Adsorption of Cu, Zn and Pb near sulfide deposits by hydrous manganese—Iron oxide coatings on stream alluvium

Chemical Geology, 33 ( 1 9 8 1 ) 6 5 - . 7 9

65

Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s

ADSORPTION OF Cu, Zn AND Pb NEAR SULFIDE DEPOSITS BY HYDROUS MANGANESE--IRON OXIDE COATINGS ON STREAM ALLUVIUM

G E N E D. R O B I N S O N

Geology Department, James Madison University, liarrisonbourg, VA 22807 (U.S.A,) (Received M a r c h 4, 1 9 8 1 ; a c c e p t e d for p u b l i c a t i o n May 22, 1981 )

ABSTRACT R o b i n s o n , G.D., 1981. A d s o r p t i o n o f Cu, Z n a n d Pb near sulfide d e p o s i t s b y h y d r o u s m a n g a n e s e - - i r o n o x i d e coatings o n s t r e a m alluvium. C h e m . Geol., 3 3 : 6 5 - - 7 9 . H y d r o u s o x i d e s o f M n a n d Fe scavenge several base metals in t h e surficial e n v i r o n m e n t by a d s o r p t i o n a n d c o p r e c i p i t a t i o n . Results o f l a b o r a t o r y i n v e s t i g a t i o n s suggest increased a d s o r p t i o n c o m p a r e d to c o p r e c i p i t a t i o n w h e r e higher c o n c e n t r a t i o n s o f m e t a l ions exist in s o l u t i o n , s u c h as generally o c c u r near a n o x i d i z i n g m i n e r a l deposit. S t r e a m c o b b l e s w i t h h y d r o u s o x i d e coatings, collected f r o m t h e Valley a n d Ridge P r o v i n c e in n o r t h w e s t e r n Virginia, were i m m e r s e d in p r e p a r e d a q u e o u s s o l u t i o n s of Cu-Z n - - P b for 50 hr. S u b s t a n t i a l q u a n t i t i e s o f each m e t a l were a b s o r b e d f r o m s o l u t i o n . Considerably greater p r o p o r t i o n s o f t h e n e w l y a d s o r b e d m e t a l c o u l d be e x t r a c t e d b y a m m o n i u m c i t r a t e t h a n b a c k g r o u n d c o n c e n t r a t i o n s o f m e t a l initially p r e s e n t in t h e coatings. Analysis o f h y d r o u s o x i d e coatings c o l l e c t e d at varying d i s t a n c e s u p s t r e a m a n d d o w n s t r e a m f r o m a small C u - - Z n - - P b - s u l f i d e d e p o s i t p r o d u c e d similar results. F o l l o w i n g t h e s a m e a n a l y t i c a l p r o c e d u r e , c o n s i d e r a b l y greater p r o p o r t i o n s o f t h e t o t a l Cu a n d Z n were e x t r a c t e d b y a m m o n i u m c i t r a t e f r o m samples near t h e m i n e t h a n f r o m t h o s e u p s t r e a m or at a greater d i s t a n c e d o w n s t r e a m . If a d s o r p t i o n o f m e t a l b y h y d r o u s o x i d e s is generally m o r e i m p o r t a n t c o m p a r e d to cop r e c i p i t a t i o n w h e r e higher c o n c e n t r a t i o n s o f m e t a l ions are available in s o l u t i o n , t h e relat i o n m i g h t be o f practical value. T h e sulfide d e p o s i t is clearly d e l i n e a t e d if Cu a n d Z n ext r a c t e d by a m m o n i u m c i t r a t e is expressed as a p e r c e n t of t o t a l Cu or Zn. Previous studies have s h o w n t h a t similar d e p o s i t s are n o t clearly d e l i n e a t e d b y analysis o f h y d r o u s o x i d e s unless ratios o f m e t a l to M n or Fe are calculated.

INTRODUCTION

The hydrous oxides of Mn and Fe (henceforth referred to as hydrous oxides) have long been recognized as important scavengers of several metals in stream sediment, soil and in the ocean (Jenne, 1968; Burns, 1976; Chao and Theobald, 1976; Nowlan, 1976, 1979; Carpenter and Hayes, 1978, 1979a). Several recent papers have shown that such alluvial forms of these materials as films, cements and coatings c o m m o n l y contain anomalously high concentrations of transition metals near oxidizing sulfide deposits and have potential

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66 as a sample medium for geochemical exploration (Carpenter et al., 1975, 1978; Whitney, 1975; Nowlan, 1976; Cerling, 1979}. The mechanism by which such concentrations of metal ions are incorporated into alluvial hydrous oxides has been inadequately investigated. The purpose of this paper is to summarize a simple laboratory experiment with hydrous oxide coatings on stream cobbles, indicating that adsorption over a period of time is a more important control of the anomalous metal concentrations reported in these materials than coprecipitation. Results of the experiment suggest that when using hydrous oxides as a geochemical sample medium, an analytical technique which excludes coprecipitated metal ions produces single-element dispersion trends that are as clearly defined as those based on ratioed data from dissolution of the oxides. The question of partitioning of metal among various coating phases is purposely avoided because the writer wanted to initially consider the coatings as a single unit, in order to focus attention on their overall behavior as metal scavengers. Partitioning relationships are currently under investigation in this laboratory and will be reported in a subsequent paper. Hydrous oxide coatings on stream alluvium are generally believed to form at an interface between oxidizing and reducing conditions such as where groundwater percolates into a stream bed. Precipitation of small quantities of the hydrous oxides has a strong autocatalytic effect on additional precipitation (Crerar and Barnes, 1974; Hem, 1977, 1978; Morris and Bale, 1979). According to Crerar and Barnes {1974), layers of 5-MnO2 should accumulate as long as sufficient dissolved 02 and Mn 2+ flow toward the deposit. Carpenter and Hayes (1980) suggest that for coated stream alluvium an equilibrium between accumulation and destruction of the hydrous oxides is achieved after 3 yr. They suggest that hydrous oxide coatings may be removed by burial in sediment (lower Eh), abrasion, desiccation and spalling during periods of low water, and biological erosion. Potter and Rossman (1979) have determined the mineralogy of hydrous oxides from several near-surface environments and Carpenter and Hayes (1980) have summarized the literature on this subject. Birnessite, (Na,Ca,K) MnvOi4.3H20 (Potter and Rossman, 1979), is the dominant Mn phase but todorokite, (Mn,Ca,Mg}Mn30~. H20, may also occur. Co-existing Fe phases probably consist of goethite and amorphous ferric-oxyhydroxide. Up to 40-50% of hydrous oxide coatings in stream alluvium can consist of trapped organic material, silt, clay and adsorbed water. The adsorption potential of these materials compared to the hydrous oxides has not been established. The literature concerning adsorption and coprecipitation of trace metals by the hydrous oxides is voluminous. Their extremely large surface area [ 160-350 m2/g for artificial 5-MnO2 according to Loganathan and Burau (1973)] is considered to be a major factor in explaining their scavenging ability (Jenne, 1977). Experiments have generally shown that adsorption of metal ions by the hydrous oxides increases with pH and with the concentration of metal ions in solution. Adsorption occurs by at least three distinct processes: specific

67 adsorption, proton exchange and diffusion exchange. Details of these processes have been discussed by Morgan and Stumm (1964), Mason (1966), Krauskopf (1967}, McKenzie (1970), Loganathan and Burau {1973), Gadde and Laitinen {1974), Murray (1975), Rose (1975) and Jenne (1977). In contrast to adsorption, which occurs after precipitation, transition-metal ions may also be scavenged during precipitation of hydrous oxides by coprecipitation (Burke, 1970). The relative importance of coprecipitation compared to adsorption has not been established, partly because of the difficulty in differentiating between the processes in laboratory experiments. Dyck (1971) has shown that hydrous oxides precipitated in a Ag solution do not scavenge more metal than previously precipitated material, suggesting that adsorption may be more important than coprecipitation. EXPERIMENTAL Cobble samples with hydrous oxide coatings were collected from small streams draining western and eastern strike belts of the Beekmantown Formation within the Valley and Ridge Province of Rockingham County, Virginia (Fig. 1). This formation was chosen because of its prominence in this portion of the Valley and Ridge Province and its association with zinc sulfide mineralization. Four samples were collected at each of the two locations and the pH of the stream water was measured by meter at the same time. The substrate of each sample consisted of chert, a c o m m o n weathering product of the Beekmantown. Several Zn prospects have been reported from the western strike belt but none are located near the sample site (Herbert and Young, 1956). No Zn occurrences are known in the eastern strike belt nor are there any apparent sources of contamination. The coatings should, therefore, contain essentially normal or background concentrations of metal. The samples were transported to the laboratory in containers of the stream water to minimize possible physical and chemical changes within the coatings. Each sample was sawed into approximately equal halves and washed in distilled--deionized water to remove loose sediment and the cutting fluid from the saw (tap water). One half of each cobble was stored for later analysis, and the other half was immersed in portions of a prepared aqueous solution having a concentration of 2 pg/ml each of Cu, Zn and Pb. Although 2 ~g/ml is somewhat greater than concentrations of these metals that are considered to be strongly anomalous in streams under slightly alkaline conditions (Cameron, 1978), this concentration was necessitated by analytical restrictions. The metal solution had been prepared 4 days before samples were collected by careful dilution of atomic absorption reference solutions supplied by Fisher Scientific ® Co. These solutions had an initial concentration of 1000 /~g/ml and were prepared as nitrate solutions. The pH of the metal solution was adjusted to the mean pH of the two streams (8.1 and 8.0, respectively) by addition of NaOH. To detect possible incipient precipitation or adsorption by container walls, a portion of the metal solution was left undisturbed in a

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Fig. 1. Map showing location of samples collected for the adsorption experiment (W and E refer to western and eastern strike belts of the Beekmantown Formation).

covered glass-walled container from preparation to the end of the experiment. Analysis of this solution showed that, within the limits of analytical error (less than 10%), no change in metal concentration had occurred. One half of each cobble was immersed in a known volume of the metal solution for 50 hr. The solutions were stirred for 30 s at the beginning of the experiment and after 6, 25 and 50 hr. Aliquots of each solution were analyzed for Cu, Zn, Pb, Mn and Fe, using a Perkin Elmer ® model 370A atomic absorption unit. After washing in distilled--deionized water, coatings were removed from both immersed and unimmersed cobble halves by a two-stage sequential leach. The initial leach consisted of a 0.1 M solution of ammonium citrate, adjusted to a pH of 8.4. Hawkes (1963) attributes ammonium citrate extraction of transition-metal ions mainly to solubilization of weakly-bonded surface ions. Samples were immersed in this reagent for 20 min. at room temperature. Previous experiments had shown that no additional metal is extracted by longer immersion times. The solution was not agitated in order to minimize spaUing o f the coating material. The ammonium citrate solutions

69 were filtered and retained for analysis. Digestion was completed, after washing the samples in distilled-deionized water, by immersion for 2 hr. in 3 M HC1 heated to 80°C. T he h y d r o u s oxide coatings were not visibly affected by the a m m o n i u m citrate solution but were c o m p l e t e l y dissolved by the acid. Silicate substrates were not appreciably attacked. T he concentrations of Cu, Zn, Pb, Mn and Fe, de t e r m i ne d by atomic absorption (AAS), were expressed as micrograms o f metal per square c e n t i m e t e r of coating surface area (~g/cm:). The coating surface area was measured by the m e t h o d of Carpenter et al. (1978), which is accurate to ~ e8%. RESULTS As shown in Table I, substantial quantities of Cu, Zn and Pb were adsorbed from prepared metal solutions during immersion of oxide-coated cobbles. Cu and Zn co n cen tr a t i ons in immersed coatings are ~ 5X greater and Pb, 2 X greater, co mp ar ed to unimmersed coatings (Table II). A comparison o f metal lost from solution (expressed as tag/cm 2 by considering the volume of the metal solution and the surface area o f each cobble excluding the sawed face) and the increase in metal in immersed coatings is presented in Table III. The bulk o f metal lost from solution apparently was adsorbed by hydrous oxide coatings although there probably has been some adsorption by the coating substrate. No experiments were c o n d u c t e d with uncoated cobbles, but Dyck (1971) f o u n d t h a t the silicate p o r t i o n of natural Mn--Fe precipitates adsorbed very little metal com pa r ed to the hydr ous oxides. Greater quantities of metal were adsorbed by coatings with higher natural concentrations o f base metal (see correlation coefficients, Table IV). This relation, which is p r o babl y a function of the Mn--Fe c o n c e n t r a t i o n in the coatings as indicated by significant correlations, would not exist unless natural adTABLE I Analysis of prepared metal solutions* after immersion of oxide-coated cobbles Sample No.

Cu (ug/ml)

Zn (ug/ml)

Pb (~g/ml)

Wl W2 W3 W4

0.23 0.24 0.32 0.51

0.15 0.20 0.20 0.41

0.1 0.3 0.1 0.4

E! E2 E3 E4

0.48 0.10 0.43 0.37

0.35 0.08 0.12 0.24

0.4 0.1 0.1 0.2

Mean

0.34

0.22

0.2

*Original concentration of each metal = 2 ug/ml.

70 TABLE II Results of adsorption test of Cu, Zn and Pb on oxide-coated cobbles

Sample Metal concentrations* (~g/cm 2) No.

unimmersed samples Cu

Zn

Pb

Wl W2 W3 W4

0.51 0.74 0.66 1.21

1.11 1.64 2.32 1.39

E1 E2 E3 E4

0.82 0.74 3.03 2.01

1.99 1.28 3.87 2.41

Mean

1.34

2.00

immersed samples Mn

1.53 3.48 4.46 4.03

Cu

Zn

818 1,502 2,322 3,155

5.68 4.28 4.67 4.18

1.88 77 693 5.90 473 7,295 33.13 4,296 4,178 16.80 721 543

6.79 5.43 16.03 6.63

8.00

404 471 607 621

Fe

959

2,563

6.71

Pb

7.62 5.70 7.05 6.71

Mn

7.69 5.17 7.47 6.97

508 261 421 501

Fe 929 465 800 1,496

10.76 9.79 75 1,006 5.65 7.36 268 3,322 19.59 57.61 4,680 5,587 8.31 17.78 439 345 8.92

14.98

894

1,743

*Combination of a m m o n i u m citrate and 3 M HCt leaches. TABLE III Metal adsorbed from prepared solutions (ug/cm 2) after immersion of oxide-coated cobbles Sample Decrease in metal concentration, No. prepared solutions Cu

Zn

Pb

Increase in metal concentration*, hydrous oxide coatings Cu

Zn

Pb

WI W2 W3 W4

6.48 9.19 6.44 7.41

6.78 9.40 6.89 7.91

6.96 8.88 7.28 7.96

5.17 3.54 4.01 2.97

6.51 4.06 4.73 5.32

6.16 1.69 3.01 2.94

El E2 E3 E4

9.29 6.36 15.07 7.42

10.09 6.34 18.05 7.82

9.78 6.36 18.24 8.00

5.97 4.69 13.00 4.62

8.77 4.37 15.72 8.00

7.91 1.46 24.48 0.98

8.46

9.17

9.18

5.49

6.92

6.08

Mean

*Combination of a m m o n i u m citrate and 3 M HCI leaches. sorption of metal by the hydrous oxides had not reached equilibrium or the a d s o r p t i o n c a p a c i t y o f t h e o x i d e s is g r e a t e r i n t h e p r e p a r e d m e t a l s o l u t i o n t h a n i n s t r e a m w a t e r . T h e l a t t e r is p r o b a b l y c o r r e c t b e c a u s e as M a s o n ( 1 9 6 6 ) s t a t e s , o n e o f t h e p r i n c i p l e s o f a d s o r p t i o n is t h a t t h e c o n c e n t r a t i o n o f a n i o n a d s o r b e d f r o m s o l u t i o n i n c r e a s e s w i t h its c o n c e n t r a t i o n i n s o l u t i o n . R e e d m a n { 1 9 7 9 ) lists t h e f o l l o w i n g as b a c k g r o u n d c o n c e n t r a t i o n s i n g r o u n d - a n d

71 TABLE IV Correlation coefficients between various element concentration pairs determined in adsorption experiment .

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Increased metal* in coatings after immersion

Metal* in unimmersed coatings Cu

Zn

Pb

Mn

Fe

Cu Zn Pb

0.779 ---

0.858 0.810 --

0.723 0.779 0.768

0.937 0.904 0.937

0.825 0.503 0.826

*Combination of ammonium citrate and 3 M HCI leaches. TABLE V Percent of total* metal extracted by ammonium citrate from cobble halves in adsorption experiment Sample

Unimmersed samples

Immersed samples

No.

Cu

Zn

Pb

Cu

Zn

Pb

Wl W2 W3 W4

<33.3 <32.4 <36.4 60.3

<4.5 <4.3 <2.6 <5.0

<15.7 <10.1 <6.7 9.2

27.3 44.9 22.7 20.5

16.7 36.9 24.7 28.6

8.0 25.4 2.7 21.4

E1 E2 E3 E4

<31.7 <23.0 <15.8 <12.9

<4.0 <3.9 <3.6 <2.9

<20.2 <4.2 <2.1 <2.2

83.8 82.1 66.5 78.1

68.1 76.9 54.1 68.1

26.2 28.8 8.9 53.1

Mean

<30.5

<3.8

<8.7

53.2

46.8

21.8

*Combination of ammonium citrate and 3 M HCI leaches. stream water: Cu, ~ 0.008 ~g/ml; Zn, ~ 0.001--0.020 #g/ml; and Pb, ~ 0.003 p g / m l . T h e c o n c e n t r a t i o n o f C u , Z n a n d P b in t h e p r e p a r e d m e t a l s o l u t i o n (2 ~ g / m l } is t h e r e f o r e c o n s i d e r a b l y h i g h e r t h a n c o n c e n t r a t i o n s n o r m a l l y o c c u r r i n g in s t r e a m s o r g r o u n d w a t e r . T h e a b i l i t y o f h y d r o u s o x i d e s t o a d s o r b m e t a l s h o u l d b e e n h a n c e d in s u c h a s o l u t i o n . A comparison between ammonium citrate extraction of metal from coatings o f i m m e r s e d a n d u n i m m e r s e d c o b b l e h a l v e s is p r e s e n t e d in T a b l e V. T h e c o n centration of metal extracted by ammonium citrate from unimmersed coatings is g e n e r a l l y v e r y l o w a n d f o r m o s t s a m p l e s , is b e l o w t h e A A S u n i t ' s l i m i t of detection; consequently, the extraction efficiency of ammonium citrate w a s d e t e r m i n e d in r e l a t i o n t o t h e a p p r o x i m a t e m i n i m u m c o n c e n t r a t i o n o f metal detectable by AAS for each sample. The proportion of metal extracted b y a m m o n i u m c i t r a t e is less t h a n t h i s q u a n t i t y b y a n u n d e t e r m i n a b l e a m o u n t . Approximately one half of the Cu and Zn adsorbed from the metal solu-

72 tions, and a fifth of the Pb w a s extracted by a m m o n i u m citrate. A much lower efficiency of extraction is evident from coatings of unimmersed oxidecoated cobbles, with an average of <3.8% of the Zn and <8.7% of the Pb being dissolved. The proportion of Cu extracted is probably within a similar range, but could not be definitely determined because of very low natural concentrations o f Cu in the coatings. No Mn or Fe could be detected by AAS of the metal solutions so, within analytical limits, no exchange of lattice Mn and Fe with metal ions has been confirmed. This is not surprising as Loganathan and Burau (1973) have shown that in experiments involving a fine suspension of artificial 5-MnO2, no exchange with structural Mn occurs at Zn concentrations of < 1 mM (6.5 ~g/ ml). DISCUSSION Natural concentrations of base metals in hydrous oxide coatings occur as both coprecipitated and adsorbed forms. Additional coprecipitated metal cannot account for increased concentrations of metal after immersion of coatings in the metal solutions because no precipitation of Mn or Fe occurred. A much greater proportion of the newly adsorbed Cu and Zn, compared to background concentrations of metal in the coatings, was extracted by amm o n i u m citrate, which suggests that adsorption by hydrous oxides is less important than coprecipitation where background concentrations of metal ions are present. Where anomalously high concentrations of metal ions occur, adsorption can be a very important process in removing metal ions from solution. The potential effect of ageing on the extraction of metal ions from the coatings was not considered in the experimental procedure. It is conceivable t h a t newly adsorbed metal ions may become more tightly bonded with time. This possibility was tested by examining metal partitioning in natural coatings collected from an environment in which metal concentrations vary sharply over a small distance. These data are part of an unpublished study conducted by the author in 1979, near the Point Cedar mine in the Ouachita Province of south-central Arkansas. Cobbles with black coatings, averaging ~ 6 × 8 cm, were collected at varying distances upstream and downstream from the mine (Fig. 2). Mineralization at the mine consists of sphalexite, galena, chalcopyrite and pyrite associated with steeply-dipping quartz veins (Stroud, 1969). Small quantities of ore have been produced from the mine but it has been inactive since 1952. A large d u m p of waste material is present on the property. Unsawed cobbles were leached by the same two-stage procedure described previously. The solutions were analyzed for Cu, Zn, Pb, M n and Fe by AAS. Although no data were obtained on metal concentrations in the stream water, results of m a n y previous studies suggest that a sharply-decreasing concentration gradient exists downstream from the sulfide deposit. Such a

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Fig. 2. Map showing location o f the Point Cedar mine and sample locations.

decreasing concentration is well-developed in minus 80 mesh sediment (Table VI). If results of the adsorption experiment are applicable to this area, adsorbed metal ions should be more abundant in coatings collected near the mine than at other locations. As shown in Table VII, the percent of total Cu and Zn extracted from the coatings by ammonium citrate follows such a pattern. Adsorption of Cu and Zn by hydrous oxide coatings is apparently a much more important process downstream from the mineralization than upstream. These results might be of practical importance in exploring for sulfide deposits. The recent interest of exploration geochemists in the hydrous oxides centers on the possibility that, by directly sampling these materials, an exploration tool more sensitive than the traditionally sampled fine-grained stream sediments becomes available. Studies which have demonstrated such a potential have generally used selective extraction procedures because some metals appear to be enriched in the Mn portion of the hydrous oxides while others may be concentrated in the Fe portion. An additional extraction specific for metal residing in organic matter occluded in the coatings might be advantageous because such material can be important in terms of metal content (L.

74 TABLE VI S u m o f metal extracted b y a m m o n i u m c i t r a t e a n d 3 M HCI f r o m m i n u s 80 m e s h s t r e a m s e d i m e n t , P o i n t Cedar, A r k a n s a s

Sam;le ..... (~,onceentration(ppm) No.*

Cu

Zn

Pb

1 2 3 4 5 6 7 8 9

<11 23 468 132 4 89 22 4 6

209 491 3,398 1,996 380 431 550 506 898

30 49 3,040 955 139 80 130 120 60

* S a m p l e n u m b e r s r e f e r t o Fig. 2. TABLE VII P e r c e n t o f t o t a l .1 m e t a l e x t r a c t e d b y a m m o n i u m c i t r a t e f r o m b l a c k c o a t i n g s o n s t r e a m c o b b l e s , P o i n t Cedar, A r k a n s a s Sample No.* 2

Cu

Zn

Pb

Mn

Fe

I 2 3 4 5 6 7 8 9

0.63 0.21 9.01 1.38 0.76 1.54 0.77 3.76 3.30

0.82 0.70 16.93 33.83 11.42 13.23 4.17 8.43 10.42

9.69 11.97 3.51 2.18 0.81 2.09 0.84 4.52 7.91

0.14 0.28 9.88 0.18 3.91 5.48 0.59 3.80 0.90

0.03 0.02 0.39 0.05 0.29 0.74 0.09 0.09 0.01

*~ C o m b i n a t i o n o f a m m o n i u m c i t r a t e a n d 3 M HC1 leaches. .2 S a m p l e n u m b e r s r e f e r to Fig. 2.

Filipek, pets. commun., 1980). These studies have also shown that ratioed data, such as Cu/Fe or Zn/Mn, must be used to avoid detecting anomalies unrelated to mineralization ("false" anomalies). In addition to requiring rather time-consuming analytical techniques, the utilization of ratioed data requires analyses for elements not directly related to the target. At Point Cedar, welldefined downstream dispersion trends and excellent anomaly/background contrast (ass-mlng samples 1 and 2 represent background concentrations of metal) for Cu and Zn result when single-element analysis of ammonium citrate extractions from coatings are expressed as percents of total coating metal (see Table VII). Trends for Pb are not apparent but previous studies

75

have shown that Pb is not strongly adsorbed by hydrous oxide coatings near sulfide deposits (Carpenter et al., 1978). Results of the adsorption experiment {Table V) show that only a small proportion of the Pb adsorbed from the metal solutions is extracted by a m m o n i u m citrate. The present study enables some speculations to be made concerning a pecularity of metal incorporation into natural hydrous oxides under alluvial condition, which was first noted by Carpenter and Hayes (1978). Their study involved placing mineral streak plates in a stream at varying distances below an oxidized sulfide deposit and determining coating and trace-metal accretion rates. After 36 days, mineralization was clearly indicated by determining Zn in the incipient coatings. An anomaly/background ratio of 10 × had been developed; however, a 20 × anomaly/background ratio was present in mature coatings on stream cobble. An anomaly/background ratio that changes with time is suggested clearly indicating non-linear scavenging of metal. Results of the present investigation suggest that adsorption can account for more rapid addition of metal as the hydrous oxides develop. It is proposed that adsorption of metal continues for a considerable time after a particle of hydrous oxide precipitates. As coating layers accumulate an increasing number of oxide particles are adsorbing metal, resulting in a nonlinear accumulation of metal. In background areas, the increase in the rate of accumulation would be less rapid than in anomalous areas due to increased adsorption where a greater

/

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Z w U Z O U _.1 w

TIME

Fig. 3. Diagram illustrating the suggested m o d e l for the scavenging o f trace metal by h y d r o u s o x i d e s in a stream e n v i r o n m e n t . The a c c u m u l a t i o n o f eopreeipitated metal is essentially linear but metal is adsorbed at a non-linear rate.

76

concentration of metal ions is available. By this process, the anomaly/background ratio would change with time. The process would be self-limiting because early-formed coating layers would eventually reach adsorption capacity so that metal accretion rates would gradually decline until a stable end-point had been reached. Coating layers in background areas would probably achieve equilibrium before those forming with a greater concentration of metal ions available. Coprecipitation of metal ions can clearly not produce a changing anomaly/ background ratio unless accumulation of hydrous oxide coatings is a nonlinear process. Studies by Carpenter and Hayes (1979b, 1980) over a 1-yr. period show coatings accumulate at an essentially constant rate. If the suggested process is important in nature, an analytical procedure specific for ions of a single metal adsorbed onto hydrous oxide coatings should be as effective in detecting significant anomalies and avoiding "false" anomalies as the generally followed procedure of utilizing ratioed data derived from coating dissolution. As shown in Fig. 3, coprecipitated metal would accumulate at an essential linear rate but metal would be adsorbed at a nonlinear rate. Accumulation of metal by coprecipitation would continue until accretion of coating material had stabilized [possibly 3 yr. as suggested by Carpenter and Hayes (1980)]. Adsorption of metal would continue until equilibrium for the local geochemical conditions had been established. Results of the adsorption experiment and at the Point Cedar mine suggest that analysis of coatings from background areas should have metal partitioning relationships similar to those shown in the lower-left portion of Fig. 3 while hydrous oxides from anomalous areas would be represented by the upper-right portion of the diagram. A significantly greater proportion of the total metal could be TABLE VHI Total *j m e t a l in black c o a t i n g s o n stream c o b b l e s , P o i n t Cedar, Arkansas Sample No.,2

.

1 2 3 4 5 6 7 8 9

.

.

.

Metal in coating ( , g / c m 2) Ratios ........................................... Cu Zn Pb Cu ........ x 100 Mn+Fe .

.

.

.

.

4.73 24.24 1.11 2.17 2.62 1.30 5.17 1.33 0.91

.

.

.

.

18.34 14.25 15.89 20.19 25.92 15.95 25.87 27.16 19.01

.

.

.

.

.

2.27 2.34 9.12 4.59 27.05 9.09 33.27 5.31 2.15

.

.

.

.

.

.

.

.

.

Zn - - - - - × 100 Mn+Fe

Pb .... × 100 Mn+Fe

0.57 0.33 9.18 1.25 8.10 14.09 2.91 3.94 0.49

0.07 0.05 5.27 0.28 8.45 23.90 3.74 0.77 0.06

.

0.14 0.56 0.64 0.13 0.82 1.15 0.58 0.19 0.02

,1 Sum o f a m m o n i u m c i t r a t e a n d HCI e x t r a c t i o n s . ,2 S a m p l e n u m b e r s r e f e r t o Fig. 2.

77 extracted from the anomalous samples than from the background samples by a m m o n i u m citrate, so t h e y should be clearly detected. If the analysis is made by dissolving the coatings, ratios o f metal/Mn or m e t a l / F e would be necessary to delineate such samples because the greater p r o p o r t i o n s of the metal detected would be coprecipitated. The c o n c e n t r a t i o n of coprecipitated metal present is in direct p r o p o r t i o n to the c o n c e n t r a t i o n o f Mn and Fe in the coatings. The comparison o f ratioed to unratioed data in Table VIII clearly illustrates this point. For the proposed model to be operative, two conditions must be fulfilled: (1) Stream water must have ready access t o previously formed layers of coatings. This co n di t i on probably exists because the hydrous oxides are known to have an " o p e n " structure with bot h internal and external surfaces being readily available for adsorption (Chao and Theobald, 1976). (2) Newly precipitated hydr ous oxides must reach adsorption capacity slowly. This condition is m or e open to question as most experiments have shown that adsorption is essentially completed after a period o f several minutes to a few days. Such studies, however, generally involved fairly high metalion co n cen tr atio n s and artificially precipitated hydrous oxides. The time required for natural h y d r o u s oxides to reach adsorption equilibrium under natural conditions has not been established. ACKNOWLEDGEMENTS T h e au th o r is grateful to his colleagues at James Madison University, particularly Cullen Sherwood and Lance Kearns, for critically reading the manuscript. He would also like to express his appreciation to T.T. Chao, Gary Nowlan and Lori Filipek for constructive suggestions which he had not considered. Finally, he would like to thank B et t y Lou H uffm an for typing the manuscript.

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