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A Boron Isotopic Study of a Mineralogically Zoned Lacustrine Borate Deposit: the Kramer Deposit, California, U.S.A

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A Boron Isotopic Study of a Mineralogically Zoned Lacustrine Borate Deposit: the Kramer Deposit, California, U.S.A ISOTOPE GEOSCIENCE ELSEVIER Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250 A boron isotopic study of a mineralogically zoned lacustrine borate deposit: the Kramer deposit, California, U.S.A. George H. Swihart a**, Eddy H. McBay b, David H. Smith b, Joseph W. Siefke c aDepartment ofGeological Sciences, University of Memphis, Memphis, TN 381.5’2. USA b Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ’ U.S. Borax, Inc., Boron. CA 93516, USA Received 27 July 1994; accepted 9 August 1995 after revision Abstract An investigation of the boron isotopic composition of hydrated borates in the Kramer lacustrine deposit of southern California was undertaken in an effort to better understand the origins of mineralogically zoned deposits of this type. Twenty-one samples from fifteen depths along a drill core through the deposit reveal that isotopic zoning accompanies mineral zoning. The range of 8’ 'B for borax through the 24.9-m-thick Na-borate core facies is + 0.1 to + 1.7%0 except for samples in and just below a clay-rich 6-m interval where 6”B varies from - 5.1 to + 2.3%0. The S”B of three cottonball ulexite samples in a 3.0- m-thick Na-Ca-borate facies above the Na-borate facies ranges from - 5.5 to - 4.6%0, whereas two samples from a 6. l-m-thick basal Na-Ca-borate facies both yield - 2.1%0. The S”B of colemanite from near the top of the upper Na-Ca-borate facies is - 8.6%0. The small range of S”B through much of the Na-borate facies indicates that the source waters of the borax-precipitating lake varied little in 6”B for a time interval of (5-7) - lo4 yr. The G”B variations within and among borax crystals in the 6-m clay- rich interval, some of which are estimated to have occurred over a period of weeks, were probably produced by a combination of pH change and Rayleigh effect during partial desiccation cycles. The distinctly different ranges of 6”B exhibited in the upper and lower Na-Ca-borate facies and the distinctive cottonball habit of the crystal aggregates indicate that ulexite originated through growth in the lake-margin muds. 1. Introduction “marsh” and playa settings (Bowser and Dickson, 1966; Xiao et al., 1992). These deposits range from Nonmarine hydrated borate deposits are the major mineralogically unzoned or poorly zoned to concentri- ores of boron. Hydrated borates are found in aprons cally or complexly zoned types. Many deposits are deposited recently around hot spring and gas vents in dominated by Na- and/or Ca-borates, but some contain South America (Muessig, 1966)) in Tertiary bedded significant proportions of Mg- and/or Mg-Ca-borates. deposits of uncertain origin (W.C. Smith, 1960; Hel- The origins of mineralogically zoned deposits are vaci and Firman, 1976)) in Tertiary to recent lacustrine unclear. The Miocene-age Kramer lacustrine deposit at deposits (Bowser and Dickson, 1966; Inan et al., 1973; Boron, California, is one of the most well-studied non- G.I. Smith, 1979), and are currently forming in marine borate deposits. It has a Na-borate core, a Na- * Corresponding author. Ca-borate intermediate zone and a discontinuous [PDI Ca-borate outer zone. Several hypotheses have been 0009.2541/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDIOOO9-2541(95)00094-l 242 G.H. Swihclrt et al. /Chemical Geology (Isotope Geoscience Section) I27 (1996) 241-250 presented for the origin of the zoning in the Kramer Depth (m) 117 deposit based on structural and petrological studies. Oi et al. ( 1989) reported a correlation between the 6”B of hydrated borate minerals from a given deposit and the ratio of 13tB to 14’B in each crystal structure. They suggested that the observed mineral-mineral dif- Mlddle Ore borax 8 claystone ferences in 6”B can be estimated from the partition function ratios of the aqueous boron species, the pro- portions of these species in solution (which are pH Lower Ore borax & claystone dependent) and the i3’B/t41B ratios of the minerals. I I The basis for this proposed mechanism is spectroscopic study which shows that in dilute aqueous solution the boron isotopes fractionate between the two major spe- cies, B (OH) 3 and B (OH), , as a function of temper- Fig. I. General stratigraphy in drill hole MD-636 ature (Kakihana et al., 1977). Given that very few data have been available and the The boron extraction and isotope analysis procedu- actual original spatial associations of the analyzed sam- res are described in Swihart (1987), Leeman et al. ples are not known, the discovery by Oi et al. (1989) ( 199 1) , Nakamura et al. ( 1992)) and other references of a systematic 6”B distribution suggests that the 6”B cited below. Briefly, the procedures used were as fol- of each mineral must be relatively constant throughout lows. Samples were characterized on the basis of crystal a deposit. As they noted, this point remains to be inves- form and habit and, when necessary, by X-ray powder tigated. If so, does this distribution result by primary diffraction. Portions of unzoned crystals were extracted precipitation from a solution with a relatively constant with a Foredom@ drill, whereas portions of macroscop- 6”B or from postdepositional alteration processes such ically zoned crystals were sampled with a very fine- as deposit-wide equilibration or modification by out- tipped teasing needle. side solutions? These problems and the implications A portion of each sample sufficient to contain - 1 for the origins of mineralogically zoned hydrated mg of boron was weighed. Borax was dissolved in borate deposits are addressed in the present study with water. Ulexite and colemanite were fused with Na,C03 spatially constrained (drill core) samples from the Kra- and then dissolved in - 15 ml of low-boron distilled mer deposit. and deionized (D&D) water. Removal of Na, Ca and trace constituents from the ulexite and colemanite solu- tions was accomplished using batch exchange followed by column cleanup. The borax sample solutions only 2. Experimental procedures required the column exchange step. For each batch exchange, - 15 ml of fresh 100-200 mesh styrene divi- The borate-bearing portion of the Kramer deposit is nylbenzene cation exchange resin were conditioned in up to - 120 m thick and has a lateral extent of about an exchange column with two bed volumes of 1 N HCl 1.6 by 6 km (W.C. Smith, 1968; Siefke, 1991). In the followed by ten bed volumes of D&D water at a rate area of the drill core utilized in the present study (MD- of l-2 ml min-‘. The cleanup column containing - 5 636) the deposit consists of ulexite-bearing layers ml of the same type of resin was regenerated and con- (Na-Ca-borate) at the top and bottom, and A zone, ditioned after each sample solution with four bed vol- Middle Ore, B zone and Lower Ore borax (Na-borate) umes of 1 N HCl followed by ten bed volumes of D&D horizons in between the ulexite intervals (Fig. 1). water at a rate of l-2 ml min- ‘. Some dispersed colemanite crystals (Ca-borate) occur After the addition of an equimolar amount of man- near the top part of the upper ulexite interval. In the nitol to prevent boron loss during evaporation (Ishi- present study, most samples were chosen at intervals kawa and Nakamura, 1990), the boron-bearing of - 3 m. Samples were sealed in plastic storage bags solution was evaporated at - 50°C on a hot plate in an to impede dehydration of borax. enclosed box equipped with KOH-saturated quartz G.H. Swihart et al. /Chemical Geology (Isotope Geoscience Section) 127 (1996) 241-250 243 Table I NIST 95 1 boric acid standard 6’lB = [ 1(“B~ ‘“B)samp~e~(“B~‘oB)standard) Analysis “BI’OB Internal precision, -l][lOOO] (1) U,,-l I 4.05697 0.00043 2 4.0.5443 0.0007 1 3 4.05606 0.00067 3. Results 4 4.0568 1 0.00049 5 4.05510 0.00048 6 4.05603 0.00203 Borax samples (12= 14) from nine depths in the Na- 7 4.05628 0.00098 borate facies of the Kramer deposit (core MD-636) were analyzed (Fig. 2; Table 2). The 6”B of samples Average 4.05595 UP-1 0.00090 (n = 7) from six depths in the A, Middle Ore and Lower Ore zones ranges from + 0.1 to + 1.7%0. Samples fiber filters (Fogg and Duce, 1985; Spivack and (n = 7) from two depths in the B zone and one in the Edmond, 1986). When just dry the residue was taken upper part of the Lower Ore zone yield a G”B range of up in 1 ml of D&D water, yielding a 1 mg ml- ’ solution - 5.1 to + 2.3%0. of boron. All procedures were carried out using low- In the Middle and Lower Ore zones, where the vol- boron laboratory equipment (polyethylene beakers, ume percent borax is high (water-soluble B,03 ranges columns, etc.; platinum crucible; Teflon@ evaporation from 25.5% to 29.4%)) the range of 6”B (excluding dishes) and the final solutions were stored in capped the sample in the Lower Ore which is near the contact 1.5ml polyethylene microcentrifuge tubes sealed with with the B zone) is + 0.8 to + 1.5%0 (5 samples from paraffin. 4 depths). In the A and B zones, where the volume The prepared solutions were analyzed by dicesium percents of clay are moderate and high (water-soluble metaborate (Cs,BO:, m/z 308 and 309) thermal ion- B,O, = 16.6% and 6.5%, respectively), the ranges of 6”B are + 0.1 to + 1.7%0 (2 samples from 2 depths) ization mass spectrometry at Oak Ridge National Lab- oratory using a VG”’ Isotopes Isomass 354 instrument.
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