S-ISOTOPIC-RATIOS OF THE SULFIDES OF THE CERRO-NEGRO MINING DISTRICT - A POSSIBLE INDICATOR OF THE CU STRATABOUND DEPOSITS ASSOCIATED WITH LACUSTRINE SEDIMENTS

Sulfides from Cerro Negro, a mining district in central Chile with str atabound (manto-type) Cu deposits hosted by sedimentary, mainly volcan oclastic rocks show a range in theta(34)S from -21.2 to -15.6 parts pe r thousand. This range is in contrast with values around 0 parts per t housand, which are typical for similar deposits in volcanic sequences, probably due to genetic differences. The S isotope ratios obtained fo r the Cerro Negro deposits can be explained by ore formation at low-te mperature, consistent with the hydrothermal alteration mineralogy of t he host rocks, deposited in a lacustrine environment containing organi c material. Presence of red beds in the sedimentary sequence hosting t he deposits suggests that circulating solutions were oxidized, allowin g transport of soluble Cu and Fe sulfates. interaction with organic ma terial would then reduce the sulfates, with precipitation of sulfides with theta(34)S S in the range measured at Cerro Negro. Bacterial acti vity may have been a contributing factor, but the available data do no t allow a conclusion. An ultimate magmatic origin for the S of the sul fates from which the sulfides were formed is indicated by of ca. +10 p arts per thousand,for barite occurring in the upper levels of the mine ralizaded bodies.