Did a quick Google search, figures China would make the most.
I'm taking a wild stab that even if there are environmentaly friendly ways of getting it that China isn't paying the extra $ to mine it right.
Rank Country Production (Metric tons)
1 China 627,000
2 Russian Federation 37,000
3 Israel 25,000
4 Kazakhstan 21,000
5 Brazil 18,000
6 Canada 16,300
7 Ukraine 2,500
8 Serbia 1,500
Though we are about in the same situation for Aluminium.
Rank Country Production (Thousand Metric tons)
1 China 12,600
2 Russian Federation 3,955.42
3 Canada 3,082.63
4 United States 2,554
5 Australia 1,960
Avg cost of each.
A quick Google couldn't find any info specific to enviromental impacts of mining Magnesium.
But then again my daughter woke up from her nap so I didn't put much time into looking.