In early February, I embarked on a second trip to the zircon deposits of Cambodia, this time with a colleague from Museum Victoria in Melbourne, Australia. Our goal this year was to visit a little-known locality 80 km north of Tbeang Meanchey, in Preah Vihear province, and then return to Ban Lung in Ratanakiri province. We would collect zircons and accessory minerals found with them for research, and buy cut gemstones for our respective national mineral collections.
Tram Poung, our first stop, is a new locality for me. It is located in the far northern province of Preah Vihear, 50 km from the Thai-Cambodian border and the hotly-disputed Prasat Preah Vihear, a Hindu temple dedicated to Shiva. This temple was built during the Khmer empire, along with Angkor Wat, between the 9th and 11th centuries.
Mining in this region happens only during the wet season, when there is enough water to be able to spray the weakly-consolidated outcrops with high-pressure water from a hose. This dislodges the precious zircon crystals.
Where the zircons in this region originated is up for debate—in Ban Lung, it is obvious that the crystals, which started life deep in the Earth’s mantle, were brought up by alkaline basalts which covered the area about one million years ago. Like diamonds in kimberlite, the zircons are not formed in the basalt, but simply hitch a ride, getting trapped in the basalt as it makes its way to the surface.
In Tram Poung; however, there are no basalts in the area—the nearest are located 100 km to the south, or possibly across the border in Thailand or neighbouring Laos. The zircons are found in gravels which are weakly consolidated (turned into solid rock) along with a host of other minerals and rock fragments.
So, where did they originally come from? This is one of the questions I will try to answer in the coming months after I return to my lab at the museum’s Natural Heritage Campus. The second question is whether or not these Tram Poung zircons came from the same part of the mantle than those in Ratanakiri province. Are they the same age? Same chemistry? Understanding the chemical characteristics of these zircons can tell us a great deal about the mantle deep below the surface.
Unlike Ratanakiri zircons, which are heat-treated to turn them a brilliant blue colour much sought after by both collectors and gemmologists, Tram Poung zircons are left in their natural state—colours range from orange-red to red-brown to orange-brown and all shades in between. As a mineralogist, I am a bit of a purist when it comes to cut gemstones—I would prefer that no mineral is treated to enhance its colour —so these stones appeal to me.
The gem dealers in town tell me that when they try to heat treat these zircons, they turn colourless or slightly yellow, not blue. One dealer, Mr. Poung, demonstrated this by placing a brown-red cut zircon into the flame of a gas burner for one minute—after removing it, the stone was completely colourless and transparent …no colour left at all!
When Ratanakiri zircons turn blue, it is because the small concentration of uranium within the atomic structure undergoes a change in charge. What causes a brown-red Tram Poung zircon to turn colourless in a flame? That’s another question to be answered. The great thing about scientific research is that there are always more questions than answers.
We left Tram Poung having bartered for a selection of rough material for research, and a couple of cut stones for our gem collection. All the stones are cut and polished by hand, by the local gem dealers and their assistants. It is rare to be able to buy a stone for which you know the exact locality from which it was mined, the miner who collected the rough zircon, and the cutter who faceted the final gem stone. In this case, it is all the same person. Having this information adds value to the stone when it is catalogued and placed into the collection.
After our wheeling and dealing to acquire rough and cut zircons, we got back in our van and prepared for the long drive back—through Tbeang Meanchey and then 400 km east, across the Mekhong River to Ban Lung in Ratanakiri province where more zircon adventures awaited!