Aseismic Refinement of Orogenic Gold Systems

Abstract

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.

Transport and coarsening of gold nanoparticles in an orogenic deposit by dissolution–reprecipitation and Ostwald ripening

“…4a-c) from the very beginning of the FIB sample thinning; (4) the gap in the FIB section widened during the FIB milling process, hence material was being removed and not added during this process (Fig. 4); (5) the Fe oxide between gold and albite narrowed during the FIB milling process ( Fig. 4b-d), reinforcing the first two points; and (6) the Fe-oxides adjacent to pyrite and on the margin of gold include goethite, hematite, and magnetite/maghemite, as confirmed by selected area electron diffraction (SAED; Fig.…”

“…rogenic Au deposits represent one of the most important Au resources on the planet, particularly those formed in Archean terranes 1,2 . As the name suggests, these epigenetic deposits formed during collisional orogeny 3 and, regardless of age and setting, are known for spectacular high-grade ore zones with coarse gold 1,[4][5][6] . There is, however, a growing recognition that the formation of such high-grade coarse gold is difficult to explain using conventional Au-in-solution processes [4][5][6][7] .…”

“…As the name suggests, these epigenetic deposits formed during collisional orogeny 3 and, regardless of age and setting, are known for spectacular high-grade ore zones with coarse gold 1,[4][5][6] . There is, however, a growing recognition that the formation of such high-grade coarse gold is difficult to explain using conventional Au-in-solution processes [4][5][6][7] . This has therefore catalyzed in recent years increased scrutiny of other possible mechanisms, such as Au enrichment via nanoparticle processes, to account for the formation of coarse gold 5,6,[8][9][10] .…”

“…There is, however, a growing recognition that the formation of such high-grade coarse gold is difficult to explain using conventional Au-in-solution processes [4][5][6][7] . This has therefore catalyzed in recent years increased scrutiny of other possible mechanisms, such as Au enrichment via nanoparticle processes, to account for the formation of coarse gold 5,6,[8][9][10] .…”

“…The aqueous transport of primary gold nanoparticles has for some time been shown to be a factor in high-grade, "bonanzatype" gold ore zones in epithermal deposits 8,10,11 and thus may also be a factor in forming high-grade gold in other deposit types 5,6,9,[11][12][13][14][15] . However, until now documenting gold nanoparticles as part of Au remobilization owing to secondary processes, as in orogenic deposits, has not been demonstrated.…”