Grain-scale permeability enhancement and reactive fluid flow during the formation of disseminated gold mineralization in orogenic gold systems

The nature of permeability and reactive fluid flow during the formation of disseminated mineralization within orogenic gold deposits remains poorly understood despite these factors being major controls on the formation of the vast majority of orogenic gold deposits. This study provides new insight into previously underexplored physicochemical processes in disseminated orogenic gold systems. To do so, we focused on the orogenic Zaozigou gold-antimony deposit, within the West Qinling Orogen of China. New structural analyses, microstructural observations, whole rock geochemical data and thermodynamic modeling of chemical reaction in mineralized dacites provide insight into permeability enhancement and reactive fluid flow during mineral deposit formation. The altered dacites proximal to auriferous quartz veins have well-defined alteration zones from intense to weak alteration with distance from the gold-bearing quartz veins in the deposit. Interconnected cracks within the weakly altered dacite and the reduction of grain size in the completely altered dacite record an increase in permeability as a result of coupled grain-scale microcracking and mineral reactions, although subsequent mineral growth indicates that the enhanced permeability is ultimately lost through mineral precipitation. Hydrothermal minerals, such as sericite and albite, are primarily distributed along cleavage planes in biotite and within newly formed pore spaces in plagioclase, with hydrothermal dolomite also forming around biotite and plagioclase pseudomorphs. The spatial distribution of these newly-formed minerals indicates that grain-scale fluid flow occurred predominantly along pre-existing cleavage planes, as well as along newly formed microcracks and pores. Whole-rock geochemical data and thermodynamic modeling indicate that sulfidation reactions enabled the precipitation of disseminated gold mineralization within altered dacite, and that compositional changes during reactive fluid flow are the primary drivers of alteration zoning during fluid infiltration into the wall-rocks adjacent to the mineralized veins. This study reveals how permeability and mineral reaction fronts evolve in disseminated systems lacking clear vein networks, providing new insights into disseminated mineralization in orogenic gold systems.