I think it might be an open question whether section (k) of
31 USC § 5112 - Denominations, specifications, and design of coins is constitutional.
(k)The Secretary may mint and issue platinum bullion coins and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary’s discretion, may prescribe from time to time.
The US Constitution article 1 section 8 begins
The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States; but all Duties, Imposts and Excises shall be uniform throughout the United States; To borrow money on the credit of the United States; To regulate Commerce with foreign Nations, and among the several States, and with the Indian Tribes; To establish an uniform Rule of Naturalization, and uniform Laws on the subject of Bankruptcies throughout the United States; To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;
It isn't clear to me that Congress can delegate the decision about the value of Platinum coins, either bullion or proof, to the Secretary of the Treasury.
See post below for why I think the fate of the world economy doesn't not depend on this.