Rockin the Rockies

Rockin the Rockies
Stowe Mtn

Thursday, May 30, 2013

Crystals in Miarolitic Cavities of Lake George Pegmatites

My rock hounding friend and I have returned to Lake George in search of crystals. We've had a slow start to the 2013 season because of the cold and snow we've had this spring. So far we haven't had much success, but perseverance and observation are key to finding the miarolitic cavities that contain the much sought after crystals. The miarolitic cavities or vugs is where the heated mineral enriched liquid cavities within the Lake George pegmatites produce crystals. As the pegmatites and cavities cooled over 1 billion years ago the crystals formed in the solidifying pockets. As the pockets contiued to cool they crack and end the crystal forming process. Due to weathering and other erosion processes many of these now collapsed crystal pockets are near the earth's surface and ready for harvesting. The difficulty in finding the collapsed pockets stems from hundreds of people digging through the pegmatites and finding the easy to get crystals. Sometimes a portion of the pegmatite is exposed to the surface and creates "float" crystals. The crystals are called float because they are on the surface and detached from their source. Inexperienced rock hounds often pick up these float crystals or keys to the location of the crystal pocket without realizing they've not only missed the mother lode, but also erased evidence for others to find the crystal pocket. One prospector at Lake George uses track hoes to dig down over 20 feet into the earth's crust to discover and follow pegmatites opening up crystal vugs. So far this year I have found 4 collapsed vugs with crystals, all within 1-3 feet of the surface. Unfortunately the pockets were small and many of the crystals were either broken or had quartz overgrowth. The breakage can be a result of either the original pocket rupturing under pressure during crystal formation, or the grinding of crystals against each other over millions of years of frost thaw cycles and other erosion processes. I believe the overgrowth of quartz over the smoky crystals is a result of a later infusion of quartz into the vug. This quartz covers the smoky crystals to varying degrees and if not gemmy, hoods the smoky crystals making them somewhat unsightly and not very desirable.  If the hooding quartz is smoky or clear, the crystals are more desirable and are described as phantom smoky crystals as the original smoky is visible through the later quartz hooding. These hooded crystals are highly prized.
This smoky cluster has a lot of overgrowth and is rather unsightly. Too bad the bluish fluorite (lower left) was not larger

A nicer crystal grouping found on Monday... no quartz overgrowth (crystals 1.5-2 inches)

Tuesday, May 14, 2013

Vug Clay at Lake George

Another day rockhounding at Lake George and another pocket of crystals. This is one of the first times I've run into blackish purple greasy clay in a crystal pocket.  Most often the clay is either reddish (iron-stained) or yellow. I've also run into grayish material. The particular pocket I was in this day had blackish purple clay and the crystals were not very collectible due to breakage and additional quartz overgrowth. I like clear quartz over smokys which gives the illusion of phantoms or ghost smoky crystals, but not the granular/rough opaque quartz overgrowth on smokys. We sometimes say the smokys are hooded when this happens. I got into a discussion with my digging buddy Bob regarding colors of clay and if that has any correlation to the quality of crystals found in the clay.  We both have had very good luck with red clay and poor luck (crystal quality) with blackish purple clay. I'm afraid our sample size is not sufficient to make any conclusions. Anecdotally crystals found in the yellowish clay are harder to clean and the smokys don't seem to have the same lustrous exterior as those found in the red clay.  This got me to thinking is the clay in the pocket from the surrounding material or is the clay a result of the soup left in the miarolitic pocket after crystal formation. Since every pocket I've ever found has been broken imploded/exploded I'm guessing it's a mixture of leftover vug material and surrounding decayed/weathered pegmatite.  I decided to do a search on the internet and not only was there a discussion about clays in miarolitic cavities but it concentrated on the Pikes Peak Batholith see Mineralogy and Provenance of clays in Miarolitic Cavities of the Pikes Peak Batholith, Colorado. by Kile.  While this article was a little above my head scientifically speaking, it seems part of my hypothesis is correct, that the clay is from both the leftover vug material and surrounding weathered pegmatite. The article mentioned the clay varieties smectite, Illite and kaolinite being predominant in the vugs (never knew there was so much to know about clay). The author also mentioned infusion of material from hydrothermal processes after the pockets form. I think some of the fluorites are formed that way. My bigger question was unanswered, does the color of the clay in a pocket have any correlation to the quality of the crystals therein. Does the color of the clay indicate a higher concentration of elements inhibiting crystal formation or luster? Enough investigation for one day. Here are some pictures of the latest dig....
Checking out the latest crystals
A pretty gemmy crystal

Darker area is pocket with broken quartz crystals

Saturday, May 11, 2013

Restaking a claim & late snow at Lake George

A friend of ours has a few claims north of Lake George and he has been sick for awhile. He asked us if we would be willing to restake his claim this year(what are friends for?). Well this turned out to be more work than we thought. Thank god for GPS. We found the first claim marker quite easily, but converting between coordinate systems and searching for vandalized posts was not what we expected. We got two claims restaked, but the weather set us back so that we will have to try again next week. We set out to restake the second set of claims only to be turned back by the weather. We thought there would be snow on the claim but didn't realize the depth and extent of the last snowstorm up in the mountains. Often a snowstorm will hit the front range mountains of Colorado and leave interior mountain areas like Lake George high and dry. Not so with the latest storm as we found out. Our trip was not totally wasted. We determined a field trip near Wilkerson Pass (9500ft) should be cancelled due to snow and instead of restaking claims we rockhounded on a sunny unclaimed area near Lake George (elev 8200ft). Elevation makes quite a difference with spring storms. I even managed to find a pocket of smoky crystals. By midafternoon over half the snow was melted and this helped me find a pocket. The pocket was small and the crystals were beaten up for the most part. Still it was my first find of the year.
Doing the forest service work, this tree has been across the rd for 6+ months!

Marker was thrown by someone into this brush pile. Supporting rock caron was about 50ft away down the hill

Pretty small crystal cache from pocket discovery, couple good ones I think

Still snow at Wilkerson Pass. Temperature was 23F on west side of pass near Hartsel. Time to go back to Lake George where the sun is shining and find those crystals depicted above!