Are there any structural-geology symbols libraries (and geology in general) for QGIS?
You may find some symbols here:
and a Howto:
I assume that the people behind those pages have the geological background to help you further.
I have over 700 point symbols created and tested for use on my geological maps, using the USGS symbol documents as a source (https://ngmdb.usgs.gov/fgdc_gds/), converting the original PDF using Inkscape. They are at http://members.iinet.net.au/~richard.langford/USGS.zip.
Alshaghdari, Mohsen. (2017). lithology symbols for geological mapping. . 10.13140/RG.2.2.19590.47687.
Provides for lithology symbols for QGIS and other app mapping.