The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Thursday, June 26, 2014

June 26. Leadville





Ore zones in black
Lead was discovered at Leadville, Colorado, in 1874, and has continued sporadically over time more or less to the present. The host rock for the ore is the Mississippian Leadville Limestone, which is one of the formations equivalent to the Madison Limestone that covered so much of North America during early to middle Mississippian time.  

As usual, the prospectors in the Leadville area were seeking gold, and the exploitation of lead was a later development.

The ore is related to igneous intrusions that forced their way into the limestone, forming sills, which are igneous bodies that are parallel, or concordant, with bedding planes in sedimentary rocks. The molten material came in along the weak zones that bedding planes created. The lead was most likely deposited by hot waters associated with the magmas. The water dissolved parts of the limestone, and the ore minerals, such as galena, lead sulfide, were deposited in the fissures and cavities that had been dissolved. The magmas are of Tertiary age, so the ores were deposited around 300 million years after the limestone formed during the Mississippian.

Zinc and silver are often associated with lead deposits, and the total cumulative production at Leadville, Colorado, includes more than 240 million ounces of silver, making Leadville more famous for silver than for lead. The district also produced more than 700 million tons of zinc, and almost a billion tons of lead.

The Leadville Limestone in Utah was eroded and dissolved by a period of karst formation, and the resulting porosity makes the Leadville a target for oil and gas exploration in Utah.
—Richard I. Gibson

Leadville history 

Cross section from Argall, in Ries (1925). Public domain.

2 comments:

  1. Very interesting post for me. I once did surveys for the rare Leadville milkvetch in that high limestone country. It's called Astragalus molybdenus so I suspect there are molybdenum mines there too. I guess it was (appropriately) called A. plumbeus originally, but that name was used earlier for another milkvetch, so the Leadville one was rechristened molybdenus. It's a place I want to go back to now that I'm much more into "plants and rocks"

    ReplyDelete
    Replies
    1. Very cool - thanks for the comment. Leadville is just a few miles down the road from Climax, at Fremont Pass - and Climax is among the largest molybdenum deposits in the world. I don't know that moly was an important byproduct at Leadville, but it might have been.

      Delete