Journal qfGeochemical Exploration, 46 (1993) 349-364 349 Elsevier Science Publishers B.V., Amste rdam

Geochemistry of ephemeral stream sediments in the Precambrian mountainous arid terrain of

southern Israel

R. Bogoch, M. Shirav, M. Beyth and L. Halicz Geological Survey of lsrael, 30 Malkhei Israel St., Jerusalem 95501, Israel

( Received 2 July 1992; accepted after revision 18 November 1992 )

ABSTRACT

Bogoch, R., Shirav, M., Beyth, M. and Halicz, L., 1993. Geochemistry of ephemeral stream sediments in the Precambrian mountainous arid terrain of southern Israel. J. Geochem. Explor., 46: 349-364.

In terms of geochemical sampling and evaluation, arid zones may be broadly classified into two categories: mountainous, where areas of rock exposures dominate over those covered by alluvium, and relatively fiat areas, where alluvium (and sand) dominate. Since the nature of diluting and mask- ing agents may differ significantly in these two types of terrain, a single stream sediment sampling technique is unlikely to be applicable for both.

An ephemeral stream sediment geochemical survey was carried out in the arid and mountainous Late Precambrian area of southern Israel. Based on an orientation survey, the -0 .15 mm sediment fraction ( - 100 mesh) was chosen for geochemical analyses. As in other studies, it was found that secondary dispersion trains are of limited extent. This, together with an attempt at reducing the "nug- get effect", led to the use of a sampling density in the order of some 3 samples per km 2. The masking effect of windblown material was found to be negligible.

The presence of six geochemical domains determined visually on single element geochemical maps and which correlate with the bedrock lithology, was supported by the distribution of single sample factor scores and inter-element correlations. Newly discovered geochemical anomalies of possible in- terest include Au-As-Sb and Y + REE.

I N T R O D U C T I O N

Stream sediment geochemistry in arid zones has been considered proble- matic mainly because of dilution related to aeolian sedimentation, mechani- cal weathering, relatively low-level chemical activity, the lack of or poor soil development, and the presence of duricrust. Several variations in sampling procedures, as compared to stream sediment geochemistry in areas of tem- perate climate, have been applied in order to overcome these masking effects,

Correspondence to: R. Bogoch, Geological Survey of Israel, 30 Malkhei Israel St., Jerusalem 95501, Israel.

0375-6742 /93 /$06 .00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

350 R. BOGOCH ET AE.

and therefore to enhance the contrast between threshold and background val- ues. Thus, recommended grain size of samples varies from fine (e.g., Beeson, 1984; Fletcher, 1985) to coarse (e.g., Li Qing et al., 1989), with preference given to the heavy fraction (Raines, 1985 ), to material from which the fines were removed by sieving (Bugrov, 1974). Similarly, recommended sampling densities vary considerably. Biogeochemical surveys in arid regions have had questionable success (Botova et al., 1963; E1 Shazly et al., 1977; Bogoch and Brenner, 1984), partly due to the sparcity of flora.

In terms of geochemical sampling and evaluation, arid zones may be broadly classified into two categories: mountainous (including "hilly"), where areas of rock exposures dominate over those covered by alluvium, and relatively flat areas, where alluvium (and sand) dominate. Since the nature of the di- luting agents may differ significantly in these two types of environments, a single stream sediment sampling technique is unlikely to be applicable for both. In this study, we report the results of a geochemical mapping program, and its orientation survey, carried out in the mountainous arid terrain of southern Israel. The purpose of the geochemical mapping project is to delin- eate regional geochemical patterns based on a multi-element analytical pro- gram and to interpret the data in terms of geochemical prospecting for metal- lic and non-metallic ores, and for anthropogenic effects.

It is suggested that the sampling and analytical techniques arrived at reduce both time and cost factors, and would be effective in regional geochemical studies in similar areas.

GEOLOGICAL SETTING

The Precambrian (late Proterozoic) of southern Israel covers some 70 km 2 and represents part of the northern exposures of the Arabo-Nubian Shield. The area has been structurally reshaped since the Late Miocene by rifting (Dead Sea Rift) and regional doming (Garfunkel, 1980) which ultimately resulted in the present pattern of a series of isolated blocks, mainly separated by graben-hosted younger sedimentary rocks or alluvial valleys. Although pe- trologically complex in detail, the Precambrian rocks in the different blocks may be sub-divided into dominant lithologies (Fig. 1 ) where, for example, granitic and granodioritic gneisses are combined with non-metamorphosed granites to allow an approximate areal comparison with the stream sediment geochemical data.

There are no known metallic ore deposits in the Precambrian of southern Israel. Minor copper mineralization, largely secondary, is locally present, and somewhat more abundant in the Timna block where it occurs mainly within rhyolitic dykes. The latter area is considered as the initial source for the nearby Cambrian stratabound Timna copper deposit (Segev and Sass, 1989; Bar

EPHEMERAL STREAM SEDIMENTS IN PRECAMBR1AN MOUNTAINOUS ARID TERRAIN, S. ISRAEL 3 51

Fig. 1. S c h e m a t i c geology o f the P r e c a m b r i a n e x p o s u r e s in s o u t h e r n Israel. D o m i n a n t l i tholo- gies: gr - - grani te ; gb - - gabbro; di - - dior i te ; rh - - rhyol i te ; cg - - c o n g l o m e r a t e ; a m - - a m p h i - boli te; gn - - i n t e r m e d i a t e gneiss , me tape l i t e .

Matthews, 1986). Barite veins are present in the Roded block (Bogoch and Weissbrod, 1986) and have been noted in the Elat block (Picard, 1943 ).

G E O M O R P H O L O G Y A N D E P H E M E R A L S T R E A M S E D I M E N T S

The Precambrian blocks have a rugged topography (Fig. 2 ). Relief is steep with the secondary ephemeral streams (wadis) dropping 100-300 m over distances of hundreds of meters. The highest point in the area is Mt. Shelomo in the Roded block (705 m a.s.1.); maximum elevations are in the order of 100-600 m. The valleys are immature and many are linear in whole or in part, suggesting that they course along structural features.

The sampled wadis vary considerably in length and width, and include highly immature, weakly incised rills. The sampling program was based al-

352 R. BOGOCH ET AL.

Fig. 2. View-of the Precambrain terrain in southern Israel (Roded Block).

Fig. 3. View of typical wadi. Note flood channels (F) and ancient alluvial terrace (T).

most entirely on tributary wadis. Most of the main wadis present sampling problems related to their large widths and to the contamination by material derived from exposures of younger sedimentary rocks, which they cross in addition to the Precambrian. Numerous, frequently braided flood channels are present in most wadis greater than 5 m in width.

EPH EMERAL STREAM SEDIMENTS IN PRECAMBRIAN MOUNTAINOUS ARID TERRAIN, S. ISRAEL 3 5 3

Depending on the main source rocks, the wadi sediments vary in colour between greyish-brown (metamorphic rocks, intermediate to basic intru- sives) and pinkish-beige (acid volcanics and intrusives). An influx of lime- stone from the surrounding areas (primarily in the main wadis) is clearly observable in the light colour of these carbonate fragments. The wadi sedi- ments are extremely heterogeneous in fragment size, varying from large boul- ders (rarely ~ 1 m in diameter) to silt. Boulders, cobbles and pebbles are sub- rounded to angular. Based on the petrology of the larger fragments, the ma- jority of these derived from proximal outcrops. Approximately 20% of the sediment is < 2 mm in diameter. Granulometric analyses of the - 2 mm frac- tion indicated that ~ 95% of the material is coarser than 0.25 mm and the fines ( <0.125 mm) make up an average of ~2.5%.

Ancient river terraces (Fig. 3 ) discontinuously line the banks of the major and many of the secondary and tertiary wadis. These consist of fairly well- bedded, friable sediments of compositions similar to the recent sediment in the wadi beds. Fragment and grain size distribution in the terrace sediments is similar to that of the wadis, but with somewhat higher contents of the finer material.

METHODS

1. Analytical techniques

Two important functions of a regional geochemical mapping program are mineral exploration and environmental geochemistry. For these purposes, the number and variety of measured elements (and accuracy) should be as broad as possible. This depends on laboratory facilities and, of course, available funding. We utilized the most routine analytical program available at the Is- rael Geological Survey Geochemical Laboratory using inductively coupled plasma-atomic emission spectrography (ICP-AES) after sintering with so- dium peroxide for the determination of 8 major- (SIO2, A1203, Fe203, CaO, MgO, TiO2, P205 and SO3) and 16 trace elements (Cu, Mo, Pb, Zn, Ba, Be, Ni, Co, Cr, V, Mn, Y, La, Ce, Eu, Yb), and atomic absorption spectropho- tometry (AAS) using aqua regia and DIBK extraction for Au, and a hydride procedure for As and Sb. For control of precision and accuracy, natural stan- dard reference materials and duplicate samples were analysed using the same procedures. U and Th were measured by neutron activation at the Soreq Nu- clear Research Centre.

2. Selection of sample fraction for analysis

The problems addressed in selecting an appropriate sediment fraction were: the possible diluting effect of aeolian material; whether the analysed fraction

354 R. BOGOCH ET AL.

is representative of the dominant Precambrian outcrops; the amount of lab- oratory time required in sample preparation; the effect of grain size on the analytical techniques.

Eight wadi sediment samples from the Timna block (orientation survey) were sieved to 4 different size fractions and analysed. The results indicate that (a) the CaO and SiO2 contents exhibit a "mirror" behaviour, where the CaO is consistently higher in the finer material, and it was thus assumed that the effect of fine windblown quartz in the area is very limited; although CaO may in part derive from windblown carbonate, Fig. 4 does not show any sig- nificant trace element dilution in the finest fractions; (b) none of the trace elements (except Ba) showed significant variations as a function of grain size (Fig. 4); (c) the trace element data could be closely related to the outcrop geology in the sampled areas (Beyth et al., 1988 ). It was therefore decided to use the - 0.15 mm fraction ( - 100 mesh ) which also conveniently avoids the need of grinding (with possible contamination) for geochemical analyses.

ppm

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Fig. 4. Box plots of selected trace elements (V, Mn, Ba, Pb) in four size fractions ( 1 ~ 10-100 mesh: (2) 100-140 mesh; (3) 140-200 mesh; (4) < 200 mesh. The vertical spread in each box represents 50% of the data between the 25 and 75 percentiles; line in each box is the median. Vertical lines extending from the boxes are bound at data points which are nearest to values equal to 1.5 times the spread of the boxes. High values are indicated by circles.

EPHEMERAL STREAM SEDIMENTS IN PRECAMBR1AN MOUNTAINOUS ARID TERRAIN, S. ISRAEL 3 5 5

Furthermore, since Au was among the elements targeted for mineral explo- ration, the well known nugget effect (Clifton et al., 1969) had to be consid- ered. Based on a study by Haycock ( 1937 ), Harris (1990) shows graphically that approximately 90% of gold grains in rock samples from fifty Canadian ores are < 100 mesh in size. Thus the use of the finer sediment samples should reduce the nugget effect. Salpeteur and Sabir (1989) found the < 80 mesh fraction to be the most effective in wadi sediments from pediplain areas of Saudi Arabia, and Saffarini and Lahawani ( 1992 ) used the < 100 mesh frac- tion with hot HC1 extraction in the wadi sediments of SW Jordan.

Artificially prepared heavy mineral enriched samples (panning or heavy liquids) have been found to be an effective method of enhancing threshold values in arid zone geochemistry (Fletcher, 1985; Raines, 1985; Ren Tian- xiang et al., 1989). Similar results were obtained during the present study (orientation survey of part of the Timna block; enrichment factor of ~ 2 X ). However, the overall geochemical pattern remained the same. Therefore, be- cause of the much longer sample preparation time required, we abandoned this technique for the regional survey.

3. Sampling density

In most regions, a sampling density in the order of 1/km 2 has been found to result in maximum information from minimum field and back-to-field time (Plant et al., 1988 ). This sampling density is effective because of the impor- tant role played by chemical weathering in the breakdown of metallic min- erals, their transportation as metal-salt phases, and their adsorption on river- bed clays. In an arid environment, the essential factors related to metal mo- bility (secondary dispersion) are resistance to weathering, resistance to ero- sion or transportation processes, and the local topography and geology. Floods, plus rock fall, are the most important determinants regarding the distance to which a particular element may travel from its source and still remain detect- able in anomalous amounts. Notwithstanding a few exceptions, dispersion trains in arid zones are < 1 km in length (Bugrov, 1974; Zeegers et al., 1985; and in the present work, < 500 m from mineralized outcrops). A relatively dense sampling network was therefore chosen to compensate for the lack of distal dispersion and for the nugget effect. The drainage network allowed for convenient sample collection in the order of 3 samples/km 2.

4. Sample sites

Test sampling was carried out to determine the best location for individual samples within the wadis. We sampled ( 1 ) single samples within recent flood channels; (2) as above, but at near surface vs. ,,, 30 cm below surface; (3) profile (or "channel") samples across the wadis in sample lengths of 2 m. Li

I 0 0

10

! As

Stream sediment ~ - Terrace samples

I I I I i ~ i i I

Ba Ce Co Cr Cu La Mn NI V Zn

356

1000 ppm

R. BOGOCH ET AL.

Fig. 5. Comparison of geochemical data between 20 near-adjacent samples of wadi sediments and alluvial terraces.

Qing et al. (1989) suggested that multi-point, cross-channel sampling is ad- visable. In our tests, no signifcant difference was found between sample types. It should also be noted that no difference was noted between samples taken at channel bends, where placers formed by hydraulic process may be present (Day and Fletcher, 1989), and samples taken in "straight" channels.

Approximately 1 to 2 cm of surface material were scraped away (and when necessary, coarse fragments were removed) using a sturdy garden hoe. For convenience, plastic containers, some 40 cm across were first filled with sed- iment, and then poured into 1 m m sieves. Some 15-25 kg of wadi sediment were required to produce ~ 1.5 kg of the - 1 m m fraction. This was later sieved in the laboratory to - 0 . 15 ram, each sample totalling a min imum of 100 g.

The ancient terraces, which apparently formed at a time of greater hydro- logical activity, are similar to overbank (or flood) deposits. They may be considered to represent a uniform deposit derived from the entire catchment basin so that a composite vertical sample would be highly representative (Ot- tesen et al., 1989 ). Vertical sections of these terraces were sampled in several wadis. Results were similar to those obtained from nearby wadi sediments (Fig. 5 ). Therefore, both because the sampling of the terraces is more diffi- cult and time consuming, and because of their sporadic occurrence, they were not used in the regional survey. It should be noted that clay-rich layers which would be the best sample medium for studying paleo-dispersion (Matheis and Kuester, 1985) are absent.

R E S U L T S

1. Data presentation

Single element geochemical maps were prepared where the highest values (100th percentile) are shown as filled squares, the 98 and 90 percentiles as

EPHEMERAL STREAM SEDIMENTS IN PRECAMBRIAN MOUNTAINOUS ARID TERRAIN, S. ISRAEL 3 5 7

open squares with respectively decreasing sizes, and the 75 percentile as a small cross (only selected figures are used to illustrate the following discus- sion. ) Interelement correlations were determined for the overall data and in- dividual blocks, together with factor analyses on log-transformed, varimax rotated data.

2. Relation between geochemical data and source rocks

Barring the availability of systematic rock geochemical data, as is the case of the Israeli Precambrian and for many other mountainous arid regions, a visual comparison between single element geochemical maps and geological maps yields a reasonable estimate of the correlation between the wadi sedi- ment data and the source rocks. In this case, the comparison should not con- sider individual samples, but areas in which "high" (or "low") concentra- tions of the elements are present. In large geographical areas, these types of spatial distributions are referred to as "geochemical terrains" (A.G. Darnley, 1992, pers. commun. - - "a region characterized by an assemblage of elements which exhibit a distinctive spatial pattern of absolute and relative geochemi- cal abundances" ). We use the term geochemical domains for the limited area of southern Israel (Table 1; Fig. 6a-g).

The geochemical maps based on the major element data also indicate vari- ations from block to block. The total Fe-oxides, MgO and CaO maps are nearly identical, and are very similar to those of the transition metals; for SiO2 and A1203, alumina is depleted relative to silica in the Timna and Amram blocks, and enriched in the central and southeastern parts of the Roded block.

3. Inter-element relations

In the overall terrain the following groups show significant correlations: F e - T i - M g - N i - C o - C r - ( V ); Y - L a - C e - Y b - U - T h . The chalcophile elements do not intercorrrelate but are variously associated with elements such as Mn, Be, U, Fe and Ti. Berrylium correlates with the LREE.

TABLE1

Geochemical domains in the Precambrian of southern Israel

Elements Distribution

1. C u + Z n + ( P b ) + (Mo) 2. N i + C o + C r + V + M n 3. Au-As-Sb 4. Y + L a + C e + Y b + U + T h 5. Eu 6. Be

Timna block Timna block (area of mafic rocks) Southeast part of Roded block South part of Elat block Roded block Neshef and Amram blocks

3 5 8

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R. B O G O C H ET AL

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E P H E M E R A L S T R E A M S E D I M E N T S IN P R E C A M B R 1 A N M O U N T A I N O U S A R I D T E R R A I N , S. I S R A E L

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Fig. 6. (a-g) Single element geochemical maps for Cu, As, Au, Be, Cr, La, Eu respectively. Precambrian terrain, southern Israel.

360 R. B O G O C H E T A L .

The correlations between Fe-Ti and the transition elements are also pres- ent in each of the Precambrian blocks (data from the Neshef and Shehoret blocks were not processed because of the small number of samples). Other correlations differ from block to block and also relate to the lithology (Fig. 7).

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RODED BLOCK

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Fig. 7. I n t e r - e l e m e n t corre la t ions (log-transformed data) for the Timna and Roded blocks . T h e former is l i t h o l o g i c a l l y dominated by granitic and lesser gabbroic rocks and the latter by quartz dior i te . T h e f i l l ed c irc les i n d i c a t e corre la t ion c o e f f i c i e n t s h igher than needed for significance at the 0.25% and 0,5% levels (large and smal l c ircles r e s p e c t i v e l y ) .

EPHEMERAL STREAM SEDIMENTS IN PRECAMBRIAN MOUNTAINOUS ARID TERRAIN, S. ISRAEL

DISCUSSION AND CONCLUSIONS

361

The geochemical domains reflect the lithological units of the Israeli Pre- cambrian, and also indicate the presence of known and newly discovered mi- neralization. Thus, gabbro in the Timna block and diorite in the Roded block are characterized by elevated levels of Ni + Cr + V + Mn, together with con- centrations of samples with high total Fe-oxide values (and conversely, rela- tively low silica). Similarly, elevated Y + La + Ce + Yb + U + Th levels char- acterize the granites. The C u + Z n + ( P b ) + ( M o ) group, and the high percentile barium distribution are largely associated with known mineraliza- tion phenomena. Results of main factor scores (Fig. 8) also support the

% ~ o ~

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~ N! Z n V ~ km .3 0 * , -~ , s Z~

< - I . 2 c-I.15 ~1 .03

Fig. 8. Map showing distribution of main factors. Note factor V, showing the "mirror" behav- iour of Si and Ca.

362 R. BOGOCH ET AL.

grouping of the data into geochemical domains. Factor I includes Y, La, Ce, Yb and Be, and may be explained by concentrations of heavy minerals such as monazite, zircon, and sphene in the wadi sediments. Samples showing high factor scores for factor I are mainly concentrated in the southern part of the Elat block (granites) and the Amram block (rhyolites). Factor II (Cr, Fe, V, Mg, Ni, Mn and Co ) relates to the ferromagnesium minerals and spinels, and is present in samples from the gabbroic Timna block and the dioritic Roded block. Factor III includes Sb, Eu, As, Ni and V. The distribution ofwadi sed- iment samples with high scores for factor III (closed squares on Fig. 8 ) occur in the unrelated Roded, Neshef and Shehoret blocks. Where present, high scores of factor IV samples (Roded, Amram, Timna), relate to chalcophile and/or barite mineralization.

The newly discovered geochemical anomalies of apparent interest include the Au-As-Sb geochemical domain in the central part of the Roded block and the Y + La + Ce + Yb + U + Th association in part of the Elat block. A prelim- inary detailed wadi sediment sampling program in both of these areas has confirmed these anomalies.

Some of the data may have petrogenetic implications. The inverse relation- ship of La, Ce and Yb vs. Eu (depleted vs. enriched respectively) in the area draining the diorites of the Roded block hints at the possible presence in these rocks of a positive Eu anomaly in a REE curve. Rhyolites dominate in the Neshef, Amram and Shehoret blocks, and are present to a lesser extent in the Elat block. Berrylium forms a discrete geochemical domain in only two of these (Neshef and Amram, Fig. 6d), suggesting a somewhat different origin for these Late Precambrian rhyolites (e.g., Griffitts and Powers, 1963, showed Be enrichment during metasomatism of rhyolites).

The use of the -0 .15 mm fraction ( < 100 mesh) of ephemeral (wadi) stream sediments in the Precambrian of southern Israel is thus a simple and rapid method for geochemical mapping and prospecting in arid areas where areas of rock exposures dominate over those covered by alluvium and sand. Because of the limited secondary dispersion in arid regions, and the impor- tance of targeting elements in which the "nuggget" effect must be reduced, a sampling density in the order of 3 samples/km 2 is recommended. The mask- ing effect of windblown material in these mountainous and "hilly" arid re- gions is negligible.

ACKNOWLEDGEMENTS

We thank Rami Madmon, Yaacov Raphael, Saadya Levy, Shelomo Ash- kenazi, Robert Knafu and Yehuda Peled for their assistance in the field, Dina Stieber and Olga Yaffe for the analyses, and S. Levy for drafting. A. Levinson is greatfully acknowledged for his comments on an early version of this paper.

EPHEMERAL STREAM SEDIMENTS IN PRECAMBRIAN MOUNTAINOUS ARID TERRAIN, S. ISRAEL 363

R.G. Garrett and C.F. Gleeson are thanked for their constructive reviews. This study represents part of the Geological Survey of Israel project 21037.

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