Download Hydrothermal Deposits

S.No.
1
2
TOPIC
INTRODUCTION
MINERALS OCCURING IN FORM OF HYDROTHERMAL DEPOSITS
3
ORIGIN OF HYDROTHERMAL FLUIDS
4
HYDROTHERMAL PROCESSES
PAGE No.
3
4-5
6
7-10
5
REQUIREMENTS FOR HYDROTHERMAL DEPOSITS TO FORM
11
6
FACTORS RESPONSIBLE IN METAL DEPOSITION FROM
HYDROTHERMAL SOLUTIONS
12-22
7
DIFFERENT FORMS OF HYDROTHERMAL DEPOSITS
23-30
8
DISSOLUTION OF METAL COMPLEXES IN SOLUTION
31-32
9
10
HYDROTHERMAL DEPOSIT FORMATION IN OCEANS
CONCLUSION
33-35
36-37
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 :
2_
_
_
Highly active fluids containing ions such as O ,F ,OH etc. to form complexes
metal ions and dissove metals in it
Suitable pathways( cracks,fissures,pores and other open spaces) for the fluid
solutions to pass through surrounding rocks
Physico-chemical factors responsible for deposition
with
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
2−
be fully coordinated with anions such as O 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
_
2_
In order to dissolve these _metal ions fluids should contain active ions such as O ,Cl
− −
−
2
OH , F , HCO3 , and CO3
etc. which forms complex ions with metals and metal is dissolved
for example :
+
Fecl2 and Fecl are the main complexes of iron involved in hydro-thermal transport of ferrous iron
+
Mangnese is disolved by forming MnCl and MnCl2
_
Molybdenum combines with hard such as O2 and OH
+
+
to form complexes MoO2 and Mo(OH)3
_
With hard bases such as
O2−, OH−,
F−, and CO3 molybdenum forms
_,
−
2
complaxes such as WO4 , HWO4 and H2WO4
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 metalcarrying 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