Though we aren’t searching for gold, some of here in the Geo department do “rush” to California, but in search of a different mineral. Zircon is the keystone to much of the petrographic and volcanological research going on this summer. As Liz wrote earlier, there are different ways to get from a chunk of outcrop to the tiny zircon crystals that we can date. Both of us have used the “heavy liquids” in the lab for a density separation, but there is also another interesting method. That’s where gold comes in.
One method we have been experimenting with to separate zircon uses a “gold table”, the same thing that bearded prospectors use to get placer gold out of fine sediment. It works on the same principles as heavy liquids, but without the effects of those “nasty” chemicals. The crushed and sieved rock gets put into the top of a channel with water flowing through it.
The particles get washed down over a set of grooves as the table shakes and vibrates. As the material moves down the table, the vibrations and flow cause the denser crystals (like zircon!) to sink and get trapped in the shallow grooves while the finer and lighter stuff continues to wash away. This leaves a concentration of heavier, albeit tiny, crystals that we can then sort out under a microscope, just like we would with the heavy liquids. The whole table is essentially a large-scale, modern version of the ubiquitous gold pan. With a table, gold miners would simply let the gold concentrate flow off the table into a designated bucket, but to be more precise, we arm ourselves with a blacklight and pipette to selectively extract the luminescent zircon. When seen under UV light, zircon glow (appropriately) gold, making them easier to spot on the table.
Finely tuning the table is still a work in progress, but we were still able to extract a separate chock full of zircon quickly and without toxic (and expensive) chemicals. But with every benefit, there are drawbacks. A gold table separation requires much more sample due to a lack of comparable efficiency to chemicals, and also carries a slightly higher risk of contamination with zircon from other sample. If one is careful and cleans thoroughly, however, a much greater amount of sample can be processed in a shorter amount of time, with less waiting. Such qualities are ideal for samples to be run on a Laser Ablation Inductively Coupled Plasma-Mass Spectrometer, which Erik and I will be doing this week.
A lot of work, from crushing and grinding, to sieving and magnetically separating, to extracting and mounting, goes into the samples, but the data acquired in the end will surely be worth the hard work. Stay tuned for updates!