2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 11
Presentation Time: 8:00 AM-12:00 PM


SWIHART, George H., Department of Earth Sciences, University of Memphis, 1 Johnson Hall, Memphis, TN 38152, CARPENTER, Steven B., Industrial Minerals, Rio Tinto Exploration, 26877 Tourney Road, Valencia, CA 91355-1847 and XIAO, Ying Kai, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008, China, gswihart@memphis.edu

About a third of the approximately 30 known borate deposits in the Furnace Creek district exhibit mineralogical zoning of borates. They feature a Na-Ca borate (ulexite and/or probertite) core (or cores) 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 post-depositional alteration.

Samples in three drill cores from the zoned Cemetery Deposit were analyzed for boron isotopic composition. One drill core from the thickest central portion of the deposit contains lower and upper colemanite intervals and a probertite core interval. The other two drill cores were selected from marginal areas of the deposit where the borate interval consists only of colemanite. Samples of marginal colemanite were chosen from depth intervals which should be time equivalent to some portion of the probertite core of the deposit. The δ11B results for colemanite samples from all three drill cores, including lower, upper, and marginal colemanite, comprise a fairly narrow range near 0 permil. The probertite samples from the deposit core region yield isotopically heavier values near 5 permil. Probertite from a mixed interval between the core and upper colemanite zones produces a similar result to the probertite samples from the core interior.

The isotopic results suggest that the lower, upper, and marginal Ca borate portions of the Cemetery Deposit all formed by the same process. The data support the formation of mineralogical zoning in the Cemetery Deposit and other zoned borate deposits of the Furnace Creek Formation by post-depositional alteration of a primary borate, presumably a Na-Ca borate. Samples from all of the analyzed colemanite-only deposits are isotopically light and many are similar in value to colemanite in the Cemetery Deposit suggesting that some of these other deposits may have formed by complete alteration from primary Na-Ca borate.