7 June 2013

Friday fold: South Pass City, Wyoming

Posted by Callan Bentley

Two summers ago, my wife (then my very new wife) Lily and I participated in a two-week workshop for teachers on the energy resources of Wyoming. We also indulged in a visit to a mineral resource site: South Pass City, a site in the southern Wind River Uplift that was the second oldest incorporated town in Wyoming (after Cheyenne). It was a gold mining site, and today it is preserved for the sake of education about history.

Here’s curator John Lane, next to a stamp mill, which would pulverize the gold ore extracted from the mine.

South Pass City is reconstructed to look like the late 1890s or early 1900s. It’s a nice place to visit for that flavor of the wild west. The town had a ratio of three saloons to one church – about the right proportions, I reckon!

What’s gold ore look like? If you’re lucky, it looks something like this:

At South Pass City’s Carissa Mine, the gold was emplaced along a shear zone that deforms some Archean-aged metagraywackes. I’m told that the graywackes are about 3.0 Ga in age, while the mineralization took place at about 2.8 Ga. The larger context is that it’s within a Neoarchean greenstone belt: the primordial signature of pre-plate tectonics crustal growth.

Here’s a look at the shear zone above the “Glory Hole”:

The “Glory Hole” of course is the access point for the mine, which excavated mineralized shear zone rocks and brought them up to be processed in a massive building.

If you ask nicely, John will take visitors inside this well-preserved structure for a look around. Visually, it’s 1946 inside here. Here’s Lily above a massive tank used for processing and concentrating the gold:

What manner of chemicals used to dwell in the tanks like this one?

A dilute cyanide solution (1:100 or 1:1000) will dissolve gold, I was told, and the resulting “pregnant solution” would be poured through a cotton “sock” with a handful of zinc pellets at the bottom. The zinc would then swap places with the element of interest, and Zn would enter solution as Au would precipitate out. Then you just bake it up and pour it into a mold about the size of a hockey puck, and you’ve got something that’s 98% pure gold.

Oh, I promised you a fold, didn’t I? Very well, here’s some of the highly sheared metagraywacke from the wall rock of the shear zone:

Happy Friday – hope you have a golden weekend!