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First experiences with a tube-excited energy-dispersive X-ray fluorescence in field laboratories
Journal of Geochemical Exploration, 21 (1984) 373--383
373
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
FIRST EXPERIENCES WITH A TUBE-EXCITED ENERGY-DISPERSIVE X-RAY FLUORESCENCE IN FIELD LABORATORIES
U. KRAMAR
Institute of Petrography and Geochemistry, University o f Karisruhe, Kaiserstr. 12, D+7500 Karlsruhe (F.R. Germany ) (Received January 9, 1984)
ABSTRACT
Kramar, U., 1984. First experiences with a tube-excited energy-dispersive X-ray fluorescence in field laboratories. J. Geochem. Explor., 21: 373--383. An energy-dispersive X-ray fluorescence system, equipped with air-cooled X-ray tube, Si(Li)-detector, a 20-sample changer, and a minicomputer was adapted to a mobile laboratory, installed in a Landrover 109. With this system different conditions for the excitation of characteristic lines of trace elements were tested and compared with the qualities of a previously used radionuclide-excited system. Up to 18 elements can be determined simultaneously in the course of one measurement (K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, W, Pb, and Th). For most elements concentrations in common rocks are clearly above the detection limits (e.g., Ni to Pb, 5 ppm; Th, 10 ppm). Two case histories (Kaiserstuhl, F.R.G. and Bastnlls, Central Sweden) demonstrate the applicability of the mobile tube XRF.
INTRODUCTION
If minimum delay between sampling and receiving results is essential in geochemical exploration, for example in anomaly follow up or overburden drilling program, field laboratories are able to save time. A field laboratory avoids the need to disrupt normal laboratory operations in central laboratories with request for "High Priority -- Rush" analyses (Fletcher, 1981). Most field laboratories are equipped for colorimetric methods, small radionuclide filter XRF (Kunzendorf, 1973; Ball et al., 1979) or atomic absorption spectrometry (AAS) (Levinson, 1974). All these methods can only be employed to determine one element at a time, and for most of these methods the elements must be transferred into solution. The USGS has used emission spectrographs installed in trucks for several years (Canney et al., 1957; Ward et al., 1963; Holman and Durham, 1967). A radionuclide-excited energy-dispersive X-ray fluorescence (EDXRF), equipped with semiconductor detectors, has been used in a mobile laboratory for several years for multi-element prospecting (Kramar and Puchelt, 1982; Kramar, 1982; Kramar et al., 1982). 0375-6742/84/$03.00
© 1984 Elsevier Science Publishers B.V.
374
Due to the increasing prejudice against the use of all types of radioactive sources, the use of radionuclides for analytical purposes is more and more restricted and decreasing. In some countries it is almost impossible to import even sealed radioactive isotopes within short times for use in field laboratories. This in part p r o m p t e d the development of the mobile tube-excited X R F reported here. The application of tube-excited E D X R F systems for multi-element analysis in mobile laboratories was difficult in the past, due to weight, size, high power requirements and the need for cooling water. Today small X-ray tubes, which are cooled by natural air convection, are available. This paper describes such a system adapted to a mobile laboratory m o u n t e d in a small four-wheel drive truck (Landrover 109). The experiences of a first field application with this laboratory are reported here. INSTRUMENTATION OF THE MOBILE LABORATORY
A schematic diagram of the complete E D X R F system is shown in Fig. 1. A view of the tube-excited E D X R F system m o u n t e d in the cargo compartm e n t of the Landrover 109 is given in Fig. 2. A 1 m A / 5 0 kV air-cooled pulsed Rh-tube is m o u n t e d together with a Si(Li)-detector in the cabinet of a 20-sample changer (TRACOR SPECT R A C E 430). The Si(Li)-detector has an active area of 30 mm 2 and an energy resolution of 145 eV (FWHM} at 6.4 keV. The detector requires cooling by liquid nitrogen during operation. The detector is m o u n t e d in a 10-1 dewar, which is sufficient to operate for a period of 6 days. If an external 100-1 dewar is used, the laboratory will be able to operate for
5°%
oppy-drive
I
I-TI-si~ent
Fig. 1. Schematic diagram of tube-excited energy-dispersive X-ray fluorescence.
375
Fig. 2. Tube E D X R F mounted into a four-wheel drive truck (Landrover 109).
6--7 weeks without further supply of liquid nitrogen. Sample changer and X-ray power supply are installed in a portable instrument desk. The instrument desk is equipped with retractable wheels for easy and fast loading and unloading. The spectroscopy amplifiers, a multichannel analyzer with 1024 channels and 100 MHz ADC (CANBERRA 8100) and a minicomputer PDP 11/04 (32 K-bytes) with dual floppy-disk drive are inserted into an instrument rack which is fixed in the laboratory truck. The power requirements for the mobile XRF laboratory of approximately 2.2 kW for the XRF system itself and 1.1 kW for an airconditioner, can be supplied by any 220 V single-phase service, or by an auxiliary 5 kVA generator. The generator needs 20 1 gasoline per day.
376 ANALYTICAL The conditions for the excitation of the characteristic radiation for all elements of interest must be optimized. A filter wheel with up to 6 different filters between tube and sample is used to minimize the radiation background in the energy region used for analysis. The widest range o f application was obtained for a tube voltage of 35 kV using a Rh-filter between tube and sample. Due to this tube voltage/ filter combination the characteristic X-rays of the sample are excited nearly exclusively by Rh-K radiation and the radiation background is drastically reduced. Therefore the peak/background ratio increases. These conditions are suitable to determine up to 18 elements in the course of one measurem e n t (K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, W, Pb, and Th). Detection limits for soil, rock, or stream-sediment samples obtained with the tube EDXRF are compared with those of radionuclide EDXRF in Table I. These detection limits are valid for a processing time of 10 min per sample. The processing time includes the time for measurement, data transfer, and data evaluation for the simultaneous determination o f all 18 elements. For several elements a clear improvement o f the detection limit was achieved with the tube-excited system. For most of the elements the stated detection limits are smaller than the concentrations f o u n d in most natural samples. With a tube voltage of 50 kV and Cu-filter between tube and sample, elements with atomic numbers 40--60 can be determined. The determination of Sn and Nd is less sensitive than with radionuclide EDXRF. In particular the determination of Ba, La, Ce, and Nd, is limited by a high Bremsstrahlung-radiation background {Table I).
TABLE I Detection limits of trace elements in rock, soil, and stream-sediment samples for mobile tube-excited EDXRF and radionuclide-exeited EDXRF (ppm) Ni
Tube EDXRF 35 kV 5O kV Radionucl. Cd I°9 EDXRF Am241
Cu
EDXRF
A m 241
Ga
As
Br
Rb
Sr
5
5
5
5
5
5
5
5
100
50
30
30
10
10
10
10
Y Tube EDXRF 35 kV 50 kV Radionucl. Cd '°9
Zn
Zr
Nb
Pb
Th
5
5
5
5
10
10
10
20
Ba
La
Ce
10 70 70 70 50 100 100 100 5 10 10
377
A comprehensive description of the analytical procedure, that is sample preparation and matrix correction, is given in Kramar and Puchelt (1982). Shortly, the sample material is sieved to - 1 7 7 ~m ( - 8 0 mesh) or finely ground. An aliquote of 5--10 g is used as bulk p o w d e r sample in a polystyrol container with 6 p m mylar window for measurement. After 500 s collecting time the spectra are transferred to a computer, and the element concentrations are calculated. Matrix differences are corrected using the Rh-Comptonpeak. The E D X R F system is calibrated using the standard addition m e t h o d or geochemical reference materials. Data evaluation is performed by a c o m p u t e r program PROCO2 (Kramar, 1982) immediately after the measurement of the corresponding spectra. During the computation the next spectrum is collected. Statistical data treatment can be easily performed in field with the computerized analytical results. Frequency distributions, element concentrations, as well as element ratios can be plotted. Using a measuring time of 400 s live time an o u t p u t of 50--60 samples can be achieved within 10 h, i.e., up to 1000 single-element determinations. The sample o u t p u t can be increased using shorter measuring times (100 samples day -1 ) if slightly reduced detection levels are acceptable. APPLICATIONS
Carbonatite mineralization (Kaiserstuhl, F.R. Germany) A geochemical overburden survey for carbonatite has been carried o u t in the Kaiserstuhl mountains at Ohrberg/Schelingen (Upper Rhine Valley, FRG) using the radioisotope-excited E D X R F system (Kramar, 1982; Kramar and Puchelt, 1983). These investigations were extended to the south using the mobile tube-excited system. The aim of this survey was to delineate the soil-covered outcrops of a Nb-bearing carbonatite dyke described in an unpublished report by R. Eigenfeld in 1947 b u t n o t n o t e d in geological maps. The Kaiserstuhl mountains are situated 60 km north of Basle and a b o u t 200 km west of Munich in the upper Rhine valley. The survey area is located east of Vogtsburg at Eichelspitze. Some 500 soil samples were taken along 7 traverses with a total length of 5 km. The distance between the sampling points was 10 m. The soil samples were collected with a Piirckhauer earth auger from a depth o f 20--90 cm from the B/C horizon, where possible. The samples were dried at 60°C and sieved to - 1 7 7 ~m. This material was used without further treatment as bulk powder for the analysis with the tube-excited system. In a single measurement Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, St, Y, Zr, Nb, Pb, and Th were determined for each sample. Figures 3 and 4 show the distribution of Ni and Nb. Strontium, Th, and Nb have nearly the same distribution pattern in the survey area. With Nb, Sr, Th, and the ratio of Fe/Mn as indicators a carbonatite d y k e with a length of at least 200 m and approximately 30 m thickness
378
I
C_,
7"42' cost
Nb < 100 ppm 1 0 0 - 200
N
ppm
200-
400 ppm
400-
800
> 800
ppm
ppm
48°06 , north I i i
"G
I i I
I
/ I /I /
i
I 0
, 100
I 200 m
// / I
/
iI
I
Fig. 3. Niobium distribution in soils of Eichelspitze, Kaiserstuhl mountains, F.R.G. (Capital letters indicate starting points of traverses).
379
could be localized (Fig. 5). With respect to petrological maps (Wimmenauer, 1966) the surrounding rocks could be outlined precisely by comparing the trace-element data of the overburden with known data of Kaiserstuhl rocks (van Wambeke, 1964). t
C-
7"4 2' east
Ni
=""=
< 50
i
ppm
5 0 - 80
ppm
8 0 - 150 ppm > 150 ppm
N
48°06 ' north I
E
~F
/
f
t/t k
/
//
/ /1
\
t
\
I /
1
i
0
100
I 200 m
/ i / /
/ *¢
1
/
/
//
!
/ r
s
q
Fig. 4. Nickel distribution in soils o f Eichelspitze, Kaiserstuhl mountains, F.G.R.
380 7°4 2' eost
cclrbonQtite
-.:,:.:~
phonolite P-::I ~TI
essexite oligocene
I
loess
i
end olluvio 48006 . north
oo
Q o o©
:o°: o:O: ,o o
~ o o
oo
,o
/
I
,
0
100
I. 200 m
/
/
I I
I
Fig. 5. Petrographical map of Eichelspitze as outlined by use of different trace-element distributions and element ratios in soil samples.
Cerite mineralization (Bastn6s, Central Sweden) A s e c o n d s u r v e y was carried o u t in c e n t r a l S w e d e n n e a r R i d d a r h y t t a n at t h e Bastn~is cerite d e p o s i t ( K r a m a r a n d Simianer, 1 9 8 4 ) . T h e R i d d a r h y t -
381
tan district is situated in the Skinnskatteberg region in the province of V~istmanland, a b o u t 150 km WNW of Stockholm. The oldest rocks in the Riddarhyttan district are leptites, mica-schists, and limestones of Precambrian age. The younger group consists more or less of gneissic granites with Searchean granite and diabase dykes. The limestones of the leptite series are often replaced by skarns accompanied b y different types of Fe ores, complex sulphides, and small occurrences of rare-earth mineralization. In order to localize cerite deposits, some 230 till and soil samples were taken. The mobile X R F laboratory was set up close to the B a s t n ~ deposit. The samples were treated as described above and analysed for Mn, Fe, Ni, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, Nb, W, Bi, Pb, and Th. Out of these elements Cu and Y showed distinct anomalies (in soils and till) displaced from the deposits in the flow direction of the glacial ice. Some 150 samples were, in a second course, analyzed for Ba, La, and Ce, using Cu filter and 50 kV tube voltage. The concentrations of Cu, Y, and Ce along one traverse are given as an example in Fig. 6, together with a map. Based on these analytical results new traverses with 50 samples were taken in order to find a connection between the anomalies. All analytical work was performed within 5 days.
,ool ,oI
60
;,,
:
~o~.i=--."" F--,.-~"
t
,o°°
,oo
,!!
"-':~...~7:~.-.,"~-;~"'-,.~", --.~:.)-._..i~__3... .... .~
600
'~
~d'i
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' :7';/
.......
(i.
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~
/B~stnes~ .... 1,o .......
,oo
,o
o
"
/
'
................................./ Illm SO00i~,. C¢ conctntrotion - ore ¢~eposlt
v
!/
,< .."'
i_s___/ I
t roverse
....
troil
Fig. 6. Traverse map and Cu, Y, and Ce distribution along one traverse in till south of the Bastniis cerite deposit, Central Sweden. Cerium concentrations measured immediately in field, are indicated at the traverse locations.
382 CONCLUSIONS
The field laboratory with tube-excited energy-dispersive X-ray fluorescence combines the flexibility and capacity for multielement determinations of a sophisticated stationary laboratory with the minimum delay time between sampling and analytical results of a field laboratory. Compared with conventional field methods the multielement analysis with tube E D X R F provides a lot of additional information. This can immediately and easily be interpreted by computerized methods. The possibility of an immediate feedback between results and further sampling will help to optimize the number of samples necessary to detect anomalies and to save time and money. The extremely short set-up time of the tubeexcited E D X R F (normally less than 1 h) enables easy and fast changing from one area to another. In remote areas the provision with liquid nitrogen is sometimes difficult to organize, but external dewars can provide an operation period of at least 6 weeks. We hope to solve the problem by using a cryogenerator or a small nitrogen liquifier. ACKNOWLEDGEMENT
These investigations were performed with financial support of grant 03 R 241 of the Ministerium fiir Forschung und Technologie (F.R.Germany). The samples for the Bastn~is prospect were taken by H. Simianer and A. Holl. Thanks are also due to Prof. Dr. H. Puchelt for improving the manuscript and all persons and institutions who thus made these investigations possible.
REFERENCES Ball, T.K., Booth, S.J., Nickless, E.F.P. and Smith, R.T., 1979. Geochemical prospecting for baryte and celestite using a portable radioisotope fluorescence analyzer. J. Geochem. Explor., 11: 277--284. Canney, F.C., Myers, A.T. and Ward, F.N., 1957. A truck-mounted spectrographic laboratory for use in geochemical exploration. Econ. Geol., 52: 289--306. Fletcher, W.K., 1981. Analytical methods in geochemical prospecting. In: G.J.S. Govett (Editor), Handbook of Exploration Geochemistry, Vol. I. Elsevier Scientific Publishing Comp., 255 pp. Hollman, R.H.C. and Durham, C.C., 1967. A mobile spectrographic laboratory. Geol. Surv. Can., Pap. 66--35, 15 pp. Kramar, U., 1982. Ein methodischer und regionaler Beitrag zur geochemischen Prospecktion auf Karbonatit. Dipl.Thesis, Univ. of Karlsruhe, 116 pp. Kramar, U. and Puchelt, H., 1981. Application of a radionuclide energy-dispersive X-ray fluorescence analysis in geochemical prospecting. J. Geochem. Explor., 15: 597-612. Kramar, U. and Puchelt, H., 1983. Geochemical multi-element prospecting for carbonatite by energy-dispersive X-ray fluorescence. J. Geochem. Explor., 19: 255--276. Kramar, U. and Simianer, H., 1984. Annual report. BMFT 03 R241.
383
Kramar, U., John, N. and Puchelt, H., 1982. Geochemische Prospektion unter Verwendung der energiedispersiven RSntgenfluoreszenzanalyse, ionensensitiver Elektroden und der instrumentellen Neutronenaktivierungsanalyse. BMFT-FB-T 82-152, FIZ, Karlsruhe, 76 pp. Kunzendorf, H., 1973. Die Isotop-RSntgenfluoreszenz Analyse und ihre Anwendung bei der geochemischen Untersuchung in GrSnland. PH.D. Thesis, T.H. Aachen. Levinson, A.A., 1974. Introduction to Exploration Geochemistry. Applied Publishing Ltd., Calgary, Alta., 612 pp. van Wambeke, L., 1964. La g~ochemie des roches du Kaiserstuhl. In: L. van Wambeke et al. (Editors), Les roches alcalines et les carbonatites du Kaiserstuhl. Euratom Publ. EUR 1827, d.e.f., Brussels, pp. 93--192. Ward, F.N., Lakin, H.W., Canney, F.C. et al., 1963. Analytical methods used in geochemical exploration by U.S. Geological Survey. U.S. Geol. Surv., Bull., 1152: 91--99. Wimmenauer, W., 1966. The eruptive rocks and carbonatites of the Kaiserstuhl, Germany. In: O.F. Tuttle and Y. Gittins (Editors), Carbonatites. Interscienees, John Wiley and Sons, Bungary, Suffolk, pp. 183--204.
373
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
FIRST EXPERIENCES WITH A TUBE-EXCITED ENERGY-DISPERSIVE X-RAY FLUORESCENCE IN FIELD LABORATORIES
U. KRAMAR
Institute of Petrography and Geochemistry, University o f Karisruhe, Kaiserstr. 12, D+7500 Karlsruhe (F.R. Germany ) (Received January 9, 1984)
ABSTRACT
Kramar, U., 1984. First experiences with a tube-excited energy-dispersive X-ray fluorescence in field laboratories. J. Geochem. Explor., 21: 373--383. An energy-dispersive X-ray fluorescence system, equipped with air-cooled X-ray tube, Si(Li)-detector, a 20-sample changer, and a minicomputer was adapted to a mobile laboratory, installed in a Landrover 109. With this system different conditions for the excitation of characteristic lines of trace elements were tested and compared with the qualities of a previously used radionuclide-excited system. Up to 18 elements can be determined simultaneously in the course of one measurement (K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, W, Pb, and Th). For most elements concentrations in common rocks are clearly above the detection limits (e.g., Ni to Pb, 5 ppm; Th, 10 ppm). Two case histories (Kaiserstuhl, F.R.G. and Bastnlls, Central Sweden) demonstrate the applicability of the mobile tube XRF.
INTRODUCTION
If minimum delay between sampling and receiving results is essential in geochemical exploration, for example in anomaly follow up or overburden drilling program, field laboratories are able to save time. A field laboratory avoids the need to disrupt normal laboratory operations in central laboratories with request for "High Priority -- Rush" analyses (Fletcher, 1981). Most field laboratories are equipped for colorimetric methods, small radionuclide filter XRF (Kunzendorf, 1973; Ball et al., 1979) or atomic absorption spectrometry (AAS) (Levinson, 1974). All these methods can only be employed to determine one element at a time, and for most of these methods the elements must be transferred into solution. The USGS has used emission spectrographs installed in trucks for several years (Canney et al., 1957; Ward et al., 1963; Holman and Durham, 1967). A radionuclide-excited energy-dispersive X-ray fluorescence (EDXRF), equipped with semiconductor detectors, has been used in a mobile laboratory for several years for multi-element prospecting (Kramar and Puchelt, 1982; Kramar, 1982; Kramar et al., 1982). 0375-6742/84/$03.00
© 1984 Elsevier Science Publishers B.V.
374
Due to the increasing prejudice against the use of all types of radioactive sources, the use of radionuclides for analytical purposes is more and more restricted and decreasing. In some countries it is almost impossible to import even sealed radioactive isotopes within short times for use in field laboratories. This in part p r o m p t e d the development of the mobile tube-excited X R F reported here. The application of tube-excited E D X R F systems for multi-element analysis in mobile laboratories was difficult in the past, due to weight, size, high power requirements and the need for cooling water. Today small X-ray tubes, which are cooled by natural air convection, are available. This paper describes such a system adapted to a mobile laboratory m o u n t e d in a small four-wheel drive truck (Landrover 109). The experiences of a first field application with this laboratory are reported here. INSTRUMENTATION OF THE MOBILE LABORATORY
A schematic diagram of the complete E D X R F system is shown in Fig. 1. A view of the tube-excited E D X R F system m o u n t e d in the cargo compartm e n t of the Landrover 109 is given in Fig. 2. A 1 m A / 5 0 kV air-cooled pulsed Rh-tube is m o u n t e d together with a Si(Li)-detector in the cabinet of a 20-sample changer (TRACOR SPECT R A C E 430). The Si(Li)-detector has an active area of 30 mm 2 and an energy resolution of 145 eV (FWHM} at 6.4 keV. The detector requires cooling by liquid nitrogen during operation. The detector is m o u n t e d in a 10-1 dewar, which is sufficient to operate for a period of 6 days. If an external 100-1 dewar is used, the laboratory will be able to operate for
5°%
oppy-drive
I
I-TI-si~ent
Fig. 1. Schematic diagram of tube-excited energy-dispersive X-ray fluorescence.
375
Fig. 2. Tube E D X R F mounted into a four-wheel drive truck (Landrover 109).
6--7 weeks without further supply of liquid nitrogen. Sample changer and X-ray power supply are installed in a portable instrument desk. The instrument desk is equipped with retractable wheels for easy and fast loading and unloading. The spectroscopy amplifiers, a multichannel analyzer with 1024 channels and 100 MHz ADC (CANBERRA 8100) and a minicomputer PDP 11/04 (32 K-bytes) with dual floppy-disk drive are inserted into an instrument rack which is fixed in the laboratory truck. The power requirements for the mobile XRF laboratory of approximately 2.2 kW for the XRF system itself and 1.1 kW for an airconditioner, can be supplied by any 220 V single-phase service, or by an auxiliary 5 kVA generator. The generator needs 20 1 gasoline per day.
376 ANALYTICAL The conditions for the excitation of the characteristic radiation for all elements of interest must be optimized. A filter wheel with up to 6 different filters between tube and sample is used to minimize the radiation background in the energy region used for analysis. The widest range o f application was obtained for a tube voltage of 35 kV using a Rh-filter between tube and sample. Due to this tube voltage/ filter combination the characteristic X-rays of the sample are excited nearly exclusively by Rh-K radiation and the radiation background is drastically reduced. Therefore the peak/background ratio increases. These conditions are suitable to determine up to 18 elements in the course of one measurem e n t (K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, W, Pb, and Th). Detection limits for soil, rock, or stream-sediment samples obtained with the tube EDXRF are compared with those of radionuclide EDXRF in Table I. These detection limits are valid for a processing time of 10 min per sample. The processing time includes the time for measurement, data transfer, and data evaluation for the simultaneous determination o f all 18 elements. For several elements a clear improvement o f the detection limit was achieved with the tube-excited system. For most of the elements the stated detection limits are smaller than the concentrations f o u n d in most natural samples. With a tube voltage of 50 kV and Cu-filter between tube and sample, elements with atomic numbers 40--60 can be determined. The determination of Sn and Nd is less sensitive than with radionuclide EDXRF. In particular the determination of Ba, La, Ce, and Nd, is limited by a high Bremsstrahlung-radiation background {Table I).
TABLE I Detection limits of trace elements in rock, soil, and stream-sediment samples for mobile tube-excited EDXRF and radionuclide-exeited EDXRF (ppm) Ni
Tube EDXRF 35 kV 5O kV Radionucl. Cd I°9 EDXRF Am241
Cu
EDXRF
A m 241
Ga
As
Br
Rb
Sr
5
5
5
5
5
5
5
5
100
50
30
30
10
10
10
10
Y Tube EDXRF 35 kV 50 kV Radionucl. Cd '°9
Zn
Zr
Nb
Pb
Th
5
5
5
5
10
10
10
20
Ba
La
Ce
10 70 70 70 50 100 100 100 5 10 10
377
A comprehensive description of the analytical procedure, that is sample preparation and matrix correction, is given in Kramar and Puchelt (1982). Shortly, the sample material is sieved to - 1 7 7 ~m ( - 8 0 mesh) or finely ground. An aliquote of 5--10 g is used as bulk p o w d e r sample in a polystyrol container with 6 p m mylar window for measurement. After 500 s collecting time the spectra are transferred to a computer, and the element concentrations are calculated. Matrix differences are corrected using the Rh-Comptonpeak. The E D X R F system is calibrated using the standard addition m e t h o d or geochemical reference materials. Data evaluation is performed by a c o m p u t e r program PROCO2 (Kramar, 1982) immediately after the measurement of the corresponding spectra. During the computation the next spectrum is collected. Statistical data treatment can be easily performed in field with the computerized analytical results. Frequency distributions, element concentrations, as well as element ratios can be plotted. Using a measuring time of 400 s live time an o u t p u t of 50--60 samples can be achieved within 10 h, i.e., up to 1000 single-element determinations. The sample o u t p u t can be increased using shorter measuring times (100 samples day -1 ) if slightly reduced detection levels are acceptable. APPLICATIONS
Carbonatite mineralization (Kaiserstuhl, F.R. Germany) A geochemical overburden survey for carbonatite has been carried o u t in the Kaiserstuhl mountains at Ohrberg/Schelingen (Upper Rhine Valley, FRG) using the radioisotope-excited E D X R F system (Kramar, 1982; Kramar and Puchelt, 1983). These investigations were extended to the south using the mobile tube-excited system. The aim of this survey was to delineate the soil-covered outcrops of a Nb-bearing carbonatite dyke described in an unpublished report by R. Eigenfeld in 1947 b u t n o t n o t e d in geological maps. The Kaiserstuhl mountains are situated 60 km north of Basle and a b o u t 200 km west of Munich in the upper Rhine valley. The survey area is located east of Vogtsburg at Eichelspitze. Some 500 soil samples were taken along 7 traverses with a total length of 5 km. The distance between the sampling points was 10 m. The soil samples were collected with a Piirckhauer earth auger from a depth o f 20--90 cm from the B/C horizon, where possible. The samples were dried at 60°C and sieved to - 1 7 7 ~m. This material was used without further treatment as bulk powder for the analysis with the tube-excited system. In a single measurement Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, St, Y, Zr, Nb, Pb, and Th were determined for each sample. Figures 3 and 4 show the distribution of Ni and Nb. Strontium, Th, and Nb have nearly the same distribution pattern in the survey area. With Nb, Sr, Th, and the ratio of Fe/Mn as indicators a carbonatite d y k e with a length of at least 200 m and approximately 30 m thickness
378
I
C_,
7"42' cost
Nb < 100 ppm 1 0 0 - 200
N
ppm
200-
400 ppm
400-
800
> 800
ppm
ppm
48°06 , north I i i
"G
I i I
I
/ I /I /
i
I 0
, 100
I 200 m
// / I
/
iI
I
Fig. 3. Niobium distribution in soils of Eichelspitze, Kaiserstuhl mountains, F.R.G. (Capital letters indicate starting points of traverses).
379
could be localized (Fig. 5). With respect to petrological maps (Wimmenauer, 1966) the surrounding rocks could be outlined precisely by comparing the trace-element data of the overburden with known data of Kaiserstuhl rocks (van Wambeke, 1964). t
C-
7"4 2' east
Ni
=""=
< 50
i
ppm
5 0 - 80
ppm
8 0 - 150 ppm > 150 ppm
N
48°06 ' north I
E
~F
/
f
t/t k
/
//
/ /1
\
t
\
I /
1
i
0
100
I 200 m
/ i / /
/ *¢
1
/
/
//
!
/ r
s
q
Fig. 4. Nickel distribution in soils o f Eichelspitze, Kaiserstuhl mountains, F.G.R.
380 7°4 2' eost
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Fig. 5. Petrographical map of Eichelspitze as outlined by use of different trace-element distributions and element ratios in soil samples.
Cerite mineralization (Bastn6s, Central Sweden) A s e c o n d s u r v e y was carried o u t in c e n t r a l S w e d e n n e a r R i d d a r h y t t a n at t h e Bastn~is cerite d e p o s i t ( K r a m a r a n d Simianer, 1 9 8 4 ) . T h e R i d d a r h y t -
381
tan district is situated in the Skinnskatteberg region in the province of V~istmanland, a b o u t 150 km WNW of Stockholm. The oldest rocks in the Riddarhyttan district are leptites, mica-schists, and limestones of Precambrian age. The younger group consists more or less of gneissic granites with Searchean granite and diabase dykes. The limestones of the leptite series are often replaced by skarns accompanied b y different types of Fe ores, complex sulphides, and small occurrences of rare-earth mineralization. In order to localize cerite deposits, some 230 till and soil samples were taken. The mobile X R F laboratory was set up close to the B a s t n ~ deposit. The samples were treated as described above and analysed for Mn, Fe, Ni, Cu, Zn, Ga, As, Br, Rb, Sr, Y, Zr, Nb, W, Bi, Pb, and Th. Out of these elements Cu and Y showed distinct anomalies (in soils and till) displaced from the deposits in the flow direction of the glacial ice. Some 150 samples were, in a second course, analyzed for Ba, La, and Ce, using Cu filter and 50 kV tube voltage. The concentrations of Cu, Y, and Ce along one traverse are given as an example in Fig. 6, together with a map. Based on these analytical results new traverses with 50 samples were taken in order to find a connection between the anomalies. All analytical work was performed within 5 days.
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Fig. 6. Traverse map and Cu, Y, and Ce distribution along one traverse in till south of the Bastniis cerite deposit, Central Sweden. Cerium concentrations measured immediately in field, are indicated at the traverse locations.
382 CONCLUSIONS
The field laboratory with tube-excited energy-dispersive X-ray fluorescence combines the flexibility and capacity for multielement determinations of a sophisticated stationary laboratory with the minimum delay time between sampling and analytical results of a field laboratory. Compared with conventional field methods the multielement analysis with tube E D X R F provides a lot of additional information. This can immediately and easily be interpreted by computerized methods. The possibility of an immediate feedback between results and further sampling will help to optimize the number of samples necessary to detect anomalies and to save time and money. The extremely short set-up time of the tubeexcited E D X R F (normally less than 1 h) enables easy and fast changing from one area to another. In remote areas the provision with liquid nitrogen is sometimes difficult to organize, but external dewars can provide an operation period of at least 6 weeks. We hope to solve the problem by using a cryogenerator or a small nitrogen liquifier. ACKNOWLEDGEMENT
These investigations were performed with financial support of grant 03 R 241 of the Ministerium fiir Forschung und Technologie (F.R.Germany). The samples for the Bastn~is prospect were taken by H. Simianer and A. Holl. Thanks are also due to Prof. Dr. H. Puchelt for improving the manuscript and all persons and institutions who thus made these investigations possible.
REFERENCES Ball, T.K., Booth, S.J., Nickless, E.F.P. and Smith, R.T., 1979. Geochemical prospecting for baryte and celestite using a portable radioisotope fluorescence analyzer. J. Geochem. Explor., 11: 277--284. Canney, F.C., Myers, A.T. and Ward, F.N., 1957. A truck-mounted spectrographic laboratory for use in geochemical exploration. Econ. Geol., 52: 289--306. Fletcher, W.K., 1981. Analytical methods in geochemical prospecting. In: G.J.S. Govett (Editor), Handbook of Exploration Geochemistry, Vol. I. Elsevier Scientific Publishing Comp., 255 pp. Hollman, R.H.C. and Durham, C.C., 1967. A mobile spectrographic laboratory. Geol. Surv. Can., Pap. 66--35, 15 pp. Kramar, U., 1982. Ein methodischer und regionaler Beitrag zur geochemischen Prospecktion auf Karbonatit. Dipl.Thesis, Univ. of Karlsruhe, 116 pp. Kramar, U. and Puchelt, H., 1981. Application of a radionuclide energy-dispersive X-ray fluorescence analysis in geochemical prospecting. J. Geochem. Explor., 15: 597-612. Kramar, U. and Puchelt, H., 1983. Geochemical multi-element prospecting for carbonatite by energy-dispersive X-ray fluorescence. J. Geochem. Explor., 19: 255--276. Kramar, U. and Simianer, H., 1984. Annual report. BMFT 03 R241.
383
Kramar, U., John, N. and Puchelt, H., 1982. Geochemische Prospektion unter Verwendung der energiedispersiven RSntgenfluoreszenzanalyse, ionensensitiver Elektroden und der instrumentellen Neutronenaktivierungsanalyse. BMFT-FB-T 82-152, FIZ, Karlsruhe, 76 pp. Kunzendorf, H., 1973. Die Isotop-RSntgenfluoreszenz Analyse und ihre Anwendung bei der geochemischen Untersuchung in GrSnland. PH.D. Thesis, T.H. Aachen. Levinson, A.A., 1974. Introduction to Exploration Geochemistry. Applied Publishing Ltd., Calgary, Alta., 612 pp. van Wambeke, L., 1964. La g~ochemie des roches du Kaiserstuhl. In: L. van Wambeke et al. (Editors), Les roches alcalines et les carbonatites du Kaiserstuhl. Euratom Publ. EUR 1827, d.e.f., Brussels, pp. 93--192. Ward, F.N., Lakin, H.W., Canney, F.C. et al., 1963. Analytical methods used in geochemical exploration by U.S. Geological Survey. U.S. Geol. Surv., Bull., 1152: 91--99. Wimmenauer, W., 1966. The eruptive rocks and carbonatites of the Kaiserstuhl, Germany. In: O.F. Tuttle and Y. Gittins (Editors), Carbonatites. Interscienees, John Wiley and Sons, Bungary, Suffolk, pp. 183--204.