For more than a decade, billionaire Ivan Glasenberg chased the White Whale of mining M&A, a merger between Glencore (OTC:GLCNF) and Rio Tinto (NYSE:RIO).

For much of the past month, it looked like Glasenberg, a former Glencore CEO and the largest shareholder, would finally succeed. The two companies were locked in the most serious talks, discussing a $200 billion+ merger, the fourth in 20 years. And then, with a U.K. takeover deadline looming, it all collapsed within 24 hours.

Close, but No Cigar

The most recent round began quietly in mid-December, when the two companies entered formal discussions. Talks accelerated in January once the approach became public, triggering U.K. takeover rules. The confirmation forced Rio to either make a formal offer or walk away by February 5. By that point, advisers had already spent weeks digging through Glencore’s operations, and both sides believed the deal had never been closer.

Yet on the day of the deadline, the negotiations broke down over a familiar issue: valuation. Glencore was pushing for its shareholders to own about 40% of the combined company. It reflected the company’s view of the long-term value of its copper assets and trading business.

Rio’s executives, unwilling to concede that level of ownership, concluded that the gap was too wide to bridge. The firm issued an