ABSTRACT

Strata-bound gold and silver deposits occur in the Devonian Jefferson Dolomite near the crest of the Little Belt uplift, Montana. There are two types of precious-metal mineralization: 1) gold-tellurium associated with hematite, pyrite, quartz, and gossan material, which is accompanied by minor silver, copper, lead, and zinc; and 2) silver-lead-zinc associated with galena, sphalerite, malachite, and minor gold. Mineralization occurs mainly as lenses or mantos up to 3 ft (1 m) thick in permeable dolomite, with replacement by silica, iron oxides, and sulfides. The deposits are mainly in a stratigraphic interval 140 ft (42 m) thick near the middle of the Jefferson Dolomite; approximately 90 ft (27 m) of this interval is permeable, petroliferous, stromotoporid-bearing dolomite that is favorable for mineralization. Ore mineral and jasperoid replacements also are present along northeast-trending vertical fractures.

The source of the gold and silver hosted by the Jefferson Dolomite in the Little Belt Mountains is not known, but the close association with lamprophyre (spessartite) dikes and syenite porphyry of the central Montana alkalic province suggests that these igneous rocks may have been involved in the mineralization.

Inasmuch as the Jefferson Dolomite hosts precious-metal deposits in numerous mining districts in Montana, we suggest that there is potential for similar strata-bound gold and/or silver deposits in some of these other areas.