The measurement and analysis of zircon crystals is used for the following objectives.

(A) Classification of geological formation

Sometimes it is difficult to confirm the original rock in cores and cuttings due to strong alteration. Zircon (ZrSiO4, Fig.5-1) is a general sub-component mineral in all kinds of igneous and metamorphic rocks and is one of main heavy minerals in sedimentary rocks. And due to its chemical stability it remains unchanged even after the rock is altered. On the other hand, the form and quantity of Zircon crystals differs from rock type to type. Therefore, it is possible to classify geological formations and rock types by classifying the type of Zircon crystal and by the quantity of its occurrence.
For example, utilizing this technique (Fig.5-2), West JEC found a small-scale subsidence zone (Fig.5-3) in a geothermal field under development. This interpretation integrated to the results of gravity survey resulted in the disclosure of a geothermal reservoir around the faults conforming this subsidence zone.

(B)Age determination

Moreover, Zircon is often used together with the Fission Track method (F.T.) for age determination to study thermal histories by analyzing the contraction of spontaneous track lengths. Recently, the thermo-luminescence method for age determination (T.L.) using Zircon became possible as mentioned in section 7.
In summary, the geological and thermal structures of a geothermal field can be disclosed by a comprehensive analysis including not only Zircon crystal forms but also by determining the age of the rocks using this mineral.

Click to image
Fig.5-1 Forms of zircon crystal
Fig.5-2 Example of zircon crystal morphology analysis
Fig.5-3 Example of application of the zircon crystal morphology analysis




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