This investigation of the Eagle Mountain Mine area, though cursory, revealed new structural, alteration, and stratigraphic relations. Eagle Mountain ores were previously an important source of iron to the western U. S. Ore (where fresh) is magnetite-pyrite rock forming two stratabound horizons virtually continuous for 11 km.
Gneissic basement rocks are overlain by two sedimentary units, separated by unconformities. The lower unit contains carbonate rocks and quartzite; the "vitreous quartzite" of previous workers, however, is an alteration feature rather than a stratigraphic one. The upper unit contains thick conglomerates. This layered sequence of rocks is deformed into a west-plunging anticline. Intrusion by Jurassic quartz monzonite apparently followed this deformation.
Quartz monzonite forms a branching network of sills, some of which dilate the contact between the upper and lower sedimentary sequences. Intrusion resulted in extensive, mostly-anhydrous skarns, but stratabound iron ore is just as closely related to some other features:
regional alteration of quartz monzonite, with iron ore adjacent to little-altered rocks along the boundary between sodic and potassic domains,
the two unconformities, which apparently formed stratigraphic traps for precipitation of stratabound iron ores,
a north-facing monoclinal plane between folds, which was preferentially replaced.
Iron ore replaces a variety of host rocks along the two unconformities, forming massive to globular bodies, and its mineralogy correlates with deuteric alteration features, not anhydrous skarn. Its pyrite contains as much as 3% cobalt.
Iron was only one of five elements that showed mobility in this region on a scale that suggests basic crustal processes. The others in probable order of flux magnitude are silica, magnesium, sodium, and potassium, to form regionally distributed “vitreous quartzite”, dolomite, and secondary feldspars, respectively.
