Gold deposits.

Gold ore deposits have a global presence, spanning the earth’s extensive geological timeline, dating back to the earliest Precambrian period over 3.1 billion years ago. Gold is a fundamental element in the earth’s metallic core, being enriched in the mantle. Melts originating from the mantle may contain elevated gold content, and with favorable ascent and evolution, gold concentrations can further increase in late melts or immiscible sulfide melts. Deep-sourced melts, enriched in gold, contribute to the formation of many sulfide deposits through igneous processes.

Evolution of Gold Deposits: Unveiling Geological Peaks.

Gold deposits exhibit a non-uniform distribution over geological time, with distinct periods of prominence. The Mesoarchean era stands out as the largest gold period, primarily due to the significant contribution of a single deposit—the Witwatersrand paleoplacers in South Africa, accounting for approximately 90,000 tonnes of gold. Subsequent peaks in gold deposits occurred in the Neoarchean, Paleoproterozoic, and Paleozoic eras, characterized by the dominance of orogenic gold deposits formed during major orogenies.

The Cenozoic era witnessed the formation of numerous and diverse gold deposit types, including epithermal, porphyry, skarn, and Carlin-type deposits. However, the observed distribution of gold periods in the Cenozoic is influenced by the preservation duration of certain deposit types. Surficial deposits, such as porphyry and epithermal deposits, are more susceptible to erosion and eventual disappearance, artificially amplifying the gold period observed in the Cenozoic compared to older periods, which may be minimized due to preservation challenges. This highlights the importance of considering preservation biases when evaluating the temporal evolution of gold deposits.

As we move closer to the earth’s surface, gold undergoes additional enrichment through two mechanisms. The first mechanism involves the resilience of gold during weathering, allowing it to withstand rock erosion and become mechanically concentrated in “placer” or alluvial deposits. The second method is through hydrothermal activity, where gold is mobilized from its primary source in the earth’s crust.

Sedimentary Gold Deposits: Origins and Challenges of Paleoplacers.

This section explores gold deposits of sedimentary origin, specifically focusing on placers. While hydrothermal processes primarily contribute to gold deposits, sedimentary processes, particularly in the form of placers, also play a significant role. Placers are secondary deposits formed through remobilization, transportation, and re-sedimentation of dense detrital minerals. Two main types of placers are distinguished based on their period of formation: neo-placers (Eocene to Quaternary) and paleoplacers, with the latter being more commonly exploited.

Paleoplacers, prevalent in deposits such as Witwatersrand in South Africa, Tarkwa in Ghana, and Jacobina in Brazil, occur in conglomerates of likely fluvial origin and rich in quartz. The Witwatersrand deposit, dominating global gold production, has sparked debates regarding its origin—whether hydrothermal, detrital, or a combination of both. Re-Os data suggest an age of gold grains preceding the sedimentary basin’s age, with recent studies favoring a detrital model, possibly with minor hydrothermal contribution.

Identifying the source of gold in paleoplacers poses a significant challenge. While some placers in the Pacific belt, Russia, or Australia have been linked to existing orogenic gold deposits, many placer sources have disappeared, complicating their identification. The study underscores the complexities involved in understanding the formation and sources of sedimentary gold deposits, particularly paleoplacers.

Hydrothermal Gold Deposits: Role of Circulating Fluids in Crustal Formation.

This section delves into gold deposits of hydrothermal origin, emphasizing their dependence on circulating hydrothermal fluids, distinct from magmatic fluids. While these deposits may share associations with magmatic intrusions, the primary role of these intrusions lies in serving as heat engines for fluid circulations. Hydrothermal deposits encompass a variety of fluid sources, including metamorphic fluids, meteoric water, sea water, and connate waters, highlighting the diverse geological processes contributing to their formation. The section underscores the significance of understanding the intricate interactions between hydrothermal fluids and crustal elements in shaping gold deposits.

Gold deposits continue to form today in active geothermal areas, showcasing the longevity of this geological process. There are six recognized “world-class” settings for magmatic-hydrothermal gold deposits, excluding secondary deposits like placers. These primary deposit types include orogenic gold deposits, Carlin-type gold deposits, epithermal deposits, porphyry copper-gold deposits, iron oxide copper-gold deposits, and gold-rich massive sulfide deposits.

In summary, gold’s presence and concentration are intricately linked to geological processes spanning billions of years, with various mechanisms contributing to its enrichment in different geological settings. The classification of world-class gold deposits further emphasizes the diversity of environments where these deposits can be found.

Selected Gold Deposits Illustrating Productivity of Diverse Geological Associations

Note: g t–1 = grams per tonne.

Geodynamics and Formation of Gold Deposits.

Gold deposits exhibit a diverse range of geologies and formation processes, with each type forming under specific geodynamic conditions. The study of gold deposits serves not only economic purposes but also provides insights into the geological events that led to their formation. Active margins, particularly subduction zones, are identified as highly effective in hosting large epigenetic gold deposits, including orogenic gold deposits, porphyry, and epithermal deposits. The concentration of gold in these deposits is influenced by geological factors such as crustal nature and lithospheric plate thickness.

Huge orogenic gold deposits are more likely to form in orogenies involving the subduction of oceanic lithosphere, where the thin continental lithosphere facilitates fluid circulation. Geodynamic processes also influence the depth of deposit emplacement, with sedimentary deposits like Placers forming at the surface, and hydrothermal and magmatic deposits emplaced at different crustal levels. Orogenic gold deposits, occurring at depths from 5 to over 25 km, show better preservation over geological time.

The largest gold deposits, excluding Placers, typically form during periods of crustal growth, characterized by important exchanges between the mantle and crust and associated heat flows favoring fluid circulation. The study of gold deposit formation indirectly contributes to understanding these geological exchanges during periods of crustal accretion. The presented information emphasizes the importance of geodynamic processes in shaping gold deposits and their significance in unraveling geological events.