I have used datasets from the follwing websites successfully for research and teaching (all will need some re-formatting for importing into statistical software):
Hubbard Brook Ecosystem Study (http://www.hubbardbrook.org/data/dataset_search.php)
USGS Mineral Resources On-Line Spatial Data (via http://mrdata.usgs.gov/)
National Geochemical Survey of Australia Project (http://www.ga.gov.au/about/projects/minerals/concluded/national-geochemical-survey)
Geochemical Atlas of Europe (http://weppi.gtk.fi/publ/foregsatlas/disclaimer.php)
I suspect there are plenty of other sources out there.