The normal distribution of the other lanthanides
Back at the Pass
The Lemhi deposit has been well-known since the 19th century, but its main interest to miners was as a copper deposit. The mineralisation has copper-rich veins, REE/Thorium-rich veins and veins of no particular interest. While Thorium usually sends investors into a tizzy it is worth noting that this is why the work was done on the deposit recent decades. Indeed, in a mini-boom for Thorium fueled reactors in the early 1970s extensive underground working were put in at the site, but more on that anon.
At Lemhi Pass, the average thoria content of some of the better explored thorium veins commonly exceeds 0.25%. The overall rare-earth content is about equal to the thoria content and varies from vein to vein and from place to place in the same vein. The ratio of total rare-earth oxides to thoria in various samples ranges from 0.05 to 5.2. Interestingly many of the samples taken there in a USGS survey were depleted in cerium and enriched in europium as compared with the normal distribution of the other lanthanides. Small amounts of silver, 0.2 to 2 ounces per ton have been found in samples from the Wonder, Black Rock, Buffalo, and Last Chance veins. More than 1% zinc occurs in some samples from the Wonder and Black Rock veins. So this is clearly a rather unique mineralisation.
The Last Chance or the Window of Opportunity
The area exciting most interest at the moment is the picturesquely named Last Chance Vein. Usually we thought Last Chance was referred for saloons. The Last Chance vein was first explored by Elkhorn Mining Co, in the 1950′s, under support by the Defense Minerals Exploration Administration (DMEA). After passing through several other owners, the property was held by the Union Pacific Railroad during the 1970′s. During this period UPR developed two adits and a shaft, and completed several drillholes to explore the vein. The adits developed by UPR consist of a northern adit 755 feet long and a southern adit 452 feet long. While the adits are not connected, both adits intersect the vein. Additional underground work by UPR included development of an 80-foot vertical shaft located between the adits which also intersected the vein with a short 22-foot tunnel.