When we talk about mineral deposits, we often don’t know accurately the geologic time when the minerals came in – it might be much, much later than the rocks in which the deposits are found. That’s changing, as we get better and better at dating techniques, but for most of these podcasts dealing with mineral deposits, we’ll probably focus on the age of the host rocks and talk about the time the minerals came in more speculatively.
The lead belt of southeastern Missouri is concentrated in Cambrian rocks, especially the Bonneterre formation, which is mostly dolomite, calcium magnesium carbonate. It’s much like limestone, calcium carbonate, but the magnesium in there skews the crystal structure, so dolomite crystals contain more intermolecular space than calcite. That makes them good candidates for oil reservoirs, or as hosts for mineral deposits.
The Bonneterre rocks originated in a warm, shallow sea during Late Cambrian time, about 495 million years ago. They might have been limestone originally, converted to dolomite by magnesium-rich water percolating through the rock at a later time. The whole process of dolomitization is complex and not thoroughly understood, at least not by me – I’d like to find someone who knows more about it to talk with, as a future podcast.
|Galena from Sweetwater Mine, Viburnum Trend District,|
Reynolds County, Missouri, USA.
Photo by Rob Lavinsky, CC-by-SA.
Waters that were heated by mountain-building activity, volcanism and the physical collision of plates, must have collected a lot of lead. Those waters were most likely driven into the Bonneterre formation from the south, possibly from as much as several hundred miles away, until they found a suitable rock in which to crystallize. There’s more galena, lead sulfide, in southeast Missouri than anywhere else in the world. The ores in Missouri are part of a class of mineral deposits called Mississippi Valley Type, which occur in sedimentary rocks.
Southeastern Missouri has produced lead since about 1721, when early French explorers began mining. They produced as much as 1,500 pounds of lead ore per day, which was shipped down the Mississippi and on to France. Production has been pretty much continuous since about 1802, when Moses Austin began smelting ore, a year before the territory became part of the United States in the Louisiana Purchase.
|Historic mine and mill buildings at the Federal Mine and Mill #3,|
now a part of Missouri Mines State Historic Site.
Note the tailings dam in the background.
Until 2011 the nearly 400 tons of lead that came from US mines was enough to make the United States a net exporter of lead, but in 2011 and 2012 the U.S. imported 2 to 4% of its lead needs. Eighty-six percent of U.S. lead consumption goes to make lead-acid batteries for cars and trucks, and thanks to recycling, we get about three times as much lead from old batteries as we do from mines in Missouri, Alaska, and Idaho.
China, the world’s leader in lead production with nearly eight times US production, and half of all the lead mined in the world, is also one of the growing consumers as their auto and battery-powered bicycle market soars.
—Richard I. Gibson
Technical paper on timing of mineral development
Galena specimen photo by Rob Lavinsky, under CC-by-SA-3.0
Mine photo by John Weber, USFWS.