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Scientific Communications |
Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405
Corresponding author: e-mail,
ripley{at}indiana.edu
Sulfur isotope measurements provide a useful means of
evaluating the source of sulfur in magmatic Cu-Ni-PGE deposits. Although low
sulfur, PGE-rich deposits (e.g., J-M reef of the Stillwater Complex,
Merensky reef of the Bushveld Complex) show little or no isotopic evidence
for the involvement of crustally derived sulfur, several high sulfur
deposits (e.g., Noril’sk,
Duluth Complex) show strong evidence for contamination of mafic magmas by
sulfur of crustal origin. Processes such as magma degassing or redox changes
accompanying crystallization are ineffective in significantly modifying
sulfur isotope signatures of sulfides that crystallize from immiscible
liquids that exsolve from the mafic magmas. For this reason 
34S
values are primarily dependent on sulfur source characteristics, and
variations can often be treated using a two-component mixing approach.
However, deposits such as those at Noril’sk
and Voisey’s Bay
that appear to have formed in magma conduit systems present another
possibility for modification of sulfur isotope signatures. These deposits
are thought to have been upgraded in metal tenor by reaction with
uncontaminated mantle-derived melts that traversed the accumulated sulfide
on their way to the surface. Our calculations illustrate the efficiency of
reactions that involve the exchange of Fe for Cu, Ni, or PGE in producing
increases in the metal tenor of the accumulated sulfide. Sulfur isotope
exchange between the crustally contaminated sulfide ore accumulation and
sulfur of mantle origin may accompany chalcophile element exchange, and is
capable of masking or limiting the degree of isotopic evidence for the
initial stage of ore genesis that involved crustal sulfur. Sulfur isotope
exchange is distinct from the addition of sulfur ( in the form of immiscible
sulfide droplets from a sulfide-saturated magma) to the accumulated sulfide,
although both additive mixing and exchange can lead to similar isotopic
effects. Stable isotopic exchange provides a mechanism in addition to
additive mixing that may lead to decoupling of different isotopic systems.
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