64
to west. Potassium-argon age dates of the Tertiary intrusive rocks range from 38 to 60.5 million
years (Lisenbee, 1985), with younger intrusions more common towards the western end of the
belt (see Figure 5.10-1). Isolated anticlinal domes are dispersed throughout the region and are
probably cored by alkaline igneous plugs of Tertiary age. On a broader scale, the Bear Lodge
intrusive complex and other Black Hills alkaline igneous bodies are part of a northerly trending
belt of scattered alkaline-igneous systems that occur from Mexico to Canada, several of which
are associated with significant gold deposits (e.g., Cripple Creek in Colorado, Zortman-
Landusky complex in Montana, and the Carache Canyon breccia pipe in New Mexico).
5.10.2 District Geology
The Bear Lodge mining district is located in the Bear Lodge Mountains, near the western end of
the northern Black Hills intrusive belt (see Figure 5.10-2). The Bear Lodge Mountains expose
and are underlain by multiple alkaline plugs, sills, and dikes intruded into Precambrian
basement and Paleozoic and Mesozoic sediments approximately 38 to 50 million years ago,
forming an elongate, northwest-trending dome about 13 km by 10 km (see Figure 5.10-2).
Figure 5.10-2
Geology of the Bear Lodge District (Richardson et al., 2010)
Several large Precambrian granitic rock bodies occur within the southern portion of the complex.
The Precambrian rock units may be roof pendants or, alternatively, may be anchored in the
basement. The Paleozoic sedimentary rocks surround the intrusive core of the dome and
consist mainly of limestone and quartzite, with minor sandstone, shale, and siltstone. Mesozoic
rocks include siltstone and shale, with minor sandstone.
Alkalic rocks of the Bear Lodge complex penetrated Precambrian granite and gneiss, and
intruded as plugs, dikes, and sills into the suprajacent Paleozoic sedimentary rocks. Sub-
volcanic trachyte and phonolite constitute the bulk of the complex. These rocks are intruded by
a variety of alkaline dikes, associated intrusion and/or contact breccias, and several diatremes
