GEOCHEMISTRY OF THE SALAR-DE-ATACAMA .1. ORIGIN OF THE COMPONENTS ANDSALT BALANCE

The Salar de Atacama in northern Chile is a sedimentary basin containi ng a 900 m thick salt crust (nucleus), about 1.100 km(2) in area, surr ounded by a 2.000 km(2) fringe of saline muds. The salt crust is fille d with a sodium chloride interstitial brine rich in Mg, K, Li, B. The main inflows to the salar drain volcanic formations of the Andean High lands at the east side of the basin. The salts dissolved in inflow wat ers have a double origin. The weathering of volcanic rocks supplies K, Li, Mg, B and, to a lesser extent, Na and Ca, The leaching of ancient evaporites beneath the volcanic formations provides additional amount s of Na, Ca, CI, SO, in the most saline inflow waters. The mass-balanc e of the upper nucleus shows a strong excess of NaCl with respect to t he bittern solutes Mg, K, Li, B, which suggests that the nucleus did n ot originate from inflow waters similar to the present ones. The exces s of NaCl is likely to be due to NaCl-rich inflow waters that formerly drained the Cordillera de la Sal, a Tertiary evaporitic ridge at the western rim of the salar, The average sedimentation rate of halite has been estimated at 0.1 mm/year from the date of an ignimbrite interbed ded in the nucleus. The same rate is obtained from the present inflow waters, which suggests that the halite of the nucleus was deposited fr om ancient inflows similar to the present ones. This is in contradicti on with the mass-balance which indicates that the former inflows were much more concentrated in NaCl. The discrepancy may be solved assuming an intermittent activity of the salar, Long dry periods of inactivity were alternating with short wet periods during which large amounts of salt were deposited. The lack of lacustrine deposits and the high pur ity of the salt suggest that the nucleus is not the remnant of an anci ent deep saline lake, but originated mostly from sub-surface and subte rraneous saline inflows.