247 Mineralogy/Crystallography (Posters)

Wednesday, 12 October 2011
9:00 AM - 6:00 PM
Hall C

HERKIMER DIAMONDS IN THE PENNSYLVANIAN TENSLEEP FORMATION MAY INDICATE HYDROTHERMAL INFLUENCES FOR MINERALIZATION OF THE RED PRYOR MOUNTAIN URANIUM/VANADIUM DEPOSITS

Paper number: 247-11
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Herkimer diamonds found in a siliceous red agate zone proximal to the Marie and Dandy abandoned uranium mines on Red Pryor Mountain, may provide evidence that the mode of mineralization was hydrothermally influenced. Similar agates were described in the Dryhead Creek area of the Pryor Mountains. Uranium content and the REE data of the Dryhead agate indicate that its formation occurred under the influence of hydrothermal fluids, or that at least a later hydrothermal overprint of the sedimentary rocks took place. The Dryhead agates show an enrichment of U which also supports the idea of hydrothermal fluids participating during agate formation and mineralization of the uranium deposits. Trace elements, in particular the REE and U, as well as the associated minerals, fluorite and REE carbonates, show that hydrothermal processes in the surrounding area may have, directly or indirectly, influenced the genesis of the Dryhead agates. The U in agates may be related directly to the U-bearing fluids which formed the U deposits, or it was released from the U-bearing rocks. A coprecipitation of dissolved U together with silica gel is suggested for the Dryhead agates as the measured trace-element profile does not correlate with Fe contents in the study. Therefore, U appears not to be associated with the Fe oxides which provide the coloring of the red zones of the agates.

The Herkimer diamonds found in the red agate zone proximal to the Marie and Dandy mines were found in vugs within the red agate zone. These euhedral crystals may help show the hydrothermal influence to the mineralization of the breccia uranium deposits in the area. Euhedral crystals of pyrite, fluorite, barite and U/V minerals in vugs may also serve as indicators of hydrothermal influence in the mineralization of the uranium vanadium deposits in the Pryor Mountains of south central Montana.

The Dryhead Creek area and the site that the Herkimer diamonds were found are both on strike with the regional Nye-Bowler lineament. This structural feature may be serving as the conduit for hydrothermal fluid circulation providing the uranium and silica to these deposits.