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TOPIC

INTRODUCTION MINERALS OCCURING IN FORM OF HYDROTHERMAL DEPOSITS ORIGIN OF HYDROTHERMAL FLUIDS HYDROTHERMAL PROCESSES REQUIREMENTS FOR HYDROTHERMAL DEPOSITS TO FORM FACTORS RESPONSIBLE IN METAL DEPOSITION FROM HYDROTHERMAL SOLUTIONS DIFFERENT FORMS OF HYDROTHERMAL DEPOSITS DISSOLUTION OF METAL COMPLEXES IN SOLUTION HYDROTHERMAL DEPOSIT FORMATION IN OCEANS CONCLUSION

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Hydrothermal deposits are important igneous ore deposits.It involves mineral deposits formed as a result of cooling of gaseous and liquid solutions (or hydrothermal fluids) when it pass through open spaces like cavities,pores, cracks and fissures etc. of the host rocks.The hydrothermal fluids contains various metallic elements like copper,tungsten,gold,molybdenum,lead,zinc, And silver etc with it.When it migrates through rocks deposition of these metallic Elements takes place and hydrothermal deposits are formed.Hydrothermal deposits vary in shapes and sizes depending upon the nature of open spaces in which deposition occurs and their distribution are also irregular in the host rocks. Processes involved in forming hydrothermal deposits include the transportation of elements dissolved in hot water and subsequent precipitation or crystallization of minerals when the water cools

Sometimes the elements precipitate in their native form such as pure gold or copper but more often they precipitate as sulphide minerals like pyrite(FeS), galena(PbS), sphalerite(ZnS),cinnabar (HgS) and chalcopyrite(CuS).

Some Copper Ores

 Some hydrothermal fluids originate from magma that has water dissolved in them.As

the magma cools and crdstallizes, the water is excluded from the growing crystals from the magma.such fluids are very hot and rich with elements dissolved from the magma.  Others are formed from rainwater or seawater that circulates deep within the crust that interacts with newly created volcanic rock on the ocean floor.

 Dehydration melting of hydrous minerals

Water enters the ground

Hydrothermal Processes

Hot Springs:

form when heated groundwater reaches the surface

Geysers:

form where a complicated plumbing system allows steam pressure to be built up, causing intermittent eruptions Heated water begins to rise

Hot spring :

Rainwater circulates deep within the crust and vaporises

The magma body heats and circulates the hydrothermal solutions carrying the metallic compounds The metals become concentrated by circulating hot fluids When the fluids become cooler (near surface or away from the magma body), the metallic minerals are deposited to form rich mineral veins

For hydrothermal deposits to form a number of criteria needed to be satisfied like :  Highly active fluids containing ions such as O 2_ ,F _ ,OH _ metal ions and dissove metals in it etc. to form complexes with  Suitable pathways( cracks,fissures,pores and other open spaces) for the fluid solutions to pass through surrounding rocks  Physico-chemical factors responsible for deposition

Factors responcible for formation of hydrothermal deosits from mineral –laden Solution are: • • • • • • Temprature Pressure Fluid mixing/dillution Fluid-rock interactions Replacement mechanisms Adsorption

Temprature : Temprature plays an important role in controlling the metal solubility in complexes

from the above graph it is evident that for metal-chloride complaxes stability increases as a function of temprature. Temperature decrease is particularly effective for destabilizing metal–chloride complexes because their solubilities are much more sensitive to temperature changes. Near surface environment, due to rapid decreases in the temperature cooling plays an important role in ore deposition.But the same does not occurs deep inside the earth surface.

Pressure : Pressure variations do not dramatically effect the solubilities of metal–ligand complexes although it is clear that pressure increases will lead to a volume reduction which, in turn, promotes the dissociation of complexes to ionic species In effect a decrease in pressure tends to favor an increase in solubility and, therefore, works in the opposite sense to temperature

Fluid mixing/dillution : The mixing of two fluids acts as another important mechanism for reducing solubility in ore-forming solutions and promoting metal precipitation. This is particularly the case when a relatively hot, metal-charged ore fluid mixes with a cooler and more dilute solution. Mixing of the two fluids would result in cooling of the hotter with modification of the prevailing ore fluid properties and destabilization of existing metal–ligand complexes. This is because mixing of a hot, saline, metal-charged magmatic fluid with a cooler, more dilute meteoric water promotes acid leaching of the host rocks, increases their permeability, and forces the fluids to condense and precipitate their dissolved metal solute.

Following digram illusterates the mixing or dillution of fluids :

Fluid-rock interactions : The interaction that occurs between a fluid and its surrounding rock promotes metal precipitation because it is another process that changes the prevailing fluid properties, especially in terms of acidity (pH) and redox state. we know that at shallow crustal levels ore deposition takes place by open space filling whereas at deeper down levels where porocity is restricted so deposition occurs by replacement mechanism. in this mechanism deposition of metals is achieved by changing the properties of hydrothermal fluids. for example oxidaton of ore fluid that is particularly effective in decreasing the solubility of metal-ion complexes.oxidation(loss of electron) cause a decrease in the pH and also the total sulfide concetration,thereby promoting metal precipitation as shown in following reaction for example:

fig. shows the increase in permeability with increase in fracture density

Replacement mechanism It occurs at deep crustal level.Replacement occurs when the original minerals in the rock are dissolved and simultaneous prcipitation of secondary mineral occurs. In contrast to more permeable near surface environment where ore depositio takes placeby open space filling, metal precipitation deeper in the earth crust occurs as fluids percolate along poorly interconnected microfractures and pore spaces Thus, the process of replacement will only persist if porosity is maintained and if the accompanying fluid–rock reactions are characterized by a reduction in the molar volume of the mineral being replaced.

Adsorption :

Ores can also form by adsorption of metal onto an existing mineral surface.

Metal deposition by adsorption can occur from fluids whose concentrations are below their saturation levels and the process may, therefore, be important in certain ore-forming environments. Adsorption occurs because a mineral surface will inevitably contain charge inbalances created by the fact that metal cations will not always be fully coordinated with anions such as O 2− or S − Sites of high charge density (either positive or negative) on a mineral surface represent the locations where adsorption of oppositely charged ions is likely to occur. In general, a mineral surface in contact with an acidic solution (i.e. a high activity of H + ) will contain an abundance of positive charge and is likely to adsorb anionic complexes. Conversely, a mineral surface in contact with an alkaline solution (with a high activity of OH − ) will show a tendency for a surplus negative charge and will adsorb cations.Thus adsorption of many different metals onto the surface of a range of silicate, oxide, and sulfide minerals can be an efficient way of forming an ore deposit.

fig. showing adsorption on mineral surfaces

Hydrothermal deposits occur in a variety of shapes and sizes depending upon the size of open spaces available in surrounding rocksthe most common forms are veins and cavity fillings .

Veins are narrow ,elongated or tabular shaped economic minerals occurring within host rock.veins are formed when mineral gets deposited along cracks or fractures available in the host rock.

The dark colored layers are chalcopyrite, sphalerite and galena – valuable ore minerals The white layers are quartz – a useless “gangue” mineral which must be removed

Hydrothermal vein deposits NOTE: The distribution of hydrothermal deposits are irregular in host rocks

• • • • Veins are further classified into following types:

Fissure-veins Ladder-veins Gash-veins Stock works

Fissure veins

Ladder veins

Gash veins

• Cavity fillings are formed when ore gets deposited in well defined open spaces Like cavities rather than along cracks or fractures available in the host rocks. • Saddle reef is an intresting cavity filling .In saddle reef cavities are associated With folded rocks and minerals get deposited in such cavities

Saddle reef

Pure water can not contain heavy metal ions In order to dissolve these metal ions fluids should contain active ions such as O 2_ ,Cl _ OH − , F − , HCO3 − , and CO 3 2_ etc. which forms complex ions with metals and metal is dissolved for example :  Fecl 2 and Fecl + are the main complexes of iron involved in hydro -thermal transport of ferrous iron  Mangnese is disolved by forming MnCl + and MnCl 2  Molybdenum combines with hard such as O 2 _ to form complexes MoO 2 + and Mo(OH) 3 + and OH _

 With hard bases such as O2−, OH−, F−, and CO3 molybdenum forms complaxes such as WO 4 2− , HWO 4 _, and H 2 WO 4  Antimony form soluble comlexe Sb(OH) 3 Similarly all the metal ions present in the solution are soluble complexes

Thus at the end we can conclude that Hydrothermal mineral deposits are those in which hot water serves as a concentrating, transporting, and depositing agent. They are the most numerous of all classes of deposit. It is interesting to know that Hydrothermal deposits are never formed from pure water, because pure water is a poor solvent of most ore minerals. Rather, they are formed by hot brines, making it more appropriate to refer to them as products of hydrothermal solutions. Brines, and especially sodium-calcium chloride brines, are effective solvents of many sulfide and oxide ore minerals, and they are even capable of dissolving and transporting native metals such as gold and silver Hydrothermal deposit various useful and precious elements like gold , copper , tungsten,molybdenum and to some extent silver , lead and zinc etc. Formation of hydrothermal ore deposits is linked not only to the generation of significant volumes of fluid in the Earth’s crust, but also to its ability to circulate through rock and be focused into structural conduits (shear zones, faults, breccias, etc.) created during deformation.

The ability of hydrothermal fluids to dissolve metals provides the means whereby ore-forming constituents are concentrated in this medium. Temperature and composition of hydrothermal fluids (in particular the presence and abundance of dissolved ligands able to complex with different metals), together with pH and fO2, control the metal-carrying capability of any given fluid. Precipitation of metals is governed by a reduction in solubility which can be caused by either compositional changes (interaction between fluid and rock, or mixing with another fluid), or changes in the physical parameters (P and T) of the fluid itself. Economically viable hydrothermal ore deposits occur when a largevolume of fluid with a high metal carrying capacity is focused into a geological location that is both localized and accessible, and where efficient precipitation mechanisms can be sustained for a substantial period of time.

INTRODUCTION TO ORE FORMING PROCESS

:by Laurence robb

ENGINEERING GEOLOGY :

by Praveen singh