New Mexico Mineral Symposium — Abstracts

For one week in July 2002 a mining operation was conducted for blue barite mineral specimens at the famous Stoneham, Colorado, barite locality. The collecting site is about 4 mi northeast of the village of Stoneham in Weld County, Colorado, which is in the high rolling plains about 100 mi northeast of Denver. The collecting site is located on state land and is currently under lease by a local resident.

The barite crystals occur in open pockets developed in the lower most member of the early Oligocene (35-37 million years old) Chadron Formation of the White River Group of sedimentary rocks. This lower most member is a 15-20-ft-thick, tan to cream claystone layer that is rich in volcanic glass shards. The Chadron Formation lies unconformably above the regionally extensive Cretaceous Pierre Shale (Modreski et al., 1990). In the project area, the crystal pockets occurred along two major fault trends, both of which strike northwest-southeast, but one dips southwest and the other dips to the northeast. Dips along the undulating fault planes were highly variable and ranged from about 10 to 45 degrees. The faults were identified in the field by striated slickensided surfaces indicating essentially vertical movement.

Open (and crystal-bearing) pockets occurred predictably in only the flat-dipping part of the faults. Modreski et al. explain that "pull-apart" movement along the flat part of the fault planes created the open pockets. Evidently only the lower most member of the Chadron Formation is sufficiently brittle (rather than plastic) to form open cavities. Pockets were not observed to have developed along the faults in either an overlying siltstone layer, or the underlying Pierre Shale, both of which are apparently more plastic in nature and are not sufficiently brittle to form open pockets when fault movement occurs.

A backhoe was used to uncover cavities as large as 6 inches high, 6 ft long, and 3 ft deep. Some pockets contained abundant free-standing barite crystals, and other cavities were devoid of barite. Individual pale to sky-blue crystals as long as 10 cm were found, and numerous smaller singles and crystal groups were also recovered. Generally, the crystals were either very clean or had only a thin clay coating and were found essentially loose in the cavities. Most crystals from this location were opaque to translucent, but on occasion fairly transparent specimens were also encountered. It was also observed that as the crystal-bearing faults approached the underlying Pierre Shale, the pockets became partially filled with a cream-colored "gooey" clay, and quality of the barite crystals decreased dramatically and became cloudy and broken. This was apparently caused by surface water migrating down the faults and accumulating or pooling in pockets at the top of the Pierre Shale, which acts as a confining layer or barrier to the further downward water migration. It is highly likely that future mining activities at this locality will result in the discovery of new pockets filled with gleaming blue barite crystals.

References:

1. Modreski, P. J., Lees, B., and Wilson, D., 1990, New explorations at the Stoneham, Colorado, barite locality: Rocks and Minerals Magazine, v. 65, no. 3, pp. 202-222.