66
along the northwest corridor, extending from the south and west flanks of Bull Hill through the
Whitetail diatreme towards the Carbon Hill diatreme.
Figure 5.10-3
Geology of the Bear Lodge Property
Major structural trends are oriented west-north-westerly, north-westerly (parallel to the axis of
the domal complex), and northeast or east-north-easterly. Widespread thin soil cover and lack
of outcrop hinder structural mapping and interpretation. Surface rock exposure in the complex is
probably on the order of 5 percent or less.
However, data obtained from detailed geologic cross sections, utilizing results of closely spaced
drilling, surface mapping of drill pads, roads, and trenches, and borehole televiewer data from
core holes, support earlier district-wide observations that indicate a predominance of both north-
westerly and east-north-easterly structural trends, as well as subordinate northerly trending
structures. Geophysical surveys (magnetics, radiometrics, and IP/resistivity) are in accord with
the field data.
Most of the rock units are affected by widespread potassic alteration and have a thick near-
surface oxidized zone. Recognizable hydrothermal alteration includes pervasive fenitization
(alkali-ferric iron metasomatism), K-feldspar-pyrite alteration, minor silicification, and localized
argillic alteration, along with superimposed oxidation and surface weathering.
Geological events that can be interpreted in the core of the complex include multiple intrusions
of dominantly porphyritic phonolite and trachyte, multiple brecciation events, carbonatite
magmatism, alkali-ferric iron metasomatism (fenitization), hydrothermal alteration, and
