Abstract

The lower portion of the Paleoproterozoic Northern Liaohe Group, Liaoning, northeast China, comprises metavolcanics (tuff-dominated) and metasediments (mostly arkoses and dolostones) that host stratiform-strata bound Fe(Cu) sulfides and Ca(Ba) sulfates. The ore-bearing sequence consists of, in ascending order, pyroclastic rocks, terrigenous clastics, and carbonates, all of which have been metamorphosed to greenschist amphibolite grade. Extensive albite-rich breccias are present at the base of the succession and as pipes crosscutting a variety of lithologic units in the Lieryu Formation. The geometry, geochemistry, and associated structures of these albite-rich rocks suggest that they are metamorphosed salt domes.The sulfide-sulfate Fe(Cu) ores occur in a transitional zone between tuffs and dolomitic carbonates, with those in the upper cycle being economically more important. The sulfides were precipitated at two sites: around venting centers as breccias, lenses, and veins; and as stratiform pyrite-chalcopyrite associated with anhydrite that is distal to the salt domes. Within each mining district the proximal ores are associated with brecciated, albite- and tourmaline-rich rocks. The sulfides in the proximal ores have delta 34 S values of 8.9 to 12.7 per mil, whereas the distal ores have delta 34 S values of 2.6 to 8.8 per mil for the sulfides and 7.9 to 19.0 per mil for the anhydrite; suggesting a sulfate-dominated sulfur source, with the pyrite delta 34 S variations arising mainly from decreasing temperatures away from the centers of the deposits.The origin of the deposits is best described by a model in which sulfate-carbonate rocks were deposited in peripheral sinks during salt diapirism. The salt domes then acted as focuses for subsequent hydrothermal venting and were the centers of sulfide mineralization. The sulfides precipitated during interaction of the reduced, metal-bearing hydrothermal fluids with the in situ sulfates that were the major source of the sulfur.

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