Solid insoluble bitumen in the Magmont West orebody, Southeast Missouri

Mo. Dep. Nat. Resour., Rolla, MO, United States
Univ. Mo., United States
U. S. Geol. Surv., United States

Solid bitumen (here used to refer to an altered, insoluble crude oil) occurs as black blebs and fracture fillings in the Magmont West orebody of the Viburnum Trend. Today, the bitumen is hard, brittle, and insoluble, but streamers show that in the past it was soft and viscous. In open cavities many of the blebs line up preferentially along organic-rich layers in the dolostone suggesting (though not proving) that the bitumen was "sweated" out of the organic-rich layers. This preferential alignment, plus the presence of sulfide chimneys, indicates that the bitumen is probably derived from an oil that was immobilized in the act of primary migration from the Cambrian Bonneterre Formation.The bitumen that we observed was most commonly found on the surface of galena and sphalerite crystals. Some bitumen was encased in calcite and dickite which are both generally considered to have formed after lead-zinc mineralization. Thus, paragenetically the bitumen is late and may have even been generated after mineralization.Elemental analyses show the blebs to be predominantly hydrocarbons with an H/C atomic ratio of around 1.4 and an NSO content of 2.6 percent. Pyrolysis gas chromatography of the blebs yields normal alkanes with up to 26 carbon atoms and no odd or even carbon preference. Trace element composition of the bitumen blebs shows enrichment in Zn, Pb, and Cu. The blebs were probably derived from a crude oil that was generated near the end of mineralization and later condensed into an insoluble macromolecular material.

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