Fluid pressure variations in relation to multistage deformation and uplift: a fluid inclusion study of Au quartz veins

Abstract

Au-bearing quartz veins result generally from a complex succession of stages of quartz crystallization in relation with long lived deformation. These stages may be related to dynamic rock-fluid interaction, and especially to the pressure variations of a suprahydrostatic fluid. Using a multidisciplinary approach including a detailed characterization of quartz microdomains and related fluid inclusions, and the determination of composition-molar volumes for each stage of fluid migration, a reconstruction of the changes in fluid pressure throughout the crack healing or filling has been carried out on the example of Hercynian quartz veins and more especially of Montagne Noire quartz veins (French Massif Central). The trapped fluids in specific networks of healed microcracks frequently show extreme fluctuation in their density although some chemical variables (salinities, CH 4 /CO 2 ratio) could display little changes. Such features result primarily from changes in fluid pressure in relation with tectonic activity or changes in the structural levels. However, some associated or superimposed processes could result in additional changes in the fluid density, complicating the reconstruction of the P-T paths: i) the local fluid immiscibility which could be accompanied or not by heterogeneous fluid trapping, ii) superimposed fluid trapping in the same quartz grains due to intense and repeated microfissuring which could cause locally post-trapping changes, iii) water leakage linked to ductile deformation of quartz. The use of selected fluid inclusion data obtained on quartz domains apparently preserved from any of these disturbances thanks to the use of SEM cathodoluminescence, appears especially useful i) to give quantitative constraints on pressure changes occurring during quartz vein deformation, ii) to test models depicting the control of fluids on deformation of rocks, especially the necessary fluid pressure (Pf) variations to get fluid-pressure-activated systems, iii) to test the models of gold deposition, involving fluid phase separation. The limitations and advantages to the use of fluid inclusions to reconstruct P changes is discussed