A VERY UNUSUAL POCKET


INTRODUCTION

The Boulder Batholith of Montana is a late Cretaceous-early Tertiary aged granitic intrusion located in southwest Montana, occupying the Butte copper district and extending northward to Helena, Montana. Compositionally, it is largely a quartz monzonite, but is slightly more mafic in places, and also leaning towards true granite in other areas. Its overall size is about forty miles long and twenty miles across; it contacts metasediments in some places, and is covered by unconsolidated sediments in other places. Throughout southwest Montana, other similar granitic intrusions of identical age and/or composition are found. It is largely equigranular granite but contains larger phenocrysts of k-spar locally and in some areas xenoliths are quite common.

The Boulder Batholith is the host rock for the greater Butte copper-lead-zinc and molybdenum deposits, along with numerous, lesser deposits of tungsten, gold, silver, and other base metals. Locally, aplites and pegmatites are common; these are generally interior-type pegmatites, that is, they are found within the granite mass rather than adjacent to it (exterior pegmatites). The pegmatites are locally vuggy and may contain good examples of crystalline granite-druse minerals, such as smoky quartz, microcline, albite, epidote, tourmaline, titanite (sphene), axinite, and a host of lesser accessory minerals.


This special occurrence has a long history: back in 1979, I was attending college and working a summer job at Stauffer’s Silver Bow Phosphorus Plant. Work was in the electric arc furnace and quite hot in the summer; I had been on graveyard shifts and just had completed a shift when I met up for the first time with Pete Knudsen. We drove up into Homestake Pass east of Butte, Montana, and took the old road towards Delmoe Lake. Near a place known as “Pappas Place”, and just at the northeast corner of Goldflint Mountain, were several small pegmatite digs that I was familiar with. Most of these had been thoroughly dug up, but I had been successful finding one small pocket of amethyst, and a small amount of smoky quartz nearby.

We had split up and were exploring the area off-road, when I found a nice, clear smoky quartz crystal lying at the bottom of a shallow draw. This crystal was flawlessly transparent, very pale from exposure, and about 4 cm. in length and 2 cm in diameter. It was good enough to warrant further investigation, so I called for Pete to join me. I was examining the hillside above the draw and noticed at its crest was an aplite outcrop, one that was prominent for several yards, then disappeared and reappeared a few feet beyond. I pointed this out to Pete, but he insisted that in order to track this specimen to its source, we had to dig up the hillside from the very bottom up, or at least until we found the pocket that it had come from.

While I knew better, I acquiesced and we proceeded to dig up the hillside. As I predicted, the pocket was found exactly where the aplite dike vanished; the vug had collapsed and the contents were intact, except for the loose crystal I had found in the float below. This pocket wasn’t very large, perhaps the size of a football; the aplite dike it was in was barely two feet thick, but the graphic granite surrounding the vug was sharp and distinct. The vug contained several smoky quartz crystals, with inclusions of golden rutile, and a couple of dark green tourmaline crystals. The largest of the rutilated quartz crystals was about 7 cm x 4.5 cm (see: ); the tourmalines were thumbnail-sized.



After completing the excavation of this small pocket, we packed up our finds and started hiking uphill. A few hundred feet above we found an area that was covered with scattered white quartz debris. We casually examined the area and decided to move on. A few dozen feet above this we found more signs of pegmatite float and dug in, finding some nice, clear smoky quartz crystals, This occurrence was a fairly weathered pocket; most of the crystals, although small, were scattered throughout the soil and not packed together.

I returned to this last site again some time later and found a little more additional material, but it was several years before I came back.



1984

After I graduated from Montana Tech, I spent the first year working the PC Mine in Basin for Japan-law twins and other quartz crystals. Early in the spring of 1984, on a rainy afternoon, a friend and I decided to make a trip up into Homestake Pass. Mike Creek was a student at Montana Tech, and the semester had just ended. We were basically bored but had no destination in mind, so we headed up to the pass and started down the road towards Delmoe Lake. I don’t remember why we chose the same place, but since I had experienced some luck here before, we ended up hiking through the woods to the place where Pete and I had found the quartz chunks five years before.

I was standing around just aimlessly looking over the ground while Mike stooped down to examine one of the masses of quartz more closely. He scraped away a thick covering of pine needles and called to me. When I went over, he had exposed the shiny face of a fairly large smoky quartz crystal, at least three inches across and as many long! This turned out to be a fine, terminated quartz crystal about 6 inches long, buried in the pine needles point down. We started to scrape away the covering of pine needles in the immediate area, finding that virtually all the exposed quartz was at least partially crystalline, and the majority of them large, terminated crystals. There were hundreds of them, all in one area about five feet across and perhaps ten feet in length. It soon became apparent that the pocket had weathered in place; the contents were completely dissociated and scattered basically just below where they had once been. They hadn’t moved downhill much; the wallrock comprised of aplite and graphic granite was exposed on one side.





We spent the rest of the afternoon in a light drizzle, removing as much as we could, and packing back to the vehicle. The next day we returned with another friend, who helped us dig out and pack the rest of the material out. I dug until the walls of the pocket became so close to each other that barely an inch separated them. At this point, the pocket was almost entirely large microcline crystals. Other than that, we found several hundred large smoky quartz crystals, all which exhibited some wear and bruising on the crystal edges. This indicated the small amount of movement they had undergone. In addition to the quartz, we found several feldspar crystals up to six inches across, several masses of bluish cleavelandite, one cubic pseudomorph of goethite after pyrite, and a couple of large black tourmaline crystals (schorl). The latter had feathery terminations consisting of tiny parallel spicules, with pinacoidal termination forms. One of these schorls measured 2 inches in length and over an inch in diameter.

After cleaning, we had a pretty impressive collection of specimens, including two smokies attached to cleavelandite matrices (see: ). My living room floor was covered in “flats” filled with crystals, and we gathered the best of these and I took a photo of Mike with them spread out on a table.













1993


It was several years before I returned to this site; by then I had moved to Missoula, where I worked for several years as a geotechnical engineer. When I moved back to Butte, to continue my education, I spent one semester in classes before spring arrived.

In May I returned to this site, having pondered over the fact that although we had thoroughly dug out this pocket, it still went on albeit a very narrow opening. Surprisingly, even though the new road to Delmoe Lake was completed a few years before, and passed only within a few hundred feet of this dig, no one had disturbed it. I started in by digging exactly where I had left off 9 years earlier, where the pocket walls nearly came together. In a very short time, the opening abruptly widened and soon I was in a separate chamber, but still a part of this pocket system. The material came out almost identically, but it was apparent that there was little smoky quartz but an abundance of microcline and cleavelandite.

This section of the pocket was completely intact, as opposed to the original lower end, which had been almost entirely breached. It was round in cross-section, and as I dug down, elongated so that the overall shape was oval in profile. At the very top of the vug I found a huge single microcline crystal, partly crowned with pale blue cleavelandite balls, and exhibiting several curious casts of some former mineral. This monster feldspar weighed 25 pounds and measured 30 x 25 x 20 cm. overall.



Another curiosity uncovered near the very top of this vug was a large, euhedral crystal that appeared to be compact greenish mica, and soon I ascertained it to be a pseudomorph of muscovite after topaz. The form, although somewhat modified by the replacement, was quite similar to many topaz crystals I had found in the Sawtooth Batholith, and later I had some of it analyzed by x-ray diffraction at Weber State by Jim Wilson, who determined it to be basically pure muscovite. A quick search of a couple of mineralogy texts informed me that “…..[topaz] is easily replaced by a compact mass of greenish muscovite”. This unusual pseudomorph was 7.0 cm long by 4.2 x 2.7 across, a fairly large and complete example of this curious phenomenon.



I spent most of the day excavating this large vug; by the end of the day I had a large amount of feldspar crystals, some up to six inches across, and quite a few masses of bluish cleavelandite. But it was clear that it wasn’t yet cleaned out, so I had to pack up what I found and return the next day.

The following day I returned and found that the US Forest Service had filled in my hole, even though I wasn’t finished with it. This is an example of their complete inefficiency, as not only did it take unnecessary time to fill in, but I had to dig it all out to continue with the excavation. It was even more surprising since no one had disturbed the original dig in a period over nine years, and since this second re-excavation took place, it hasn’t been filled in by them since!

On the second day I returned with an abundance of “flats”, a good lunch, beverage, and even portable music to keep me company. I spent the rest of the day digging further and further down, until I finally reached the bottom. Here, the rock became quite solid and resisted all attempts to dig down further; while the rest of the pocket was excavated with relative ease.

In all I recovered several flats of lightly etched microcline crystals, a few black tourmalines (all which appeared to be etched forms of much larger crystals originally), one fine cluster of several intergrown cubes of goethite after pyrite, and no less than 35 flats of cleavelandite masses! These were pale bluish in color, coarsely bladed hemispherical masses measuring up to 8 inches across, and every one of them exhibiting multiple casts that were apparently formerly occupied by topaz crystals! The overall size of the pocket was at least three feet deep (straight down) and over two feet in diameter.





The presence of the topaz pseudomorph and the forms of the crystal’s cross-sections strongly hinted that at one time this vug was full of topaz crystals, but they had all been “etched” or dissolved away completely. The one pseudo survived apparently because it was near the very top of the vug; I surmised that perhaps the upper end of the vug had at one time been a gaseous fraction rather than liquid; those topaz crystals below the liquid-gas interphase had dissolved completely in the solution. This was also supported by the coarse crystals of silvery muscovite that dotted the outer portions of the feldspar crystals, and in some cases, lining the very casts left by the topaz crystals.


After cleaning off and examining the total contents of this pocket, I was able to determine the exact path that the crystallization sequence had apparently taken: The large cluster of goethite after pyrite and the topaz pseudomorph helped to solve the problem.

When the pyrite was altered to goethite, the exchange of ions was that the sulfur was replaced by hydrogen and oxygen, which liberated the sulfur into the pocket solution (which is largely water). The reaction between the water and the sulfur would create sulfuric acid, which topaz is soluble in! This would explain the disappearance of so many topaz crystals in the pocket. Which brings up this: what if the topaz had NOT dissolved? This would have been quite the find!!

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