Siegenite with Pyrite

specimen number:
2133386

location:
Magmont Mine, Iron Co., Missouri, USA

description:
Sure red lustrous twinned crystals of siegenite measuring to 0.5 cm in size on matrix with crystallized pyrite.

The Magmont mine, located near Bixby in Iron County, Missouri, was a major underground operation situated within the world-class Viburnum Trend, or New Lead Belt. Geologically, the deposit is classified as a Mississippi Valley-type (MVT) lead-zinc-copper deposit, where mineralization occurs primarily as replacements and open-space fillings within the Cambrian-aged Bonneterre Dolomite. Hydrothermal fluids migrating through the regional sedimentary strata deposited heavy concentrations of galena, sphalerite, and chalcopyrite along breccia zones, reef structures, and paleochannels. Jointly developed by Cominco American and Magnet Cove Barium Corporation (Magcobar), from which the name Magmont was derived, the mine began regular industrial production in 1968. It operated continuously as one of the cornerstone producers of the district for over two decades until the economic ore reserves were completely depleted, leading to the permanent cessation of all mining and milling operations in 1994.

Siegenite is a cobalt nickel sulfide mineral belonging to the thiospinel linnaeite group, crystallizing in the isometric system where it typically forms octahedral crystals or granular masses. Geologically, it occurs as a primary hydrothermal mineral in low-to-medium temperature stratabound deposits, frequently associating with other sulfides such as chalcopyrite, galena, and marcasite. The mineral displays a bright metallic luster with a light steel-gray to silvery color, which often takes on a yellowish or reddish tarnish over time. It was originally described in 1850 from its type locality in the Siegerland region of North Rhine-Westphalia, Germany, but it achieved notable mineralogical prominence within the Viburnum Trend of Missouri, where the local MVT solutions were uniquely enriched in cobalt and nickel, allowing for the formation of distinct, well-crystallized specimens.

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