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I took those photos in a hurry (sorry, the color is a bit off due to low light, the first picture is the most distorted one).

It struck me that the discolorations seem to follow the fissures in the rock. Could somebody explain me what caused the red-brown colorations along the grooves?

IIRC the bedrock shown is glacially polished Rhyolite, location is the "Gleif church in Eppan, Italy". Further research told me that the glacial polishing happened in the Würm glaciation, ending 12000 years ago.

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  • $\begingroup$ If you are comfortable working with geologic maps you could take a look at the 1:50000 Foglio Appiano, from the Geologic Map of Italy. The cross section C goes through the town of Cornaiano, which is very close; also the Schema tettonico (look for Appiano) shows this church is probably right near the contact between the Atesino volcanic group and a very faulted sedimentary section; geothermal fluid circulation through these fractures causing Hydrothermal alteration at surface is a plausible explanation. Hopefully a good start. $\endgroup$ – MyCarta Jan 10 '19 at 15:54
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    $\begingroup$ @MyCarta You should post an answer. $\endgroup$ – Spencer Jan 10 '19 at 23:50
  • $\begingroup$ I will see what I can do this evening. Without proper access to more detailed geological works, it will remain a cursory look. $\endgroup$ – MyCarta Jan 11 '19 at 15:11
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This answer is based on a cursory look at the 1:50000 Foglio Appiano, from the Geologic Map of Italy (in Italian Language only, as far as I can tell). As I pointed out in comment, I intend it as just a start for further investigations, which should be done with more detailed geological works and possibly more field work. I'd recommend contacting the office of the Geological survey of the Provincia di Bolzano as they certainly know the area well and may be able to answer on the phone or by email.

This is what I think, based on my cursory look.

The cross section C goes ~E-W just north of the town of Cornaiano, relatively close to the location of the Church, which is just West of San Michele (based on Google map link); I indicated its approximate whereabouts with yellow circle in the figure below. It looks like the rocks in your picture could be an outcrop of the ORA formation (in orange in the figure), a Rhyolitic tuff of the Atesino volcanic group.

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Also the Schema tettonico below (look for Appiano) shows the Atesino volcanic group and is in contact with a very faulted sedimentary section (Successione sedimentaria permocenozoica); I'd say geothermal fluid circulation through these fractures causing hydrothermal alteration at surface is a plausible explanation for the coloration you captured. enter image description here

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    $\begingroup$ Thank you very much. Geology isn't my strength. A further visit to this site is no problem. Perhaps you could purpose further research steps possible for a layman? $\endgroup$ – Martin Jan 15 '19 at 12:07
  • $\begingroup$ I was going to recommend perhaps a look at the notes accompanying the Geologic map, but unfortunately they are in Italian (a lot of the material published for this area is in either Italian or German), to which you add you do not have a background in Geology. You could try and contact the Geological Survey of Bolzano (email here under Contatti segreteria) and see if they can get you in touch with some of the folks involved in the survey, but I'd be surprised if that got you there. $\endgroup$ – MyCarta Jan 15 '19 at 20:46
  • $\begingroup$ Probably your best chances are to talk to folks in the local museums involved in outreach and educational activities, particularly if on minerals and mining. Try the Museum of Natural Science of South Tyrol (staff here, contacts here) or the Museo delle Scienze di Trento, perhaps this resercher, who's involved in exactly those activities. Good luck. $\endgroup$ – MyCarta Jan 15 '19 at 20:52
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    $\begingroup$ @MyCarta I actually think sending an email to a local geological survey is a great idea. There's a high chance someone knows and will answer you. $\endgroup$ – Gimelist Jan 16 '19 at 5:58
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    $\begingroup$ @MyCarta sorry, forgot about it. Thank you for your effort $\endgroup$ – Martin Feb 8 '19 at 20:51

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