Friday, January 20, 2012

Bob Conkwright Reflects on his Giant Maryland Corundum

Our recent Jan. 6 topic, "An Humongous Maryland Corundum," gleaned more hits than any previous Mineral Bliss post has ever received. How did this specimen get to where Bob Conkwright, Jr. found it back in 1960 amidst the dumps of one of the Devries Quarries, which even then were long defunct? Regardless of whether the piece was actually quarried there, which is open to question, we can be quite certain at least that it formed close by, somewhere near this Henryton Tunnel less than a mile north of Marriottsville.

When we spoke late last December in Bob's office at the Maryland Geological Survey, he referred to this find as "my heritage," and needless to say it had much to do with how and why he went on to make geology his career. Surely Bob has given plenty of consideration to where this amazing specimen came from and how. Here are his thoughts:

There are many references available on corundum digenesis. Looking at page 90, “Geochemical Methods of Prospecting for Non-metallic Minerals”, (Komov, I.L., 1994)*, the author indicates six recognized primary corundum deposits. Corundum is commonly associated with mafic and ultra-mafic rocks (silica-poor), pegmatites (which are typically not silica-poor), and marble (which can be relatively silica-poor). If you look through the various scenarios for corundum formation, a couple of items stand out. Bedrock in the Patapsco Valley has pegmatite and marble, and in places, pegmatites intruding marble. According to the 1928 Carroll County geologic map, the easternmost Henryton pegmatite dike is in contact with Cockeysville marble. The pegmatite/marble associations in the valley are known to include albite, diopside, phlogopite and pyrite, all minerals listed as associated with corundum. Given the size of the crystals, the corundum mass is almost certainly pegmatitic in origin, and the Cockeysville marble could have supplied the necessary silica-poor environment for the reaction to occur. Given the large size of albite and diopside crystals known to exist in these formations, I think it is not unlikely the pegmatites that injected into the marble are a likely source.

I have not found any mapped occurrences in this part of the State where pegmatites intrude silica-poor ultramafics or serpentinites. This has occurred around the Pennsylvania border, and corundum is known to occur in those formations. Red corundum crystals were reported in pegmatite-associated serpentine, which also produced talc, actinolite, chlorite and other silica-poor minerals **. There are soapstone quarries in the Patapsco area, so perhaps there are pegmatite-injected serpentinites that have not yet been identified.

Since the large crystal size would indicate a pegmatite source, but no known pegmatite dikes are injected into these local, silica-poor rocks, I suggest the specimen came from the local marble. This leaves the mystery of the reddish color, known to be derived from trace chromium. I have an idea, which might be a bit lame, but it is the only one I have for now. According to the 1993 Geologic Map of Howard County, John Edwards interprets the Patapsco Valley pegmatites as Silurian in age. The mafic and ultramafic complexes in the region are interpreted by Edwards as older than the pegmatitic injection. Could it be that the injected magma picked up some chromium ions during the intrusive event into the older marbles? Perhaps.

I am certain the corundum mass was loose in dump material, or possibly just in the soil/saprolite layer. Given the hardness of corundum and the rapidity with which calcite and feldspar weather, it is not unreasonable to assume the crystals, if not derived from quarried material, weathered out of the bedrock. There are some parts of the crystal mass which look to me like they have been weathered and polished, which would take a long time to do to corundum. This makes me think the specimen is eroded rather than quarried. With that in mind, maybe there are more!

The more I think about it the more intrigued I am with the possibility that some of the Howard county soapstone bodies may be serpentinite hydrothermally altered by pegmatite injection, similar to those in the MD/PA line area. Although pegmatites have not been mapped near most of these bodies, it is entirely possible they were not observed. For instance, the Henryton pegmatites are clearly seen on the 1928 Carroll County geologic map, but are absent on the 1993 Howard County map, right across the river. Did those dikes really do a dead-stop at the river’s edge? I don’t think so.

Contributed by Bob Conkwright

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