Abstract
Polymetallic deposits occur in the different morphotectonic units of the Cretaceous magmatic arc of the Chilean Andes. A preliminary regional study shows that the sulphur isotope composition (δ 34S cdt) of galena samples from 16 mines in these units in the sector between 24°11'S and 32°11'S shows distinct ranges: −27.4 to −35.0%, 8.1 to −0.1% and −2.1 to −18.9% for margin, basin, and arc, respectively. This variation reflects the control of the tectonic setting on the source of sulphur and the sulphur isotope fractionation processes. The very low ratios in the marginal marine setting are typical of bacterial sulphate reduction processes. Chemical and petrographical features of the host rock affect the α 34S cdt within individual morphotectonic units. The deposits in the magmatic arc hosted by lavas have a relatively high and restricted range of compositions (−2.1 to −3.8%) reflecting dominantly a magnetic source. Breccia hosted deposits are intermediate (−6.4 to −10.4%), while the lowest ratio (−18.9%) occurs in deposits hosted by laminated tuffs, reflecting an increased contribution from bacterially reduced sulphur. Carbonate hosted basin deposits have high ratios (up to −0.1%). The variation in sulphur isotope composition stands in marked contrast to the homogeneity of the lead isotope composition throughout these deposits.
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