The Tar Creek region was once one of the world’s richest fields.

Lead and Zinc.

Explanation:

Already at the beginning of the XX century, this area was significant for the production of lead and zinc. During World War I, the region supplied 45 % of the point and 50 % of the zinc used by American zinc and lead was used for bullets during both World Wars. During the 1920s, it became the world's largest source of lead and zinc, employing 11,000 men in nearly 250 mills.

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