I downloaded the USGS lithologic patterns from the Univeristy of Otago. At the same time, I installed inkscape software to work with these patterns.
Steps to use these patterns in QGIS:
- Open USGS's patterns file in Inkscape -> Select the pattern you want to use -> File: export as a PNG image.
I recommend a minimun size of 1125x900 (1500ppp). Don't forget to press 'Export'.
Before converting the PNG image to svg file, crop the borders using any image/photo editor. Otherwise, you will get as a fill pattern a grid of boxes containing the pattern. Now you have a PNG image, go to imagen.online-convert.com/es/convertir-a-svg and convert your png image to svg file.
I don't know why but if you save your file as svg in Inkscape you can't use it in QGIS.
Now with your pattern as a svg file, open QGIS, go to 'Options' -> system -> add a new svg path to the folder where you saved your files. Accept.
Select a polygon layer and select 'properties' -> style -> select a svg fill. Your folder should appear on the list. Select the pattern you want to use.
About the policy, USGS says that you just have to mention them:
"Suggested citation U.S. Geological Survey, 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization (PostScript Implementation): U.S. Geological Survey Techniques and Methods 11-A2 [pubs.usgs.gov/tm/2006/11A02/]."