Evolutionary Trends for the Auriferous Hydrothermal Fluids Inclusions: A Microstructural Study of the Emplacement of Banded Quartz Veins in the Birimian Rocks in Southern Ghana

Abstract

Using polarizing microscope, the Scanning Electron Microscope (SEM) and the LINKAM THM 600, techniques, genetic connections between opening of space, the filling with mobile hydrothermal fluid phase, of banded gold quartz veins from the Birimian rocks were studied. The internal structure and the filling along the vein-wall contact suggest a crack-seal mechanism of emplacement. Observations of inhomogeneous extinction of bands under crossed polars, show the opening direction across the vein. The ion content of the fluid inclusion that find their way into the interstice are controlled by low frequency and temperature. Microthermometric data on primary inclusions indicate that there are two main types of fluids: partly aqueous (CO2-rich ) fluid, and aqueous (H2O- rich,) fluid. The limited range of the degree of fill (0.5 to 0.7) for the CO2 – rich and H2O – rich inclusions indicates the original homogeneous fluid. The CO2-rich fluids homogenize at a higher temperatures (350 – 400°C) than the H2O-rich fluids (310 – 370°C) and had salinity ≈5 wt % NaCl equiv. while H2O-rich fluid had salinity ≈16 wt % equiv. Dominant trace gases in both fluids are CH4 and N2. Observations suggest that the geometric peculiarities of macroscopic growth mechanism may account for capillary effects. Based on this consideration, a simple quantitative model of quartz vein formation is proposed. This model provides a possible origin for the enhancement and maintenance of a diffusional mass transfer from the matrix to the position of the crack. The study of the development of such veins and dikes with discontinuities, is a vital tool to the discovery of hydrothermal gold and other economic minerals. It can also helps to understand crystal growth in localized weak planes within massive blocks of quartz, which in time and space may preferentially accommodate further brittle deformation; a phenomena which often affects long term sustainability of engineering projects such as hydro-dams and tunnels.