As the name says, the only way to work with ground truth data is to collect information in the field. I see, per definition, no way to derive training or validation data from the image itself, because this information then is no longer retrieved independently from the image.
Maybe you can find external sources which provide reference information for your area of interest, such as PANGAEA (https://www.pangaea.de) or you use spectral profiles of minerals (https://www.usgs.gov/energy-and-minerals/mineral-resources-program/data-tools) and try to identify the minerals of interest with spectral unmixing.