MAGNESIUM-RICH CLAYS OF THE MEERSCHAUM MINES IN THE AMBOSELI BASIN, TANZANIA AND KENYA

The Sinya Beds of the Amboseli Basin in Tanzania and Kenya consist lar gely of carbonate rocks and Mg-rich clays that are intensely deformed where exposed in and near former meerschaum mines. The carbonate rocks consist of limestone and dolomite in Tanzania, but only dolomite has been identified in Kenya. Sepiolite and mixed-layered kerolite/stevens ite (Ke/St) are subordinate constituents of the carbonate rocks. The c arbonate rocks and overlying bedded sepiolite were deposited in a semi arid lake basin at the foot of the large volcano Kilimanjaro. Calcite and dolomite of the carbonate rocks have delta(18)O values 4-6 parts p er thousand lower than calcite and dolomite of the late Pleistocene Am boseli Clays, suggesting that the Sinya Beds were deposited in the mid dle or early Pleistocene under a different climatic regime when meteor ic water had lower delta(18)O values than at present. Mg-rich clay min erals form veins and fill cavities in the Sinya Beds. The principal cl ay minerals are sepiolite and Ke/St, some of which contains substantia l Al and Fe (Al-Ke/St). NEWMOD(R) modeling and other X-ray diffraction (XRD) data suggest that most of the Ke/St contains 25-50 percent kero lite layers, but minor amounts of kerolite-rich Ke/St are present in s ome samples. Illite with an inferred high content of Fe or Mg is a min or constituent of the samples with Al-Ke/St. The cavity-filling clays were chemically precipitated, as shown by field relationships and SEM study. The early-deposited clays of veins and cavities are principally Ke/St with minor sepiolite, and the latest clay is sepiolite (meersch aum), generally with minor Ke/St. The delta(18)O values of cavity-fill ing Ke/St range from 22.5-25.6 parts per thousand and correlate with m ineral composition, with the highest values associated with the highes t content of stevensite and the lowest values with the highest content of kerolite. This relation suggests that high salinities favored stev ensite and low salinities favored kerolite. delta(18)O values of sepio lite (meerschaum) fall in the middle of the range for Ke/St, suggestin g that salinity was not the main control on sepiolite precipitation. H igh values of a(SiO2)/a(Mg2+) may have been a major factor in sepiolit e precipitation. Different mixtures of dilute ground water and saline, alkaline lake water in pore fluids may largely account for the differ ences in clay mineralogy of cavity-filling clays. Sepiolite is the dom inant clay mineral in lacustrine sediments of the Amboseli Basin, and the cavity-filling sepiolite may reflect a high proportion of lake wat er. The low-Al Ke/St may have formed from fluids with a higher proport ion of ground water. Detrital clay was very likely a factor in forming the Al-Ke/St, for which delta(18)O values suggest a saline environmen t.