Dear Brenton, 

from my point of view, this is clearly related to the Pan-African Orogeny. The early events in the Damara Belt were recognized at ca. 600-590 Ma (Lehmann et al. 2016 Tectonics). To the east of the Kalahari Craton, the Pan-African Orogeny is related to the Mozambique Belt (e.g., review work of Fritz et al. 2013 J Afr Earth Sci). 

Dear Sebastian,

Thank you for you help. The onset of Damara-related convergence may be a little late? The area of concern is much closer to the western edge of the Kaapvaal Craton - surely too far to have been affected by activity associated with the Mozambique Belt?

Dear Brenton, 

yes, the onset of the Damara convergence is slightly younger: 600-590 Ma according to Lehmann et al. and may be recorded in the northern branch. However, there are some events older than 600 Ma recorded in the Pan-African belts that sorround the Kalahari Craton (e.g., Frimmel et al. 2011 Int J Earth Sci and references therein), which may coeval with the hydrothermal activity you described. 

What type of ore deposits? Exactly where and how far from the Damaran and Kaapvaal margin. There is evidence of salt basins, thrusting with high-T brine release on a decollement, slightly higher metamorphic grade rocks thrust over younger lower-grade rocks - along the sounthern margin of the northeast Damaran branch. Brine cann remain trapped and be released hundreds of millions of years later. I'm not clear how tightly constrained your ages of tectonism are - you seem to be implying a very tight dating despite a lack of granites on the southern margin.

We have preliminary ages for hydrothermal feldpars in karst-hosted manganese ores that give an average Ar-Ar age of 620 Ma. The deposits are located near Postmasburg in the Northern Cape.

Hi Brenton. That would be one hell of a long migration across a craton from where I discussed. I did my Doctorate on basin-hosted Namibian deposits (a long time ago). It seems to me it is also a long way from the west coast Damaran and I am not sure if the Damaran-Katangan salt basins extended there - there is much literature on the area but I am out of date. My suspicion would be yes, because of my prejudices re Rosh Pinah-Skorpion etc. However that would still be a long way from the margin. I admit my ignorance, I thought the main manganese was older than 620 Ma....and far to the east...is the karst material metamorphosed and is their also older manganese as I thought? Do you have fluid inclusions? I'm out of my comfort zone, I did look at Pb-Zn MVT (presumably brine-related) in the Malmani just east of the manganese but I asume it is older? Guess I can't really assist more.

Hi Martin,

I agree that it is a long way! Hence my problem. The manganese "protore" would have probably been older. It is derived from residual enrichment by dissolution of the manganiferous Campbellrand dolomites and all evidence is pointing to hydrothermal alteration around the age discussed above. Its original genesis is unrelated to the formation of the giant chemical sedimentary manganese deposits of the Kalahari Manganese Field. No fluid inclusions I'm afraid - and unlikely to be analysed either.  Although some of the MVT deposits that you refer to formed in the same karst environment (same stratigraphic level) as the manganese deposits, I believe the source of metals is associated with the Bushveld complex? And that ore formation was much earlier - Palaeoproterozoic. Thank you for you contribution Martin - you have, at the very least, got me thinking a bit more!  

Hmm - I would not have thought that they were related to the Bushveld but I guess that is a MVT prejudice of mine - but I thought they would have been older as you say. I have found some massive anhydrite in the Malmani years ago - even analysed its sulphur isotopes - but did nothing further (plus ten per mil from memory). No Pb minerals in your Mn as there are plenty of common lead isotope results for occurrences in the dolomite including my unpublished data (for comparison with the MVT)? Not accurate for age but good for fingerprinting differences and similarities - and coronadite etc are fairly common in some Mn deposits. Good luck.

Thanks Martin. I did quite a considerable mineralogical/petrographic study of both the ore and gangue phases - an assemblage that is quite complex. But I did not find any Pb phases. I might have a look through the literature of those MVT deposits and see if there are any similarities. Thanks again.

Dear Brenton,

Perhaps the next suggestion is strange at the first hearing: perhaps you will get a better understanding if you try to find a tectonic place which is new but has some common features with your studied area… It seems that exist a similar area in South America, in Argentina In Zone of Cordoba… In general, the all South America tectonics shows similar sign with tectonics of the zone mentioned are by you: But something happened no-known process which is not discrybed in actual tectonics literature (actually I do not have any idea about it)

From this can be presumed that the response to your question:   ‘What major tectonic processes were occurring on the (present) western, southwestern and northwestern margins of the Kaapvaal Craton at ca. 620 Ma?’ – Same process which we are observing in the Andes the west part of South America… only for you seems more important geology in zona of Cordoba.

Regards,

Laszlo