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Short course for the SEG student
chapter at Universidad Nacionalde Ingenieria, Lima, Peru
Skarn deposits
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Universidad Nacional de Ingenieria, Lima, Peru
30 June 2013
Skarn deposits
Zhaoshan Chang
EGRU, JCUZhaoshan.chang@jcu.edu.au
Skarn deposits
1. Introduction, definition and mineralogy
2. Classification and terminology
3. Skarn-forming processes & evolution stages
4. Zonation in skarn systems
5. Factors affecting the formation of skarns andzonation patterns
6. Exploration (quiz)
Why skarns?Au, Cu, Sn, W, Pb, Zn, Mo, Fe, minor Ag, B, Be, Bi, Co, F, REE and
U
Common: >1400skarn deposits described in literatures Major source of W and Sn Significant source of base metals and Au, e.g., Antamina, Peru
(1521 Mt @ 0.9% Cu, 0.5% Zn, 0.02% Mo, 10.9g/t Ag); Ertsbergdistrict, Indonesia (716 Mt @ 1.2% Cu, 1.2 g/t Au)
High grade sweetener in porphyry deposits
Antamina, Peru. Photo courtesy of Cam McCuaig
Why skarns?
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What is a skarn?
Defined by mineralogy: Ca-rich garnet and/or pyroxene
Contact betweenintrusions and
carbonatefavorable but notnecessary; notpresent at typelocality
Skarn mineralogy
Garnet A32+B23+C34+O12 Pyroxene A12+B12+C24+O6 Olivine A12+B14+O4 Pyroxenoid A12+B14+O3 Epidote A22+B33+C34+O12(OH)2 Amphibole A11+B22+C52+D84+O22(OH)2 Mica A1B2-3C4O10(OH, F)2 Carbonate A12+CO3 Others
Spessartine Mn3Al2(SiO4)3
Almandine Fe3Al2(SiO4)3Pyrope Mg3Al2(SiO4)3
Subcalcic
garnet
Skarn mineralogy
Garnet A32+B2
3+C34+O12
Grossular Ca3Al2(SiO4)3Andradite Ca3Fe2(SiO4)3
Grandite
Pyroxene A12+B1
2+C24+O6
Diopside CaMgSi2O6Hedenbergite CaFe Si2O6Johannsenite CaMnSi2O6
Salite
Garnet and pyroxene composition diagrams
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Inappropriate pyroxene composition diagram Skarn mineralogy
Pyroxenoid A12+B1
4+O3
Ferrosilite FeSiO3
Rhodonite MnSiO3
Wollastonite CaSiO3
Pyromangite
Bustmite
Epidote A22+B3
3+C34+O12(OH)2
Epidote Ca2FeAl2Si3O12(OH)Clinozoisite Ca2AlAl2Si3O12(OH)
Classification of skarns
Major economic metal:
- Fe, Cu, Pb-Zn, Au, W, Sn, Mo
Magnetite is abundant in many types:
- Cu, Au, Sn
Pb-Zn skarns mostly polymetallic (e.g., with Ag)
Classification of skarns
Endoskarn vs. exoskarn:
Exoskarn is typically much more abundant than endoskarn:
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Endoskarnat Mt Colin
Classification of skarns
Ca-skarn: protolith = limestone- Garnet, pyroxene, wollastonite, scapolite, Ca-rich plagioclase,
vesuvianite, epidote, amphibole, ilvaite, chlorite, prehnite, biotite,
quartz, carbonate
Mg-skarn: protolith = dolomitic- Olivine, pyroxene, humite, clinohumite, periclase, amphibole,
phlogopite, chlorite, serpentine, talc, burcite
Mn-skarn:- Johannsenite, spessartine, rhodonite, pyromangite, bustmite
Distal locations; Pb-Zn skarn;a subgroup of Ca-skarn
Magmas contain volatiles(H2O, HCl, SO2, CO2, HF,B, ) and metals
White Island, New Zealand:~300 t Au & ~1 Mt Cu discharged toatmosphere in the past ~10,000 yrs Courtesy of Jeff Hedenquist
Skarn-forming processes Water solubility and exsolution
Burnham (1979, 1981, 1997)
Water in aqueousphase - MagmaticHydrothermal fluid
Water dissolvedin melt
Solubility curve
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Water solubility and exsolution
Burnham (1979, 1981, 1997)
Water dissolvedin melt
Water in aqueousphase - Magmatic
Hydrothermal fluid
Secondboiling
First boiling
Rapid ascent;adiabaticdepressurisation
Cooling /crystallisationin a chamber
Slow ascendingaccompanied bycooling
Burnham (1979, 1981, 1997)
Second boiling
First boiling
Rapid ascending;adiabatic depres-
surisation
Cooling /crystallisation in achamber
Slow ascendingaccompanied by
cooling
Water solubility and exsolution
Water dissolvedin melt
Water in aqueousphase - Magmatic
Hydrothermal fluids
Water exsolution
Burnham (1979, 1981, 1997)
1. Water slowly exsolves;
P increases; water not
released from chamber
yet due to solid carapace
Quartz USTs, NorthParkes, NSW
Buoyant hydrothermal fluid migrates upwards through the meltand accumulates in the roof of the inwardly-crystallizing magma
Bands of distinctly prismatic crystals (e.g., qz, mt) growdownwards from the roof of the volatile pocket
Volatile accumulationand UST growth
UST quartz form here.
Coalesced waterbubbles. If caught inmelt, miarolitic cavitieswill form.
Slide from Dave Cooke
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Miarolitic cavities
Candela and Blevin (1995); Candela (1997)
Coarser grained domain; crystals terminated in or project into void Pockets of magmatic hydrothermal fluids; inward growth Maybe interconnected!water migration channel in magma
Field of view = 10cm
Field of view = 2.5 cm
Cavity
USTquartz
Lowenstern and Sinclair (1996)
Water exsolution
Burnham (1979, 1981, 1997)
2. Internal P > (lithostatic P + carapace
tensile strength, ~100 bar), !Hydro-
fracturing; P drops by ~75%. Form
magmatic hydrothermal breccia and
quartz veins/networks.
Burnham (1979, 1981, 1997)
3. Pressure drop causes decrease
of water solubility !rapid
degassing; Water takes away a lot
heat, causing quenching (pressure
quenching); Form porphyry texture
Waterexsolution
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Water exsolution
Burnham (1979, 1981, 1997)
4. Seal; Magma at
depth continues to
crystallise; another
cycle
4. Seal; Magma at
depth continues to
crystallise; another
cycle
Water exsolution
Burnham (1979,1981, 1997)
5. Water exsolves from
deep seated magma
through second boiling
5. Wall rock is ductile atdepth; no explosion, only
expansion; pegmatite
Transitional stage physicalprocesses
Burnham (1997)
FWS: H2O solubility as mass
fractionation
PtVt: max. mechanical energyreleased upon complete crystallisation
MTS: max. tensile strength of rock,equiv. 100 bar
For Bingham quartzmonzonite with 2.7 wt% initial water,hydrofracturing occursat depth < ~ 4-5km
Transitional stage chemical processes
Burnham (1981, 1997)
Crystal + melt + aqueous fluid
"Aqueous fluid extracts metals and volatiles from melt"Interaction between water and crystals (e.g., Chang and
Meinert, 2004)"Crystallisation continues from melt
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Transitional stage chemical processes
Burnham (1981, 1997)
Elemental partitioning between aqueous phase andmelt
"Cl: coefficient = 40:1; Cl in melt: ~0.03 of H2O wt% in melt!NaCl wt% in aqueous fluid = 2 x H
2O wt% in melt
!if 3-6 wt% H2O in melt, then ~6-12 wt% NaClinaqueous fluid
"S (SO2vs. H2S; redox state), CO2"F: 5:1"Most metals prefers aqueous phase, e.g., Cu: 9:1 (Candellaand Holland, 1984)
Timing of water exsolution
Burnham (1981, 1997)
Proportion of melt crystallised at water exsolution
Starting conditions: Dioritic to granitic melt, 2 wt% H2O,
Pressure Depth H2O solubility % of crystalli. at water saturation
5000 bars 20 km 10-11 wt% 80%
2000 bars 8 km 6.3 wt% 68%
500 bars 2 km 3.0 wt% 33%
Water exsolution may occur at much highertemperatures than magma solidus temperatures
Burnham (1981, 1997)
Timing of exsolution affects metal concentration inaqueous fluids
"Some elements can be taken into igneous minerals, e.g.,Fe, and some Cu !early exsolution favourable
"Some trace elements may only become enriched enoughat the end of crystallisation (high degree of crystalli.), e.g.,Be !late exsolution favourable L+V
V L
Single phase
H+V
H+L
L+H+V
80% 20%
Evolution of magmatic fluids
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Magmaticfluid
At shallow depths (< 4 km)aqueous magmatic fluids
split into two separate phases
GAS PHASElow density
high SO2, HCl
low NaCl, metals
LIQUID PHASEhigher density
low SO2, HCl
high NaCl, metals
Partitioning changeswith changes in P, T
Magmatic vapor Water solubility and exsolution
Burnham (1979, 1981, 1997)
Water dissolved inmelt
Water in aqueousphase - Magmatic
Hydrothermal fluid
Carbonate-rich area:magma assimilatescarbonate!CO2increases!H2Osolubility decreases!exsolution occurs
earlier (Meinert, 1995)
Hypogene, up to 850 C
vapors, with HCl, SO2,
CO2, H2S
White Island, New Zealand; Courtesy of Jeff Hedenquist
High temperature degassing
" Such magmatic hydrothermal fluids rich in Si and Fe" Al typically has low solubility, except when acidity is
high or F content is high
Sn-associated Mole granite;Audetat et al., 2000
Fe up to 21.6 wt% Empire Cu-Zn skarn;Chang and Meinert, 2004
Skarn-forming processes
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Evolution ofskarns - 1
Thermal metamorphism:
- Isochemical
Meinert, 1992
Before magmatic wateris released out frommagma chamber; nofluid flow
Meinert, 1992
Metamorphism:Thermal metamorphism
Calcareous: containing carbonateCarbonaceous: containing organic C
Bimetasomatism:
Calc-silicatehornfels
Biotitehornfels
Pyroxenehornfels
Biotitehornfels
"Very fine grained" Low permeability;
block fluid flow;none to weaklymineralized
" Garnet rich in Al
Meinert, 1992
Metamorphism:Thermal metamorphism
Bimetasomatism:
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Reaction skarn
Zoned; fine-grained; low permeability; garnet Al-rich; miner. poor
Marble
Garnet
Pyroxene
Garnet
Marble
Reaction skarnZoned; fine-grained; low permeability; garnet Al-rich; miner. poor
Reaction skarn
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Evolution ofskarns - 1b
Skarnoid- Fine-grained,- High Al, low Fe
Meinert, 1992
Small scale fluidflow along fractures,promoting minormass exchange
Skarnoid at smallscale
Reaction skarn and skarnoid
Metasomatic skarn,prograde
- Garnet, pyroxene,wollastonite,
- Olivine, pyroxene,
periclase,
Meinert, 1992
- Large scale fluid flow- Massive; protolith texture
obliterated- Higher Fe in skarn minerals- Zoned- May over print calc-silicate
hornfels, reaction skarns
Evolution ofskarns - 2
- Protolith texture mostly obliterated- Ground preparation
Massive garnet skarn
0.5 cm
Calcite to garnet:CO2lost; density increased; mineral tends to be granular !mineral volume decreased, open space created,permeability and porosity increased
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Mt Colin
Massivepyroxene
skarn
Elaine
Replacement of hornfels
Evolution ofskarns - 3
Retrograde skarn andmineralisation
- ep, act/tr, qtz, + mt/hm
- chl, qtz, cal, + sulfides- cal
Mostly in skarn but may gobeyond
Meinert, 1992
ep: epidote; act: actinolite; tr: tremolite; cal: calcite; qtz:quartz; chl: chlorite; mt: magnetite; hm: hematite
" Metals dissolved as complexes (e.g., with Cl);" High solubility at higher temperatures;" Not deposited until lower temperatures or losing the ligands
(e.g., by dilution)
" Deposition when solubility decreases dramaticallyMagnetite
Chalcopyrite SphaleriteGalena
500-400C ? 400-250C?
300-200C ?
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Williams-Jones et al. (2010)pH
Base metals in hydrothermal fluids
- Transported as Cl complexes(e.g., Wood and Samson, 1998)- Deposition causes acidity increaseZnCl4
2-+ H2S = ZnS + 4Cl-
+ 2H+
Concentrated vs.dispersed: wall rock
important
Carbonate-rich regiongood for base metaldeposition !higher grade
Retrograde alteration and mineralization
Garnet
EpidoteQuartz
Epidote replaces and pseudomorphs garnet
Retrograde alterationand mineralization
Pyroxene replaced by ilvaite, quartz, and siderite
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Ilvaite replaced by siderite+quartz
Pyroxene replacedby quartz, siderite,and sphalerite
Magnetite is later than prograde skarn;
Sulfides later than oxides
Retrograde alteration andmineralization
Garnet
Chl + mt
Qtz-cal-py-cpy
GarnetGarnet
GarnetMt + chl
Py + cpyPy + cpy
Retrograde alteration and mineralization
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Note thin chlorite between sulfides and garnet Black chlorite selectively replaces bands ofgarnet. Note sulfides are associated withchlorite-quartz-carbonate.
Ga: garnet; Cc: calcite; Qz: quartz; Chl: chlorite; Cpy: chalcopyrite;Py: pyrite; Bn: bornite
Quartz-calcite-sulfides replace some
bands of garnet and fill the interstices
CpyCpy
Cpy
Cpy+ChlCpy+Chl
Qz+Cc
Retrograde alteration andmineralization
Chang and Meinert, 2008
Retrograde alteration andmineralization may bepresent beyond skarn
Chl
Po
Cpy+Py+Au
Retrograde alteration & mineralization
Chl: chlorite; Cpy: chalcopyrite; Py: pyrite; Po: pyrrhotite
Cpy
Graniteporphyry
Hornfels
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Skarns are typically zoned
Transfer of heat and mass from intrusions or fluid conduitsIntrusion Calcareous wall rock
Endoskarn Massive/metasomatic skarn
Bleached
marbl
e
Marble Limestone
Hornfels
Fluid escape structures
Garnet/pyroxene ratio decrease
Garnet colour lighter
Pyroxene colour darker
Cu Zn-Pb
Exoskarn is typically much more abundant than endoskarn
Reaction skarn
The closest intrusion is not necessarily the causative intrusion!
Zonation in a Cu skarn
gar: garnet; pyx: pyroxene; cp: chalcopyrite; wo: wollastonite;ves: vesuvianite; bn: bornite; po: pyrrhotite; py: pyrite
Carr Fork, Bingham, USA
Atkinson andEinaudi, 1978
38.3m Endoskarn.Dark red garnet 45.2m Red garnet 46.3m Brown garnet 48.2m Yellow garnet
54.2m Light yellowgarnet
Zoning away from intrusion in a Cu skarn
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Zonation in a Cu skarn
Carr Fork, Bingham, Cu skarn
Zonation in a Zn skarn
Marble
Groundhog,USA;
Meinert,
1987
Gar: garnet; Pyx: pyroxene; Hd: hedenbergite; Jo: Johannsenite
Zonation in a Au skarn Fortitude, USA
Meyer and Meinert, 1991
Zonation at marble front in a Au skarn, Mexico
Darkmarble
Bleached marbleWollastoniteGarnet
Fluid escape structures distalfeatures beyond skarn
Meinert et al., 2005
Pyroxene
Garnet
Veins
Ve
ins
Garnet
Garnet veinsGarnet veins
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Meinert et al., 2005
50-100m away from skarn
Meinert et al., 2005
200m away from skarn
Stylolite
Calcite vein offset by stylolite fluid escape structure
Meinert et al., 2005 Meinert et al., 2005
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Fluid escape structures distalfeatures beyond skarn Fluid escape structures distal features beyond skarn
Beyond bleached marble, there are white, bleachedveins with diffusive sulfide veins in the middle. The veindensity and width decrease away from skarn
Dark marble
Diffusive veins(po-cpyMo)Bleached
halo
Fluid escape structures distal features beyond skarn Fluid escape structures distal features beyond skarn
Diffusive veins(po-cpyMo)
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Factors affecting the formation of skarns
Redox state gradient between magma and wall rock
Causative magmaVolatilesDegree of fractionationRedox state
Wall rockCompositionRedox statePermeability
Depth of formation
Distance from magma
Grossular Ca3Al2(SiO4)3Andradite Ca3Fe2(SiO4)3
Diopside CaMgSi2O6Hedenbergite CaFeSi2O6
Redox state gradient
Oxidizing Reducing
Fe3+!garnet Fe2+!pyroxene
Intrusion Calcareous wall rock
Endoskarn Massive/metasomatic skarn
Bleached
marble
Marble Limestone
Hornfels
Fluid escape structures
Garnet/pyroxene ratio decrease
Garnet colour lighter
Pyroxene colour darker
The zoning pattern is based on:
If both the magma and the wall rocks are reducing
Ettlinger, 1990; Ray et al., 1996
Ilmenite-bearing
Fe2O3/(Fe2O3+FeO)= 0.15
Nickle Plate ,Hedley District,Canada
If both the magma and the wall rocks are oxidizing
A Cu skarn prospect, Philippines
High inmagnetite
DioritePorphyry Conglomeratemarble
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38.3m Endoskarn.Dark red garnet 45.2m Red garnet 46.3m Brown garnet 48.2m Yellow garnet
54.2m Light yellowgarnet
Specularite
If both the magma and the wall rocks are oxidizing Effect of magmatic volatiles - F
Empire Cu-Zn skarn, USA
Chang and Meinert, 2004, 2008
Unusual features: 1) Abundant endoskarn, > exoskarn2) Proximal Zn minearlisation
Effect of magmatic volatiles - F
High F content in the magmatic-hydrothermal system as indicated by:
Empire Cu-Zn skarn mine, USA
Chang and Meinert, 2004, 2008
1.53-2.46 wt% F in magmatichornblende
1.43-3.87 wt% F in magmatic biotiteFluorite as igneous accessory mineralFluorite as daughter mineral in fluid
inclusions
1.29-2.42 wt% F in hydrothermalvesuvianite
Fluorite in skarns
Effect of magmatic volatiles - F
Reasons for these unusual features:
- F greatly facilitates the dissolution of silicates- F decreases the solidus temperatures ofmagmas. When the late-stage fluids exsolved
from them, the fluids were already at lowtemperatures, therefore only shorttransportation distance was needed for thefluids to be cool enough to deposit sphalerite
Empire Cu-Zn skarn mine, USA
Chang and Meinert, 2004, 2008
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Textures indicating high F
Chang and Meinert, 2004, 2008
Effect of causative magma compositions
Intrusions associated with
Fe and Au skarns: moreprimitive
Associated with Mo and Snskarns: more evolved and/orcrustally contaminated
Mostly calc-alkaline;tholeiitic and alkaline notcommon
Meinert, 1995
Meinert, 1995
Intrusions associated with Sn skarns: peraluminous
Sn and Au-only skarns: more reduced causative intrusionsCu, Zn, and Mo skarns: more oxidized causative intrusions
Meinert, 1995
Effect of causative magma compositions
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Sn skarns
Intrusive rocks peraluminous, more evolved and/or with morecrustal contamination, and more reduced
Suite of trace elements: Sn, F, B, Be, Li, W, Mo, and Rb
Strong greisen alteration
Sn can be incorporated into silicate minerals, e.g., garnet (up to 6wt%), vesuvianite, and titanite; extensive retrograde or greisenalteration needed to release such Sn; most attractive ore bodiesdistal
Economic Sn in cassiterite, stannite, vonsenite (4FeOFe2O3B2O3),etc.
Plane-polarized
Pyroxene
Vonsenite
Plane-polarized
Hornblende
Garnet
Calcite
A Sn skarn in western Tasmania, Australia
GarnetCalcite
Hornblende
VonseniteVonsenite
Residual Hb
Fe-rich hb withbio replacement
Bio withresidual hb
Cassiteritein this band
Biotite
CassiteriteCassiterite
Biotite
Fluorite
Pyrrhotite
A Sn skarn in western Tasmania, Australia
Redox
W
W
W
Composition Ca skarn vs. Mg skarn
Porosity, composition
CaCO3+ SiO2(aq)= CaSiO3+ CO2
W: wollastonite P: pyroxene G: garnet
C + O2= CO2
Effect of wall rocks
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Geometry
- Massive/irregular vs. stratiform
Effect of wall rocks
Geometry Massive/irregular vs.
stratiform
Effect of wall rocks
Chang et al., 2007
Stratiform skarn
Chang et al., 2007
Effect of wall rocks
Meinert, 1992
Depth of formation
-Ambient temperature- Metamorphism- Retrograde alteration
-PermeabilityW skarn vs. Cu skarn
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W skarn
Musc - hm - chl - qtz
Musc - hm - chl - qtz
Bio - sill - alm - ilm cord ksp qtz
Granodiorite
Bio - sill - ilm cord
Bio - adl - sill - ilm
Wskarn
Wskarn
Granodioriteporphyry
Skarn
Cuskarn
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Distance from causative intrusion
Fluid temperatures
Distal skarnsare rich in Mn
Johannsenite,rhodonite,pyromangite,
bustmite
Meinert, 1992
Zonation in skarns - summary
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QuizA granodiorite intrusion contains ilmenite but no magnetiteFe2O3/(Fe2O3+FeO) < 0.2
The wall rocks ~800m awayfrom the intrusion are dark incolour
Sandstone
Shale
Limestone
Quiz
If there are skarns between such an intrusion andwall rocks, how wide would the skarn zone likelybe?
1) A few meters
2) Tens of meters3) Hundreds of meters
Quiz
In the zoning pattern, which zone would be themajor one?
1) Garnet > pyroxene zone2) Garnet pyroxene zone
3) Garnet < pyroxene zone
Quiz
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Around the intrusion some biotite hornfels containingdisseminated pyrrhotite were found. What kind ofeconomic metal(s) may the skarn contain?
1) Fe 2) Cu 3) Au
4) Pb-Zn 5) W 6) Sn
Biotitehornfels
Pyroxenehornfels
Quiz
In the intrusion there are some garnet veins with greenpyroxene halo close to contact, but there is no greisenalteration. With this additional information, please estimate
again what economic metal(s) the skarn may contain:
1) Fe 2) Cu 3) Au4) Pb-Zn 5) W 6) Sn
Quiz
Estimate the composition of the garnet. Is it
1) Rich in Al (grossularitic), or
2) Rich in Fe (andraditic), or3) Rich in Mn (spessartine)
Quiz
Narrow garnetmargins; majority
grossularitic
Garnet composition
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1
2
34
Where wouldyou drill?Why?
Quiz
134
Quiz
Is the graniteporphyry thecausative
intrusion forthe skarns?Why or whynot?
Quiz