Calculations of fluid-mineral equilibria in the Aspo Hard Rock Laboratory Page: 3 of 13
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Calculations of Fluid-Mineral Equilibria
in the Asp5 Hard Rock Laboratory
Carol J. Bruton
Lawrence Livermore National Laboratory
Livermore, California 94550
The purpose of this report is to evaluate the utility of the EQ3/6 geochemical codes (Wol-
ery, 1992; Wolery and Daveler, 1992) in describing mineral-fluid equilibria in the low
temperature (<25 C) systems at the Aspo Hard Rock Laboratory (HRL). Data on fluid
chemistry and on fracture-filling mineralogy with depth were obtained from Smellie and
Laaksoharju (1992). Average temperatures in the HRL boreholes are generally less than
20 C. EQ3/6 was used to evaluate the extent to which equilibrium is achieved between
minerals and fluids in these systems.
Smellie and Laaksoharju (1992) used the PHREEQE geochemical modeling code to cal-
culate saturation indices for fracture-lining minerals in boreholes KASO2, KASO3, KASO4
and KAS06 in order to "support the presence or absence of the major fracture minerals".
They noted that only calcite and gypsum may be expected to attain equilibrium under the
low temperature conditions at Asp6. However, they used closeness to equilibrium as an
indicator of "stable conditions, long bedrock residence/reaction times and slow to stagnant
flow in the system." EQ3 (Wolery, 1992) was used to calculate mineral saturation indices
for comparison, and EQ6 (Wolery and Daveler, 1992) was used to try to predict the min-
eral assemblages coexisting with fluids.
Fluid analyses from boreholes KASO2, KASO3, KASO4 and KASO6 from Smellie and
Laaksoharju (1992) were used in the EQ3 simulations. Waters that were categorized as
both representative and not representative in Table A from Smellie and Laaksoharju
(1992) were included. As will be shown below, some of the contamination in the "not rep-
resentative" samples may be sufficiently small that its impact on computed rock-fluid
equilibria may be insignificant.
The initial EQ3 runs were made assuming that the measured redox potential represented a
condition of homogeneous redox equilibrium throughout the system. Measured SO4 con-
centrations were input as total sulfur, neglecting measured S- concentrations in the pre-
liminary calculations. Measured SO4 concentrations greatly dominated measured S-
A number of the fluid analyses lacked Eh values. In KASO2, the Eh values for the 308-344
and 314-319 m samples were assumed to equal -260 mV, and that of the 802-924 m sam-
ple was assumed to equal -320 mV. In KASO3, all missing values were assumed to equal
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Bruton, C.J. Calculations of fluid-mineral equilibria in the Aspo Hard Rock Laboratory, report, May 1, 1995; California. (digital.library.unt.edu/ark:/67531/metadc793421/m1/3/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.