Dear Dr. Swain,

before starting a lengthy interpretation I wonder some data of the Cr-bearing illite and its determination. Normally Cr-bearing muscovite is found associated with Au in some epithermal deposits or with spinel and (precious) corundum. I have not seen yet such a layer-cake texture of barite and chromium illite. Can you also give some more data on the minerals associated in time and space with these nice mineral couple?

Best regards

H.G.Dill

Dear colleagues,

tidal bundles are different from what we can see on this image. The question on the formation of Cr-bearing illite is still left unanswered, as it is the barite concentration. While barite can still be accounted for with some assumptions, fuchsite is still enigmatic and hard to explain.

Best regards

H.G.Dill

Hello,

On a different note, it would be interesting to evaluate if you are looking at 'metamorphic differentiation' resulting in interlayering. The Fuchsite-quartzite that I have seen in the Bababudan Group show very tight, isoclinal folding.

Suggest assess if either of the above is valid before looking up 'sedimentological explanations'.

Lastly, in case the barite deposits are of strata-bound origin, one would expect such banding....

SEM and microprobe data should be able to help you decipher the situation better.

All the best.

I see fuchsitic quartzite a lot in Archean and Proterozoic sequences in association with all sorts of mafic rocks - it is common with gold deposits but much more widely distributed than that. For example, in fuchsitic siliciclastics in Cu VMS amphiobolite sequences such as in the Damaran. I assume it is fuchsite and not a V mica such as roscoelite? I am very confused by the reference to it being tidal - I would have though it more likely to be exhalative sea floor activity near a vent. But I suppose it could be later Ba-replacement of gypsum.

whether the rock sequence is forming part of  older  Sargurs,  high grade supracrustal rocks underlying the basal unconformity of the Dharwar Craton

or part of Dharwar super super group rocks for which many have evoked sedimentation and volcanism  in deep marine environments.

Bedded barytes and fuschite quartzites are typical of Sargur Group of rocks which are  metamorphosed ranging from middle amphibolites facies in central Karnataka to upper amphibolites and granulie facies in Southern Karnataka.

the banding probably may imply a volcano-sedimentary sequence.

the baryte could be of volcanogenic origin similar to Mangampeta Baryte of Cuddapah basin which is bedded.

Bedded barite is believed to have formed as emanations from seafloor sediments, as diagenetic replacements of preexisting minerals, or as direct precipitants due to biological fixation of barium in the water column. Geochemical studies, particularly those that have employed δ34S and 87Sr/86Sr analyses of the barite, have proven very useful in understanding bedded barite genesis.

S isotopes could give some clues (or they might not) if the del S of contemporaneous sea water is known. Oxidation of reduced sulphur in hot exhalative fluids would be rather different to simple lower-temperature warming near a vent (perhaps the more likely alternative). However replacement of evaporitic gypsum would be little different to the latter. But you probably do not know contemporaneous sea water values?

 http://rruff.info/doclib/cm/vol37/CM37_603.pdf

This is one of the earlier work on association of barite and chromiferous quartzite.

So many thanx....

But its very interesting that this figure shows the layers, of both Anoxic and oxic conditions during Archean period...

Hydrothermal alteration zone. Check for Gold!

Dear colleagues:

We need more hard facts. Tidal regimes, volcano-sedimentary sequences, the source of Cr and the host of Cr, as well as  exhalations are very difficult to be put together to give me a coherent picture. Or shall we cast aside the slogan "The present is the key to the past"  for these very old rocks ?  A chemical approach using the V/Cr ratio in the argillaceous layers (so-called fuchsite) may give an answer to the variation in the redox conditions. V increases with lowering of the Eh (more reducing) and Cr responds in the opposite direction (more oxidizing). If it gives a more coherent picture together with barite you get a bit closer to the large-scale environment. I would be very much reluctant to jump too fast into an interpretation of the depositional environment without proper knowledge of the mineralogy and chemistry. Nevertheless, a nice story, to think about.

Best regards

H.G.Dill

Dear Sagar: the answer you posed lacks information. I assume that this layer is part of a volcano-sedimentary sequence in a greenstone belt in Darwar Craton. Is that right?Adding to what Dr. Dill said, the source of Cr should have been, almost certainly, some ultramafic rock, perhaps peridotite-serpentinite of even komatiite. Cr is a cation you find in very specific hosts, mainly chromite, it is not like, for example, Cu, which can be found in a great variety of rocks and deposits. The association barite-fuchsite-quartz suggests quite probably some sort of exhalative or sedex deposit in an oceanic realm, quite probably a back-arc type oceanic basin. You don't speciy which is the host rock of this mineralized layer you found, and the picture sent really doesn't show it. So, until more information is provided, your question comes to a stand still. With regards, Sebastian.

I agree with Sebastian completely. But first of all, are you absolutely sure it is a Cr-mica and not a green V-mica like roscoelite, which occurs in exactly the same environment (e.g. I have seen it in the Oroya gold ore shoot in Kalgoorlie).

Good day Sagar

Have you analysed any of the green 'fuchsite' mica? This may be a long shot, and I am by now means an expert on green micas, but I have seen several examples of what appeared to be fuchsitic quartzite associated with barite in the Namaqua Metamorphic Province of South Africa, where the green fuchsite turned out to be a highly Ba-enriched mica. These occurrences were typically interbanded along with other metasediments, and represented distal equivalents of sediment-hosted massive sulphide deposits, deposited under generally oxidizing conditions.

Excelent answer that of Tarryn-Kim: not all green micas are fuchsite, as this mineral is a Cr-rich variety of muscovite or phengite, just as Turkish kammererite is a Cr-rich crimson-red variety of chlorite. So, the first thing should be to analyze this mineral and certify it is really a Cr-bearing mica. There are many easy and cheap Cr-tests available in anaylitic chemistry books, one of them which I remember quite well is called " the blue ring": a deep blue "ring" which forms when a bichromate-sulphuric acid solution enters in contact with hydrogen peroxide inside a test tube. So, the first thing is to dissolve the mica and submit the solution to a very oxidizing agent which could transform the Cr3+ into bichromate anion [Cr2O7]2-. Then add the dense sulphuric acid letting it slide in the side of the test tube and let it rest in the bottom of the solution, shake a little, and then and drop by drop add the hydrogen peroxide in the same manner. If the "blue ring" forms in the contact of the dense sulphuric solution and the top H2O2, then Cr is present. All these substances are normally found in chemistry labs. Also, XRD can be used to identify the mineral. 

The distal sedimentary-exhalative origin of the Ba-rich layers is quite probable, and this could also mean that a sizeable body of MVS could be outcropping nearby, or buried in the vicinity... If it really turns out to be fuchsite, then an ultramafic-mafic source of Cr should also be associated to the sequence. Regards, Sebastian.  

Dear Sagar: Accidentally, looking for something else, I found a very interesting and old separata regarding really unusual rock types in Precambrian greenstone belts in Zimbabwe and Australia, where a very important Cr-enrichment took place and produced really outstanding metasedimentary rocks, rich in aluminosilicates (andalusite or kyanite), corundum, fuchsite and quartz. The proposed origins of these strange rocks point to volcanic-hydrothermal alunite alteration of former komatiites, hence the Cr-enrichment seen, which then suffered low grade metamorphism in the greenstone belts. The paper states than the corundum abbundance was responsible for over 80% of world production between 1958-1965, and that the Zimbabwe rocks in the O'Brien Claim were considered as the most alumina-rich rocks in the planet, with over 80% Al2O3, even excluding the possibility that they could have been metamorphosed bauxites, which never have more than 53-55% alumina! No barite is mentioned, but the paper is worth reading because it is very illustrative of how metasomatic alteration can produce exotic protoliths which give rise to very unusual metamorphic rocks, such as these. Here's is the reference, I hope you will be able to find the paper in your library:

Shereyer, Bochum (1982) "Fuchsite-Aluminium Silicate Rocks in Archaean Greenstone Belts: Are they Metamorphosed Alunite Deposits? Sonderdruck aus der Geologischen Rundschau Band 71: 347-360. Ferdinand Enke Verlag, Stuttgart. 

If you can't find it, I can scan it and send it to you, it's only 13 pages long.

Interesting, but my understanding is that these are fuchsite-quartz rocks (assuming it is actually fuchsite) and such rocks are not at all exotic - very common really. Fuchsite is not a highly Cr-rich mineral overall, and presumably only requires some Cr transport in fluids producing the assemblage - which occur in environments where we suspect or know there has probably been large-scale fluid convection associated with mafic-ultramafic sequences. Cr+3 and Ba+2 are present in vent fluids in oceanic crust and barite readily precipitates in contact with sea water sulphate - the Cr oxidises to Cr+6 and is recorded enriched in Cr-rich vent pyrite and also Mn oxide crusts etc. Contained Cr isotopes are used in interpreting oxidation in sea water. Cr has some mobility even at high concentrations in cold weathering fluids during oxidation of lead ores in ultramafic rocks (to form the lead chromate crocoite). So the presence of Cr in various fluids at different temperatures in association with mafic-ultramafic rocks would not seem surprising and would substitute into fuchsite during white mica alteration. Barite and fuchsite occur together in ores thought to be exhalative, such as the barite deposits at North Pole, Western Australia, Barberton in South Africa, and various localities in India.

Looks like the hostrock is chert or quartzite ? Is it Cr or V rich regards the fuchsite determination there ?  I assume the 'foliation' is shear fabric ? Is there any anomalous Au also I wonder ?

I would not particularly expect gold in the absence of base metal sulphides (need some thio-complexing, so dissolved H2S not just Ba2+ ion in the reduced fluids). However the barite-rich fringes of VMS base metal bodies tend to be the most gold-rich parts with mean gold of up to 3.3 g/t eg the small Que River body (1.5-2 g/t for larger Rosebery, Hellyer, Woodlawn), although some can be just pyrite-barite averaging up to 60 g/t for millions of tons overall (eg in BC). I noticed on a recent trip to Tasmania that the Hellyer zinc-lead-silver mine sign has now changed to "Hellyer Gold Mine" for that reason.

Two different issues though - in that case the gold is exhalative, whereas the fuchsite-rich lodes at Kalgoorlie and elsewhere are mesothermal quartz-sulphide veins formed during major deformation, commonly immediately post or syn metamorphism. However barite in the photo is more-likely exhalative, as at North Pole and Barberton.