This study was undertaken to determine whether wallrocks around the Twin
Creeks Carlin-type gold deposits exhibit oxygen isotope haloes similar
to those found around other types of hydrothermal deposits.
Mineralization at Twin Creeks is hosted by Ordovician Sequence shales
containing some carbonate minerals and by Pennsylvanian-Permian Etchart
Formation limestone. Analysis of orthophosphate-soluble carbonate from
these rocks shows that oxygen isotope haloes are detectable in
Ordovician Sequence shales but not in Etchart Formation limestone. The
soluble fraction of Ordovician Sequence shales at Twin Creeks has
delta(18)O values of 12 to 24 parts per thousand and delta(13)C values
of 0 to -10 parts per thousand. Most samples fall along a poorly defined
trend that extends from delta(18)O of about 24 parts per thousand and
delta(13)C values of about 0, which are typical of unaltered limestones,
toward lower values for both isotope systems, which are typical of rocks
that have undergone alteration by hydrothermal fluids. Plots of these
values along two sections through the ore body show that delta(18)O
values of wallrocks are lowest in the ore zone and increase outward,
forming a halo several hundred meters in size. In the same plots,
delta(13)C values of the wallrocks do not show systematic spatial
variations. The soluble fraction of Etchart Formation limestones at Twin
Creeks have delta(18)O values of 25 to 5 parts per thousand and
delta(13)C values of 4 to -10 parts per thousand, but do not show any
systematic spatial variation relative to mineralization at the scale of
our samples. Failure of the Etchart Formation samples to show detectable
haloes is probably related to deposition of post-ore carbonate minerals
or lower ore fluid:rock ratios. Material balance calculations used to
model the isotopic composition of average Ordovician Sequence shales
indicate that changes in temperature and water:rock ratio were probably
not sufficient to account for the wide range of isotope compositions
observed in these rocks. The most likely additional factor contributing
to this range of values was a change in the composition of the altering
fluid, probably by mixing of the ore fluid with surrounding meteoric
water. These results suggest that Carlin-type gold deposits are
surrounded by haloes of low delta(18)O values, but that detection of
these haloes could be complicated by local compositional variations and
post-ore modification of the wallrocks. (C) 1998 Elsevier Science B.V.
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