Application of studies on fluid inclusion planes and fracture systems in the reconstruction of the fracturing history of granitoid rocks I

Introduction to methods and implications for fluid-mobilisation events in the Velence Hills

Abstract

This study provided obvious macroscopic evidence of fluid flow and alteration zones which were the result of
fluid/rock interaction and mineralized veins. These are typically the main channels of fluid migration in a rock unit.
However, fluid percolation is not only controlled by thick mineralized veins but also by micro cracks. These micro cracks
can be healed during the fluid/rock interaction and trap fluids in inclusions therefore these healed cracks are called fluid
inclusion planes (FIP). FIPs can be present in seemingly fresh rocks without any alteration or a vein system, this is
obvious evidence of former fluid migration events. The mapping and geometric analysis of FIPs — coupled with fluid
inclusion microthermometric studies — represents a relatively new method. The geometric properties of FIPs in the rock
forming quartz of granite provide important information about the stress field in which the fluid percolation and
fracturing of a rock occurred. Furthermore microthermometry of fluid inclusions in FIPs provides information about the
temperature-pressure-composition properties of fluids. Results concerning FIPs and macro-fracture analyses in the
eastern part of the Velence Hills (W Hungary) are presented in this paper as examples of the application of the methods
mentioned above. This area is built up of a Variscan monzogranite intrusion which was affected by Variscan, Triassic and
multiple Palaeogene hydrothermal processes. Open micro- and macro-fissures in the granite have two main orientations
with NE–SW and NW–SE strike-directions; these represent the initial, cooling related fracture system of the granite
body. The younger Triassic and Palaeogene fluids also migrated mostly into these re-opened fractures, but a new set of
fractures with an E–W orientation was also formed due to the different stress field. In the eastern part of the granite body
the border of the Palaeogene fluid migration was mapped out; this was also the border of the most permeable zones during
the Palaeogene fluid migration in the granite. The latter was based on the densities of FIPs. By means of fractal analysis
studies carried out on quartz vein swarms, the study established the fractal properties of vein growth and differentiated
the vein systems (based on their maturity). The orientation of the recent fracture system of the granite is similar to those
of the hydrothermal fracture systems; thus it can be concluded that the older fracture systems had a determining
influenced on the orientation of the younger ones.