Fluid inclusion planes as tectonic and ore formation indicators: an example the Antei uranium deposit (se Transbaikalia)

Ustinov S.A., Petrov V.A., Poluektov V.V.

The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, Russia

stevesa@mail.ru

Migration of the dissolved substances or colloidal particles in fluid flows under the influence of gravitation, stress and temperature is carried out by faults and cracks, from which substances and particles both penetrate to surround volume of rocks and migrate back to discontinuities. Paleofluid pathways can be observed not only at macro- but also at microscale by studying microcracks of various generations and types.

Cracks in rocks can be considered as Open Cracks (O.C.: not filled with secondary mineral phases) or Filled Cracks (F.C: filled with secondary mineral phases or Fluid Inclusions). F.C. are often partially filled or reopened after a first time of filling. The best records of fluid percolation are paleofluids trapped as fluid inclusions in healed microcracks of the rock-forming minerals [2]. Usually such fluid inclusions with liquid, vapor and solid phases form differently oriented systems, known as fluid inclusion planes (FIPs). FIPs result from the healing of former open cracks and appear to be fossilized fluid pathways [3]. FIPs are totally sealed and do not present secondary opening.

Microcracks should provide valuable information about the local stress in rocks and can be assumed to be (σ₁σ₂)

planes [4]. The FIP are mode I cracks that occur in sets with a predominant orientation perpendicular to the least principal compressive stress axis σ₃. These mode I cracks propagate in the direction which favors the maximum decrease in the total energy of the system. They do not disrupt the mechanical continuity of mineral grains and do not exhibit evidence of shear displacement like mode II and III cracks. The FIP are usually observed and characterized in minerals which crack according to the regional stress field, independently of their crystallographic properties, and may easily trap fluids as fluid inclusions when healing. In some minerals (carbonates, feldspars), the fluids are not always preserved due to alteration or dissolution and cracks display more complex patterns resulting from the presence of cleavages, subgrain boundaries or twin planes. The rate of healing is rapid in quartz (compared to geological times), so it is more informative for studying FIP systems [1].

Frequently, FIP form well defined networks which allow the determination of a chronology. After a first generation of FIP, a second crack family can be formed with the trapping of different fluid. This second FIP generation commonly cross-cuts the first one. Thus, FIPs provide good records of successive episodes of crack initiation and fluid migration.

So orientation of FIPs is defined by reorganization of the local stress field and it is possible to use them as geostructural markers for reconstruction of porosity and paleopermeability of rocks, geometry of fluids migration pathways, reconstruction of fluid migration stages and for studying dynamics of change of PT, physical and chemical conditions at various events of deformation of geological objects and ore formation.

FIP generations were studied at the Antei uranium deposit. It is located in Eastern Transbaikalia within the Streltsovskaya caldera, generated in process of late-Mesozoic tectonic and magmatic activation of the region. Samples were taken from all fault zones – from the central part (core), the zone of its dynamic effect (failure), and undestroyed wall rock (protolith) at the9th to 11th horizons. Also for microfissural mapping of natural cracks and allocation of FIP systems, samples must be oriented in space, north direction must be fixed. The chronology of FIP generations and their spatial parameters (extension, dip angle) can be established by means of the classical microstructural analysis (Fedorov’s stage), or by method of statistical analysis of 2D and 3D digital images of thin sections by means of the special software [2]. Besides, this analysis allows quantifying paleofluid flow porosities and permeabilities by the reconstruction of the crack network consisting of cracks described as discs using the geometry of the crack network. So for each FIP system, one can determine dip direction, length, thickness, porosity and paleopermeability using microstructural analysis. The data on composition and properties of fluid inclusions trapped in the cracks (temperature, pressure, salinity, phase content) to separate different sets of FIP was found out using microthermometry and Raman spectroscopy.

As the main ore component of the Antei deposit is uranium, we can use sufficiently effective method for reconstruction fluid filtration processes and stages of intraore tectonics during the past geological events – fission-track radiography (FTR) of thin sections. As detector lavsan fibre was used. Thin section and lavsan were placed into reactor and irradiated with a stream of slow neutrons. By means of this method it is possible to establish characteristics of uranium distribution at microscale, and also to calculate its concentration in mineral phases, microcracks and pores. The thin sections for FTR were the same, as for microstructural analysis. So it allows to define spatial distribution of uranium concentrations, their relation to already revealed systems and types of microcracks and other microstructures, and also to allocate various stages of ore formation.

The comparative characteristic of orientation of open microcracks, fluid inclusion planes and linear distributions of uranium concentrations was carried out by construction of roses-diagrams for each type of objects. It allowed to define spatial parameters of the chosen linear objects, allocate stages of fluid migration and to confirm conception about polystage development of intraore tectonic processes.

After getting parameters of each FIP generation, making their interpretation and finding out distribution of uranium ore the conclusion can be drawn:

1) Ore forming process at the Antei uranium deposit took place through several (minimum 2) stages of fluid migration;

2) FIPs change their orientation from the north-northeast to the east-northeast on a vertical interval from 9th to 11th horizon, this fact confirms that reorientation (about 30 degrees) of the horizontal paleostress axis take place at this interval;

3) This approach helps us to model paleopermeability of crystalline massifs (tectonics, fluid flow pathways, fluid chemical composition, etc.) as a function of stressed-strained and temperature state in space-time context.

This work was financially supported by the Russian Foundation of Basic Researches (grant № 12-05-00504).

References:

1. Lespinasse M. Are fluid inclusion planes useful in structural geology? // J. Struct. Geol. 1999. N 21. P. 1237-1243.

2. Lespinasse M., Désindes L., Fratczak P., Petrov V. Microfissural mapping of natural cracks in rocks: implications on fluid transfers quantification in the crust // Chemical Geology Spec. Issue, 223, 170-178 (2005).

3. Roedder E. Fluid Inclusions. Review of Mineralogy, 12 (1984).

4. Tuttle O. F. Structural petrology of planes of liquid inclusions. J. The journal of geology. 1949. N 57. P. 331-356.