MG-BEARING AND K-BEARING BORATES AND ASSOCIATED EVAPORITES AT EAGLE BORAX SPRING, DEATH-VALLEY, CALIFORNIA - A SPECTROSCOPIC EXPLORATION

Efflorescent crusts at the Eagle Borax spring in Death Valley, Califor nia, contain an array of rare Mg and K borate minerals, several of whi ch are only known from one or two other localities. The Mg- and/or K-b earing borates include aristarainite, hydroboracite, kaliborite, mcall isterite, pinnoite, rivadavite, and santite. Ulexite and probertite al so occur in the area, although their distribution is different, from t hat of the Mg and K borates. Other evaporite minerals in the spring vi cinity include halite, thenardite, eugsterite, gypsum-anhydrite hexahy drite, and bloedite. Whereas the first five of these minerals are foun d throughout Death Valley, the last two Mg sulfates are more restricte d in occurrence and are indicative of Mg-enriched ground water. Minera l associations observed at the Eagle Borax spring, and at many other b orate deposits worldwide. can be explained by the chemical fractionati on of borate-precipitating waters during the course of evaporative con centration The Mg sulfate and Mg borate minerals in the Eagle Borax ef florescent crusts point to the fractionation of Ca by the operation of a chemical divide involving Ca carbonate and Na-Ca borate precipitati on in the subsurface sediments. At many other borate mining localities , the occurrence of ulexite in both Na borate (borax-kernite) and Ca b orate (ulexite-colemanite) deposits similarly reflects ulexite's copre cipitation with Ca carbonate at an early concentration stage. Such ule xite may perhaps be converted to colemanite by later reaction with the coexisting Ca carbonate -the latter providing the additional Ca2+ ion s needed for the conversion, Mg and Ca-Mg berates are the expected lat e-stage concentration products of waters forming ulexite-colemanite de posits and are therefore mist likely to occur in the marginal zones or nearby mud facies of ulexite-colemanite orebodies. Under some circums tances, Mg and Ca-Mg borates might provide a useful prospecting guide for ulexite-colemanite deposits, although the high solubility of Mg bo rate minerals may prevent their formation in lacustrine settings and c ertainly inhibits their geologic preservation. The occurrence a of Mg borates in borax-kernite deposits is also related to fractionation pro cesses and points to the operation of an Mg borate chemical divide, ch aracterized by Mg borate precipitation ahead of Mg carbonate. All of t hese considerations imply that Mg is a significant chemical component of many borate-depositing ground waters, even though Mg borate mineral s may not be strongly evident in borate orebodies. The Eagle Borax spr ing borates and other evaporite minerals were studied using spectrosco pic and X-ray powder diffraction methods, which were found to be highl y complementary. Spectral reflectance measurements provide a sensitive means for detecting borates present in mixtures with other evaporites and can be used to screen samples rapidly for X-ray diffraction analy sis. The apparently limited occurrence of Mg and K borate minerals com pared to Ca and Na borates may stem partly from the inefficiency of X- ray diffraction methods for delineating the mineralogy of large and co mplex deposits. Spectral reflectance measurements can be made in the l aboratory, in the field, on the mine face, and even remotely. Reflecta nce data should have an important role in studies of existing deposit mineralogy and related chemical fractionation processes,and perhaps in the discovery of new borate mineral resources.