Is there a public source with an SVG library of symbols for structural geology mapping in QGIS ?
There is the plugins "QGIS Resource Sharing" that offer you to install some geology symbol.
You may also have a look at FGDC Compliant Geology Symbology for QGIS it is designed for use in QGIS
In order to adhere as closely as possible to the specifications set out in the FGDC document, it was necessary to design a substantial number of svg symbols and combine these in QGIS to arrive at the required symbols
The set of symbols uses some the FGDCGeoAge fonts: the font files can be obtained from the website http://pubs.usgs.gov/tm/2006/11A02