skarn short course 1 intro
TRANSCRIPT
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Exploration for Skarn Deposits
Larry Meinert Department of Geosciences Smith College Northampton, MA 01063 USA Email: [email protected]
UNESCO Skarn Deposit Short Course���
Exam Questions
Based upon the two samples on the left: 1) What metals are likely to be present? 2) Is there ore potential and if so, what is the
possible tonnage and grade? 3) What direction and how far away is the
intrusion? 4) What direction and how far away is the ore
zone?
Skarns and Skarn Deposits 1) General processes of skarn formation
2) Examples of major skarn types: W, Cu, Zn, Fe, Au
3) Mineralogy and petrogenesis 4) Zonation models & guides for exploration
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Processes - metamorphism and metasomatism
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Interpretation of Skarn Deposits
• Depth Intrusive textures, stratigraphy, mineral equilibria
• Temperature Mineral equilibria, fluid inclusions
• Oxidation State Igneous and skarn mineralogy, wall rock composition
• Mineralogy Mineral composition, association, timing
• Alteration Stages Igneous, metamorphic, prograde, retrograde
Depth of Formation - W Skarns • Tonnage and grade 63 Mt of 0.95 WO3
• Igneous Composition calc-alkaline granodiorite to granite
• Igneous geometry c.g. batholith with pegmatite, megacrysts
• Igneous alteration Fresh, minor myrmekite, Ca endoskarn
• Metamorphism Extensive metamorphic aureole (km)
with high T mineral assemblages
and reaction skarns
(bimetasomatism)
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Depth of Formation - Cu Skarns • Tonnage and grade 500 Mt of 0.8% Cu
• Igneous Composition calc-alkaline diorite to quartz monzonite
• Igneous geometry m.g. to porphyritic stocks and dikes
• Igneous alteration K-silicate, sericitic, epidote endoskarn
• Metamorphism Variable metamorphic aureole (cm to
km) lacks high T mineral
assemblages
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Distal Features - Zn Skarns • Tonnage and grade 50 Mt of 10% Zn, 5% Pb, 150 g/t Ag
• Igneous Composition granodiorite to granite to unknown
• Igneous geometry batholith to stocks to dikes to unknown
• Igneous alteration Extensive epidote±garnet endoskarn
• Metamorphism Bleaching and recrystallization,
no high T metamorphic aureole
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Distal features Fluid escape structures
Carbon veins Distal brown pyroxene skarn
“stylolites”
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SR 173-133m
Calcite vein cut & offset by stylolite fluid escape structure
Stylolite fluid escape structure
Red fluorescence – partly reflects Mn