NEW HORIZONS — America’s ex-smokers

Deep in the dark privacy of the ocean floor, a group of smokers puff away proudly; what they exhale is a far cry from second-hand smoke.

“Black smokers,” or massive chimney-like structures that vent hot streams of minerals from deep cracks in the ocean floor, were first observed in deep rifts by submersibles with underwater cameras. The hot hydrothermal solutions vented by the smokers solidify as they mix with the cold sea water and form glistening towers, or ribs, of massive sulphide minerals such as chalcopyrite, sphalerite, galena and pyrite.

Massive sulphide deposits around the world have long been sources of copper, zinc, lead and silver. Many of the biggest mines are found in rocks in continental shields (in the interior areas of continents) that are billions of years old.

Black smokers have been hailed as modern-day massive sulphides forming on the ocean floor, but why haven’t nature’s formerly active chimneys been found in the thousands of deposits mined from land?

Some have just been found. Recent detailed mapping in the open pit of the Phelps Dodge-owned United Verde mine in Jerome, Ariz., has found a stack of smoker chimneys near the top of the ore body. The mine is one of the largest massive sulphide deposits in the world.

Discovered by geologists Paul Lindeberg and Pat McGehen, the smoker chimneys are thought to be the first fossil smokers discovered in the United States and only the second to be discovered in the geologic record. The smokers are cylindrical, about 10 inches in diameter, and consist of concentric rings of pyrite and quartz, with small amounts of zinc and copper minerals. Most of the smokers are completely filled with minerals, but a few still have porous cores made of quartz.

According to the geologists, the smokers were difficult to see in the pit wall. It had been thought that such structures were never found in deposits because they collapse into rubble as the deposit grows. These smokers were found at the top of the system and formed during the last stages of hydrothermal action.

The rare discovery once again demonstrates that the age-old principal of geology — that the present is the key to the past — holds true, even over billions of years of geologic time. The processes we see shaping the earth today were active in the geologic past, and the United Verde chimneys that quit “smoking” ages ago represent exciting new evidence.

— This article first appeared in Mining Voice, the publication of Washington, D.C.-based National Mining Association.