Hi Herminator,
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--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.
Shall we move the mark accordingly??
Greetings,
Ranzo