June 10. Tri-State Lead-Zinc District

The Tri-State lead-zinc district lies around the common corner of Missouri, Kansas, and Oklahoma. Lead and zinc were produced from Mississippian rocks that historically were called the Boone Formation, but have been subdivided and given other names including Reeds Spring, Keokuk, and Warsaw formations.

The rocks that host the lead-zinc deposits are mostly limestones that were deposited in warm, shallow seas, the more or less standard for middle America during the Mississippian Period. It’s not 100% limestone – there’s a lot of chert, fine-grained silica, interbedded with it and as nodules in the limestone, as well as some shale in places. 

The mineralization appears to be related to faults and fractures in the limestone, complicated by dissolution and collapse of the limestone in some places. The fractures provided abundant pores and passages for mineral-rich waters to flow through, but the ultimate origin of those fluids is debated. The most common view is probably that the mineral-rich fluids rose from some deep, magmatic source until they were stopped by an impermeable layer and found the open fractures in which to crystallize the lead and zinc minerals.

Sphalerite photo by Rob Lavinsky, iRocks.com
CC-BY-SA-3.0

The deep magma from which the hot mineral-rich water rose might have been related somehow to the flank of the Ozark Uplift, which got its start as long ago as the Ordovician, but there was some ongoing, relatively gentle tectonic activity over many millions of years, including during the Mississippian. As is often the case, it’s difficult to pin down the time the minerals came in with accuracy. It has to be younger than the rocks that contain the minerals, of course – they and their fractures had to be there first. And there’s good evidence that the deposition of minerals must have taken a long time, probably in many episodes, possibly spanning as much as many tens of millions of years.

The most common minerals are galena, lead sulfide, and sphalerite, zinc sulfide. The minerals were so important to the early history of this region there’s even a town in Kansas named Galena. Mining began about 1848 and continued until the Eagle-Pitcher Mine in Oklahoma shut down in 1967. Over the century from 1850 to 1950, the district produced about half the Zinc mined in the United States and 10% of the lead. One mine complex, the Pitcher Field in Oklahoma, was the most prolific producer, yielding about 60% of the total which amounts to more than 15 million tons of lead and zinc over the history of the mining district.

—Richard I. Gibson

Resources:
Report from Oklahoma Historical Society 
Report from Kansas Geological Survey part 1; part 2


The Geology and Ore Deposits of the Tri-State District: D.C. Brockie et al., in Ore Deposits in the United States, John D. Ridge, ed., American Inst. Of Mining, Metallurgical and Petroleum Engineers, 1968, p. 400.

Sphalerite photo by Rob Lavinsky, iRocks.com – CC-BY-SA-3.0