The Empire Mine near Mackay, Idaho USA began operation in 1902, and by 1975 (mostly before 1930) produced more than 42,000 ounces of gold, 1,200,000 ounces of silver, 30,000 tons of copper, as well as zinc and lead (Moran, 2021, Geology and historic and present activity of the Empire Mine Project, Custer County, Idaho: Road Log: Northwest Geology, The Journal of the Tobacco Root Geological Society, v. 50, p. 63-67).
Today the operation is focused on copper exploration in skarns in the Mississippian (Lower Carboniferous) White Knob Limestone. The skarns were produced in the limestone by an Eocene intrusion, the Mackay Stock. Significant faulting served as conduits for the mineralizing fluids (Moran, 2021).
Chrysocolla, copper silicate, is one of the primary ores in this project. Chrysocolla is usually a minor accessory mineral, but here it is abundant enough to serve as ore. By molecular weight, chrysocolla is about 33% copper. Â
Garnets (typically grossular or possibly andradite) are also common in skarns, resulting from the combination of calcium from the limestone with aluminum and silica from the intrusion. At the Empire Mine, garnet has been replaced by magnetite, producing some very cool pseudomorphs.
Magnetite, iron oxide, crystallizes in the isometric (cubic) system. It most commonly forms octahedrons (usually at relatively high temperature), more rarely cubes (usually lower temperature), and even more rarely as dodecahedrons, 12-sided forms, that have been attributed to intermediate temperatures of crystallization.
The magnetite dodecahedrons (some are deformed) in the 14-cm specimen above probably have nothing to do with the temperature of crystallization. These dodecahedrons used to be garnets, which have been faithfully replaced by magnetite. High on the list of strangest rocks I’ve seen.
Some of the rocks at the Empire Mine are essentially nothing but light brown garnet and black magnetite. One example, highly brecciated, is shown in the photo above with the hammer.
In some other deposits it seems that magnetite replaces garnet during retrograde metamorphism – not the heating up phase, but the cooling down from a peak high temperature. Alternatively it’s possible that a second heating, to a temperature lower than the previous peak, might have been a factor in the alteration to magnetite.
Please note that this project is on private property. Photos are from a field trip during the 2021 Tobacco Root Geological Society field conference when we had permission to visit.
Great read, thanks😊