Amateur Geology in Five Springs Cave

- by Devin Kouts
Author stands next to the igneous dyke spotted by Ben Schwartz in Five Springs Cave - copyright D. Kouts, click to enlarge.
During a September 29, 2001 survey trip into Five Springs Cave Ben Schwartz spotted a rare geologic formation in the far reaches of the cave. An igneous dyke appears in the wall of the cave passage very near the end of the current survey. [Ed note: Since writing this article I have learned that this dyke was first spotted by Carol Peterson during an earlier survey trip.]

Initial observations at the dyke indicate a relative strike of 140 degrees (observed, not measured) and a dip of approximately 85 degrees (observed, not measured). The material of the dyke is very weathered but crystals were readily visible. Some of the largest mineral crystals are visible even in the low resolution photograph taken at the time (dark spots in center of photograph). The predominantly observed minerals were quartz, mica and biotite. The light color of the mafic rock intruding into the country rock could be an indication of a "felsic dyke".

Three small samples of the igneous material were removed from the cave for closer examination. These will be disposed of in accordance with the cave owner's wishes. These samples were examined by David Hubbard of the State of Virginia Dept. of Geology and much of the text below is based upon my recollection of his observations.

If you would like to make comment on the analysis of this material, or share what you know on the subject of similar igneous dykes, please contact the author at devinkouts@yahoo.com

Some very interesting observations have been made by various contributors to a newsgroup discussion I started on the sci.geo.geology newsgroup.

Mitchell Isaacs from Australia wrote:

I wouldn't be surprised if there's a bit of calcite in the dyke... The dark spots are probably xenoblasts - although, could be weathered phenocrysts.

xenoblasts are basically bits of rock that have broken off and mixed into the magma as it has moved. xeno=foreign, they are like foreign bodies. (not crystallized from the surrounding magma). Phenocrysts are large crystals...

...the orangey sort of colour is largely from K-spar (orthoclase) weathering.

...the sample described as "mafic" [Ed. in sample photos below] is actually about as far from mafic as you get. In the close up, I can make out Qtz, K-spar, I would guess there would be some plag, and there are a few black specks. A mafic sample would be dark in colour - in a mafic enclave in a rock like this, you would expect mostly biotite & hornblende (which is what I suspect your dark spots are largely composed of).

Another poster suggested tourmaline - however, hornblende as your non-biotite dark mineral is much more likely. Tourmaline is often associated with pegmatitites, which are much coarser grained that this dyke.

...it looks like you have a felsic dyke, broadly granitic in composition, with numerous mafic xenoblasts.

KMR from Austria (I think) says:

What you describe sounds very much like a felsic dike. Often, the very lightest elements in the Magma accumulate at the top of the Magma-Chamber, thus intruding first into the above crust , as felsic dikes. Those typically show coarse crystallisation. Granitic composition is also typical, as you described (feldspar, quarz, mica), ... I'd speculate that the black minerals which are no[t] biotites, are black turmaline or some amphibolites.

Neat stuff, huh? If you have any thoughts on this subject please feel free to join the newsgroup discussion (linked above) or send an email to my yahoo account (also listed above).

Quartz pebble found in igneous dyke within Five Springs Cave, dark spots are biotite, each square represents 1/20 of an inch - copyright D. Kouts
10X view of of the same quartz pebble from Five Springs Cave - copyright D. Kouts
10X view of biotite found in quartz pebble from Five Springs Cave - copyright D. Kouts
This piece of material from the igneous dike is light yellow in color and very light. In places it shows some foliation (not visible) but is largely composed of quartz and biotite. - copyright D. Kouts
10X view of the same sample from Five Springs Cave igneous dyke shows quartz crystalline nature of material - copyright D. Kouts

Mica or biotite sample from igneous dyke in Five Springs Cave, each square represents 1/20 of an inch - copyright D. Kouts
10X view of of the same sample from Five Springs Cave - copyright D. Kouts

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