Hi Nasser, favourable wall rocks should either contain high Fe contents (i.e. mafic rather than felsic compositions) or organic material (e.g. black shales), because they act as a reducing agent on the hydrothermal fluids which can trigger the gold precipitation. Cheers Daniel

Dear Daniel

Thank you for following up this question

Based on your statement, high CO2 could unstable Au bearing complexes and consequently precipitated Au in hydrothermal flouids.

So can it be concluded that CO2 do not play important role in separation of Au ion from magma,

Is it true?

Dear Meysam

Thank you very much for following up this question and your description about the mechanisms of entering CO2 to magma

generally, gold is soluble (as Au(HS)2-) when CO2-H2O fluid is gomogenous. As soon as CO2 separated into gas phase, HS- converted into H2S, and gold precipitates

Dear Timur

Thank you

Dear Nasseri,

You could also look at these articles and the references therein: Article The role of carbon dioxide in the transport and fractionatio...

Article Combined effect of carbon dioxide and sulfur on vapor-liquid...

Dear Maria,

Thank you for following up this question and your perfect article but your article and most of studies are focused on carbon which added to magma by digesting carbonate rocks such as limestone, and it does not mention the role of carboniferous host rocks in precipitation of gold. I'm looking to understand the role of carbon (could be transformed to activated carbon due to metasomatism of organic matters from shales in surrounding of Intruded magma) in trapping gold specially in Intrusion related Gold Systems -IRGS.

Dear Nasser, carbonaceous host rocks can change the chemical equilibrium of migrating hydrothermal fluids, because they can buffer their pH and also reduce their fO2. This can trigger the precipitation of sulfides. If the original hydrothermal fluid contained metals such as gold, it can precipitate auriferous sulfides.

Hence, when exploring for low-S epithermal gold deposits, the composition of the wall rocks can make an important difference.

Dear Daniel, I think organic matter like coal specially in shale and sandstone cannot play as a buffering agent

Beside of gold mineralization in an intrusion related gold systems in parallel sheeted veins, host rocks such as shale have high background geochemical anomalies of Au for example in my study area more than 200 ppb

Dear Nasser, organic matter in shales can definitely act as a buffer. A prominent example is the Carbon leader reef at the Witwatersrand in South Africa. Due to its high gold grades it has been mined out already many years ago at Venterspost and Kloof gold mines. The Witwatersrand does not only contain placer gold (primary), but some of that gold has subsequently been re-mobilized and re-crystallized at: (1) the lithological contact to hot andesites at about 2.1 Ga (i.e. Ventersdorp contact reef), and (2) the Carbon leader reef, respectively. The native gold flakes of the Carbon leader reef even had the shapes of algae, because the gold had precipitated along its contacts. Paul Ramdohr (author of "The ore minerals and their intergrowths") has documented this in the sixties already.

Dear Daniel,

Fuchs et al., 2014 investigate the role of organic matter in the gold mineralization in Carbon Leader Reef of the Witwatersrand deposit using Micro PIXE and micro-EBS analyses. 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.

Dear Ioan,

Thank you for introducing an interesting article.

Thank you Jorge

I don't agree that gold could form stable complexes with carbonate ligands. The principle difference between gold and uranium is that gold is a soft metal according to the Pearson classification, whereas uranium is a hard metal. Carbonate is a hard anion and tends to combine with hard metals. Up to date no experimental confirmation exist that gold could form stable complexes with carbonate and bicarbonate ions. E.g., in the recent article of Kusebauch et al. (2019) (Science Advances, 5, 5, DOI: 10.1126/sciadv.aav5891) carbonate complexes of gold are completely ignored because of their weakness.

Dear Naser

you can find some data about CO2 source in magmatism, in the work of Whitely et al, 2019.

"Crustal CO2 contribution to subduction zone degassing recorded through calc-silicate xenoliths in arc lavas"

Regards

Afshin

Dear Afshin

thank you for recommending a particular article about sources of CO2 in magma but I am seeking about transportation geochemistry of gold from magma to host rock.

Dear Nasser Nasseri:

It is not very clear for the role of CO2 playing in precipitation or transport of Au for intrusion-related Au systems, although the ore fluids are commonly low salinity carbonic fluids that often contain CH4, suggesting that interaction of the intrusions with reduced-carbon bearing country rocks be essential in reduction of the magmatic-hydrothemal systems. Basically, the granitoid intrusions are dominantly ilmenite-series I-type, which are most likely caused by assimilation/contamination of the country rocks that are rich in organic carbon - sometimes graphite-bearing if metamorphosed.

Best regards,

Xue-Ming Yang

It can directly affect the pH of the environment and this modifies the solubility of the Au for forming complexes with species