SULFUR ISOTOPE GEOCHEMISTRY OF THE ORES AND COUNTRY ROCKS AT THE ALMADEN MERCURY DEPOSIT, CIUDAD REAL, SPAIN

Seventy-four new S isotope analyses of ore minerals and country rocks are given for the Hg deposit of Almaden. The spread of the cinnabar deltaS-34 is narrow within each of the three orebodies, but the deltaS-34 average values differ sufficiently between them (mean deltaS-34: San Nicolas = 0.2 +/- 1.1 parts per thousand, San Francisco = 8.1 +/- 0.7 parts per thousand, San Pedro = 5.9 +/- 1.0 parts per thousand) to indicate three different mineralization episodes and possibly processes. The unweighted mean for all cinnabar samples is 5.6 parts per thousand and the S source is considered to be the host-rocks. either the Footwall Shales (deltaS-34 = 5.5 parts per thousand) or the spilites (deltaS-34 = 5.1 +/- 1.3 parts per thousand). For geometric and chronologic reasons. the former seem the best potential source. However, the high deltaS-34 values of the San Francisco cinnabar cannot be explained without addition of heavy S from reduction of seawater sulphate. Orderly distributions of the deltaS-34 values are observed in all three orebodies: (1) their increase from the stratigraphic bottom to the top in the San Pedro orebody is explained by a Rayleigh process, and (2) the maxima in the centres of the San Francisco and San Nicolas orebodies are explained by mixing of the S transporting hydrothermal fluids with seawater within the sediments. Associated pyrite and cinnabar were deposited under isotopic disequilibrium, probably because the low solubility of cinnabar caused rapid precipitation of cinnabar. The different morphological pyrite types have their own isotopic deltaS-34 signatures. The spilites are notably enriched in S (n = 3; average S content = 0.56%) compared to normal basalts ( 1000 ppm) and have an average deltaS-34 = 5.1 +/- 1.3 parts per thousand. The linear relationship between the deltaS-34 and the S content of the spilites is interpreted as a mixing line between mantle S and a constant S source, probably an infinite open reservoir. An incomplete basalt-seawater reaction at nearly constant temperature is the best explanation for this relation. The S (predominantly pyrite) of the black shales (n = 3; deltaS-34 from -6.7 parts per thousand to 14.4 parts per thousand) is interpreted as resulting from bacterial seawater sulphate reduction under variable euxinic conditions. The stratabound pyrite (n = 16) found in the sediments near, but independent of the Hg ores, also has a wide range of deltaS-34 between 15.0 parts per thousand and 16.4 parts per thousand, with a mode of about 8 parts per thousand; the pyrite rims of the diagenetic dolomite nodules (n = 2; deltaS-34 = -14.7 +/- 0.35 parts per thousand) in the Hanging Wall Shales display the most negative values found at Almaden.