MINERALOGIA, QUIMICA E INCLUSIONES FLUIDAS EN LOS DEPOSITOS DE NITRATOS DE MARIA ELENA, IIa. REGION, CHILE

the deposit of maría elena in the atacama desert, is at present the only important nitrate ore deposit mined in chile. the mineralogy, chemistry and fluid inclusions of this mineralization are described in this paper. nitrate ores occur as stratiform seams below the desert surface and are hosted mainly by breccias and sandstones which include volcaniclastic and tuff facies. salt minerals cement these clasts or form bands at the mm-m scale. the ore is largely composed of nitratite and halite. nitrate contents vary between 7.5 to more than 20 wt % and nacl between 8.6 to 40 wt %. locally abundant are the sulphates thenardite, anhydrite, bloedite, polyhalite, hexahidrite and glauberite, which reflect part of 7.5 to 19.7 wt % of na2so4. other salts present in minor contents include a varity of sulphates (kieserite, woodwardite, loeweite); nitrates (humberstonite, darapskite and niter); borates (kaliborite, stroncioborite); chlorides (sylvite, nantokite) and iodates (bruggenite). the io3 vary between 0.026 to 0.074 wt %. preliminary fluid inclusions studies show that the homogenization temperature in liquid rich type inclusions in thenardite from the banded ores in these deposits vary between 216° to 239 °c. the salinity of the fluid phase varies between 4.4 to 10.6 wt % of na cl equivalent. this suggests the existence of thermal processes during salt formation. therefore evaporation may have not been the only mechanism of importance during the genesis of these salt deposits