The deposit located above the village of Chamoson is an oolitic iron ore deposit formed from oxfordian-callovian (Dogger) sediments. The ore consists mainly of chamosite, magnetite, limosite and carbonates and forms a lens of about 250 m width and a few meter heights. The iron content of the ore was in the range of 32-39%. The mine operation can be traced back to 1814 and lasted until 1857. It was reactivated in 1942-44 without much underground work.
The outcrop is the type locality for chamosite, already discovered there in 1820.
Polished slab of the ore. Width about 14 cm
Enlarged part from the upper picture (4800dpi scan). This sample contains a substantial amount of euhedral magnetite and belongs to the richer part of the ore body. The magnetite content determined by image classification is about 10-12 Vol% (see below).
The polished section with Nic +, Obj. 20x, air, 100 µm bar. Oolite of mainly chamosite, surrounded by magnetite crystals and carbonates. This foto was made by HDR tone mapping of six single exposures.
Magnetite crystals. Nic //, Obj. 20x, air, 100 µm bar
Biogenic pyrite, forming ball and cloud structures (Nic //, Obj. 20x, air, 100 µm bar):
Same as above, enlarged view:
Determination of the magnetite content: 10 random images were taken with 10x Obj, Nic // at 420 nm and subjected to unsupervised image classification using geospatial software. Due to the high difference between magnetite and the silicate and carbonate reflectance, three classes were sufficient to classify the magnetite crystals. See below a sample image and the overlay of the magnetite class in blue. The magnetite class was present between 10.1 and 12.3 %.
Enlarged parts of an other sample shows the accuracy of the classification concerning fragments and hollow crystals:
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For further informations on the region this publications might be interesting:
Déverin, L.: Die Eisen und Manganerze der Schweiz. 2. Band: Etude pétrographique des minerais de fer oolithique du Dogger des Alpes suisses. Kümmerly & Frey, 1945
Rickenbach, L:, Fehlmann, E.: Die Eisen und Manganerze der Schweiz. 7. Band: Die eisenhaltigen Doggererze der Schweiz. Kümmerly & Frey, 1962