Q: What is the role of CO2 fugacity in trapping Au in magma and hydrothermal fluids?

There are many school of thoughts prevailing in the role of CO2 playing in the formation of gold bearing ores. As on today there is a heavy gold rush, it has become very important to know the present day scenario on the genesis of formation of gold and associated metals and the various possibilities of locating potential gold deposits through the latest findings and proposed views. The general review on the role of CO2 in making gold bearing sulphide deposits are listed out below.

Reply by Dr V.Ganesan, former Director, GSI

Role of CO2 playing in the formation of gold bearing ores:

There are many school of thoughts prevailing in the role of CO2 playing in the formation of gold bearing ores. As on today there is a heavy gold rush, it has become very important to know the present day scenario on the genesis of formation of gold and associated metals and the various possibilities of locating potential gold deposits through the latest findings and proposed views. The general review on the role of CO2 in making gold bearing sulphide deposits are listed out below.

1.There is a common belief that the presence of CO2 in a supercritical fluid may enhance mobility or massive precipitation of some metals, depending on salinity and sulfur content. There is a strong chemical bonding between gold ions and CO2 species which has a direct role in gold transport.

2. CO2 buffer the pH value of the solution and crystallize and concentrate base metal ore by complexation in reduced sulphur. It is seen in Carbonaceous host rocks change chemical equilibrium in hydrothermal fluids by buffering the pH values which in turn reduces the fO2 and trigger precipitation of sulphides.

3.The mechanism in hydrothermal fluids must be similar in CO2 complexes and transport gold until a certain location where the complex become unstable, low P and /or T, and then Au is precipitated. Based on mineralogical evidence and elemental imaging, scientists concluded that gold was mobilized in hydrothermal fluids and precipitated around bitumen nodules due to reduction of the ore fluid by organic matter.

4. Many studies on uranium leaching in rocks informed that the prevailing environmental CO2 plays a major role in complexing the formation of U as well as heavy metals. 

5. CO2 induces immiscibility in aqueous liquid and hydrothermal solution and acts as reducing agent which trigger the gold precipitation.

6. Continuous generation and supply of crustal CO2 to subduction zone where degassing is recorded through calc-silicate xenoliths in arc lavas which facilitated the flow of hydrothermal solutions. Similarly subducted oceanic crust and overlying sediments supply CO2 to arc magmas. At high level crust magma and wall rock mixing and interaction may produce CO2 which enrich the hot aqueous liquid with CO2.

8. It is reported that Fe content in felsic rocks and organic remains in black shale (carbonaceous) act as reducing agent and react with hydrothermal fluids and ppt gold.

9. Some different views are also reported that organic matter like coal, specially in shale and sandstone cannot play as a buffering agent whereas gold mineralization in an intrusion related gold systems in parallel sheeted veins hosted rocks such as shale have high background geochemical anomalies of Au >200 ppb.

10.Though placer golds are reported as major occurrence, the lithological contact to hot andesites showing remobilization and recrystallisation nature as seen in Venterspost Kloof gold mines and Carbon leader reef at the Witwatersrand in South Africa. Gold leaf in the shape of algae is also noticed in Carbon leader reef, indicating the influence of CO2 in gold concentration.

To understand the exact nature of the process, it is very much essential to know the following basic concepts of fO2 and fCO2.

In solution thermodynamics, Gilbert Newton Lewis carried out the study of different phases and chemical reaction equilibria involving gases (O2, CO2 ) at high pressures and introduced the term “fugacity” to represent the behavior of real gases in that. In the system under isothermal condition, there may be infinitesimal reversible changes occurring in ideal or non ideal gas and liquid mixtures. So fugacity explains the same relationship to chemical potential as the partial pressure of an ideal gas, that is the escaping tendency of the gas from one phase to other.

A high fugacity of water or oxygen means a high chemical potential of water or oxygen, respectively. A high chemical potential of water or oxygen indicates a “wet” or “oxidized” system, respectively. The oxygen fugacity (fO2) of a melt is a critical controlling parameter of the magmatic processes, as it controls the iron redox state of the melt (Kilinc et al. 1983; Botcharnikov et al. 2005) and strongly influences the crystallization sequences and composition of the crystallizing minerals. Even in a silicate melt, the dissolved FeO and Fe2O3 define the fO2. So we can define fO2 for groundwaters, mineral assemblages, gases, melts, metamorphic rocks etc.,.

Similarly, carbon dioxide (CO2) can change the pH of water and the formation of carbonate and bicarbonate act as good buffering agents making the hot liquids in more alkaline and facilitate to precipitate the sulphides in hydrothermal solutions.

         Carbon dioxide dissolves slightly in water to form a weak acid called carbonic acid, H2CO3, according to the following reaction:
                                    CO2 + H2O –> H2CO3 (carbonic acid)

         After that, carbonic acid reacts slightly and reversibly in water to form a hydronium cation, H3O+, and the bicarbonate ion, HCO3-, according to the following reaction:
           H2CO3 + H2O –>                               HCO3                                          + H3O+
carbonic acid + water  –>  bicarbonate (hydrogen carbonate-buffering agent)  +   hydronium oxide  

                                                                                                                      (acidic).

Carbonate and Bicarbonate act as good buffering agents in strong and weakly basic condition respectively. In aqueous solutions, carbonate, bicarbonate (HCO3), carbon dioxide, and carbonic acid exist together in equilibrium. In strongly basic conditions, the carbonate ion predominates, while in weakly basic conditions, the bicarbonate ion predominates. So fCO2 play an indirect role for the precipitation of sulphides under alkaline condition.

For a very long period, many scientists experimented on this, published their findings and suggested their views. They are listed below for a quick review and understanding. As it requires detailed study to orient the younger generation, the existing critical problems and the future course of studies to unravel the complicated natural facts on ore formation are briefed below.

The role of CO2    in gold bearing sulphide deposits as observed from different workers:

SNAuthorsFindings / OpinionSupportive lab / field  evidences
1Geoffrey neil Philips and Katharine anni Evans (2004)  1.Shown that much of global gold production has come from deposits with uneconomic concentrations of base metals, such as copper, lead and zincSupported with their     geochemical modellings and compared the determined pH values of samples from different geological setup.
  2. Chemical bonding between gold ions and CO2 species is not strong, and so it is unlikely that CO2 has a direct role in gold transport.They show that effective Au transport requires both H2S and CO2, which must be  incorporated at the source of Au. It constrains the genesis of ore-bearing fluids.
  3.CO2 Buffer the pH value of the solution and crystallize and concentrate basemetal ore by complexation in reduced sulphur. 
  4.K and Na metasomatism observed in gold deposit wallrocks indicates that both elements are intimately involved in the depositional process.It is therefore likely that these pH values derived indirectly using measurements of Na and K relate to intermediate depositional processes rather than values during gold transport.
  Characteristics determined at the source will be consistent throughout a group of gold deposits, for example, Yandal gold province, whereas those related to depositional sites, such as high iron or carbon content, will account for some of the variability between deposits.The effect of H2O–CO2 immiscibility may be important in gold deposition.
2Fuchs et al., (2014 )Based on mineralogical evidence and elemental imaging, they conclude that gold was mobilized in hydrothermal fluids and precipitated around bitumen nodules due to reduction of the ore fluid by organic matter. Investigated the role of organic matter in the gold mineralization in Carbon Leader Reef of the Witwatersrand deposit using Micro PIXE and micro-EBS analyses. It supports the Paul Ramdohr findings.
3Meysam Akbari (2019)Fe content in felsic rocks and organic remains in black shale act as reducing agent and  react with hydrothermal fluids and ppt goldSource of CO2 from different magma have been explained
  Subducted oceanic crust and overlying sediments supply CO2 to arc magmas, magma and wall rock mixing and interaction may produce CO2 and enrich the hot aqueous liquid with CO2. CO2 induce immiscibility in aqueous liquid and hydrothermal solution and act as reducing agent which trigger the gold pption. 
4Daniel Muller(2019)Carbonaceous host rocks change chemical equilibrium in hydrothermal fluids by buffering the pH values which in turn reduces the fO2 and trigger precipitation of sulphides.Accepts Philips and Evans view
5Whitely et al, (2019)Reported generation of CO2 ie from Crustal CO2 contribution to subduction zone degassing recorded through calc-silicate xenoliths in arc lavas.Carbonate cycle from upper crust to lower crust through subduction zone and mingling with arc lavas and supply CO2 gases.
6Jorge Costa-de-Moura (2019)Informed many studies of uranium leaching in rocks demonstrated that environmental CO2 complexes U as well as heavy metals. The mechanism in hydrothermal fluids, supercritical fluids, must be similar in CO2 complexes and transport gold until a certain location where the complex become unstable, low P and/or T, and then Au is precipitatedFavouring CO2 induced transportation to hydrothermal fluids to find a favourable zone of gold precipitation.
7Xue-Ming Yang (2019)The author finds that the ore bearing fluids are commonly low salinity carbonic but often contain CH4. The interaction of the intrusions with reduced-carbon bearing country rocks be essential in reduction of the magmatic-hydrothemal systems.The author questions the role of CO2 playing in precipitation or transport of Au for intrusion-related Au systems in a CH4 rich low salinity carbonic liquid?
8Gregg Morrison (2019)Mentioned about the source of origin of carbon-di-oxide. Combustion of fossil fuels, volcanic eruptions add carbon dioxide to air. In the aquatic ecosystem carbon dioxide can be stored in rocks and sediments but it will take a long time to be released through weathering of rocks or geologic processes. Carbon dioxide that is stored in water will be present as either carbonate or bicarbonate ions. These ions are an important part of natural buffers that prevent the water from becoming too acidic or too basic.  Favours the CO2 act as a buffering agent in hot acqueous liquid.     
9Maria A. Kokh, N. N. Akinfiev  and Stefano Salvi  et alWorked on the role of carbon dioxide in the transport and fractionation of metals by geological fluids. 1. Performed laboratory scale solubility measurement experiments using a flexible-cell reactor equipped with a rapid sampling device.
  They proved that (1) solubilities of Si, Au, Mo, Pt and Cu either decrease (within <1 log unit) or remain constant upon CO2 increase, whereas those of Fe, Zn and Sn increase significantly (>1 log unit) with CO2 contents in the fluid increasing from 0 to 50 wt%.  (2) The transport of gold is unfavorable in the presence of CO2, only in S-rich (>0.5 wt% S) fluids in which Au forms the negatively charged Au(HS)2– and Au(HS)S3– complexes. Whereas, it is weakly affected in S-poor (<0.1 wt% S) acidic-to-neutral fluids in which the uncharged Au(HS)0 complex predominates.
  In more saline-oxidizing and S-rich fluids such as those in magmatic porphyry Cu-Au deposits, the Fe, Cu, and Au solubilities in the presence of CO2 decrease by ∼1 order of magnitude with CO2 increasing to 20-30 wt%, following the decrease in the stability of their dominant charged species (FeCl2O-, CuCl2-, Au(HS)2– and Au(HS)S3-), but stay almost constant at higher CO2 contents (30-70 wt%) as controlled by the neutral species (FeCl2O, Cu(HS)O and Au(HS)O).  
   Based on these above experiments they conclude that “This study does not support the common belief that the presence of CO2 in a supercritical fluid may lead to enhanced mobility or massive precipitation of some metals, depending on salinity and sulfur content, and, more generally, no significant fractionations happening between different metals.” 
10Maria A. Kokh, 2016S-free CO2 present in vapour-liquid systemEnhance Au and Pt partioning in vapour and  slight change in Fe,Cu, but no change in Zn,Mo,Si,K, Sn.
  In low S bearing CO2 present in vapour –liquid system.CO2 favours the vapour-liquid immiscibility  extends even at higher T-P conditions and  the reduced sulphur in the system enhanced partiniong of sulphur loving metals (Au,Pt and Mo) into the vapour phase and concentration of alkali, basemetals (Na, K,Fe,Cu) and chloride into the salt rich liquid phase.
  The above study reveals that CO2 presence plays the role in hydrothermal system .Influence the depth of ore deposition and Vertical metal zonation in hydrothermal system. 
11 Nasser Nasseri (2019) Reported organic matter like coal specially in shale and sandstone cannot play as a buffering agent whereas gold mineralization in an intrusion related gold systems in parallel sheeted veins, host rocks such as shale have high background geochemical anomalies of Au >200 ppb. Denying the organic matter act as buffer in shale and sandstone bearing horizon but opined that the hot liquid dissolves the gold present in the host rock.
12Paul Ramdohr (2019)Though placer golds are reported, the lithological contact to hot andesites showing remobilization and recrystallisation nature in Venterspost and Kloof gold mines, Carbon leader reef at the Witwatersrand in South Africa and gold leaf in the shape of algae also noticed Carbon leader reefSuggested in his article “The ore minerals and their intergrowths” ie organic matter in shales can definitely act as a buffer. 
13 Qing Wei,Hongrui Fan,Jacques Pironon andXuan Liu,(2020) Since the auriferous quartz veining due to CO2 content variations and decompressional cooling are noticed in Linglong goldfield in Jiaodong, it is targeted to investigate gold-producing quartz veining process. Quartz is the most common gangue mineral in hydrothermal veins. Coupled with capacities of hosting fluid inclusions and recording varieties of microtextures, its solubility behavior may provide unparalleled insights into hydrothermal processes. Through Quartz Solubility, SEM-CL (cathodoluminescence)  and Fluid Inclusion Analyses, it was brought out that there are three episodes of quartz deposition, intervened by an episode of quartz dissolution.This research may elucidate gold formation processes in orogenic intrusion—related deposits, and points to imperative CL-based in situ analyses for future studies.
  The earliest quartz (Qz1) was controlled by CO2 content increase and subordinately affected by decompressional cooling, leading to the formation of the early thick gold-barren veins (V1); 
  The second generation of quartz (Qz2a) was formed by the same fluids that may have been diluted and cooled by meteoric water, leading to a greatly reduced quantity of quartz and the deposition of pyrite and gold; 
  The third generation of quartz (Qz2b) was deposited along with polymetallic sulfides, due to fluid cooling following a quartz dissolution event likely induced by cooling in retrograde solubility region and/or CO2 content decrease.

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