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Long story short: if and when Glencore loses its Investment Grade rating, it's more or less game over, if not for the company's already mothballed mining operation then certainly for its trading group, where "junking" would lead to numerous collateral shortfalls and margin call waterfalls, reminiscent of the ratings agency downgrade of AIG that culminated with the US bailout of the insurer.
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Why is Glencore's IG rating so critical? As explained above, Glencore is really not so much the Lehman as the AIG of the commodity world: without an investment grade rating, a self-reinforcing collapse will begin that could ultimately terminate Glencore's trading desk, in the process liquidating one of the world's biggest commodity trading counterparties.
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For those who enjoy playing with numbers, here is Goldman's real "Doomsday" scenario: the one which sees Glencore's IG rating stripped. As Goldman admits, all it would take is a small 5% drop in commodity prices from here:
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http://www.zerohedge.com/news/2015-09-24/goldman-preparing-sacrifice-next-lehman
TBTD (too big to downgrade)... Expect credit rating agencies to get creative in their need to readjust definitions, thresholds and requirements for various ratings if they need to...