Dear James, Illite, Mg-Chlorite and/or chamosite among others are clay minerals indicators of hydrothermal processes and can be discriminated from supergene clays by difference in geochemical composition, majors, traces and REEs (e.g., see the paper joined). See also the text book of Georges Millot (1970): The Geology of clays, Weathering, Sedimentology, Geochemistry. Springer Verlag, pp. 427. All the best, Anicet

Thank you.  That's a very interesting and helpful paper, especially the reference list.  I'll track down the book as well.

Best regards, Jim

I wondered if the results in the paper mentioned above were specific to the seafloor vents, and might not apply in my case, with andesitic tuffs and marine incursions.  Unfortunately, we don't have a way to do REE analysis right now.  We do have a lot of smectite, and occasional cristobalite, but no zeolites (yet).  All of the quartz coatings in vugs and veinlets is chalcedony, with no fluid inclusions, and we have not yet found a mineral assemblage that would conclusively indicate more than ambient temperatures. Some of the Mn oxide textures (pyrolusite and ramsdellite by XRD) have well-crystallized grains visible to the naked eye (albeit small), along with cryptomelane and coronadoite).  I would think the visibly crystallized MnOx would be consistent with a hot fluid, but not conclusive of hot spring temperatures.

I do agree with Kenneth that Illite, Mg-Chlorite and/or chamosite are not indicator for hydrothermal processes. these minerals can form also during diagenesis within older sediments. Chamosite is also important constituents of many sedimentary Phanerozoic ironstones.

I think the presence of flint clay with secondary veins of calcite, silica, and gypsum and mobilized Fe are evidence of hydrothermal activity, where flint clay (kaolinite) can be formed in-situ via alteration of a calcareous shale. Kindly see the attachment

I think you can find ij the hydrothermal deposit Pb-Zn, halloysite is formed as a result of alteration processes in temperartura low.

Carbonate clumped isotopes will give you crystallization temperatures for the calcite and dolomite and would indicate, very generally, if you are in the right temperature range for hot springs activity, at least for the formation of the carbonate-bearing assemblage(s).  With T in hand, you can directly calculate 18O(water) which would give you add'l info about whether or not this was a hot-springs environment.  Best of luck (and drill deep)...

Hi James,

I've used XRD clay mineralogy on my mineral exploration projects in tropical Papua New Guinea since the mid-1980s. Typical hydrothermal clays and micas include illite-smectite (100-200oC/pH 5-6), illite (200-250oC/pH 5-6), and sericite (>250oC/pH 5-6).  As you'd expect temperature and pH are important controls on hydrothermal alteration, and I have included in the above list this detail for each mineral.

Suggest you read the work of the late Terry M. Leach (Chapter 4 in particular) contained in Society of Economic Geologists Special Publication No. 6 'Southwest Pacific Rim Gold-Copper Systems: Structure, Alteration and Mineralization'. My PNG clay work was done with Terry, and he was ahead of his time with his work. Regards, David

flint clay, please see this reference

Dear Dr. Murowchick,

many of the 7, 10 and 14 Ångstrom clay minerals be it of the swelling or non-swelling clan are ambiguous as to their physical-chemical conditions and they can form under near-ambient conditions (supergene alteration) and hypogene alteration, alike. Only where you have continuous sections you might be able to draw a line between the two groups if there are further indicators.

There are some exceptions from the rule. Among the kandite/ kaolinite group dickite and nacrite used to form under hydrothermal conditions but I have never observed them in a supergene alteration zone. On the other hand there exists a varied group of Ni-Co-bearing chlorites, serpentines, smectites and hormites which are exclusive to tropical weathering. A critical mineral is the Cr illite/ muscovite which I found in deep-seated lineamentary fault zones at the brink from hypogene to supergene alteration sparked by deeply circulating fluids and those tapping biolites enriched in Cr and V (there are also V-bearing analogues among the micaceous minerals).

For further reading on this matter see:

1. DILL, H.G., KUS, J., DOHRMANN, R. and TSOY, Y. (2008) Supergene and hypogene alteration in the dual-use kaolin-bearing coal deposit Angren, SE Uzbekistan.- International Journal of Coal Geology, 75: 225-240.

2. DILL, H.G., DOHRMANN, R. , KAUFHOLD, S. and ÇIÇEK, G. (2015) Mineralogical, chemical and micromorphological studies of the argillic alteration zone of the epithermal gold deposit Ovacik, Western Turkey: Tools for applied and genetic economic geology.- Journal of Geochemical Exploration, 148: 105-127.

3. DILL, H.G. (2016) Kaolin: soil, rock and ore From the mineral to the magmatic, sedimentary, and metamorphic environments.- Earth Sciences Reviews 161: 16-129.

4. DILL, H.G. (2017) An overview of the pegmatitic landscape from the pole to the equator – Applied geomorphology and ore guides.- Ore Geology Reviews 91: 795 – 823.

5. 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.

In these papers you may find catenae from the primary rock through the zone of argillitization into the duricrusts .

Another paper on phyllosilicate marker minerals is currently under review.

The above papers are available on the RG server on request.

With kind regards H.G.Dill