Cathodoluminescence petrography and fluid inclusion microthermometry of vein calcites from Laxemar borehole KLX-01, Sweden

This report provides results and interpretations of the cathodoluminescence (CL) and fluid inclusion
microthermometric properties of fracture-filling calcite cements from the Swedish Nuclear Waste
Management company (SKB) Laxemar-01 borehole (KLX-01). The work was undertaken on
behalf of Terralogica AB as part of the Palaeohydrogeological Data Analysis and Model Testing
(PADAMOT) project funded as part of the Framework Five programme of the European Union.
CL imaging has allowed the recognition of five calcite generations which show evidence for
significant fracture rejuvenation, reworking, brecciation, replacement and cementation by later
calcite generations. These generations are consistent with those recognised during earlier work.
which on this borehole. Fluid inclusion microthermometric data has been acquired from all the
generations.
The fluids are shown to have variable chemistries, with relatively low-salinity (brackish water,
typically <2 wt% CaCl2 equiv.) inclusions occurring in close proximity to very saline solutions (up
to 24 wt% CaCl2 equiv.), even within the same calcite generation. The higher salinity inclusions
are NaCl-CaCl2-H2O brines, with high CaCl2 / (NaCl+CaCl2) ratios inferred for the highest salinity
inclusions. The chemical species present in the relatively low salinity inclusions are unknown.
The presence of high salinity fluids even at shallow depths in the borehole (well within the present
day freshwater zone that extents down to approximately 1000 m), indicates that the transition zone
between the fresh and saline fluids was closer to ground level at some time during precipitation of
these cements. Similarly the absence of fresh water inclusions below approximately 840 m,
suggests that the transition zone was never much deeper than 840 m during calcite precipitation.
Some, apparently systematic, variations in fluid chemistry are evident between the calcite
generations but interpretation is hindered by the difficulty in distinguishing between primary and
secondary inclusions due to the altered nature of the earlier generations.