When companies loathe the idea of being bought by unwanted suitors, they can use poison pills to defend themselves. Those moves are designed to make a hostile takeover pricey by, for instance, issuing discounted equity to the shareholders of the target company.

Teck Resources TECK-B-T, the last Canadian mining company of any size still standing after an orgy of foreign takeovers in the past two decades, early this month agreed to merge with a larger rival, Anglo American NGLOY of London. Competing bids for Teck are possible, given that it's a rising star in the world of copper, the hottest of the critical metals that are underwriting the drive to electric power.

But Teck is determined to merge with Anglo, even if another bid is pitched at a fat premium (the "merger of equals" with Anglo comes with no premium). There is no one reason for Teck's peculiar stand, though a biggie is Anglo's commitment to make Vancouver the head office of the enlarged company, to be called Anglo Teck. Teck is not equipping itself with a traditional poison pill to ward off competing bids - it doesn't need one. Its insistence, backed by the federal and British Columbia governments, that Anglo Teck is run from Vancouver, not London, is poison pill enough for most would-be buyers (Anglo's legal domicile and primary stock market listing would remain in London).

All of the diversified mining companies that are big and ambitious enough to buy Teck are in foreign lands. BHP's home is London, as is Rio Tinto's, Vale's is Rio de Janeiro, Glencore's just outside of Zurich. Would BHP, the world's biggest mining company, move its headquarters to Vancouver, a mining backwater, to gain regulatory and political approval to own a business with a market value one-seventh of its own? That would be like General Motors decamping to Stockholm in order to take control of Volvo Cars. Not gonna happen.

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It is impossible to imagine that any bidder with barely disguised plans to downgrade the Teck office to a branch plant would get takeover approval from Ottawa. The hands-off era is over. Canada has lost far too many big-name headquarters, especially in the resources industry. Inco, Falconbridge, Alcan, among many others, have vanished from the corporate landscape. The Globe and Mail reported this week that Prime Minister Mark Carney had insisted that Anglo move its headquarters to Canada before lending support to the Teck deal, and Anglo agreed.

So Anglo's deal with Teck is airtight? No, not quite.

Anyone who wants to bust up the Anglo-Teck merger would no doubt go after Anglo, not Teck. Doing so would negate the Canadian head office requirement; Anglo is not Canadian and has virtually no assets or employees in Canada. Plus owning Anglo would still land the owner with copious amounts of copper, along with iron ore and a troubled fertilizer mine in Britain (Anglo's coal and diamond businesses are on the auction block).

The kicker is that buying Anglo would theoretically still give the new owner access to Teck's copper.

In northern Chile, Anglo owns 44 per cent of Collahuasi, the second-biggest copper mine in the world (Glencore also owns 44 per cent). Teck's smaller but still enormous Quebrada Blanca (QB) mine, which is 60-per-cent owned by Teck, lies only 15 kilometres away. For years, the owners of Collahuasi and QB have dreamed of putting the two mines together to save costs and boost profits by sharing infrastructure and operations. Anglo and Teck said that running the two mines as one would boost collective annual pretax operating profits by US$1.4-billion.

But no one has to own Teck to put the mines together. A joint venture would create similar synergies. Teck and any new owner of Anglo would be highly motivated to form a JV that would be pro-rata owned by Teck, Anglo's new owner and the minority Chilean and Japanese partners of the two mines.

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Who might go after Anglo? The logical buyer would be Glencore. Were Glencore to buy Anglo, it would own 88 per cent of Collahuasi. It would then present a JV plan to Teck, which the Canadian company could not ignore, given the enormous cost overruns and technical problems at QB. Teck needs a way to boost cash flow at QB and a JV would do it, though at the expense of losing control of QB.

The other logical buyer would be BHP, which went after Anglo last year but was repelled. BHP is also a huge copper player in northern Chile, through its control of the Escondida mine, the world's single largest copper producer. BHP also covets Collahuasi and would also be keen to form a JV with the QB mine.

You can see where this might be going. The requirement to keep Teck's head office intact in Canada hands the advantage to Anglo. But it also makes Anglo itself vulnerable to a takeover. If Anglo disappears, so does Teck's merger plans, casting the Canadian company adrift in a sea of whales.