fluvial origin. Volcanic (basalts) and plutonic (granites) rocks are located north and west of the studied area. Ultramafic rocks, a potential PGE source, have recently been discovered along a north-south-oriented shearzone, northeast of Loei. Conglomerates, sandstone and sands covering the area about 20 km west of Nong Khai, represent likely recycled sediments forming a palaeoterrace of the Mekong River. A cross-section has been sampled
Fig. 1. (a) Pt-Fe alloy; (b) Ir-Os-Pt 300 fkm.
alloy; (c) Au-Ag
Polished thin sections were prepared from each grain and were embedded in 12 X 10 X 10 mm targets. The morphology and major-element composition were first studied by scanning-electron microscopy (SEMI and standardless EDX to characterize each grain. Four types of noble metal alloys (Fig. 1) can be distinguished: - Pt-Fe, - Ir-Os-Pt, * Au-Ag, and . Ag-Pd-Au. The SEM-investigations showed that Pt-Fe alloys contain euhedral Ir-0s and Ir-(OS)-As inclusions (Fig. la,b). The grains have been analysed with the Heidel-
alloy; (d) Ag-Pd-Au
alloy containing
Pt and In. Grain diameters
XII. GEOLOGY
are. approximately
AND PLANETARY
SCIENCE
Table I Sampling
depth of the analysed
particles
Depth in cm from surface
Grain types
Number of grains
Number of analysed points
O-IO 10-20
Au-Ag Au-Ag Pt-Fe Au-Ag Ag-Pd-Au
1
6
1 1 2 1 I 1 1
5 4+4 12/6 5 5 6 7
I
5
l 2
5 6/18 22
20-40
40-50 SO-70 70-90 90-100
Ir-OS-Pt Au-Ag Pt-Fe Au-Ag Pt-Fe Au-Ag R-Fe
1
berg proton microprobe [12]. Table 1 shows sample location of the analysed particles.
4. Experimental ses
the
problems during the PIXE analy-
Routine PIXE analyses of mineralogical samples usually deal with silicates and/or sulfides. The spectra of these targets are dominated mainly from Si/S and Fe/Cu lines, ranging up to 9 keV, whereas the interesting elements normally have higher energies. In these cases, secondary fluorescence phenomena may be neglected. In our case the energy regions of the major and trace elements overlap, with the consequence that secondary fluorescence from contaminations may falsify the trace-element results. Additionally, our detector shows artefacts (ghost lines and bumps) at higher energies (> 15 keV).
Comment
Fig. Ic Fig. Id Fig. lb
Fig. la
(high purity Ge), respectively; these are in the range of the trace-element data for most of the analysed grains. Consequently we discarded all Ni and Ta data and most of the Cu data. A comp~ison of different absorbers suggests a Ni, Cu and Ta contamination in the absorber stage and/or in the window stage of the detector, whereas an additional Ta contamination results from the AI absorber. 4.2. Ghost lines and abnormal line shapes In the high-energy region (E, > 15 keV), the Si(Li) detector from Link (called ‘old’ detector) shows peaks which cannot be explained by any elemental K- or L-lines. This can be seen in Fig. 3 where the pure element spectra from Pd and Ag are plotted. These occurring ‘ghost’ lines increase the
4.1. Secondary fluorescence phenomena Secondary fluorescence artefacts for Cu, Ni, and Ta were observed. To check the influence of possible contaminations, a high purity Ge target and a GaAs mono crystal were anaIysed. The energies of the K-lines of Ga, Ge, and As are very similar to those of the L-lines of Au and Pt (Fig. 2). The absorption edges of Cu, Ni, and Ta are also plotted. The spectra show clear Cu, Ni, and Ta peaks with corresponding concentrations of about 300 ppm (Ni and Cu> and 2000 ppm Ta (GaAs mono crystal) and 3700 ppm Ta
9.0
9.5
10.0
10.5
11.0
31.5
12.0
energy[keV]
Fig. 2. Comparison of the Ge, Ga, and As K-lines with the L-lines of Au and Pt. Additionally, the absorption edges of Ni, Cu. and Ta are shown.
M.
19
20
21
22
Muetz et
Instr. and
al. /iVucf.
23
24
25
lath.
26
energy[keV]
Fig. 3. Spectra of pure Pd and Ag. The occurring ‘ghost’ lines worsen the limits of detection for Ru and Rh.
1”
104 B 2
”
I
”
”
/
13
tector), another interesting feature typical for the high-energy region can be identified. Instead of ghost lines a broad peak (‘bump’) in the low-energy tailing region of the K-lines is approaching (Fig. 4). This ‘bump’ also affects the limits of detection in the energy region of interest (here Rh). The strength of this ‘bump’ is at maximum after the bias is switched on and decreases with measuring time, unfortunately, at the cost of an increased low energy tailing (Fig. 5). For the grain analyses only the ‘old’ Si(Li) detector with the ghost lines was available because of technical reasons.
5.1. Au-Ag
= IO' z4 t 10'
100
energy [k&J] Fig. 4. Analyses of a pure Ag target with two Si(Li) detectors. The ‘old’ detector shows ghost lines in the region of interest, the ‘new’ one a ‘bump’.
limits of detection mainly for the elements Ru or Rh in matrices containing Pd or Ag as a major or minor element. In the spectrum of a pure Ag target taken with our second Si(Li) detector from Link (called ‘new’ de-
10' 17
18
19
20
21
22
23
669
5. Results and discussion
103
15
666-670
‘I
2 b
2 0
in Phys. i&s. B 130 ff997)
24
25
26
27
energy [k&/j Fig. 5. Time resolved spectra of pure Ag taken with the ‘new’ detector. The ‘bump’ in the Iow-energy region of the K-lines decreases with measuring time at the cost of low-energy tailing.
grains
The grains contain between 81 .O and 99.7 wt% Au and between 0.2 and 13.0 wt% Ag, whereas Ag is mostly located in micrometer and submicrometer sized pure Ag inclusions. Fe is very inhomogeneously distributed at the grain scale with concentrations up to 17 wt%. A few data points show small amounts (up to 870 ppm) of Pd. Detector artefacts (ghost lines) allow no reliable Rh data for the Au-Ag grains. The elements OS, Ir, and Pt were not detected. 5.2. Pt-Fe
grains
The average major-element composition is approximately 90 wt% Pt and 7.5 wt% Fe. Trace elements are Pd (1800-9000 ppm), Rh (800-8700 ppm), Ir (3700-9500 ppm), and OS (1400-5800 ppm). Some grains contain up to 1000 ppm Ag. The ghost lines of Pd increase the limit of detection for Ru. Nevertheless, a few data points show Ru contents up to 940 ppm. Two particles show large amounts of Cu (2000-6000 ppm). Euhedral inclusions composed of Ir-OS-AS-S with Rh and Ru in the percent range and micrometer- and submicrometer-sized Ir-Os-inclusions were observed. The Pt-Fe alloys host Ir, Ru, OS, Rh, Pd, and Ag with the Platinum-ME (PPGE) Pt, Pd, Rh being higher concentrated than the Iridium-PGE (IPGE) Ir, Ru, OS. The fractionation into IPGE and PPGE is commonly observed in rocks derived from fractional crystallizaXII. GEOLOGY AND PLANETARY SCIENCE
tion. It is suggested that Ir, OS, and Ru were derived, at least in part, from an ultramafic source rack. 5.3. Ir-Os-Pt
grains
The grains consist on average of 63 wt% Ir, 27 wt% OS, and 9.3 wt% Pt. Trace elements are Ru (2000-~~ ppm> and Rh (8~-19~ ppm). Ag and Pd were not detected. 5.4. Ag-P&Au
grains
Constituting elements are Ag (32.2-36.0 wt%>, Au (27.3-30.8 wt%), Pd t21.8--23.4 wt%), Pt t&710.2 wt%>, and In (4.91-5.18 wt%o). The large amounts of Ag and Pd do not allow us to get reliable results for Ru and Rh. The spongy texture of the native Ag grains suggests that they represent neoformations, precipitated from the meteoric water. Similar textures have been observed in the ultramafic rocks which were affected by initial weathering. Those features were clearly neocrystallizations from the circulating meteoric water [13]. Thus, the Pt and Pd-traces of the Ag-aggregates were also dissolved in the circulating waters.
6. Conclusions Although all levels of the sampled profile contain the two types of noble metal alloys (Pt-Fe and Au-Ag), their composition differs in each level with the minor and trace elements being inhomogeneously distributed at the grain scale. Artefacts from the Heidelberg setup, especially seconds-~uorescence
phenomena and the abnormal line shape of the Si(Li) detector in the high-energy region, increase the limits of detection for some trace elements in individual grains to several 100 ppm, and in the case of Ta, up to 2000 ppm. Nevertheless, these first PIXE analyses of natural platinum-group alloys give direct evidence for weathering processes.
References [l] G.T. Nixon, L.J. Cabri and J.H.G. Laflamme, Canadian Mineralogist 28 (1990) 503. 1’21J.F.W. Bowles, Bulletin de M~n~~ogie 104 (1981) 478. 131 J.F.W. Bowles, Econ. Geol. 81 (1986) 1278. [4] K.P. Burgath, in: Geo-Platinum 87, eds. H.M. Prichard, P.J. Potts, J.F.W. Bowles and S.J. Cribb (Elsevier, New York, 1988) p. 383. [5] B. Orberger and J. Alleweldt, Journal of Southeast Asian Sciences 9 (3) (1994) 229. [6] L.J. Cabri, G.J.H. Campbell, R.G. Laflamme, R.G. Leigh, J.A. Maxwell, J.D. Scott and K. Tmxel, Canadian Mineralogist 23 (1985) 133. [7] B. Orberger and K. Traxel, Nucl. Instr. and Meth. B 54 (1991) 304. [8] D.L. Huston, S.H. Sie, G.F. Suter, D.R. Cooke and R.A. Both, Econ. Geol. 90 (1995) 1167. 191 D.D. Hickmott and W.S. Baldridge, Econ. Geol. 90 (1995) 246. [IO] C. Ballhaus, C.G. Ryan, T.P. Memagh and D.H. Green, GCA 58 (2) (1994) 811. [Ill S.H. Sie, S. Murao and G.F. Suter, Nucl. Instr. and Meth. B 109/l IO (1996) 633. [12] K. Trnxel, P. Amdt, J. Bohsung, K.U. Braun-Dull~us, M. Maetz, D. Reimold, H. Schiebler and A. Wallianos, Nucl. Instr. and Meth. B 104 (1995) 19. [I31 B. Orberger, G. Friedrich and E. Waxen, in: GeoPlatinum 87, eds. H.M. Prichard, P.J. Potts. J.F.W. Bowles and S.J. Cribb (Elsevier, New York, 1988) p. 361.
&
ELSEVIER
RIONil B
Beam Interactions with Materials 8 Atoms Nuclear Instruments
and Methods in Physics ResearchB 130 (1997) 666-670
Proton-microprobe analyses of platinum-group element and gold grains M. Maetz a3*, B. Orberger b*c,K. Traxel d, P. Wathanakul
e, S. Pitragool
f
a Max-Planck-lnstitutf~r Kernphysik. P.O. Box 103980, 69029 Heidelberg. Germany h Laboratoire de Gkochmnologie, lnstitut de Physique du Globe de Paris, Universite’ Denis Diderot. Paris, Frunce ’ LEPCAT. Unicersitl de La Rochelle. La Rochelle. France * Physikalisches Institut, Unioersitiit Heidelberg, Heidelberg. Germuny ’ Department qj’General Sciences, Kasetsart Unioersity, Bangkok, ThaiLund ’ Department of Geological Sciences, Chiang Mai Uniuersiv, Chiang Mai. Thailund
Abstract Noble metal grains were separated from sediments of a paleoterrace of the Mekong river, northeast Thailand. Preliminary scanning-electron microscopy investigations show that all levels of the sediment are characterized by the coexistence of Au and Pt grains. This coexistence in one geological site is rare. Proton-microprobe analyses were carried out in order to detect trace elements which are indicative of their source region(s) and the physico-chemical conditions during formation. Special attention was given to the trace-element contamination of the used absorbers and to the influence of the response function of our Si(Li)-detector on the data evaluation. Both effects play an important role in judging the reliability of our results. 0 1997 Elsevier Science B.V.
1. Introduction
Platinum-group minerals (PGM) have long been considered as resistant to chemical attack during weathering. Three interacting processes have been proposed for the formation of alluvial noble metals: (a) simple erosion departing from the hostrock (e.g. ultramafic for PGM and granite for Au) and translocation of the particles [I]; (b) erosion followed by overgrowth of PGE/Au which were dissolved in the meteoric water [2-41; (c> complete dissolution and neocrystallization of PGM/Au from aqueous solutions [5].
* Corresponding author: Tel: +49 6221 516210; 6221 516540; e-mail: [email protected]
fax:
+49
The paleoplacer of the ancient Mekong terrace consists mainly of Pt-Fe and Au-Ag alloys. The objective of this study is the determination of the trace-elements contents, particularly the other noble metals (Rh, Ir, OS, Ru, Pd) and those indicative of the geologically favourable source region (e.g. As, Hg, Cu, Ni, etc.). This helps to constrain the conditions of the formation. Trace elements down to levels of a few ppm can be analysed by the proton microprobe. Platinumgroup elements have been analysed by PIXE mainly in sulfides [6-91 or in fluid inclusions [lo]. Native gold contains traces of Ag (1.4-30%), Hg, Bi, Cu, Pd, As and Mn [ll]. This study presents the first PIXE analyses on platinum-group element (PGE) alloys and tries to enhance the knowledge of trace
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SO168-583X(97)00263-2
661
M. Maetz et al. / Nucl. Instr. and Meth. in Phys. Res. B 130 (1997) 666-670
elements in native gold. Some detector artefacts like ghost lines and bumps and secondary fluorescence phenomena that distort the evaluation of some traceelement data will be discussed.
over 1 m from the surface (sample distance: Each level contains Au and Pt alloys.
3. Mineralogy, methods
sample preparation
20 cm>.
and analytical
2. Geological setting The Mekong river forms the border between Laos and the northern and northeastern part of Thailand. Its ancient river bed was probably located south of the actual stream, in the region of Loei. The geology around the studied area consists of sedimentary rocks of marine or fluvial origin. Volcanic (basalts) and plutonic (granites) rocks are located north and west of the studied area. Ultramafic rocks, a potential PGE source, have recently been discovered along a north-south-oriented shearzone, northeast of Loei. Conglomerates, sandstone and sands covering the area about 20 km west of Nong Khai, represent likely recycled sediments forming a palaeoterrace of the Mekong River. A cross-section has been sampled
Fig. 1. (a) Pt-Fe alloy; (b) Ir-Os-Pt 300 fkm.
alloy; (c) Au-Ag
Polished thin sections were prepared from each grain and were embedded in 12 X 10 X 10 mm targets. The morphology and major-element composition were first studied by scanning-electron microscopy (SEMI and standardless EDX to characterize each grain. Four types of noble metal alloys (Fig. 1) can be distinguished: - Pt-Fe, - Ir-Os-Pt, * Au-Ag, and . Ag-Pd-Au. The SEM-investigations showed that Pt-Fe alloys contain euhedral Ir-0s and Ir-(OS)-As inclusions (Fig. la,b). The grains have been analysed with the Heidel-
alloy; (d) Ag-Pd-Au
alloy containing
Pt and In. Grain diameters
XII. GEOLOGY
are. approximately
AND PLANETARY
SCIENCE
Table I Sampling
depth of the analysed
particles
Depth in cm from surface
Grain types
Number of grains
Number of analysed points
O-IO 10-20
Au-Ag Au-Ag Pt-Fe Au-Ag Ag-Pd-Au
1
6
1 1 2 1 I 1 1
5 4+4 12/6 5 5 6 7
I
5
l 2
5 6/18 22
20-40
40-50 SO-70 70-90 90-100
Ir-OS-Pt Au-Ag Pt-Fe Au-Ag Pt-Fe Au-Ag R-Fe
1
berg proton microprobe [12]. Table 1 shows sample location of the analysed particles.
4. Experimental ses
the
problems during the PIXE analy-
Routine PIXE analyses of mineralogical samples usually deal with silicates and/or sulfides. The spectra of these targets are dominated mainly from Si/S and Fe/Cu lines, ranging up to 9 keV, whereas the interesting elements normally have higher energies. In these cases, secondary fluorescence phenomena may be neglected. In our case the energy regions of the major and trace elements overlap, with the consequence that secondary fluorescence from contaminations may falsify the trace-element results. Additionally, our detector shows artefacts (ghost lines and bumps) at higher energies (> 15 keV).
Comment
Fig. Ic Fig. Id Fig. lb
Fig. la
(high purity Ge), respectively; these are in the range of the trace-element data for most of the analysed grains. Consequently we discarded all Ni and Ta data and most of the Cu data. A comp~ison of different absorbers suggests a Ni, Cu and Ta contamination in the absorber stage and/or in the window stage of the detector, whereas an additional Ta contamination results from the AI absorber. 4.2. Ghost lines and abnormal line shapes In the high-energy region (E, > 15 keV), the Si(Li) detector from Link (called ‘old’ detector) shows peaks which cannot be explained by any elemental K- or L-lines. This can be seen in Fig. 3 where the pure element spectra from Pd and Ag are plotted. These occurring ‘ghost’ lines increase the
4.1. Secondary fluorescence phenomena Secondary fluorescence artefacts for Cu, Ni, and Ta were observed. To check the influence of possible contaminations, a high purity Ge target and a GaAs mono crystal were anaIysed. The energies of the K-lines of Ga, Ge, and As are very similar to those of the L-lines of Au and Pt (Fig. 2). The absorption edges of Cu, Ni, and Ta are also plotted. The spectra show clear Cu, Ni, and Ta peaks with corresponding concentrations of about 300 ppm (Ni and Cu> and 2000 ppm Ta (GaAs mono crystal) and 3700 ppm Ta
9.0
9.5
10.0
10.5
11.0
31.5
12.0
energy[keV]
Fig. 2. Comparison of the Ge, Ga, and As K-lines with the L-lines of Au and Pt. Additionally, the absorption edges of Ni, Cu. and Ta are shown.
M.
19
20
21
22
Muetz et
Instr. and
al. /iVucf.
23
24
25
lath.
26
energy[keV]
Fig. 3. Spectra of pure Pd and Ag. The occurring ‘ghost’ lines worsen the limits of detection for Ru and Rh.
1”
104 B 2
”
I
”
”
/
13
tector), another interesting feature typical for the high-energy region can be identified. Instead of ghost lines a broad peak (‘bump’) in the low-energy tailing region of the K-lines is approaching (Fig. 4). This ‘bump’ also affects the limits of detection in the energy region of interest (here Rh). The strength of this ‘bump’ is at maximum after the bias is switched on and decreases with measuring time, unfortunately, at the cost of an increased low energy tailing (Fig. 5). For the grain analyses only the ‘old’ Si(Li) detector with the ghost lines was available because of technical reasons.
5.1. Au-Ag
= IO' z4 t 10'
100
energy [k&J] Fig. 4. Analyses of a pure Ag target with two Si(Li) detectors. The ‘old’ detector shows ghost lines in the region of interest, the ‘new’ one a ‘bump’.
limits of detection mainly for the elements Ru or Rh in matrices containing Pd or Ag as a major or minor element. In the spectrum of a pure Ag target taken with our second Si(Li) detector from Link (called ‘new’ de-
10' 17
18
19
20
21
22
23
669
5. Results and discussion
103
15
666-670
‘I
2 b
2 0
in Phys. i&s. B 130 ff997)
24
25
26
27
energy [k&/j Fig. 5. Time resolved spectra of pure Ag taken with the ‘new’ detector. The ‘bump’ in the Iow-energy region of the K-lines decreases with measuring time at the cost of low-energy tailing.
grains
The grains contain between 81 .O and 99.7 wt% Au and between 0.2 and 13.0 wt% Ag, whereas Ag is mostly located in micrometer and submicrometer sized pure Ag inclusions. Fe is very inhomogeneously distributed at the grain scale with concentrations up to 17 wt%. A few data points show small amounts (up to 870 ppm) of Pd. Detector artefacts (ghost lines) allow no reliable Rh data for the Au-Ag grains. The elements OS, Ir, and Pt were not detected. 5.2. Pt-Fe
grains
The average major-element composition is approximately 90 wt% Pt and 7.5 wt% Fe. Trace elements are Pd (1800-9000 ppm), Rh (800-8700 ppm), Ir (3700-9500 ppm), and OS (1400-5800 ppm). Some grains contain up to 1000 ppm Ag. The ghost lines of Pd increase the limit of detection for Ru. Nevertheless, a few data points show Ru contents up to 940 ppm. Two particles show large amounts of Cu (2000-6000 ppm). Euhedral inclusions composed of Ir-OS-AS-S with Rh and Ru in the percent range and micrometer- and submicrometer-sized Ir-Os-inclusions were observed. The Pt-Fe alloys host Ir, Ru, OS, Rh, Pd, and Ag with the Platinum-ME (PPGE) Pt, Pd, Rh being higher concentrated than the Iridium-PGE (IPGE) Ir, Ru, OS. The fractionation into IPGE and PPGE is commonly observed in rocks derived from fractional crystallizaXII. GEOLOGY AND PLANETARY SCIENCE
tion. It is suggested that Ir, OS, and Ru were derived, at least in part, from an ultramafic source rack. 5.3. Ir-Os-Pt
grains
The grains consist on average of 63 wt% Ir, 27 wt% OS, and 9.3 wt% Pt. Trace elements are Ru (2000-~~ ppm> and Rh (8~-19~ ppm). Ag and Pd were not detected. 5.4. Ag-P&Au
grains
Constituting elements are Ag (32.2-36.0 wt%>, Au (27.3-30.8 wt%), Pd t21.8--23.4 wt%), Pt t&710.2 wt%>, and In (4.91-5.18 wt%o). The large amounts of Ag and Pd do not allow us to get reliable results for Ru and Rh. The spongy texture of the native Ag grains suggests that they represent neoformations, precipitated from the meteoric water. Similar textures have been observed in the ultramafic rocks which were affected by initial weathering. Those features were clearly neocrystallizations from the circulating meteoric water [13]. Thus, the Pt and Pd-traces of the Ag-aggregates were also dissolved in the circulating waters.
6. Conclusions Although all levels of the sampled profile contain the two types of noble metal alloys (Pt-Fe and Au-Ag), their composition differs in each level with the minor and trace elements being inhomogeneously distributed at the grain scale. Artefacts from the Heidelberg setup, especially seconds-~uorescence
phenomena and the abnormal line shape of the Si(Li) detector in the high-energy region, increase the limits of detection for some trace elements in individual grains to several 100 ppm, and in the case of Ta, up to 2000 ppm. Nevertheless, these first PIXE analyses of natural platinum-group alloys give direct evidence for weathering processes.
References [l] G.T. Nixon, L.J. Cabri and J.H.G. Laflamme, Canadian Mineralogist 28 (1990) 503. 1’21J.F.W. Bowles, Bulletin de M~n~~ogie 104 (1981) 478. 131 J.F.W. Bowles, Econ. Geol. 81 (1986) 1278. [4] K.P. Burgath, in: Geo-Platinum 87, eds. H.M. Prichard, P.J. Potts, J.F.W. Bowles and S.J. Cribb (Elsevier, New York, 1988) p. 383. [5] B. Orberger and J. Alleweldt, Journal of Southeast Asian Sciences 9 (3) (1994) 229. [6] L.J. Cabri, G.J.H. Campbell, R.G. Laflamme, R.G. Leigh, J.A. Maxwell, J.D. Scott and K. Tmxel, Canadian Mineralogist 23 (1985) 133. [7] B. Orberger and K. Traxel, Nucl. Instr. and Meth. B 54 (1991) 304. [8] D.L. Huston, S.H. Sie, G.F. Suter, D.R. Cooke and R.A. Both, Econ. Geol. 90 (1995) 1167. 191 D.D. Hickmott and W.S. Baldridge, Econ. Geol. 90 (1995) 246. [IO] C. Ballhaus, C.G. Ryan, T.P. Memagh and D.H. Green, GCA 58 (2) (1994) 811. [Ill S.H. Sie, S. Murao and G.F. Suter, Nucl. Instr. and Meth. B 109/l IO (1996) 633. [12] K. Trnxel, P. Amdt, J. Bohsung, K.U. Braun-Dull~us, M. Maetz, D. Reimold, H. Schiebler and A. Wallianos, Nucl. Instr. and Meth. B 104 (1995) 19. [I31 B. Orberger, G. Friedrich and E. Waxen, in: GeoPlatinum 87, eds. H.M. Prichard, P.J. Potts. J.F.W. Bowles and S.J. Cribb (Elsevier, New York, 1988) p. 361.