Sulfur sources and magmatic sulfide mineralization in the Fraser … · Sulfur sources and magmatic...
Transcript of Sulfur sources and magmatic sulfide mineralization in the Fraser … · Sulfur sources and magmatic...
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Sulfur sources and magmatic sulfide
mineralization in the Fraser Zone
Alex Walker, Katy Evans, Christopher Kirkland
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• Sulfur isotopes allow tracing of sulfurthrough the Earth
• A powerful tool with which to fingerprint sulfur incorporated into mineral prospects
• Tracking of sulfur sources and fertility
Magmatic sulfidesand sulfur isotopes
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Sulfur Isotope 101
• δ34𝑆 =34𝑆/32𝑆
𝑠𝑎𝑚𝑝𝑙𝑒− 34𝑆/32𝑆
𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒34𝑆/32𝑆
𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒𝑥 1000
• Magmatic sulfide deposits sulfurisotopes:
• characterize origin
• degree of mixing between distinct sources
• Can be used to identify Archean input
• Ratios of sulfur isotopes 32S, 33S, 34S can be used to characterize sulfur reservoirs
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1. Emplacement of magma containing mantle sulfur2. Assimilation of sulfur with sedimentary signature3. Resultant magmatic sulfur has mixed signature
Sedimentary sulfur
Magmatic sulfur
Assimilation
Mixing of magmatic/sedimentary
sulfur signatures
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Albany–Fraser Orogen
• Orogenic belt sandwiched between Yilgarn Craton and Madura Province
• Divided into Northern Foreland and Kepa Kurl Booya Province
• Reworked cratonic vs mixed cratonic and juvenile material
Albany–Fraser Orogen
Yilgarn Craton
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Fraser Zone
• Interpreted as mid-crustal hot zone formed by mantle upwelling
• Metagabbros/metagranites, ultramafics and metasediments
• Prospective for magmatic sulfidemineralization
Albany–Fraser Orogen
Yilgarn Craton
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Petrography
• Mafic and metasedimentary rocks
• Sulfides analysed primarily pyrrhotite, pentlandite and chalcopyrite (± secondary pyrite)
• Breccias, disseminated, net textured and massive sulfides at Octagonal
• Disseminated and blebby sulfides at Plato
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Methodology
• Petrography
• In-situ analysis via IMS1280
• Thin section ‘pucks’ embedded into epoxy
• Standard materials either embedded or mounted alongside
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-0.87 – 3.11‰ range (1.05‰) 2.25 – 5.93‰ range (4.31‰)
4.44 – 7.88‰ range (6.12‰)
δ34S isotope data
OctagonalPlato
metasediment
magmatic
Man
tle
Man
tle
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δ34S isotope data
• Sulfides analysed primarily pyrrhotite, pentlandite and chalcopyrite (± pyrite)
• Minimal fractionation – likely coeval formation
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δ34S isotope data
Increasing metasedimentary
component
(7.07‰)
• Plato• -0.87 – 3.11‰ range (1.05‰)
• Octagonal• 2.25 – 5.93‰ range (4.31‰)
• Assimilation of external sulfur
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δ34S isotope data
Increasing metasedimentary
component
(7.07‰)
• Plato• -0.87 – 3.11‰ range (1.05‰)
• Octagonal• 2.25 – 5.93‰ range (4.31‰)
• Assimilation of external sulfurHow much?
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• Simple mixing model between mantle derived magmas and Snowys Dam Formation metasediments
• Mineralized material ~4.25‰
• Assimilation of component equal to ~30% mass of sulfur in magma
• Incongruent melting
Sulfur mixing models
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Sulfur mixing models
• Simple mixing model between mantle derived magmas and Snowys Dam Formation metasediments
• Mineralised material ~4.25‰
• Assimilation of component equal to ~30% mass of sulfur in magma
• Incongruent melting
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Sulfur mixing models
• Simple mixing model between mantle derived magmas and Snowys Dam Formation metasediments
• Plato material ~1.05‰
• Assimilation of component equal to ~5% mass of sulfur in magma
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Archean component in the Fraser Zone
• Whole rock geochemistry, Hf and Ndisotopes and dated inherited zircon grains establish a Biranup basement to Fraser Zone
• Biranup is itself reworked Archean lithologies
• Arid Basin includes sparse pre-1500 Ma detrital zircons
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Juvenile input
Archean sediment (and Biranup basement material) assimilated by the Fraser Zone magmas during emplacement; also present in Snowys Dam metasediments
Archean sulfur in the Fraser Zone ReworkingJuvenile inputReworkingJuvenile input
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Peter Sawyer
Archean sulfur and Δ33S
Thiemens, 2013
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Archean sulfur in the Fraser Zone
• Archean sulfur may be identified via MIF signature
• Deviation from MDF line reflects MIF; Δ33S
• Collected data indicates overall Δ33S signature of –0.03 ± 0.08‰
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Archean sulfur in the Fraser Zone
• Modelling suggests dilution not the answer
• Likely stripping of sulfides from material via sedimentary processes
• Phases more resistant to weathering (e.g. zircon) survive
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What else we can do with sulfides?
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Sulfide mapping
• TIMA analysis: identification/quantification of mineralogy
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Sulfide mapping
Mn
• Elemental mapping of sulfide surfaces via laser ablation
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Testing of spatial relationships
• Test images were used to assess how this technique can be applied and whether results are realistic
r = 0.45
M1/M2 = 0.95/0.27
r = 0.07
M1/M2 = 0.55/0.14
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Colocalization analysis• Statistically significant relationship
between distributions of fractures and Mn
• Fluid flow and remobilization of metals via fracture networks is a means by which mineral deposits may be upgraded or destroyed
Fracture map
Mn elemental distribution
r = 0.30
M1/M2 = 0.85/0.34
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Conclusions
• More positive sulfur isotopic signature at mineralized Octagonal relative to poorly mineralized Plato
• Mineralization linked to greater degree of country rock sulfur assimilation
• Absence of Archean sulfur within the Fraser Zone
• Laser ablation elemental mapping of sulfides
• Development of technique to quantify spatial relationships• Metal remobilization may upgrade or destroy mineralization