Mississippi Valley-type (MVT) fluorite-barite-sphalerite depou are are important Zn and Pb reserves and resources in the world. Silver, barite, gypsum, and fluorite may also be economically recovered from these deposits. Fluorites, by-products from mid-United States MVT districts are one of classic mineral specimens for collectors.
Fluorites are hosted in the upper Knox Group (Lower Ordovician) carbonate sequenc which is the source of Ca. However, source of F is still on debating. Fluorine has two main sources: sedimentary environments (such as black shales); And igneous environments (volatile-rich alkaline rocks). Some research on other MVT deposits suggested that F came from the local sedimentary rocks rather than igneous Or mantle derived, evidence by Sm-Nd isotopic and fluid inclusion studies. Although granite on the basement may or may not contribute significant amount of F, as the heat source that allowed fluid to dissolve and penetrate wall-rock and precipitation of ore-forming and gangue minerals.
The famous Illinois style is blue, purple and yellow zoned cubic fluorite with or without associations. However, some of the highly lustrous fluorites (many of others were etched) are even harder to get.
Fluorite, 4cm, Minerva No. 1 Mine, Ozark-Mahoning group, Cave-In-Rock Mining Sub-District, Hardin County, Illinois, USA
Jingnan (Tom) Zhang Coll & Photo
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