Cathedral Rock Pluton, Southern Ravalli County, Montana


Over a period exceeding 15 years, I have explored an unmapped Tertiary age miarolitic granite located at the extreme southern border of Ravalli County, Montana. I coined the name “Cathedral Rock Pluton” after the only named feature in the area, a prominent granite outcrop located near the eastern extremity of this intrusion. It outcrops at the head of Woods Creek and westward nearly to Reynolds Lake; it extends for several miles into Idaho, but only in places shallowly into Montana. Here, collectors have worked over areas with numerous miarolitic cavities, in a granite very similar to the Sawtooth Batholith or the Lolo Batholith to the north of this area. One place appears to be an aplitic body riddled with cavities; here people have dug hundreds of holes to intersect the smoky quartz-lined vugs. Other minerals we have identified here include: smoky quartz, microcline, albite, magnetite, fluorite, bertrandite, zinnwaldite, zircon, allanite and epidote. The zircon crystals are small but bright green; some of the microcline is found in Baveno twins, well-formed and sharp, and some highly prismatic.
I have been examining some of the material from a large pocket I found and collected several years ago, and have discovered numerous stilbite crystals, often in close association with the tiny epidote crystals (the latter identified by SEM/EDX at Montana tech by Dr.Paul Miranda), and also some of the specimens exhibit a secondary overgrowth of hyalite opal, which fluoresces a vivid green. At least one unknown mineral has been recognized, but more work needs to be done with this one. The crystals are small (up to about 1/2 mm), grayish in color, transparent to transluscent, and somewhat equant in form, but it is not clear exactly what crystal forms are evident here.

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