Getting to the bottom of copper's rise to the top

Getting to the bottom of copper's rise to the top

Kennecott has been spruced up quite a bit since my last visit there nearly 30 years ago.

Even so, many crumbling buildings remain. Some have been shored up by the National Park Service, which now manages the site as Kennecott Mines National Historic Landmark in Wrangell-St. Elias National Park and Preserve. A few other buildings have been restored.

Several exhibits show the history and workings of the once mighty mine, which produced copper from high-grade ore from 1911 to 1938. The Park Service displays and signs that visitors see today were not there on our first visit.

I doubt any of us on our family trip back in 1997 or 1998 knew we were likely walking on the source of it all.

Copper. Lots of it, including masses of nearly pure chalcocite, a mineral that is almost 80% copper by weight.

On our visit in mid-July of the current year, we learned a lot about copper production from our guide at St. Elias Alpine Guides. We had signed up for a two-hour tour of the 14-story mill, the building that dominates almost any Kennecott postcard.

That got me wondering why Kennecott had so much copper.

What geologic processes created this abundance? Why was the ore so rich in copper?

Alan M. Bateman and D.H. McLaughlin had some of the answers in 1920, when their research into Kennecott appeared in the science journal Economic Geology. They titled their work, "Geology of the ore deposits of Kennecott, Alaska."

Bateman and McLaughlin worked in the field together in 1915, just four years after the mine started production.

Bateman, who worked for the Kennecott Mining Co., continued field work for four more years. McLaughlin did laboratory work as part of a mining industry effort to better understand how natural processes can make an ore deposit richer after it forms.

"The copper deposits of the famous Bonanza and Jumbo Mines at Kennecott, Alaska, are unique for the character of their ores and their purity and size," the pair wrote more than a century ago. "Their occurrence presents many peculiar and interesting features, and the origin of the great masses of chalcocite has long been a puzzle."

•••

Our guide, Molly, led us up the winding slope and past the restored mine office. She stopped at a small spot on the trail among the trees. Not far above us sat some rusty machinery from a failed 1970s attempt by some Anchorage lawyers and doctors to restart mining.

She then gave us a little geology talk.

From her pocket she pulled two similar-sized chunks of ore and passed them around. One was about 30% heavier than the other. The heavier piece was rich in chalcocite, which was the main copper sulfide at Kennecott. The other contained chalcopyrite, which is about 35% copper.

•••

The story of Kennecott's copper begins with two rock formations: the Nikolai Greenstone below and the Chitistone Limestone above. Both formed about 230 million years ago.

The Nikolai Greenstone contains more copper than most rocks and can be 9,000 feet thick or more in the Kennecott region, according to a 1999 U.S. Geological Survey report.

Above it sits the Chitistone Limestone. It formed from mud and other material that built up in a shallow sea. The 1999 USGS report put its greatest thickness at about 2,000 feet.

Much later, the rocks were buried, heated and changed. Fluids moving through the Nikolai Greenstone picked up some of its copper and carried the dissolved metal upward through cracks and faults into the Chitistone Limestone.

There, the copper-rich fluid mixed with salty, sulfur-bearing water moving through the limestone. The chemical reaction caused copper minerals to form as solids. Over time, this process gathered copper that had been spread through a huge amount of greenstone and packed it into much smaller areas. Those became the rich ore bodies that made Kennecott famous.

•••

On our tour, Molly stopped us on Kennecott's main road and turned toward the hillside. She pointed out an angled green streak.

It was a splash of color showing copper in the rock - the same geology that gave rise to Kennecott's Bonanza, Jumbo, Mother Lode, Erie and Glacier mines.

Why green?

Chalcocite is a dark gray to black copper mineral. But when copper-bearing rock is exposed to oxygen, water and carbon dioxide, some of its copper can become part of bright green malachite. Malachite can coat cracks, fill small openings and stain the surrounding limestone.

•••

That century-old study by Bateman and McLaughlin is filled with not only science but also excitement.

"The Bonanza vein outcropped as a great mass of rich ore, mostly solid chalcocite, along the top of a knife edge ridge projecting out from Kennecott Spur," they wrote, adding that "The rock fragments vary from small grains up to the size of one's head."

Of Kennecott as a whole, they wrote that "the great masses and purity of this mineral are one of the striking features of the deposits."

A century later, my wife, Julie Stricker, and I were just as struck as Bateman and McLaughlin. Copper was indeed under our feet as we poked among the rock and gravel alongside National Creek, the tumbling water source that cut through Kennecott Mines.

Julie stopped, bent down and picked up a little bit of green no bigger than a lima bean. How rich it is in copper, we don't know. We do know, however, that it is rich in history.

Since the late 1970s, the University of Alaska Fairbanks' Geophysical Institute has provided the Alaska Science Forum column free in cooperation with the UAF research community. Rod Boyce is a science communicator at the UAF Geophysical Institute.