A contribution to geochemical prospecting for fluorite

Journal of Geochemical Exploration, 15 (1981) 6 2 5 - - 6 3 4

625

Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- Printed in The Netherlands

A CONTRIBUTION TO GEOCHEMICAL PROSPECTING FOR FLUORITE

N. J O H N and H. P U C H E L T

Institute of Petrography and Geochemistry, University of Karlsruhe, 7500 Karlsruhe, Kaiserstr. 12 (F.R. Germany) (Received D e c e m b e r 22, 1980)

ABSTRACT J o h n , N. and Puchelt, H., 1981. A c o n t r i b u t i o n to geochemical prospecting for fluorite. In: A.W. Rose and H. G u n d l a c h (Editors), G e o c h e m i c a l E x p l o r a t i o n 1980. J. Geochem. Explor., 15: 6 2 5 - - 6 3 4 . With fluorine as indicator, eight different e x t r a c t i o n m e t h o d s were tested for their suitability in geochemical prospecting for fluorite in stream sediments. Since this material can be regarded as coarse-grained, clastic fragments derived f r o m the bedrock -i.e. granites, gneisses, and fluorite veins as well -- it was c o n c l u d e d that the fluorine c o n t e n t of the stream sediments c o m e s m o s t l y f r o m fluorine-bearing b e d r o c k minerals. A n o m a l o u s samples contain fluorine p r e d o m i n a n t l y as fluorite f r o m fluorspar mineralization. Five o f the c o m m o n e s t fluorine-bearing minerals -- fluorite, mica, apatite, a m p h i b o l e and t o u r m a l i n e - - were treated with the different extractants and with various times of reaction and mineral grain size. F l u o r i n e analysis was by ion-selective electrode. The ratio cx-F (fluorite) vs. cx-F (mica, apatite) was f o u n d to be a useful measure o f the applicability of the respective m e t h o d . In order to test this conclusion 41 stream s e d i m e n t samples (--177 microns ~ - - 8 0 mesh) f r o m the area u n d e r investigation were treated with eight different extractants. The m o s t suitable e x t r a c t i o n solution - - boric acid solution - - was used for the reconnaissance stream s e d i m e n t survey in an area o f 400 km 2 in the southeastern part o f the Black F o r e s t ( F . R . Germany).

INTRODUCTION

The fluoride c o n t e n t of stream waters, stream sediments, and soils has been reported b y several authors as a direct indicator of fluorite mineralization (Van Alstine, 1965; Chrt, 1969; Friedrich and Plfiger, 1971; Schwartz and Friedrich, 1973; Lalonde, 1974; Farrell, 1974). The purpose of this investigation was to develop a geochemical m e t h o d for fluorite prospecting applicable in the southeastern part of the Black Forest (FRG) and similar granite-gneiss areas with fluoride as an indicator. Therefore, a new approach was made by testing the extraction methods first with monomineralic samples o f the fluorine-bearing c o m p o u n d s in

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© 1981 Elsevier Scientific Publishing C o m p a n y

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the respective stream sediments. Thereafter, the extracting solutions were applied to stream sediments of the working area. DESCRIPTION OF THE WORKING

AREA

The area selected for the survey is situated in the southeastern part of the Black Forest covering approximately 400 km 2 (Fig. 1). GeologicaUy it is composed mainly of Variscian granites and porphyries and of Prevariscian gneisses. Close to the southeastern boundary of the investigated area, Triassic sediments occur, chiefly limestones and sandstones. Several north-south striking, steeply dipping fluorite-quartz-barite veins in this area are described in the literature (Metz et al., 1957; Metz, 1980).

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Triassic sediments 0 1 2 3kin Schists, graywackes / Granites ~ Quorternarygrovel Fig. 1. Geological map of the southeastern Black Forest (simplified after Metz et al., 1957). INVESTIGATIONS ON STREAM SEDIMENTS

One aim of this investigation was to find a suitable specific reagent for fast cold extraction of stream sediments in this area. Therefore at the very beginning it appeared reasonable to review the stream sediments for their

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overall mineral content, grain size distributions, total fluoride concentration, and heavy mineral content. With studies on eleven different samples it was shown that most of the stream sediments are coarse-grained clastic weathering products of the underlying rocks and mineral veins. Consequently, the fluoride in these samples is bound almost exclusively within the fluorine-bearing minerals from the underlying rocks. These are mainly micas, apatite, and amphiboles. In the samples taken in the vicinity of fluorite mineralization, the largest part of this element was bound in fluorite. In addition to these clastic sediments, limonitic grains and crusts w e r e observed in minor amounts. Organic matter ranges up to 4 weight-per-cent. DISSOLUTION EXPERIMENTS WITH VARIOUS EXTRACTANTS MONOMINERALIC SAMPLES

ON

Since it was found in previous investigations that fluoride anomalies in these stream sediments are essentially due to clastically dispersed fluorite from mineral veins, an attempt was made to find an extractant that was relatively specific for fluorite but that did not attack other fluorinebearing minerals to a significant degree. Eight different extractants were tested for their dissolving capability with mineral concentrates of the most frequent fluorine-bearing minerals: fluorite, biotite, apatite, amphibole, and tourmaline. Five of these extractants have been described in literature previously (Pliiger and Friedrich, 1972; Schwartz, 1972; Lalonde, 1974; Farrell, 1974). The extractants tested were: (1) H20 (2) TISAB* (3) 10-' N HC1 (4) 75% HNO3/25% HC104,5 times diluted (5) 10 -3 N NaOH (6) H3BO3 solution, 0.5% B, pH 3.7 (7) Na2B407 solution, 0.5% B, pH 10.0 (8) A1C13 solution, 2.5 ppm A1, pH 4.3 Two further parameters have been varied during the experiments: (1) time of extractions (from 5, 10, 20 to 30 min); and (2) grain size of the minerals to be employed (40--100 microns and 100--250 microns). The applicability of a given extract~qt is characterized by a coefficient calculated according to the following equation: *TISAB (Total Ionic Strength Adjustment Buffer) is widely used for the determination of fluoride by means of an ion-selective electrode. 1 liter contains 58 g NaCI, 57 ml glacial acetic acid, and 5 g CDTA (1,2-cyclohexylene dinitrilotetra-acetic acid). Added to an aqueous sample it adjusts pH and ionic strength, and avoids interferences due to complexing.

632

A -

cx-F (CaF2) cx-F (X)

, X: biotite or apatite

This ratio is a measure of the usefulness of an e x t r a c t a n t for the described purposes. A high value means t hat an extractant is highly suitable, since relatively mu ch fluoride is dissolved from the fluorite and relatively little from the other fluorine-containing minerals. According to these investigations the selection of a m e t h o d depends on the minerals to be e x p e c t e d in the sample. Only very small amounts of fluoride were dissolved from amphibole and tourmaline during the experiments, so that their presence in the sample can be neglected. T ha t means the choice of an e x t r a c t a n t had to consider -- besides fluorite itself -- biotite and apatite only. Thus a sample scheme can be established which allows the choice of the proper e x t r a c t a n t provided the fluorine in the samples is bound in these minerals (Fig. 2).

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NaOH

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NaOH

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DISSOLUTION OF F L U O R I N E I N S T R E A M SEDIMENTS WITH DIFFERENT

EXTRACTANTS In order to compare the results obtained in the previous investigations with natural samples, 41 stream sediment samples from a part of the working area were treated with all the extractants. The samples had been air-dried and sieved to --80 mesh. Some of these were taken close to fluorite veins. Consequently, t h e y showed higher fluoride concentrations. The suitability of the respective m e t hods was determined from the contrast b e t w e e n these anomalous samples and sediments with background concentrations. Procedure 500 mg o f the sample were placed into a 50 ml plastic beaker and 25 ml o f the e x t r a c t a n t added. It was stirred on a magnetic stirrer for 30 min. T h en 10 ml o f the extracting solution were transferred to a n o t h e r plastic

633

beaker and 10 ml of TISAB buffer solution were added. In this mixture the potential between the fluoride-selective and reference electrodes was measured after 5 min and compared with a similar standard solution. In the case of the HNO3]HC104 extraction a procedure similar to that of Farrell (1974) was used: 250 mg of the sample were treated with 25 ml of the extractant. Again it was stirred for 30 minutes on a magnetic stirrer. Afterwards 25 ml of TISAB buffer solution were added followed by 15 ml of 5M NaOH. The solution was then made up to 100 ml with the buffer and electrode potentials measured after 5 min. The extractable fluoride c o n t e n t was calculated by comparing these potentials with similar standard solutions. During these investigations it was found t h a t -- in agreement with the experiments on monomineralic samples -- the extractants H3BO3, HNO3/ HC104, and H20 showed good results. The NaOH was not as suitable as the other extractants. Most striking was the inconsistency of TISAB behaviour between the two series of experiments: with the monomineralic samples it proved to be less suitable, while with stream sediments it was among the extractants which exhibited the best contrasts. It is assumed that a small part of the total fluoride is not bound to one of the fluorinebearing minerals from underlying rocks and veins, but is contained in secondary compounds, which are readily leached by TISAB. That would also explain the good correlation between the TISAB soluble fluoride contents of the stream sediments and the fluoride contents of the stream waters. As H3BO3 showed good results in both series of experiments it was used for the reconnaissance survey throughout. R E G I O N A L RECONNAISSANCE SURVEY

In an area of approximately 400 km 2 , water and stream sediment samples were collected at 470 sampling points. The sediments were air-dried, sieved to --80 mesh, and this material was used for the extractions. Indcators used were -- as previously mentioned - - t h e fluoride concentrations of the waters and the H3BO3-extractable fluoride content of the stream sediments. From Fig. 3 it is evident that nearly all known fluorite veins are indicated by the fluoride concentrations of the waters. In addition the fluoride contents of the waters suggest t h a t some of the veins m a y have extensions b e y o n d their known occurrence. In still other locations (northwest corner of the map) it is to be assumed from the pattern of the fluoride distribution t h a t the higher fluoride concentrations in water are not caused by vein-type mineralization. In some of these areas glacial deposits are mapped, the local extension of which corresponds closely with the positions of those sampling points. From this obserVation it is concluded that glaciers have eroded outcropping fluorite veins and that the scraped-off minerals of the veins were transported with the glacier.

634

Fig. 4 shows that the HaBOa-extractable fluoride of the stream sediments indicates almost without exception all known fluoride veins. A drawback of the H3BO3 ~xtractable fluoride is that a rather strong dependence exists on grain size and mineral composition of the sediment. Special care has to be taken therefore in cases where limestone in the bedrock causes distinctly increased background values. Examples of this behaviour have been observed in the limestone area in the southeastern corner of the investigation area. On the other hand H3BO3 ~xtractable fluoride can be observed in higher concentrations even at locations where fluorine anomalies in waters are suppressed due to dilution. This occurs especially at places where the streams cross the mineral vein in an unfavourable angle of about 90 ° . In any case it proved to be useful to apply both fluoride in water and cx-F in sediments since the data obtained are complimentary and thus make interpretation easier. ACKNOWLEDGEMENTS

The authors wish to thank the Ministerium ffir Forschung und Technologie of the Federal Republic of Germany for financial support by Grant NTS 0114/0 and all persons and institutions who thus made these investigations possible. Constructive comments by Dr. H.E. Hawkes are greatly appreciated.

REFERENCES Chrt, J., 1969. Methoden der Prospektion und Erkundung auf Flussspat in der C.S.S.R. Berg. Hiittenm~nn. Monatsch., 114: 459--464. Farrell, B.L., 1974. Fluorine, a direct indicator of fluorite mineralization in local and regional soil geochemical surveys. J. Geochem. Explor., 3 : 227--244. Friedrich, G.H. and Plilger, W.L., 1971. Geochemical prospecting for barite and fluorite deposits. In: R.W. Boyle and J.I. McGerrigle (Editors), Geochemical Exploration. Can. Inst. Min. Metall., Spec. Vol., 11: 151--156. Lalonde, J.P., 1974. Research in geochemical prospecting methods for fluorite, Madoc area, Ontario. Geol. Surv. Can., Pap. 73-38, 56 pp. Metz, R., 1980. Geologische Landeskunde des Hotzenwaldes. Mortiz Schauenburg Verlag, Lahr, 1116 pp. Metz, R., Richter, M. and Schiirenberg, H., 1957. Die Blei-Zink-Erzg~inge des Schwarzwaldes. GeoL Jarhb., Beih., 2 9 , 2 7 7 pp. Plilger, W.L. and Friedrich, G.H., 1972. Determination of total and cold-extractable fluoride in soils and stream sediments with an ion sensitive fluoride electrode. In: M.J. Jones (Editor): Geochemical Exploration 1972. Proc. 4th Int. Geochem. Explor. Syrup., Inst. Min. Metall., London, pp. 421--427. Schwartz, M.O. and Friedrich, G.H., 1973. Secondary dispersion patterns of fluoride in the Osor area, Province of Gerona, Spain. J. Geochem. Explor., 2: 103--114. Van Alstine, R.E., 1965. Geochemical prospecting in the Browns Canyon fluorspar district, Chaffee County, Colorado. U.S. Geol. Surv., Prof. Pap., 525D: D 59--64.