Temporal variation of metal concentrations in biogeochemical samples over the Royal Tiger Mine, Colorado, Part II. Between-year variation

Journal of Geochemical Exploration, 27 {1987) 53-62

53

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Temporal Variation of Metal Concentrations in Biogeochemical Samples over the Royal Tiger Mine, Colorado, Part II. Between-Year Variation

J.D. STEDNICK 1 and W.C. RIESE 2

~Department of Earth Resources, Colorado State University, Fort Collins, CO 80523, U.S.A. 2ARCO Exploration Company, Houston, TX 77251, U.S.A. (Received December 1, 1986; revised and accepted April 10, 1987)

ABSTRACT Stednick, J.D. and Riese, W.C., 1987. Temporal variation of metal concentrations in biogeochemical samples over the Royal Tiger Mine, Colorado, Part II. Between-year variation. J. Geochem. Explor., 27: 53-62. Samples of lodgepole pine (Pinus contorta Dougl. ex Loud.) were collected every two months for nine sampling periods, May 1983 to September 1984. Needle, twig, and wood samples were taken at 65 points at 15-m intervals along a 1-kin transect line and analyzed for Cu, Zn, Pb, Mo, Au, and Ag. In the 18-month study period, there were significant variations in Au concentrations within and between years.Gold anomalies,as measured by Au concentrationsin plantorgans,were observed in lessthan halfthe sampling periods.Gold concentrationsin needleswere consistentlylower,but paralleledAu concentrations in the twigs. Mineralized areas were identifiedby high Cu and Zn concentrationsin needle and twig samples over allsampling periods,although the relativeconcentrationsvaried over time. Molybdenum concentrations in wood were mostly below detection limitsand not indicativeof mineralization.Similarly,Ag concentrations were frequentlybelow detectionlimitsand were not useful for mineralization identification. Pb in needles ranged from 3 to 112 #g g - ~ (ppm), and up to 450/~g g - 1 in twigs, but did not correspond to mineralization. Between-year variation of metal concentrations in biogeochemical samples was not significant for Zn and Cu, except in wood samples. Between-year variation in Au concentrations was significant in biogeochemical samples. Lead and Mo variation between years was difficult to assess since background/anomaly contrasts were not identified for these metals. Biogeochemical sampling has proven to be useful for delineating mineralization from year to year. However, temporal variationsof metal concentrations in plants within and between years must be recognized.Processesthat regulatebioavailabilityneed to be identifiedin order to increase the utilityof biogeochemical sampling.

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54 INTRODUCTION Biogeochemical methods of prospecting for mineral deposits have been used successfully in correlating gold concentrations in plants with the Au concentrations in underlying substrata. In areas of transported soils or overburden, there may be no geochemical relationship between the underlying bedrock mineralogy and overlying soil material. Because plants may sample and obtain nutrients from bedrock in addition to soil, vegetation can be an advantageous sampling medium to assist in locating underlying mineral deposits not reflected in the soil chemistry. In order for an element to be taken up by plant roots, it must first be in a chemically available form. However, the forms in which trace elements are held in soil are complex, and only a small amount of the total element content is available for plant uptake (Mitchell, 1964, 1972; Brooks, 1983). Little research has been conducted on the metal concentrations of specific plant organs or metal concentration variation over time in biogeochemical prospecting. In trees, the literature has indicated a tendency for many elements {including Au) to be concentrated in leaves and twigs, with the lowest levels in sapwood and heartwood of tree trunks ( Carlisle and Cleveland, 1958; Khotamov et al., 1966; Brooks, 1983). A number of researchers have noted seasonal variations in the element concentrations in plants, with the largest concentrations generally occurring in the springtime (Guha, 1961; Aripova and Talipov, 1966; Khotamov et al., 1966; Schiller et al., 1973; Stednick et al., 1987). No research has been done on the element concentration variation between years in biogeochemical prospecting. Element concentrations in plant organs and temporal variations in these concentrations are important considerations in planning an exploration biogeochemical survey, and additional research in these areas is warranted. This study investigated temporal variation over a sampling period of 18 months in concentrations of Au and associated metals in specific organs of lodgepole pine (Pinus contorta Dougl. ex Loud. ) growing in a mineralized area. The study hypothesis was that the metal concentrations in the plant organs would vary seasonally but exhibit similar patterns between years. The most appropriate tree organ to collect and analyze for exploration purposes, the optimum time frame for sample collection, and relation of metal concentrations in tree samples to the underlying soil and geology have already been examined ( Stednick et al., 1987 ). AREA GEOLOGY This study was performed over the workings of the now abandoned Royal Tiger Mine in the Breckenridge District, Colorado, U.S.A. ( Fig. 1 ). This area was selected because the previous mining activity provided a record of excellent underground control. Additionally, Anaconda Minerals Company was at

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The vegetation transect traversed the Royal Tiger Mine workings and extended into terrain initially believed to be unmineralized between Golden Run Gulch and Rock Island Gulch. Sixty-five sample sites were established at 15-m intervals along this line. At each sample site, needle, twig, and wood samples were collected near midmonth in May, July, September, November 1983 and January, March, May,

56 July, and September 1984. Sampling methodologies were detailed in Part I ( Stedriick et al., 1987). Duplicate samples for each sample type were collected during each sampling period as a check on laboratory analytical precision, which was found to be satisfactory (Klem, 1985). ANALYTICALPROCEDURES Needle, twig, and soil analyses were performed by the Anaconda Geoanalytical Laboratory in Tucson, Arizona. Plant samples were dried at 80 ° C, ashed at 550 ° C, and then digested with 30 mL of aqua regia, evaporated to incipient dryness and the residue taken up in 25 ml of 25% HC1, after filtration. Ash solutions were analyzed directly for Cu,Pb, Zn,and Mo by inductively coupled argon plasma emission spectroscop~} (ICPES) and for Ag by graphite furnace atomic absorption spectrophotometry (FAA). A 10-mL aliquot of ash solution was extracted with 4 mL of methyl isobutyl ketone (MIKB) and the backwashed phase was analyzed for Au by graphite furnace AA ( C.K. Unni, pers. commun., 1984 ). Whole wood samples were analyzed by neutron activation by Nuclear Activation Services Limited, in Hamilton, Ontario, Canada. RESULTS AND DISCUSSION Summary statistics were calculated for all samples from all months (Table 1 ). To illustrate within- and between-year variation, scan sheets were prepared for each metal by plant organ and month {Figs. 2-4). Each element will be discussed separately.

Copper Twig samples contained the largest Cu concentrations (Table 1). Copper concentrations in needles consistently paralleled those in twig samples at roughly half the concentration level of twigs. The minimal variation in Cu concentrations within- and between-years may be due to the role of Cu as an essential element in plant nutrition, or alternatively, may reflect minimal changes in the amount of Cu available for plant uptake and/or plant redistribution. Copper concentrations in wood samples generally did not reflect Cu concentrations in soil samples (Stednick et al., 1987) nor was the Cu. behavior in wood consistent within or between years.

Lead Twigs contained higher Pb concentrations than needles, but both concen-

57 TABLE 1 Summary statistics for annual metal concentrations for plant organs and soil (values in #g g-1, except Au ng g- 1) Element

Statistic

Cu

minimum maximum mean standard dev. sample size

Pb

minimum maximum mean standard dev. sample size

Zn

minimum maximum mean

standard dev. sample size Mo

minimum maximum mean

standard dev. sample size

Au

minimum maximum mean

standard dev. sample size

Ag

minimum maximum mean standard dev.

sample size

Needles I (ashed)

Twig 2 (ashed)

Wood

Soil

21 191 82 18 558

56 1154 171 61 558

<0.5 13.0 1.5 1.1 578

29 225 75 56 65

3 112 50 18 556

52 450 163 61 556

-

28 314 96 265 65

21 7560 2110 1280 536

14 7420 1870 1031 536

3 55 19 9.3 578

138 2360 600 510 65

0.7 13.8 4.6 1.9 557

5 32 14 4 557

<0.05 13.00 0.11 0.62 578

<5 22 5 2 65

<1 302 19 36 558

<1 883 41 94 558

<0.1 35.0 1.2 2.6 578

<5 644 79 119 65

<0.5 12.6 1.4 1.1 558

<0.5 18.2 2.9 1.4 558

<0.5 3.0 0.9 3.4 65

1Average ash weight for needles for winter months was 0.8%. 2Average ash weight for twigs for winter months was 1.6%.

trations were erratic across the transect and over time. Lead concentrations exhibited variability between years in both needle and twig samples. Whether t h e s e r e s u l t s a r e d u e t o l o w P b c o n c e n t r a t i o n s i n t h e soil o r a n e x c l u s i o n m e c h a n i s m b y t h e p l a n t is n o t k n o w n , b u t P b w a s a n u n s u i t a b l e e l e m e n t for b i o g e o chemical sampling in this area.

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Fig. 3. Scan sheet of metal concentrations in twigs for 9 sampling periods. Fault-boundedarea of mineralization is denoted by M in geology cross-section. Zinc concentrations in wood samples were lower and less consistent than those in needle and twig samples but patterns can be recognized between years (Fig. 4). The data bases are incomplete for Zn because 22 Zn analyses were lost for needle and twig samples for the July 1983 sampling period and were plotted as the average of data points on either side.

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Molybdenum Molybdenum concentrations were generally higher in the twigs than the needles ( Table 1 ). Wood samples usually had Mo concentrations below detection limits and those results are not presented in this analysis. It is not possible

61 to assess the suitability of Mo as a sampling element in this area due to the low Mo concentrations in both soil and plant samples. Although Mo is an essential element for plants and uptake barrier mechanisms are improbable, concentrations were fairly consistent between years. Its non-parallel behavior relative to Cu and Zn may reflect different metabolic functions and rates. Gold

Highest Au concentrations occurred in the twigs (Table 1 ). Gold concentrations in needles paralleled those in twigs at roughly half the concentration. Gold concentrations in twig and needle samples identified sharp background/ anomaly contrasts, although large temporal and spatial variations occurred over the nine sampling periods (Figs. 2-4 ). The highest Au concentrations in twig and needle samples occurred in September 1983 when a large anomalous zone was defined between station numbers 52 and 65. The anomaly was a massive me.tal-bearing skarn with a high Au content dipping underneath the transect (Stednick et al., 1987). This anomaly demonstrated the ability of the trees to absorb significant quantities of available Au and locate a deep Au source not reflected in soil samples. The optimum sampling period of spring and summer months identified in Part I was not repeated in the second year of sampling, and neither was the anomaly detected over the skarn in September 1983 seen again in 1984. This apparent temporal variation in plant available Au suggests that an optimum biogeochemical sampling window may be very short, such as 1-2 weeks. These data suggest that bioavailability may vary through time as a function of lithology. This lithologic dependence will be examined in a later article. Silver

Low soil Ag concentrations and low Ag concentrations in biogeochemical samples precluded their utility for exploration purposes and these data are not presented. CONCLUSIONS Lodgepole pine (Pinus contorta Dougl. ex Loud. ) was determined to be useful for biogeochemical prospecting. Gold, Zn, and Cu concentrations in needle and twig samples defined background/anomaly contrasts, but an optimum biogeochemical sampling window may be very narrow for Au. Biogeochemical sampling for Au was successful in identifying mineralization, but was not reproducible between years. Two distinct sampling windows were identified for biogeochemical prospecting for Au. One sampling window was in the spring as identified earlier, and another was observed in the fall. These data suggest that

62 m e t a l b i o a v a i l a b i l i t y to t h e s e t r e e s m a y v a r y as a f u n c t i o n of lithology. T h e r e was no t i m e d e p e n d e n c e for b i o g e o c h e m i c a l s a m p l i n g for Cu or Zn a n d conc e n t r a t i o n t r a c e s were c o n s i s t e n t b e t w e e n y e a r s s a m p l e d . L e a d a n d Ag are n o t e s s e n t i a l for p l a n t growth, h a d low c o n c e n t r a t i o n s in soil a n d m o s t b i o g e o c h e m i c a l s a m p l e s , a n d a r e a s o f m i n e r a l i z a t i o n could n o t be d e t e c t e d u s i n g t h e s e e l e m e n t s . Availability a n d u p t a k e of s o m e m e t a l s b y p l a n t s varies over t i m e a n d a p p e a r s to e x h i b i t a lithologic d e p e n d e n c e , while o t h e r m e t a l s are c o n s i s t e n t b e t w e e n years. A d d i t i o n a l soil g e o c h e m i c a l a n d b i o c h e m i c a l p r o c e s s e s t h a t m a y be r e s p o n s i b l e for t h i s e x h i b i t e d b e h a v i o r are still b e i n g addressed. REFERENCES Aripova, K.H and Talipov, R.M., 1966. Concentration of gold in soils and plants in the southern part of the Tamdytau Mountains. Uzb. Geol. Zh., 10:45-51 (in Russian). Brooks, R.R., 1983. Biological Methods of Prospecting for Minerals. John Wiley and Sons, New York, N.Y., 322 pp. Carlisle, D. and Cleveland, G.B., 1958. Plants as a guide to mineralization. Calif. Dep. Nat. Resour., Div. Mines, Spec. Rep., 50, 31 pp. Cocker, M.D. and Pride, D.E., 1979. Hydrothermal alteration-mineralization of a subvolcanicvolcanic complex, Breckenridge Mining District, Colorado (U.S.A.). Nev. Bur. Mines,Geol. Rep., 33: 141-149. Guha, M., 1961. A study of the trace-element uptake of deciduous trees. Ph.D. Diss., Univ. of Aberdeen, Scotland (unpubl.). Khotamov, S., Lobanov, E.M. and Kist, A.A., 1966. The problem of the concentration of gold in organs of plants within ore fields. Dokl. Akad. Nauk Tadzh. SSR, 9:27-30 (in Russian). Klem, R.B., 1985. Biogeochemical prospecting for gold in the Breckenridge Mining District, Colorado, U.S.A.M.Sc. Thesis, Dep. Earth Resour., Colorado State Univ., 149 pp. Mitchell, R.L., 1964. Trace elements in soils. In: F.E. Bear (Editor), Chemistry of the Soil. 2nd ed. Am. Chem. Soc., Monogr. Set., 160: 320-368. Mitchell, R.L., 1972. Trace elements in soils and factors that affect their availability. Geol. Soc. Am. Bull., 83: 1069-1076. Schiller, P., Cook, G.B., Kitzinger-Skalova, A. and Wolf, E., 1973. The influence of seasonal variation for gold determination in plants by neutron activation analysis. Radiochem. Radioanal. Lett., 13: 238-286. Stednick, J.D., Klein, R.B. and Riese, W.C., 1987. Temporal variation of metal concentrations in biogeochemical samples over the Royal Tiger Mine, Colorado, Part I: Within-year variation. In: R.G. Garrett (Editor), Exploration Geochemistry 1985. J. Geochem. Explor., 29: 75-88. Tweto, 0., 1968. Geologic setting and interrelationships of mineral deposits in the mountain province of Colorado and South-Central Wyoming. In: J.D. Ridge (Editor), Ore Deposits of the United States, 1933-1967 (Graton Sales Volume). A.I.M.E., 1: 55-588. Tweto, O., 1975. Laramide {Late Cretaceous-earth Tertiary) orogeny in the Southern Rocky Mountains. In: B.F. Curtis (Editor), Cenozoic History of the Southern Rocky Mountains. Geol. Soc. Am. Mere., 144: 1-44.