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The Neglected Involvement of Organic Matter in Forming Large and Rich Hydrothermal Orogenic Gold Deposits

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The Neglected Involvement of Organic Matter in Forming Large and Rich Hydrothermal Orogenic Gold Deposits geosciences Review The Neglected Involvement of Organic Matter in Forming Large and Rich Hydrothermal Orogenic Gold Deposits Damien Gaboury Laboratoire de Métallogénie Expérimentale et Quantitative (LAMEQ), Université du Québec à Chicoutimi (UQAC), 555 Boulevard de l’Université, Chicoutimi, QC G7H 2B1, Canada; [email protected] Abstract: Orogenic gold deposits have provided most of gold to humanity. These deposits were formed by fluids carrying dissolved gold at temperatures of 200–500 ◦C and at crustal depths of 4–12 km. The model involves gold mobilization as HS− complexes in aqueous solution buffered by CO2, with gold precipitation following changes in pH, redox activity (f O2), or H2S activity. In this contribution, the involvement of carbonaceous organic matter is addressed by considering the formation of large and/or rich orogenic gold deposits in three stages: the source of gold, its solubilization, and its precipitation. First, gold accumulates in nodular pyrite within carbonaceous- rich sedimentary rocks formed by bacterial reduction of sulfates in seawater in black shales. Second, gold can be transported as hydrocarbon-metal complexes and colloidal gold nanoparticles for which the hydrocarbons can be generated from the thermal maturation of gold-bearing black shales or from abiotic origin. The capacity of hydrocarbons for solubilizing gold is greater than those of aqueous fluids. Third, gold can be precipitated efficiently with graphite derived from fluids containing hydrocarbons or by reducing organic-rich rocks. Black shales are thus a key component in the Citation: Gaboury, D. The Neglected formation of large and rich orogenic gold deposits from the standpoints of source, transport, and Involvement of Organic Matter in precipitation. Unusual CO2-rich, H2O-poor fluids are documented for some of the largest and richest Forming Large and Rich orogenic gold deposits, regardless of their age. These fluids are interpreted to result from chemical Hydrothermal Orogenic Gold reactions involving hydrocarbon degradation, hence supporting the fundamental role of organic Deposits. Geosciences 2021, 11, 344. https://doi.org/10.3390/ matter in forming exceptional orogenic gold deposits. geosciences11080344 Keywords: orogenic gold deposits; carbonaceous organic matter; hydrocarbons; graphite; CO2-rich fluids Academic Editors: Jesus Martinez-Frias and Gianluca Groppelli 1. Introduction Received: 14 May 2021 For at least 6000 years, the use of gold [1] marked a change for humanity: the use of Accepted: 13 August 2021 metals. Gold was the first metal used because it occurs in a native form. Gold is insoluble Published: 17 August 2021 under surface conditions and non-oxidable, so its physical properties are conserved. Pre- senting a shining yellowish color like that of the sun and exhibiting extreme malleability, Publisher’s Note: MDPI stays neutral gold was first used for ornamental purposes and later as a medium of exchange and with regard to jurisdictional claims in coinage. The first exploitations were from rivers or dried riverbeds, where gold was physi- published maps and institutional affil- cally concentrated as nuggets. Later, gold was extracted from quartz vein outcroppings at iations. 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., [2]). Consequently, wars, invasions, colonizations, and territorial conquests (gold rushes) were established and Copyright: © 2021 by the author. driven. Gold, as has any other substance, positively and negatively impacted human Licensee MDPI, Basel, Switzerland. development. Artisanal gold extractions are still providing revenue for 15–20 million This article is an open access article persons worldwide [3], whereas hundreds of mines are producing gold commercially in distributed under the terms and more than 42 countries (www.gold.org (accessed on 21 March 2021)). conditions of the Creative Commons In the inorganic world of metals, minerals, and rocks, consideration of the roles of Attribution (CC BY) license (https:// organic matter in accumulating, solubilizing, and precipitating gold in lodes was not creativecommons.org/licenses/by/ a natural way of thinking for geologists. At first, it may appear paradoxical that the 4.0/). Geosciences 2021, 11, 344. https://doi.org/10.3390/geosciences11080344 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, x FOR PEER REVIEW 2 of 13 Geosciences 2021, 11, 344 In the inorganic world of metals, minerals, and rocks, consideration of the roles 2of of or- 13 ganic 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 ad- ultimatevances regarding noble metal the role requires of carbon-rich organic matterorganic for matter concentration. in forming rich In this and contribution, large gold de- I addressposits in recentthree stages: advances (1) regardingthe source thestage, role when of carbon-rich gold in seawater organic accumulates matter in forming in organic- rich andrich sediments; large gold (2) deposits the mobilization in three stages: stage, wh (1)en the gold source is solubilized stage, when by hydrocarbon-metal gold in seawater accumulatescomplexes and in organic-richcolloidal nanoparticles sediments; for (2) hydr the mobilizationothermal transport stage, when along gold faults; is solubilizedand (3) the by hydrocarbon-metal complexes and colloidal nanoparticles for hydrothermal transport precipitation stage. It is demonstrated that unusual CO2-rich, H2O-poor fluids, documented alongfor some faults; of the and largest (3) theandprecipitation richest orogenic stage. goldIt deposits, is demonstrated are the result that of unusual chemical CO reactions2-rich, Hinvolving2O-poor hydrocarbonfluids, documented degradat forion, some hence of the demonstrating largest and richest the fundamental orogenic gold role deposits, of carbona- are theceous result organic of chemical matter. reactions involving hydrocarbon degradation, hence demonstrating the fundamental role of carbonaceous organic matter. 2. Orogenic Gold Deposits 2. Orogenic Gold Deposits “Gold is where you find it” was the adage used for explaining occurrences of gold. “Gold is where you find it” was the adage used for explaining occurrences of gold. Decades of research has provided a better understanding of the distribution of gold de- Decades of research has provided a better understanding of the distribution of gold de- posits inin spacespace andand timetime andand thethe physicochemicalphysicochemical processesprocesses involvedinvolved inin theirtheir formationformation (e.g.,(e.g., [[4–7]).4–7]). ApproximatelyApproximately 75% ofof goldgold extractedextracted byby humanityhumanity [[8]8] comescomes fromfrom aa geneticgenetic type calledcalled “orogenic“orogenic goldgold deposit”deposit” [[9].9]. GoldGold primarilyprimarily occursoccurs inin quartz-bearingquartz-bearing veinsveins (Figure(Figure1 ),1), formed formed at at depths depths between between 4 and4 and 12 km12 km and and with with temperatures temperatures ranging ranging from from 200 to200 500 to ◦500C; these °C; these are hosted are hosted in metamorphic in metamorphic volcano-sedimentary volcano-sedimentary belts. Gold belts. is Gold typically is typi- the onlycally metalthe only extracted metal economically,extracted economically, but silver can but be silver a by-product, can be a andby-product, Au/Ag ratiosand Au/Ag range fromratios 5 range to 10 from [10]. Typical5 to 10 [10]. gold Typical grades gold are in grades the range are in of the 4–8 range ppm Auof 4–8 [11 ],ppm and Au deposit [11]), sizesand deposit range from sizes tonnes range from to hundreds tonnes to of hundreds tonnes of in-situof tonnes gold. of in-situ gold. FigureFigure 1. TypicalTypical aspect aspect of of orogenic orogenic gold-bearing gold-bearing quartz quartz veins, veins, with with low low sulfide sulfide content content and anda strong a strong iron-carbonate iron-carbonate alter- alteration,ation, inducing inducing an orange an orange tint to tint the tohost the rocks. host ( rocks.A) Extensional, (A) Extensional, en echelon, en low-dippi echelon, low-dippingng quartz vein quartz network vein developed network developedin a competent in a competentrock bordered rock by bordered shear zo bynes, shear New zones, Rock mine, New RockSudan. mine, Yellow Sudan. field Yellow book (red field arrow) book (redas scale arrow) (20 ascm). scale (B) (20Laminated cm). (B) quartz Laminated vein quartzhosted veinin a re hostedverse inshear a reverse zone, shearNegeim zone, mine, Negeim Sudan. mine, Sudan. Orogenic gold deposits are hostedhosted inin accretionaryaccretionary oror collisionalcollisional orogens,orogens, rangingranging inin age fromfrom Meso-ArcheanMeso-Archean toto tertiarytertiary times.times. Modern analogues are presentlypresently formingforming atat depth, such as alongalong thethe AlpineAlpine FaultFault inin NewNew ZealandZealand [[12].12]. TheseThese orogens,orogens, commonlycommonly termed greenstonegreenstone belts,belts, areare composedcomposed mostlymostly ofof oceanicoceanic rocks,rocks, suchsuch asas basalticbasaltic lavaslavas andand sedimentary rocks. Although orogens cover th thee entire geological age spectrum [13], [13], oro- genic gold deposits are distributed
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