New Mexico Mineral Symposium — Abstracts

In the past 10 years several collectors including this author have made an effort to search out wulfenite occurrences throughout southwest Montana. Nearly all of these finds are of microcrystals, and many of the finds have been limited to just a few specimens. Many additional locations have yet to be investigated. Wulfenite is found at lead-silver mines and prospects containing oxidized ores. These lead deposits typically occur at or near the contact of igneous intrusive rocks with limestone sediments. In the early days of mining the zinc minerals hemimorphite and sphalerite were considered worthless and were thrown out onto the waste rock dumps. A common factor in nearly all of the wulfenite finds is the presence of hemimorphite, which can be used as an indicator mineral. Whenever hemimorphite is present the locality bears further investigation, if sphalerite is present the mine was not in an oxidized zone and little of interest is likely to be found. Although wulfenite is not always present at locations containing hemimorphite, there are a few mines that have other interesting secondary minerals associations. In particular the Black Pine, Elkhorn, and Scratch Awl mines are worthy of mention.

While most of the wulfenite finds have been very limited in size and quantity of specimens, there are a few localities that have been very productive of specimens. Beaverhead and Broadwater Counties in particular have several localities that continue to yield wulfenite and other interesting minerals including cerussite, chlorargyrite, descloizite, mimetite, rosasite, pyromorphite, and vanadanite. A summary of the occurrences that the author has visited is presented in the following table; it is about half of the known wulfenite occurrences in the state. More finds continue to be made.

For the field collector I would recommend the Bob Ingersoll, Gar, Stapleton prospect, Santa Anita, and Jo Dandy mines as being the most likely for finding more specimens of wulfenite.

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