COVER— Silhouette of an oak tree in the California Mother Lode. Photo by Mary Hill. STATE OF CALIFORNIA GOODWIN I. KNIGHT. Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 GORDON B. OAKESHOTT. Chief N FRANCISCO SPECIAL REPORT 50 1958 PLANTS AS A GUIDE TO MINERALIZATION By DONALD CARLISLE and GEORGE B. CLEVELAND Price 50rate certain metals more readily than others. If the 1 its are systematically collected and carefully ana- ' d, the results may indicate the possibility of a buried r deposit. This technique of sampling, analyzing, and lrpreting the plant cover is called biogeochemical iipecting. lthough the method has the advantage of rapidity r is relatively inexpensive, the numerous factors (such ! ge and organ of the plant, soil pH, geochemistry of ) and its exchange capacity) that bear on the concen- ••ion of metals within plants must be considered. ;onomists and other research workers have provided >e data; biogeochemical field work has added more, hree known molybdenum deposits in California were upected in the course of this study. At the 'Bour ivbdenum mine, San Diego County, molybdenite is ieminated as flakes in an aplite dike that cuts grano- : anuscript submitted for publication December 1956. le authors are indebted to Owen R. Lunt of the University of California at Los Angeles for help through discussions, and for reviewing part of the manuscript. ' ssociate Professor of Geology, University of California, Los Angeles. ' eologist, California State Division of Mines. diorite. Plants growing above the dike showed an ab- normally high concentration of molybdenum ; the soil in which they were rooted was also high in molybdenum, but not so anomalously as the plants. Flakes of molybdenite were found in the Cosumnes mine, El Dorado County, along the contact of an igneous body and metasedimentary rocks of the Carboniferous (?) Calaveras group, though the mine itself was not primarily worked for molybdenum. Plant samples from the area showed the highest concentration of molyb- denum to be over the contact zone, as geological field studies also indicated. At the Tyler Creek tungsten mine, Tulare County, the analysis of samples collected along a fault zone that traversed several rock types indicated that the underly- ing rock type exercised some control over the molyb- denum concentration. No definite pattern of mineraliza- tion could be determined from the plant analysis ; higher values were seemingly distributed at random. Several factors other than the metal content of the soil may influence the metal content of a plant. If these factors vary within the area of survey, and if they have a significant effect on metal uptake, the biogeochemical results are misleading. Soil moisture and drainage in- fluence the availability of some metals although, in general, the influence of the physical properties of soil is probably not great. Even the amount of sunlight and shade has a slight effect for some plants. The largest effects, however, are associated with the chemical prop- erties of the soil both as an influence on the metabolism of plants and as a control of the state of the metal in the soil, as most of the metal in the soil is present not in minerals or in solution but as ions sorbed or ex- changed in the soil materials, especially on clay min- erals and organic matter, or else as complex ions. Con- sequently, the ability of the plant to absorb metals is moderately to profoundly influenced by the kind and- concentration of exchange materials, by the soil pH, by the kind and concentration of other ions, and by organic materials and micro-organisms. The exchange capacity of a soil also largely determines the amount of matal that will be taken and retained from various sources. Available data suggest that the clay or organic content of a soil can be responsible for variations of a few fold in the metal content of that soil. BIOGEOCHEMICAL PROSPECTING Biogeochemical prospecting, like most geochemical and geophysical methods of exploration, is an adjunct to other techniques for the discovery of economic mineral deposits. As with other geochemical and geophysical methods, clues to the location of hidden deposits are sought through relatively inexpensive procedures per- formed on readily accessible materials at the earth's surface. In biogeochemical prospecting the procedures are (1) the observation of the distribution of plants (in- dicator plants) that tolerate or fail to tolerate abnor- mally high concentrations of certain, metals in the soil (this method is sometimes called botanical prospecting) ; or (2) the sampling and analysis of plants that con- centrate or of plants that merely acquire certain metals (3) Special Report 50 RAINFALL ZO - 100" Figure 1. Maps of California showing the close relationship be- tween soil pH and amount of rainfall. Modified from Jenny (1951). from the soil, (accumulator plants)* and (3) the inter- pretation of the data thus obtained. The basic assump- tions, then, are that a high metal content in the soil may reflect a deposit of metal somewhere at depth and that the high metal content in the soil may be reflected in some way in the plants. Indicator plants, though of limited occurrence, are very useful when present and offer an extremely cheap method for reconnaissance of large areas to show variations in metal content of the soil. An example is the use of some species of Astragalus, Stanleya, and other selenium- and sulfur-tolerant plants in the search for uranium deposits (Cannon, 1952; Cannon and Starrett, 1956 ; Dorn, 1937 ; Lidgey, 1897 ; Hawkes, 1950). The second technique, systematic sam- pling and analysis of plants for their metal content is more widely applicable, though best suited to small areas. It is with this technique and some of the problems in- volved that this report is concerned. The term biogeo- chemical prospecting will be used for this second tech- nique. Unusually high concentrations of metal commonly do develop in the soil near ore deposits. For example, the metal in the soil may be the residuum of a metal deposit now eroded. Grains of a primary ore mineral may have persisted through the weathering process or may have been concentrated through removal of more easily weathered gangue minerals. Residual concentrations of cassiterite, gold, and other minerals are sometimes pro- duced in this way. If the primary ore mineral weathers readily a secondary mineral such as azurite, malchite, cerrusite or garnierite may remain in the soil. Under still other circumstances only traces of the ore metal may remain as a contaminant but in concentrations sufficiently large to distinguish the soil from other soils nearby. Or the metal in the soil may have been intro- duced or dispersed either as solid mineral particles carried by water, wind, or ice, or as dissolved ions in ground waters. In this event the source of the metal might not be nearby but might nevertheless be discovered • The use of the term accumulator plant for all plants that can be sampled for a given metal is misleading. Many plants used suc- cessfully by blogeochemists contain lesser concentrations of the metal sought than does the underlying soil. The plants sample the soil but they do not necessarily accumulate the metal. through careful study of the dispersion pattern pears also that metal ions are able to migrate i ward directly from hidden deposits by proces well understood. Presumably they travel in pa upward percolating ground water and in part fusion within ground water. Ion exchange an< reactions with soil materials probably play a rol within the reach of plant roots, metals may be concentrated at the surface very effectively by themselves. Goldschmidt (1937, pp. 670-71) has out how absorption of metals by plant roots, tr( tion to the upper organs of the plant, and retun earth when leaves are shed or the plant dies tend centrate them in the upper layers of the soil. Agronomists have long recognized the ability ol to accumulate some metals from the soil, especial] metals useful or necessary to the plant and als ability to absorb other metals present in the so though these appear to serve no useful purpose plant. Probably every element could be found in if sensitive enough analytical techniques were use( the synthetic element, plutonium, is absorbed anc located within plants (Jacobson and Overstreet, In some cases an unusually large amount of i metal in the plant can be correlated with an un high concentration of the metal in the soil in wh plant is growing. The abnormal amounts of go copper reported in horsetails is a classic example analyses of many of the world's coals did m prompt an early interest in the relationship t plant cover and the subsurface geology. Milner reported vanadium from Scottish coal and gol< coals at Cambria, Wyoming. Germanium has beer in many of the world s coals by Goldschmidt (193 more recently by others ( Stadnichenko et al., 195! work of Palmquist and Brundin reported by the S Prospecting Company in 1939 was among the first eal applications of biogeochemical principles. Tl method was employed with some success by Rank the vicinity of the nickel ore deposits at Nei northern Finland (Rankama, 1940). The value geochemistry was demonstrated by many investig* Scandinavia and has been applied successfully ii land, Greece, U. S. S. R., Spain, Australia, South Nigeria, Canada, Venezuela, Poland, Brazil, Ge and the United States. ;) "- Prospecting Procedure. The procedure for chemical prospecting in an area is as follows :'ii samples are collected according to a grid patten >r intervals along traverses. The same species of pla , t same organ of the plant and if possible plants same age are sampled at each station. Some worke p fer to use only twigs of the previous year's grt others use only leaves. Sampling stations may be f a feet to 1000 feet apart depending upon the expect) < of the target and the size of the area involved It general practice to collect a compound sample fed! station by gathering the leaves, twigs or whatever ig is sampled from two, three or four trees or plant? ro ing within a radius of perhaps 5 to 25 feet of the st« All sampling is done within a single day or a feviH Where no single plant species grows over the 'h< area some workers have sampled two kinds of pli ts Plants as a Guide to Mineralization aempt to establish a correlation in metal uptake i n them through sampling both plants together at •id locations. Usually, though, geological considera- sor the distribution of already known deposits tthe size of the area to be sampled and a single a e plant species can be found throughout, b purpose of an investigation dictates, in most in- |k the number of samples to be collected. If the lion of a known ore body is the object of an in- intion, perhaps then fewer samples will be required qieve the objective than if a totally unknown area ieig investigated. In the former case the general i? of the ore body affords a starting place from I traverses can radiate. Moreover, if the deposit has jir trend the problem is narrowed to sampling the ileyond the extremities. If the area is entirely un- I it is necessary to sample quite intensively. In II a widely spaced grid pattern will give, at the .i of a program, some indication of where the f avor- iocalities lie. After this preliminary attempt, de- jjwork in likely areas may establish the limits of a \t. Various means have been employed to collect 1 in samples such as the leaf punch, which cuts out l area of plant material, but this technique fails to jnto account the variable thickness of the leaf. 13, cutters and other garden devices have all been |>ut none with any particular advantage. The type img tool to use is best left up to the individual and | the metal from which it is made contributes to the jnination of a sample, little concern need be given ihase of the operation. f?ood deal of attention has to be given to the jrs of contamination. Plants can become contami- | in two important ways before they are sampled. I been demonstrated that wind blown material from a and smelters poses a serious problem in some . and soil contamination through rain-spattering onstant hazard when plants are sampled too close [ surface of the ground. In areas where contamina- i likely, organs such as twigs should be sampled in rence to leaves, because the ratio of total contami- area to the volume of the organ is much less in ise of twigs than in the case of leaves. If there is oubt as to whether a sample is contaminated or t can be carefully washed in a dilute solution of chloric acid. This procedure will be sufficient in cases to remove contaminants. Some prospectors le bark from twigs to remove surficial dust. Others ■ r wash the samples in distilled water with or with- detergent. Lest this phase of the operation be too lightly, it has been demonstrated that the it of metal picked up by the hand from turning a doorknob is sufficient to produce a positive metal m with the reagent dithizone. plant samples are taken to the laboratory in paper bags where they are dried and ashed under ird conditions. Since the metal content is usually ited in terms of the ash weight (agronomists corn- use "dry weight") and since the weight of "ash" eed from a given amount of dried plant depends the length of time and the intensity of heating specially important that this procedure be stand- d. Ashing temperatures above 450° centigrade may volatilization of some elements. The ash is weighed and analysed for the metal sought by one of several techniques. Rapid, accurate wet chemical procedures have been developed for zinc, copper, "heavy metals," molybdenum, uranium, and nickel. Spectrographic analysis is sometimes used where the equipment is avail- able. It provides a permanent record, has a high degree of sensitivity, and can analyze for several elements at once. Contamination is a serious problem at this stage also. It has been suggested that the high concentration of lead and other metals in wall paints, for example, makes them important sources of error where uncovered samples are placed near old flaking paint. The analyses, usually reported as parts per million (ppm) of metal in the plant ash, are plotted on a map and interpretations may then be made for areas of possible mineralization. The technique is thought to have some advantages and some disadvantages as compared with analysis of soils directly for their metal content or with other pros- pecting techniques. Advantages (1) The metal content of several plants at each station is likely to be a more representative sample of the area than is the metal content of a few grams of soil. The plant roots themselves sample a large volume of soil. (2) Sampling is rapid and easily done, and is therefore, inex- pensive. (3) Plant roots have access to large amounts of soil well below the surface. This can be an important factor because the concentra- tion of the metal near the surface of a soil may tend to correlate with purely surficial conditions and be unrelated to the concentra- tions deeper in the soil. Moreover the migration of metals in soil can be extremely slow. Lundblad et al. (1949) have shown, for example, that of the copper supplied in large amount (250 Kgm per hectare) to the surface of a peaty copper deficient soil, only 2 percent had penetrated to a depth 5 centimeters below the soil surface 6 years after application. Most of the copper was fixed in the upper peaty layers. Nevertheless crops grown thereon did take up copper. (4) If plants do accumulate metal concentrations greater than those present in the soil the analysis of plants may be simpler than direct analysis of the soil. Usually this is not a major factor. Disadvantages (1) The added step of ashing the plant for analysis is not required in soil analysis unless organic matter interferes with the analytical procedure. (2) The unknown influences of many complex and many un- known factors in addition to metal content of the soil that affect the uptake and storage of metals by plants. The more important known factors will be discussed below and an attempt will be made to estimate whether such factors are likely to produce variations in the metal content of plants so large that under some natural conditions they may obscure the varia- tions in metal content in the soil. It should be pointed out, how- ever, that soil analysis itself is not free of such complications. Surficial soil can be enriched in metal though the accumulation of plant debris, "Goldschmidt enrichment" ; and is therefore sub- ject also to the complications of plant metabolism. According to Mitchell (1955, p. 269-70), Goldschmidt enrichment appears to apply to Scottish moorland soils, on an ignition weight basis, for Ag, Cu, Mo, Pb, Sn, and Zn, but not for Co, Ni, Cr, V, Mn, Ti, Ga, Li, Rb, Ba or Sr. The effect is strongest where the metal enters into insoluble combinations with organic or inorganic ma- terials. In addition the metal content of surficial soil may be in large part dependent upon the abundance of soil materials that are capable of selectively absorbing and binding or fixing metal ions. Micro-organisms in the soil commonly are major factors in controlling metal mobility in the soil. Inasmuch as direct soil analysis has become a standard adjunct to other exploration tech- niques in many places, and since the results obtained may depend in large part upon biological enrichment of the upper layers of the soil, it is desirable that geologists and others involved in this work Special Report 50 sampler plants for biogeochem ical prospecting t Plant name Common Green alder Willow Scrub oak Emory oak Mountain mahogany Mountain hemlock Balsam fir Giant cedar Lodgepole pine Black cottonwood Larch Aspen Soolpolallie Douglas fir Western red cedar Western hemlock Rocky mountain juniper. Juniper Pinyon pine Ponderosa pine White bark pine Englemann spruce Sitka spruce White spruce Dwarf juniper Rocky mountain fir Scrub birch Mountain birch Silver birch Sy ringa orange Mountain maple Apple Poplar Western red birch Grand fir Willow Red alder Broad leaf maple Vine maple Citrus Choke cherry Flowering dogwood Sumac Soft maple. Pignut Hickory Flowering dogwood - Black walnut Red cedar Osage orange Sycamore Black cherry White oak Red oak Bur oak Blackjack oak Pin oak Post oak Black oak Sumac Black locust.. Nees Winged elm... American elm. Slippery elm.. Pine Hickory. Sugar maple . _ Princess pine.. Pecan Brazil nut Chestnut Oak Scientific Alnus sinuata Salix sp Quercus tubinella Quercus emoryi Cercocarpus ledifolius.. Tsuga mertensiana Abies amabilis Thuya plicata Pinus contorta Populus trichocarpa Larix occidentalis Populus tremuloides — Shepherdia canadensis. Pseudotsuga taxifolia.. Thuja plicata Tsuga heterophylla Juniperus scopularum. Juniperus sp Pinus monophylla Pinus ponderosa Pinus albicaulis Picea engelmanni Picea sitchensis Picea glacua Juniperus communis Abies lasiocarpa Betula glandulosa Betula occidentalis Betula popyrifera Philadelphus lewisii Acer glabrum Populus grandidintata. Betula f ontinalis Abies grandis Salix scouleriana Alnus rubra Acer macrophyllum Acer cireinatum Prunus demissa Cornus nuttallii Rhus typhina Sassafras variifolium Acer saccharinum Carya cordif ormis Carya o valis obvalis Carya ovata Cornus florida Juglans nigra Juniperus virginiana Maclura pomifera Platanus occidentalis Prunus serotina Quercus alba Quercus borealis maximus. Quercus macrocarpa Quercus marilandica Quercus muhlenbergii Quercus palustris Quercus stellata Quercus velutina Rhus copallina Robinia pseudoacacia Sassafras albidum Ulmus alata Ulmus americana Ulmus fulva Acer saccharum Lycopodium fabelliforme. Quercus wislizeni Pseudotsuga douglasii. Af zelia af ricana Rubiacaea sp Bahia nitida Parinari curatellif olia . Albizzia zygia Lophira alata Vitex cuneata Parkia oli veri Millettia sp Newbouldia lavis Trichilia prievriana Anacardiaceae sp Sp. indet Metal detected Plants as a Guide to Mineralization SAMPLER PLANTS FOR BIOGEOCHEMICAL PROSPECTING— Continued Plant name Metal detected Common Scientific Quercus douglasii Artemisia tridentata Gaultheria shallon Vaccinium parvifolium Arctostaphylos u va-ursa Pachystima myrsinites Corylus californica Vaccinium sp Echinopanax horridus Pteridium aquilinum Holodiscus discolor Prosopis juliflora Astragalus sp Oryzopsis hymenoides Atriplex confertifolia Vaccinium vitis idaea Ledum palustre Equisetum sp Vaccinium ovalifolium Symphoricarpos racemosus- Artemisia trifida Viola calaminaria* Thlaspi calaminaria* Amorphia canascens* Viola tricolor* Thlaspi alpestre* Asparagus officinalis Linaria vulgaris Zea mays Euphorbia maculata Solidago sp Equisetum arvense Lobelia inflata Lobelia syphilitica Plantago lanciolata Pycnanthemum flexuosa Ambrosia artemisiifolia Amelanchier canadensis Smilacina racemosa Tomanthera auricula ta Viola sagittata Daucus carota Achillea millefolium Physalis sp. Celtis occidentalis Persicaria hydropiper Ambrosia coronopif olia Ambrosia elatior Ambrosia trifida Andropogon furcatus Andropogon scoparius Apocynum sibiricum Gnaphalium obtusifolium Helianthus mollis Liatrus aspera Desmodium sessilif olium Phytolacca decandra Salvia pitcheri Sorghastrum nutans Symphoricarpos orbiculatus. Tridens flavis Vernonia interior Amelanchier alnif olia Calluna rulgaris. Polycarpia spirostyles. Thymus serpyllum Viola hirto Astragalus racemosus. Oonopsis condensata. - Astragalus bisulcatus.. Xylorhiza parri Astragalus pectinatus. Astragalus grayii Aplopappus fromantii. Special Report 50 SAMPLER PLANTS FOR BIOGEOCHEMICAL PROSPECTING— Continued Plant name Common Dandelion. Rothrock gramma California poppy _. Carnation Wild rose Horsetail Bush cinquefoiL Scientific Stanleya pinnata Gutierrezia sarothrae- Arctostaphylos viscida Adenostoma fasciculatum. Vivia americana Medicagohispida Lotus corniculatus Trif olium repens Trifolium fragiferum Trif olium subterraneum_. Melilotus alba Melilotus indica Melilotus sativa Tritieum vulgare A vena sativa Hordeum vulgare Chloris gayana Lolium perenne Bautiloua rothrockii* Eschscholtzia mexicana*__ Holostium umbellatum Viscaria alpina* Melandrium dioecum* Polycarpaie spirostylus*. _ Rosa woodsi* Equisetum variegatum*__ Dasiphara f ruticosa* Cneoridium dumosum Metal detected t Compiled from various published and unpublished sources. have some knowledge of the main factors influencing the uptake and storage of metals by plants. Present Status of the Method Elements that have been successfully detected by biogeochemical methods are : zinc, copper, lead, tin, tungsten, molybdenum, nickel, chromium, uranium, be- ryllium, manganese, gallium, strontium, silver, barium, mercury, cobalt, titanium, iron, and gold. A review of the agricultural data shows that aluminum, arsenic, silicon, selenium, germanium, boron, lithium, vanadium, iodine, magnesium, and the rare earths are known to accumulate in plants and therefore can be detected by biogeochem- ical means. The accompanying table lists plants that have been successfully used in biogeochemical studies and others that, according to the agricultural data, probably could be used. The table includes both indicator and accumula- tor plants although the indicators represent only 8 per- cent of the total. Further, the table shows the dominant use of trees over all plants except that there are no indi- cators represented among the trees. Previous Work. The areas in which biogeochemical studies have been conducted are many and diverse. The greatest activity has centered in North America, princi- pally in Canada, with much of the work being done by the University of British Columbia and the Canadian mining industry. Pioneer work in the United States has been led by the U. S. Geological Survey. European con- tributions have come from Finland and Sweden. Russia has undoubtedly done considerable work in this field in an effort to develop her industrial potential. The number of commercial organizations using geo- chemical prospecting in their exploration programs is unknown but apparently very large. Soil sampling and analysis is more popular than biogeochemistry and has become a standard prospecting technique for many pri- vate groups in Canada and in the United States. The U. S. Geological Survey, as well as several universities, is actively engaged in the development and imp; ment of geochemical techniques. The U. S. Geological Survey has developed quick water, and plant tests for various metals that ca performed in the field. Recently the Survey has us portable grating spectrograph mounted on a two truck. Geochemical programs have been carried 01 at least ten major mining districts and in the dev ment of uranium exploration in the southwest. '. have been developed for the detection of zinc, ni uranium, and molybdenum in plants. These tests designed to give plus or minus 30-50 percent accu at the minimum rate of 24 to 30 samples per man- Under favorable conditions this figure can be raise 60 or as many as 80 determinations per man-day. Survey has also conducted basic research in plant metal relationships in an effort to expand the us ness of biogeochemistry. The University of British Columbia, with the coo] tion of the Canadian mining industry, has perfc various analytical testing techniques using dithizor an indicator for heavy metals. Some 5,000 separate alyses have been run on plant samples to detect ani lous concentrations of copper, zinc, lead, molybdei iron, manganese, silver, and gold. On the Colorado Plateau trees growing in sedimen formations have yielded samples indicating that urai may be concentrated at a depth as great as 60 feet (' non, 1953) ; the results of Canadian studies s mineralization can be detected through 50 feet of gl; drift or 60 feet of muskeg swamp (Webb, 1953, p. 3 The successful applications of biogeochemical ] pecting fall into two groups: studies over known bodies and the actual location of new ore deposits the former case the record is marked by varying de£ of success. The work of Webb and Millman, Vogt, i baugh and others in some cases has shown a less clear cut support of biogeochemical principles. "W on the other hand Warren and Delavault, Soke Plants as a Guide to Mineralization i, Hawks, Rankama and many more have achieved Hcant success in their investigations. The great ma- il of evidence cited in the literature supports the cses on which biogeochemical prospecting is based, ah undoubtedly there have been a great many un- S;sful attempts not reported in the literature. ';>ical examples of successful applications over p ore deposits are found in the work of Warren lawatson (19-17) on copper and zinc mineralization "nada. Studies were made in several copper camps I extensive mining operations had definitely out- t positive and negative areas. Tree samples were teed and the results between positive and negative •(were compared. In one area five different tree types kowed marked variation in copper concentration ?en the mineralized and nonmineralized areas; one Is differed nearly a hundred fold in concentration \ the smallest variation was threefold. The average ir concentration for all five trees in the positive ^was 13,080 ppm copper as opposed to 560 ppm i r in the negative area. > geochemistry has played an essential role in lo- t small bodies of uranium (Cannon, 1953), vana- i (Cannon, 1953), and iron, and chromium (Buck, I. This method has also been successful in locating l:d, pinched, and otherwise hidden extensions of in ore bodies. A number of near-commercial de- \\, such as the chalcopyrite mineralization in the jiroo mining district of South Australia, have been (l (Harbaugh and Starrett, p. 32, 1953; Webb, p. JL953). Cannon and Starrett (1956) have combined ise of plants as indicators and as samplers for ium. file the discoveries of new ore are not as spectacular ne might expect from the success of empirical es, it has been stated that they more than paid for ntire cost of all exploratory and experimental work within the last several years (Webb, p. 344, 1953). the other hand the results from biogeochemical >ecting have not been uniformly reliable. Anoma- variations in the metal content of plants have been 1 which are so haphazard in their distribution that 'could not be used safely to plan more expensive irface exploration. The uptake of some minor ele- ;s is not necessarily proportional to their relative ints in the soils. ABSORPTION OF METALS BY PLANTS ants obtain mineral salts from the soil by (1) ion [it accumulation, (2) ionic exchange, (3) chemical lination, and (4) by diffusion. The absorption jss is highly selective, some cations, anions and cules being effectively accumulated while others are ided to a remarkable degree. Ion accumulation is process by which ions may accumulate in much ter concentrations within the plant cells than the sntration of the same ion in the medium surround- ;he plant, This is an energy-expending process tak- slace largely in young growing cells especially near tips and continuing only while oxygen supply and lerature are conducive to aerobic respiration in the t. Since aerobic respiration is dependent in turn i the supply of carbohydrates and other organic irials produced by photosynthesis in the upper or- gans of the plant, the rate of ion accumulation tends to fall with any conditions that reduce markedly the rate of photosynthesis. Ion accumulation may tend to vary also with the rate of transpiration by the plant and in addition, ions of one species may have an effect upon the uptake of ions of other species. Ionic exchange mechanisms permit the exchange of anions or cations from the root surfaces of the plant for anions or cations in the soil. Ionic exchange is prob- ably not a metabolic process, does not require expendi- ture of energy by the plant and occurs independently of aeration or temperature. Carbonic acid or possibly organic acids produced by the roots and absorbed on the root surfaces releases hydrogen ions which then may be exchanged for other cations in the soil fluid or those sorbed on soil materials. Entry of ions by combination is similar to their entry by ion exchange. In this process, however, the ion may become a part of a metal-organic complex. Iron and magnesium are so absorbed; copper, zinc, manganese, molybdenum, cobalt and aluminum may enter such compounds, though knowledge of this process is very limited. Simple diffusion from soil to plant is not now thought to be an important primary process for mineral salt as- similation though it may contribute in some favorable circumstances. Nor do mineral nutrients merely travel along with the water absorbed. Diffusion plays an im- portant role in the soil quite outside the plant and assists in the translocation of ions once they have entered the plant. Thus the amount of metal taken up from the soil by a plant is influenced by factors that control the health and metabolism of the plant and also by many factors that control the availability of metal ions to the plant. If these factors vary markedly along a sample traverse in a biogeochemical survey, or if they vary throughout the area sampled, the analyses of the plant samples will reflect these factors as well as, or perhaps to the exclu- sion of, variations in metal content of the soil. Pre- sumably, then, these factors might produce misleading anomalies in plant metal content, suggesting a concen- tration of metal in the soil that does not exist. Or they may mask a real concentration of metal in the soil above a metal deposit. On the other hand, the effect of some of them may be slight in comparison with the variation observed in plants from ore-bearing to barren areas. In any event prospectors attempting to use the biogeo- chemical method should be aware of these factors and should have a rough idea of their possible influence on survey results. Unfortunately the subject is complicated and the data for many metals of interest to prospectors is meager. Nevertheless some suggestions might be ob- tained as to the approximate size of the variation in plant metal content that is needed to justify additional expenditures for detailed exploration. It will be obvious in the following that the authors, who are geologists, are heavily indebted to agricultural- ists and plant physiologists as well as biogeochemists for the data summarized. Factors Related to the Plant Kind of Plant. Warren and Delavault have con- cluded, after considering the results obtained from hun- dreds of plant analyses, that practically all trees and 10 Special Report 50 LOLIUM PERENNE MELILOTUS INDICO MEDICAGO HESIPDA 200 100 200 100 200 100 ^""^ 5 10 PRM. OF MOLYBDENUM IN THE SOIL Figure 2. Graphs showing how different types of plants, growing under the same conditions, will take up different amounts of molybdenum from the soil. Data from Barshad (1951). smaller plants are capable of reflecting mineralization. Any number of plant types can be used in sampling a given area but each will give a different biogeochemical anomaly which may vary considerably. While the pattern of the anomaly will generally be preserved, there is a chance that an anomaly could be overlooked if the plant type sampled absorbs only minor amounts of a given metal. The relative absorption of molybdenum by three different plant types serves to illustrate this problem. If at the beginning of a prospecting program one chose to sample Melilotus indica and during the course of a traverse changed to Medicago hespida the molybdenum content would increase by as much as 125 parts per million. This increase could be interpreted to mean the existence of an underlying ore-body whereas the metal content of the soil actually had remained constant. If on the other hand Lolium perenne were sampled at the same point a drop of 40 parts per million would be expected, and one might conclude that a mineralized area lay be- hind. Yet the plant type would be the only factor respon- sible for the variation. Table 1. Nutrient Requirements of Different Plants. Plant Element (%) Ca K Mg Sunflower 2.18 5.01 .640 Beans 2.11 4.02 .594 Wheat .79 6.73 .406 Barley 1.87 6.92 .540 Peas 1.55 5.25 .504 Corn .51 3.87 .402 After Newton (1928) An even more striking example is the accumulation of selenium by certain species of Astragalus. In one investi- gation (Trelease, 1945), the average content of these accumulators was about 800 ppm, some plants containing as much as 15,000 ppm (l| percent), many times the amount that would be lethal to ordinary plants. Wheat from the same soil averaged less than 5 ppm of selenium. Large differences may exist between families, species, or varieties in their ability to absorb a given element under a given set of conditions. Many plants require abnormal amounts of molybdenum; horsetails absorb silica ; and members of the family Lycopodioceac accumu- late aluminum. Even the normal uptake of major U ents varies within wide limits among plants of dii groups. Table 1, modified from Newton (1928) illus the varying requirements of calcium, potassiun magnesium by six common plants. All six plants grown in nutrient solution for 56 days, with five individuals of each type in the same crock. On green portion of each plant was analyzed. The data of Collander (1941) showing variatic uptake between plants are even more striking. The mum uptake for Na and Mn is about 20 to 60 I greater than the minimum; for Li, Mg, Ca, anl about 3 to 5 times the minimum; for K, Rb, an Q about 2 to 3 times the minimum. A given plan w found to be consistently rich or poor in a given el .a regardless of the year of cultivation or the compcii of the culture solution. An interesting relationship I by Collander and also by Hurd-Karrer (1939) great similarity in the uptake by a given plant c«l ments that are chemically alike. Rubidium and cl were found to be absorbed almost as rapidly as j sium; strontium almost as rapidly as calcium; seltl as fast as sulfur, and arsenic as fast as phospll Some plants appear to be able to screen out or to si themselves against excessive uptake of some eleul Plants do not contain silicon and aluminum, for (I pie, in proportion to their abundance in the soil. Ml algae generally contain abundant potassium andl little sodium, although the concentration of thestl ments in sea water is in the reverse ratio. Pea roots! found to absorb 2 to 3 times as much calcium I Na-Ca bentonite as did barley roots, while at the! time barley roots absorbed 4 to 5 times as mucl dium as did pea roots (Elgabaly and Wiklander, ll This is an effect that varies markedly between types, is different for different groups of elements! soil materials and is likely to become known only I extensive experimental work. Unfortunately almost I ing is known about selective uptake of the micronutil or of metals present in minor amounts. In the cai sodium and calcium just described the disproporti(l uptake is reduced when the concentration of sodium! calcium outside the plant is increased. Plants as a Guide to Mineralization 11 Ie roots of rice were found to protect themselves I further absorption of copper ions after they had ;-bed a certain amount (Isizuka, 1940). In another Melandrinm silvestre plants that had already grown ipper-rich soils were found to grow better than Is under experimental conditions of high copper ly and in still another case large increases in the c nt of zinc in the soil did not result in proportionate mses in the zinc content of the oat plant. I are not aware of any cases, however, where selec- e uptake or shielding by healthy plants has been •a to result in a reverse relationship between metal i:nt of a plant and metal content of the soil in which )lant is growing, all other conditions remaining iant, although there are cases where the variation pnt metal content is negligible. Ie experience of biogeochemical prospectors indicates trees in general are more consistent in reflecting netal concentration of the soil than are smaller 3S. But the lesser plants in many instances show a jer contrast in metal concentration than do the trees ling in a given area. The giant ragweed Ambrosia (a sampled by Harbaugh (1950, pp. 564-565) in the (itate district showed a variation in zinc content \ 5000 ppm to 1500 ppm between the mineralized iand the barren area. Among the trees the black :y (Prunus serotina) showed the largest variation; iged from 2200 ppm zinc in the mineralized area to bpm zinc in the barren area. Webb and Millman k, p. 495) noted in their biogeochemical studies in !a, that there was little variation in metal content ien species. They showed that the background s were especially constant among different plants i buted over wide areas. knt Organ Sampled. After the absorption of soil Irials by the roots, each element travels to specific ! of the plant structure. The translocation, or route Hestination, of these nutrients is important in choos- which organ to sample. In general, the newly ab- Id materials travel to the younger parts of the I. Therefore, these parts contain the highest concen- fm of metal and are the most satisfactory to sample. liver, Warren and Delavault (1949, p. 541) working I copper and zinc in British Columbia, found that feaves, cones, and needles of trees gave inconsistent its. They recommend the use of twigs which were I I to 3 years old and an eighth to a quarter of an Sin diameter. The data summarized in table 2 are i on nine biogeochemical investigations which in- d 75 tree samples, 13 samples of smaller plants, and [ iety of metals. The organs are arranged in order of 'asing metal concentration. In the tree samples the k showed the highest concentration of metal in 49 •nt of the analyses, and the twigs were highest in ercent of the samples. The leaves contained the ust concentration in 77 percent of the smaller plants. toradiographs taken by Biddulph and reported by g (1951, pp. 261-275) substantiate the results tabu- above. Biddulph introduced radioactive phosphorus a nutrient solution which contained young bean Is ; half of these were sampled after a period of four and the others were transferred to fresh nutrient Table 2. Organs with the highest metal concentration (in order of decreasing concentration). Trees Smaller plants 1. Leaves 1. Leaves 2. Twigs 2. Roots 3. Cones 3. Stems 4. Wood 5. Roots 6. Bark solution which contained no phosphorus. The plants sampled after the first four days showed that the phos- phorus had moved directly through the roots to the younger portion of the plants. The remaining plants were sampled after an additional four days and auto- radiographs were made of them. The second set of graphs showed that a considerable amount of the phos- phorus deposited in the younger organs, at the time of transplanting, had moved to the new plant organs devel- oped during the second four-day period. Thus, not only is there a continuous influx of material from the roots to the younger organs but there is also a constant redistri- bution of material to the younger leaves and twigs. Age of Plant and of Plant Organ. After a plant reaches a certain minimum age, which is different for each plant type, the uptake of material from the soil — all other things being equal — becomes constant and the re- quirements of new growth cause only gradual increases. While the plant as a whole may show no sharp variation in nutrient content, the individual organs developed during different growing seasons exhibit marked varia- tions. Table 3, from Barshad's work with molybdenum, contrasts the metal content of new tree leaves with those of the previous year's growth. Tree organs from 1 to 3 years old give consistent results in addition to con- taining the maximum concentration of metal. Younger organs fluctuate in metal content while the oldest wood consistently shows low concentrations. The smaller plants reach a similar optimum at the time of flowering when the metal concentration is at a peak and the leaf functions are fully developed (Lun- degarth, 1951). For these reasons the whole of a trav- erse or area should be sampled at about the same time. The combined effect of sunlight, rainfall, air tempera- ture, as well as the plant itself is responsible for the seasonal variation of metal uptake by plants. Robinson (1943, p. 6) has checked the same trees for rare earths on three different occasions. It was found on June 1 that the trees contained 174 ppm rare earths, on July 4 the total ppm had risen to 634, and by October 1, to 981 ppm. The same worker reported boron in samples of hickory leaves doubled between spring and fall (Robin- son et al., 1942). Barshad shows molybdenum in certain plants varies three-fold from spring to fall. Warren et al. (1947, p. 806) have noticed the seasonal variation of copper and zinc in tree leaves. Health of the Plant. If plant roots are injured through toxic soil conditions or in other ways they may lose their ability to absorb elements selectively or they may absorb excessive quantities of some elements. A single plant thus injured might yield an execessively high assay which would not be duplicated in neighboring plants and should therefore be rejected. But if all the 12 Special Report 50 PH IOOO 2000 3000 MGS. OF MOLYBDENUM ABSORBED PER KG. OF DRY COLLOID Figure 3. Graph showing amount of molybdenum absorbed by four types of clay minerals at different pH. Modified from Barshad (1951). Table 3. Molybdenum content of new and old tree leaves. Plant Molybdenum in leaves (mgm./kgm.) Lemon tree Current growth 1.3 1.2 Previous year's growth 2.3 Mesquite.. .. 3.6 systems extend to depths of 80 feet and more, in gen deep roots are exceptional. Weaver (1919) limits root depth of herbs and shrubs growing on the f< floor to the upper 18 inches. Kelley (1923, p. 15) rej that the absorbing system of most woodland plan concentrated within the soil's upper 12 cm and depth of 30-70 cm. Further, he questions the ger assumption that trees have deeply penetrating tap r The root depth appears to be less important in s ing out high metal concentrations in the soil than the forces that raise the ions from depth. Many bic chemical studies have utilized both trees and h plants in a single investigation (Warren and Howat 1947). There was no evidence to suggest that trees i able to tap high metal concentration at depths reached by shallow-rooted plants. a. us Q GD o— o o o () G o o o o o o 7 SOIL pH Modified after Barshad (1951) Figure 4. Graph showing amount of molybdenum taken up by alfalfa at different pH. Assimilation increases with decreasing soil acidity. Data from Evans, Purvis, and Bear (1951). plants within one segment of the area under survey are injured in the same way, this segment would yield high assays and unless the condition of the plants was recog- nized from some fairly obvious abnormality the anoma- lous segment might be treated as a valid suggestion of mineralization. Biogeoehemical anomalies that corre- spond with marked differences in the physical character of the soil should be suspect. Root Depth. The maximum depth at which a biogeo- ehemical anomaly can be detected at the surface is determined principally by two factors: the depth to which the plant roots extend downward and the ion- dispersing forces working from the ore body upward. The root systems of most plants extend into the soil only a few inches or feet at the most. While some root Chemical and Physical State of the Metal Metals in the Soil Solution and in Mineral On The soil is the principal source of the macronutrfci water, phosphorus, sulfur, potassium, calcium, mfll sium and iron; the micronutrients, copper, zinc, mol denum, manganese, boron, and possibly silicon, coll and others, as well as the many metals not yet sbl to be of nutrient value to plants. Metals may simply be dissolved in soil water and sPb>Ni>Co>Zn >Ba>Ca has been suggested (Mitchell, 1955 p. 265) ; but in view of the several causes of ion exchange, probably no one order of replaceability applies for all exchange materials. Generally, but not universally, the difficulty of displacing a cation appears to increase with valence and with size of the ion, and tends to decrease with hydration of the ion. Reducing the size of the clay parti- cles, as in grinding, tends to increase the capacity of the clay to exchange cations — especially clays in which the main cause of cation exchange is broken bonds. The cation-exchange capacities of clay minerals, meas- ured in milligram equivalents (m.e.) of cation held per hundred grams of exchange material at pH 7, ranges from 3 to 15 for kaolinite, from 5 to 50 for halloysite, from 80 to 150 for montmorillonite and vermiculite, and from 10 to 40 for illite, chlorite, and attapulgite groups (Grim, 1953, p. 129). Natural soils are reported with a cation-exchange capacity of 200 m.e. or more per 100 grams of soil. Peaty soils are reported to have the high- est capacities. Inorganic soils rarely have exchange ca- pacities exceeding 75 m.e., and the great majority have less than 50 m.e. per 100 grams of soil. A soil with an exchange capacity of 50 m.e. per 100 grams could absorb over 15,000 ppm of zinc. Other ions, if also present, would, of course, compete with the zinc for exchange positions. Exchange capacity increases with pH ; that is, exchange capacity is higher under alkaline conditions. Generally the number of ions held tends to vary with its concentration outside the exchange material. In addition to, or instead of being absorbed as cations, some metals such as boron, molybdenum, selenium, and vanadium can appear as complex anions which are solu- ble in water and can be absorbed as anions by anion- exchange materials. Others such as zinc, copper, ferric iron, and thorium may form basic salts with clay as well as being absorbed as cations (Mitchell, 1955, p. 264). And in general, the higher the percentage of saturation by a given cation, the more easily that cation can be displaced and absorbed by plants (Jenny and Ayers, 1939). Thus, in a kaolinitic soil with an exchange capacity of 8 m.e. per 100 g, of which 0.08 m.e. (1 percent) was copper, the amount of copper available to plants should be greater than in a montmorillonitic soil with a cation-exchange capacity of 40 m.e. per 100 g, of which 0.2 m.e. (0.5 percent) was copper. In general, the heavy metals are less readily available to plants from montmorillontic clays than from kaolinitic clays. The situation is complex where several ions compete f< » change positions, the degree of saturation having fo tively little effect on availability where the other s jw ions are held very tightly but having a very large ect on availability where the other ions are held loose i common result of this is that the availability of a 1 n metal decreases as the ratio of sorbed sodium to s< J calcium increases. Metals Held in the Soil Other Than by Ion Exch Through a process called "chelation" some metal able to form stable complex ions with certain or substances produced in soils. The metal thus ehe does not enter into reactions of its own but is ca along with the complex ion. It may be either less s able or more available to plants than it was b chelation. Relatively insoluble compounds of co zinc, iron, manganese, aluminum and calcium ca converted to soluble forms through chelation. Th< appearance of zinc and manganese deficiency synrj in plants under conditions favoring greater mio ganic activity, or when organic matter especially plant roots is added to the soil has been ascribed t formation of organic chelating compounds which the needed metals available (Mehlich and Drake, p. 322). Iron versenate and ferric citrate are che forms of iron that provide an available source of for plants. On the other hand, Crooke (1954) has s that nickel supplied as nickel versenate to oats in culture did not increase the nickel content of the p over that of control plants whereas nickel supplit the same amounts as the soluble ion from nickel si} increased the nickel content in the plants more 10 fold, producing toxic effects. Therefore, the nick the nickel-toxic soil studied by Crooke he thought the chelate form associated with the soil organic m and thus was readily available to plants. Under some conditions metals present as soluble may form relatively insoluble precipitates with si anions such as hydroxide, carbonate, phosphate or cate. That zinc, manganese, copper and cobalt ar< creasingly available as the alkalinity increases result from this. Bivalent iron and manganese ca made much less soluble through oxidation to oxid' hydrous oxides. Fixation. The term fixation does not appear to a precise meaning. It is used by some people to ind merely retention of a metal in the soil in a form resists leaching by meteoric water and by others to cate greatly reduced availability to plants. Bei metals can be held in the soil in so many ways ar such varying degrees there is no sharp distinctioi tween those that are exchangeable or otherwise avai and those that are not. If the energy of binding particular cation to a clay mineral, for example, ex< the cation exchange energy of a plant root, the | is unavailable to the plant. But other cations of the element sorbed in a different way may be avail Some of the zinc ions sorbed in some magnesium are thought to become incorporated within the cr lattice of the clay mineral and are not exchang' (Elgabaly, 1950). The amount of nonexchangeable on kaolinite was found to increase with grinding o Plants as a Guide to Mineralization 15 tj attice structure to destruction. Appreciable fixation j'tassium on montmorillonite occurs on drying and traction of the lattice. Chemical Properties of the Soil fe chemical composition of the soil, more than any- ixi else, determines the chemical and physical state dbence the availability of metals to plants. It also bnces the amount of metal taken up by plants. Cf-organisms may compete for some nutrient metals, nty produce substances that make metals either more hs soluble in soil water. Of the several chemical var- rij, the abundance of competitive or complementary isind the pH of the soil can have marked effects on 3 fetal content of plants. A ! :dity or alkalinity of a soil is probably the most phtant single factor likely to produce misleading re- It' in biogeochemical prospecting. It has some effect vigor of plants and determines the distribution of plant types that are sensitive to a particular in pH. Most plants that have been studied by riilturists, however, tolerate a wide range of pH for oth if the necessary nutrients are available. Probably I as its greatest effect on plant growth though con- )lhg the chemical state and availability of nutrient bsmces. It may completely immobilize a metal in the i|>y causing its precipitation as an insoluble com- ujl. For example, carbonates and phosphates of cal- irtf and magnesium and carbonates, oxides and hy- dies of iron, copper, manganese, and zinc tend to •a under alkaline conditions. Soil pH affects the ion 3hhge properties of clays and other exchange ma- tt. In soils with pH varying in depth, the part of )ot system that absorbs nutrients is governed by ariation. T;- normal range of pH in soils is between 3 and 9, rah locally the pH may be beyond this range. Cer- mlants thrive on abnormally acid or alkaline soils, '. le salt bush (Atriplex) and glasswort (Silicornia) tic 1 prefer strongly alkaline soils. In general, plants i hst suited to neutral or slightly acid soils. The soil I not constant and tends to become more alkaline fMepth. Kelley, working in Chester County, Pennsyl- ioi* reports the soil increases in acidity to a depth of citimeters then decreases (1948, p. 53). More com- tedata by Barshad is summarized in table 4 (1948, if). •H$. Examples of the relation of pH to depth in various soils. ->., „ (After Barshad) Jeptn range (inches) pH range to 80 7.3 to 7.5 t to 60 7.3 to 9.1 to 36 7.6 to 8.1 to 48 7.5 to 6.8 to 60 7.3 to 8.1 to 36 8.4 to 9.7 to 48 7.9 to 9.3 to 72 7.9 to 9.4 to 30 6.4 to 6.7 to 56 5.6 to 5.5 to 43 5.6 to 5.2 its themselves acidify soils by exchanging hydro- >ns at the root surface for soil nutrients. Parent ;ends to influence soil pH especially of residual lsyhere rainfall is low. Carbonate rocks, for example, tend to yield alkaline soils. But in areas of high rainfall the increased vegetation, the resulting humus and the leaching of more soluble alkaline components tends to produce acid soils. The mineral belts in California are, in general, in mountainous regions where the soils tend to be acid. Weathering of sulfide ore deposits tends also to acidify soils. Acidic surface waters are fairly com- mon near ore deposits. There is evidence to suggest that copper is fixed in alkaline soils. Experiments of Peech reported by Jami- son (1942, p. 296) show that only 17 percent of copper added to soil remains available at pH 4.0 and only a trace at pH 6.0. A similar test by the same worker showed 40 percent recovery at pH 3.1 and 10 percent at pH 5.0 (1942, p. 295). However, Jamison reports somewhat different results, with a soil pH of 4.0 he re- covered all of the copper and at pH of 6.0 only 33 per- cent (1942, p. 296). In spite of the lack of agreement in these data, it has been established that copper precipita- tion begins at near neutral pH and is complete in the alkaline range. Warren et al. (1952, p. 483), in field prospecting for copper, noted a similar influence of pH on the solubility of this metal. The influence of pH on the solubility of zinc is much the same as on copper. Zinc is concentrated in plants growing in soils with a pH range of 3.5 to 7, with the majority of the samples having a pH of 4 to 5 (Fulton, 1950, p. 667). Robinson et al. (1947, p. 577) report that nut and fruit trees resist the absorption of zinc in alka- line soils. The solubility of zinc in alkaline soils can be materially increased by the addition of acid to the soil (Aldrich and Turrell, 1951, p. 89). Manganese, like the previous two metals, is absorbed more readily by plants in an acid environment. Mulder and Gerretsen (1952, p. 229) have shown that manga- nese is highly soluble below pH 5.5 but with increasing pH the metal is converted to the manganic oxides (Mn +++ and Mn ++++ ) and as a result becomes less avail- able to the plant. Brenchley et al. (1936, p. 182) have observed a similar reaction where manganese is unavail- able to plants in alkaline soils. The same conclusion was reached by Fujimoto (1948, p. 132) working in Ha- waiian soils and also by Aldrich and Turrell (1951, p. 89). Aluminum is reported to be available to plants in two pH ranges: 4.5 and below, and 8.0 and above (Meh- lich, 1942, p. 121). However, previous work showed aluminum as the silicate is precipitated between pH 3 and 5 and also at pH 9 to 14 (Hartsock and Pierce, 1952). Both tungsten and molybdenum are available in the alkaline pH range. Tungsten and molybdenum are pre- cipitated as ochers in a pH range of 3 to 5 (Hartsock and Pierce, 1952; Clark, 1924). Molybdenum is ex- tremely sensitive to slight pH changes in certain ranges. Barshad has shown that a pH change of 0.8 in an acid sandy loam resulted in a hundred percent increase of available molybdenum, and a pH change of 2.7 in a clay soil more than doubled the available molybdenum (1948, p. 189). Other metals such as barium and boron have been studied by various workers. Roberts (1948, p. 74) reports that he found no correlation between barium content and pH. However, Mehlich (1942, pp. 121-122) reports the absorption of barium in inorganic soils 16 Special Report 50 reaches a maximum between pH 8 and 9 ; in organic soils between pH 6 and 8. Brenchley (1936, p. 175) states that boron is unavailable to plants in the alkaline range (1936, p. 175). The accompanying table summarizes the effect of soil pH on the availability of certain elements to plants. Table 5. Effect of pH on the Availability of Elements to Plants Elements more avail- Elements more avail- able in acid soils able in alkaline soils Aluminum * Aluminum Iron Calcium Manganese Magnesium Copper Sodium Zinc Potassium Boron Molybdenum Phosphorus f Tungsten Selenium Silicon Barium • Aluminum is available to plants in two pH ranges, (1) 8.0 and above, (2) 4.5 and below. t Most available in mildly acid soils pH 6-7. Increasingly unavailable at pH < 5.0. The uptake of a metal also can be influenced very strongly by other elements or radicals for reasons rang- ing from simple competition for exchange positions to very complex metabolic relationships that are not well understood. For example, addition of phosphate in amounts likely to be used in ordinary fertilization practice has been found to cause the increase of the molybdenum content of crop plants as much as 10 fold. Addition of sulfate as gypsum decreased the molybdenum content of the plants as much as 6 fold (Stout, et al., 1951). Barshad (1951, II, p. 397) found the effect of phosphorous on molybdenum to be much more pronounced in acid than alkaline soils. Manganese sulfate inhibits the uptake of molybdenum by cauliflower and lettuce but not by spin- ach, tomato or white clover (Mulder, 1954). The zinc content of oats decreased two- to four-fold with a 20-fold increase in the amount of calcium car- bonate in the soil and decreased almost two-fold with a 15-fold increase in phosphate (Rogers and Chih-Hwa Wu, 1948). Zinc combining with phosphoric acid in the soil forms ZnP0 4 which is unavailable to plants (Robin- son, et al., 1947, p. 597; Davis, 1941, p. 84). Increased soil nitrogen and potassium decreased the zinc content of orange leaves by as much as 5 percent (Reuther and Smith, 1950). Variations of two- or three-fold in the distribution of manganese in potato plants and in the manganese content were caused by altering the amounts of iron, potassium, phosphate, or calcium carbonate in the soil (Bolle Jones, 1955). Manganese tends to be available in the presence of sulfur, chlorides and iodides, but is fixed by fluorides and bromides (Fujimoto and Sherman, 1948). A significant increase in the uptake of nickel by oats was caused by an increase in the rate of supply of phosphorous but not by an increase in other major nutrients (Crooke and Inkson, 1955). Copper sul- fate added to peaty soils decreased uptake of iron enough to cause chlorosis of corn plants (Willis and Piland, 1936). Similar interaction between iron and manganese, copper and nickel, copper and zinc, copper and molybde- num, copper and manganese, manganese and aluminum and calcium and manganese are also reported (Mehlich and Drake, 1955). The presence of such ions as Ca ++ , * Ba ++ , Al +++ and Fe +++ has been observed to stimulal k uptake of K + , Rb + , Cs + and Br - under some cond iq (Overstreet and Jacobson, 1952, p. 190). Certain elements apparently are able to protect j i| against the toxic effect of excess metals without red a uptake of the toxic metal by the plant. Thus alum u has been found to decrease the toxicity of excess ci ier in citrus plants but does not inhibit the uptake of c I by the plants. An increase in the rate of supply of : i gen, potassium, calcium or magnesium decrease; I toxic nickel symptoms in oat plants but does not 4 the uptake of nickel (Crooke and Inkson, 1955). ' Physical Properties of the Soil The soil structure is a controlling factor in the n tion, drainage and temperature of the soil. The strn ■ is partly dependent upon the size and shape of thfl particles ; soils containing a high percentage of the i«r fractions tend to be compact, to have poor drainag oi to contain relatively little soil air. Soil temperati 1 affected by the structure, water, and humus conten nil color of the soil as well as climatic factors. Since n tion and temperature affect the rate of minera all accumulation in plants and since drainage mayB affect the local supply of aerated water and metal a these physical properties can be expected to infll the metal content of plants. The size of soil particles, metal content and availability commonly are related. Soil samplin heavy metals (lead, copper, zinc) in northern Sask; wan showed a close correlation between clay cc and amount of metal; clay soil containing up to 8 as much heavy metal as sandy soil over an unminer area (Byers, 1956). Roberts (1949, p. 73) has s the particle size of the soil may correlate with its bi content. Soil samples with an average size of 0.0' contained several percent more barium than the 1.65 mm. Barshad (1948, p. 189), working with n denum, reports that particle size has little effect c concentration of this metal in the soil. On the ba 27 samples Lovering and Huff (1950, p. 507) repc; average copper concentration at San Manuel of 361 in the silt and clay fraction of the alluvium an ppm copper in the more coarse fraction. Rudrin pp. 19-25) has demonstrated that the fine fracti gray limestone soils contain more phosphorous th£ coarse fraction. Since the clay fraction commonly mulates in certain layers or horizons in soils, size si is partly responsible for the uneven distribute trace elements in soil profiles. "Without questio greater metal content of clay-sized material I mainly from the greater ion exchange capacity in as well as the leaching of metals from more peril sandy soils. Reitemeier (1946, p. 211) working in arid re] has demonstrated the effect of increasing soil moisti) solubilities of different ions. By increasing the mcl in six different soils he found that calcium and n| sium increased in four soils and continually dec:, in one. Chloride and nitrate decreased by 32 to 5 cent on dilution. Plants as a Guide to Mineralization 17 it effect of soil moisture on pH is described by Hass )4, p. 39) ; he found pH decreased as moisture de- a d. \Ichell (1955, p. 271) has reported a remarkable bization of Co, Ni, Fe, V, Cu, and Mn in soils that > pry poorly drained. Several times as much metal sxtracted by 2.5 percent acetic acid or neutral nor- 1 mmonium acetate from poorly drained soil. Fuji- tdnd Sherman's work (1948, p. 144) with manganese in| that this metal is released by a waterlogged soil, ecby a damp soil and released by a dry soil. In- >ai:d solubility of molybdenum, silver and lead is yfli in other poorly drained profiles. The cause of this bization is probably chemical though it is also known it epeated and intensive drying of clay soils increases ■ capacity to fix some metals. A|hough the magnitude of the effect caused by varia- in,in physical properties of the soil along a traverse is kj)wn, biogeochemical anomalies that coincide with ai;es in soil structure or with soil moisture content cl;as from dry ground to swampy ground should be ■e.imined very critically. Environment Sfce photosynthesis controls salt accumulation, it is tl,r likely that plants growing in the shade will ab- rtjiifferent amounts of metals than plants growing in e im and in the same soil. The uptake of zinc by jvjr was found to increase with temperature and dura- )npf light (Ferres, 1951). Experiments with Cuban teco have shown that these plants accumulate more in'-al matter when shaded than when they are in the 1 Barshad 's conclusion is supported, at least for jftrations of less than 300 ppm, by the high molyb- a- content of many plants: corn 46.8 ppm, alfalfa pm, sunflower 15.9 ppm, tomato 34.0 ppm, and plis 281.0 ppm, none of which exhibited toxic symp- sjitiles (1946, p. 134) reported the following molyb- I concentrations in different plants: clover 156 timothy 30 ppm, rye grass 54 ppm, and Yorkshire 8 ppm. m minor amounts of molybdenum are required for »:hy plant ; but many plants concentrate this metal ■n':h as a thousand fold from the soil in which they w(Killeffer and Linz, 1952, p. 112). Lewis, as re- t( by Stiles (1946, p. 134), showed clover, a member hlegume family, assimilated more molybdenum than oier plants by ratios of as much as seventeen to one .' | an average of by five to one. ia'shad, working with 22 plants in soil of pH 8, has ldstrated that there is a difference between total yjlenum content of the soil and the fraction that .viable to the plant (average ratio 2:1). Further, sltws the accumulative powers of those plants in rr great quantities of molybdenum when the soil U is but minor amounts. iei. Average molyldenum content of plants as related to molybdenum content of soils. PLANT 2 Concentration* : 01 01 a d< n 1C ul al it - s ill Mo.. 10 6 220 200 150 150 175 80 80 80 30 27 21 18 40 33 9 7 11 5 5 3 100 40 80 50 30 28 50 15 5 17 14 10 2 2 3 4 2 54 47 50 50 77 55 30 16 ~7 13 18 7 9 4 7 13 1.5 0.8 36 16 12 12 18 14 15 13 6 5 7 2 5 2 4 2 0.5 trace 8 11 5 5 22 5 13 5 4 4 trace ale 1 Mo trace 0.8 4 3 4 subterraneum alba 5 2 7 1.3 2 n vulgare •yana. erenne 2 cicutarium -- Mod dry matter; first one fot of total soluble at pH cate; stems and leaves. ified aftei t of soil. !. Barshad (1951) Background values have to be determined in barren areas for each biogeochemical study. However, certain tentative limits have been established for molybdenum in plants by Barshad and by Robinson. Barshad 's values range from a trace to 220 ppm ; Robinson gives a range of a trace to 137 ppm. In general the average will be below 100 ppm and in most cases above 10 ppm. As a general rule, analyses of most plants growing in soils containing high concentrations of molybdenum will show a biogeochemical anomaly. However, the lesser plants, especially the legume family, reflect the soil con- centration most effectively. The following plants are well suited to biogeochemical prospecting and have been used successfully in molybdenum studies : Table 8. Plants useful in prospecting for molybdenum. Coreothrogyne filaginifolia (Wooly aster) Devils paint brush Vicia americana Medicago hespida Lotus corniculatus Trifolium repens Trifolium fragiferum Trifolium subterraneum Melilotus alba Melilotus indica Melilotus sativa Triticum vulgare Avena saliva Horedum vulgare Chloris gayana Lolium perenne Cneoridium dumosum Quercus weslenzenii (Interior live oak) Quercus douglasii (Blue oak) Misleading Anomalies Exchange Material pH, and Other Ions. As the pH and exchange material in the soil are closely related, they have a combined effect on the availability of molyb- denum to plants. Clay is the most common and effective exchange mate- rial in the soil. It is possible that clays, through differ- ential absorption, may nullify the effect of a large concentration of molybdenum in the soil. The amount of absorption depends primarily on the soil pH and sec- ondarily on the type of clay. Organic exchange materials become important only when the organic content of the soil is abnormally high. An accompanying figure, taken from Barshad 's studies with molybdenum, illustrates the relationship between pH, clay type and availability of molybdenum to plants. The pH controls molybdenum availability through its effect on solubility and through its effect on absorption of molybdenum by clay minerals. The solubility of molybdenum minerals increases di- rectly with decrease in pH of the soil solutions. As the molybdenum minerals near an ore deposit have not completely lost their identity, much of the soil molyb- denum will be in the mineral form. The acid conditions common near ore deposits may, however, promote the decomposition of the mineral compounds to release the molybdenum. An accompanying table illustrates the solubility of molybdenum minerals in water, sodium hydroxide and hydrochloric acid. Once freed from the other elements in the mineral compound, molybdenum forms complex ions with oxy- gen, the most common of which is the divalent oxide 20 Special Report 50 Table 9. Solubility of molybdenum minerals in various reagents. (Mo in ppm) Reagent Molybdenite M0S1 Ferrimolybdite Fe 2 (MoO«)3-7.5H 2 Wulfenite PbMoO« Molybdic oxide M0O3 H 2 --- 0.05 0.44 2.84 5.4 88.0 168.0 0.014 27.4 3.0 250 O.IN Na(OH)_ O.IN HC1 Very sol. 130 After Barshad (1951) Mo0 4 = . It is at this point in the chemical history of molybdenum ions that they are likely to be absorbed, by the clay fraction of the soil. Barshad (1951, p. 299) stated that soil molybdenum is in three forms: (1) as a soluble molybdenum salt, (2) as a component of soil organic matter, and (3) as the absorbed exchange anion Mo0 4 = . The newly formed complex ion is of such size that it can enter into exchange relationships with the OH radical in the clay fraction of the soil. There is a potential set up between the OH ions in the clays and the molybdenum complex which favors exchange and is governed by the pH. To illustrate this, Barshad (1951, p. 298) has shown that a removal of molybdenum from acid clays results in an increase in pH. The greater the pH, the smaller the amount of molybdenum absorbed into the crystal lattice of the clay until at pH 7.5 virtually no molybdenum is absorbed (Barshad, 1951, p. 298). Therefore, only in alkaline soils does this ion become available to plants in any significant amount. Table 10. Relationship of molybdenum in plants to soil pH. Increase of Mo in plant (ppm) 10 66 58 39 26 96 74 15 Plant pH range Ladino clover 5 to 7 Ladino clover 5.3 to 7 Ladino clover 6 to 7 Ladino clover 5.3 to 7 Ladino clover 5.3 to 7.5 Lotus corniculatus 5.7 to 7.5 Lotus corniculatus 5.3 to 7.5 Rhodes grass 7.3 to 7.9 Rye grass 7.1 to 8.5 —3 After Barshad (1951) The work of Evans, Purvis and Bear (1951, p. 118) on alfalfa in New Jersey's Nixon loam demonstrated that the molybdenum concentration in alfalfa increased with higher pH. Their results are shown in an accom- panying figure, and show clearly the changes in concen- tration with increase in pH. When the pH exceeds 7.5, molybdenum is again ab- sorbed but in smaller amounts. An accompanying table shows the reabsorption of molybdenum in the alkaline range as well as the general increase of alkalinity with depth shown by most mature soils. The concentration of molybdenum at the surface could also be attributed to enrichment by many cycles of plant growth which ab- sorbed molybdenum and subsequently contributed it to the surface soil layer after death. Walker (1948, p. 475), working with the serpentine soils of northern California, has shown these soils to be abnormally low in available molybdenum. He believed this to be a general condition of all primary soils derived from serpentine parent material. He thought it to result partly from the inherently low molybdenum content of Table 11. Relation of available molybdenum to pH at Depth Available s Soil type (inches) pH molybdenum (ii Sandy loam 0-12 7.3 3.3 12-24 7.9 2.6 24-36 7.4 2.1 48-60 8.1 1.8 Clay loam 0-12 7.3 6.8 36-48 8.7 5.8 48-60 9.1 3.0 Clay loam 0-12 7.5 2.9 12-24 7.2 2.5 36-48 6.8 1.3 Modified after Barshad serpentines; it may also result partly from the fiji of molybdenum in clay molecules. Clays derived serpentine rocks are high in magnesium, a metal chemical activity and ionic radius are similar to n denum. Table 12. Some ions that affect the uptake of molybdenum by plants. Ions that stimulate Ions that suppress the uptake of Mo the uptake of Mo PO« COa CI HCOa NOs OH SO* s N FeO Compiled from several sources. Changes in the chemical makeup of soil are dii to recognize in the field. Only under optimum cond: is it possible to correct errors that are the result fl lack of knowledge of the exact soil chemistry. 0: oi the most common sources of change in the ionic con • tion of the soil is due to a change in underlying I type, and the investigator who crosses a contact bet m two rock types without recognizing it may recor an anomaly where none exists. There are many rocks tat could introduce sufficient foreign ions to produce I a misleading anomalous effect ; some of these are ! * in an accompanying table. Table 18. Sources of detrimental ions. Rock type Ion supplied to the I Limestone and dolomite CO3, HCO3 Playa deposits (salines) S0 4 , CI, OH, N(I Basalt P0 4 Gossan S, SO, Season. Plants, as the result of cyclic chang'! their physiology, contain different amounts of mi "i material during different times of the year. The ( lii change in the assimilation of molybdenum by plant * been studied by Barshad ; his results are summariz ii the accompanying table. This cyclic change with rel to molybdenum as reported in the lesser plants ill reflected in deciduous trees. Age of the Plant and Organ. As the plant dev(pl certain chemical and physical changes take place y M alter the amount and kind of material that the ]d will require from the soil. Similar changes are repi d in the development of each organ. Plants as a Guide to Mineralization 21 a\e 14- Effect of season on molybdenum content of plants. mi Plant Location* Molybdenum in leaves Spring' Summer 2 Fall' Ife Coloma Buttonwillow Pond 16.0 9.2 18.1 30.0 20.6 8.1 2.5 4.3 1.7 3.6 3.9 4.0 20.0 11.3 32.0 44.8 43.6 31.0 14.0 4.3 7.5 5.0 4.9 28.4 tfd 18.0 77.5 Greenfields Greenfields Wasco Buttonwillow Greenfields.. .- 9 riiiculatus _ ,8 rniculatus 92.9 72.0 25.0 6.5 10.0 Pond Shatter Buttonwillow Greenfields 9.8 8.2 7.6 i iiTn County, Californ iteier-October. After Barshad (1 ia. 951) Its necessary, therefore, to sample growth of the uage. Second year stems and leaves have proven b'the most satisfactory. Tvnslocation. After absorption by the root system, ! iitrient materials are transported to different parts tji plant structure. Molybdenum, as evidenced by idiographs, moves directly into the interveinal ?ks of new leaves in most plants (Stout and Meagher, 18 p. 473). In legumes, accumulation is centered at ? fed pods (Killeffer and Linz, 1952, p. 112). Stout dfleagher (1948, p. 437) state that molybdenum is t jisorbed in the stems of plants to any large degree, t farshad found, after testing 14 plant types and 34 ufc, the highest molybdenum concentration was found bun the leaves of 16 of the plants, in the stems of »d in the roots of four. Field Studies Tj-ee areas in California were studied to determine wiiogeochemical prospecting can be used to indicate 'umally high concentrations of molybdenum in the 1 Factors (reported in the agricultural literature) itnfluence the concentration of molybdenum in soils cblants were observed in the field and their impor- tu to biogeochemical prospecting evaluated. Molyb- ntn, in the minerals molybdenite or powellite, was ra to be present in all areas. In all but one area i letal was concentrated in definite zones. The plant ^ soil, geology, climate, and topography differ ajy in the three areas. Spcimens of several species of trees and smaller ■p were collected. At certain stations old and new oth was collected for comparison, as were different [lis of the same plant. In each area soil samples Tj taken where feasible and the pH determined. The iripal clay in the soil at the Bour molybdenum mine s dentified by X-ray methods and its concentration lcflated. Molybdenum concentration in the soil in [area was also determined. To methods of plant analysis were used in these Mtes. Quantitative spectrographic methods were used Jalyze samples collected at the Bour molybdenum Table 15. Variation of molybdenum content of plant leaves of various species. Milligrams of Mo per Plant kilogram of dry matter Desert athel (tamarisk) 34.0 Black locust 15.9 Eucalyptus 1.4 Cottonwood 1.6 Yellow sweet clover 59.3 Cowpeas 141.0 Alfalfa 20.7 Perennial rye grass 11.6 Tall fescue 14.9 Prairie brown grass 8.7 Wheat stubble 26.4 Sweet corn 46.8 Mullet 50.4 Sunflower 15.9 Watermelon 14.1 Cantaloupe 1.3 Tomato 34.0 Malvarotundifolia 66.0 Cotton 21.6 Romie 11.8 After Barshad (1951) All grown in Merced clay loam, Buttonwillow area, Kern County, California. Surface foot contained 6 mgm of Mo/Kgm. pH 7.2. mine, whereas a wet chemical technique, developed by the U. S. Geological Survey, was used to analyze the samples from the other two areas. Areas for study were chosen where diverse soil, plant, and climatic conditions could be expected. These areas demonstrate, to a limited degree, the variation in results that can be expected when prospecting for molybdenum using biogeochemical techniques. Bour Molybdenum Mine At the Bour molybdenum mine, 6 miles west of Ra- mona, San Diego County, California, molybdenite is disseminated as flakes in a northwest-trending aplite dike that cuts the Cretaceous "Woodson Mountain gran- odiorite. The ore is reported to average 0.5 to 1.0 percent molybdenite. The dike ranges from 30 to 300 feet in width. A 240-foot adit driven from the northeast inter- sects the dike 200 feet below the surface. Two open cuts are south of the adit. Two biogeochemical traverses were made across the dike; one 260 feet long, bearing N. 68° E., was 300 feet south of the adit; the other, 135 feet long, bearing N. 80° E., was 700 feet south of the adit and entirely within the dike. The samples were collected in Febru- ary 1954. Biochemistry. Leaves and stems of Cneoridium dumosum were collected at each station. This plant, which is widely distributed throughout the region, is more abundant on the granodiorite, where a deeper soil is developed, than on the aplite dike. No attempt was made to sample only the youngest growth, as new growth was not obvious. The plant was in bloom at the time of sampling. Soil samples were collected at all but three stations. About 1 pound of soil was collected from 3 inches below the surface. No true soil profile is developed anywhere in the area, and existing soil forms only a thin veneer in protected depressions. The aplite dike, being more resistant to erosion, stands above the country rock and is almost devoid of soil. 22 Special Report 50 LEGEND WOODSON MTN. GRANODIORITE I 2 '♦' TRAVERSE WITH * STATIONS AND ► STRIKE GEOLOGIC SKETCH MAP OF BOUR MOLYBDENUM MINE SAN DIEGO COUNTY, CALIFORNIA 300 600 SCALE IN FEET Figure 6. Analysis. The plant and soil samples were analyzed in accordance with procedure outlined by Nichols and Rogers (1944). The plants were dried overnight at 100° C, ground in an agate mortar, then ashed in a pot furnace at 450° C. for 2 hours. Silica crucibles were used to avoid salting the samples. Suitable molybdenum standards were prepared with beryllium added as an internal standard, and a working curve was developed. Twenty mg. samples of ash were ignited between car- bon electrodes in a D.C. arc using a 4mm. gap at 15 amps. Spectrographs were obtained on a Jarrell-Ash 3.4-meter Wadsworth-mounting grating spectrograph. Line transmission was measured on a Jarrell-Ash micro- photometer, converted to line intensity, and the molyb- denum content of the ash obtained to the nearest 10 ppm by comparison with the working curve. The molyb- denum line at 3170 A was used for comparison. The soil was ground and analyzed in 100 mg. samples in the same manner as outlined for the plants. A de- termination of soil pH was made in accordance with the method outlined by the Soil Survey Manual of the De- partment of Agriculture (1951, p. 237). A 1:1 ratio of soil to water was shaken vigorously and allowed to stand for 30 minutes, reshaken and checked with a pH meter. The soil pH was slightly acid in all cases and varied only by one full unit. X-ray analyses determined the principal clay mineral present and the relative amounts in each sample. Results. The results of plant analysis show a definite anomaly over the metalliferous dike. Background values average 15 ppm molybdenum in the plant ash (Stations Bour molybdenum mine, San Diego County, Californu Traverse A-A' Traverse E Station Station 1 2 3 4 5 1 2 Molybdenum concentra- tion in Cnearidium du- mosum (ppm) 10 20 150 200 10 200 200 1 Molybdenum concentra- tion in soil 10 20 20 no soil 10 140 no soil pH of soil 6.00 6.50 6.90 no soil 5.90 6.10 no soil 6. Relative concentration of illite in soil (percent) 70 100* 60 no soil 90 85 no soil * Standard (100 percent). 1 and 2, traverse A- A') . At Station 3, which i] feet from the dike, the concentration reached 15< and at Station 4, near the center of the dike, the : denum concentration rose to 200 ppm, falling off to 10 ppm at Station 5, some 50 feet beyond thf The molybdenum concentration in the soil sho & similar but weaker anomaly over the ore body, a mum concentration was 120 ppm over background ■ imum concentration 10 ppm, in the six soil sample fl| lyzed. On the average the plant samples taken ne I ore body contained 100 ppm more molybdenum ash than did the soil samples. The soil pH ranged from 5.9 to 6.9 and had 1 apparent effect on the molybdenum anomaly. The ni cipal clay mineral present in the soil was illite. It id markedly in concentration ; the highest value w I corded at Station 2 along traverse A-A' and the a trary value recorded at that station was taken fl standard (100 percent). The lowest concentration 17 percent of the standard ; all other concentration m above 50 percent. Like the soil pH, the clay cont J the soil had no apparent effect on the anomaljl high concentration of molybdenum may have satii the exchange material in the relatively small volufl soil, rendering this effect negligible. Howeveil slightly acid condition of the soil may have inc:| the exchange eapacitv of the illite and lowered t| tensity of the anomaly. Cosumnes Mine The Cosumnes mine, 15 miles southeast of Place i El Dorado County, in section 25, T. 9 N., R. 12 E.I is on the Middle Fork of the Cosumnes River il central foothills of the Sierra Nevada. Mineralization is along the periphery of an igl] body which intrudes metasedimentary rocks of the 1 veras group. Molybdenite occurs in minor amoml flakes from one millimeter to a centimeter or m< J greatest diameter. Copper, lead, silver, and gold mi: 1 are also found with the molybdenite, which is n| northeast-trending quartz vein and contact metamel zone. The oldest rocks in the area are the metasedifl of the Carboniferous (?) Calaveras group composl a quartz-biotite-muscovite schist and recrystallized i Plants as a Guide to Mineralization 23 LEGEND &Qal#s Alluvium •if i a I Quartz vein < i Jqd + iQuartz diorite i , i ,Cls I ' I ■ I :h_Cqs"- > Calaveras group ( limestone, quartz schist)^ Toography from U.S.G.S. Omo Ranch Quadrangle, 1952 Contour interval 40 feet 200 SCALE IN 400 FEET 600 Geologic contact Mine 5 (No.) io ( Mo cone.) Sample Station 3DL0GIC SKETCH MAP OF THE COSUMNES COPPER MINE, ELDORADO CO., CALIFORNIA Figure 7. North is toward top of sketch. leTntrusive into these rocks is a biotite-hornblende rl diorite of Upper Jurassic age. A contact meta- •pic zone is developed along the southwest contact ■T«n the quartz diorite and schist. Massive grossu- teand epidote are common in this zone. A quartz I to 15 feet wide follows the igneous-metamorphic ta; and thin veins have invaded the contact zone igninor fractures. Molybdenite was not observed in qiirtz vein on the surface, being confined principally h«eontact zone. hraine has been developed along the quartz vein by sits driven from the east and a small open cut at srface. Two large dumps attest to considerable in activity. khemistry. Soil samples and leaves and twigs of 'tjT Live Oak (Quercus weslenzenii) were collected I Stations in the mine area, and at one station 2 « istant for a background sample. The samples were xd on a grid pattern with the stations spaced 50 «art in an east-west direction and 150 feet apart i merally north-south direction. A short traverse a^o made across the quartz vein. An effort was made o^ct only young growth, but not current growth, from trees of approximately the same age. The Interior Live Oak is abundant around the mine and little diffi- culty was experienced in selecting suitable samples. A deep soil covers the quartz diorite and schist but cover on the limestone is scant and there is none on the quartz vein near the mine. About 1 pound of soil was collected 6 inches below the surface at each station. Eleven of the stations were located on the quartz diorite, three on schist, one each on the limestone and quartz vein. The samples were collected in November 1954. Analysis. The plant samples were analyzed by a wet chemical technique developed by the U. S. Geological Survey (Reichen and Ward, 1951). Only the leaves were analyzed. The plant material was dried at room tem- perature for several weeks, then ashed in silica crucibles over a low bunsen flame for 1 hour. A 10 mg. sample of ash was fused with lithium nitrate to destroy the carbon- aceous material; the fusion was dissolved in HC1, neu- tralized, and reacidified. Potassium ferrocyanide and a saturated solution of stannous chloride were added. The solution was shaken with peroxide-free isopropyl ether to abstract the molybdenum complex into the ether layer. Suitable standards of molybdenum were prepared from 24 Special Report 50 • 9 LEGEND surface Tactite contacts from Krauskopf (1953) 16 %° BBS 50 SCALE 100 =3 150 FEET at de( Tactite Granodiorite sq Schist, quartzite, and m< largely covered by so Geologic contact Fault Dump Open pit and adit • 23 Sample station GEOLOGIC SKETCH MAP OF THE TYLER CREEK TUNGSTEN MINE, TULARE COUNTY, CALIFORNIA Figure 8. a 0.002 percent molybdenum solution of molybdic oxide. The concentration of molybdenum in the unknown samples was determined to the nearest 10 ppm. by colori- metric comparison with the standards. The pH of the soil samples was checked in accordance with the procedure outlined by the U. S. Department of Agriculture (Soil Survey Manual, 1951, p. 273). Results. The concentration of molybdenum in the tree samples increased near the mineralized zone from west to east. Concentration in samples collected over the quartz diorite doubled over the background (10 ppm) while concentration in samples from the schist increased 12 to 14 times over the values recorded on the quartz diorite. There was no significant increase in content in the trees growing on the quartz diorite from north to south. The principal center of molybdenum concentra- tion appears to be in the contact metamorphic zone rather than the quartz vein. This relationship is in keep- ing with the field observations. A short traverse over the quartz vein, where it was hidden by a deep soil cover several hundred feet west of the mine, showed no anoma- lous increase in molybdenum. The soil pH was higher in soil samples collectei the quartz diorite than over the other rocks. It i from 6.25 to 7.00 on the quartz diorite and from i 6.20 on the other rocks. The pH could not be shown to have a consistent! on the availability of molybdenum to the plants^ sample collected on the soil with the lowest pH contained the highest concentration of molybdenun ppm) which is contrary to what might be ex{ Moreover, the second highest molybdenum concent (120 ppm) was recorded from a tree rooted in soi a pH of 6.20. Tyler Creek Mine The Tyler Creek tungsten mine (west workin; sec. 35, T. 23 S., E. 30 E., MD., is 2 miles west oi fornia Hot Springs, Tulare County, California, elevation of 3000 feet. The tungsten mineralization is localized along si zone that cuts a roof pendant of schist, quartzitl limestone probably of the Kaweah series (Trias 1 and granodiorite (Jurassic ?) country rock (Dji Plants as a Guide to Mineralization 25 7 — •- 10 20 30 40 Mo CONCENTRATION IN BLUE OAK (PPM) 50 ■it 9. Graph showing absence of clear relationship between o entration of molybdenum in Blue oak trees and soil pH. { jKrauskopf , 1953). Seheelite and powellite are icin the pendant along a discontinuous tactite zone clj strikes generally northeast. The ore is reported vrage 0.77 percent W0 4 over the mineralized area ; eti samples assayed 29 percent. The seheelite con- is dmixed powellite in sufficient amounts (probably ri: than 0.4 percent) to draw penalties at the It (Jess Acker, lessee, personal communication, 5 j 'hi mine has been worked principally along two adits ! ( ) <> o - 1 7 O in ( ( ■> () o r-< L— --< r - — i H r J i c ) c > <) ° 6 c 1 50 <> 2*0 C Ol 0- < > <> < 30 O UI O _1 20 m o <) z c 1 ( > < > <) O o o o <> o <) o '0 < > i J I 2 6 14 15 17 IB 29 30 5 7 II 20 21 26 27 28 3 4 8 9 10 12 13 16 19 22 23 24 25 SAMPLE STATIONS — — ——— Average Overall average Figure 10. Graph showing concentration of molybdenum in Blue oak trees growing in soils of different pH overlying granodiorite, limestone, and quartzite and schist. The parent rock from which the soil is derived affects the soil pH, which in turn affects the amount of molybdenum taken up by the trees. Table 17. Comparison of new and old growth. Station Molybdenum concentration (in ppm) Current growth Previous years' growth 2 15 15 20 15 3 10 27 25 Table 19. Data from biogeochemical survey for mo Tyler Creek mine, Tulare County, California. Blue oak twigs and leaves of previous years' growth from Station 11 each contained 20 ppm molybdenum. The soil ranged in pH from 6.20 to 8.10, averaging 6.90 at 30 stations in the mine area. As might be ex- pected, the soil overlying granodiorite was the most acid; the soil derived from quartzite and schist was intermediate; and limestone soil was the most basic. The discontinuity of the tactite bodies, the movement of soil by creeping, and the disturbance of the ground by mining operations may account for the lack of any Table 18. The pH of soil overlying different rock types. Number of samples PH Rock type Range Average Granodiorite 9 13 8 6.21-7.10 6.40-7.75 6.65-8.10 6 67 Quartzite and schist 6 86 Limestone 7.22 Station Molybdenum content of Blue oak* SoilpH Underlying 1 40 15 10 15 20 15 10 15 40 15 20 15 15 10 5 45 15 35 15 30 30 15 20 10 15 35 25 25 20 10 6.60 6.75 6.65 6.60 7.80 6.95 7.20 7.15 6.85 6.80 6.65 7.60 6.40 6.20 6.50 7.75 7.10 6.65 7.20 7.70 6.70 6.40 6.80 6.40 6.60 6.70 8.10 6.90 6.50 6.75 Granodiorite Granodiorite Quartzite and Quartzite and Limestone Granodiorite i Limestone Quartzite and Quartzite and Quartzite and Limestone Quartzite and Quartzite and Granodiorite Granodiorite Quartzite and Granodiorite Granodiorite Quartzite and Limestone? Limestone Quartzite and Quartzite and 1 2 3 4 5 6 7 8 9... 10 11 12 13 14 15 16 17.. .. ._ 18 19 -__ 20 21 22 23 24.. ._ . _ Quartzite and 1 1 25 Quartzite and 1 1 26 Limestone 27.. Limestone? 28 Quartzite and 1 1 29'. Granodiorite J 302 Granodiorite | * Twigs only (in ppm). 1 Background sample approximately J mi. from mine. 2 Background sample approximately I ml. from mine. Plants as a Guide to Mineralization 27 fiite molybdenum concentration pattern in this area. wver, as a relatively small area was sampled, it is e" that the entire mineralized zone is positive for ibdenum and that some factors other than the molyb- atn concentration in the soil have partly determined ? lolybdenum concentration in the trees. Sice this study was made, the present operators of j yler Creek mine have driven an adit N. 60° E. for i:tance of about 300 feet near the former workings. halite mineralization was encountered at depth be- ei Stations 20 and 21 where concentrations of 30 n molybdenum were found in the plants. At Station , inhere the highest molybdenum concentration (45 n was recorded in the plants, an ore body was found leveraged 0.5 percent W0 3 . The total extent of this d; is not known ; however, it has been exposed for let in length and 15 feet in width. These discoveries r made without knowledge of the biogeochemical !('. SUMMARY AND CONCLUSIONS B)geochemical prospecting has not developed to the h where its full potentialities can be realized. If this tibecome a reliable method of prospecting, then cer- xrefinements in application and interpretation are cesary. A least 19 factors other than the metal concentra- >nin the soil play an important part in determining 3 letal concentration in plants ; at least seven of them e ontrolling factors. These seven are : soil pH, ex- a:?e material, other ions, plant type, age of plant and K, translocation, and season. Te field studies reported here provided a means for it,ig some of these factors. If the effect of one factor tfbe separately evaluated in the field, then areas must = t-osen where the other related factors remain con- u. Te field studies made for this report, made at lo- bes where, as nearly as possible, four of the seven n oiling factors were constant, showed that the anom- 7 etected was due primarily to the high molybdenum a'-ntration in the soil. It was, therefore, a direct fl'tion of the underlying ore zone, or was closely tad to it. Additional proof that the underlying ore psit was the responsible factor was provided by soil ia"ses made from samples taken in the Bour mine w, which showed that the concentration of molyb- mn in the soil fluctuated in the same manner as the lKntration of molybdenum in the plants, providing •ay identical anomalies. The overall concentration in e oil, however, was lower than in the plants, showing a the plants were a more sensitive prospecting instru- 3J. 1 e data collected at the Cosumnes mine showed con- l«able variation in soil pH, and soil ions, which may iv been due to the three underlying rock types. The 'eige soil pH was definitely higher over the quartz ote than over the metamorphic rocks. However, this e'r had little apparent influence on the metal con- a )f the plants as the highest molybdenum values were cded from soils with the lowest pH. Molybdenum nmtration increased as the ore zone was approached :cot at one station where there is some evidence that the suppressing effect of C0 3 and HC0 3 ions, derived from the underlying limestone, reduced the uptake of molybdenum. As at the Bour mine, the high concentra- tion of molybdenum in the soil had a greater effect on the metal concentration in the plants than did the other controlling factors. More than half of the plant samples collected at the Tyler Creek mine showed no anomalous concentration of molybdenum and there was no uniform concentration pattern related to the ore zone. The data point to the underlying rock type as a principal influence in regulat- ing the soil pH; therefore the rock type might be ex- pected to have some influence on molybdenum concen- tration in plants. Unfortunately, the data here are insufficient to demonstrate such an influence conclusively. Nevertheless, it will probably be necessary in future investigations to apply a correction factor when trav- ersing from one rock type to another. The anomalous molybdenum content of the plants was considerably lower in the Tyler Creek mine area than at the other two mine areas studied. This may have been due in part to the molybdenum concentration in the soil although, in view of the rather erratic results, it seems likely that the plant type sampled or the high concentration of C0 3 and HCO3 ions in the soil at certain stations may have reduced the assimilation of molybdenum by the plant. This area then illustrates to a limited degree how the other factors can obscure and disperse a biogeochemi- cal anomaly. The fact that most of the plant samples show a greater than normal molybdenum concentration is evidence that the soil molybdenum concentration is still the principal controlling factor in this area. Future work in biogeochemistry should be directed toward the accumulation of data on plant-metal-soil relationships for all of the economic metals. The char- acter of the metal as it is related to the soil chemistry and to the geology appears more vital than the influence exerted by the plant. It is true that certain physiological features of plants exert a significant influence in biogeo- chemistry, but on the whole all plants react with a certain degree of consistency. The important controls lie in the soil chemistry. The behavior of molybdenum is not yet completely understood and there is a real need to extend the studies begun in this investigation. 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