BBC Mine, El Dorado County, California, USAi

Regional Level Types
BBC Mine	Diggings (Active)
El Dorado County	County
California	State
USA	Country

The BBC Mine lies on a faulted contact between two groups of granitic rocks of various lithologies. They are an older, deformed rock generally of quartz diorite to diorite composition, and a younger, less deformed rock generally of granodiorite to quartz-monzonite composition. The older dioritic rocks occur in the western part of the area and the rocks occur in the east part. The darker dioritic rocks contain large amounts of biotite and hornblende, which vary greatly in relative proportions over short distances. These bodies are often brecciated and intruded by plugs and dikes of aplite, pegmatite, and granite. The plugs and dikes generally cut across the foliations in the host rock, and are relatively undeformed (although pockets within the pegmatite dikes have been heavily effected by tectonic/shearing events) indicating they are substantially younger than the diorite. Foliations and outcrop patterns suggest the pluton is tabular and has been folded. Radiometric dates indicate the dioritic rocks are early Cretaceous (-140 million years old). The younger granodiorite and monzonite are much lighter and coarser-grained. Dark minerals within the east portion are biotite and hornblende. The most common lithology within the claim boundaries is a biotite-hornblende granodiorite.

The small aplite/pegmatite seams have yielded a number of large pockets with a variety of accessory minerals. The pockets have many features that are resemblant of Alpine Type Deposits, even though they are hosted within granite/pegmatite. These pegmatites have also yielded a number of rare albite twin specimens. Although none have been officially examined and documented, they have certain experts in the field of feldspar twinning confirm that they are indeed uncommon/rare twin forms.

Discovery
The pegmatite zone occurrence was discovered in 2017 somewhat by accident. In August of that year I was hiking out to prospect for gold after work and I decided to hike up to a cliff where I had observed a number of pegmatite seams. Because of the steepness of the terrain, relative inaccessibility, and my limited knowledge of pegmatites and granite hosted Quartz bodies, I had passed it by. Upon checking the scree at the bottom of the face, I noticed some small bits of albite with numerous small light colored quartz crystals. This was enough to spur me to climb up to the lowest of the seams, the seam jutted out bout a foot from the rock face and the entire underside was covered in broken but large smoky quartz crystals. There was also a portion of an exposed pocket was that had other feldspar and quartz crystals. I pulled some of the loose pegmatite apart and right away found a small vug of nice small smoky Quartz crystals, the first I’d ever found. Because I only had a gold pan and some spades, I decided to return the next day and explore this spot some more.

I hiked out early the next day and started where I had left off the day before. I was finding not more signs of Quartz as I worked towards the crystals on the exposed ledge, and so decided to dig towards a big plug of Quartz sitting on a ledge above me. I cut in hand and toe holds into the soft granite wall and began disassembling the seam. The pegmatite crumbled easily, and it did take long to remove the grus and loose pegmatite. I was about 2’ away from the large quartz plug when I pulled a chunk of peg that was about 2’ square that I had been working around for about a half hour. This I tossed down the cliff and watched as it went careening down the hill. I turned around to resume digging and notice a small hole behind where the big block has sat. The hole was just wide enough that my first though was that it was a gopher hole… but, a gopher in solid/friable rock…? I quickly grabbed my iPhone and turned the light on and shoved it inside. I could see almost nothing, but that the hole definitely got bigger inside. So I turned my camera on video and stuffed my arm and camera into the hole as far as I could reach. When I brought it back out and reviewed the video, I about fell off of the cliff. Inside the hole were very large quartz crystals in a large open cavity. It didn’t take long to open the pocket door wide enough for me to start removing the detritus and some quartz crystals. Many were broken, some were around 2” while others were up to 18”. I worked this pocket off and on for close to 6 months. The largest crystal we found was towards the end of the pocket and weighed 107 lbs. Removing a little at a time and trying to keep the slope above it as safe as possible. In the winter of that year, the cliff face suffered another landslide event and the seam was buried. I have not explored this specific zone any more because slope stability is a real safety concern.

Due to the large size, very dark tint, and mildly phallic nature of the crystals, we decided to comically call the pocket the BBC Pocket, short for Big Black Crystal Pocket. In time, with other large crystal pocket discoveries, we decided to file a claim and name the mine The BBC Mine, and the irreverent name has since stuck.

Some of the significant discoveries are listed here, with a rundown of minerals they contained.

BBC Pocket, 2017-2018 This large pocket had numerous large, very dark, and very lustrous crystals with size ranges from 2” to 24” and weights from a few ounces to 107lbs. The quartz crystals from this pocket tended to be heavily coated with beautiful green and nearly pristine muscovite florets on 2-3 of the prism faces. We also found the interesting hoppered albite formations in this pocket, along with red rutile inclusions and monazite-(Ce).

Saturday Pocket, 2018 This pocket was the first pocket we discovered with smoky quartz and pocket schorl, we pulled several pounds of needle like schorl with the largest piece standing at 4” and about 1” in diameter. This was also the first we noticed a slight baby-blue color in a weird blocky feathered form of albite. The feathered albite we later discovered (per analysis from McGill university and CalTec) was a form of cleavelandite. Interesting side note is that the blue colored cleavelandite is only found in pockets that also contain schorl.

Colin’s Pocket 2018 This pocket yielded a number of short stout smokies which we later repaired into a 13 piece cluster of beautiful smokies. This was also the same pocket where we first observed limonite cubes.

Blue Albite Vein, 2018-2019 This was a pegmatite seam that we noticed was surrounded by bright red clay, unusual in a drab gray granite environment. As soon as we began digging this, we started pulling thick schorl chunks and smoky quartz crystals. As we got deeper into the seam we found multiple plates of vivid blue albite that we later discovered, from Dr. Rossman, was actually a form of cleavelandite. This seam produces a number of small pockets, but only two that contained schorl and often garnets.

Scepter Pocket, 2020 This pocket was a fairly small pocket of smoky quartz crystals, with one corner of the pocket having ferruginous amethyst scepters capping low grade smoky quartz crystals. Less than a foot from this pocket we opened another pocket containing beautiful smoky quartz, schorl, small garnets, and some of the most vivid baby blue cleavelandite we have found to date.

Vein of Twins Pocket System 2021-2022 A pegmatite seam we are still analyzing and trying to better understand the albite numerous twins we found there along with very light colored smoky quartz. While most of the crystals are in the 1” range, they are oriented at +/-90° on the albite twin structures.

Lupine Pocket, 2022 This pocket yielded several hundred pounds of dark and lustrous smoky quartz crystals with almost no feldspar plates. The largest of the crystals is 13” with the majority being in the 6” range.

Boulder Pocket, 2018 This was a small fist size pocket on the side of a pegmatite boulder, the pocket only had a few quartz crystals that were grown together. The crystals were all amethyst scepters, the largest was about 1.5” tall. This was the second amethyst discovery we had made, and this material did not have the color saturation of the first (see photo album).

Other Regions, Features and Areas containing this locality