The ore mineral that is mined at the Erzberg is called Siderite, an iron carbonate with an average iron content of 33.5%.
The genesis of this mineral can be dated back 400 million years (Devonian period). More than a half billion tons of siderite were deposited at the Erzberg. Therefore, the Erzberg is considered the biggest siderite deposit of the world. According to Geologists, 80 million tons of economically usable ore is still hiding in this ferrous mountain. Thus, mining at the Erzberg can be done for another 30 to 40 years.
The Erzberg is at the Northern fringes of the Eastern greywacke zone, a band of Paleozoic metamorphosed sedimentary rocks that forms an east-west band through the Austrian Alps.
The following is a simplified description of the constitution of the deposit. The footwall or “basement” of the Erzberg is the so-called Blasseneck porphyroid, a magmatic sedimentary rock with a greenish color. The second layer is called Sauberger Kalke. It consists mostly of limestone, but in this layer you can find the two ore bodies of the Erzberg: the upper- and lower ore body. The top layer of the Erzberg, the hangingwall, is the Werfen beds, consisting mainly of purple-colored schist. The next figure is the “Seigerschnitt”, a geological map of the Erzberg:
The ore deposit was heavily influenced by two orogenic phases. Those lead to striking deformations and faults in the existing layers. As a result, the high quality ore bodies are distributed unequally within the deposit, which makes mining very demanding. The figure below is a deposit model made by the geologists of the VA Erzberg GmbH. It shows the local distribution of the different ore grades and waste. Important sources for this model are historical maps and exploration drillings.