I'm afraid you're off by about a mile to the North.
The Mining site of Petite Spiennes lies around 50° 25' 11" N/ 003° 58' 45" E. The Camp-à-cayaux site is situated around 50° 25' 07" N/ 003° 59' 18". At those last coordinates you can even see the chalky fill of the prehistoric shafts in the field.