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,*Department of Geology, Washington State University, Pullman, Washington 99164-2812
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401-1887
Geological Survey of Japan, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan
Geological Survey of Japan, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan
Corresponding author: email,meinert{at}wsu.edu
Most skarn ore deposits are characterized by two distinctly different
alteration styles. An early prograde stage with anhydrous minerals, such as
garnet and pyroxene, forms from relatively high-temperature, hypersaline liquid.
A later retrograde stage with hydrous minerals, such as epidote, amphibole, and
chlorite plus sulfide ore minerals, forms from lower temperature, lower salinity
fluids. These two alteration stages commonly have been thought to reflect a
dominance of magmatic and meteoric water, respectively, with relevance to the
source of ore metals. We report data from two different skarn systems, one being
part of the world’s largest Cu-Au resource. Stable isotope compositions of
anhydrous and hydrous alteration minerals from both deposits indicate a magmatic
source for both the prograde and retrograde stages: 
18O
averages 5.0 per mil for garnet (range, 3.4–7.2
), 6.5 per mil for pyroxene
(4.3–8.2), and 7.1 per mil for amphibole (4.3–8.7). The D
values of late amphibole are more complex, with magmatic values (–77 to –78)
for one deposit and both magmatic and lighter values for another deposit that
could be explained either by magmatic degassing or by limited mixing with
meteoric water. We conclude that the differences in fluid composition—prograde
versus retrograde stages—resulted from a magmatic fluid that intersected its
solvus during the early stage, creating vapor and hypersaline liquid, whereas in
the later stage this magmatic fluid did not intersect its solvus because it
followed a different cooling path. This late, low- salinity liquid only boiled
once its vapor-pressure curve was reached, causing sulfide ore to precipitate
during the retrograde stage.
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