Aseismic refinement of orogenic gold systems

Abstract

©2020 Society of Economic Geologists, Inc.

Orogenic Au deposits have contributed the majority of Au recovered globally throughout history. However, the mechanism that concentrates Au to extremely high bonanza grades in small domains within these deposits remains enigmatic. The volume of fluid required to provide extreme Au endowments in localized occurrences is not reflected in field observations (e.g., in the extent of quartz veining or hydrothermal alteration). Detailed optical, scanning and transmission electron microscopy, nanoscale secondary ion mass spectrometry, and 3-D neutron tomography have been used to investigate the processes responsible for development of anomalously high grade ore (upward of 3% Au) found in quartz veins at Fosterville gold mine (Victoria, Australia). Distinct textural settings of visible Au include (1) Au concentrated along pressure solution seams associated with wall-rock selvages, (2) as nano- to microscale dusty Au seams parallel to pressure solution seams, and (3) in microscale tension fractures perpendicular to stylolitic seams. The distribution of Au in arsenopyrite and pyrite hosted within pressure solution seams changes as a function of the extent of deformation. Sulfides in highly deformed pressure solution seams exclusively host Au as nano- to micrometer-sized clusters within features associated with corrosion and brittle failure, whereas sulfides in mildly deformed pressure solution seams have Au bound in the crystal structure. It is proposed that Au supersaturation in fluids introduced during seismic periods led to the deposition of abundant Au nanoparticles in quartz-carbonate veins. Subsequent pressure dissolution of vein quartz and carbonate during interseismic intervals allowed for episodic increase in the Au/ quartz ratio and permitted liberation and migration of Au nanoparticles, promoting Au grain growth in favorable textural settings. Galvanic corrosion and brittle fracturing of auriferous sulfides during the interseismic period allowed additional remobilization and/or enrichment of sulfide-hosted Au. Repetition of this mechanism over the time scale of deposit formation acted to concentrate Au within the lodes. This Au ore upgrading model, referred to as “aseismic refinement,” provides a new insight for the genesis of ultrarich Au mineralization and, based on textures reported from many Au deposits, may be a globally significant component in the formation of orogenic Au deposits.