Field mapping and economic geology

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Field mapping and economic geology
http//www.ontariominerals.com/on.gross.jpg

• Skarns

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References/Links

• Material from this lecture is taken directly from
  Larry Meinerts website Skarns
  http//www.wsu.edu8080/meinert/skarnHP.html
• http//en.wikipedia.org/wiki/Skarn
• Chapter 14 Evans, 1997. An Introduction to
  Economic Geology and its Environmental Impact.
  Blackwell.

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Skarns

• Formed by contact metamorphism (mineralogy
  change) metasomatism (addition or subtraction
  of material via hydrothermal fluids)
• Develop most often, but not invariably, at the
  contact of intrusive plutons and carbonate
  country rocks
• Forms marbles, calc-silicate hornfels and skarns
• Majority of skarns are devoid of mineralization
• What are calc-silicate minerals?

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Mineralogy
Andradite

• Calc-silicate minerals (calcic skarns)
• diopside px CaMgSi2O6
• andradite grnt Ca3Fe32(SiO4)3
• wollastonite CaSiO3
• Skarns divided into magnesian or calcic according
  to their dominant mineralogy eg if Mg-silicates
  such as forsterite (ol. - Mg2SiO4) dominate gt
  magnesian skarn
• Majority of economic skarn deposits occur in
  calcic-skarns
• Skarn deps are categorised according to their
  dominant metal or mineral constituent eg Cu, Au,
  Fe, W, Zn, Pb, Mo, Sn, Talc etc

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Garnet and Pyroxene Composition
Triangular plots commonly are used to express
variations in compositionally complex minerals
such as garnet and pyroxene
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Composition of Garnet and Pyroxene in Skarn
Deposits
http//www.wsu.edu8080/meinert/min.html
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Garnet
Andradite
http//www.ontariominerals.com/on.gross.jpg

• grossularite Gr Ca3Al2(SiO4)3
• Andradite Ad Ca3Fe2(SiO4)3
• spessartine Sp Mn3Al2(SiO4)3
• almandine Al Fe3Al2(SiO4)3
• pyrope Py Mg3Al2(SiO4)3

http//www.wsu.edu8080/meinert/min.html
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Pyroxene

• diopside Di CaMgSi2O6
• hedenbergite Hd CaFeSi2O6
• johannsenite Jo CaMnSi2O6
• fassaite Fas Ca(Mg,Fe,Al)(Si,Al)2O6

http//www.wsu.edu8080/meinert/min.html
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Olivine

• larnite Ln Ca2SiO4
• forsterite Fo Mg2SiO4
• fayalite Fa Fe2SiO4
• tephroite Tp Mn2SiO4

http//www.wsu.edu8080/meinert/min.html
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Pyroxenoid

• ferrosilite Fs FeSiO3
• rhodonite Rd MnSiO3
• wollastonite Wo CaSiO3

http//www.wsu.edu8080/meinert/min.html
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Amphibole

• tremolite Tr Ca2Mg5Si8 O22(OH)2
• ferroactinolite Ft Ca2Fe5Si8 O22(OH)2
• manganese actinolite Ma Ca2Mn5Si8 O22(OH)2
• hornblende Hb Ca2(Mg,Fe)4Al2Si7 O22(OH)2
• pargasite Pg NaCa2(Mg,Fe)4Al3Si6 O22(OH)2
• cummingtonite Cm Mg2(Mg,Fe)5Si8 O22(OH)2
• dannemorite Dm Mn2(Fe,Mg)5Si8 O22(OH)2
• grunerite Gru Fe2(Fe,Mg)5Si8 O22(OH)2

http//www.wsu.edu8080/meinert/min.html
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Epidote

• piemontite Pm Ca2(Mn,Fe,Al)3(SiO4)3(OH)
• allanite All (Ca,REE)2(Fe,Al)3(SiO4)3(OH)
• epidote Ep Ca2(Fe,Al)3(SiO4)3(OH)
• clinozoisite Cz Ca2Al3(SiO4)3(OH)

http//www.wsu.edu8080/meinert/min.html
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http//www.cliftonmining.com/Fig3ac.JPG
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Ertsberg-Grasberg
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"Classic" skarn associated with plutons
Zonation of most skarns reflects the geometry of
the pluton contact and fluid flow. Such skarns
are zoned from proximal endoskarn to proximal
exoskarn, dominated by garnet. More distal skarn
usually is more pyroxene-rich and the skarn
front, especially in contact with marble, may be
dominated by pyroxenoids or vesuvianite.
http//www.wsu.edu8080/meinert/min.html
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Skarnoids
Skarnoid results from metamorphism of impure
lithologies with some mass transfer by small
scale fluid movement. The fluid-controlled
metasomatic skarn typically is coarser grained
and does not as closely reflect the composition
or texture of the immediately surrounding rocks.
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Metamorphism and Bimetasomatism (reaction skarns)
Types of skarn formation A) Isochemical
metamrophism involves recrystallization and
changes in mineral stability without significant
mass transfer. B) Reaction skarn results from
metamorphism of interlayered lithologies, such as
shale and limestone, with mass transfer between
layers on a small scale (bimetasomatism).
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Copper Skarns
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Gold Skarn - Fortitude
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Zinc Skarn
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Evolution of a Skarn
Evolutionary stages of pluton-associated skarn
deposits A) Initial intrusion causes
metamorphism of sedimentary rocks. B) Metamorphic
recrystallization and phase changes reflect
protolith compositions with local bimetasomatism
and fluid circulation forming diverse
calc-silicate minerals (reaction skarns and
skarnoid) in impure lithologies and along fluid
boundaries. Note that metamorphism is more
extensive and higher temperature at depth than
adjacent to the small cupola near the top of the
system. C) Crystallization and release of a
separate aqueous phase result in fluid-controlled
metasomatic skarn. Note that skarn at depth is
small relative to the size of the metamrophic
aureole. It is also vertically oriented compared
to the laterally extensive skarn which locally
extends beyond the metamorphic aureole near the
top of the system. D) Cooling of the pluton and
the possible circulation of cooler, oxygenated
meteroic waters cause retrograde alteration of
metamorphic and metasomatic calc-silicate
assemblages. Note that retrograde alteration is
more extensive in shallow zones.
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Tectonic Setting
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Study Questions

• As a field geologist, what rock types would look
  for if you were exploring for skarn deposits?
• What clues (mineralogy, metamorphic grade) would
  help you determine whether you are getting closer
  or further away from the intrusion? Why is this
  important in terms of Cu and Au mineralisation?
• Describe the processes involved in the evolution
  of a skarn deposit.