Aseismic Refinement of Orogenic Gold Systems

Orogenic gold deposits contribute the majority of gold recovered globally throughout history. However, the mechanism that concentrates gold to extremely high “bonanza” grades within these deposits remains enigmatic. The volume of fluid experimentally required to provide such endowments of gold in a localised area is not reflected in field observations (e.g., in the extent of quartz veining or hydrothermal mineral alteration). To explain the genesis of bonanza-grade gold lodes in orogenic settings, a model termed “aseismic refinement” has been developed through characterisation of ultra high-grade (upwards of ~3% Au) ore found in quartz veins at Fosterville gold mine (Victoria, Australia). Three distinct textural settings of visible gold have been distinguished and include (i) gold along stylolitic wallrock selvage pressure-solution seams (PSS); (ii) as a fine-grained “dusty gold seams” parallel and in the same orientation as PSS; and (iii) in tensile fractures which

are perpendicular to PSS. As visibile gold is most abundant when in conjunction with these features, it follows that pressure-dissolution may govern gold distribution and inter-seismic processes are likely to be signifcant during oregenesis. It is proposed that depressurization and fluid unmixing during seismic periods led to the deposition of abundant Au nanoparticles (Au NPs) in quartz-carbonate veins at Fosterville. Subsequent pressure-dissolution of quartz during interseismic intervals allowed for episodic removal of gangue material and the amalgamation of these Au nanoparticles around pressure-dissolution features. Repetition of this mechanism over the time scale of deposit formation acts to concentrate gold to anomalously high grades within the host vein. This mechanism provides a new insight for the genesis of ultra-rich gold mineralisation and,

based on textures reported from many gold deposits, may be an important step in the formation of orogenic gold deposits.