I.e., barite (BaSO4, the major barium mineral) containing small amounts of radium as RaSO4. Both have extremely low solubility in water (under normal conditions), so any radium salt that gets dissolved quickly precipitates back out, together with the more abundant barium, as soon as it encounters sufficient sulfate ions in solution. Here's a 2010 post on that:
http://www.democraticunderground.com/discuss/duboard.php?az=view_all&address=115x249200



Note that's per gram, not per kilogram.