Dear Dr. Camprubi,

there are numerous Cr-bearing phyllosilicates, many of which are supergene in origin and not all of the "green glittering stuff" in the argillaceous alteration zone of mineral deposits qualifies for being named Cr illite or fuchsite. We have studied some argillaceous alteration zones with high Cr contents and micaceous and smectitic material.

I would like to refer to the paper published only recently:

DILL, H.G. (2017)  Residual clay deposits on basement rocks: The impact of climate and the geological setting on supergene argillitization in the Bohemian Massif (Central Europe) and across the globe.- Earth Sciences Reviews 165: 1-58.

There you can find all the common Cr-bearing phyllosilicates and what you can deduce from their presence as to the supergene part of alteration and their parent rocks.

Chromium has favourable sites to be concentrated, the environs of ultramafic rocks and paleosols sensu lato.

Best regards

H.G.Dill

Thanks for the answer!!! And for the fresh reference, of course.

We have no doubt that it is fuchsite (even got nice plateau Ar/Ar ages from them) and that it is hydrothermal in origin, rimming 'core' potassic alteration assemblages rich in K-feldspar. What intrigues me is the specific occurrence of Cr-rich muscovite in IOCG deposits—it might mean nothing at all anyway, but I have only vague ideas and not previously existing frame of knowledge about its occurrence in hydrothermal environments other than those related to metamorphism on ultramafic-mafic rocks...

try to see the gravity profile of the region.as fuchsite  is mostly fro ultramafic-mafic and by metamorphism there is a possibility the fuchsite in hydrothermal may  be an inlier and and belongs to the basement below.

Dear Dr. Comprubí,

an example of hydrothermal origin for fuchsite after metamophism of ultrabasic rocks, can be seen in Mendoza province, Argentina. I would to refer to an old publication in that area:

Dristas, J.A. y Hayase, K. (1974) Presencia de muscovita crómica en la mina La Mendocina, quebrada de Santa Elena, departamento Uspallata, provincia de Mendoza, Argentina. Revista de la Asociación Argentina de Mineralogía y Petrología, tomo 5, 1-2.

Kind regards,

J.A. Dristas

Has anybody explored the possibility that Cr in hydrothermal deposits does not necessarily come from preexisting concentrations in ultramafic rocks?

Antoni, In the Black Hills of South Dakota (ca. 2000-1710ma), there are very-late tectonic shear zones and vein (carbonate-quartz mostly) complexes that include some concentration of fuchsite. The nearby protoliths are ocean floor exhalatives (C-Fe-S-Si slates, volcanogenic metagraywackes, and metabasalt). There are no ultramafic materials anywhere in the region and the Cr (and Ni) content is generally quite low. The fluids responsible for the veins show evidence of input from hydrous, aluminous, S-type granitic intrusion. And so, the fuchsite occurs among original mafic potential contributions (for the Cr enrichment) but also in the context of granitic fluids with elevated K, B, F, Al, Na. I have a manuscript in revision that describes the mineralogy of the veins and speculates about their tectonic-petrologic affiliation.

Thanks, Jeffrey. This case of yours certainly says something about the mobility of Cr and may bear interesting clues to my problem. I will be very grateful to you if you were so kind to provide your manuscript once it is ready for some stranger's eyes...

Antoni,

The paper in process has some interesting ideas but not much data. I can send you a preprint at some point. However, if you have any questions about my initial comments, I'll be happy to see if I can help. Of course, the famous occurrence of emerald and Cr-rich tourmaline is interpreted as the mingling of oceanic-mafic fluids with continental-sialic ones. The total geochemistry of chromium has got to be more complex than just its association with ultramafic materials.

Any more ideas that may not require Cr to be derived from the country rocks?

No way in conventional thinking that Cr concentrates from a granitic source. The granite in this case is anatectic from wet-aluminous metasediments and metabasalts. Mantle metasomatism seems very unlikely #1 because this is a relatively shallow, low-P supracrustal package without any identifiable "basement", and #2 there are no ultramafites anywhere exposed in the region. The only potential source for Cr and Mg, are the earlier basalts.