Biogeochemical anomalies

ContentsContents

IntroductionGeochemical anomaliesBiogeochemical anomaliesBiogeochemical surveyGeobotanical Indicators Geobotanical Survey TechniquesConclusion References

Introduction Introduction Geochemical exploration also known as geochemical prospecting and

exploration geochemistry, is the search for economic deposits of minerals or petroleum by detection of abnormal concentrations of chemical elements or hydrocarbons in surficial materials such as soils, waters and plants.

Geochemical prospecting for buried ore deposits is an ancient technique. For thousands of years prospectors have sought iron and copper stains on rocks as possible indicators of mineralization below; the strains are geochemical anomalies that arise as a result of the interaction of the atmosphere and rainwater with a sulphide mineral deposit, and the dispersion of the oxidized products so formed.

The various methods of geochemical exploration may be classified according to the type of sample in the search for ore deposits

Litho geochemical method Pedo geochemical method Hydro geochemical method Biogeochemical method

Geochemical anomaliesGeochemical anomalies

Geochemical anomalies have been identified by setting threshold values, which mark the upper and lower limits of normal variation for a particular population of data. Values within the threshold values are referred to as background values and those above or below as anomalies. In mineral exploration interest is generally in positive anomalies, on the assumption that ore deposits and their weathering have increased element abundances above normal crustal levels. However, negative anomalies can also be important, for example where they reflect depletion in some elements during host rock alteration accompanying ore formation.

At many locations within and on the earth concentrations of some element or set of elements are found which are much higher than the background concentrations normally encountered in the host material. The host can be any naturally occurring substance such as rock, soil, a stream or lake sediment, glacial debris, vegetation, or water, and the abnormal concentration.

Biogeochemical anomaliesBiogeochemical anomalies

Biogeochemical anomalies are areas where vegetation contains an abnormally high concentration of metals.

Different species of plant take up different amounts of inorganic material from the soil and the peculiar habit of each plant must be understood.

Deep rooted plants that obtain their water from the zone of saturation below the water table phreatophytes are in contrast to shallow rooted xerophytes that can survive exclusively on vadose water derived from rainfall.

The contrast between the background and anomalous values are related to the mobility of element in soil solutions where the plant is deep rooted directly over an ore deposit or in a superjacent soil anomaly. In such cases, the biogeochemical anomaly will also be superjacent.

It has been observed that many species of plants take up anomalous metals from an ore body located at depth of 10-15mts.For e.g., in Arizona live roots have been reported at depths of 55 m in an unconsolidated overburden overlying porphyry copper deposits.

The concentrations of trace elements decreases in the order of leaves, twigs, root and bark.

In cases where the parent anomaly is lateral groundwater or hydromorphic soil anomaly the resulting biogeochemical anomaly will also show the corresponding displacement with respect to the ore.

Controlling factors: the plant species sampled, the plant organ sampled, age of the plant or organ depth of the root system, health of the plant amount and direction of sunlight, soil, Eh & pH, soil moisture and interferences due to other elements in the soil.

Biogeochemical survey Biogeochemical survey Biogeochemical surveys are used in cases where vegetation can be used as a test medium. The first stage is orientation survey during which the following aspects are determined.

The optimum species based on distribution, contrast of anomalies, ease of recognition, ease of sampling and depth of the root system.

Part of the plant to be sampled: twigs, leaves, roots, bark, wood. Best indicator elements Effects of sunlight, drainage and interference of other elements Amount of vegetation required to give adequate ash. Contamination from dust or other sources Sampling pattern and intergrowth.

In biogeochemical survey deep rooted plants are generally preferred to shallow rooted plants. In areas where densely populated vegetation are to be sampled the sampling points can be plot as geometric grid of profile lines. In areas where vegetation is widely scattered, sample points can be in places where they are available. During the chemical analysis burning of plant organs should be carried out with a relatively low oxygen supply so that the sample does not burst into flames but with sufficient oxygen supply to minimize volatization of an oxidized organic matter.

Geobotanical Indicators Geobotanical Indicators

Geobotanical studies have been used since ancient times as a prospecting tool. It has long been known that specific plants or plant communities are indicative of a high concentration of a given chemical element in the soil.

All living plants respond to changes in physical, chemical and biological environment which is reflected by their characteristic habit of growth. Geobotany is the study of plant in relation to their geological environment many factors other than geologic factor can have a major influence on the health and major distribution like sunlight, length of season.

Morphological changes in plants due to anomalous concentration of trace elements include dwarfism, gigantism, mottling [dots on leaves] and chlorosis [yellowing of leaves], abnormal fruits, changes of flower color and disturbances in the rhythm of growth and flowering. These abnormal colors and morphological changes of plants are caused by the presence of poisonous element in the nutrient solution and referred to as toxicity.

There are two types of indicator plants:

Universal indicators Local indicators

Universal indicators are found only on mineralized soils and do not grows elsewhere. Universal indicators are extremely useful within limited areas of similar climate, geology and topography.

e.g., Calamine violet which grows only on soils rich with anomalous Zn content.

Local indicators are species that grow preferentially on mineralized ground within limited areas but grow over non-mineralized areas in other regions.

e.g., a local indicator of copper in Arizona is the California poppy, observed over outcrop of the san manual copper porphyry deposit and at other deposits.

Calamine violet

California poppy

Crotalaria cobalticola [cobalt]

Astragalus [selenium]

Deschampsia flexnousa [lead]

Eriogonum ovalifolium [copper]

Oceum homblei [copper]

Elements Effects

Aluminum Stubby roots, mottling.

Boron Dark foliage ,deformed internodes .

Chromium Chlorosis of leaves .

Elements EffectsCobalt White dead patches on leaves .

Copper Purple stem, stunted roots, chlorosis of leaves and dwarfism .

Iron Darkening of leaves, thickened roots.

Elements EffectsManganese Curling of leaf margins,

distortion of laminae .

Molybdenum

Stunting, yellow -orange coloration.

Uranium Abnormal number of chromosome in unusual shape of fruits.

Zinc Dead areas on leaf tips , stunted roots .

Geobotanical Survey Geobotanical Survey Techniques Techniques

i. Population map of all species growing both in the vicinity of known but undisturbed mineralization and in barren areas of other similar characters to be made.

ii. Indicators plants are best found and mapped when they are in bloom, mapping plants at other times of the year may be done but work progresses at a slower rate and there is a chance of missing certain useful species as indicators of ore deposits.

iii. Once a system of identifying geobotanical indicators has been established, a geobotanical survey consists merely in plotting their occurrence on a map and then sketching the outlines of areas where they occur.

iv. Remote sensing techniques can be developed for a rapid survey because the IR radiations are very sensitive incase of plants it has been established that vegetation with metal toxicity has a different reflection spectrum than normal vegetation.

Geobotanical surveys are useful in groundwater exploration

because desert plant respond spectacularly to the availability of water. Phreatophytes indicate a water table within the reach of their root system ,in contrast xerophytes have shallow roots which depends on occasional rains for their survival ferns of considered to indicate water log soil or ground water seepage areas. A high salinity is indicated by the presence of halophytes plants with their roots in bituminous soil tend to have peculiar form distinguished by their gigantism and deformed form of these plant show a tendency for abnormal blooming.

Geobotanical anomalies have the advantage of rapid scanning of large areas at low cost however the problems in such method are related to identification of species, plant spacing, depth and type of over burden etc.,

Phreatophytes

Xerophyte

Halophyte

Conclusion Conclusion

Biogeochemical exploration constitutes an important component of mineral exploration.

It compliments other methods of mineral exploration when effectively applied in the search for ore deposits.

References References

Books: Geochemistry in mineral exploration by A W. Rose, H E.Hawkes &

J S Webb. Academic press. pp 657. Courseware of Geological survey of India 2005. Prospecting and Exploration of Mineral Deposits by M. Kuzvert &

M. Bohmer. Elsevier publication, 1986. pp 51-110.

Websites :www.google.com www.wikipedia.com www.geology.com