Dear Christopher, please try these:

Tomkins 2002 Mobilization of Gold as a Polymetallic Melt during Pelite Anatexis at the Challenger Deposit, South Australia: A Metamorphosed Archean Gold Deposit, Economic Geology, 97:1249-1271

and

Engstrom 2013 Metal mobility during metamorphism.....MSc thesis

regards

I can recommend following paper

Pitcairn, I.K., Teagle, D.A.H., Craw, D., Olivo, G.R., Kerrich, R., Brewer, T.S. (2006) Sources of metals in orogenic gold deposits: insights from the Otago and Alpine Schists, New Zealand. Economic Geology, v. 101, p. 1525–1546

I would worry about the composition of fluids that may have gone through your rocks, which of course means that there is no blanket answer to this question.The presence of potential ligands in the fluids may be an important factor.

Thanks Evandro, Tobias and Marlina.

Marlina, yes I was wondering about that. I guess mobility would vary if, for example, it was a high sulfur or low sulfur system. However, I was also wondering whether, whatever the ligands, there are structural constraints as well? Whether increasing the metamorphic grade progressively shuts down much mobility until retrograde metamorphism sets in?

It is very clear that the carbon mobility, shearing, sulphur and arsenic concentrations shows intimate relationship with gold mobility in meta-sediments. Several researches by scientists uploaded on researchgate also very helping.

Dear Dr. Brough,

some papers have been cited to provide us with an insight into this matter. But let me frankly say that I am in doubt of the results obtained and the interpretation based upon them, despite the prestigious journals they have been published. Chemical and mineralogical changes in a progressive metamorphic system are much more difficult to track than those systems evolving in the opposite direction. If we deal with relics it may still work under certain circumstances, but in the Q&A it is the mobilized part of the story which we are talking about. Therefore, to expect a coherent chain of processes in a more or less closed system that might give us a proper insight into the way of mobilization is in my opinion a bit too close to wishful thinking. So we have to take refuge in different metamorphic compartments being juxtaposed to each other so that we can see if and how gold is mobilized. This is more based on practical experience using different commodities rather than on a list of papers focused on gold. But I am open for examples contradicting my personal view. I am a constant learner and even an old dog can learn new tricks.

Best regards

Harald G. Dill

Dear Prof. Dill,

Thank you for your comments. If I understand you correctly you are suggesting that at best the literature cited above can only be taken as indicative of the process of gold mobilisation during progressive metamorphism? Do you think there are no general rules that we can point to, or are we restricted to case study examples?

The question itself arises from a recent project where gold mineralisation was distinctly pre-peak but despite reaching granulite facies during peak metamorphism there has been limited movement of the gold (to be sure some micro-fracture movement, but the mineralisation carries the fold geometry of the orogenic event). This development seems distinctly counter-intuitive to me, and as Engstrom notes increasing metamorphic grade in the Dalradian is associated with increasing mobilisation of gold.

Thanks, 

Chris

Fluid flow related to high-grade metamorphism would mainly come from the dehydration of hydroxyl-bearing phases such as the micas and amphiboles. That fluid may not have contained the suitable ligands for gold mobilisation. And once granulite facies has been reached, everything is dry, and gold mobilisation becomes very difficult.

Dear Christopher,

there is no easy anwer to your question because the factors controlling Au solubility in geological fluids are many (T, P, fluid composition, permeability, connectivity of fracture networks, etc.), and you need to make many assumptions to generalize. A good summary of how these factors depend on each other can be extracted from recent papers of Gleb Pokrovski (Pokrovski et al. 2014: "Gold speciation and transport in geological fluids: insights from experiments and physical chemical modeling" The Geological Society of London. Spec. Pub. no. 402, 9-70). See if that work focussing on experiental data can be inspiring for your specific question.

Also, consider that at T higher than about 300 °C Au is soluble also in a S-bearing VAPOR phase (Pokrovski et al. 2013: Thermodynamics of Geothermal Fluids. Mineralogical Society of America vol. 76, 165-218). This is important because a vapor-rich geological fluid would be much more compressible than a liquid-only fluid, and therefore very sensitive to pressure variations. This might explain why Au mineralizations are often found at the intersection of fracture sets in many orogenic deposits.

Finally, consider that the chemical composition of orogenic gold fluids - i.e., those of vein-hosted deposits - is very consistent throught he geological record (Garofalo et al., 2014: The Geological Society of London. Spec. Pub. no. 402). This suggests that remobilization in that environment does not modify much fluid composition 

Best regards

Dear Dr. Brough,

to answer this question in a way to satisfy each and everybody is difficult, because if we look at a rock section in a metamorphic environment containing gold as stringers, veinlets or whatever type of morphology, we can describe to some degree what the parent material was like, using e.g. the chemical composition, but we cannot precisely give a level of gold which the process of mobilization started off from. We postulate an isochemical process as to the major components but we meditate on an open system as to the minor components, such as gold.

There are some papers where you can find concentrations of elements like uranium in metamorphic rocks which underwent different grades of regional metamorphism. Some interpret this as enrichment. The major shortcoming is we do not know the background where all began. I investigated black shales (better call it slates) at the brink from very-low-grade stage to low-grade stage and I could claim an enrichment of gold by about six times as much during this process with native gold being transferred from disseminated to the fissure bound type. The host rocks are stratigraphically well determined and the facies relations are very homogeneous. So the setting was favourable.

Stepping up the process to a more advanced level of medium grade or even high grade, the unknown parameters (facies changes, tectonic disturbances) increase while the number of equations does not. So the system simply by logic does not offer any precise solution anymore.

There are many papers as to the element mobilization of elements from the super-dry high grade metamorphic rocks of the granulite facies, which are "overcooked" .

I have deliberately used this term, well known to petroleum geologists when they deal with the source rock potential of rocks for gas and oil. I am fervent admirer of this approach taken by the HC people and we can learn a lot from them because of the handling of migration of fluids. Even if we have to deal with two different types of fluids called subcritical and supercritical.

My suggestion is if we want to learn how gold and some other elements get mobilized we must step down to the level where it all began (very-low to low-grade, or even lower in the field of diagenesis). Here we know about the background, the mobilizing agents and have a lot more tools than necessary to get a solution for  the equation. Starting somewhere in the medium- or even-high grade terrains means putting the cart before the horse. So far I have not been in touch with a study showing me in a convincing way how gold is mobilized or enriched along the progressive pathway of metamorphism. The retrograde pathway is much easier to go and in my opinion logical and persuasive data can be given in satisfactory way.

Why no acting  like a crystallographer and using the reciprocal lattice/structure for the explanation of the entire system. Sometimes we have to leave the beaten tracks.

Beste regards

H.G.Dill

Below mentioned documents are helpful for your research.

1. Three mechanisms of ore re-mobilisation during amphibolite facies metamorphism at the Montauban Zn–Pb–Au–Ag deposit

By Andrew G. Tomkins

Mineralium Deposita

August 2007, Volume 42, Issue 6, pp 627-637

Date: 08 Mar 2007

2. http://www.diva-portal.org/smash/get/diva2:638149/FULLTEXT01.pdf

Thanks all for the additional comments. I'll reply in turn below.

Marlina,

For some reason the dry nature of the granulite facies had slipped my mind, Thanks for the reminder. Factoring the 'dry' nature of the granulite facies means that the bulk of the gold mineralisation could have been during the more mobile Greenchist / Amphibolite prograde metamorphism with the gold eventually being constrained during the peak metamorphism and not re-mobilised during the retrograde step. I guess once the system has been dried up and sutured it could effectively be 'frozen' during retrogression.

Paolo,

Some more helpful papers, I am grateful. In this instance, despite the host in micro-fractures the gold is not really in a typical vein hosted scenario. The dominant association is with Bi, in re-healed feldspar microfractures. There is a sub-ordinate association with sulfides that is often spatially associated but far less frequent. 

Thanks Prof. Dill

I appreciate what you are saying and agree that if we could unpick the timing of the pre-peak mineralisation then much more could be said. I am assuming that upgrading has occurred simply because concentrations are significantly elevated above mean crustal abundance (it is a potential mine after all), but how far it has travelled in that direction is fundamentally uncertain. Equally, knowing which pre-peak facies the gold mineralisation occurred in is completely unknown at this stage. I am not even sure what you could look at to help answer that question?

Regards,

Chris