The conservative deal valuation reflects current market prices of about $475 per metric ton for a total of 5 million metric tons.

The contract is take-or-pay, backed by a letter of credit.

The agreement also provides an option for American West to sell and Sichuan Chemical to purchase an additional mount of potash.

Prospect Global said it is believed to be the largest-ever purchase and sale contract - in price and volume - for a potash mine under development in the United States. It added that the deal is also believed to be one of the largest export contracts in United States history.

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(Copyright 2012 The Denver Post)