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Organic Matter in Forming Gold-Deposits
Carbonaceous organic matter occurs under various phases and forms, where its fine characterization is mostly restricted to petroleum and coal geology. As a consequence, few studies have integrated the complete link between various forms of organic matter and metals to decipher hydrothermal ore concentrating processes. The study of Dill et al., integrating the concentration of sulfides and oxides with the interaction of silicates and organic matters, is an example of the next step to reach for defining the complex role of organic matter for the formation of orogenic gold deposits.
For at least 6000 years, the use of gold marked a change for humanity: the use of metals. Gold was the first metal used because it occurs in a native form. Gold is insoluble under surface conditions and non-oxidable, so its physical properties are conserved. Presenting a shining yellowish color like that of the sun and exhibiting extreme malleability, gold was first used for ornamental purposes and later as a medium of exchange and coinage. The first exploitations were from rivers or dried riverbeds, where gold was physically concentrated as nuggets. Later, gold was extracted from quartz vein outcroppings at surfaces. Both types of gold extraction are still in use today.
Humans have always had a fascination and irrational relationship with gold. Gold was and is still a physical means for conserving values (e.g., ). Consequently, wars, invasions, colonizations, and territorial conquests (gold rushes) were established and driven. Gold, as has any other substance, positively and negatively impacted human development. Artisanal gold extractions are still providing revenue for 15–20 million persons worldwide , whereas hundreds of mines are producing gold commercially in more than 42 countries ( www.gold.org (accessed on 21 March 2021)).
In the inorganic world of metals, minerals, and rocks, consideration of the roles of organic matter in accumulating, solubilizing, and precipitating gold in lodes was not a natural way of thinking for geologists. At first, it may appear paradoxical that the ultimate noble metal requires organic matter for concentration. In this contribution, I address recent advances regarding the role of carbon-rich organic matter in forming rich and large gold deposits in three stages: (1) the source stage, when gold in seawater accumulates in organic-rich sediments; (2) the mobilization stage, when gold is solubilized by hydrocarbon-metal complexes and colloidal nanoparticles for hydrothermal transport along faults; and (3) the precipitation stage. It is demonstrated that unusual CO2-rich, H2O-poor fluids, documented for some of the largest and richest orogenic gold deposits, are the result of chemical reactions involving hydrocarbon degradation, hence demonstrating the fundamental role of carbonaceous organic matter.
2. Fluid Composition and Generation
The fluid composition of orogenic gold deposits was estimated from the study of fluid inclusions for more than 70 years (e.g., ). Hundreds of studies have detailed the mineralizing fluids from worldwide examples covering all ages (e.g., ). The fluids are aqueous with low salinity (<5 wt% NaCl equiv.), ubiquitous CO2, and variable contents of N2, CH4, and, in some cases, H2, C2H6, H2S, He, and Ar. Thermodynamic calculations have demonstrated that metamorphic dehydration of seafloor rocks is a viable mechanism for producing abundant aqueous-carbonic and low-salinity fluids. Elmer et al.  and Phillip and Powell  demonstrated that seafloor rocks, hydrated initially by hydrothermal seawater convection cells at oceanic ridges , release fluids at the metamorphic transition of greenschist to amphibolite, mostly when chlorite is converted to amphibole. Metamorphic fluids have a more diverse volatile composition than other fluids, such as seawater, magmatic fluids, or meteoric fluids, because they are generated by devolatilization of lithologies, where organic compounds in sedimentary rocks contribute to C-O-H-S-N contents .
Of particular interest, CO2-rich and H2O-poor fluid inclusions have been documented from some world-class gold districts and deposits, such as those at the Red Lake , Ashanti  and Tarkwa goldfields , and the Detour Gold and Wona deposits . Fluids for these deposits also contain CH4, N2, and C2H6. The origins of these fluids are still debated (e.g., ).
For the Paleozoic Ashanti gold belt, Western Africa, Goldfarb et al.  suggested that devolatilization of abundant carbonaceous schists and cherts could lead to a variety of carbon-bearing molecular components within metamorphic C-O-H-S fluids bearing gold. Such a sedimentary source is confirmed by the compositions of stable carbon isotopic mixture in quartz-hosted, CO2-rich fluid inclusions . These unusual fluids are thus likely derived from the metamorphism of carbonaceous-rich sedimentary rocks. Nonetheless, these fluids are associated with either very high-grade or very large gold deposits, suggesting that CO2-rich and H2O-poor fluids have unrecognized potential for forming exceptional orogenic gold deposits.
3. Sources of Gold
The sources of gold for orogenic deposits have been reviewed by numerous authors (e.g., ). Gold can be sourced from intrusion degassing (e.g., ) and oceanic basalt devolatilization (e.g., ). However, carbonaceous- and pyrite-rich sedimentary rocks, commonly referred to as black shales, are considered one of the most important sources (e.g., ).
Gold and other trace metals occurring in sedimentary pyrite can be liberated by recrystallization and hydrothermal replacement processes occurring under metamorphic conditions, corresponding to the pyrite–pyrrhotite transition . Gold concentrations in nodular pyrite average 0.09 ppm  but can reach 10 ppm Au in orogenic gold districts (e.g., ). Considering that nodular pyrite can constitute up to 20% of black shales and that black shales are very extensive marine sediments, these rocks may constitute a significant volume for providing gold. In addition, because pyrrhotite (Fe1-xS) has a lower S content (37.67% vs. 53.45%) than pyrite (FeS2), the conversion can also provide S in solution for solubilizing gold . Gaboury  stated that, in addition to gold, fluids and ligands (HS-) can all be sourced from the metamorphism of black shales and associated rocks under amphibolite conditions.
Using a solid-probe mass spectrometer system , Gaboury  documented that ethane (C2H6) is present in fluid inclusions from orogenic gold deposits, ranging in age from ~2800 Ma to ~100 Ma. Ethane is sourced from thermally degraded organic matter because the values of CH4/(C2H6 + C3H8), expressed as C1/C2+ in hydrothermal fluids, are lower than 100 . Consequently, ethane provides a reliable tracer for the involvement of carbonaceous and pyritic shales at depth in the formation of gold deposits .
The ultimate support for a sedimentary gold source model is provided by the link between gold dissolved in oceans and the temporal distribution of orogenic gold deposits. This was first proposed by Tomkins  and later documented by Large et al.  by using gold concentrations in primary pyrites from black shales. Oxidizing seawater conditions are favorable for gathering gold in nodular sedimentary pyrite in black shales . The lack of major orogenic gold deposits from the middle to late Proterozoic (~1800 to 800 Ma—the boring period) is interpreted as being related to low levels of Au in the oceans . During this period, the deep oceans were anoxic and sulfidic , hence limiting the bacterial reduction of sulfate and the incorporation of gold in primary pyrite . The occurrence of orogenic gold deposits in Neoproterozoic time, such as those in Sudan, which also contain ethane , coincides with the reappearance of oxygenic conditions in the oceans .
4. The Fundamental Involvement of Organic Matter
The entry is from 10.3390/geosciences11080344
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