Mathematics – Logic
Scientific paper
Oct 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3765s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 10, pp.3765-3780
Mathematics
Logic
2
Scientific paper
Seventy-four new S isotope analyses of ore minerals and country rocks are given for the Hg deposit of Almadén. The spread of the cinnabar 34 S is narrow within each of the three orebodies, but the 34 S average values differ sufficiently between them (mean 34 S : San Nicolas = 0.2 ± 1.1 %., San Francisco = 8.1 ± 0.7%., San Pedro = 5.9 ± 1.0%.) to indicate three different mineralization episodes and possibly processes. The unweighted mean for all cinnabar samples is 5.6%. and the S source is considered to be the host-rocks, either the Footwall Shales ( 34 S = 5.5%.) or the spilites ( 34 S = 5.1 ± 1.3%.). For geometric and chronologic reasons, the former seem the best potential source. However, the high 34 S values of the San Francisco cinnabar cannot be explained without addition of heavy S from reduction of seawater sulphate. Orderly distributions of the 34 S values are observed in all three orebodies: 1. (1) their increase from the stratigraphic bottom to the top in the San Pedro orebody is explained by a Rayleigh process, and 2. (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 34 S 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 34 S = 5.1 ± 1.3%.. The linear relationship between the 34 S 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; 34 S from -6.7%. to 14.4%.) 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 34 S between 15.0%. and 16.4%., with a mode of about 8%.; the pyrite rims of the diagenetic dolomite nodules ( n = 2; 34 S = -14.7 ± 0.35%.) in the Hanging Wall Shales display the most negative values found at Almadén.
Arnold Michel
Saupé Francis
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