The relevant test here is whether or not the Congressional delegation of authority is accompanied by an "intelligible principle" by which the agency can guide its decision-making.
The Federal Reserve Act, which establishes and empowers the Fed, is a long bill full of instructions from Congress about what the Fed may and may not do. (In fact, the Fed's powers were changed recently, under the Dodd-Frank Act.) Most importantly, the Act directs the Fed to implement monetary policy to meet some very specific goals.
The Board of Governors of the Federal Reserve System and the Federal Open Market Committee shall maintain long run growth of the monetary and credit aggregates commensurate with the economy's long run potential to increase production, so as to promote effectively the goals of maximum employment, stable prices, and moderate long-term interest rates.
That's an extremely explicit "intelligible principle." The Fed cannot make policy willy-nilly.It must conform its policy to increase production while promoting maximum employment, stable prices and moderate long-term interest rates. The Fed has a lot of latitude when it comes to how best to pursue those goals but the goals are very clear.
The statute authorizing the Treasury Secretary to mint platinum coins is very different. Here's what the relevant section says:
The Secretary may mint and issue bullion 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.
There's no intelligible principle here at all. Nothing to guide the Secretary of the Treasury about how to exercise this authority, no goal at which his discretionary decisions are aimed. This is just too broad to fit into our constitutional framework.