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

618 OLIVO ET AL.
FIG. 9. Photomicrographs showing fluid inclusions in the North Dipper and North Dipper-related veins. A. Fluid inclusions
in healed fractures of various orientations; most of the inclusions trails transect quartz grains boundaries and reworked
grains. They are rarely intragranular. B. Deformed, necked down, and decrepitated fluid inclusions along grain boundaries.
C. Two- (AC LL) and three-phase (AC LLV) aqueous-carbonic subtypes with variable volumetric proportions coexisting in the
same healed fracture. D. Two-phase and monophase carbonic fluid inclusions in the same healed fracture.
30°C, with the lower temperatures measured in the North
Dipper vein. The densities of the carbonic phase, which were
calculated from the graphs of Valadovich and Altunin (1968)
and Swanenberg (1979), using values of T h(CO2 ), are high for
liquid H 2 O and liquid CO 2 inclusions (avg 0.85 g/cm 3 , AC LL )
and moderate for all liquid H 2 O, liquid CO 2 , and vapor CO 2 -
H 2 O inclusions (avg 0.65g/cm 3 , AC LLV ). The molar proportions
of CH 4 are estimated to be less than 0.2 mol percent,
based on the Heyen et al. (1982) method, which uses the relationship
between T m(CO2 ) and T h(CO2 ), and the highest values
were obtained in the North Dipper-related veins. Most of the
clathrates melted between 7° and 8°C (T m(C) ) in both North
Dipper and North Dipper-related veins, yielding average
salinities of 4 to 5 wt percent NaCl equiv, according to the
method outlined in Colins (1979). Although some inclusions
in the North Dipper-related veins with high T m(C) (and consequently
the lowest salinities) had low T m(CO2 ) values, other inclusions
with equally low T m(CO2 ) yielded average T m(C) . The
first and last ice-melting temperatures were very difficult to
document due to the formation of large clathrates during
cooling experiments.
CO 2 -rich fluid inclusions (C): The temperatures of melting
of the carbonic phase (T m(CO2 )) in the CO 2 -rich inclusions
from the North zone North Dipper, North Dipper, and North
Dipper-related veins are consistently –56.7° ± 0.2°C, indicating
pure CO 2 . However, in inclusions from the North Dipperrelated
veins, T m(CO2 ) values less than –57°C are commonly
observed, suggesting the presence of minor CH 4 , similar to
the aqueous-carbonic fluid inclusions. T h(CO2 ) is commonly
greater than –10°C in carbonic inclusions (C) and carbonic
inclusions with a small amount of H 2 O (C AQ ) from North Dipper
veins and as low as –45°C in inclusions from North Dipper-related
veins. The calculated densities of the carbonic
phase average 0.86 and 0.80 g/cm 3 for the C and C AQ types,
respectively, and the CH 4 molar proportion is low (