vein deposits

1. Vein deposits of tin and tungsten
Definition: Vein Deposits
• Veins are mineral deposits which form when a
preexisting fracture or fissure within a host rock
is filled with new mineral material.
•The deposition of minerals is typically
performed by circulating aqueous solutions.
•Many ore deposits of economic importance
occur in veins.

2. • Vein deposits are believed to form when
aqueous solutions carrying various elements
migrate through fissures in rock and deposit
their burden onto the fissure walls.
• Hot, rising water escaping from cooling igneous
plutons may deposit minerals as it ascends
through the crust.
• As heated magmatic waters rise, the
temperature and pressure of their environment
drop and minerals exsolute and crystallize.

3. • Meteoric ground water may also percolate
down through the earth's crust, dissolving
surface minerals and gaining heat from the
geothermal gradient or from nearby igneous
intrusions.
• At greater depths the dissolved substances may
precipitate and crystallize along the walls of the
fissures and cavities through which the water
travels.

4. • Most vein deposits are formed as new mineral
species are precipitated onto rock walls which
themselves remain unaltered.
• In such cases mineral deposits fill the original
crack or fissure in the host rock but do not
extend into the host rock itself.
• The boundary between host rock wall and
deposited vein minerals therefore remains
clearly delineated.

5. • Vein deposits of this nature are a type of
hydrothermal deposit because the mineral
species which compose the veins were
precipitated by hot waters.
• However, sometimes the preexisting rock wall
which contains the vein undergoes alteration.
• Portions of the host rock may either dissolve and
be transported away or else react chemically with
the circulating volatile fluids or the newly formed
mineral species.
• In this case the boundary between vein deposit
and original rock wall will be unclear.

6. • If most of the mineralization process occurs
within the space once occupied by unaltered wall
rock then the vein is termed a hydrothermal
replacement deposit.
• A hydrothermal replacement deposit occurs
when hot circulating aqueous solutions replace
the original rock with new mineral species.
• This typically occurs in more soluble rocks such as
limestone.
• Hydrothermal replacement deposits are a form of
hydrothermal metamorphism or metasomatism.

7. Tungsten deposits
• Wolframite, opaque mineral composed of
ferrous and manganous tungstate, (Fe, Mn)
WO4, and existing as an isomorphous mixture of
the minerals ferberite, FeWO4, and huebnerite,
MnWO4.
• Tungsten deposits occur in the form of veins,
stockworks, skarns and placers.
• The vein deposits are numerous and occur
among intrusive granitic masses, generally in
their convex parts and in rocks overlying the
intrusive body.

8. • Stockwork deposits occurs in all outcrops of granitoids
and are usually large, measuring hundred of meters in
length and tens to hundreds of meters across.
• Mineralization is confined to a network of small cracks,
filled with quartz streaks containing wolframite, pyrite,
arsenopyrite, scheelite, molybdenite, beryl and
sometimes cassiterite.
• The metal content of the ore is usually low (fraction of
1%) but the reserves as a rule are large aggregating tens
of thousands of tones.
• Despite their low wolframite content, deposits of this
type are attractive economically because they can be
worked by the cheap strip mining technique.

9. • Skarns deposits are usually found in the contact
zone of intrusive bodies.
• The ore bodies are of irregular shapes.
• In some places mineralization is disseminated for
the entire thickness of skarn, while in other it
occur in patches.
• In skarns, mineralization is usually represented by
scheelite, molybdenite, and partly tinstones, gold
and chalcopyrite.
• The ore carry varying amount of tungsten.
• The deposits are often large and of great
economic importance.

10. • Placer deposits are loose or cemented
accumulation of fragmental materials,
containing grains or crystals of heavy resistant
mineral such as scheelite, wolframite, tinstone
and gold.
• Tungsten placers are derive through the
destruction of vein, stockwork and skarn
deposits as well as of rocks in which these
minerals occur as fine impregnation.

11. • Tungsten mineralization is universally
associated with granitic rocks and occurs in or
near them.
• All the granitic rocks in the younger orogenic
belts have higher tungsten contents in
comparison to the similar rock of older
orogenies.
• The mineralization associated with older
orogenies has dominantly Scheelite Province
whereas the minimization associated with
younger orogenies has dominantly wolframite
province.

12. • The study of tungsten minimization in term of
plate tectonic indicates that practically all
productive tungsten occurrences are located
either on the boundaries of the two orogenic
cycles (Convergent and divergent plate
boundaries) or on the point which could be called
triple Junction.
• Such junctions are the locations of hotspot
activities and magma generation.
• Tungsten ore may have formed under condition
of a medium to high temperature by gas
emanations or hydrothermal solution.
• The deposits so formed are fissure veins,
replacement contact metasomatic and
pegmatitic.

13. Tin Deposits
• The Tin ore occur mainly in veins, stockworks,
dissemination, replacement and placer. Depending
upon the mode of their formation they are classified
as:
• (i) Tin- bearing pegmatite
• (ii) Cassiterite-quartz
• (iii) Cassiterite-sulphide ores and
• (iv) Tins-bearing sand

14. • Pegmatite type:
• The ores of this type form irregular deposits
generally confined to the central section of
granitic mass and occasionally in
metamorphosed sandstone and schist. The
cassiterite deposit of Orissa is in Pegmatite-
veins, intrusive into the Bengpal
matasediments.

15. • Cassiterite-quartz:
• This type is widespread and is represented
mainly by light grey and milky white quartz,
books of light green mica and light grey
feldspars.
• The ore bodies occur in granite masses or near
their margin.
• They occur as veins and stockworks, which are
ore-bodies characterised by thin irregular
fissures filled with gangue and ore minerals.
• Ore veins are of irregular size and persist to a
great depth.

16. • Stockworks are large ore bodies, several
hundreds of meters long and tens and
sometimes hundreds of meters wide.
• The ore bodies consist of breciated rocks,
intersected by a dense network of veinlets.
• The metal content is usually low but bigger
deposits contain thousands of tones of metal.
• The Tosham cassiterite deposits of Haryana
are examples of this type.

17. Cassiterite-sulphide type
• It is characterised by the presence of green-blue
chlorite, dark tourmaline, pyrite, pyrrhotite,
arsenopyrite and inclusions of fine crystalline
cassiterite.
• This type differs from the ores of cassiterite-quartz
type by its dark grey colour.
• Its tin content is high.
• The ore bodies occur as veins and stockworks in
sandstone and schists, generally in the vicinity of
granitoids masses away from the margin.

18. Tin bearing sands (Placer)
• These placers are formed as a result of destruction
of cassiterite-quartz, and sometimes, Pegmatitic
and cassiterite-sulphide deposits as well as rocks
impregnated with cassiterite.
• They are the biggest source of tin.
• The placers may be eluvial, deluvial, depending
upon the conditions of their formation.
• The three elements distinguished in the structure
of placers are top-soil, metal bearing gravels, and
the bed rocks.
• The important tin province of the world is the belt
of placers found along the Malaya peninsula.