Fluid inclusions in the Stripa granite and their possible influence on the groundwater chemistry

Geochimica et Cosmochimica Acta

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Fluid inclusions in quartz and calcite of the Proterozoic Stripa granite, central Sweden, demonstrate that the rock and its fracture fillings have a complex evolutionary history. The majority of inclusions indicate formation during a hydrothermal stage following emplacement of the Stripa pluton. Total salinities of quartz inclusions range from 0-18 eq.wt% NaCl for unfractured rock and from 0-10 eq.wt% for fractured rock. Vein calcites contain up to 25 eq.wt% NaCl but the inclusion size is larger and the population density is lower. Homogenization temperatures are 100-150??C for unfractured rock and 100-250?? for fractured rock. Pressure corrections, assuming immediate post-emplacement conditions of 2 kbar, give temperatures about 160??C higher. Measurements of fluid-inclusion population-densities in quartz range from about 108 inclusions/cm3 in grain quartz to 109 inclusions/cm3 in vein quartz. Residual porosity from inclusion densities has been estimated to be at least 1% which is two orders of magnitude greater than the flow porosity. Breakage and leaching of fluid inclusions is proposed as an hypothesis for the origin of major solutes (Na-Ca-Cl) in the groundwater. Evidence for the hypothesis is based on (1) mass balance-only a small fraction of the inclusions need to leak to account for salt concentrations in the groundwater, (2) chemical signatures- Br Cl ratios of fluid inclusion leachates (0.0101) match those ratios for the deep groundwaters (0.0107), (3) leakage mechanisms-micro-stresses from isostatic rebound or mining activities acting on irregular-shaped inclusions could cause breakage and provide connection with the flow porosity, and (4) experimental studies-water forced through low permeability granites leach significant quantities of salt. This hypothesis is consistent with the available data although alternate hypotheses cannot be excluded. ?? 1989.

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Fluid inclusions in the Stripa granite and their possible influence on the groundwater chemistry
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Geochimica et Cosmochimica Acta
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Geochimica et Cosmochimica Acta
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