About a third of the approximately 30 known deposits in the Furnace Creek, California, borate district reveal mineralogical zoning of borates. The deposits feature an Na-Ca borate (ulexite and/or probertite) inner zone (or zones) surrounded by Ca borate (colemanite). The remaining deposits consist essentially of colemanite. Investigators have debated the origin of the mineralogical zoning and the relationship, if any, between the zoned and colemanite-only deposits. The Ca and Na-Ca borate zoning has been characterized as either primary or formed by postdepositional alteration.
Boron isotope analysis was applied to drill core samples from two zoned deposits, one non-zoned deposit, and selected samples from six other deposits accessed from surface and subsurface mine workings. Probertite samples from the inner regions of two zoned deposits yield δ11B values near 5‰, whereas most colemanite samples from both zoned and non-zoned deposits yield results near −1‰. The small δ11B range of most of the colemanite samples and their distribution from a grouping near 0‰ to isotopically lighter values suggest a pattern formed by alteration from a common initial value.
Based on the new isotopic data and existing sedimentologic and stratigraphic knowledge of the Furnace Creek borate deposits, it is proposed that the isotopically heavier boron of probertite in the inner zone of zoned deposits reflects the composition of the ancestral lake and its springs during an early stage of deposition of the laminated series of the Furnace Creek Formation. Furthermore, the isotopically lighter B of colemanite in zoned deposits reflects the alteration of earlier formed Na-Ca borate deposits by downward migrating lake water at a later stage of deposition of the laminated series. Some non-zoned Ca borate deposits may have been precipitated directly during the later stages.