Olivo, G.R., Chang, F., and Kyser, T.K., 2006. Formation of the ...

626 OLIVO ET AL.
The fact that most of the gold in the North Dipper and
North Dipper-related veins occurs with calcite filling late
fractures that cut quartz and tourmaline suggests that most of
the gold postdated the main stage of vein filling and likely was
transported by CO 2 -bearing aqueous fluids. Significantly, the
auriferous veins have higher abundances of CO 2 -rich and
H 2 O-CO 2 fluid inclusions in healed fractures (i.e., the same
paragenesis as gold and calcite) than the barren vein (Table
2). Gold was detected only in the CO 2 -bearing fluid inclusion
of the North Dipper-related vein (Table 3, Fig. 11).
A small proportion of gold, which occurs locally as micrometer-sized
inclusions in pyrite, precipitated early, during the
main vein-filling stage in the North Dipper veins. The composition
of this gold also differs from the late gold (i.e., the
early gold contains 6 wt % Ag, whereas the late gold contains
8–17 wt % Ag). If gold is transported as Au (HS) – 2 complexes
in these systems, as postulated by Benning and Seward (1996)
and Mikucki (1998), gold precipitation in the main vein-filling
stage and in the wall rock may have been caused by a decrease
of the H 2 S concentration in the hydrothermal fluid
caused by the precipitation of pyrite. However, in the barren
North zone North Dipper vein and its wall rock, the content
of sulfide is very low (