Rare-Earth-Element and Isotopic Evidence for the Genesis of the Bajiazi Stratabound Sulfide Ore Bodies of Northeast China

Abstract

This study examines the rare-earth-elemenl and isotopic geochemistry of sulfide ores and associated rocks in the Bajiazi sulfide deposits of northeastern China. The distribution and concentration of rare earth elements (REE) in sulfide ores and associated rocks from the Bajiazi deposits have been determined by inductively coupled plasma spectroscopy (ICP) methods. Birdwing-shaped rare-earth-element profiles are only observed in granitic rocks at Bajiazi. Diverse rare-earth-element profiles of the ores and dolostones are different from those of granite and are interpreted to reflect an early sedimentary-diagenetic mineralization stage during which rare earth elements were added to the ores and host rocks. The characteristic features of the abundances of REE in the Bajiazi samples, including granite and sedimentary host rocks, are demonstrated by a variety of Eu anomalies in the chondrite-normalized REE pattern. Eu is the most mobile element, exhibiting increasingly negative anomalies in granite rock samples relative to the depletion of the sulfide ores and associated sedimentary host rocks, which are considered to be caused by preferential scavenging of Eu from sea water to sediments. The δ34S values of sulfide minerals at Bajiazi generally range from −12.3 to 14.2%, suggesting reduction both of marine sulfate and biogenic sources. The δ34S values from different ore types, sulfide minerals, and mines indicate a sedimentary exhalative origin, although they were, to some extent, homogenized during late overprinting. The late-stage sulfides from Bajiazi are isotopieally remarkably homogeneous and are significantly, although slightly, enriched in δ34S. These results imply multiple sources of sulfur derived from biogenic reduction, and/or sulfate in oceanic and/or connate waters, or from marine evaporites, and/or from magmatic hydrothermal sources. Isotopic temperatures from intersulfide fractionations (pyrite, sphalerite, and galena) range from 75° to 542°C, indicating the Bajiazi sulfide minerals have experienced different mineralizing stages. Galena in the Bajiazi Proterozoic sediment-hosted Pb-Zn sulfide deposits has a very uniform Pb-isotope composition, with 206Pb/204Pb = 16.07 to 16.58, 207Pb/204Pb = 15.00 to 15.66, and 208Pb/204Pb = 36.13 to 36.92, suggesting that it all formed from a common mineralizing fluid. The variation of lead-isotopic values of ores is similar to that of their host sedimentary strata. The lead in the various ore types and host rocks within the Bajiazi district is virtually identical. Lead-isotope signatures, which generally plot very close to model crustal growth curves, characterized by a model age of ∼1350 Ma, indicate a normal, nonradiogenic origin and an Early Proterozoic upper-crustal source for the lead in the ores. That lead was unrelated to the radiogenic lead in the Mesozoic granite. The δ13C values range from +1 to −5% PDB, and the δ18O values from −6 to −15% PDB. Later crystallization generations are enriched in the light isotopes of carbon and oxygen compared to early generations. The characteristics of carbon and oxygen isotopes of the Bajiazi deposits indicate that: (1) primary ores were precipitated in the Proterozoic marine environment; (2) both ores and host carbonates from the Bajiazi district are remarkably homogeneous in their carbon- and oxygen-isotope compositions during later tectonic, conlact-metamorphic stages; (3) although the variations in isotopic composition of carbon and oxygen are relatively small, a significant evolution toward a lighter isotopic composition with advancing diagenetic, tectonic, and contact metamorphic processes is observed; and (4) this evolution is independent of the presence or absence of ore minerals.