The Premier Mine diamontiferous kimberlite is 1.25 Ga old and is cut by a 1.15 diabase dyke.  It is located northeast of Johannesburg and intrudes the 2.5 Ga Transvaal Supergroup, the eastern lobe of the 2.0 Ga Bushveld Complex, and the 1.9 Ga Waterburg Group fluvial sediments.  The uppermost diatreme zone has been completely excavated down to the diabase sill (seal).  Below the diabase, underground mining continues.   Because the kimberlitic rock mass is not so coherent, mine infrastructure is sited in the adjacent intruded rock.  Exploratory boreholes penetrate the kimberlite to depths of 1 km.  From one of these boreholes our water sample was collected.