Orogenic treasures: Ziller Valley garnets and their transformation from petrogenetic indicator to gemstone

Garnet has been utilized for various applications for centuries. It is an important phase for interpreting geological and mineralogical processes due to its wide range of stability and occurrence in various geochemical environments. In the late 18^th century, the Zemmgrund in the innermost parts of the Zillertal became an important source of raw material for garnet jewelry. The garnets are found within ductile shear zones in the “Zentralgneisse” of the Venediger nappe system, which is part of the Penninic Tauern Window. There are several comparable shear zones in the innermost Zillertal, with the most relevant for industrial purposes being at the Rossrugg ridge (Zemmgrund). The shear zone rocks can be described as garnet-bearing chlorite mica schists from a petrographic perspective. To better comprehend the deposit's genesis, geochemical composition analyses were conducted on Rossrugg samples using µ-XRF, EPMA, and LA-ICP-MS. The garnet samples exhibit continuous zoning, with the almandine component having the highest proportions of 60 mol% (core) and 73 mol% (rim). The distribution of trace elements, such as Co, Zn, or Zr, correlates with this pattern. In contrast, Ti, HREEs and Y shows a decreasing concentration towards the core. The calculations for garnet formation conditions were performed using Thermocalc v3.45 and TC_Comb, resulting in T=612±34°C and P=7.5±1.4 kbar. The PerpleX and Theriak-Domino software was used to calculate garnet isopleths, which showed that garnet growth occurred at decreasing pressure and increasing temperatures.

This mineral has been utilized by mankind for thousands of years due to its abundance in a wide variety of rocks, especially as a gemstone. Almandine garnets from the Zemmgrund (Ziller Valley, Tyrol) and Radenthein (Carinthia) have played a significant role in jewelry production in the Alpine region since the end of the 18^th century. Until the early 20^th century, a small industry was established in the Zillertal, which supplied raw materials to Bohemia. Due to the export to Bohemia, the 'Tyrolean' garnets were mixed with the pyropes and lost their identity to some extent. However, there are various methods available to determine the origin of the raw materials used. Besides size and color, another criterion for differentiation is the inclusion pattern of the garnets. For example, Zillertal garnets typically contain chlorite, zircon, apatite, quartz, ilmenite, and epidote as inclusions. In contrast, the garnets from Radenthein exhibit oriented inclusion growth of ilmenite and rutile. However, to clearly differentiate between various garnets from the alpine deposits in a piece of jewellery, destruction-free chemical analyses using suitable methods (e.g. µ-XRF) are necessary. By using PCA and comparing specific oxides (e.g. CaO-MgO), individual deposits can be effectively distinguished. However, local differences, due to different small deposits of the Zemmgrund, presents a challenge as the chemical differences are subtle because of similar geological conditions and little differences in local bulk compositions. Nonetheless, by applying PCA to the collected data, it can be subdivided into five groups, including samples from individual deposits and a warehouse in Zell am Ziller.