GEOCHEMICAL CYCLE AND MOBILITY OF ELEMENTS

CONTENTS INTRODUCTIONGEOCHEMICAL ENVIRONMENTGEOCHEMICAL CYCLEGEOCHEMICAL DISPERSIONGEOCHEMICAL MOBILITYHYPOGENE MOBILITYSUPERGENE MOBILITYASSOCIATION OF ELEMENTSCONCLUSIONREFERENCE

INTRODUCTION Geochemistry refers to the

determination of the absolute abundant of elements in the earth.

Geochemistry as applied in mineral exploration is mainly concerned with the preparation and interpretation of geochemical maps rater than the investigation of basic principles

Geochemical prospecting can be realized only through a understanding of the principles that govern the distribution of element in the earth

CntdGeochemically and geologically the

earth is a is dynamic system in which materials move from one place to another and change in composition by a variety of geological process like melting , crystallization, erosion, dissolution, precipitation etc….

GEOCHEMICAL ENVIRONMENT Based on the differential pressure

temperature and chemistry the geochemical environment of the earth can be broadly classified into deep seated environment and surfacial environment .

The deep seated environment extends downwards from the lower levels of circulating meteoric water to the deepest level at which normal rocks can be formed.

It is an environment of high temperature and pressure, restricted circulation of fluids, and relatively low free oxygen content

Continued

The surfacial environment is characterized by the process of weathering, erosion, and sedimentation at the surface of the earth.

It is characterized by low temperatures, nearly constant low pressure, free movement of solutions, and abundant free oxygen, water, and CO2.

Geochemical cycle The movement of earth materials

from one environment to another can be conveniently visualized in terms of a closed cycle referred to as the geo chemical cycle

Geochemical cycle

continued In the geochemical cycle starting on the right hand

side of the diagram and moving clockwise direction Sedimentary rocks are progressively metamorphosed

as they are subjected to increasing temperature, pressure, and increments of new materials from out side the system.

The may eventually attain a state of fluidity such that on recrystallization they can differentiate into various kinds of igneous rocks and hydrothermal extracts.

When erosion brings the resulting suite of rocks into the surficial environment again, the component elements are relative solubility in water. redistributed by weathering agencies primarily in accordance with their

A new series of sedimentary rocks is then deposited ,and the cycle is closed

The geochemical cycle therefore includes both deep seated process of metamorphism and igneous differentiation and the surficial process of weathering, erosion, transportation and sedimentation

GEOCHEMICAL DISPERSION

Any material on the earth is constantly subjected to transformation as process through different environment in the geochemical cycle.

This process in which atoms and particals move to new locations and geochemical environment is referred to as geochemical dispersion.

Geochemical dispersion may occur by mechanical process or even chemical and biological factors .

continued

The response of an elements to dispersion depends on its mobility that is how easily it can be dispersed relative to other associated material.

continued

The distribution of elements of the constituents rocks and minerals in response to various geological process are referred to as geochemical mobility.

GEOCHEMICAL MOBILITY Fundamentally, the response of an element to

dispersion process is governed by its mobility, that is the ease with which it may be move in any given environment

Mobility depends an the mechanical properties of the mobile phase, and factors such as the viscosity of magmas and solutions and the size shape , and density of clastic grains in following scheme water.

The relation between mobility and chemical differentiation, an other hand is rather more involved .

Here the prime factors are the changes in relative stability of the various coexiting fluid (mobile) and crystalline (immobile) phases that result from changes in the environmental condition.

MOBILITY UNDER HIGH P-T CONDITIONS (HYPOGEN MOBILITY)

In crystallization of magmas ,the constituents that can not be accommodated in the lattice structures of stable rock forming minerals are concentrated in mobile residual fluide.

Water, which cannot be entirly accommodated in the minerals of igneous rock , is progressively enriched in the residual fluids .

Hydrothermal vein deposits may be precipitated at this stage in the primary geochemical cycle .

At high P-T the hydrous minerals of sedimentary rocks becomes unstable and water is released to become principal constituent of the mobile phase.

Continued

The occurrence of an element as a characteristic constituents of deposits formed from fluids of various kinds,either vapors,supercritical solutions,or normal aqueous solutions,may also be an indication of their hypogene mobility.

In this group are the elements of complex pegmatites,hydrothermal vein deposits,juvenile water, and gaseous emanation.

Supergene mobility of elements siliceous sulfide-free environmentRelative mobility

Very mobile Moderately mobileModerately immobileVery immobile

Major elements

S,clCa,na,mg ,k

Si,mnFe,Al,Ti

Minor elements

Br,l,mo,b,seZn,ba,u

Ni,co,cu,as,pbCr,Rare earths

ASSOCIATION OF ELEMENTSIn a geochemical survey an elements

measured to detect and ore body is referred to as an “INDICATOR ELEMENTS”.

In many situation the indicator elements is the same element as the ore deposits. Example-Element gold for gold deposits.

If the ore element is difficult to analyze then another element associated with the ore will be useful which is referred to as a “PATHFINDER ELEMENT”

Example- Arsenic for gold ,

PATHFINDERS TABLE Pathfinder Material Ore

typeelements sampled

. As Wall rock , residual soil vein type Au ore Hg Wall rock & soil Complex Pb-Zn- Ag ores Se Gossan , residual soil Epigenetic

sulfides Ag Residual soil Ag-bearing Au ore So4 Water sulfide deposits

CONCLUSION

The movement of earth material from one environment to another can be explained by this geochemical cycle.

Geochemical exploration is mainly governed by mobililty of elements and the geochemical cycle and geochemical environments help understand the mobility of elements.

REFERENCE

Geochemistry in mineral exploration H . E . HAWKES and J . S . WEBB, HARPER and ROW,PUBLISHERS.

( 1979). Pp. 10 - 23 WEB SITE -WWW.GOOGLE.COM http://en.wikipedia.org http://geochemical cycle.org

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