Borate minerals of the penobsquis and millstream deposits, Southern New Brunswick, Canada

The borate minerals found in two potash deposits, at Penobsquis and Millstream, Kings County, New Brunswick, are described in detail. These deposits are located in the Moncton Subbasin, which forms the eastern portion of the extensive Maritimes Basin. These marine evaporites consist of an early carbonate unit, followed by a sulfate, and finally, a salt unit. The borate assemblages occur in specific beds of halite and sylvite that were the last units to form in the evaporite sequence. Species identified from drill-core sections include: boracite, brianroulstonite, chambersite, colemanite, congolite, danburite, hilgardite, howlite, hydroboracite, kurgantaite, penobsquisite, pringleite, ruitenbergite, strontioginorite, szaibelyite, trembathite, veatchite, volkovskite and walkerite. In addition, 41 non-borate species have been identified, including magnesite, monohydrocalcite, sellaite, kieserite and fluorite. The borate assemblages in the two deposits differ, and in each deposit, they vary stratigraphically. At Millstream, boracite is the most common borate in the sylvite + carnallite beds, with hilgardite in the lower halite strata. At Penobsquis, there is an upper unit of hilgardite + volkovskite + trembathite in halite and a lower unit of hydroboracite + volkovskite + trembathite-congolite in halite-sylvite. At both deposits, values of the ratio of B isotopes [delta(11)B] range from 21.5 to 37.8 parts per thousand [21 analyses] and are consistent with a seawater source, without any need for a more exotic interpretation. The assemblage at Penobsquis is consistent with a fore-basin deposition by evaporation, whereas that at Millstream, a more inland and restricted basin, is indicative of a late-stage periodic influx of concentrated brines from the fore-basin. The evaporites are intensely folded, but the degree of metamorphism remains unknown.