Boron Mineralization in Louann Salt and Norphlet Shale, Clarke County, Alabama

W. B. Simmons

A suite of unusual boron minerals is present in the upper Louann Salt and immediately overlying Norphlet Shale in Clarke County, Alabama. Core samples come from a depth of about 12,000 ft in a well located on the flank of a nonpiercement salt dome. The suite consists of calcium and magnesium borates similar to those occurring in the Zechstein salt deposits of Germany. Well-developed micron-size to millimeter-size crystals were recovered from water-insoluble residue from the salt.

The minerals identified include boracite (modified pseudoisometric cubes), hilgardite (prismatic crystal aggregates), szaibelyite (acicular crystal aggregates), and volkovskite (plates, rare prisms). Associated minerals are anhydrite, gypsum, magnesite, phlogopite, talc, and quartz. Boracite and hilgardite have boron isotopic compositions indicative of marine evaporite deposits.

Danburite occurs in irregular nodules up to 2 cm in diameter in the overlying Norphlet Shale. The nodules constitute up to 30% of the Norphlet immediately adjacent to the Louann but disappear within about 1 m from the contact. The danburite appears to be the result of boron-rich fluids derived from the underlying marine evaporite sequence, infiltrating and reacting with the shale.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.