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Constraints on the genesis of lode-style tin mineralization: evidence from
the San Rafael tin-copper deposit, Peru
Michael S.J. MlynarczykMay 2005
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Plan of the presentation
Tin metallogeny in the Central Andes
The San Rafael Sn-Cu deposit:- structural geology- alteration
- mineralization- fluid inclusions- stable isotopes
Genetic model
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The metallogeny of the Central Andes
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A collisional model for tin metallogeny
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The San Rafael tin deposit
~ 1,000,000 tonnes Sn metal, average grade ~ 5 wt.% Sn
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Geology of the San Rafael area
granite
slate
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One major vein-breccia system (the San Rafael lode)
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Variation in normative mineralogy: early sericitic andtourmaline alteration versus main-stage chloritization
Freshgranite
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Chemical zoning of hydrothermal tourmaline:from early dravite (Mg) to ore-stage schorl (Fe)
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Physico-chemical conditions of chloritic alteration
associated with main-stage cassiterite deposition
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Ore mineralogy and metal zoning
cas
Upper zone: sulfides dominate, complex mineralogy(chalcopyrite, pyrite, arsenopyrite, sphalerite, galenapyrrhotite, needle-tin cassiterite, quartz, chlorite)
Lower zone: oxides/silicates dominate, simple mineralogy(massive cassiterite, quartz, chlorite wolframite)
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Fluid inclusion studies
18 m
15 m
qtz
cas
Ore fluids of moderate T(360-230C) and fluctuatingsalinity (21-0 wt.% NaCl eq.)
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Stable isotope studies
34 S V-CDT ()
18 O V-SMOW ()
18 O V-SMOW ()
cas
qtz
chl wol
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Isotopic evolution of hydrothermal fluids at San Rafael
Calculated valuesof 18 O and D forwater in equilibriumwith the different oreand gangue minerals(at the temperatureof their formation),exhibit a clearparagenetic trendof 18 O decrease.
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Mineral stabili ty relationships for the tin stage (II) and thecopper stage (III) of the San Rafael paragenesis
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Effect of cooling and mixing of magmatic and meteoricfluids on the oxygen isotope composition of
precipitating quartz (A) and cassiterite (B)
Only a scenario of mixing of a hot brine with a cooler,dilute fluid can reproduce the range of oxygen isotope
compositions of quartz and cassiterite from the deposit
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A genetic model for San Rafael
qtz
cas
Early stage: lithostatic conditions, hot magmatic brines produce strongsericite + tourmaline alteration and sealed, barren, tourmaline-quartz veins.
Main ore stage: mixing of magmatic brines and meteoric waters in the fault- jogs oxidation, dilution, cooling, and acid neutralization of the ore fluids massive precipitation of cassiterite as localized, high-grade ore shoots.
Tectonic activity reopens the vein system transition to hydrostatic
conditions + influx of heated meteoric waters.
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