"Your board has serious concerns about the impact of this proposed new tax," du Plessis wrote, "not only for Rio Tinto and the mining industry, but indeed for the whole of Australia's future economic prosperity."

The tax, du Plessis went on, "has been developed in a vacuum and is divorced from the day-to-day realities of business."

Meanwhile, BHP Billiton's equity received a downgrade Wednesday from JPMorgan ( JPM) mining analysts in London, who cited valuation relative to peers and sheer diversity -- BHP's sprawling operations mean that it mines just about every kind of metal and mineral that it's possible to mine -- as the chief reasons for the cut. At the moment, the analysts said they believe that a rally will continue in "higher beta stocks" -- that is, mining companies with leverage to one kind of metal, copper especially.

BHP's New York-listed American depositary receipts were trading Wednesday morning at $66.81, down 1.5%. Rio Tinto shares were losing 0.8% to $49.55.

The JPMorgan analysts, who cut their rating on BHP to underweight, struck a cautious, almost fearful note in their 59-page report Wednesday, which tackled the whole of the metals space. It's a sentiment that seems to have become increasingly common among commodities players.

Though the firm remains bullish, the analysts wrote, "The the risk to this call is the very real and present danger that Q3 this year could see a hard landing in China aligned with peak softness in Western markets."

Translated, China's recent moves to end rampant speculation in its real estate markets and stave off the creation of a dangerous asset bubble will lead to a slower-growing economy in the People's Republic. Because Chinese industry has come to dominate global trade in raw materials, any softening there will without question hurt metals prices.

If you liked this article you might like

What BHP Means for Oil; How ADP Could Have Avoided a Showdown: Best of Cramer

Market Rally Fades as Dow, S&P 500 and Nasdaq Finish Lower

Wall Street Holds Lower as Consumer, Defense Names Decline