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Trace metal adsorption potential of phases comprising black coatings on stream pebbles
Journal o[ Geochemical Exploration, 17 (1982) 205--219
205
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
TRACE METAL ADSORPTION POTENTIAL OF PHASES COMPRISING BLACK COATINGS ON STREAM PEBBLES
GENE D. ROBINSON
Geology Department, James Madison University, Harrisonburg, Virginia (U.S.A.) (Received November 25, 1981; revised and accepted June 24, 1982)
ABSTRACT Robinson, G.D., 1982. Trace metal adsorption potential of phases comprising black coatings on stream pebbles. J. Geochem. Explor., 17 : 205--219. A sequential extraction of Cu and Zn was completed on 8 samples of black coatings on coarse stream alluvium before and after immersion in prepared metal-rich solutions. Such immersion resulted in a substantial change in the partitioning of the metals among the various phases present. Most of the samples were collected from small streams in areas of the Valley and Ridge, Blue Ridge, and Piedmont provinces of north~entral Virginia without known mineral deposits, and partitioning relationships were determined by a five-step selective extraction procedure. The following order of partitioning was determined in the samples:
Crystalline Fe oxides Mn oxides Amorphous Fe oxides Weakly-bonded ions Organic matter
Cu
Zn
Mn
Fe
1 2 3 4 5
2 1 3 4 5
2 1 4 3 5
1 3 2 5 4
Over 90% of the total Cu and Zn in the coatings resides in the Mn and Fe oxide phases. The coating samples were immersed for 1 week in aqueous solutions containing 50 ug/ 1 Cu and 400 ug/1 Zn (pH 6.0). After this time, almost all of the metal had been adsorbed from the solutions. The organic portion of the coatings and metal present as weaklybonded ions became significantly more important as residence sites for both Cu and Zn. These results suggest that the partitioning of the anomalous concentrations of metal previously reported in black coatings near weathered sulfide deposits may be distinct compared to that occurring in samples collected from areas containing background concentrations of metal.
INTRODUCTION
Precipitates of Mn and Fe oxides are c o m m o n in m a n y streams, particularly in the form of black coatings on rock substrates. Recent interest in these materials is mainly due to their ability to "scavenge" a number of trace
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© 1982 Elsevier Scientific Publishing Company
206 metals from stream or ground water. Several recent studies, including those of Carpenter and Hayes (1979), Filipek et al. (1981), and Whitney {1981) have shown that Mn-Fe oxide coatings contain anomalous concentrations of metal near sulfide deposits and have suggested that directly sampling the precipitates may be advantageous in geochemical exploration. The "scavenging" of metal ions by Mn-Fe oxides has been widely investigated and is believed to include both coprecipitational and adsorptional processes (Chao and Theobald, 1976). Data presented by Robinson (1981) indicates that adsorption may be more important than coprecipitation in producing the anomalous levels of metal that occurs in these materials near mineral deposits. Although apparently simple, Mn-Fe oxide coatings actually represent a complex, multiphase system consisting of various Mn and Fe oxides and hydroxides (with differing degrees of crystallinity) plus a mixture of organic components and silicates, such as clay minerals (Potter and Rossman, 1979 and Filipek et al., 1981). Only a few studies have been made of trace metal partitioning relationships among the various phases comprising coatings. Some of these investigations have not attempted a complete determination of metal partitioning but instead have emphasized only the Mn and Fe components. For example, Carpenter et al. (1978), using a two-step sequential extraction, determined that the partitioning of Cu, Zn, and Co between the Mn and Fe oxide fractions of boulder coatings collected upstream from an oxidizing sulfide deposit is distinctly different from samples downstream of the mineralized zone. More recently, Filipek et al. (1981) investigated trace metal partitioning among all major components of boulder coatings and found that the organics, which had not been previously emphasized, are a major trace metal sink. A knowledge of trace metal partitioning is essential in understanding the basic geochemical processes operating during the weathering of a mineral deposit. Such information is also necessary in choosing the most effective technique of geochemical exploration for a particular area. A thorough understanding of the geochemical factors which influence partitioning of metal within coatings in various geological environments is needed before an assessment can be made of their potential use in detecting mineralization. The present study represents an initial attempt to provide such information. The main goals of the investigation are twofold: (1) To determine the partitioning of Cu, Zn, Mn, and Fe among the major phases of black coatings on coarse stream alluvium collected from diverse geological environments. (2) To investigate the influence of stream water pH and metal concentration on the partitioning of these metals in Mn-Fe oxide coatings. SAMPLING AND ANALYTICAL METHODS Mn-Fe oxide coatings on stream pebbles (approximately 5--7 cm diameter) were used in this study. Sampling was purposely non-random, that is, an
207
effort was made to collect pebbles that visually appeared to have the most well-developed coatings. Sample locations were chosen to represent the varying geological conditions occurring in the Piedmont, Blue Ridge, and Valley and Ridge provinces of north-central Virginia (Fig. 1). Sampled streams were small with drainage areas generally less than 20 km 2. Stream water pH was measured by portable meter at the time of sample collection, and water samples were collected in 1000 ml linear polyethylene bottles. The water samples were not acidified to avoid extraction of metal from particulates that might be present. Cameron (1978) has shown that even after several months, Cu and Zn concentrations in unacidified lake waters do not differ significantly from samples which were acidified at the time of collection. In the present study, water samples were analyzed within 2 days by atomic absorption spectrophotometry after filtering through 2 ~m filter paper. Coating samples were transported to the lab in containers of the stream water and were kept immersed until used in experiments. This precaution was deemed necessary to minimize possible physical and chemical changes within the coatings such as those described by Jeffries and Stumm (1976). With the exception of P-l, none of the sample locations are near any known mineral deposits. P-1 was collected from Contrary Creek near Mineral, Virginia, in the historic "Gold-Pyrite Belt." Numerous old mines, dumps, and prospects occur in the area. Due to the release of H2SO4 from the oxidation of pyrite, chalcopyrite, and sphalerite within the dumps, the pH of Contrary Creek is very low. Samples from Contrary Creek were included in the study to determine how partitioning of metal among coating phases would be affected by such anomalous conditions.
Fig. 1. Index map showing sampling locations.
208 The coating samples were used in experiments designed to determine natural partitioning relationships of Cu, Zn, Mn and Fe as well as the effect of stream water metal concentration on metal partitioning. The procedure involved a determination of changes in metal partitioning among coating phases resulting from immersing samples in prepared metal solutions as compared to the natural partitioning. In order to minimize variability at each sample site so that changes in metal partitioning could be better evaluated, each sample was sawed in half and washed in DD H~O. The samples were then dried for 24 hours at 40°C and weighed. One sample half was subjected to a five-step selective extraction procedure to determine natural metal partitioning. The other half was leached by the same procedure after being immersed for 1 week in 350 ml. of an aqueous solution containing 50 ug/1 of Cu and 400 gg/1 Zn, with the pH adjusted to 6.0. The metal concentration of this solution was chosen to emulate levels of Cu and Zn in stream water which, according to Reedman (1979}, would be considered anomalous. A previous study has shown that under the experimental conditions, Mn-Fe oxide coatings adsorb substantial quantities of the metal in solution but adsorption onto container walls is almost nfl (Robinson, 1983). To evaluate possible metal adsorption onto the sawed faces, one of the samples, from which the coating had previously been removed chemically, was subjected to the same adsorption experiment and extraction procedure. The sequential extractions were carried out by totally immersing each washed sample half in various reagents chosen for the selectivity of their attack. The following order of extraction was followed: (1) Weakly-bonded metal ions were extracted with a 0.1 M solution of ammonium citrate (Ac), as described by Robinson (1981). (2) Metal associated with the organic fraction was removed by 2 extractions with sodium hypochlorite (hyp) adjusted to a pH = 9.5 using a procedure adapted from Lavkulich and Wiens (1970}. According to Lavkulich and Wiens (1970} sodium hypochlorite is a more efficient oxidizer of organic matter than the more c o m m o n l y used hydrogen peroxide. Their procedure was modified by slowly evaporating the final sodium hypochlorite solution to dryness and dissolving the residue in 10% HCI. Mn and Fe determinations were made by diluting this solution as needed, but due to interferences, Cu and Zn were determined in a MIBK extract, prepared after chelating dissolved metal with APDC. (3) The Mn oxide portion of pebble coatings was dissolved in a 0.1 M solution of hydroxylamine hydrochloride (Hxl) in 0.01 M HNO3 (pH = 2.0} using the procedure described by Chao (1972). Filipek et al. (1981) dissolved the Mn oxide portion of boulder coatings before oxidizing the organic matter. In the present study, this order was reversed to avoid the possibility of losing fine particulate forms of organic material which might be attached to coatings. Such a form of organic matter could be released into the hydroxylamine hydrochloride solution if portions of the coating were dissolved.
209 (4) Acid ammonium oxalate (Aao) adjusted to a pH = 3.75 was used to dissolve the amorphous iron oxide portion of the coatings. This extraction was performed in darkness as described by Hoffman and Fletcher (1979). According to Rose (1975) this reagent is an effective reducing agent for the dissolution of amorphous or poorly crystalline oxide phases. (5) The crystalline iron oxide fraction was dissolved in a 0.7 M solution of hydrazine hydrochloride (Hz) adjusted to a pH of 2.1 with 12 M HCI (Gatehouse et al., 1977). Cu, Zn, Mn, and Fe in the above solutions were determined with a Perkin Elmer Model 370 atomic absorption unit using an air-acetylene flame. Standards were prepared from the same reagents used in sequential extractions. Metal concentrations are expressed in relation to the total weight loss resulting from dissolution of the coatings. These concentrations are only approximations due to the rapid equilibration of the dried samples with atmospheric moisture; however, the proportion of metal associated with each sample fraction is not affected because each is based on weight loss resulting from dissolution of the entire coating rather than individual fractions. RESULTS AND DISCUSSION Physical descriptions of the coating samples used for selective extractions and basic geochemical data on the stream water at each sample site are shown in Table I. As might be expected, considering the diverse geological environments represented by these samples, the coatings vary considerably in appearance. The coating sample from Contrary Creek (P-l), which has formed under very acidic conditions, does not resemble the other samples. Metal concentrations in all of the streams except Contrary Creek are at background levels. The tow pH of Contrary Creek, resulting from the oxidation of sulfides in mine dumps, enables extremely high concentrations of Cu and Zn to remain in solution. Natural partitioning relationships for Cu, Zn, Mn, and Fe, as determined by the sequential extraction scheme, are shown in Table II. Because established procedures were. used for each of the sequential extractions, no attempts to establish reagent specificity were made. Although the partitioning relationships ~ o n g the Various samples is extremely variable, a clear order of partitioning for each metal is apparent. Based on the calculated means, the following order of partitioning for each metal occurs:
Crystalline Fe oxides Mn oxides A m o r p h o u s Fe oxides Weakly-bonded ions Organic matter
Cu
Zn
Mn
Fe
1 2 3 4 5
2 1 3 4 5
2 1 4 3 5
1 3 2 5 4
210 TABLE I Physical description of coating samples and basic geochemical data on stream water Sample No. Physical description of coating
P-1 P-2 P-3 P-4
BR-1 BR-2 VR-1
VR-2
Yellow-yellowish brown, "fluffy" appearance, apparently thick and uniform Fine-grained, continuous, brownish-black, spotty on bottom, greenish algae abundant Dark gray-black, uniform and thick Light brown-gray-black, spotty in places. Unidentified, rodshaped organic material present, brown color Dark gray (top)-reddish brown (bottom). Mottled appearance, apparently thin coating Gray, powdery appearance, poorly developed Brown-brownish black (top), brownish red (bottom). Unidentified rod-shaped organic material present Dark gray (top), reddish brown (bottom)
Stream water Stream water pH metal concentration, ug/l
3.4
Cu
Zn
3400
7800
7.5
2.4
4.5
7.4
1.2
3.9
7.4
1.8
5.1
7.6
0.03
3.0
7.5
6.6
4.2
7.9
1.8
4.2
8.0
2.1
5.7
Only insignificant concentrations o f metal are extracted from the blank (uncoated sample) by the various reagents suggesting t hat using sawed pebbles has n o t significantly affected the det erm i ned partitioning order. The partitioning of Cu and Zn among the various coating phases o f P-1 is distinct from the o t h e r samples. Organic m a t t e r is a more i m port ant residence site for these metals than in samples from background areas, and t h e Mn oxides, which oc c ur in very low concentrations, are less important. Most o f the Mn in this sample pr obabl y occurs adsorbed o n t o the iron oxide coatings, as suggested by the large per cent o f Mn extracted by hydrazine and a m m o n i u m citrate and the low per cent o f Mn ext ract ed by h y d r o x y l amine hydrochloride. Because P-1 does represent ext rem el y contaminated conditions, such partitioning is probably n o t representative of coatings in streams draining unmined mineral deposits. T he inclusion of P-1 considerably affects calculations of the mean per cent o f each metal extracted by the various reagents, particularly for Mn; however, the affect on metal partitioning orders is much smaller, causing only mi nor shifts.
0.02 1.49 4.40 1.80 1.56 1.90 4.61 4.76 2.57 1.77 2.93
13.9 0.8 0.5 2.9 1.2 1.5 0.5 0.6 2.7 4.6 1.1
<0.1 <0.1 3.3 5.2 <0.1 1.9 4.7 0.7 2.0 2.2 2.3
4.0 68.9 62.7 83.4 84.6 67.8 39.5 19.0 53.7 29.8 60.8
Hxl <0.1 1.5 3.5 2.6 1.9 1.1 1.5 6.6 2.5 1.8 2.7
Aao 82.1 28.7 30.0 5.9 12.3 27.7 53.8 73.0 39.2 27.7 33.1
Hz 38.24 42.09 18.66 15.42 40.36 11.46 40.81 46.68 31.72 14.03 30.78
(%)*
Total
Hyp
Ae
(%)*
990 385 280 150 505 190 525 385 425 265 345 Fe
90.7 95.0 64.4 65.2 73.4 <0.1 84.6 <0.1 59.2 38.2 54.7
% Extracted by:
<0.1 <0.1 2.9 <0.1 <0.1 <0.1 4.4 41.7 6.1 14.5 7.0
(ppm)
Total
3.0 3.9 26.8 22.8 14.7 76.5 9.0 41.7 24.8 24.6 27.9
Hz
Mn
4.8 0.4 2.9 7.6 3.7 8.8 1.3 <0.1 3.7 3.2 3.5
Aao 5.1 4.9 2.1 2.0 0.6 <0.1 0.4 <0.i 1.9 2.1 1.4
Hyp 20.4 44.6 58.7 80.0 49.3 55.6 27.0 31.3 45.9 19.5 49.5
Hxl
1.1 <0.1 < 0.1 0.2 0.1 <0.1 <0.1 0.6 0.3 0.4 0.1
Ae
3.9 <0.1 0.6 <0.1 0.1 <0.1 <0.1 0.4 0.6 1.3 0.2
Hyp
3.3 1.1 4.4 7.8 2.5 1.5 1.0 1.9 2.9 2.3 2.9
Hxl
% Extracted by:
3.0 1.6 <0.1 <0.1 6.3 8.5 <0.i <0.I 2.4 3.3 2.3
Ac
% Extracted by:
9.0 2.0 8.0 14.4 0.5 6.9 3.6 9.7 6.8 4.6 6.4
Aao
2.5 <0.1 <0.1 18.0 <0.1 13.8 8.2 18.7 7.7 8.2 8.4
Aao
82.7 96.8 87.0 77.6 96.7 91.7 95.3 87.3 89.4 7.0 90.3
Hz
69.0 49.0 39.1 <0.1 43.8 22.2 64.4 50.0 42.2 22.4 38.4
Hz
*Total ffi sum of all extractions. Metal extracted from blank (pebble wi t hout a coating): Cu & Zn ffi below detection limits (all extractions); Mn = 1 p p m (all extracted by Hz); Fe = 9 ppm (94% extracted by Hz).
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Standard dev. Mean (P-1 omitted)
Sample No.
1.4 0.6 2.9 4.4 8.3 14.7 0.8 16.7 6.2 6.4 6.9
Hxl
Total
Hyp
% Extracted by:
Total
(ppm) Ac
Zn
Cu
P-1 2230 P-2 925 P-3 205 P-4 160 BR-1 110 BR-2 35 VR-1 390 VR -2 70 Mean 515 Standard dev. 750 Mean (P-1 omitted) 270
Sample No.
Natural partitioning of Cu, Zn, Mn, and Fe among various phases comprising coatings on stream pebbles
TABLE II
b3
212
Several environmental factors including the relative abundance of coating phases, the concentration of dissolved metal in the stream, and the stream water pH, can be important in controlling metal partitioning relationships. Over 90% of the total Cu and Zn in the coatings resides in the Mn and Fe oxide phases. The degree of control exerted by these phases on the concentration of Cu and Zn can be seen in Table III where the ratio of Cu and Zn to total Mn+Fe has been calculated. If P-1 is excluded from the calculations, much of the original variability apparent in Table II is lost; instead, a fairly consistent ratio between the trace metals and these major partitioning sites occurs. T A B L E III R a t i o s o f Cu a n d Z n t o t o t a l M n + F e
Sample No.
[Cu/(Mn + Fe)] × I00
[Zn/(Mn + Fe)] × i00
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Mean (P-1 omitted) Standard deviation (P-1 omitted)
0.58 0.21 0.09 0.09 0.12 0.14 0.12 0.08 0.18 0.12 0.04
0.26 0.09 0.12 0.09 0.12 0.14 0.12 0.08 0.13 0.11 0.02
Variations in stream water concentrations of Cu and Zn and stream water pH appear to be unimportant in controlling trace metal concentrations in all of the coating samples except P-1. This is probably because all of the samples but P-1 represent background conditions with only minor variations in stream water pH and metal concentrations. The anomalous nature of P-1 can be directly attributed to the fact that substantial quantities of sulfides occur in the drainage basin of Contrary Creek. The oxidation of these minerals releases H2SO4 into Contrary Creek, causing the pH of the water to be very low. This allows extremely high concentrations of dissolved Cu and Zn to exist in the water but does not allow Mn oxides to accumulate as pebble coatings. Inspection of the Eh-pH diagram presented by Rose (1975) shows that under oxidizing conditions typical of streams, Fe oxides are stable at low pH's, but Mn oxides are stable only under neutral to basic conditions. Organic matter and weakly-bonded ions are substantially more important as partitioning sites of Cu and Zn after immersion of coating halves in solutions o f Cu and Zn for 1 week than before (Table IV). Based on calculated
.004 1.90 4.27 1.62 1.84 3.13 4.36 1.29 2.30 1.50 2.65
<0.1 2.3 1.2 4.2 1.8 0.6 1.1 7.3 2.3 2.4 2.6
<0.1 <0.1 0.1 0.2 0.1 0.1 0.3 0.2 0.1 0.1 0.2
<0.1 35.0 58.5 78.1 54.4 12.0 45.4 45.6 41.1 25.3 47.0
<0.1 0.8 1.6 1.9 1.1 1.8 1.3 1.4 1.2 0.6 1.4
Aao
3.6 0.2 1.6 <0.1 1.5 0.5 3.6 <0.1 1.4 1.5 1.1
Aao
100.0 61.9 38.9 15.6 42.6 85.5 51.9 45.6 55.2 26.9 48.8
Hz
81.9 91.6 65.1 57.4 37.9 85.9 73.6 22.6 64.5 24.2 62.0
I-Iz
53.04 48.22 22.66 15.31 29.85 49.44 33.46 15.36 33.4 15.30 30.6
(%)
Total*
Fe
890 1445 355 600 920 955 890 1090 895 320 895
(ppm) 29.8 35.9 9.3 39.3 13.1 22.2 16.8 18.3 23.1 10.8 22.1
Hyp 14.6 29.9 56.3 40.0 54.5 41.2 44.4 37.7 39.8 13.4 43.4
Hxl
0.3 <0.1 <0.1 0.1 < 0.1 <0.1 <0.1 <0.1 0.1 0.1 <0.1
Ae
0.5 <0.1 0.9 0.3 0.7 0.8 1.0 1.7 0.7 0.5 0.8
Hyp
16.7 2.8 4.8 6.6 2.2 1.3 2.1 1.5 4.8 5.2 3.0
Hxl
% Extracted by
<0.1 4.1 2.8 6.3 7.3 4.0 6.4 32.3 7.9 10.1 9.0
Ac
% Extracted by
13.8 1.6 4.8 8.3 7.8 4.9 4.2 0.9 5.8 4.1 4.6
Aao
11.7 5.1 5.1 6.2 4.7 5.4 4.5 5.9 6.1 2.4 5.3
Aao
68.8 95.5 89.4 84.7 89.3 92.7 92.8 95.9 88.6 8.8 91.5
Hz
43.9 25.0 26.5 8.2 20.5 27.3 27.8 5.7 23.1 12.1 20.1
Hz
*Total ffi sum of all extractions. Metal extracted from blank (pebble w it hout a coating): Cu = below detection limits, all extractions; Zn = < 1 ppm, Hxl & Aao, other extractions below detection limits; Mn = below detection limits, all extractions; Fe ffi 1 ppm (Hxl), 4 p p m (Aao), other extractions below detection limits.
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Standard dev. Mean (P-1 omi tte d)
(%)
6.4 5.2 23.4 21.3 27.4 7.2 16.2 29.9 17.1 9.9 18.7
Hxl
Ac
Total* Hyp
% Extracted by
Mn
No.
6.2 1.9 5.8 11.6 15.1 3.9 5.4 16.1 8.2 5.3 8.5
Sample
1.8 1.0 2.7 9.7 18.3 2.5 1.3 31.4 8.6 11.0 9.6
Hxl
Ac
(ppm)
Hyp
Zn Total*
% Extracted by
Cu
Total*
P-1 1425 P-2 1600 P-3 260 P-4 155 BR-1 220 BR-2 750 VR-1 665 VR-2 260 Mean 665 Standard dev. 565 Mean (P-1 omitted) 560
Sample No.
Partitioning of Cu, Zn, Mn, and Fe among various phases comprising coatings on stream pebbles after immersion for 1 week in prepared solutions of Cu and Zn (pH 6.0)
TABLE IV
¢,o
b~
214
means for each extraction, the following partitioning order is apparent:
Crystalline iron oxides M n oxides
Amorphous Fe oxides W e a k l y - b o n d e d ions Organic m a t t e r
Cu
Zn
Mn
Fe
1 2 5 3 4
3 1 5 4 2
1 1" 3 2 4
1 3 2 5 4
*Excluding P-I
Concentrations o f metal extracted from the blank are very small indicating that adsorption of metal by the exposed, uncoated faces is unimportant. A measure of how each coating phase has been affected by the high concentrations of Cu and Zn used in the metal solutions is obtained by dividing the concentration of metal extracted from immersed coatings by the concentration extracted from unimmersed coatings in each step of the sequential extraction. For example, 1.82 would mean that 1.82 times as much metal was extracted by a certain reagent from a coating that was immersed in the metal solution than from the other half that contained only natural concentrations of metal. Such adsorption ratios (Table V) clearly show that organic matter and weakly-bonded ions are significantly more important as residence sites for Cu and Zn adsorbed from the prepared metal solutions than for metal sorbed under natural conditions. Apparently concentrations of metal ions in solution that would be considered anomalous in streams have greatly enhanced the adsorption potential of these coating phases. Such a conclusion is consistent with results of recent studies. Filipek et al. (1981) showed that organic matter is a major partitioning site in boulder coatings collected from a stream draining a polymetallic sulfide deposit. Concentrations of Cu and Zn in the stream water are above background levels at most sample sites. Similarly, results obtained by Robinson (1981} show that the adsorption of metal in a weakly-bonded form is more pronounced downstream from an oxidizing sulfide deposit than upstream. These two phases may generally be important contributors to the overall higher concentrations of trace metal commonly occurring in Mn-Fe oxide precipitates near sulfide deposits. Because the solutions used in the adsorption experiments contained no Mn or Fe, little change in the concentration of these metals was expected. As Table V shows, this is generally true, b u t significant variations are found for a few of the samples. There is, however, no general positive or negative variation and the mean adsorption ratios are close to unity for each metal. Natural concentrations of Cu and Zn in these samples probably have a similar degree of variability. No attempt was made to determine the effect of such variability on the adsorption ratios shown in Table V, but it is probably small. All of the immersed samples, except P-l, have adsorption
1.07 0.49 1.07
1.15 0.40 1.26
4.25 7.26 4.77
4.77 4.39 4.77
NC 9.83 NC NC 2.09 2.38 NC NC
Hxl
33.68 28.29 39.35
2.29 1.05 2.53
5.32 0.64 27.26 2.51 5.50 1.21 59.00 2.00 30.00 2.01 NC 3.75 75.00 2.79 NC 3.40
Hyp
1.87 1.35 1.30
4.16 NC NC 1.37 NC 2.00 0.93 0.90
Aao
1.63 2.08 1.81
0.57 1.91 0.85 NC 0.85 6.21 0.73 0.32
Hz
2.66 1.48 2.91
0.90 3.74 1.26 3.97 1.81 5.06 1.70 2.83
Overall
0.92 0.49 1.03
0.2 1.28 0.97 0.90 1.18 1.65 0.95 0.27
1.36 1.23 1.36
1.31 1.15 1.21 0.99 0.74 4.31 0.82 0.33
Mn Fe Overall Overall
NC: Not calculated because one or both o f the extractions produced metal concentrations below the limit of detection. Such determinations are not included in calculating means.
2.15 1.00 2.26
0.64 1.73 1.27 0.98 1.99 22.09 1.70 3.63
4.55 2.83 5.17
0.58 1.67 1.29 0.86 1.04 NC 1.48 NC
3.09 1.94 3.41
NC NC 0.73 NC NC NC 1.41 NC
Mean Stand. dev. Mean (P-1 omitted)
1.34 2.33 1.11 0.92 3.75 2.08 3.09 2.60
0.82 6.00 2.50 1.50 6.60 4.25 7.20 NC
0.84 2.67 1.17 2.14 4.44 3.80 2.83 6.83
P-1 P-2 Po3 P-4 BR-1 BR-2 VR-1 VR-2
Hz
Overall Ac
Aao
Hyp
Ac
Hxl
Zn
Sample No.
Cu
Adsorption ratios for Cu, Zn, Mn, Fe
TABLE V
¢j1
216
ratios greater than 1 for at least one of the metals. Analyses of the metal solutions, made at the conclusion of the adsorption experiments {Table VI), show that the increased concentrations of Cu and Zn in immersed samples can be directly attributed to adsorption. Robinson (1983) previously showed that under the conditions used in the experiment, adsorption by container walls is minor. T A B L E VI C o n c e n t r a t i o n * o f Cu a n d Z n in metal s o l u t i o n s after c o n c l u s i o n o f t h e a d s o r p t i o n experiment S a m p l e No.
Cu (#g/l)
Z n (~g/l)
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2
55 3 2 3 4 3 4 5
570 12 <1 1 <1 <1 1 2
*Original c o n c e n t r a t i o n = 4 0 0 ug/1 Z n a n d 50 ug/1 Cu.
EXPLORATION SIGNIFICANCE
Selective chemical extractions have proven valuable to geochemists in understanding processes operating during the weathering of mineral deposits and in choosing optimum sampling-dissolution procedures. In order for such techniques to be adapted in routine surveys, significant practical advantages, such as greater sensitivity, must be demonstrated. The potential advantages of applying such techniques to Mn-Fe oxide precipitates in streams, as compared to a procedure which effects a total dissolution, has not yet been generally demonstrated. Filipek et al. (1981), in a study involving samples collected upstream and downstream from mineralization, did show that a sequential extraction of metal from boulder coatings produced greater anomaly contrast for Cu and Zn than had been reported in other studies of this area (Carpenter et al., 1975, 1978). However, the results of Carpenter and Hayes {1979} suggest that applying sequential extractions to boulder coatings may not always be advantageous. They achieved greater anomaly contrast for Zn using a simple 1:4 HCI digestion, which quickly dissolves the coatings with only minor attack on rock substrates, than from using hydroxylamine hydrochloride. The experimental procedure used in the present study is not ideal for assessing the feasibility of directly using selective extractions of Mn-Fe oxide coatings in exploration because all of the samples, except P-l, represent
217
background concentrations of metal. Although anomalous conditions are represented b y this sample, it is unlikely that streams with a pH this low and a dissolved concentration o f Cu and Zn this high would be encountered in normal exploration programs, unless areas contaminated b y old mine workings were included. Bearing these limitations in mind Table VII, showing anomaly contrast achieved b y each extraction, was prepared by dividing the mean natural concentration o f each metal extracted by the various reagents from all background samples into the concentration of metal extracted from P-1. Ratios o f each metal to Mn and Fe were calculated and treated in a similar manner. For comparison, anomaly contrasts were also calculated from analyses of the stream waters. As Table VII shows, none o f the extractions of metal from the coatings produces an anomaly/background ratio as great as that achieved by analyses of the stream waters; however, when ratioed data is utilized, even greater anomaly contrasts for both Cu and Zn result from the hydroxylamine hydrochloride extraction. TABLE VII A n o m a l y c o n t r a s t * p r o d u c e d b y each reagent used in t h e sequential e x t r a c t i o n p r o c e d u r e
Cu Zn Cu/Mn Cu/Fe Zn/Mn Zn/Fe
Ammonium citrate
Sodium hypochlorite
Hydroxylamine hydrochloride
Acid a m m o n i u m oxalate
Hydrazine hydrochloride
Overall
4.4 7.5 30 0.1 9.1 0.02
36 17 NC 0.7 NC 0.37
2 1.3 3400 0.7 1836 0.55
NC 1.0 NC NC NC 0.5
16 5 134 6.7 103 4.3
8.3 2.9 1116 6.4 396 2.3
mean concentration mean concentration NC = n o t c a l c u l a t e d b e c a u s e e x t r a c t i o n below t h e limits of detection. A n a l y s e s o f the stream w a t e r s p r o d u c e d * A n o m a l y c o n t r a s t -~
o f m e t a l in P - I o f m e t a l in all samples e x c e p t P-1 o f o n e o r m o r e o f t h e m e t a l s f r o m P-1 b y this reagent w a s
t h e f o l l o w i n g a n o m a l y c o n t r a s t : Cu = 1 4 9 4 ; Z n = 1 7 8 4 .
CONCLUSIONS
Based on results of a sequential extraction procedure, the Mn-oxide and crystalline iron oxide phases o f black coatings on stream alluvium, collected from areas containing background levels of metal, are the major residence sites for Cu and Zn. The partitioning o f these metals within a pebble coating collected from an area contaminated by past mining activities differs significantly. Organic matter is more important as a residence site for b o t h Cu and Zn than in samples from background areas and the Mn-oxides are less important. Because of the extreme contamination represented b y this sample (very low stream water pH and high metal concentrations) the determined partitioning m a y n o t generally occur near unmined mineral deposits. The partitioning of Cu and Zn within coatings from background areas changed significantly after the samples were immersed for 1 week in prepared
218
solutions containing concentrations of Cu and Zn that would be considered anomalous in streams. The concentration of metal occurring as weaklybonded ions and adsorbed on organic matter increased sharply suggesting that these fractions may be significant contributors to the anomalous concentrations of metal which often occur in coatings near oxidizing sulfide deposits. These results suggest that the partitioning of Cu and Zn within Mn-Fe oxide coatings on alluvium may be distinct, depending on whether the coatings have precipitated in streams near mineral deposits or in background areas. The few samples studied in the present investigation do not allow this generalization to be made, but if such a relationship is widespread in nature, it should be useful in exploration geochemistry for such purposes as distinguishing anomaly types and increasing the sensitivity of geochemical methods. ACKN OWLEDGEMENTS
This work was supported by a grant from the James Madison University Program of Grants for Faculty Research.
REFERENCES Cameron, E.M., 1978. Hydrogeochemical methods for base metal exploration in the northern Canadian shield. J. Geochem. Explor., 10: 219--243. Carpenter, R.H. and Hayes, W.B., 1979. Fe-Mn coatings in routine geochemical surveys. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration, 1978. Proceedings of the Seventh International Geochemical Exploration Symposium. Assoc. Explor. Geocl~em., pp. 277--282. Carpenter, R.H., Pope, T.A. and Smith, R.L., 1975. Fe-Mn oxide coatings in stream sediment geochemical surveys. J. Geochem. Explor., 4: 349--363. Carpenter, R.H., Robinson, G.D. and Hayes, W.B., 1978. Partitioning of manganese, iron, copper, zinc, lead, cobalt, and nickel in black coatings on stream boulders in the vicinity of the Magruder mine, Lincoln Co., Georgia. J. Geochem. Explor., 10: 75--89. Chao, T.T., 1972. Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Soil Sci. Soc. Am. Proc., 36: 764--768. Chao, T.T. and Theobald, Jr., P.K., 1976. The significance of secondary iron and manganese oxides in geochemical exploration. Econ. Geol., 71: 1560--1569. Filipek, L.H., Chao, T.T. and Carpenter, R.H., 1981. Factors affecting the partitioning of Cu, Zn, and Pb in boulder coatings and stream sediments in the vicinity of a polymetallic sulfide deposit. Chem. Geol., 33: 45--64. Gatehouse, S., Russell, D.W. and Van Moort, J.C., 1977. Sequential soil analysis in exploration geochemistry. J. Geochem. Explor., 8: 483--494. Hoffman, S.J. and Fletcher, W.K., 1979. Selective sequential extractions of Cu, Zn, Fe, Mn, and Mo from soils and sediments. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration, 1978. Proceedings of the Seventh International Geochemical Exploration Symposium. Assoc. Explor. Geochem., pp. 289--299. Jeffries, D.S. and Stumm, W., 1976. The metal adsorption chemistry of buserite. Can. Mineral., 14: 16--22.
219 Lavkulich, L.M. and Wiens, J.H., 1970. Comparison of organic matter destruction by hydrogen peroxide and sodium hypochlorite and its effects on selected mineral constituents. Soil Sci. Soc. Am. Proc., 34: 755--758. Potter, R~I. and Rossman, G.R., 1979. Mineralogy of manganese dendrites and coatings. Am. Mineral., 64: 1219--1226. Reedman, J.H., 1979. Techniques in Mineral Exploration. Applied Science Publishers, London, 533 pp. Robinson, G.D., 1981. Adsorption of Cu, Zn, and Pb near sulfide deposits by hydrous manganese-iron oxide coatings on stream alluvium. Chem. Geol., 33: 65--79. Robinson, G.D., 1983. Heavy metal adsorption by ferro-manganese coatings on stream alluvium: natural controls and implications for exploration. Chem. Geol., 38. Rose, A.W., 1975. The mode of occurrence of trace elements in soils and stream sediments applied to geochemical exploration. In: I.L. EUiott and W.K. Fletcher (Editors), Geochemical Exploration 1974. Elsevier, Amsterdam, pp. 691--705. Whitney, P.R., 1981. Heavy metals and manganese oxides in the Genesee Watershed, New York State: effects of geology and land use. J. Geochem. Explor., 14: 95--118.
205
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
TRACE METAL ADSORPTION POTENTIAL OF PHASES COMPRISING BLACK COATINGS ON STREAM PEBBLES
GENE D. ROBINSON
Geology Department, James Madison University, Harrisonburg, Virginia (U.S.A.) (Received November 25, 1981; revised and accepted June 24, 1982)
ABSTRACT Robinson, G.D., 1982. Trace metal adsorption potential of phases comprising black coatings on stream pebbles. J. Geochem. Explor., 17 : 205--219. A sequential extraction of Cu and Zn was completed on 8 samples of black coatings on coarse stream alluvium before and after immersion in prepared metal-rich solutions. Such immersion resulted in a substantial change in the partitioning of the metals among the various phases present. Most of the samples were collected from small streams in areas of the Valley and Ridge, Blue Ridge, and Piedmont provinces of north~entral Virginia without known mineral deposits, and partitioning relationships were determined by a five-step selective extraction procedure. The following order of partitioning was determined in the samples:
Crystalline Fe oxides Mn oxides Amorphous Fe oxides Weakly-bonded ions Organic matter
Cu
Zn
Mn
Fe
1 2 3 4 5
2 1 3 4 5
2 1 4 3 5
1 3 2 5 4
Over 90% of the total Cu and Zn in the coatings resides in the Mn and Fe oxide phases. The coating samples were immersed for 1 week in aqueous solutions containing 50 ug/ 1 Cu and 400 ug/1 Zn (pH 6.0). After this time, almost all of the metal had been adsorbed from the solutions. The organic portion of the coatings and metal present as weaklybonded ions became significantly more important as residence sites for both Cu and Zn. These results suggest that the partitioning of the anomalous concentrations of metal previously reported in black coatings near weathered sulfide deposits may be distinct compared to that occurring in samples collected from areas containing background concentrations of metal.
INTRODUCTION
Precipitates of Mn and Fe oxides are c o m m o n in m a n y streams, particularly in the form of black coatings on rock substrates. Recent interest in these materials is mainly due to their ability to "scavenge" a number of trace
0375-6742/82/0000--0000/$02.75
© 1982 Elsevier Scientific Publishing Company
206 metals from stream or ground water. Several recent studies, including those of Carpenter and Hayes (1979), Filipek et al. (1981), and Whitney {1981) have shown that Mn-Fe oxide coatings contain anomalous concentrations of metal near sulfide deposits and have suggested that directly sampling the precipitates may be advantageous in geochemical exploration. The "scavenging" of metal ions by Mn-Fe oxides has been widely investigated and is believed to include both coprecipitational and adsorptional processes (Chao and Theobald, 1976). Data presented by Robinson (1981) indicates that adsorption may be more important than coprecipitation in producing the anomalous levels of metal that occurs in these materials near mineral deposits. Although apparently simple, Mn-Fe oxide coatings actually represent a complex, multiphase system consisting of various Mn and Fe oxides and hydroxides (with differing degrees of crystallinity) plus a mixture of organic components and silicates, such as clay minerals (Potter and Rossman, 1979 and Filipek et al., 1981). Only a few studies have been made of trace metal partitioning relationships among the various phases comprising coatings. Some of these investigations have not attempted a complete determination of metal partitioning but instead have emphasized only the Mn and Fe components. For example, Carpenter et al. (1978), using a two-step sequential extraction, determined that the partitioning of Cu, Zn, and Co between the Mn and Fe oxide fractions of boulder coatings collected upstream from an oxidizing sulfide deposit is distinctly different from samples downstream of the mineralized zone. More recently, Filipek et al. (1981) investigated trace metal partitioning among all major components of boulder coatings and found that the organics, which had not been previously emphasized, are a major trace metal sink. A knowledge of trace metal partitioning is essential in understanding the basic geochemical processes operating during the weathering of a mineral deposit. Such information is also necessary in choosing the most effective technique of geochemical exploration for a particular area. A thorough understanding of the geochemical factors which influence partitioning of metal within coatings in various geological environments is needed before an assessment can be made of their potential use in detecting mineralization. The present study represents an initial attempt to provide such information. The main goals of the investigation are twofold: (1) To determine the partitioning of Cu, Zn, Mn, and Fe among the major phases of black coatings on coarse stream alluvium collected from diverse geological environments. (2) To investigate the influence of stream water pH and metal concentration on the partitioning of these metals in Mn-Fe oxide coatings. SAMPLING AND ANALYTICAL METHODS Mn-Fe oxide coatings on stream pebbles (approximately 5--7 cm diameter) were used in this study. Sampling was purposely non-random, that is, an
207
effort was made to collect pebbles that visually appeared to have the most well-developed coatings. Sample locations were chosen to represent the varying geological conditions occurring in the Piedmont, Blue Ridge, and Valley and Ridge provinces of north-central Virginia (Fig. 1). Sampled streams were small with drainage areas generally less than 20 km 2. Stream water pH was measured by portable meter at the time of sample collection, and water samples were collected in 1000 ml linear polyethylene bottles. The water samples were not acidified to avoid extraction of metal from particulates that might be present. Cameron (1978) has shown that even after several months, Cu and Zn concentrations in unacidified lake waters do not differ significantly from samples which were acidified at the time of collection. In the present study, water samples were analyzed within 2 days by atomic absorption spectrophotometry after filtering through 2 ~m filter paper. Coating samples were transported to the lab in containers of the stream water and were kept immersed until used in experiments. This precaution was deemed necessary to minimize possible physical and chemical changes within the coatings such as those described by Jeffries and Stumm (1976). With the exception of P-l, none of the sample locations are near any known mineral deposits. P-1 was collected from Contrary Creek near Mineral, Virginia, in the historic "Gold-Pyrite Belt." Numerous old mines, dumps, and prospects occur in the area. Due to the release of H2SO4 from the oxidation of pyrite, chalcopyrite, and sphalerite within the dumps, the pH of Contrary Creek is very low. Samples from Contrary Creek were included in the study to determine how partitioning of metal among coating phases would be affected by such anomalous conditions.
Fig. 1. Index map showing sampling locations.
208 The coating samples were used in experiments designed to determine natural partitioning relationships of Cu, Zn, Mn and Fe as well as the effect of stream water metal concentration on metal partitioning. The procedure involved a determination of changes in metal partitioning among coating phases resulting from immersing samples in prepared metal solutions as compared to the natural partitioning. In order to minimize variability at each sample site so that changes in metal partitioning could be better evaluated, each sample was sawed in half and washed in DD H~O. The samples were then dried for 24 hours at 40°C and weighed. One sample half was subjected to a five-step selective extraction procedure to determine natural metal partitioning. The other half was leached by the same procedure after being immersed for 1 week in 350 ml. of an aqueous solution containing 50 ug/1 of Cu and 400 gg/1 Zn, with the pH adjusted to 6.0. The metal concentration of this solution was chosen to emulate levels of Cu and Zn in stream water which, according to Reedman (1979}, would be considered anomalous. A previous study has shown that under the experimental conditions, Mn-Fe oxide coatings adsorb substantial quantities of the metal in solution but adsorption onto container walls is almost nfl (Robinson, 1983). To evaluate possible metal adsorption onto the sawed faces, one of the samples, from which the coating had previously been removed chemically, was subjected to the same adsorption experiment and extraction procedure. The sequential extractions were carried out by totally immersing each washed sample half in various reagents chosen for the selectivity of their attack. The following order of extraction was followed: (1) Weakly-bonded metal ions were extracted with a 0.1 M solution of ammonium citrate (Ac), as described by Robinson (1981). (2) Metal associated with the organic fraction was removed by 2 extractions with sodium hypochlorite (hyp) adjusted to a pH = 9.5 using a procedure adapted from Lavkulich and Wiens (1970}. According to Lavkulich and Wiens (1970} sodium hypochlorite is a more efficient oxidizer of organic matter than the more c o m m o n l y used hydrogen peroxide. Their procedure was modified by slowly evaporating the final sodium hypochlorite solution to dryness and dissolving the residue in 10% HCI. Mn and Fe determinations were made by diluting this solution as needed, but due to interferences, Cu and Zn were determined in a MIBK extract, prepared after chelating dissolved metal with APDC. (3) The Mn oxide portion of pebble coatings was dissolved in a 0.1 M solution of hydroxylamine hydrochloride (Hxl) in 0.01 M HNO3 (pH = 2.0} using the procedure described by Chao (1972). Filipek et al. (1981) dissolved the Mn oxide portion of boulder coatings before oxidizing the organic matter. In the present study, this order was reversed to avoid the possibility of losing fine particulate forms of organic material which might be attached to coatings. Such a form of organic matter could be released into the hydroxylamine hydrochloride solution if portions of the coating were dissolved.
209 (4) Acid ammonium oxalate (Aao) adjusted to a pH = 3.75 was used to dissolve the amorphous iron oxide portion of the coatings. This extraction was performed in darkness as described by Hoffman and Fletcher (1979). According to Rose (1975) this reagent is an effective reducing agent for the dissolution of amorphous or poorly crystalline oxide phases. (5) The crystalline iron oxide fraction was dissolved in a 0.7 M solution of hydrazine hydrochloride (Hz) adjusted to a pH of 2.1 with 12 M HCI (Gatehouse et al., 1977). Cu, Zn, Mn, and Fe in the above solutions were determined with a Perkin Elmer Model 370 atomic absorption unit using an air-acetylene flame. Standards were prepared from the same reagents used in sequential extractions. Metal concentrations are expressed in relation to the total weight loss resulting from dissolution of the coatings. These concentrations are only approximations due to the rapid equilibration of the dried samples with atmospheric moisture; however, the proportion of metal associated with each sample fraction is not affected because each is based on weight loss resulting from dissolution of the entire coating rather than individual fractions. RESULTS AND DISCUSSION Physical descriptions of the coating samples used for selective extractions and basic geochemical data on the stream water at each sample site are shown in Table I. As might be expected, considering the diverse geological environments represented by these samples, the coatings vary considerably in appearance. The coating sample from Contrary Creek (P-l), which has formed under very acidic conditions, does not resemble the other samples. Metal concentrations in all of the streams except Contrary Creek are at background levels. The tow pH of Contrary Creek, resulting from the oxidation of sulfides in mine dumps, enables extremely high concentrations of Cu and Zn to remain in solution. Natural partitioning relationships for Cu, Zn, Mn, and Fe, as determined by the sequential extraction scheme, are shown in Table II. Because established procedures were. used for each of the sequential extractions, no attempts to establish reagent specificity were made. Although the partitioning relationships ~ o n g the Various samples is extremely variable, a clear order of partitioning for each metal is apparent. Based on the calculated means, the following order of partitioning for each metal occurs:
Crystalline Fe oxides Mn oxides A m o r p h o u s Fe oxides Weakly-bonded ions Organic matter
Cu
Zn
Mn
Fe
1 2 3 4 5
2 1 3 4 5
2 1 4 3 5
1 3 2 5 4
210 TABLE I Physical description of coating samples and basic geochemical data on stream water Sample No. Physical description of coating
P-1 P-2 P-3 P-4
BR-1 BR-2 VR-1
VR-2
Yellow-yellowish brown, "fluffy" appearance, apparently thick and uniform Fine-grained, continuous, brownish-black, spotty on bottom, greenish algae abundant Dark gray-black, uniform and thick Light brown-gray-black, spotty in places. Unidentified, rodshaped organic material present, brown color Dark gray (top)-reddish brown (bottom). Mottled appearance, apparently thin coating Gray, powdery appearance, poorly developed Brown-brownish black (top), brownish red (bottom). Unidentified rod-shaped organic material present Dark gray (top), reddish brown (bottom)
Stream water Stream water pH metal concentration, ug/l
3.4
Cu
Zn
3400
7800
7.5
2.4
4.5
7.4
1.2
3.9
7.4
1.8
5.1
7.6
0.03
3.0
7.5
6.6
4.2
7.9
1.8
4.2
8.0
2.1
5.7
Only insignificant concentrations o f metal are extracted from the blank (uncoated sample) by the various reagents suggesting t hat using sawed pebbles has n o t significantly affected the det erm i ned partitioning order. The partitioning of Cu and Zn among the various coating phases o f P-1 is distinct from the o t h e r samples. Organic m a t t e r is a more i m port ant residence site for these metals than in samples from background areas, and t h e Mn oxides, which oc c ur in very low concentrations, are less important. Most o f the Mn in this sample pr obabl y occurs adsorbed o n t o the iron oxide coatings, as suggested by the large per cent o f Mn extracted by hydrazine and a m m o n i u m citrate and the low per cent o f Mn ext ract ed by h y d r o x y l amine hydrochloride. Because P-1 does represent ext rem el y contaminated conditions, such partitioning is probably n o t representative of coatings in streams draining unmined mineral deposits. T he inclusion of P-1 considerably affects calculations of the mean per cent o f each metal extracted by the various reagents, particularly for Mn; however, the affect on metal partitioning orders is much smaller, causing only mi nor shifts.
0.02 1.49 4.40 1.80 1.56 1.90 4.61 4.76 2.57 1.77 2.93
13.9 0.8 0.5 2.9 1.2 1.5 0.5 0.6 2.7 4.6 1.1
<0.1 <0.1 3.3 5.2 <0.1 1.9 4.7 0.7 2.0 2.2 2.3
4.0 68.9 62.7 83.4 84.6 67.8 39.5 19.0 53.7 29.8 60.8
Hxl <0.1 1.5 3.5 2.6 1.9 1.1 1.5 6.6 2.5 1.8 2.7
Aao 82.1 28.7 30.0 5.9 12.3 27.7 53.8 73.0 39.2 27.7 33.1
Hz 38.24 42.09 18.66 15.42 40.36 11.46 40.81 46.68 31.72 14.03 30.78
(%)*
Total
Hyp
Ae
(%)*
990 385 280 150 505 190 525 385 425 265 345 Fe
90.7 95.0 64.4 65.2 73.4 <0.1 84.6 <0.1 59.2 38.2 54.7
% Extracted by:
<0.1 <0.1 2.9 <0.1 <0.1 <0.1 4.4 41.7 6.1 14.5 7.0
(ppm)
Total
3.0 3.9 26.8 22.8 14.7 76.5 9.0 41.7 24.8 24.6 27.9
Hz
Mn
4.8 0.4 2.9 7.6 3.7 8.8 1.3 <0.1 3.7 3.2 3.5
Aao 5.1 4.9 2.1 2.0 0.6 <0.1 0.4 <0.i 1.9 2.1 1.4
Hyp 20.4 44.6 58.7 80.0 49.3 55.6 27.0 31.3 45.9 19.5 49.5
Hxl
1.1 <0.1 < 0.1 0.2 0.1 <0.1 <0.1 0.6 0.3 0.4 0.1
Ae
3.9 <0.1 0.6 <0.1 0.1 <0.1 <0.1 0.4 0.6 1.3 0.2
Hyp
3.3 1.1 4.4 7.8 2.5 1.5 1.0 1.9 2.9 2.3 2.9
Hxl
% Extracted by:
3.0 1.6 <0.1 <0.1 6.3 8.5 <0.i <0.I 2.4 3.3 2.3
Ac
% Extracted by:
9.0 2.0 8.0 14.4 0.5 6.9 3.6 9.7 6.8 4.6 6.4
Aao
2.5 <0.1 <0.1 18.0 <0.1 13.8 8.2 18.7 7.7 8.2 8.4
Aao
82.7 96.8 87.0 77.6 96.7 91.7 95.3 87.3 89.4 7.0 90.3
Hz
69.0 49.0 39.1 <0.1 43.8 22.2 64.4 50.0 42.2 22.4 38.4
Hz
*Total ffi sum of all extractions. Metal extracted from blank (pebble wi t hout a coating): Cu & Zn ffi below detection limits (all extractions); Mn = 1 p p m (all extracted by Hz); Fe = 9 ppm (94% extracted by Hz).
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Standard dev. Mean (P-1 omitted)
Sample No.
1.4 0.6 2.9 4.4 8.3 14.7 0.8 16.7 6.2 6.4 6.9
Hxl
Total
Hyp
% Extracted by:
Total
(ppm) Ac
Zn
Cu
P-1 2230 P-2 925 P-3 205 P-4 160 BR-1 110 BR-2 35 VR-1 390 VR -2 70 Mean 515 Standard dev. 750 Mean (P-1 omitted) 270
Sample No.
Natural partitioning of Cu, Zn, Mn, and Fe among various phases comprising coatings on stream pebbles
TABLE II
b3
212
Several environmental factors including the relative abundance of coating phases, the concentration of dissolved metal in the stream, and the stream water pH, can be important in controlling metal partitioning relationships. Over 90% of the total Cu and Zn in the coatings resides in the Mn and Fe oxide phases. The degree of control exerted by these phases on the concentration of Cu and Zn can be seen in Table III where the ratio of Cu and Zn to total Mn+Fe has been calculated. If P-1 is excluded from the calculations, much of the original variability apparent in Table II is lost; instead, a fairly consistent ratio between the trace metals and these major partitioning sites occurs. T A B L E III R a t i o s o f Cu a n d Z n t o t o t a l M n + F e
Sample No.
[Cu/(Mn + Fe)] × I00
[Zn/(Mn + Fe)] × i00
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Mean (P-1 omitted) Standard deviation (P-1 omitted)
0.58 0.21 0.09 0.09 0.12 0.14 0.12 0.08 0.18 0.12 0.04
0.26 0.09 0.12 0.09 0.12 0.14 0.12 0.08 0.13 0.11 0.02
Variations in stream water concentrations of Cu and Zn and stream water pH appear to be unimportant in controlling trace metal concentrations in all of the coating samples except P-1. This is probably because all of the samples but P-1 represent background conditions with only minor variations in stream water pH and metal concentrations. The anomalous nature of P-1 can be directly attributed to the fact that substantial quantities of sulfides occur in the drainage basin of Contrary Creek. The oxidation of these minerals releases H2SO4 into Contrary Creek, causing the pH of the water to be very low. This allows extremely high concentrations of dissolved Cu and Zn to exist in the water but does not allow Mn oxides to accumulate as pebble coatings. Inspection of the Eh-pH diagram presented by Rose (1975) shows that under oxidizing conditions typical of streams, Fe oxides are stable at low pH's, but Mn oxides are stable only under neutral to basic conditions. Organic matter and weakly-bonded ions are substantially more important as partitioning sites of Cu and Zn after immersion of coating halves in solutions o f Cu and Zn for 1 week than before (Table IV). Based on calculated
.004 1.90 4.27 1.62 1.84 3.13 4.36 1.29 2.30 1.50 2.65
<0.1 2.3 1.2 4.2 1.8 0.6 1.1 7.3 2.3 2.4 2.6
<0.1 <0.1 0.1 0.2 0.1 0.1 0.3 0.2 0.1 0.1 0.2
<0.1 35.0 58.5 78.1 54.4 12.0 45.4 45.6 41.1 25.3 47.0
<0.1 0.8 1.6 1.9 1.1 1.8 1.3 1.4 1.2 0.6 1.4
Aao
3.6 0.2 1.6 <0.1 1.5 0.5 3.6 <0.1 1.4 1.5 1.1
Aao
100.0 61.9 38.9 15.6 42.6 85.5 51.9 45.6 55.2 26.9 48.8
Hz
81.9 91.6 65.1 57.4 37.9 85.9 73.6 22.6 64.5 24.2 62.0
I-Iz
53.04 48.22 22.66 15.31 29.85 49.44 33.46 15.36 33.4 15.30 30.6
(%)
Total*
Fe
890 1445 355 600 920 955 890 1090 895 320 895
(ppm) 29.8 35.9 9.3 39.3 13.1 22.2 16.8 18.3 23.1 10.8 22.1
Hyp 14.6 29.9 56.3 40.0 54.5 41.2 44.4 37.7 39.8 13.4 43.4
Hxl
0.3 <0.1 <0.1 0.1 < 0.1 <0.1 <0.1 <0.1 0.1 0.1 <0.1
Ae
0.5 <0.1 0.9 0.3 0.7 0.8 1.0 1.7 0.7 0.5 0.8
Hyp
16.7 2.8 4.8 6.6 2.2 1.3 2.1 1.5 4.8 5.2 3.0
Hxl
% Extracted by
<0.1 4.1 2.8 6.3 7.3 4.0 6.4 32.3 7.9 10.1 9.0
Ac
% Extracted by
13.8 1.6 4.8 8.3 7.8 4.9 4.2 0.9 5.8 4.1 4.6
Aao
11.7 5.1 5.1 6.2 4.7 5.4 4.5 5.9 6.1 2.4 5.3
Aao
68.8 95.5 89.4 84.7 89.3 92.7 92.8 95.9 88.6 8.8 91.5
Hz
43.9 25.0 26.5 8.2 20.5 27.3 27.8 5.7 23.1 12.1 20.1
Hz
*Total ffi sum of all extractions. Metal extracted from blank (pebble w it hout a coating): Cu = below detection limits, all extractions; Zn = < 1 ppm, Hxl & Aao, other extractions below detection limits; Mn = below detection limits, all extractions; Fe ffi 1 ppm (Hxl), 4 p p m (Aao), other extractions below detection limits.
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2 Mean Standard dev. Mean (P-1 omi tte d)
(%)
6.4 5.2 23.4 21.3 27.4 7.2 16.2 29.9 17.1 9.9 18.7
Hxl
Ac
Total* Hyp
% Extracted by
Mn
No.
6.2 1.9 5.8 11.6 15.1 3.9 5.4 16.1 8.2 5.3 8.5
Sample
1.8 1.0 2.7 9.7 18.3 2.5 1.3 31.4 8.6 11.0 9.6
Hxl
Ac
(ppm)
Hyp
Zn Total*
% Extracted by
Cu
Total*
P-1 1425 P-2 1600 P-3 260 P-4 155 BR-1 220 BR-2 750 VR-1 665 VR-2 260 Mean 665 Standard dev. 565 Mean (P-1 omitted) 560
Sample No.
Partitioning of Cu, Zn, Mn, and Fe among various phases comprising coatings on stream pebbles after immersion for 1 week in prepared solutions of Cu and Zn (pH 6.0)
TABLE IV
¢,o
b~
214
means for each extraction, the following partitioning order is apparent:
Crystalline iron oxides M n oxides
Amorphous Fe oxides W e a k l y - b o n d e d ions Organic m a t t e r
Cu
Zn
Mn
Fe
1 2 5 3 4
3 1 5 4 2
1 1" 3 2 4
1 3 2 5 4
*Excluding P-I
Concentrations o f metal extracted from the blank are very small indicating that adsorption of metal by the exposed, uncoated faces is unimportant. A measure of how each coating phase has been affected by the high concentrations of Cu and Zn used in the metal solutions is obtained by dividing the concentration of metal extracted from immersed coatings by the concentration extracted from unimmersed coatings in each step of the sequential extraction. For example, 1.82 would mean that 1.82 times as much metal was extracted by a certain reagent from a coating that was immersed in the metal solution than from the other half that contained only natural concentrations of metal. Such adsorption ratios (Table V) clearly show that organic matter and weakly-bonded ions are significantly more important as residence sites for Cu and Zn adsorbed from the prepared metal solutions than for metal sorbed under natural conditions. Apparently concentrations of metal ions in solution that would be considered anomalous in streams have greatly enhanced the adsorption potential of these coating phases. Such a conclusion is consistent with results of recent studies. Filipek et al. (1981) showed that organic matter is a major partitioning site in boulder coatings collected from a stream draining a polymetallic sulfide deposit. Concentrations of Cu and Zn in the stream water are above background levels at most sample sites. Similarly, results obtained by Robinson (1981} show that the adsorption of metal in a weakly-bonded form is more pronounced downstream from an oxidizing sulfide deposit than upstream. These two phases may generally be important contributors to the overall higher concentrations of trace metal commonly occurring in Mn-Fe oxide precipitates near sulfide deposits. Because the solutions used in the adsorption experiments contained no Mn or Fe, little change in the concentration of these metals was expected. As Table V shows, this is generally true, b u t significant variations are found for a few of the samples. There is, however, no general positive or negative variation and the mean adsorption ratios are close to unity for each metal. Natural concentrations of Cu and Zn in these samples probably have a similar degree of variability. No attempt was made to determine the effect of such variability on the adsorption ratios shown in Table V, but it is probably small. All of the immersed samples, except P-l, have adsorption
1.07 0.49 1.07
1.15 0.40 1.26
4.25 7.26 4.77
4.77 4.39 4.77
NC 9.83 NC NC 2.09 2.38 NC NC
Hxl
33.68 28.29 39.35
2.29 1.05 2.53
5.32 0.64 27.26 2.51 5.50 1.21 59.00 2.00 30.00 2.01 NC 3.75 75.00 2.79 NC 3.40
Hyp
1.87 1.35 1.30
4.16 NC NC 1.37 NC 2.00 0.93 0.90
Aao
1.63 2.08 1.81
0.57 1.91 0.85 NC 0.85 6.21 0.73 0.32
Hz
2.66 1.48 2.91
0.90 3.74 1.26 3.97 1.81 5.06 1.70 2.83
Overall
0.92 0.49 1.03
0.2 1.28 0.97 0.90 1.18 1.65 0.95 0.27
1.36 1.23 1.36
1.31 1.15 1.21 0.99 0.74 4.31 0.82 0.33
Mn Fe Overall Overall
NC: Not calculated because one or both o f the extractions produced metal concentrations below the limit of detection. Such determinations are not included in calculating means.
2.15 1.00 2.26
0.64 1.73 1.27 0.98 1.99 22.09 1.70 3.63
4.55 2.83 5.17
0.58 1.67 1.29 0.86 1.04 NC 1.48 NC
3.09 1.94 3.41
NC NC 0.73 NC NC NC 1.41 NC
Mean Stand. dev. Mean (P-1 omitted)
1.34 2.33 1.11 0.92 3.75 2.08 3.09 2.60
0.82 6.00 2.50 1.50 6.60 4.25 7.20 NC
0.84 2.67 1.17 2.14 4.44 3.80 2.83 6.83
P-1 P-2 Po3 P-4 BR-1 BR-2 VR-1 VR-2
Hz
Overall Ac
Aao
Hyp
Ac
Hxl
Zn
Sample No.
Cu
Adsorption ratios for Cu, Zn, Mn, Fe
TABLE V
¢j1
216
ratios greater than 1 for at least one of the metals. Analyses of the metal solutions, made at the conclusion of the adsorption experiments {Table VI), show that the increased concentrations of Cu and Zn in immersed samples can be directly attributed to adsorption. Robinson (1983) previously showed that under the conditions used in the experiment, adsorption by container walls is minor. T A B L E VI C o n c e n t r a t i o n * o f Cu a n d Z n in metal s o l u t i o n s after c o n c l u s i o n o f t h e a d s o r p t i o n experiment S a m p l e No.
Cu (#g/l)
Z n (~g/l)
P-1 P-2 P-3 P-4 BR-1 BR-2 VR-1 VR-2
55 3 2 3 4 3 4 5
570 12 <1 1 <1 <1 1 2
*Original c o n c e n t r a t i o n = 4 0 0 ug/1 Z n a n d 50 ug/1 Cu.
EXPLORATION SIGNIFICANCE
Selective chemical extractions have proven valuable to geochemists in understanding processes operating during the weathering of mineral deposits and in choosing optimum sampling-dissolution procedures. In order for such techniques to be adapted in routine surveys, significant practical advantages, such as greater sensitivity, must be demonstrated. The potential advantages of applying such techniques to Mn-Fe oxide precipitates in streams, as compared to a procedure which effects a total dissolution, has not yet been generally demonstrated. Filipek et al. (1981), in a study involving samples collected upstream and downstream from mineralization, did show that a sequential extraction of metal from boulder coatings produced greater anomaly contrast for Cu and Zn than had been reported in other studies of this area (Carpenter et al., 1975, 1978). However, the results of Carpenter and Hayes {1979} suggest that applying sequential extractions to boulder coatings may not always be advantageous. They achieved greater anomaly contrast for Zn using a simple 1:4 HCI digestion, which quickly dissolves the coatings with only minor attack on rock substrates, than from using hydroxylamine hydrochloride. The experimental procedure used in the present study is not ideal for assessing the feasibility of directly using selective extractions of Mn-Fe oxide coatings in exploration because all of the samples, except P-l, represent
217
background concentrations of metal. Although anomalous conditions are represented b y this sample, it is unlikely that streams with a pH this low and a dissolved concentration o f Cu and Zn this high would be encountered in normal exploration programs, unless areas contaminated b y old mine workings were included. Bearing these limitations in mind Table VII, showing anomaly contrast achieved b y each extraction, was prepared by dividing the mean natural concentration o f each metal extracted by the various reagents from all background samples into the concentration of metal extracted from P-1. Ratios o f each metal to Mn and Fe were calculated and treated in a similar manner. For comparison, anomaly contrasts were also calculated from analyses of the stream waters. As Table VII shows, none o f the extractions of metal from the coatings produces an anomaly/background ratio as great as that achieved by analyses of the stream waters; however, when ratioed data is utilized, even greater anomaly contrasts for both Cu and Zn result from the hydroxylamine hydrochloride extraction. TABLE VII A n o m a l y c o n t r a s t * p r o d u c e d b y each reagent used in t h e sequential e x t r a c t i o n p r o c e d u r e
Cu Zn Cu/Mn Cu/Fe Zn/Mn Zn/Fe
Ammonium citrate
Sodium hypochlorite
Hydroxylamine hydrochloride
Acid a m m o n i u m oxalate
Hydrazine hydrochloride
Overall
4.4 7.5 30 0.1 9.1 0.02
36 17 NC 0.7 NC 0.37
2 1.3 3400 0.7 1836 0.55
NC 1.0 NC NC NC 0.5
16 5 134 6.7 103 4.3
8.3 2.9 1116 6.4 396 2.3
mean concentration mean concentration NC = n o t c a l c u l a t e d b e c a u s e e x t r a c t i o n below t h e limits of detection. A n a l y s e s o f the stream w a t e r s p r o d u c e d * A n o m a l y c o n t r a s t -~
o f m e t a l in P - I o f m e t a l in all samples e x c e p t P-1 o f o n e o r m o r e o f t h e m e t a l s f r o m P-1 b y this reagent w a s
t h e f o l l o w i n g a n o m a l y c o n t r a s t : Cu = 1 4 9 4 ; Z n = 1 7 8 4 .
CONCLUSIONS
Based on results of a sequential extraction procedure, the Mn-oxide and crystalline iron oxide phases o f black coatings on stream alluvium, collected from areas containing background levels of metal, are the major residence sites for Cu and Zn. The partitioning o f these metals within a pebble coating collected from an area contaminated by past mining activities differs significantly. Organic matter is more important as a residence site for b o t h Cu and Zn than in samples from background areas and the Mn-oxides are less important. Because of the extreme contamination represented b y this sample (very low stream water pH and high metal concentrations) the determined partitioning m a y n o t generally occur near unmined mineral deposits. The partitioning of Cu and Zn within coatings from background areas changed significantly after the samples were immersed for 1 week in prepared
218
solutions containing concentrations of Cu and Zn that would be considered anomalous in streams. The concentration of metal occurring as weaklybonded ions and adsorbed on organic matter increased sharply suggesting that these fractions may be significant contributors to the anomalous concentrations of metal which often occur in coatings near oxidizing sulfide deposits. These results suggest that the partitioning of Cu and Zn within Mn-Fe oxide coatings on alluvium may be distinct, depending on whether the coatings have precipitated in streams near mineral deposits or in background areas. The few samples studied in the present investigation do not allow this generalization to be made, but if such a relationship is widespread in nature, it should be useful in exploration geochemistry for such purposes as distinguishing anomaly types and increasing the sensitivity of geochemical methods. ACKN OWLEDGEMENTS
This work was supported by a grant from the James Madison University Program of Grants for Faculty Research.
REFERENCES Cameron, E.M., 1978. Hydrogeochemical methods for base metal exploration in the northern Canadian shield. J. Geochem. Explor., 10: 219--243. Carpenter, R.H. and Hayes, W.B., 1979. Fe-Mn coatings in routine geochemical surveys. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration, 1978. Proceedings of the Seventh International Geochemical Exploration Symposium. Assoc. Explor. Geocl~em., pp. 277--282. Carpenter, R.H., Pope, T.A. and Smith, R.L., 1975. Fe-Mn oxide coatings in stream sediment geochemical surveys. J. Geochem. Explor., 4: 349--363. Carpenter, R.H., Robinson, G.D. and Hayes, W.B., 1978. Partitioning of manganese, iron, copper, zinc, lead, cobalt, and nickel in black coatings on stream boulders in the vicinity of the Magruder mine, Lincoln Co., Georgia. J. Geochem. Explor., 10: 75--89. Chao, T.T., 1972. Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Soil Sci. Soc. Am. Proc., 36: 764--768. Chao, T.T. and Theobald, Jr., P.K., 1976. The significance of secondary iron and manganese oxides in geochemical exploration. Econ. Geol., 71: 1560--1569. Filipek, L.H., Chao, T.T. and Carpenter, R.H., 1981. Factors affecting the partitioning of Cu, Zn, and Pb in boulder coatings and stream sediments in the vicinity of a polymetallic sulfide deposit. Chem. Geol., 33: 45--64. Gatehouse, S., Russell, D.W. and Van Moort, J.C., 1977. Sequential soil analysis in exploration geochemistry. J. Geochem. Explor., 8: 483--494. Hoffman, S.J. and Fletcher, W.K., 1979. Selective sequential extractions of Cu, Zn, Fe, Mn, and Mo from soils and sediments. In: J.R. Watterson and P.K. Theobald (Editors), Geochemical Exploration, 1978. Proceedings of the Seventh International Geochemical Exploration Symposium. Assoc. Explor. Geochem., pp. 289--299. Jeffries, D.S. and Stumm, W., 1976. The metal adsorption chemistry of buserite. Can. Mineral., 14: 16--22.
219 Lavkulich, L.M. and Wiens, J.H., 1970. Comparison of organic matter destruction by hydrogen peroxide and sodium hypochlorite and its effects on selected mineral constituents. Soil Sci. Soc. Am. Proc., 34: 755--758. Potter, R~I. and Rossman, G.R., 1979. Mineralogy of manganese dendrites and coatings. Am. Mineral., 64: 1219--1226. Reedman, J.H., 1979. Techniques in Mineral Exploration. Applied Science Publishers, London, 533 pp. Robinson, G.D., 1981. Adsorption of Cu, Zn, and Pb near sulfide deposits by hydrous manganese-iron oxide coatings on stream alluvium. Chem. Geol., 33: 65--79. Robinson, G.D., 1983. Heavy metal adsorption by ferro-manganese coatings on stream alluvium: natural controls and implications for exploration. Chem. Geol., 38. Rose, A.W., 1975. The mode of occurrence of trace elements in soils and stream sediments applied to geochemical exploration. In: I.L. EUiott and W.K. Fletcher (Editors), Geochemical Exploration 1974. Elsevier, Amsterdam, pp. 691--705. Whitney, P.R., 1981. Heavy metals and manganese oxides in the Genesee Watershed, New York State: effects of geology and land use. J. Geochem. Explor., 14: 95--118.